Abstract:
A heat transfer device having a working fluid capable of circulating around a fluid flow path, the circulation around the fluid flow path bringing the working fluid in and out of thermal contact with a heat source, the heat transfer device comprising: a fluid containing portion internally defining a working fluid flow path; a heat source at least partially in thermal contact with the fluid containing portion; a gas substance generator at least partially within the fluid containing portion, and arranged to generate bubbles of vapor capable of driving the working fluid along a portion of the working fluid flow path in thermal contact with the heat source; wherein, in use, the driven working fluid absorbs heat from the heat source and transports the heat away from the heat source; and the driven working fluid returns to the gas substance generator to be recycled about the fluid flow path.

Description:
RELATED APPLICATIONS 
     This application is a U.S. Utility Patent Application which claims priority to British Patent Application No. GB1104722.2, filed on Mar. 21, 2011, the entirety of which is herein incorporated by reference. 
     FIELD OF THE INVENTION 
     This invention relates to heat transfer devices and in particular heat transfer devices for use in solar energy converter devices which convert incident solar energy into heat and electricity. 
     Devices converting solar energy into electricity are known. One means of converting solar energy into electricity is the use of photovoltaic arrays. Photovoltaic arrays generally consist of semi-conductor materials appropriately encapsulated, and arranged to generate electricity when exposed to solar radiation. 
     Separately, devices converting solar energy into useable heat are known. A variety of thermal collection devices are known which absorb heat energy when exposed to solar radiation. These thermal solar collectors heat up as they absorb heat energy from solar radiation and this heat energy may then be extracted for use, for example by pumping a liquid flow, such as water, through the thermal collector in order to heat the liquid. 
     It has been proposed to combine these two technologies to provide a hybrid solar energy collector converting solar energy simultaneously into both electricity and heat. Such hybrid devices have been found to suffer from the problem that the elements of the photovoltaic array become hot when the device is operating. In general, the efficiency of photovoltaic elements drops as their temperature increases. Also, in general, photovoltaic elements subject to high temperatures may suffer degradation resulting in a permanent decrease in efficiency. As a result, in use, the electricity generating efficiency of the photovoltaic arrays of such hybrid devices tends to be low, and tends to reduce over time. 
     Accordingly, a heat transfer device suitable to transfer heat away from a solar collector is desirable. 
     SUMMARY OF THE INVENTION 
     A first aspect provides a heat transfer device comprising: 
     a fluid flow means extending between a first surface and a second surface, at least a part of the fluid flow means being inclined to the horizontal; 
     the fluid flow means being partially filled with a liquid and being arranged so that the first surface is in thermal contact with the liquid in at least a first part of the fluid flow means inclined to the horizontal and containing the liquid; and 
     the first part of the fluid flow means being divided into a first fluid flow channel and a second fluid flow channel arranged so that the liquid in the first fluid flow channel is in better thermal contact with the first surface than the liquid in the second fluid flow channel; 
     wherein the part of the fluid flow means above the surface of the liquid is at least partially evacuated; 
     whereby, when the first surface is hotter than the second surface, heat energy from the first surface causes the liquid in the first fluid flow channel to vaporize, and the vapor travels through the liquid in the first fluid flow channel to the surface of the liquid, such that the liquid circulates around the first fluid flow channel and the second fluid flow channel; 
     vapor travels from the surface of the liquid to the second surface and condenses at the second surface; and 
     condensed liquid returns from the second surface to the first part of the fluid flow means; whereby heat energy is transported from the first surface to the second surface. 
     Preferably, the first fluid flow channel is closer to the first surface than the second fluid flow channel. 
     Preferably, at least a part of the first fluid flow channel is located between the first surface and the second fluid flow channel. 
     Preferably, the first fluid flow channel lies between the first surface and the second fluid flow channel. 
     Preferably, each of the first and second fluid flow channels has a section bounded by a perimeter, and a proportion of the perimeter of the first fluid flow channel which is in thermal contact with the first surface is greater than a proportion of the perimeter of the second fluid flow channel which is in thermal contact with the first surface. 
     Preferably, the cross sectional area of the first fluid flow channel and the cross sectional area of the second fluid flow channel are equal. 
     Preferably, the first fluid flow channel is in thermal contact with the first surface across a greater area than the second fluid flow channel. 
     Preferably, the first part of the fluid flow means is inclined to the horizontal by an angle of up to 90°. 
     Preferably, at least a portion of at least one surface of the first fluid flow channel in thermal contact with the first surface comprises features arranged to promote vapor bubble nucleation. 
     Preferably, at least a portion of at least one surface of the first fluid flow channel in thermal contact with the first surface has a surface texture adapted to promote vapor bubble nucleation. 
     Preferably, said portion of at least one surface has a roughened surface texture. 
     Preferably, the roughened surface texture is provided by a solder layer. 
     Preferably, the first part of the fluid flow means is divided into a plurality of first fluid flow channels. 
     Preferably, the first part of the fluid flow means is divided into a plurality of first fluid flow channels and a plurality of second fluid flow channels. 
     Preferably, the number of first fluid flow channels is the same as the number of second fluid flow channels. 
     Preferably, the first and second fluid flow channels are located side by side with first fluid flow channels and second fluid flow channels interleaved. 
     Preferably, the or each first and second fluid flow channel has an upper end and a lower end, and the lower ends of the first and second fluid flow channels are connected together. 
     Preferably, the or each first and second fluid flow channel has an upper end and a lower end, and the upper ends of the first and second fluid flow channels are connected together. 
     Preferably, the upper ends of the first and second fluid flow channels are connected together by a manifold. 
     Preferably, the upper ends of the first and second fluid flow channels are connected together by a vapor manifold. 
     Preferably, vapor traveling from the surface of the liquid to the second surface passes through the manifold. 
     Preferably, condensed liquid returning from the second surface to the first part of the fluid flow means passes through the manifold. 
     Preferably, the liquid comprises water. 
     Preferably, the liquid comprises ethanol. 
     Preferably, the liquid comprises a mixture of water and ethanol. 
     Preferably, the mixture comprises up to 25% ethanol. 
     Preferably, the second surface is located above the first surface such that the condensed liquid returns from the second surface to the first part of the fluid flow means by gravity. 
     Preferably, at least a portion of a surface of the first fluid flow channel in thermal contact with the first surface has a dimpled surface profile. 
     Preferably, the dimpled surface profile comprises a regular array of dimples. 
     Preferably, the regular array of dimples comprises dimples arranged in rows separated by flat strips without dimples. 
     Preferably, the first and second fluid flow channels are located between first and second spaced apart plates. 
     Preferably, the first plate is in thermal contact with the first surface and forms a surface of the or each first fluid flow channel. 
     Preferably, there are a plurality of first fluid flow channels and a plurality of second fluid flow channels located side by side with first fluid flow channels and second fluid flow channels arranged alternately, and each first fluid flow channel is separated from an adjacent second fluid flow channel by a partition extending between and attached to the first plate and the second plate. 
     Preferably, the first plate has a dimpled surface profile comprising a regular array of dimples arranged in rows separated by flat strips without dimples, and each partition is attached to the first plate at a position located in one of the flat strips. 
     Preferably, the part of each partition extending between the first plate and the second plate is substantially flat. 
     Preferably, a plurality of the partitions are formed by a third plate. 
     Preferably, all of the partitions are formed by a single third plate. 
     Preferably, the third plate is corrugated. 
     Preferably, each of the plates comprises a metal or a metal alloy material. 
     Preferably, each of the plates comprises mild steel. 
     Preferably, each of the plates comprises tin coated mild steel. 
     Preferably, the plates are bonded together by a bonding technique including at least one of: soldering; spot welding; roller welding; and an adhesive. 
     Preferably, the plates are bonded together by solder joints and at least a part of the first plate forming a surface of each first fluid flow channel is coated with solder. 
     Preferably, the heat transfer device comprises a substantially rigid heat conducting structure. 
     Preferably, the part of the fluid flow means above the surface of the liquid is at a pressure of 40 mbar or less. 
     Preferably, the part of the fluid flow means above the surface of the liquid is at a pressure of 2 mbar or less. 
     Preferably, the part of the fluid flow means above the surface of the liquid is at a pressure of 1 mbar or less. 
     Preferably, the part of the fluid flow means above the surface of the liquid is at a pressure of 10 −2  mbar or less. 
     Preferably, the part of the fluid flow means above the surface of the liquid is at a pressure of 10 −3  mbar or less. 
     Preferably, the part of the fluid flow means above the surface of the liquid is at a pressure of 10 −6  mbar or less. 
     A second aspect provides a heat transfer device comprising: 
     a first fluid flow channel inclined to the horizontal and containing a liquid; 
     a second fluid flow channel connected to the first fluid flow channel and containing the liquid; and 
     a first surface in thermal contact with the liquid in the first fluid flow channel; 
     wherein heat energy from the first surface causes liquid in the first fluid flow channel to vaporize; 
     the vapor travels upwardly along the first fluid flow channel; and 
     the vapor drives a flow of liquid from the second fluid flow channel to the first fluid flow channel and upwardly along the first fluid flow channel; 
     whereby heat energy is transported away from the first surface. 
     Preferably, the first fluid flow channel is closer to the first surface than the second fluid flow channel. 
     Preferably, at least a part of the first fluid flow channel is located between the first surface and the second fluid flow channel. 
     Preferably, the first fluid flow channel lies between the first surface and the second fluid flow channel. 
     Preferably, each of the first and second fluid flow channels has a section bounded by a perimeter, and a proportion of the perimeter of the first fluid flow channel which is in thermal contact with the first surface is greater than a proportion of the perimeter of the second fluid flow channel which is in thermal contact with the first surface. 
     Preferably, the cross sectional area of the first fluid flow channel and the cross sectional area of the second fluid flow channel are equal. 
     Preferably, the cross sectional area of the first fluid flow channel and the cross sectional area of the second fluid flow channel are equal. 
     Preferably, the first fluid flow channel is in thermal contact with the first surface across a greater area than the second fluid flow channel. 
     Preferably, the first fluid flow channel is inclined to the horizontal by an angle of up to 90°. 
     Preferably, at least a portion of at least one surface of the first fluid flow channel in thermal contact with the first surface comprises features arranged to promote vapor bubble nucleation. 
     Preferably, at least a portion of at one surface of the first fluid flow channel in thermal contact with the first surface has a surface texture adapted to promote vapor bubble nucleation. 
     Preferably, said portion of at least one surface has a roughened surface texture. 
     Preferably, the roughened surface texture is provided by a solder layer. 
     Preferably, the heat transfer device comprises a plurality of first fluid flow channels. 
     Preferably, the heat transfer device comprises a plurality of first fluid flow channels and a plurality of second fluid flow channels. 
     Preferably, the number of first fluid flow channels is the same as the number of second fluid flow channels. 
     Preferably, the first and second fluid flow channels are located side by side with first fluid flow channels and second fluid flow channels interleaved. 
     Preferably, wherein the or each first and second fluid flow channel has an upper end and a lower end, and the lower ends of the first and second fluid flow channels are connected together. 
     Preferably, the heat transfer device further comprises: 
     a second surface; 
     at least one vapor channel connecting the first and second fluid flow channels to the second surface; 
     whereby, when the first surface is hotter than the second surface, vapor travels from a surface of the liquid to the second surface through the vapor channel and condenses at the second surface; and 
     condensed liquid returns from the second surface to the first and second fluid flow channels; 
     whereby heat energy is transported away from the first surface to the second surface. 
     Preferably, wherein the or each first and second fluid flow channel has an upper end and a lower end, and the upper ends of the first and second fluid flow channels are connected together. 
     Preferably, the upper ends of the first and second fluid flow channels are connected together by a manifold. 
     Preferably, the upper ends of the first and second fluid flow channels are connected together by a vapor manifold. 
     Preferably, vapor traveling from the surface of the liquid to the second surface passes through the manifold. 
     Preferably, condensed liquid returning from the second surface to the first part of the fluid flow means passes through the manifold. 
     Preferably, the liquid comprises water. 
     Preferably, wherein the liquid comprises ethanol. 
     Preferably, the liquid comprises a mixture of water and ethanol. 
     Preferably, the mixture comprises up to 25% ethanol. 
     Preferably, the second surface is located above the first surface such that the condensed liquid returns from the second surface to the first part of the fluid flow means by gravity. 
     Preferably, at least a portion of a surface of the first fluid flow channel in thermal contact with the first surface has a dimpled surface profile. 
     Preferably, the dimpled surface profile comprises a regular array of dimples. 
     Preferably, the regular array of dimples comprises dimples arranged in rows separated by flat strips without dimples. 
     Preferably, the first and second fluid flow channels are located between first and second spaced apart plates. 
     Preferably, the first plate is in thermal contact with the first surface and forms a surface of the or each first fluid flow channel. 
     Preferably, there are a plurality of first fluid flow channels and a plurality of second fluid flow channels located side by side with first fluid flow channels and second fluid flow channels interleaved, and each first fluid flow channel is separated from an adjacent second fluid flow channel by a partition extending between and attached to the first plate and the second plate. 
     Preferably, the first plate has a dimpled surface profile comprising a regular array of dimples arranged in rows separated by flat strips without dimples, and each partition is attached to the first plate at a position located in one of the flat strips. 
     Preferably, the part of each partition extending between the first plate and the second plate is substantially flat. 
     Preferably, a plurality of the partitions are formed by a third plate. 
     Preferably, all of the partitions are formed by a single third plate. 
     Preferably, the third plate is corrugated. 
     Preferably, each of the plates comprises a metal or a metal alloy material. 
     Preferably, each of the plates comprises mild steel. 
     Preferably, each of the plates comprises tin coated mild steel. 
     Preferably, the plates are bonded together by a bonding technique including at least one of: soldering; spot welding; roller welding; and an adhesive. 
     Preferably, the plates are bonded together by solder joints and at least a part of the first plate forming a surface of each first fluid flow channel is coated with solder. 
     Preferably, the heat transfer device comprises a substantially rigid heat conducting structure. 
     Preferably, the heat transfer device above the liquid is at least partially evacuated. 
     Preferably, the heat transfer device above the liquid is at a pressure of 40 mbar or less. 
     Preferably, the heat transfer device above the liquid is at a pressure of 2 mbar or less. 
     Preferably, the heat transfer device above the liquid is at a pressure of 1 mbar or less. 
     Preferably, the heat transfer device above the liquid is at a pressure of 10 −2  mbar or less. 
     Preferably, the heat transfer device above the liquid is at a pressure of 10 −3  mbar or less. 
     Preferably, the heat transfer device above the liquid is at a pressure of 10 −6  mbar or less. 
     A third aspect provides a heat transfer device comprising: 
     a first surface; 
     a second surface; 
     a liquid reservoir in thermal contact with the first surface and containing a liquid; and 
     a tube connecting the liquid reservoir to the second surface; 
     wherein the liquid reservoir comprises a first fluid flow channel inclined to the horizontal and containing the liquid and a second fluid flow channel connected to the first fluid flow channel and containing the liquid; 
     the first surface is in thermal contact with the liquid in the first fluid flow channel; and 
     at least a part of the tube is at least partially evacuated; 
     whereby, when the first surface is hotter than the second surface, heat energy from the first surface causes liquid in the first fluid flow channel to vaporize; 
     the vapor travels upwardly along the first fluid flow channel and through the tube, and condenses at the second surface; 
     the vapor drives a flow of liquid from the second fluid flow channel to the first fluid flow channel and upwardly along the first fluid flow channel; and 
     condensed liquid returns from the second surface to the liquid reservoir; 
     whereby heat energy is transported away from the first surface to the second surface. 
     Preferably, the first fluid flow channel is closer to the first surface than the second fluid flow channel. 
     Preferably, at least a part of the first fluid flow channel is located between the first surface and the second fluid flow channel. 
     Preferably, the first fluid flow channel lies between the first surface and the second fluid flow channel. 
     Preferably, each of the first and second fluid flow channels has a section bounded by a perimeter, and a proportion of the perimeter of the first fluid flow channel which is in thermal contact with the first surface is greater than a proportion of the perimeter of the second fluid flow channel which is in thermal contact with the first surface. 
     Preferably, the cross sectional area of the first fluid flow channel and the cross sectional area of the second fluid flow channel are equal. 
     Preferably, the first fluid flow channel is in thermal contact with the first surface across a greater area than the second fluid flow channel. 
     Preferably, the first fluid flow channel is inclined to the horizontal by an angle of up to 90°. 
     Preferably, at least a portion of at least one surface of the first fluid flow channel in thermal contact with the first surface comprises features arranged to promote vapor bubble nucleation. 
     Preferably, at least a portion of at least one surface of the first fluid flow channel in thermal contact with the first surface has a surface texture adapted to promote vapor bubble nucleation. 
     Preferably, said portion of at least one surface has a roughened surface texture. 
     Preferably, the roughened surface texture is provided by a solder layer. 
     Preferably, the liquid reservoir comprises a plurality of first fluid flow channels. 
     Preferably, the first part of the fluid flow means is divided into a plurality of first fluid flow channels and a plurality of second fluid flow channels. 
     Preferably, the number of first fluid flow channels is the same as the number of second fluid flow channels. 
     Preferably, the first and second fluid flow channels are located side by side with first fluid flow channels and second fluid flow channels arranged alternately. 
     Preferably, the or each first and second fluid flow channel has an upper end and a lower end, and the lower ends of the first and second fluid flow channels are connected together. 
     Preferably, the or each first and second fluid flow channel has an upper end and a lower end, and the upper ends of the first and second fluid flow channels are connected together. 
     Preferably, the upper ends of the first and second fluid flow channels are connected together by a manifold. 
     Preferably, the upper ends of the first and second fluid flow channels are connected together by a vapor manifold. 
     Preferably, vapor traveling from the surface of the liquid to the second surface passes through the manifold. 
     Preferably, condensed liquid returning from the second surface to the first part of the fluid flow means passes through the manifold. 
     Preferably, the liquid comprises water. 
     Preferably, the liquid comprises ethanol. 
     Preferably, the liquid comprises a mixture of water and ethanol. 
     Preferably, the mixture comprises up to 25% ethanol. 
     Preferably, the second surface is located above the first surface such that the condensed liquid returns from the second surface to the first part of the fluid flow means by gravity. 
     Preferably, at least a portion of a surface of the first fluid flow channel in thermal contact with the first surface has a dimpled surface profile. 
     Preferably, the dimpled surface profile comprises a regular array of dimples. 
     Preferably, the regular array of dimples comprises dimples arranged in rows separated by flat strips without dimples. 
     Preferably, the first and second fluid flow channels are located between first and second spaced apart plates. 
     Preferably, the first plate is in thermal contact with the first surface and forms a surface of the or each first fluid flow channel. 
     Preferably, there are a plurality of first fluid flow channels and a plurality of second fluid flow channels located side by side with first fluid flow channels and second fluid flow channels arranged alternately, and each first fluid flow channel is separated from an adjacent second fluid flow channel by a partition extending between and attached to the first plate and the second plate. 
     Preferably, the first plate has a dimpled surface profile comprising a regular array of dimples arranged in rows separated by flat strips without dimples, and each partition is attached to the first plate at a position located in one of the flat strips. 
     Preferably, the part of each partition extending between the first plate and the second plate is substantially flat. 
     Preferably, a plurality of the partitions are formed by a third plate. 
     Preferably, all of the partitions are formed by a single third plate. 
     Preferably, the third plate is corrugated. 
     Preferably, each of the plates comprises a metal or a metal alloy material. 
     Preferably, each of the plates comprises mild steel. 
     Preferably, each of the plates comprises tin coated mild steel. 
     Preferably, the plates are bonded together by a bonding technique including at least one of: soldering; spot welding; roller welding; and an adhesive. 
     Preferably, the plates are attached together by solder joints and at least a part of the first plate forming a surface of each first fluid flow channel is coated with solder. 
     Preferably, the heat transfer device comprises a substantially rigid heat conducting structure. 
     Preferably, the tube is at a pressure of 40 mbar or less. 
     Preferably, the tube is at a pressure of 2 mbar or less. 
     Preferably, the tube is at a pressure of 1 mbar or less. 
     Preferably, the tube is at a pressure of 10 −2  mbar or less. 
     Preferably, the tube is at a pressure of 10 −3  mbar or less. 
     Preferably, the tube is at a pressure of 10 −6  mbar or less. 
     A fourth aspect provides a heat transfer device comprising: 
     a first surface; 
     a second surface; 
     a liquid reservoir in thermal contact with the first surface and containing a liquid; and 
     a tube connecting the liquid reservoir to the second surface; 
     wherein at least a part of the tube is at least partially evacuated; 
     whereby, when the first surface is hotter than the second surface, heat energy from the first surface causes liquid in the liquid reservoir to vaporize; 
     the vapor travels through the tube and condenses at the second surface; and 
     condensed liquid returns from the second surface to the liquid reservoir; 
     whereby heat energy is transported from the first surface to the second surface. 
     Preferably, at least a portion of a surface of the fluid reservoir in thermal contact with the first surface comprises features arranged to promote vapor bubble nucleation. 
     Preferably, at least a portion of a surface of the fluid reservoir in thermal contact with the first surface has a surface texture adapted to promote vapor bubble nucleation. 
     Preferably, said portion of the surface has a roughened surface texture. 
     Preferably, the roughened surface texture is provided by a solder layer. 
     Preferably, condensed liquid returning from the second surface to the fluid reservoir travels through the tube. 
     Preferably, the liquid comprises water. 
     Preferably, the liquid comprises ethanol. 
     Preferably, the liquid comprises a mixture of water and ethanol. 
     Preferably, the mixture comprises up to 25% ethanol. 
     Preferably, the second surface is located above the first surface such that the condensed liquid returns from the second surface to the fluid reservoir by gravity. 
     Preferably, at least a portion of a surface of the fluid reservoir in thermal contact with the first surface has a dimpled surface profile. 
     Preferably, the dimpled surface profile comprises a regular array of dimples. 
     Preferably, the regular array of dimples comprises dimples arranged in rows separated by flat strips without dimples. 
     Preferably, the tube is at a pressure of 40 mbar or less. 
     Preferably, the tube is at a pressure of 2 mbar or less. 
     Preferably, the tube is at a pressure of 1 mbar or less. 
     Preferably, the tube is at a pressure of 10 −2  mbar or less. 
     Preferably, the tube is at a pressure of 10 −3  mbar or less. 
     Preferably, the tube is at a pressure of 10 −6  mbar or less. 
     Preferably, the heat transfer device comprises a substantially rigid heat conducting structure. 
     A fifth aspect provides a heat transfer device having a working fluid capable of circulating around a fluid flow path, the circulation around the fluid flow path bringing the working fluid in and out of thermal contact with a heat source, the heat transfer device comprising: 
     a fluid containing portion internally defining a working fluid flow path; 
     a heat source at least partially in thermal contact with the fluid containing portion; 
     a gas substance generator at least partially within the fluid containing portion, and arranged to generate bubbles of vapor capable of driving the working fluid along a portion of the working fluid flow path in thermal contact with the heat source; 
     wherein, in use, the driven working fluid absorbs heat from the heat source and transports the heat away from the heat source; and 
     the driven working fluid returns to the gas substance generator to be recycled about the fluid flow path. 
     Preferably, the gas substance generator comprises a hot vapor generation surface configured to at least partially heat-vaporize the working fluid such that vapor bubbles generated within the working fluid drive the working fluid along the fluid flow path defined internally of the fluid containing portion. 
     Preferably, the heat source is the hot vapor generation surface of the gas substance generator. 
     Preferably, the fluid flow path defined internally of the fluid containing portion is arranged such that the driving of the working fluid along the portion of the working fluid flow path in thermal contact with the heat source is unimpeded by the returning of the driven working fluid to the gas substance generator. 
     Preferably, the fluid flow path comprises a plurality of portions of the working fluid flow path in thermal contact with the heat source. 
     Preferably, the fluid flow path comprises a plurality of return portions returning the driven working fluid to the gas substance generator. 
     Preferably, the heat transfer device is configured so that the driven working fluid travels with an upward component of direction along the portion of the working fluid flow path in thermal contact with the heat source, and returns to the gas substance generator at least partially under the action of gravity. 
     Preferably, at least a partial vacuum is maintained in the fluid containing portion above the working fluid. 
     Preferably, the rate of gas substance generation is determined, at least in part, by an operating temperature of the heat transfer device. 
     Preferably, the heat transfer device is configured so that the operating temperature achieving a predetermined rate of gas substance generation is controllable by means of varying a pressure level in the fluid containing portion above the working fluid. 
     Preferably, the heat transfer device further comprising at least one photovoltaic element having a first light incident surface and a second heat emitting surface, wherein said heat source is configured to be provided with heat from the heat emitting surface of the at least one photovoltaic element. 
     Preferably, the heat transfer device comprises a plurality of photovoltaic elements. 
     Preferably, the plurality of photovoltaic elements are comprised in an array. 
     Preferably, the heat emitting surface of the at least one photovoltaic element is thermally coupled to the heat source across a predetermined area such that, in use, the heat source is configured to be provided with heat from all, or substantially all, of the heat emitting surfaces of the array of photovoltaic elements. 
     Preferably, the heat transfer device is configured such that the heat source maintains a substantially uniform temperature across the predetermined area. 
     Preferably, the heat transfer device further comprises a heat exchanger configured so that, in use, the heat exchanger cools the working fluid. 
     Preferably, the heat transfer device is configured so that, in use, the working fluid is at least partially heat-vaporized to generate vapor, the vapor passes through the fluid containing portion to the heat exchanger and condenses at the heat exchanger, whereby the heat exchanger cools the working fluid. 
     Preferably, the heat transfer device is configured so that, in use, the heat exchanger is at least partially in contact with the working fluid. 
     Preferably, at least a part of the heat transfer device is located in an envelope under at least a partial vacuum. 
     Preferably, the envelope is one of: a cylindrical tube; an elliptical tube. 
     Preferably, the envelope is formed, at least in part, of glass. 
     Preferably, a plurality of tubes are mounted in a solar energy collecting array. 
     Preferably, at least one of the plurality of tubes is rotatable to track light incident on the solar energy collecting array. 
     Preferably, the plurality of tubes are rotatable to track light incident on the solar energy collecting array. 
     Preferably, the heat transfer device comprises a substantially rigid heat conducting structure. 
     A sixth aspect provides an energy generator comprising a heat transfer device according to any preceding claim, and at least one photovoltaic element, the energy generator having an electrical output and a heated fluid output. 
     The invention further provides systems, devices and articles of manufacture for implementing any of the aforementioned aspects of the invention. 
    
    
     
       DESCRIPTION OF FIGURES 
       The invention will now be described in detail with reference to the following figures in which: 
         FIG. 1  is a diagram of a first embodiment of a hybrid solar energy converter according to the invention; 
         FIG. 2  is a diagram of a tube useable in the hybrid solar energy converter of  FIG. 1 ; 
         FIG. 3  is a diagram of a solar energy collector assembly useable in the hybrid solar energy converter of  FIG. 1 ; 
         FIG. 4  is a side view of the solar energy collector assembly of  FIG. 3 ; 
         FIG. 5  is a cut away diagram of the solar energy collector assembly of  FIG. 3 ; 
         FIG. 6  is a transverse cross-sectional diagram of the solar energy collector assembly of  FIG. 3 ; 
         FIG. 7  is a longitudinal cross-sectional diagram of the solar energy collector assembly of  FIG. 3 ; 
         FIG. 8  is a longitudinal cross section diagram of a heat exchange assembly useable in the hybrid solar energy converter of  FIG. 1 ; 
         FIG. 9  is a cut away diagram of an alternative arrangement of a part of the heat exchange assembly of  FIG. 8 ; 
         FIG. 10  is a diagram of a second embodiment of a hybrid solar energy converter according to the invention; 
         FIG. 11  is a cut away diagram of a solar energy collector assembly useable in the hybrid solar energy converter of  FIG. 10 ; 
         FIG. 12  is a transverse cross-section along the line A-A of the solar energy collector assembly of  FIG. 11 ; 
         FIG. 13  is a longitudinal cross-sectional diagram along the line B-B of the solar energy collector assembly of  FIG. 11 ; 
         FIG. 14  is a diagram of a central sheet useable in the solar energy collector assembly of  FIG. 11 ; 
         FIG. 15  is an explanatory diagram illustrating the operation of the solar energy collector assembly of  FIG. 11 ; 
         FIG. 16  is a transverse cross section along the line C-C of the solar energy collector assembly of  FIG. 11 ; 
         FIG. 17A  is an explanatory diagram of the solar energy collector assembly of  FIG. 11 ; 
         FIG. 17B  is an explanatory diagram of the solar energy collector assembly of  FIG. 11 ; 
         FIG. 18A  is a detailed plan view of a part of the solar energy collector assembly of  FIG. 11 ; 
         FIG. 18B  is a cross section along the line D-D of a the part of the solar energy collector assembly of  FIG. 11 ; 
         FIG. 19  is a diagram showing a part of the solar energy collector assembly of  FIG. 11  with the photovoltaic elements removed; 
         FIG. 20  is a diagram of a third embodiment of a hybrid solar energy converter according to the invention; 
         FIG. 21  is a cut away diagram of a part of a solar energy collector assembly useable in the hybrid solar energy converter of  FIG. 20 ; 
         FIG. 22  is a diagram of an alternative transparent tube useable in a hybrid solar energy converter according to the invention; 
         FIG. 23  is a diagram of a solar energy collector arranged for rotation about a single axis; and 
         FIG. 24  is a diagram of a solar energy collector array arranged for rotation about two axes. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     Apparatus according to a first embodiment of the present invention is illustrated in  FIG. 1 .  FIG. 1  shows a general exterior view of a first embodiment of a hybrid solar energy converter  1  according to the present invention. 
     Overview 
     In the first embodiment, the hybrid solar energy converter  1  includes a solar energy collector assembly  2  housed within a sealed transparent tube  3 . The solar energy collector assembly  2  includes a heat transport element  4  and an array of photovoltaic elements  5  mounted on an upper surface of the heat transport element  4 . The hybrid solar energy converter  1  also includes a heat exchange assembly  6  at one end of the transparent tube  3 . One end of the solar energy collector assembly  2  is connected to the heat exchange assembly  6 . In one example the photovoltaic elements  5  may be formed of silicon. In another example the photovoltaic elements  5  may be formed of gallium arsenide. In other examples, photovoltaic elements formed of other semiconductor materials may be used. In other examples organic photovoltaic elements may be used. In other examples hybrid photovoltaic elements may be used. 
     Photovoltaic elements may also be referred to as photovoltaic cells, solar cells or photoelectric cells. For the avoidance of doubt, in the present application the term photovoltaic element is used to refer to any element which converts incident electromagnetic radiation into electrical energy. 
     In the first embodiment, the heat exchange assembly  6  includes a primary heat exchanger  7  arranged to transfer heat energy from the heat transport element  4  to a first fluid, and a secondary heat exchanger  8  arranged to transfer heat energy from the heat transport element  4  to a second fluid. The primary heat exchanger  7  and the secondary heat exchanger  8  are separated by a heat transfer control valve  9  able to selectively allow, or prevent, the transfer of heat energy from the heat transport element  4  to the secondary heat exchanger  8 . 
     In one possible example, in use the hybrid solar energy converter  1  may be mounted on a roof. In the illustrated embodiment mounting brackets  10  and  11  are provided. Mounting bracket  10  supports the transparent tube  3  and mounting bracket  11  is attached to the heat exchange assembly  6 . The transparent tube  3  is secured to the mounting bracket  10  by a strap or clip  12  of a plastics material in order to reduce the risk of accidental damage to the transparent tube  3 . 
     An overview of operation of the hybrid solar energy converter  1  is that the solar energy, in other words sunlight, incident on the hybrid solar energy converter  1  passes through the sealed transparent tube  3  and is incident on the photovoltaic elements  5  of the solar energy collector assembly  2 . The photovoltaic elements  5  convert a part of the energy of the incident solar energy into electrical energy, and convert a part of the energy of the incident solar energy into heat energy. A further part of the incident solar energy may be incident on any parts of the solar energy collector assembly  2  which are not covered by the photovoltaic elements  5 . This further part of the incident solar energy may also be converted into heat energy. In general, it is desirable to maximize the proportion of the surface of the solar energy collector assembly  2  exposed to incident solar energy which is covered by the photovoltaic elements  5 , and to minimize the proportion which is not so covered. However, in some circumstances it may be preferred to leave some parts of this exposed surface uncovered, for example to simplify manufacture and/or assembly of the solar energy collector assembly  2  and attachment of the photovoltaic elements  5  to the solar energy collector assembly  2 . Usually, in the first embodiment the surface of the solar energy collector assembly exposed to incident solar energy will be the upper surface. 
     The electrical energy produced by the photovoltaic elements  5  is carried along the heat transport element  4  by electrical conductors (not shown in  FIG. 1 ) and away from the solar energy converter  1  for use. The heat energy absorbed by the photovoltaic elements  5  is transferred into the heat transport element  4 , cooling the photovoltaic elements  5 , and then carried to the heat exchange assembly  6 . 
     As explained above, the heat transfer control valve  9  is able to selectively allow, or prevent, the transfer or transport of heat energy from the heat transport element  4  to the secondary heat exchanger  8 . Accordingly, at the heat exchange assembly  6  the heat energy from the heat transfer element  4  is selectively passed under the control of the heat transfer control valve  9  either to the primary heat exchanger  7  only, or to both the primary heat exchanger  7  and the secondary heat exchanger  8 . By selecting whether the heat energy is transferred to the primary heat exchanger  7  only, or to both the primary heat exchanger  7  and the secondary heat exchanger  8  the degree of cooling applied to the photovoltaic elements  5  can be varied. 
     In one typical arrangement, the hybrid solar energy converter  1  may be used in a domestic situation, such as on a household roof, to generate electricity for household use and/or for export, and to generate hot water for a domestic hot water and/or heating system. In this arrangement the heat energy transferred to the primary heat exchanger  7  is transferred into a pumped water supply flowing through the primary heat exchanger  7  to heat the water. This heated water is then used by the domestic hot water and heating system, and the electrical energy produced by the photovoltaic elements is supplied to a domestic electrical supply system. In this arrangement the heat energy transferred to the secondary heat exchanger  8  is transferred into ambient air and allowed to escape into the atmosphere. The secondary heat exchanger  8  is used, under the selective control of the heat transfer control valve  9 , to release heat energy into the atmosphere in order to regulate the temperature of the solar energy collector assembly  2 . 
     The efficiency of semiconductor photovoltaic elements generally drops as the temperature of the semiconductor material rises. The temperature above which efficiency drops with increasing temperature and the rate at which efficiency drops with increasing temperature will vary for different semiconductor materials and different designs of photovoltaic element. For silicon photovoltaic elements the efficiency of electrical energy generation generally drops by about 0.35% to 0.5% for each degree centigrade of temperature increase above 25° C. 
     Transparent Tube 
     In the first embodiment illustrated in  FIG. 1  the sealed transparent tube  3  is formed by a cylindrical glass tube having one open end  3   a  and one closed domed end  3   b . The sealed transparent tube  3  is illustrated in more detail in  FIG. 2 . The open end  3   a  of the cylindrical glass tube is sealed by a metal cap  12  which is bonded to the glass tube with adhesive to form an air tight seal. The interior of the tube  3  is at least partially evacuated. That is, the interior of the tube is at a pressure below normal atmospheric pressure. The pressure of the vacuum within the tube  3  may be 10 −3  mbar. 
     The open end  3   a  of the cylindrical glass tube sealed by the cap  12  is attached to the heat exchange assembly  6  and the closed domed end  3   b  is remote from the heat exchange assembly  6 . 
     Insulated electrical conductors  21  pass through the metal cap  12  to carry the electrical energy generated by the photovoltaic elements  5  away from the solar energy collector assembly  3 . The heat transport element  4  of the solar energy collector assembly  2  has a projecting tube  13  which passes through the metal cap  12  in order to carry heat energy from the solar energy collector assembly  3  to the heat exchange assembly  6 . 
     As discussed above, the solar energy collector assembly  2  housed within the transparent tube  3  includes photovoltaic elements  5 . Typically, photovoltaic devices are made from semiconductor materials which may suffer from oxidation and other environmental effects adversely affecting their performance and lifetime when exposed to the atmosphere. The use of an evacuated tube  3  may protect the semi-conductor materials of the photovoltaic elements  5  from such environmental damage. This may allow the cost of encapsulating the photovoltaic elements to be avoided. 
     The use of an evacuated tube may also increase the efficiency with which heat can be collected from incident solar energy by the solar energy collector assembly  2 . Having the solar energy collector assembly  2  surrounded by an evacuated tube  3  may reduce or effectively prevent convective heat loss from the solar energy collector assembly  2  into the material of the transparent tube  3  and the air around the hybrid solar energy converter  1 . 
     In alternative example a different vacuum pressure may be used. In some examples the vacuum pressure may be in the range 10 −2  mbar to 10 −6  mbar. In general, it is expected that lower vacuum pressure, or in other words a harder vacuum, will provide greater insulating benefits. Further, it is expected that lower vacuum pressure, or in other words a harder vacuum, will provide greater protection from environmental damage in examples where the photovoltaic elements are not encapsulated. In practice the benefits of using a lower vacuum pressure may need to be balanced against the increased cost of achieving a lower vacuum pressure. In some examples a vacuum pressure of 10 −2  mbar, or lower, may be used. 
     In an alternative example the sealed transparent tube  3  may be filled with an inert gas instead of being evacuated. In particular, the inert gas may be nitrogen. 
     In another alternative example the sealed transparent tube  3  may be filled with an inert gas at a reduced pressure. In some examples this may be achieved by filling the tube  3  with the inert gas and then evacuating the tube  3 . In particular, the inert gas may be nitrogen. 
     In the illustrated first embodiment the tube  3  is cylindrical having a circular cross section. The use of a circular cross section shape may increase the strength of the evacuated tube to resist the atmospheric pressure acting on the evacuated tube. In alternative examples the tube may have other shapes. In some examples the cross sectional size and/or shape of the tube may vary at different positions along its length. 
     In an alternative example the tube may have an elliptical cross section. In particular, the tube  3  may have an elliptical cross section with the long axis of the ellipse aligned with the plane of the solar energy collector assembly  2 . The use of a tube  3  having an elliptical cross-section with the long axis of the ellipse aligned with the plane of the solar energy collector assembly may reduce the amount of glass required by the tube  3  and may reduce reflection losses due to the reflection of incident solar energy from the tube  3 . 
     In the illustrated first embodiment the tube  3  is formed of glass. The use of glass may allow the vacuum within the tube  3  to be maintained longer because the rate of migration of gas molecules from the atmosphere through glass is, in practice, effectively zero. In alternative examples suitable transparent plastics materials or laminated structures may be used to form the tube  3 . 
     In the illustrated first embodiment the tube  3  is transparent. In alternative examples the tube may be only partially transparent. 
     In the illustrated first embodiment the metal end cap  12  is bonded to the glass tube  3  by adhesive. In other examples alternative glass to metal bonding techniques may be used, for example welding, brazing or soldering. 
     In the illustrated first embodiment the tube  3  has a metal end cap  12  at one end. In alternative examples the end cap  12  may be made of other materials. In some examples the end cap  12  may be made of glass. This may reduce conductive heat losses from the collector assembly  2 . 
     Collector Assembly 
     The solar energy collector assembly  2  according to the first embodiment is illustrated in  FIGS. 3 and 4 . The solar energy collector assembly  2  includes a heat transport element  4  and an array of photovoltaic elements  5  mounted on one surface of the heat transport element  4 . In order to allow radiant solar energy to be incident on the photovoltaic elements  5  the array of photovoltaic elements  5  will usually be mounted on the surface of the heat transport element  4  exposed to the incident radiant solar energy in operation of the hybrid solar energy converter  1 . This will usually be the upper surface of the heat transport element  4 . 
     In some arrangements the surface of the heat transport element  4  exposed to the incident radiant solar energy may not be the upper surface. In particular, this would be the case if the solar energy collector assembly  2  was located in a vertical, or substantially vertical, plane, or if the incident solar radiant energy was incident horizontally or from below, for example after redirection by an optical system, such as a mirror. Accordingly, references to upper and lower surfaces, and similar directional terminology in this description, should be understood as referring to the situation illustrated in the figures where the solar energy collector assembly is in a plane at an angle to the horizontal and radiant solar energy is incident from above. 
     In the illustrated example of the first embodiment, the solar energy collector assembly  2  is supported by a cylindrical tube  13  of the heat transport element  4 . The cylindrical tube  13  passes through the end cap  12  and into the heat exchange assembly  6 , as will be explained in more detail below. Where the cylindrical tube  13  passes through the end cap  12  the cylindrical tube  13  is soldered to the end cap  12  to retain the cylindrical tube  13  in place and support the solar energy collector assembly  2 . 
     In alternative examples the cylindrical tube  13  may be secured to the end cap  12  in other ways. In one example the cylindrical tube  13  may be welded to the end cap  12 . 
     The supporting of the solar energy collector assembly  2  by a single physical connection through the cylindrical tube  13  may increase the efficiency with which heat can be collected from incident solar energy by the solar energy collector assembly  2 . Having the solar energy collector assembly  2  supported by a single physical connection through the cylindrical tube  13  may reduce conductive heat loss from the solar energy collector assembly  2  into the supporting structure outside the transparent tube. 
     In the first embodiment, the heat transport element  4  is substantially trapezoid in cross section, having a substantially flat upper surface  4   a  and a substantially flat lower surface  4   b . Each of the photovoltaic elements  5  is square, and the width of the heat transport element  4  is the same as the width of each square photovoltaic element  5 . In the illustrated embodiment, seven square photovoltaic elements  5  are mounted side by side to one another along the length of the heat transport element  4 . Substantially the entire upper face of the heat transport element  4  is covered by the photovoltaic elements  5 . Covering a large proportion of the heat transport element with photovoltaic elements may increase the efficiency of the hybrid solar energy converter. 
     The photovoltaic elements  5  are bonded to the substantially flat upper surface  4   a  of the heat transport element  4  using a layer  49  of heat conducting adhesive. This thermally conductive adhesive bonding layer  49  is shown in  FIG. 7 . The adhesive bonding layer  49  is electrically insulating. The adhesive bonding layer  49  between the photovoltaic elements  5  and the heat transport element  4  is arranged to be thin. This may improve the degree of thermal conduction between the photovoltaic elements  5  and the heat transport element  4 . This may increase the rate of heat transfer laterally across the photovoltaic elements  5 . An adhesive material loaded with solid spheres of a predetermined size may be used to form the adhesive bonding layer  49 . This may allow a thin adhesive layer  49  to be consistently and reliably formed. The adhesive bonding layer  49  is formed of a flexible or “forgiving” adhesive material. This may relieve stresses in the assembled solar energy collector assembly  2  and reduce any stress applied to the photovoltaic elements  5 . 
     The photovoltaic elements  5  are semiconductor photovoltaic elements formed of silicon. In one embodiment the photovoltaic elements are formed of single-crystal silicon. In one embodiment the photovoltaic elements are formed of amorphous silicon. In one embodiment the photovoltaic elements are formed of polycrystalline silicon, or polysilicon. In other embodiments alternative types of semiconductor photovoltaic elements may be used. 
     As discussed above, in operation of the hybrid solar energy converter  1  the photovoltaic elements  5  are cooled by the heat transport element  4 . This cooling may allow the temperature of the photovoltaic elements  5  to be maintained at a desired value. 
     This cooling may provide the advantage that the appearance of hot spots or regions in the photovoltaic elements  5  can be reduced or eliminated, and the temperature of the photovoltaic elements  5  maintained at a uniform desired value. Such hot spots or regions may for example be produced by heating by incident solar radiation, by inhomogeneities or faults in the photovoltaic elements  5 , or by a combination of, or interaction between, these causes. 
     Such hot spots or regions can reduce the efficiency of the photovoltaic elements  5 . It is believed that hot spots in the photovoltaic elements  5  may reduce the efficiency of the photovoltaic elements  5  in the short term, and may also degrade the performance of the photovoltaic elements  5  in the longer term. As discussed above, the efficiency of photovoltaic elements reduces as the temperature increases. In the short term a hot spot in a photovoltaic element may reduce the output of the photovoltaic element because the material forming the hot spot is at a higher temperature than the rest of the photovoltaic element, and so has a reduced efficiency compared to the rest of the photovoltaic element. Further, in the longer term the degrading of the performance of the photovoltaic element may also take place more rapidly at a hot spot because the material forming the hot spot is at a higher temperature than the rest of the photovoltaic element. 
     Accordingly, maintaining the photovoltaic elements  5  at a more uniform temperature value and reducing, or eliminating, hot spots or regions may improve the efficiency of the photovoltaic elements  5  at a specific temperature, and may reduce the amount of degradation of the photovoltaic elements  5  caused by higher temperatures. 
     This may allow the photovoltaic elements  5  to operate at a higher overall temperature than would otherwise be the case. This may be understood by considering that where hot spots exist in the photovoltaic elements  5  it may be the temperature induced reduction in efficiency and temperature induced degradation in these hot spots that limits the maximum operating temperature of the photovoltaic element  5  as a whole. As a result, reducing, or eliminating, these hotspots may allow the maximum operating temperature of the photovoltaic element  5  as a whole to be raised. 
     The illustrated example of the first embodiment has a solar energy collector assembly  2  supported by a single physical connection through the cylindrical tube  13 . In other examples alternative supporting arrangements may be used. In some examples the solar energy collector assembly  2  may be supported by two physical connections, one at each end of the solar energy collector assembly  2 . In some examples, one of the two physical connections may be the through the cylindrical tube. In general, it is advantageous to minimize the number of physical supports in order to minimize the escape of heat from the solar energy collector assembly by conduction through the physical supports. 
     In other examples the number of photovoltaic elements  5  mounted on the heat transport element  4  may be different. In one example, twelve photovoltaic elements  5  may mounted on the heat transport element  4 . In one example, eighteen photovoltaic elements  5  may mounted on the heat transport element  4 . In other examples the relative sizes of the photovoltaic elements  5  and the heat transport element  4  may be different. 
     In some examples the adhesive layer  49  may comprise an epoxy resin which remains non-brittle after curing. 
     In other examples the adhesive layer  49  may be formed by a double sided adhesive tape. 
     Heat Transport Element 
     The heat transport element  4  according to the first embodiment is shown in more detail in a cut away view in  FIG. 5 , and in transverse and longitudinal cross-sectional views in  FIGS. 6 and 7  respectively. 
     In the first embodiment, the heat transport element  4  is substantially trapezoid in cross section and has an upper surface  4   a  formed by an upper sheet  14  and a lower surface  4   b  formed by a lower sheet  15 . The sides of the heat transport element  4  are formed by upwardly bent parts of the lower sheet  15 . The photovoltaic elements  5  are bonded to the upper sheet  14 . The upper sheet  14  and the lower sheet  15  are sealed together around their respective edges by welding and define three fluid passages  16  between them. The upper sheet  14  and the lower sheet  15  are separated by 1 mm so that each of the passages  16  is 1 mm thick. Each of the passages  16  is divided into an upper portion  16   a  and a lower portion  16   b  by a partition sheet  17 . The partition sheets  17  tend to guide fluid flow along the passages  16  along either the upper portions  16   a  or the lower portions  16   b  of the passages  16 . However, the partition sheets  17  do not extend entirely across the passages  16 . The upper portion  16   a  and the lower portion  16   b  of each fluid passage  16  are not sealed from one another. The partition sheets  17  are located and secured in place by being spot welded to dimples  19  projecting upwardly from the lower sheet  15 . 
     The heat transport element  4  is a substantially rigid structure. This may reduce the physical stress applied to the photovoltaic elements  5  by flexing of the heat transport element. This may extend the working life of the photovoltaic elements  5 . 
     In the illustrated example of the first embodiment the upper, lower and partition sheets  14 ,  15  and  17  are formed of 0.2 mm thick tin coated mild steel. The use of mild steel may avoid or reduce problems produced by differential thermal expansion of the silicon semiconductor photovoltaic elements  5  and the heat transport element  4  because the coefficients of thermal expansion of silicon and mild steel are similar. 
     The upper sheet  14  is bent to form two longitudinal recesses in its upper surface which forming two parallel troughs  18  running along the upper surface  4   a  of the heat transport element  4 . In these recesses the upper sheet  14  contacts the lower sheet  15  and the two sheets  14  and  15  are bonded together. This may increase the rigidity of the heat transport element  4 . Electrically conductive ribbons or wires  20  run along the troughs  18  between the heat transport element  4  and the photovoltaic elements  5 . The wires  20  are electrically connected to the photovoltaic elements  5  and to the conductors  21  which pass through the cap  12  to provide a conductive path to carry the electrical power generated by the photovoltaic elements  5  out of the sealed transparent tube  2 . This electrical power may be supplied to an inverter for voltage conversion and/or for conversion to alternating current for supply to a domestic or mains electrical system. 
     At an end of the heat transport element  4  adjacent the open end of the glass tube  3  and the end cap  12  the generally trapezoid cross sectional shape of most of the length of the heat transport element  4  transitions to a projecting cylindrical tube  13 . The upper and lower sheets  14  and  15  are sealed to the cylindrical tube  13  so that the interior of the heat transport element  4  is sealed. The cylindrical tube  13  passes through the end cap  12  and into the heat exchange assembly  6 . The central bore of the cylindrical tube  13  is connected to the passages  16  and acts to carry heat energy from the heat transport element  4  to the heat exchange assembly  6 , as will be explained below. The cylindrical tube  13  physically supports the solar energy collector assembly  2  within the sealed transparent tube  3 . 
     The passages  16  are filled with degassed distilled water  22  as a working fluid and the interior of the heat transport element  4  including the passages  16  and the tube  13  is at least partially evacuated. That is, the interior of the heat transport element  4  is at a pressure below normal atmospheric pressure. The interior of the heat transport element may be under a vacuum at a pressure of 10 −3  mbar. The heat transport element  4  is arranged to be inclined to the horizontal with the end of the heat transport element  4  adjacent the heat exchange assembly  6  higher than the end of the heat transport element  4  remote from the heat exchange assembly  6 . As a result, the passages  16  within the heat transport element  4  are similarly inclined to the horizontal. The amount of water  22  in the passages  16  is sufficient that the lower surface of the upper sheet  14 , that is, the surface forming the top of the passages  16 , is below the surface of the water  22  at a position corresponding to the location of the nearest part of any of the photovoltaic elements  5  to the tube  13 . The inclination angle to the horizontal may be small. The inclination angle may be 5° or more. An inclination angle of about 5° is sufficient. Larger angles of inclination may be used if desired. An angle of inclination up to and including 90° may be used, i.e. the heat transport element  4  may be arranged longitudinally vertically. 
     The heat transport element  4  is a substantially rigid structure. This may minimize changes in the level of the surface of the water  22  due to flexing of the components of the heat transport element  4 , such as the upper and lower sheets  14  and  15 . Such changes in the level of the surface of the water  22  may affect the efficiency of the cooling of the photovoltaic elements  5 . 
     In operation of the first embodiment, when the solar energy collector assembly  2  is exposed to incident solar radiative energy the photovoltaic elements  5  absorb some of this energy, converting a part of the absorbed energy into electrical energy. The remainder of the absorbed energy is converted into heat energy, raising the temperature of the photovoltaic elements  5 . The absorbed heat energy flows from the photovoltaic elements  5  into the heat transport element  4 , flowing through the upper sheet  14  and into the water  22  inside the channels  16 , which water is in contact with the lower surface of the upper sheet  14 . 
     The liquid water  22  inside the passages  16  absorbs the heat energy and vaporizes, producing bubbles  23  of steam or water vapor. The liquid water may vaporize and produce bubbles as a result of either or both of convection boiling and nucleation. At the vacuum pressure of 10 −3  mbar inside the passages  16  the water boils from around 0° C., so that the water  22  vaporizes readily at the normal operating temperatures of the hybrid solar energy converter  1 . The bubbles  23  of water vapor are less dense than the liquid water  22 . As explained above, the passages  16  are inclined to the horizontal, and as a result, this density difference causes the bubbles  23  of water vapor to travel upwards along the passages  16  towards the upper surface of the water  22 . The roughening of the surface of the sheet  14  produced by the tin coating may provide nucleation sites, increasing the tendency of the liquid water  22  to vaporize and form bubbles  23  of water vapor. 
     When a bubble  23  of water vapor reaches the surface of the water  22  the vapor is released into the vacuum above the water  22 . The bursting of the bubbles of water vapor at the water surface may generate droplets of liquid water and may project at least some of these water droplets upwardly from the water surface into the vacuum above the water surface. As a result, the heat transfer mechanism may be a multi-phase system comprising liquid water, water vapor and droplets of liquid water, and not just a two-phase system comprising liquid water and water vapor only. The presence of such droplets of water in the vacuum may enhance the rate of vaporization by increasing the surface area of the liquid water exposed to the vacuum. 
     The water vapor in the vacuum travels at a very high speed through the vacuum along the cylindrical tube  13  and into the heat exchange assembly  6 . The travel speed of the hot water vapor in the vacuum is very fast, approximating to the thermal speed of the water vapor molecules. Inside the heat exchange assembly  6  the water vapor condenses on a heat exchange surface of one of the primary and secondary heat exchangers  7  and  8 . The condensed water flows back out of the heat exchange assembly  6  down the tube  13  and back into the water  22  within the passages  16 . 
     The bubbles  23  of water vapor will tend to move upwardly through the liquid water  22  in the passages  16  because of the lower density of the water vapor compared to the liquid water, which will result in an upward buoyancy force on each bubble  23 . Further, the movement of the bubbles  23  of water vapor will tend to drive the liquid water  22  in the passages  16  upwardly. As a result, the bubbles  23  in combination with the partition sheet  17  cause the water  22  in each passage  16  to circulate with relatively hot water  22  and bubbles  23  of water vapor flowing upwards along the upper portion  16   a  of the passage  16  and relatively cool water  22  flowing downwards along the lower portion  16   b  of the passage  16 . This circulation is driven primarily by the difference in density between the water vapor of the bubbles  23  and the liquid water. However, this circulation may also be driven by convection as a result of the difference in density between the relatively hot water in passage  16   a  and the relatively cool water in passage  16   b , in a similar manner to a thermosiphon. This density driven circulation may form a highly effective heat transport mechanism because water has a relatively high enthalpy of vaporization, so that the bubbles  23  of water vapor may carry a large amount of heat energy additional to the heat energy carried by the circulation of the relatively hot water in passage  16   a  and the relatively cool water in passage  16   b.    
     As the bubbles  23  of water vapor travel upwardly along the passages  16  the pressure head acting on the bubbles  23  decreases, so that the bubbles  23  tend to expand. As a result, the tendency of the vapor bubbles  23  to collapse and implode is reduced by the effects of the expansion and decreasing pressure as the bubbles  23  move upwardly. When considering this point, it should be remembered that when the heat transport element  4  is operating the bubbles  23  will be forming within an established density driven circulation fluid flow and will move upwardly carried by this flow in addition to the bubbles movement due to their own buoyancy relative to the liquid water. Further, it is believed that expansion of the bubbles  23  as they move upwardly will further increase the speed of the density driven circulation flow by increasing the buoyancy of the expanding bubbles  23 . 
     In general, the speed of the density driven circulation increases and the effectiveness of the heat transport mechanism increases as the temperature of the upper sheet  14  of the heat transport element  4  increases. 
     The density driven circulation of the water  22  within the passage  16  is a vapor driven circulating or rolling flow. 
     The density driven circulation of the water  22  within the passage  16  becomes particularly vigorous, and becomes particularly effective as a heat transport mechanism, when the water  22  within the passage  16  enters a rolling boil state. The effectiveness of the heat transport mechanism significantly increases when rolling boiling of the water  22  commences. In general, when other parameters of the system remain constant, entry into the rolling boil state will take place when the temperature of the upper sheet  14  of the heat transport element  4  reaches a specific temperature. 
     In the illustrated example using water, the water  22  within passage  16  may enter a rolling boil state at a temperature of about 40° C. 
     In the illustrated example of the first embodiment the heat transport element  4 , the cylindrical tube  13  and the primary and secondary heat exchangers  7  and  8  are all arranged in a straight line. Accordingly, the channels  16  and  17  within the heat transport element  4 , the internal passage of the cylindrical tube  13 , and the internal passages of the heat exchange assembly  6  are all inclined at the same angle to the horizontal. This is not essential. In some examples it may be preferred to have these components inclined at different angles to the horizontal. In particular, in some examples it may be preferred to have channels  16  and  17  within the heat transport element  4  inclined at a first angle to the horizontal selected to optimize the efficiency of the density driven circulation, and to have the internal passages of the cylindrical tube  13  and the heat exchange assembly  6  inclined at a second angle to the horizontal selected to optimize the return flow of condensed water to the heat transport element  4 . 
     In the illustrated first embodiment three fluid passages  16  are defined within the heat transport element  4 . In other examples there may be different numbers of fluid passages. In particular, some examples may have only a single passage. 
     In the illustrated example of the first embodiment 0.2 mm thick tin coated mild steel sheets are used to form the heat transport element  4 . In alternative examples other thicknesses may be used, in particular 0.1 mm thick sheets may be used. The use of a thinner upper sheet may improve the rate of heat energy transfer from the photovoltaic elements to the water inside the channels. In other examples sheets having different thicknesses may be used for the different sheets. In other examples different materials may be used, in particular sheets of other metals, such as copper or brass, may be used. In other examples the upper, lower and/or partition sheets may be formed from materials which are not metals. In other embodiments there may be openings in the upper sheet allowing the water inside the channels to directly contact the back surfaces of the photovoltaic elements to maximize heat transfer. In such examples the thickness or material used to form the upper sheet could be selected without having to take thermal conductivity into account. 
     The sheets used to form the heat transport element  4  may be shaped by pressing. 
     In the illustrated first embodiment the partition sheets are secured to dimples projecting from the lower sheet  15 . In alternative examples other support arrangements may be used. In particular dimples projecting from the partition sheets may be secured to the lower sheet. 
     In the illustrated first embodiment each of the passages is 1 mm thick. In alternative examples different passage thicknesses may be used. In particular a passage thickness of 0.8 mm may be used. In particular a passage thickness of 1.2 mm may be used. 
     In the illustrated first embodiment the thickness of each of the portions  16   a  and  16   b  of a passage  16  is approximately equal. In alternative examples the thicknesses of the portions  16   a  and  16   b  in a passage  16  may be different. In particular the thickness of the portion  16   a  carrying the bubbles of vapor  23  may be greater than the thickness of the other portion  16   b.    
     In the illustrated first embodiment the upper sheet  14  is flat where it contacts the photovoltaic elements  5 . In alternative examples the upper sheet  14  may be patterned to stiffen it, to reduce flexing of the upper sheet  14  due to thermal expansion or contraction when the temperature of the heat transport element  4  changes. Such flexing may place damaging stress on the photovoltaic elements. 
     In the illustrated first embodiment the different sheets are welded together. In alternative examples different bonding techniques may be used. In some examples the different sheets may be bonded by techniques including spot welding, roller welding, solder or adhesive. 
     In the illustrated first embodiment the partition sheet  17  dividing each passage  16  into portions  16   a  and  16   b  is flat. In alternative examples the partition sheet may have other profiles. In particular the partition sheet may have a corrugated or wave profile. The partition sheet may divide the passage  16  into a plurality of portions  16   a  and a plurality of portions  16   b    
     In the illustrated first embodiment the heat transport element  4  and the passages  16  are formed by shaped sheets. In alternative examples the heat transport element and the passages may be formed in other ways. In particular, the heat transport element and the passages may be formed by flattened tubes. 
     In the illustrated first embodiment the tube  13  is a cylindrical tube. In other examples the tube  13  may have other cross sectional shapes. In some examples the tube  13  may have a cross sectional shape that varies along its length. 
     In the illustrated example of the first embodiment the flow of water vapor and liquid water through the heat transport element  4  tends to keep the cooled upper surface of the heat transport element  4  at a uniform operating temperature during operation. That is, the cooled upper surface of the heat transport element  4  tends to be kept isothermal. The isothermal nature of the cooled upper surface of the heat transport element  4  tends to give rise to isothermal cooling of the photovoltaic elements  5 , where hotter parts of the photovoltaic elements  5  tend to be preferentially cooled so that the photovoltaic elements  5  themselves tend to become isothermal. 
     Such isothermal cooling provides further advantages in addition to those provided by cooling. 
     Isothermal cooling may provide the advantage that the appearance of hot spots or regions in the photovoltaic elements  5  produced by heating by incident solar radiation can be reduced or eliminated. Such hot spots or regions can reduce the efficiency of the photovoltaic elements  5 . 
     Isothermal cooling may simplify the control and wiring arrangements of the photovoltaic elements  5  by reducing or eliminating any requirement for compensation for differences in the performance of the different parts of the photovoltaic elements  5  that are at different temperatures. 
     Isothermal cooling tends to reduce, or prevent, the formation of hot spots or regions in the photovoltaic elements  5 . As is explained above, this may allow the efficiency of the photovoltaic elements  5  to be improved at a specific temperature. Further, this may reduce the amount of degradation of the photovoltaic elements  5  caused by higher temperatures. 
     Still further, this may allow the photovoltaic elements  5  to operate with a given degree of efficiency at a higher temperature than would otherwise be the case. This may allow the solar energy collector assembly  2  including the photovoltaic elements  5  to be operated at a higher temperature without reducing the efficiency with which the photovoltaic elements  5  produce electrical energy. 
     One example of this effect of isothermal cooling is that the general figure quoted above for silicon photovoltaic elements that the efficiency of electrical energy generation generally drops by about 0.35% to 0.5% for each degree centigrade of temperature increase above 25° C. may not apply to silicon photovoltaic elements that are isothermally cooled. Such isothermally cooled silicon photovoltaic elements having hotspots eliminated or reduced may have a higher threshold temperature at which the efficiency of electrical energy generation begins to drop and/or may have a reduced rate of reduction in efficiency for each degree centigrade of temperature increase above the threshold temperature. Further, the temperature at which there is a risk of permanent degradation of the silicon photovoltaic elements may also be increased for isothermally cooled silicon photovoltaic elements. Similar effects may be found in photovoltaic elements formed of other semiconductor materials. 
     In some examples, one or more layers of heat conductive material may be located between the upper sheet  14  and the photovoltaic elements  5 . Such layers of heat conductive material may increase the rate of heat transfer between the photovoltaic elements  5  and the upper sheet  14 , and thus the rate of heat transfer between the photovoltaic elements  5  and the liquid within the passages  16 . Such layers of heat conductive material may also increase the rate of heat transfer laterally across the photovoltaic elements  5 . 
     Accordingly, providing a layer of heat conductive material may increase the degree of isothermal cooling and further tend to reduce, or eliminate, the formation of hot spots or regions in the photovoltaic elements  5 . 
     The heat transport element may be used in other applications separately from the rest of the solar energy converter. 
     Heat Exchange Assembly 
       FIG. 8  illustrates a cross sectional diagram of the heat exchange assembly  6  according to the first embodiment. As explained above, the heat exchange assembly  6  includes a primary heat exchanger  7  and a secondary heat exchanger  8  separated by a heat transfer control valve  9 . 
     The tube  13  of the heat transport element  4  is connected to the heat exchange assembly  6 . The tube  13  is connected to the primary heat exchanger  7 . The primary heat exchanger  7  is formed by a cylindrical copper tube  24  having a plurality of heat transfer fins  25  extending outwardly from the tube  24 . The heat transfer fins  25  extend into a flow channel carrying a first operating fluid. In the illustrated example of the first embodiment the first operating fluid is a pumped flow of water forming part of a domestic hot water and/or heating system. 
     The secondary heat exchanger  8  is formed by a cylindrical copper tube  26  having a plurality of heat transfer fins  27  extending outwardly from the tube  26 . The heat transfer fins  27  extend into a second operating fluid. In the illustrated example of the first embodiment the second operating fluid is ambient air. 
     The copper tube  26  of the secondary heat exchanger  8  is separated from the copper tube  24  of the primary heat exchanger  7  by a length of glass tube  28 . The glass tube  28  forms a thermal break between the primary and secondary heat exchangers  7  and  8 . This thermal break may minimize the conduction of heat energy between the primary and secondary heat exchangers  7  and  8 . The copper tube  24  of the primary heat exchanger  7 , the glass tube  28  and the copper tube  26  of the secondary heat exchanger  8  define a fluid flow passage  29  extending from the tube  13  through the primary heat exchanger  7  and the heat transfer control valve  9  to the secondary heat exchanger  8 . 
     The interiors of both of the primary and secondary heat exchangers  7  and  8  in communication with the tube  13  are sealed and at a vacuum pressure of 10 −3  mbar. A vacuum pipe  35  is provided at the end of the secondary heat exchanger  8  to allow the primary and secondary heat exchangers  7  and  8  and the connected channels  16  within the heat transport element  4  to be evacuated during manufacture. This vacuum pipe  35  is blocked to provide a seal after the evacuation. 
     The fluid flow passage  29  is selectively blockable between the primary and secondary heat exchangers  7  and  8  by a valve element  30  of the heat transfer control valve  9 . In a closed condition the valve element  30  bears against a valve seat  31  formed by a circumferential inwardly extending ridge in the copper tube  24  of the primary heat exchanger  7 , blocking water vapor flow along the fluid flow passage  29 . In an open condition the valve element  30  is separated from the valve seat  31  defining an annular gap allowing water vapor flow along the fluid flow passage  29 . 
     The valve element  30  is urged towards the closed position by a toggle spring  32 . A bellows  33  partly filled with an actuating liquid  34  is arranged so that as the temperature increases the vapor pressure of the actuating fluid increases and the increased pressure causes the bellows  33  to urge the valve element  30  towards the closed position with a force that increases with increasing temperature. At a predetermined trigger temperature the force applied by the bellows  33  will exceed the force applied by the toggle spring  32  and the valve element  30  will move to the open position, allowing water vapor flow along the fluid flow passage  29 . 
     Accordingly, at temperatures below the trigger temperature the fluid flow passage  29  will be closed and at temperatures above the trigger temperature the fluid flow passage  29  will be open. As explained above, the hot water vapor moves very quickly in the vacuum conditions within the fluid flow passage  29  so that when the valve  9  opens the time delay before heat energy is transferred to the secondary heat exchanger  8  may be very short. 
     In the illustrated embodiment the trigger temperature is the intended maximum temperature of the hot water supplied to the domestic hot water and/or heating system, 65° C. 
     In operation, when the temperature of the first operating fluid and the primary heat exchanger  7  is below the trigger temperature of the heat transfer control valve  9  the hot water vapor from the heat transport element  4  passes along the tube  13  and into the primary heat exchanger  7 . The hot water vapor is prevented from reaching the secondary heat exchanger  8  by the closed control valve element  30 . The hot water vapor condenses on an interior surface of the copper tube  24  of the primary heat exchanger  7 , releasing heat energy which passes through the heat transfer fins  25  and into the first operating fluid. In the illustrated embodiment the heated first operating fluid provides a flow of heated water to a domestic hot water and/or heating system. 
     When the temperature of the first operating fluid and the primary heat exchanger  7  reaches or exceeds the trigger temperature of the heat transfer control valve  9 , the heat transfer control valve  9  opens, allowing the hot water vapor from the heat transport element  4  to pass along the tube  13  and into both the primary heat exchanger  7  and the secondary heat exchanger  8 . Accordingly, in addition to passing into the primary heat exchanger  7  as discussed above, the hot water vapor is also able to reach the secondary heat exchanger  8  through the open control valve element  30 . The hot water vapor condenses on an interior surface of the copper tube  26  of the secondary heat exchanger  8 , releasing heat energy which passes through the heat transfer fins  27  and into the second operating fluid. In the illustrated embodiment the heated second operating fluid provides a convection flow of heated air carrying heat away from the hybrid solar energy converter  1 . This may allow the hybrid solar energy converter  1  to use the atmosphere as a heat sink. This may prevent further heating of the first operating fluid to be reduced or prevented. 
     This may allow the problem of stagnation encountered in solar water heating systems to be avoided or reduced. In solar water heating systems stagnation may occur when the water being heated reaches a maximum desired temperature. Generally, the pumping of the water to be heated through the solar water heater is then stopped to avoid overheating of the water, which could otherwise result in damage to the system supplied with the heated water. However, when the pumping of the water to be heated is stopped the stationary water near the solar water heater may then be heated to a very high temperature by the solar heater, resulting in undesirable overheating and pressurization of the water system. 
     The primary heat exchanger  8  is surrounded by a casing  50  including an inlet opening  51  and an outlet opening  52  for the water to be heated as the first operating fluid. Inlet and outlet supply pipes for the water may be attached to the inlet opening  51  and the outlet opening  52 . The casing  50  is formed of a foamed plastics material with a hard outer shell to provide thermal insulation of the primary heat exchanger  7  and the first operating fluid and to provide weather resistance. 
     In some examples the casing  50  may be formed of other materials instead of foamed plastics. In some examples the casing may be formed of an electrically and thermally insulating material with good resistance to water and weathering. In particular, the casing may be formed of glass, ceramic, or concrete. 
     In one example the primary heat exchanger  8  may include a bleed valve to allow any air trapped within the first operating fluid in the primary heat exchanger  8  to be bled out. In other examples the bleed valve may not be provided. In examples where the first operating fluid is pumped through the primary heat exchanger the primary heat exchanger may be arranged to be pump purged of air by the pumped fluid flow. 
     As explained above, in the illustrated example of the first embodiment the trigger temperature of the heat transfer control valve  9  is predetermined. In some examples the trigger temperature may be settable in use, or on installation or manufacture of the hybrid solar energy converter  1 . In some examples the trigger temperature may be settable to different values depending on the intended maximum water temperature of the water to be heated. In particular, in some examples the trigger temperature may be settable to 65° C. when the hybrid solar energy converter is to be used to heat water for a domestic hot water system and may be settable to 135° C. when the hybrid solar energy converter is to be used to heat water for an industrial hot water system. 
     In some examples the trigger temperature of the heat transfer control valve may be selected to maximize the generation of electrical energy by the photovoltaic elements  5 . In some examples the trigger temperature value may be selected to increase the amount of heat energy transferred to the first operating fluid. In some examples the trigger temperature may be selected to optimize the overall production of energy, taking into account both the amount of electrical energy produced by the photovoltaic elements  5  and the amount of heat energy transferred to the first operating fluid. In some examples the optimizing may maximize the total production of energy. In some examples the optimum overall production of energy may take into account the relative demand for, or value of, the different types of energy, rather than simply maximizing the total amount of energy produced. 
     As explained above, the isothermal cooling tends to reduce, or prevent, the formation of hot spots or regions in the photovoltaic elements  5 . This may allow the solar energy collector assembly  2  including the photovoltaic elements  5  to be operated at a higher temperature without reducing the efficiency with which the photovoltaic elements  5  produce electrical energy. This may allow the temperature of the collector assembly to be increased to produce more useable heat energy without the increase in temperature reducing the efficiency with which the photovoltaic elements  5  produce electrical energy. This may allow the trigger temperature to be increased. 
     In some examples the trigger temperature may be set to different temperatures during use of the hybrid solar energy converter  1 . This may allow the temperature of the collector assembly to be controlled to produce different amounts of useable heat energy or electricity depending upon which type of energy is most in demand at a specific time. 
     An alternative arrangement to operate the valve  9  is shown in  FIG. 9 . In this alternative arrangement the valve element  30  is urged into the closed position by a toggle spring as before. In this arrangement the valve element  30  may be selectively urged into an open position by a solenoid  36 . In this alternative arrangement the solenoid may be controlled based on a measured temperature of the primary heat exchanger or of the water to be heated in order to limit the maximum temperature reached. 
     Alternatively, the solenoid may be controlled in whole, or in part, based upon the current requirements of a user. For example, when hot water is more in demand than electricity the valve  9  may be closed to pass hot water vapor from the heat transport element  4  only to the primary heat exchanger  7  to maximize the amount of heat applied to the water acting as the first operating fluid regardless of any temporary reduction in efficiency of the photovoltaic elements  5  as a result of any resulting increase in temperature of the collector assembly. Further, when hot water is less in demand than electricity, the valve  9  may be opened in order to pass hot water vapor from the heat transport element  4  to both of the primary and secondary heat exchangers  7  and  8  in order to cool the photovoltaic elements as much as possible and maximize the efficiency of electricity generation regardless of the effects on the temperature of the water acting as the first operating fluid. 
     In the illustrated example of the first embodiment the temperature of the solar energy collector assembly  2 , and thus the temperature of the photovoltaic elements  5 , is controlled by operating the heat transfer control valve  9  to selectively enable or disable the transfer of heat energy from the solar energy collector assembly  2  to the secondary heat exchanger  8 . 
     In other examples other control methods can be used additionally or alternatively to control the temperature of the solar energy collector assembly  2 . In some examples the temperature of the solar energy collector assembly  2  may be controlled by changing the rate of removal of heat energy from the solar energy collector assembly  2 . 
     In some examples the rate of removal of heat energy from the solar energy collector assembly  2  can be controlled by altering the flow rate of the first operating fluid passing through the primary heat exchanger  7 . In some examples the rate of removal of heat energy from the solar energy collector assembly  2  can be controlled by altering the surface area over which the first operating fluid is in contact with the primary heat exchanger  7 , for example by selectively opening or closing fluid flow passages of the first operating fluid within the primary heat exchanger  2 . 
     In some examples the rate of removal of heat energy from the solar energy collector assembly  2  can be controlled by altering the vacuum pressure within the tube  3 . This may change the rate of convective heat loss from the solar energy collector assembly  2  to the tube  3 . In general, heat transferred to the tube  3  will be rapidly lost to the outside environment by convection and/or conduction. 
     In some examples the rate of removal of heat energy from the solar energy collector assembly  2  can be controlled by altering the vacuum pressure within the heat transport element  4 . In general, the tendency of the liquid water within the passage  16  to vaporize and form bubbles of vapor  23  will increase as the vacuum pressure is reduced, and the tendency of the liquid water within the passage  16  to vaporize and form bubbles of vapor  23  will decrease as the vacuum pressure is increased. As is explained above, the density driven circulation of water around the passages  16  and the transport of heat energy along the tube  13  are both driven by water vapor. Accordingly, altering the tendency of the liquid water to vaporize by altering the vacuum pressure may allow the rate of removal of heat energy from the solar energy collector assembly  2 , and the rate of removal of heat energy from the photovoltaic elements  5  to be controlled, and so allow the temperature of the solar energy collector assembly  2  and photovoltaic elements  5  to be controlled. 
     Further, the temperature at which rolling boiling of the water  22  within the passage  16  commences will tend to increase as the vacuum pressure is increased, and will tend to decrease as the vacuum pressure is decreased. Accordingly, in examples where the vacuum pressure within the heat transport element  4  is altered the temperature at which the water  22  within the passage  16  commences rolling boiling can be changed. 
     As is explained above, the density driven circulation of water around the passages  16  becomes particularly vigorous, and becomes particularly effective as a heat transport mechanism, when the water  22  within the passage  16  enters a rolling boil state. Accordingly, altering the temperature at which the water  22  within the passage  16  commences rolling boiling by altering the vacuum pressure may allow the rate of removal of heat energy from the solar energy collector assembly  2  and photovoltaic elements  5  to be controlled, and so allow the temperature of the solar energy collector assembly  2  and photovoltaic elements  5  to be controlled. 
     In some examples the temperature of the solar energy collector assembly  2  may be controlled by changing the amount of solar energy incident on the solar energy collector assembly  2 , and so changing the rate of absorption of heat energy by the solar energy collector assembly  2 . 
     In some examples the amount of incident solar energy may be controlled by changing the orientation of the solar energy collector assembly relative to the direction of the incident solar energy. This can be carried out using a drive mechanism able to rotate the solar energy collector assembly about one or more axes. 
     In some examples the amount of incident solar energy may be controlled using adjustable light intercepting or blocking mechanisms in the path of the incident solar energy. In some examples variable filters, shutters, stops, or the like may be used. In some examples these adjustable light intercepting or blocking mechanisms may comprise physical devices. In some examples these adjustable light intercepting or blocking mechanisms may comprise devices having electronically controlled optical characteristics, such as liquid crystals. 
     In examples where the temperature of the solar energy collector assembly and/or the photovoltaic elements are to be controlled, a temperature sensor and a temperature controller may be provided, together with a temperature control mechanism arranged to carry out one, some, or all, of the methods of controlling temperature described above. 
     The temperature sensor is arranged to measure the temperature of the solar energy collector assembly and provide this temperature value to the temperature controller. The temperature controller can then operate the temperature control mechanism in a suitable manner to control the temperature of the solar energy collector assembly to the desired value. 
     Examples where the temperature of the photovoltaic elements is to be controlled a temperature sensor arranged to measure the temperature of a photovoltaic element or elements and provide this temperature value to the temperature controller may be provided. This may be additional to, or instead of, the temperature sensor arranged to measure the temperature of the solar energy collector assembly. The temperature controller can then operate the temperature control mechanism in a suitable manner to control the temperature of the photovoltaic element or elements to the desired value. 
     In some examples the temperature sensor can be provided on the upper surface of the solar energy collector assembly. In some examples the temperature sensor can be formed on the same semiconductor wafer as a photovoltaic element. 
     Conveniently, the temperature controller may be a suitably programmed general purpose computer. 
     In the illustrated first embodiment copper is used in the heat exchangers. This may enhance the efficiency of the heat exchangers because copper has a relatively high thermal conductivity. In alternative examples other materials may be used. 
     In alternative examples different types of valve may be used. In particular a valve may be used with a valve element acting as a piston moving within a valve seat acting as a cylinder whereby the valve is closed when the valve element is within the valve seat and the valve is opened when the valve element is outside the valve seat. 
     In the illustrated first embodiment the fluid flow passage  29 , tube  13 , and the passages  16  within the heat transport element  4  are evacuated through a vacuum pipe  35  at the end of the secondary heat exchanger  8 . In alternative examples a differently located vacuum pipe may be provided. In particular a vacuum pipe may be provided at the end of the heat transport element remote from the heat exchange assembly, as shown in  FIG. 3 . 
     The illustrated first embodiment is a hybrid solar energy converter comprising photovoltaic elements and arranged to convert incident solar radiation into outputs of both electrical energy and hot water. In other examples the photovoltaic elements may be omitted to provide a solar energy converter arranged to convert incident solar radiation into an output of hot water. 
     Second Embodiment 
     Apparatus according to a second embodiment of the present invention is illustrated in  FIG. 10 .  FIG. 10  shows a general exterior view of a second embodiment of a hybrid solar energy converter  101  according to the present invention. 
     Overview 
     In the second embodiment, the hybrid solar energy converter  101  includes a solar energy collector assembly  102  housed within a sealed transparent tube  103 . The solar energy collector assembly  102  includes a heat transport element  104  and an array of photovoltaic elements  105  mounted on an upper surface of the heat transport element  104 . The hybrid solar energy converter  101  also includes a heat exchange assembly  106  at one end of the transparent tube  103 . One end of the solar energy collector assembly  102  is connected to the heat exchange assembly  106 . Similarly to the first embodiment, in different examples the photovoltaic elements  105  may be formed of silicon, or gallium arsenide, or other suitable semiconductor materials. In other examples organic photovoltaic elements may be used. In other examples hybrid photovoltaic elements may be used. 
     In the second embodiment, the heat exchange assembly  106  includes a primary heat exchanger  107  arranged to transfer heat energy from the heat transport element  104  to a first fluid, and a secondary heat exchanger  108  arranged to transfer heat energy from the heat transport element  104  to a second fluid. The primary heat exchanger  107  and the secondary heat exchanger  108  are separated by a heat transfer control valve  109  able to selectively allow, or prevent, the transfer of heat energy from the heat transport element  104  to the secondary heat exchanger  108 . 
     In one possible example, in use the hybrid solar energy converter  101  may be mounted on a roof. Accordingly, mounting brackets similar to those of the first embodiment may be provided. 
     In overview, the operation of the hybrid solar energy converter  101  of the second embodiment is similar to operation of the hybrid solar energy converter  1  of the first embodiment. Solar energy incident on the hybrid solar energy converter  101  passes through the sealed transparent tube  103  and is incident on the photovoltaic elements  105  of the solar energy collector assembly  102 . The photovoltaic elements  105  convert a part of the energy of the incident solar energy into electrical energy, and convert a part of the energy of the incident solar energy into heat energy. A further part of the incident solar energy may be incident on any parts of the solar energy collector assembly  102  which are not covered by the photovoltaic elements  105 , and this further part of the incident solar energy may also be converted into heat energy. In general, it is desirable to maximize the proportion of the surface of the solar energy collector assembly  102  exposed to incident solar energy which is covered by the photovoltaic elements  105 , and to minimize the proportion which is not so covered. However, in some circumstances it may be preferred to leave some parts of this exposed surface uncovered, for example to simplify manufacture and/or assembly of the solar energy collector assembly  102  and attachment of the photovoltaic elements  105  to the solar energy collector assembly  102 . Usually, in the second embodiment the surface of the solar energy collector assembly exposed to incident solar energy will be the upper surface. 
     The electrical energy produced by the photovoltaic elements  105  is carried along the heat transport element  104  by electrical conductors and away from the solar energy converter  101  for use. The heat energy absorbed by the photovoltaic elements  105  is transferred into the heat transport element  104 , cooling the photovoltaic elements  105 , and then carried to the heat exchange assembly  106 . 
     Similarly to the first embodiment, the heat transfer control valve  109  is able to selectively allow, or prevent, the transfer or transport of heat energy from the heat transport element  104  to the secondary heat exchanger  108 . Accordingly, the degree of cooling applied to the photovoltaic elements  105  can be varied. 
     In one typical arrangement, the hybrid solar energy converter  101  may be used to generate electricity, and to generate hot water. Similarly to the first embodiment, in this arrangement the heat energy transferred to the primary heat exchanger  107  is transferred into a pumped water supply flowing through the primary heat exchanger  107  to heat the water. This heated water is then used by a domestic or industrial hot water system, and the electrical energy produced by the photovoltaic elements  105  is supplied to an electrical supply system. In some arrangements the heat energy transferred to the secondary heat exchanger  108  is transferred into ambient air and allowed to escape and the secondary heat exchanger  108  is used, under the selective control of the heat transfer control valve  109 , to release heat energy in order to regulate the temperature of the solar energy collector assembly  102   
     Transparent Tube 
     In the second embodiment illustrated in  FIG. 10  the sealed transparent tube  103  is similar to the sealed transparent tube  3  of the first embodiment, having one closed domed end and one open end sealed by a metal end cap  120 . The interior of the tube  103  is at least partially evacuated. That is, the interior of the tube is at a pressure below normal atmospheric pressure. 
     The pressure of the vacuum within the tube  103  may be 10 −3  mbar. Other vacuum pressures may be used, as discussed regarding the first embodiment. In some examples the vacuum pressure may be in the range 10 −2  mbar to 10 −6  mbar. In general, it is expected that lower vacuum pressure, or in other words a harder vacuum, will provide greater insulating benefits. Further, it is expected that lower vacuum pressure, or in other words a harder vacuum, will provide greater protection from environmental damage in examples where the photovoltaic elements are not encapsulated. In practice the benefits of using a lower vacuum pressure may need to be balanced against the increased cost of achieving a lower vacuum pressure. In some examples a vacuum pressure of 10 −2  mbar, or lower, may be used. 
     In an alternative example the sealed transparent tube  103  may be filled with an inert gas instead of being evacuated. In particular, the inert gas may be nitrogen. 
     In another alternative example the sealed transparent tube  103  may be filled with an inert gas at a reduced pressure. In some examples this may be achieved by filling the tube  103  with the inert gas and then evacuating the tube  103 . In particular, the inert gas may be nitrogen. 
     In the illustrated second embodiment the tube  103  is cylindrical having a circular cross section. Similarly to the first embodiment, in alternative examples the tube  103  may have other shapes. In some examples the cross sectional size and/or shape of the tube  103  may vary at different positions along its length. In an alternative example the tube  103  may have an elliptical cross section. In particular, the tube  103  may have an elliptical cross section with the long axis of the ellipse aligned with the plane of the solar energy collector assembly  102 . 
     In the illustrated first embodiment the tube  103  is formed of glass. In alternative examples suitable transparent plastics materials or laminated structures may be used to form the tube  103 . 
     In the illustrated second embodiment the tube  103  is transparent. In alternative examples the tube may be only partially transparent. 
     In the illustrated second embodiment the metal end cap  120  may be bonded to the glass tube  103  by adhesive. In other embodiments alternative glass to metal bonding techniques may be used, for example welding, brazing or soldering. 
     Similarly to the first embodiment the tube  103  has a metal end cap  120  at one end. In alternative examples the end cap  120  may be made of other materials. In some examples the end cap  120  may be made of glass. This may reduce conductive heat losses from the collector assembly  102 . 
     Collector Assembly 
     In the second embodiment, the solar energy collector assembly  102  includes a heat transport element  104  and an array of photovoltaic elements  105  mounted on a surface of the heat transport element  104 . In order to allow radiant solar energy to be incident on the photovoltaic elements  105  the array of photovoltaic elements  105  are mounted on the surface of the heat transport element  104  which is exposed to the incident radiant solar energy in operation of the hybrid solar energy converter  101 . This will usually be the upper surface of the heat transport element  104 . 
     In some arrangements the surface of the heat transport element  104  exposed to the incident radiant solar energy may not be the upper surface. In particular, this would be the case if the solar energy collector assembly  102  was located in a vertical, or substantially vertical, plane, or if the incident solar radiant energy was incident horizontally or from below, for example after redirection by an optical system, such as a mirror. Accordingly, references to upper and lower surfaces, and similar directional terminology in this description, should be understood as referring to the situation illustrated in the figures where the solar energy collector assembly is in a plane at an angle to the horizontal and radiant solar energy is incident from above. 
     In the illustrated example of the second embodiment, the solar energy collector assembly  102  is supported by a cylindrical tube  119  of the heat transport element  104 . The cylindrical tube  119  passes through the end cap  120  and into the heat exchange assembly  106 , as will be explained in more detail below. Where the cylindrical tube  119  passes through the end cap  120  the cylindrical tube  119  is soldered to the end cap  120  to retain the cylindrical tube  119  in place and support the solar energy collector assembly  102 . 
     In alternative examples the cylindrical tube  119  may be secured to the end cap  120  in other ways. In one example the cylindrical tube  119  may be welded to the end cap  120 . 
     The supporting of the solar energy collector assembly  102  by a single physical connection through the cylindrical tube  119  may increase the efficiency with which heat can be collected from incident solar energy by the solar energy collector assembly  102 . Having the solar energy collector assembly  102  supported by a single physical connection through the cylindrical tube  119  may reduce conductive heat loss from the solar energy collector assembly  102  into the supporting structure outside the transparent tube. 
     In the second embodiment the heat transport element  104  has a substantially flat upper surface  104   a . Each of the photovoltaic elements  105  is square, and the width of the heat transport element  104  is the same as the width of each square photovoltaic element  105 . Five square photovoltaic elements  105  are mounted side by side to one another along the length of the heat transport element  104 . Substantially the entire upper face of the heat transport element  104  is covered by the photovoltaic elements  105 . Covering a large proportion of the upper surface  104   a  of the heat transport element  104  with photovoltaic elements  105  may increase the efficiency of the hybrid solar energy converter  101 . 
     In one example the square photovoltaic elements  105  may each be a 125 mm by 125 mm square and 0.2 mm thick. In another example the square photovoltaic elements may each be a 156 mm by 156 mm square. In other examples, photovoltaic elements having other sizes or shapes may be used. 
     The photovoltaic elements  105  are bonded to the substantially flat upper surface  104   a  of the heat transport element  104  using a layer  149  of heat conducting adhesive in a similar manner to the first embodiment. This thermally conductive adhesive bonding layer  149  is shown in  FIG. 11 . The adhesive bonding layer  149  is electrically insulating. The adhesive bonding layer  149  between the photovoltaic elements  105  and the heat transport element  104  is arranged to be thin. This may improve the degree of thermal conduction between the photovoltaic elements  105  and the heat transport element  104 . This may increase the rate of heat transfer laterally across the photovoltaic elements  105 . An adhesive material loaded with solid spheres of a predetermined size may be used to form the adhesive bonding layer  149 . This may allow a thin adhesive layer  149  to be consistently and reliably formed. The adhesive bonding layer  149  is formed of a flexible or “forgiving” adhesive material. This may relieve stresses in the assembled solar energy collector assembly  102  and reduce any stress applied to the photovoltaic elements  105 . 
     The photovoltaic elements  105  are semiconductor photovoltaic elements formed of silicon. In one embodiment the photovoltaic elements are formed of single-crystal silicon. In one embodiment the photovoltaic elements are formed of amorphous silicon. In one embodiment the photovoltaic elements are formed of polycrystalline silicon, or polysilicon. In other embodiments alternative types of semiconductor photovoltaic elements may be used. 
     As discussed above, in operation of the hybrid solar energy converter  101  the photovoltaic elements  105  are cooled by the heat transport element  104 . This cooling may allow the temperature of the photovoltaic elements  5  to be maintained at a desired value. 
     This cooling may provide the advantage that the appearance of hot spots or regions in the photovoltaic elements  105  can be reduced or eliminated, and the temperature of the photovoltaic elements  105  maintained at a uniform desired value. Such hot spots or regions may for example be produced by heating by incident solar radiation, by inhomogeneities or faults in the photovoltaic elements  105 , or by a combination of, or interaction between, these causes. 
     Such hot spots or regions can reduce the efficiency of the photovoltaic elements  105 . It is believed that hot spots in the photovoltaic elements  105  may reduce the efficiency of the photovoltaic elements  105  in the short term, and may also degrade the performance of the photovoltaic elements  105  in the longer term. As discussed above, the efficiency of photovoltaic elements reduces as the temperature increases. In the short term a hot spot in a photovoltaic element may reduce the output of the photovoltaic element because the material forming the hot spot is at a higher temperature than the rest of the photovoltaic element, and so has a reduced efficiency compared to the rest of the photovoltaic element. Further, in the longer term the degrading of the performance of the photovoltaic element may also take place more rapidly at a hot spot because the material forming the hot spot is at a higher temperature than the rest of the photovoltaic element. 
     Accordingly, maintaining the photovoltaic elements  105  at a more uniform temperature value and reducing, or eliminating, hot spots or regions may improve the efficiency of the photovoltaic elements  105  at a specific temperature, and may reduce the amount of degradation of the photovoltaic elements  105  caused by higher temperatures. 
     This may allow the photovoltaic elements  105  to operate at a higher overall temperature than would otherwise be the case. This may be understood by considering that where hot spots exist in the photovoltaic elements  105  it may be the temperature induced reduction in efficiency and temperature induced degradation in these hot spots that limits the maximum operating temperature of the photovoltaic element  105  as a whole. As a result, reducing, or eliminating, these hotspots may allow the maximum operating temperature of the photovoltaic element  105  as a whole to be raised. 
     The illustrated example of the second embodiment has a solar energy collector assembly  102  supported by a single physical connection through the cylindrical tube  119 . In other examples alternative supporting arrangements may be used. In some examples the solar energy collector assembly  102  may be supported by two physical connections, one at each end of the solar energy collector assembly  102 . In some examples, one of the two physical connections may be the through the cylindrical tube. In general, it is advantageous to minimize the number of physical supports in order to minimize the escape of heat from the solar energy collector assembly by conduction through the physical supports. 
     In other examples the number of photovoltaic elements  105  mounted on the heat transport element  104  may be different. In other examples the relative sizes of the photovoltaic elements  105  and the heat transport element  104  may be different. 
     In some examples the adhesive layer  149  may comprise an epoxy resin which remains non-brittle after curing. 
     In other examples the adhesive layer  149  may be formed by a double sided adhesive tape. 
     Heat Transport Element 
     The heat transport element  104  according to the second embodiment is shown in more detail in a cut away view in  FIG. 11 , and in transverse and longitudinal cross-sectional views in  FIGS. 12 and 13  respectively. The transverse cross section of  FIG. 12  is taken along the line A-A in  FIG. 11 . The longitudinal cross section of  FIG. 13  is taken along the line B-B in  FIG. 11 . 
     In the second embodiment, the heat transport element  104  is generally rectangular. The heat transport element  104  has a flat upper surface  104   a  and a lower surface  104   b  which is flat across most of its area, and has an outwardly projecting section  110  along one edge  104   c  of the heat transport element  104 . The outwardly projecting section  110  contains and defines a vapor manifold  111 . In operation the heat transport element  104  is arranged to be transversely sloping, so that the side edge  104   c  of the heat transport element  104  bearing the outwardly projecting section  110  is higher than the opposite side edge  104   d  of the heat transport element  104 , for reasons which will be explained in detail below. The inclination angle of the heat transport element  104  to the horizontal may be small. An inclination of about 5° is sufficient. 
     Larger angles of inclination may be used if desired. An angle of inclination up to and including 90° may be used, i.e. the heat transport element  104  may be arranged transversely vertically. 
     The heat transport element  104  has an upper surface  104   a  formed by an upper sheet  114  and a lower surface  104   b  formed by a lower sheet  115 . A central sheet  116  is located between the upper sheet  114  and the lower sheet  115 , so that fluid flow passages  117  and  118  running transversely across the heat transport element  104  are defined between the central sheet  116  and each of the upper sheet  114  and the lower sheet  115 . The fluid flow passages  117  and  118  are sloped along their lengths. In the illustrated example the heat transport element  104  is transversely sloping, and as a result the fluid flow passages  117  and  118  running transversely across the heat transport element  104  will be sloped along their lengths. 
       FIG. 14  shows the profile of the central sheet  116  in more detail.  FIG. 14  shows a longitudinal cross section along the line B-B in  FIG. 11 . The central sheet  116  is formed with a corrugated profile having ridges and troughs which run transversely across the heat transport element  104 . The cross-sectional profile of the corrugated central sheet  116  can be understood as a zig-zag profile with the points of the zig-zag forming the peaks and troughs being flattened. Accordingly, the upper and lower fluid flow passages  117  and  118  are interleaved. The upper and lower fluid flow passages  117  and  118  are arranged side by side in a planar array with upper fluid flow passages  117  and lower fluid flow passages  118  arranged alternately. 
     To be more specific, in the illustrated example of the second embodiment the central sheet  116  comprises a plurality of flat surfaces connected by folds running transversely across the heat transport element  104 . The central sheet  116  comprises a first series of first coplanar surfaces  116   a  spaced apart equidistantly in a first plane C and a second series of second coplanar surfaces  116   b  spaced apart equidistantly in a second plane D, each of the first and second coplanar surfaces  116   a  and  116   b  having the same width, and the separation between successive coplanar surfaces  116   a  or  116   b  of each of the first and second series of first and second coplanar surfaces  116   a  and  116   b  being larger than the width of the coplanar surfaces  116   a  and  116   b . The first and second planes C and D are parallel and spaced apart. The first and second series of coplanar surfaces are arranged so that in plan view, i.e. when viewed perpendicularly to the first and second planes C and D, each of the first coplanar surfaces  116   a  is located equidistantly between two of the second coplanar surfaces  116   b , and vice-versa. The first and second coplanar surfaces  116   a  and  116   b  are interconnected by a first series of first parallel linking surfaces  116   c  and a second series of second parallel linking surfaces  116   d.    
     As is shown particularly in  FIG. 13 , the central sheet  116  is arranged with the first surfaces  116   a  contacting an inner face of the upper sheet  114  and the second surfaces  116   b  contacting an inner face of the lower sheet  115 . The first surfaces  116   a  of the central sheet are bonded to the upper sheet  114  and the second surfaces  116   b  of the central sheet  116  are bonded to the lower sheet  115 . Accordingly, the upper lower, and central sheets  114 ,  115 ,  116  define a plurality of trapezoid cross-section upper fluid flow channels  117  and lower fluid flow channels  118  between them. The upper fluid flow channels  117  are defined between the upper sheet  114  and the central sheet  116 . The lower fluid flow channels  118  are defined between the lower sheet  115  and the central sheet  116 . The trapezoid upper fluid flow channels are arranged so that the larger one of the two parallel faces of the trapezoid channel is formed by the upper sheet  114 . 
     The edges of the heat transport element  104  are formed by upwardly bent parts of the lower sheet  115 , which are bonded to the upper sheet  114 . The photovoltaic elements  105  are bonded to the upper sheet  114 . At the edges of the heat transport element  104 , the upper sheet  114  is bonded directly to the lower sheet  115 , the central sheet  116  is not located between the upper and lower sheets  114  and  115  at their edges. 
     In some examples the central sheet  116  may extend at least partially between the upper and lower sheets  114  and  115  at the end edges of the heat transport element  104  so that the upper and lower sheets  114  and  115  are both bonded to the central sheet  116 . This may assist in locating and securing the central sheet  116  relative to the upper and lower sheets  114  and  115 . 
     As discussed above, the heat transport element  104  has an outwardly projecting section  110  along the upper side edge  104   c  of the heat transport element  104 . The outwardly projecting section  110  is substantially semi-cylindrical and is formed by an outwardly projecting part of the lower sheet  115 . The outwardly projecting section  110  defines a vapor manifold  111 . The fluid flow channels  117  and  118  connect to the vapor manifold  111 . It should be noted that the central sheet  116  extends across most of the width of the vapor manifold  111 . Accordingly, the upper fluid flow channels  117  defined between the upper sheet  114  and the central sheet  116  connect to the vapor manifold  111  towards the top of the vapor manifold  111 , while the lower fluid flow channels  118  defined between the lower sheet  115  and the central sheet  116  connect to the vapor manifold  111  towards the bottom of the vapor manifold  111 . All of the upper and lower fluid flow channels  117  and  118  are interconnected by the vapor manifold  111 . 
     At the lower side edge  104   d  of the heat transport element  104  opposite the outwardly projecting section  110 , there is a gap  123  between the edge of the central sheet  116  and the side edge  104   c  of the heat transport element  104  formed by an upwardly bent part of the lower sheet  115 . This gap  123  allows water to flow between different ones of the fluid flow channels  117  and  118 . The gap  123  extends along the side edge  104   d  of the heat transport element  104 , and forms a fluid manifold  124  interconnecting all of the upper and lower fluid flow channels  117  and  118 . 
     At an end of the heat transport element  104  adjacent the open end of the glass tube  103  and the end cap  120  the substantially semi-cylindrical outwardly projecting section  110  extending most of the length of the heat transport element  104  transitions to a projecting cylindrical tube  119 . The upper and lower sheets  114  and  115  are sealed to the cylindrical tube  119  so that the interior of the heat transport element  104  is sealed. The cylindrical tube  119  passes through the end cap  12  and into the heat exchange assembly  106 . The central bore of the cylindrical tube  119  is connected to the vapor manifold  111  and acts to carry heat energy from the heat transport element  104  to the heat exchange assembly  106 , as will be explained below. 
     The cylindrical tube  119  physically supports the solar energy collector assembly  102  within the sealed transparent tube  103 . There is no other physical support of the solar energy collector assembly  102 . This may reduce conductive heat losses from the solar energy collector assembly  102 , which may increase the amount of useful heat energy produced by the hybrid solar energy converter  101 . 
     The fluid flow channels  117  and  118  are at least partially filled with degassed distilled water  121  as a working fluid and the interior of the heat transport element  104  including the fluid flow channels  117  and  118 , the vapor manifold  111 , and the tube  119  are at least partially evacuated. That is, the interior of the heat transport element  104  is at a pressure below normal atmospheric pressure. The interior of the heat transport element  104  may be under a vacuum at a pressure of 10 −3  mbar. The heat transport element  104  is arranged to be laterally inclined to the horizontal with the side  104   a  of the heat transport element  104  where the vapor manifold  111  is located being arranged to be higher than the opposite side  104   b  of the heat transport element  104 . 
     In the illustrated second embodiment the amount of water  121  in the fluid flow channels  117  and  118  is such that an upper surface  132  of the water  121  in the lower fluid flow channels  118  is level with the ends of the lower fluid flow channels  118  where the lower fluid flow channels  118  connect to the vapor manifold  111 . In the illustrated second embodiment the level of the surface  132  of the water  121  in the upper fluid flow channels  117  and lower fluid flow channels  118  is substantially the same. Accordingly, in the illustrated second embodiment the lower fluid flow channels are filled with liquid water, while the upper fluid flow channels  117  are only partially filled with liquid water. 
     In other examples the level of the water  121  may be different. In some examples the upper surface  132  of the water  121  in the lower fluid flow channels  118  may be below the vapor manifold  111 . In some examples the upper surface  132  of the water  121  in the lower fluid flow channels  118  may be above the bottom of the vapor manifold  111 , with some water being present in the bottom of the vapor manifold  111 . 
     It is expected that in practice the heat transport element  104  will operate most efficiently with the upper surface  122  of the water being at, or close to, the point where the lower fluid flow channels  118  contact the vapor manifold  111 . If the level of the water in the heat transport element  104  is too high, so that the upper surface  122  of the water is too high within the vapor manifold  111 , the efficiency of operation of the heat transport element  104  may be reduced, as will be discussed in more detail below. 
     The upper surface  132  of the water  121  in the upper fluid flow channels  117  may be higher than in the lower fluid flow channels  118  as a result of capillary action. The extent of this capillary effect in any specific example will depend upon the dimensions of the upper fluid flow channels  117 . In the illustrated second embodiment some of the inner surface of the upper sheet  114 , that is, the surface forming a part of the upper fluid flow channels  117 , is above the surface of the water  121 . In some examples the upper fluid flow channels  117  may have a small enough cross-sectional area that the upper surface  123  of the water  121  in the upper fluid flow channels  117  is at the ends of the upper fluid flow channels  117  due to capillary action. 
     It should be noted that, unlike the first embodiment, it is not necessary that the inner surface of the upper sheet  114 , that is, the surface forming a part of the upper fluid flow channels  117 , is below the upper surface  132  of the water  121  at a position corresponding to the location of the uppermost parts of the photovoltaic elements  105 . However, in some embodiments this may be the case. 
     In operation of the second embodiment, when the solar energy collector assembly  102  is exposed to incident solar radiative energy, the photovoltaic elements  105  absorb some of this energy, converting a part of the absorbed energy into electrical energy. The remainder of the absorbed energy is converted into heat energy, raising the temperature of the photovoltaic elements  105 . The absorbed heat energy flows from the photovoltaic elements  105  into the heat transport element  104 , being transmitted through the upper sheet  114  and into the water  121  inside the upper fluid flow channels  117 , which water is in contact with the inner surface of the upper metal sheet  114  across the larger parallel faces of the trapezoid upper fluid flow channels  117 . 
     The liquid water  121  inside the upper fluid flow channels  117  absorbs the heat energy from the photovoltaic elements  105  passing through the upper sheet  114  and vaporizes, producing bubbles  122  of steam or water vapor, as shown in  FIG. 15 . The liquid water may vaporize and produce bubbles as a result of either or both of convection boiling and nucleation. At the vacuum pressure of 10 −3  mbar inside the upper fluid flow channels  117  water boils from around 0° C., so that the water  121  vaporizes readily at the normal operating temperatures of the hybrid solar energy converter  101 . 
     The bubbles  122  of water vapor are less dense than the liquid water  121 . Further, as explained above the upper fluid flow channels  117  are sloping along their lengths. Accordingly, as a result of this density difference the water vapor bubbles  122  travel upwards along the upper fluid flow channels  117  towards the upper side edge  104   c  of the heat transport element  104  and the surface of the water  121 . When a bubble  122  of water vapor reaches the surface of the water  121  the vapor is released into the vacuum above the water  121  in the vapor manifold  111 . Further, as a bubble  122  travels upwards along a fluid flow channel  117  the bubble  122  will act as a piston to drive the liquid water, and any other bubbles  122  above it, upwardly along the upper fluid flow channel  117 . This pistonic driving may tend to accelerate the speed with which the vapor bubbles  122  move upward along the upper fluid flow channels  117 . This pistonic driving may act to pump liquid water upwards along the upper fluid flow channels  117  to the ends of the upper fluid flow channels  117 , where the liquid water will be ejected from the upper fluid flow channels  117  into the vapor manifold  111 . In the illustrated second embodiment, where some of the inner surface of the upper sheet  114  is above the surface of the water  121 , this pumping of liquid water upwards along the upper flow channels  117  ensures that the part of the inner surface of the upper sheet  114  above the surface of the water  121  is in contact with a flow of water so that it can be cooled. 
     The amount of the pistonic driving produced by the bubbles  122  will depend upon the relative sizes of the bubbles  122  compared to the cross-sectional areas of the upper fluid flow channels  117 . The amount of pistonic driving produced by the bubbles  122  may be increased where the size of the bubbles is relatively large compared to the cross-sectional areas of the upper fluid flow channels  117 . The pistonic driving produced by the bubbles  122  may be particularly effective in examples where the size of the bubbles  122  of water vapor is equal to, or only a little smaller than, the cross sectional areas of the upper fluid flow channels  117 . 
     In practice the sizes of individual water vapor bubbles will vary. However, the likely average sizes of the bubbles and the likely variability in their sizes can be determined in any specific case, based on the operating parameters to be used in the hybrid solar energy converter. 
     The bursting of the bubbles of water vapor at the water surface and any pistonic pumping of liquid water out of the ends of the upper fluid flow channels  117  may generate droplets of liquid water, and may project at least some of these water droplets into the vacuum within the vapor manifold  111  above the water surface. As a result, the heat transfer mechanism may be a multi-phase system comprising liquid water, water vapor and droplets of liquid water, and not just a two-phase system comprising liquid water and water vapor only. The presence of such droplets of water in the vacuum, and any pumping of liquid water out of the ends of the upper fluid flow channels  117 , may enhance the rate of vaporization by increasing the surface area of the water exposed to the vacuum. 
     Similarly to the first embodiment, the water vapor in the vacuum within the vapor manifold  111  travels at a very high speed through the vacuum along the vapor manifold  111 , along the tube  119  and into the heat exchange assembly  106 . The travel speed of the hot water vapor in the vacuum is very fast, approximating to the thermal speed of the water vapor molecules. Inside the heat exchange assembly  106  the water vapor condenses on a heat exchange surface of one of the primary and secondary heat exchangers  107  and  108 . The condensed water flows back out of the heat exchange assembly  106  down the tube  119 , along the bottom of the vapor manifold  111 , and is returned back into the water  121  within the lower fluid flow channels  118 . This generating of hot water vapor within the upper fluid flow channels  117  and the vapor manifold  111 , and subsequent travel of hot water vapor from the vapor manifold  111  to the heat exchange assembly  106  where it condenses, followed by return of the condensed water, transfers heat energy from the heat transfer element  104  to the operating fluids in the heat exchange assembly  106 . 
     Any liquid water ejected from the upper fluid flow channels  117  into the vapor manifold  111  which does not vaporize will also fall to the bottom of the vapor manifold  111 , and is returned back into the water  121  within the lower fluid flow channels  118 . 
     As is explained above, all of the upper and lower fluid flow channels  117  and  118  are interconnected by the fluid manifold  124  formed by the gap  123 . Accordingly, it is not important which of the lower fluid flow channels  118  is entered by any liquid water returning from the vapor manifold  111 . 
     As is clear from the description above, the vapor manifold  111  generally includes liquid water in addition to water vapor when the hybrid solar energy converter  101  is operating. However, as is also discussed above, if the level of the water in the heat transport element  104  is too high, so that the upper surface  122  of the water is too high within the vapor manifold  111 , the efficiency of operation of the heat transport element  104  may be reduced. This reduction in efficiency of operation may occur because there is insufficient space within the vapor manifold  111  above the surface of the water for the movement and evaporation of the droplets of liquid water. This reduction in efficiency of operation may occur because the droplets of liquid water and waves and splashing upwardly of the liquid water surface may reduce the open, or water free, cross sectional area of the vapor manifold at some locations to a relatively small amount, or even to zero, momentarily closing the vapor manifold. This reduction in the open, or water free, cross sectional area of the vapor manifold may interfere with the movement of the water vapor in the vacuum within the vapor manifold  111 . 
     The bubbles  122  of water vapor will tend to move upwardly through the liquid water in the upper fluid flow channel  117  because of the lower density of the water vapor compared to the liquid water  121 , which will result in an upward buoyancy force on each bubble  122 . Further, the movement of the bubbles  122  of water vapor will tend to drive the liquid water  121  in the upper fluid flow channel  117  upwardly, particularly in examples where pistonic driving takes place. As a result, the bubbles  122  of water vapor cause the water  121  in the upper and lower fluid flow channels  117  and  118  to circulate, with relatively hot liquid water and bubbles  122  of water vapor flowing upwards along the upper fluid flow channels  117 , and relatively cool liquid water flowing downwards along the lower fluid flow channels  118 . The upper and lower fluid flow channels  117  and  118  are interconnected by the vapor manifold  111  and the fluid manifold  124 , as explained above. Accordingly, the relatively hot liquid water flowing upwards along the upper fluid flow channels is continuously replaced by relatively cool liquid water from the lower fluid flow channels  118 . This circulation is driven primarily by the difference in density between the water vapor and the liquid water. However, this circulation may also be driven by convection as a result of the difference in density between the relatively hot liquid water in the upper fluid flow channels  117  and the relatively cool liquid water in the lower fluid flow channels  118 , in a similar manner to a thermosiphon. Accordingly, the upper fluid flow channels  117  may be regarded as riser channels, while the lower fluid flow channels  118  may be regarded as sinker channels or return channels. 
     As the bubbles  122  of water vapor travel upwardly along the upper fluid flow channels  117  the pressure head acting on the bubbles  122  decreases, so that the bubbles  122  tend to expand. As a result, the tendency of the vapor bubbles  122  to collapse and implode is reduced by the effects of the expansion and decreasing pressure as the bubbles  122  move upwardly. When considering this point, it should be remembered that when the heat transport element  104  is operating the bubbles  122  will be forming within an established density driven circulation fluid flow and will move upwardly carried by this flow in addition to the bubbles movement due to their own buoyancy relative to the liquid water. Further, it is believed that expansion of the bubbles  122  as they move upwardly will further increase the speed of the density driven circulation flow by increasing the buoyancy of the expanding bubbles  122 . In some examples expansion of the bubbles as they move upwardly may also increase the degree of pistonic driving. 
     This density driven circulation may form a highly effective heat transport mechanism because water has a relatively high enthalpy of vaporization, so that the movement of the bubbles  122  of water vapor may carry a large amount of heat energy, in addition to the heat energy carried by the movement of relatively hot water out of the upper fluid flow channels  117 , and its replacement by cooler water. In arrangements where pistonic driving of the flow of the liquid water by the water vapor bubbles takes place the effectiveness of the heat transport mechanism may be further increased by the increase in the flow rate of the liquid water caused by the pistonic driving. This pistonic driving is a component of the overall density driving producing the density driven circulation. The pistonic driving is caused by the density difference between the liquid water and the bubbles of water vapor. 
     In general, the speed of the density driven circulation increases and the effectiveness of the heat transport mechanism increases as the temperature of the upper sheet  114  of the heat transport element  104  increases. 
     The density driven circulation of the water  121  within the fluid flow channels  117  and  118  is a vapor driven circulating or rolling flow. 
     The density driven circulation of the water  121  within the fluid flow channels  117  and  118  becomes particularly vigorous, and becomes particularly effective as a heat transport mechanism, when the temperature of the upper sheet  114  of the heat transport element  104  becomes sufficiently high that the water  121  within the fluid flow channels  117  and  118  enters a rolling boil state. The effectiveness of the heat transport mechanism significantly increases when rolling boiling of the water  121  commences. In general, when other parameters of the system remain constant, entry into the rolling boil state will take place when the temperature of the upper sheet  114  of the heat transport element  104  reaches a specific temperature. 
     In the illustrated example using water, the water  121  within fluid flow channels  117  and  118  may enter a rolling boil state at a temperature of about 40° C. 
     The arrangement of fluid flow channels  117  extending laterally across the heat transport element  104  may allow the vertical height of the liquid water in the heat transport element  104  to be reduced compared to embodiments in which the density driven flow extends along the length of a heat transport element, and so reduce the pressure head acting on the liquid water at the bottom of the heat transport element  104 . In general, increased pressure reduces the tendency of liquids to vaporize and so increases the boiling point of liquids. Accordingly, reducing the pressure head acting on the liquid water at the bottom of the heat transport element  104  may increase the tendency of the liquid water  121  towards the lower ends of the upper fluid flow channels  117  to vaporize and produce bubbles  122 , and so may improve the efficiency and effectiveness of the heat transport element  104 . 
     In particular, the reduction of the pressure head acting on the liquid water at the bottom of the upper fluid flow channels  117  may reduce any temperature differential along the lengths of the upper fluid flow channels between their the top and bottom ends by reducing any difference in the tendency of the liquid water to vaporize due to differences in pressure. This may reduce temperature differentials between the different points on the heat transport element  104  and may assist in reducing or avoiding the formation of hot spots in the photovoltaic elements  105 . 
     In general the forming of hot spots in the photovoltaic elements  105  is undesirable because these may lead to a reduction in the efficiency of electrical energy generation in the photovoltaic elements  105 , which reduction in efficiency may be permanent. 
     The arrangement of the upper fluid flow channels  117  extending laterally across the heat transport element  104  and interconnected by a vapor manifold  111  extending longitudinally along the heat transport element  104  may allow a very rapid flow of heat energy along the heat transport element  104  away from any upper fluid flow channel  117  having a higher temperature. This may reduce temperature differentials between the different points on the heat transport element  104  and may reduce, or avoid, the formation of hot spots in the photovoltaic elements  105 . 
     The provision of the two separate heat transport mechanisms of the movement of water vapor along the vapor manifold  111  and the density driven flow of liquid water and water vapor along each of the upper fluid flow channels  117 , respectively acting longitudinally and transverse the length of the heat transport element  104 , may tend to equalize the temperature across the entire upper surface of the heat transport element, and thus tend to equalize the temperature across the photovoltaic elements  105  and reduce, or avoid, the formation of hot spots. 
     The movement of water vapor along the vapor manifold  111  provides a very rapid heat transport mechanism that tends, by the vaporization and condensation of water, to move heat energy from relatively hot locations to relatively cold locations. As a result, the movement of water vapor along the vapor manifold  111  may tend to equalize the temperature of the liquid water surface at different positions along the heat transfer element  104 , in addition to transporting heat energy from the heat transport element  104 , and specifically from the upper surface  104   a  of the heat transport element  104 , to the heat exchange assembly  106 . This temperature equalization may have the effect of removing more heat energy from hotter parts of the upper surface  104   a  of the heat transport element  104 , and so tending to equalize the temperature across the upper surface  104   a . It is clear that such isothermal cooling will tend to reduce, or avoid, the formation of hot spots, for example in any photovoltaic element attached to the upper surface  104   a.    
     The lower sheet  115  of the heat transport element  104  has a plurality of hollow ridges  125  extending between the flat part of the lower surface  104   b  and the semi-cylindrical surface of the outwardly projecting section  110 . Each hollow ridge  125  has a ‘V’ profile, and the hollow ridges  125  are located spaced apart at regular intervals along the length of the heat transport element  104 .  FIG. 16  shows a transverse cross section of the heat transport element  104  taken along the line C-C in  FIG. 11 . The line C-C of  FIG. 16  is parallel to the line A-A of  FIG. 12 , but passes through one of the hollow ridges  125 . The hollow ridges  125  act as supports for the outwardly projecting section  110 , acting as buttresses and helping to keep the curved part of the lower sheet  115  forming the outwardly projecting section  110  fixed relative to the flat part of the lower metal sheet  115  and the other parts of the heat transport element  104 . 
     The hollow ridges  125  also act as drains to return liquid water from the vapor manifold  111  into the lower fluid flow channels  118 , as will be explained in more detail below. 
     As explained above, the vapor manifold  111  is semi-cylindrical, being defined by the semi-cylindrical outwardly projecting section  110  formed by a curved part of the lower sheet  115 . Further, as explained above, the heat transport element  104  is transversely sloping so that the side edge  104   c  of the heat transport element  104  bearing the outwardly projecting section  110  is higher than the other side edge  104   d  of the heat transport element  104 . As a result, depending upon the transverse inclination angle of the heat transport element  104  there may, or may not, be parts of the vapor manifold  111  which are located below the ends of the lower fluid flow channels  118  where the lower fluid flow channels  118  connect to the vapor manifold  111 . 
       FIGS. 17A and 17B  are explanatory diagrams, each showing a transverse cross sectional view of the heat transport element  104  corresponding to the view shown in  FIG. 12 .  FIG. 17A  shows the heat transport element  104  inclined at a relatively large angle to the horizontal, while  FIG. 17B  shows the heat transport element  104  inclined at a relatively small angle to the horizontal. 
     When the heat transport element is inclined at a relatively small angle to the horizontal, as shown in  FIG. 17A , the lower fluid flow channels  118  connect to the vapor manifold  111  at the lowest point of the semi-cylindrical outwardly projecting section  110  of the lower sheet  115  defining the vapor manifold  111 . In this position all liquid water within the vapor manifold  111  will drain directly into the lower fluid flow channels  118 . In contrast, when the heat transport element  104  is inclined at a relatively large angle to the horizontal, as shown in  FIG. 17B , the part of the semi-cylindrical outwardly projecting section  110  of the lower sheet  115  defining the vapor manifold  111  is located below the point at which the lower fluid flow channels  118  connect to the vapor manifold. In this position, in the absence of the hollow ridges  125 , some liquid water within the vapor manifold  111 , specifically liquid water below the horizontal line  126 , could be retained within the vapor manifold  111  and not drain into the lower fluid flow channels  118 . 
     The hollow ridges  125  form a drain path for liquid water in the vapor manifold  111  to return to the lower fluid flow channels  118  and so prevent the retention of a reservoir of liquid water within the vapor manifold  111  which might otherwise occur. 
     As discussed above, the heat transport assembly  104  can operate with liquid water within the vapor manifold  111 . However, in the absence of the hollow ridges  125  the existence and size of any reservoir of liquid water retained in the vapor manifold  111  will vary depending on the angle of inclination to the horizontal of the heat transport element  104 , and the resulting changes in the liquid water level in the fluid flow channels  117  and  118  at different angles of inclination may adversely affect the operation of the heat transport element  104  at some angles of inclination and so limit the range of angles of inclination at which the heat transport element  104  can be used. 
     Accordingly, the hollow ridges  125  may extend the range of angles of inclination at which the heat transport element  104  can be used. 
     Depending upon the geometry of the different parts of the heat transport element  104  in any specific design, even when the hollow ridges  125  are used there may still be a minimum angle of inclination at which the heat transport element  104  can operate without the retention of liquid water in the vapor manifold  111  having adverse effects on operation of the heat transport element  104 . 
     In the illustrated example of the second embodiment the hollow ridges  125  act as supports for the outwardly projecting section  110  and also act as drains to return liquid water from the vapor manifold  111  into the lower fluid flow channels  118 . In some examples these functions may be carried out by separate dedicated structures. 
     The corrugated profile of the central sheet  116  and the bonding of the first and second surfaces  116   a  and  116   b  of the central sheet  116  to the upper sheet  114  and the lower sheet  115  so that the linking surfaces  116   c  and  116   d  of the central sheet  116  interconnect the upper and lower sheets  114  and  115  increases the strength and rigidity of the heat transport element  104 . This may make the heat transport element  104  a more rigid structure. This may tend to reduce the amount of flexing of the heat transport element  104  in use. This may prevent damage to the photovoltaic elements  105  by reducing the amount of mechanical stress applied to the photovoltaic elements  105 . This may allow the upper, lower, and/or central metal sheets  114 ,  115 ,  116 , to be thinner, which may reduce weight and costs. This may allow the upper metal sheet  114  to be thinner, which may improve the transfer of heat from the photovoltaic elements  105  into the liquid water within the upper fluid flow channels  117 . 
     The heat transport element  104  is a substantially rigid structure. This may minimize changes in the level of the upper surface  132  of the water  121  due to flexing of the components of the heat transport element  104 , such as the upper and lower sheets  114  and  115 . Such changes in the level of the upper surface  132  of the water  121  may affect the efficiency of the cooling of the photovoltaic elements  105 . 
     As is explained above, the interior of the heat transport element  104  is evacuated, and the heat transport element  104  is located within an evacuated tube  103 . Usually the heat transport element  104  and the evacuated tube  103  are evacuated to the same pressure. In the illustrated example of the second embodiment described above this pressure may be 10 −3  mbar. 
     When the water within the heat transport element  104  is heated the proportion of the water in a vapor phase will increase and the proportion in a liquid phase will decrease. As a result the pressure within the heat transport element  104  will increase, producing a pressure differential between the interior and exterior of the heat transport element  104 . This pressure differential may cause the upper and lower metal sheets  114  and  115  to ‘balloon’, or bend outwards. The interconnection of the upper and lower metal sheets  114  and  115  by the linking surfaces  116   c  and  116   d  of the central metal sheet  116  may resist such ballooning of the upper and lower metal sheets  114  and  115  and reduce or prevent ballooning. Arranging for the linking surfaces  116   c  and  116   d  of the central metal sheet  116  to be straight may increase the resistance to ballooning. Reducing or preventing ballooning may prevent damage to the photovoltaic elements  105  by reducing the amount of mechanical stress applied to the photovoltaic elements  105 . This may allow the upper metal sheet  114  to be thinner, which may reduce weight and costs and/or may improve the transfer of heat from the photovoltaic elements  105  into the liquid water within the upper fluid flow channels  117 . 
     The above description of the operation of the heat transfer element  104  according to the second embodiment describes the transfer of heat energy from the photovoltaic elements  105  through the upper metal sheet  114  and into the water within the upper fluid flow channels  117 . In addition, in the regions of the upper metal sheet  114  bonded to the first surfaces  116   a , some heat energy will pass through the upper metal sheet  114  and the central metal sheet  116  into the water within the lower fluid flow channels  118 . Although this transfer of heat energy will cool the photovoltaic elements  105 , the heating of the water in the lower fluid flow channels  118  is generally undesirable because it will tend to counteract and slow the density driven circulation of water produced by the heating of the water in the upper fluid flow channels  117  described above. Accordingly, it is preferred for the sizes of the first surfaces  116   a  of the central metal sheet  116  in contact with the upper metal sheet  114  to be as small as possible, subject to the contact area between the first surfaces  116   a  and the upper metal sheet  114  being sufficiently large to form a reliable bond of the required strength. 
     Unlike the first embodiment, it is not necessary for the heat transport element  104  according to the second embodiment to be inclined to the horizontal along its longitudinal axis. In other words, unlike the first embodiment, it is not necessary for the end of the heat transport element  104  adjacent the heat exchange assembly  106  to be higher than the end of the heat transport element  104  remote from the heat exchange assembly  106 . 
     In the illustrated second embodiment the heat transport element  104  is arranged to be horizontal along its longitudinal axis. That is, the end of the heat transport element  104  adjacent the heat exchange assembly  106  should be at the same height as the end of the heat transport element  104  remote from the heat exchange assembly  106 . However, in practice some deviation from the horizontal may be tolerated without significant impact on the operation of the heat transport element  104 . Such deviation from the horizontal will result in differences in the level of the liquid water surface relative to the structure of the heat transport element  104  at different positions along the length of the heat transport element  104 . As is explained above, the level of the liquid water surface may be varied. Accordingly, the minor differences in level caused by small deviations from the horizontal may be accommodated. 
     In some examples the hybrid solar energy converter  101  may be arranged so that the tube  119  and the internal passages of the heat exchanger assembly  106  are inclined at an angle to the horizontal downwardly from the heat exchanger assembly  106  towards the heat transport element  104  in order to assist the return flow of condensed liquid water from the primary and secondary heat exchangers  108  and  109  to the vapor manifold  111  of the heat transport element  104 . 
     In the illustrated example, each of the upper and lower sheets  114  and  115  has a dimpled profile. This dimpled profile is shown in more detail in  FIGS. 18A and 18B .  FIG. 18  A shows a plan view from above of a part of the upper sheet  114 .  FIG. 18B  shows a cross section through the upper sheet  114  along the line D-D in  FIG. 18A . 
     As is shown in  FIG. 18A , a plurality of dimples  127  are formed in the flat upper surface  104   a  of the heat transport element  104  in the upper sheet  114 . The dimples  127  are formed in straight rows and columns to form a regular two dimensional square array, and are spaced apart leaving a flat strip  128  between each row of dimples  127 . 
     Each dimple  127  comprises a looped recess  127   a  having a circular inner perimeter  127   b  and a square outer perimeter  127   c . The square outer perimeter  127   c  has rounded off corners  127   d . Within the circular inner perimeter  127   b  a circular region  127   e  is raised relative to the looped recess  127 . The circular region  127   e  is at the same level as the surface  104   a  of the flat strips of the upper sheet  115  outside the dimple  127 . 
     The flat strips  128  run transversely across the upper sheet  114  and have the same width as the width of the first coplanar surfaces  116   a  of the central sheet  116 . The flat strips  128  provide flat areas for bonding with the first surfaces  116   a  of the central sheet  116 . The flat strips  128  may allow reliable and strong bonds to be made between the first surfaces  116   a  and the upper sheet  114 . The flat strips  128  may allow a good seal to be formed between adjacent upper fluid flow passages  117 . 
     A plurality of dimples  129  are formed in the lower sheet  115 . The dimples  129  are formed in straight rows and columns to form a regular two dimensional square array, and are spaced apart leaving a flat strip  130  between each row of dimples  129 . The dimples  129  in the lower sheet  115  are the same as the dimples  127  in the upper sheet  114 . The flat strips  128  run transversely across the upper metal sheet  114  and have the same width as the width of the first and second coplanar surfaces  116   a  and  116   b . The flat strips  130  provide flat areas for bonding with the second surfaces  116   b  of the central sheet  116 . The flat strips  130  may allow reliable and strong bonds to be made between the second surfaces  116   b  and the lower sheet  115 . 
     In the illustrated example of the second embodiment of the invention both the dimples  127  in the upper sheet  114  and the dimples  130  in the lower sheet  115  are formed by downward recesses. Accordingly, the dimples  127  in the upper sheet  114  have recesses extending into the heat transport element  104 , while the dimples  130  in the lower sheet  115  have recesses extending out of the heat transport element  104 . In other examples the dimples  127  and  130  may be formed by recesses extending upwardly, or by recesses extending in opposite directions. 
     The array of dimples  130  on the lower metal sheet  115  extends across the flat part of the lower sheet  115 , but does not extend into the semi-cylindrical surface of the outwardly projecting section  110 . Further, the array of dimples  130  on the lower sheet  115  has dimples omitted from the array at the locations of the hollow ridges  125 . 
     The dimples  127  and  130  may increase the rigidity of the upper and lower sheets  114  and  115 . This may tend to reduce the amount of flexing of the heat transport element  104  in use. This may prevent damage to the photovoltaic elements  105  by reducing the amount of mechanical stress applied to the photovoltaic elements  105 . This may allow the upper, lower, and/or central sheets  114 ,  115 ,  116 , to be thinner, which may reduce weight and costs. This may allow the upper sheet  114  to be thinner, which may improve the transfer of heat from the photovoltaic elements  105  into the liquid water within the upper fluid flow channels  117 . 
     The surfaces of the dimples  127  may provide additional nucleation sites for the formation of water vapor bubbles  122 , which may improve efficiency. 
     In examples where adhesive is used to attach the photovoltaic elements  105  to the heat transport element  104  the dimples  127  on the flat upper surface  104   a  of the heat transport element  104  may provide reservoirs for the adhesive. This may allow more secure attachment of the photovoltaic elements  105 . This may allow a thinner layer of adhesive to be used, which may improve the transfer of heat from the photovoltaic elements  105  into the liquid water within the upper fluid flow channels  117 . 
     As discussed above the heat transport element  104  has a flat upper surface  104   a  formed by an upper sheet  114  with a dimpled profile. In addition the upper sheet  114  is has two longitudinal recesses  129  running across in its upper surface  104   a  which form two parallel troughs running along the upper surface  104   a  of the heat transport element  104 .  FIG. 19  shows one of these recesses  129 . Electrically conductive ribbons or wires  130  run along the longitudinal recesses  129  between the heat transport element  104  and the photovoltaic elements  105 . The wires  130  are electrically connected to the photovoltaic elements  105  and to the conductors  21  which pass through the cap  12  to provide a conductive path to carry the electrical power generated by the photovoltaic elements  105  out of the sealed transparent tube  103 . This electrical power may be supplied to an inverter for voltage conversion and/or for conversion to alternating current for supply to a domestic or mains electrical system. 
     In examples where adhesive is used to attach the photovoltaic elements  105  to the heat transport element  104 , an electrically insulating adhesive can be used to electrically insulate the electrically conductive ribbons or wires  130  from the photovoltaic elements  105  and from the upper surface  104   a  of the heat transport element  104 . The electrically insulating adhesive can also be used to electrically insulate the photovoltaic elements  105  from the upper surface  104   a  of the heat transport element  104 . 
     In the second embodiment the longitudinal recesses  129  run perpendicularly to the fluid flow channels  117  and  118 . Accordingly, each of the first surfaces  116   a  of the central metal sheet  116  has two recesses to receive the longitudinal recesses  129 . 
     In the illustrated example of the second embodiment each dimple  127  comprises a looped recess with a circular inner perimeter  127   b  and a square outer perimeter  127   c , with the circular region  127   e  at the same level as the surface  104   a  of the flat strips of the upper metal sheet  115  outside the dimple  127 . In some examples the circular region  127   e  may not be at the same level as the surface  104   a  of the flat strips of the upper metal sheet  115  outside the dimple  127 . In other examples different dimple shapes and/or profiles may be used. In some examples the perimeters may have different shapes. In some examples the circular region  127   e  may not be at the same level as the surface  104   a  of the flat strips of the upper metal sheet  115  outside the dimple  127 . In some examples the dimples may simply comprise a recessed region, rather than a recessed outer region surrounding a relatively raised inner region. 
     In the illustrated example of the second embodiment 0.2 mm thick tin coated mild steel sheets are used to form the different sheets of the heat transport element. In alternative examples other thicknesses may be used, in particular 0.1 mm thick tin coated mild steel sheets may be used. The use of a thinner upper metal sheet may improve the rate of heat energy transfer from the photovoltaic elements to the water inside the upper fluid flow channels. In other examples the different sheets may have different thicknesses. 
     In the illustrated example of the second embodiment the spacing between the upper sheet  114  and the parallel lower sheet  115  is 1.8 mm at the locations of the longitudinal recesses  129 . Accordingly, the thickness of the fluid flow channels  117  and  118  at the locations of the longitudinal recesses  129  is 1.6 mm, since the thickness of the central sheet is 0.2 mm. 
     The use of mild steel may avoid or reduce problems produced by differential thermal expansion of the silicon semiconductor photovoltaic elements  105  and the heat transport element  104  because the coefficients of thermal expansion of silicon and mild steel are similar. 
     The sheets used to form the heat transport element may be shaped by pressing. 
     In other examples different materials may be used, in particular sheets of other metals or metal alloys, such as copper or brass may be used. In other examples the upper, lower and/or partition sheets may be formed from materials which are not metals. In other examples there may be openings in the upper sheet allowing the water inside the upper fluid flow channels to directly contact the back surfaces of the photovoltaic elements to maximize heat transfer. In such examples the thickness or material used to form the upper sheet could be selected without having to take thermal conductivity into account. 
     In the second embodiment of the invention the roughening of the surfaces of the upper sheet  114  produced by the tin coating may provide nucleation sites, increasing the tendency of the liquid water  121  to vaporize and form bubbles  122  of water vapor. In the second embodiment of the invention the roughening of the surfaces of the central sheet  116  produced by the tin coating may provide nucleation sites, increasing the tendency of the liquid water  121  to vaporize and form bubbles  122  of water vapor. 
     In some examples other coatings may be added to the surfaces of the upper sheet  114  in order to promote or increase nucleation and formation of bubbles of water vapor. In some examples these coatings may be of metals, or plastics. In some examples these coatings may be of PTFE. 
     In the illustrated example of the second embodiment the different sheets are soldered together. In alternative embodiments different bonding techniques may be used. In some examples the different sheets may be bonded by techniques including spot welding, roller welding or adhesive. 
     In the illustrated example of the second embodiment inner faces of the upper and lower sheets  114  and  115 , and both faces of the central metal sheet  116 , are coated with a solder layer. In the illustrated example the solder layers are 2 to 6 microns thick. Other examples may have different thicknesses. 
     The edges of the upper and lower sheets  114  and  115  are then soldered together to form a gas tight seal between them, and to form a gas tight seal between the upper and lower sheets  114  and  115  and the tube  119 . As is explained above, the central metal sheet  116  is not located between the upper and lower metal sheets  114  and  115  at their edges. 
     The heat transport element  104  is then heated in an oven to a sufficiently high temperature to reflow the solder layers on the upper, lower and central sheets  114 ,  115 ,  116 , and is simultaneously evacuated. 
     This manufacturing procedure may ensure good solder bonding between the central sheet  116  and the upper and lower sheets  114  and  115 . This manufacturing procedure may allow a better level of vacuum to be achieved within the heat transport element  104  by evacuating the heat transport element  104  at a high temperature when out-gassing by the metal sheets and solder is taking place. 
     The solder may microscopically roughen the surfaces of the upper and central sheets  114  and  116 , This may provide nucleation sites, increasing the tendency of the liquid water  121  to vaporize and form bubbles  122  of water vapor. 
     In other examples, a solder layer is formed on the central sheet  116  only on the parts of the central metal sheet which contact the upper or lower sheets  114  and  115 . As can be understood from a comparison of  FIGS. 13 and 14  this will be the contact faces of the first and second surfaces  116   a  and  116   b . Similarly, in some examples a solder layer is formed on the surfaces of the upper sheet  114  and the lower sheet  115  only on the parts of the surfaces which will contact one of the other sheets. Reducing the amount of solder used may reduce costs. 
     In one example the upper sheet  114  only is coated in solder across its entire surface, while the central sheet and lower sheet  116  and  115  are coated in solder only on the parts of the surfaces which will contact one of the other sheets. This may allow the solder layer to provide nucleation sites on the surface of the upper sheet  114  forming parts of the upper fluid flow channels, while reducing the total amount of solder used. 
     As explained above, in the illustrated example of the second embodiment the flow of water vapor and liquid water through the heat transport element  104  tends to keep the cooled upper surface of the heat transport element  104  at a uniform operating temperature during operation. That is, the cooled upper surface of the heat transport element  104  tends to be kept isothermal. The isothermal nature of the cooled upper surface of the heat transport element  104  tends to give rise to isothermal cooling of the photovoltaic elements  105 , where hotter parts of the photovoltaic elements  105  tend to be preferentially cooled so that the photovoltaic elements  105  themselves tend to become isothermal. 
     Such isothermal cooling provides further advantages in addition to those provided by cooling. 
     Isothermal cooling may provide the advantage that the appearance of hot spots or regions in the photovoltaic elements  105  produced by heating by incident solar radiation can be reduced or eliminated. Such hot spots or regions can reduce the efficiency of the photovoltaic elements  105 . 
     Isothermal cooling may simplify the control and wiring arrangements of the photovoltaic elements  105  by reducing or eliminating any requirement for compensation for differences in the performance of the different parts of the photovoltaic elements  105  that are at different temperatures. 
     Isothermal cooling tends to reduce, or prevent, the formation of hot spots or regions in the photovoltaic elements  105 . As is explained above, this may allow the efficiency of the photovoltaic elements  105  to be improved at a specific temperature. Further, this may reduce the amount of degradation of the photovoltaic elements  105  caused by higher temperatures. 
     Still further, this may allow the photovoltaic elements  105  to operate with a given degree of efficiency at a higher temperature than would otherwise be the case. This may allow the solar energy collector assembly  102  including the photovoltaic elements  105  to be operated at a higher temperature without reducing the efficiency with which the photovoltaic elements  105  produce electrical energy. 
     One example of this effect of isothermal cooling is that the general figure quoted above for silicon photovoltaic elements that the efficiency of electrical energy generation generally drops by about 0.35% to 0.5% for each degree centigrade of temperature increase above 25° C. may not apply to silicon photovoltaic elements that are isothermally cooled. Such isothermally cooled silicon photovoltaic elements having hotspots eliminated or reduced may have a higher threshold temperature at which the efficiency of electrical energy generation begins to drop and/or may have a reduced rate of reduction in efficiency for each degree centigrade of temperature increase above the threshold temperature. Further, the temperature at which there is a risk of permanent degradation of the silicon photovoltaic elements may also be increased for isothermally cooled silicon photovoltaic elements. Similar effects may be found in photovoltaic elements formed of other semiconductor materials. 
     In some examples, one or more layers of heat conductive material may be located between the upper sheet  114  and the photovoltaic elements  105 . Such layers of heat conductive material may increase the rate of heat transfer between the photovoltaic elements  105  and the upper sheet  114 , and thus the rate of heat transfer between the photovoltaic elements  105  and the liquid within the upper fluid flow channels  117 . Such layers of heat conductive material may also increase the rate of heat transfer laterally across the photovoltaic elements  105 . 
     Accordingly, providing a layer of heat conductive material may increase the degree of isothermal cooling and further tend to reduce, or eliminate, the formation of hot spots or regions in the photovoltaic elements  105 . 
     The heat transport element may be used in other applications separately from the rest of the solar energy converter. 
     Heat Exchange Assembly 
     The heat exchange assembly  106  of the second embodiment may be essentially the same as the heat exchange assembly  6  of the first embodiment. As explained above, in the second embodiment the heat exchange assembly  106  includes a primary heat exchanger  107  and a secondary heat exchanger  108  separated by a heat transfer control valve  109 . These are similar to, and operate similarly to, the heat exchange assembly  6  including a primary heat exchanger  7  and a secondary heat exchanger  8  separated by a heat transfer control valve  9  according to the first embodiment. 
     In the illustrated example of the second embodiment the trigger temperature of the heat transfer control valve  109  is predetermined. In some examples the trigger temperature may be settable in use, or on installation or manufacture of the hybrid solar energy converter  101 . In some examples the trigger temperature may be settable to different values depending on the intended maximum water temperature of the water to be heated. In particular, in some examples the trigger temperature may be settable to 65° C. when the hybrid solar energy converter is to be used to heat water for a domestic hot water system and may be settable to 135° C. when the hybrid solar energy converter is to be used to heat water for an industrial hot water system. 
     In some examples the trigger temperature of the heat transfer control valve may be selected to maximize the generation of electrical energy by the photovoltaic elements  105 . In some examples the trigger temperature value may be selected to increase the amount of heat energy transferred to the first operating fluid. In some examples the trigger temperature may be selected to optimize the overall production of energy, taking into account both the amount of electrical energy produced by the photovoltaic elements  105  and the amount of heat energy transferred to the first operating fluid. In some examples the optimizing may maximize the total production of energy. In some examples the optimum overall production of energy may take into account the relative demand for, or value of, the different types of energy, rather than simply maximizing the total amount of energy produced. 
     As explained above, the isothermal cooling tends to reduce, or prevent, the formation of hot spots or regions in the photovoltaic elements  105 . This may allow the solar energy collector assembly  102  including the photovoltaic elements  105  to be operated at a higher temperature without reducing the efficiency with which the photovoltaic elements  105  produce electrical energy. This may allow the temperature of the collector assembly to be increased to produce more useable heat energy without the increase in temperature reducing the efficiency with which the photovoltaic elements  105  produce electrical energy. This may allow the trigger temperature to be increased. 
     In some examples the trigger temperature may be set to different temperatures during use of the hybrid solar energy converter  101 . This may allow the temperature of the collector assembly to be controlled to produce different amounts of useable heat energy or electricity depending upon which type of energy is most in demand at a specific time. 
     For example, when hot water is more in demand than electricity the valve  109  may be closed to pass hot water vapor from the heat transport element  104  only to the primary heat exchanger  107  to maximize the amount of heat applied to the water acting as the first operating fluid regardless of any temporary reduction in efficiency of the photovoltaic elements  105  as a result of any resulting increase in temperature of the collector assembly. Further, when hot water is less in demand than electricity, the valve  109  may be opened in order to pass hot water vapor from the heat transport element  104  to both of the primary and secondary heat exchangers  107  and  108  in order to cool the photovoltaic elements  105  as much as possible and maximize the efficiency of electricity generation regardless of the effects on the temperature of the water acting as the first operating fluid. 
     In the illustrated example of the first embodiment the temperature of the solar energy collector assembly  102 , and thus the temperature of the photovoltaic elements  105 , is controlled by operating the heat transfer control valve  109  to selectively enable or disable the transfer of heat energy from the solar energy collector assembly  102  to the secondary heat exchanger  108 . 
     In other examples other control methods can be used additionally or alternatively to control the temperature of the solar energy collector assembly  102 . In some examples the temperature of the solar energy collector assembly  102  may be controlled by changing the rate of removal of heat energy from the solar energy collector assembly  102 . 
     In some examples the rate of removal of heat energy from the solar energy collector assembly  102  can be controlled by altering the flow rate of the first operating fluid passing through the primary heat exchanger  107 . In some examples the rate of removal of heat energy from the solar energy collector assembly  102  can be controlled by altering the surface area over which the first operating fluid is in contact with the primary heat exchanger  107 , for example by selectively opening or closing fluid flow passages of the first operating fluid within the primary heat exchanger  102 . 
     In some examples the rate of removal of heat energy from the solar energy collector assembly  102  can be controlled by altering the vacuum pressure within the tube  103 . This may change the rate of convective heat loss from the solar energy collector assembly  102  to the tube  103 . In general, heat transferred to the tube  103  will be rapidly lost to the outside environment by convection and/or conduction. 
     In some examples the rate of removal of heat energy from the solar energy collector assembly  102  can be controlled by altering the vacuum pressure within the heat transport element  104 . In general, the tendency of the liquid water within the upper fluid flow channel  117  to vaporize and form bubbles of vapor  122  will increase as the vacuum pressure is reduced, and the tendency of the liquid water within the upper fluid flow channel  117  to vaporize and form bubbles of vapor  122  will decrease as the vacuum pressure is increased. As is explained above, the density driven circulation of water around the upper and lower fluid flow channels  117  and  118  and the transport of heat energy along the vapor manifold  111  and the tube  119  are both driven by water vapor. Accordingly, altering the tendency of the liquid water to vaporize by altering the vacuum pressure may allow the rate of removal of heat energy from the solar energy collector assembly  102 , and the rate of removal of heat energy from the photovoltaic elements  105  to be controlled, and so allow the temperature of the solar energy collector assembly  102  and photovoltaic elements  105  to be controlled. 
     Further, the temperature at which rolling boiling of the water  121  within the upper fluid flow channel  117  commences will tend to increase as the vacuum pressure is increased, and will tend to decrease as the vacuum pressure is decreased. Accordingly, in examples where the vacuum pressure within the heat transport element  104  is altered the temperature at which the water  121  within the upper fluid flow channel  117  commences rolling boiling can be changed. 
     As is explained above, the density driven circulation of water around the upper and lower fluid flow channels  117  and  118  becomes particularly vigorous, and becomes particularly effective as a heat transport mechanism, when the water  121  within the upper fluid flow channel  117  enters a rolling boil state. Accordingly, altering the temperature at which the water  121  within the upper fluid flow channel  117  commences rolling boiling by altering the vacuum pressure may allow the rate of removal of heat energy from the solar energy collector assembly  102  and photovoltaic elements  105  to be controlled, and so allow the temperature of the solar energy collector assembly  102  and photovoltaic elements  105  to be controlled. 
     In some examples the temperature of the solar energy collector assembly  102  may be controlled by changing the amount of solar energy incident on the solar energy collector assembly  102 , and so changing the rate of absorption of heat energy by the solar energy collector assembly  102 . 
     In some examples the amount of incident solar energy may be controlled by changing the orientation of the solar energy collector assembly relative to the direction of the incident solar energy. This can be carried out using a drive mechanism able to rotate the solar energy collector assembly about one or more axes. 
     In some examples the amount of incident solar energy may be controlled using adjustable light intercepting or blocking mechanisms in the path of the incident solar energy. In some examples variable filters, shutters, stops, or the like may be used. In some examples these adjustable light intercepting or blocking mechanisms may comprise physical devices. In some examples these adjustable light intercepting or blocking mechanisms may comprise devices having electronically controlled optical characteristics, such as liquid crystals. 
     In examples where the temperature of the solar energy collector assembly and/or the photovoltaic elements are to be controlled, a temperature sensor and a temperature controller may be provided, together with a temperature control mechanism arranged to carry out one, some, or all, of the methods of controlling temperature described above. 
     The temperature sensor is arranged to measure the temperature of the solar energy collector assembly and provide this temperature value to the temperature controller. The temperature controller can then operate the temperature control mechanism in a suitable manner to control the temperature of the solar energy collector assembly to the desired value. 
     Examples where the temperature of the photovoltaic elements is to be controlled a temperature sensor arranged to measure the temperature of a photovoltaic element or elements and provide this temperature value to the temperature controller may be provided. This may be additional to, or instead of, the temperature sensor arranged to measure the temperature of the solar energy collector assembly. The temperature controller can then operate the temperature control mechanism in a suitable manner to control the temperature of the photovoltaic element or elements to the desired value. 
     In some examples the temperature sensor can be provided on the upper surface of the solar energy collector assembly. In some examples the temperature sensor can be formed on the same semiconductor wafer as a photovoltaic element. 
     Conveniently, the temperature controller may be a suitably programmed general purpose computer. 
     The illustrated second embodiment is a hybrid solar energy converter comprising photovoltaic elements and arranged to convert incident solar radiation into outputs of both electrical energy and hot water. In other examples the photovoltaic elements may be omitted to provide a solar energy converter arranged to convert incident solar radiation into an output of hot water. 
     Third Embodiment 
     Apparatus according to a third embodiment of the present invention is illustrated in  FIG. 20 .  FIG. 20  shows a general exterior view of a third embodiment of a hybrid solar energy converter  201  according to the present invention. 
     Overview 
     In the third embodiment, the hybrid solar energy converter  201  includes a solar energy collector assembly  202  housed within a sealed transparent tube  203 . The solar energy collector assembly  202  includes a heat transport element  204  and an array of photovoltaic elements  205  mounted on an front surface of the heat transport element  204 , the front surface being the surface exposed to incident solar radiation in use. The hybrid solar energy converter  201  also includes a heat exchange assembly  206  at one end of the transparent tube  203 . One end of the solar energy collector assembly  202  is connected to the heat exchange assembly  206 . Similarly to the first and second embodiments, in different examples the photovoltaic elements  205  may be formed of silicon, or gallium arsenide, or other suitable semiconductor materials. In other examples organic photovoltaic elements may be used. In other examples hybrid photovoltaic elements may be used. 
     In the third embodiment, the heat exchange assembly  206  includes a primary heat exchange assembly  207  arranged to transfer heat energy from the heat transport element  204  to a first fluid, and a secondary heat exchange assembly  208  arranged to transfer heat energy from the heat transport element  204  to a second fluid. The primary heat exchange assembly  207  and the secondary heat exchange assembly  208  are separated by a heat transfer control valve assembly  209  able to selectively allow, or prevent, the transfer of heat energy from the heat transport element  204  to the secondary heat exchange assembly  208 . 
     In one possible example, in use the hybrid solar energy converter  201  may be mounted on a wall. Accordingly, suitable mounting brackets may be provided. 
     In overview, the operation of the hybrid solar energy converter  201  of the third embodiment is similar to operation of the hybrid solar energy converter  1  and  101  of the first and second embodiments. Solar energy incident on the hybrid solar energy converter  201  passes through the sealed transparent tube  203  and is incident on the photovoltaic elements  205  of the solar energy collector assembly  202 . The photovoltaic elements  205  convert a part of the energy of the incident solar energy into electrical energy, and convert a part of the energy of the incident solar energy into heat energy. A further part of the incident solar energy may be incident on any parts of the solar energy collector assembly  202  which are not covered by the photovoltaic elements  205 , and this further part of the incident solar energy may also be converted into heat energy. 
     In general, it is desirable to maximize the proportion of the surface of the solar energy collector assembly  202  exposed to incident solar energy which is covered by the photovoltaic elements  205 , and to minimize the proportion which is not so covered. However, in some circumstances it may be preferred to leave some parts of this exposed surface uncovered, for example to simplify manufacture and/or assembly of the solar energy collector assembly  202  and attachment of the photovoltaic elements  205  to the solar energy collector assembly  202 . 
     The electrical energy produced by the photovoltaic elements  205  is carried along the heat transport element  204  by electrical conductors and away from the solar energy converter  201  for use. The heat energy absorbed by the photovoltaic elements  205  is transferred into the heat transport element  204 , cooling the photovoltaic elements  205 , and then carried to the heat exchange assembly  206 . 
     Similarly to the first and second embodiments, the heat transfer control valve  209  is able to selectively allow, or prevent, the transfer or transport of heat energy from the heat transport element  204  to the secondary heat exchanger  208 . Accordingly, the degree of cooling applied to the photovoltaic elements  205  can be varied. 
     In one typical arrangement, the hybrid solar energy converter  201  may be used to generate electricity, and to generate hot water. Similarly to the first embodiment, in this arrangement the heat energy transferred to the primary heat exchange assembly  207  is transferred into a pumped water supply flowing through the primary heat exchange assembly  207  to heat the water. This heated water is then used by a domestic or industrial hot water system, and the electrical energy produced by the photovoltaic elements  205  is supplied to an electrical supply system. In some arrangements the heat energy transferred to the secondary heat exchange assembly  208  is transferred into ambient air and allowed to escape and the secondary heat exchange assembly  208  is used, under the selective control of the heat transfer control valve assembly  209 , to release heat energy in order to regulate the temperature of the solar energy collector assembly  202 . 
     Transparent Tube 
     In the third embodiment illustrated in  FIG. 20  the sealed transparent tube  203  is similar to the sealed transparent tube  3  of the first embodiment, having one closed domed end and one open end sealed by an end cap  220 . The interior of the tube  203  is at least partially evacuated. That is, the interior of the tube  203  is below normal atmospheric pressure. 
     The pressure of the vacuum within the tube  203  may be 10 −3  mbar. Other pressures may be used, as discussed regarding the first and second embodiments. In some examples the vacuum pressure may be in the range 10 −2  mbar to 10 −6  mbar. In general, it is expected that lower vacuum pressure, or in other words a harder vacuum, will provide greater insulating benefits. Further, it is expected that lower vacuum pressure, or in other words a harder vacuum, will provide greater protection from environmental damage in examples where the photovoltaic elements are not encapsulated. In practice the benefits of using a lower vacuum pressure may need to be balanced against the increased cost of achieving a lower vacuum pressure. In some examples a vacuum pressure of 10 −2  mbar, or lower, may be used. 
     In an alternative example the sealed transparent tube  203  may be filled with an inert gas instead of being evacuated. In particular, the inert gas may be nitrogen. 
     In another alternative example the sealed transparent tube  203  may be filled with an inert gas at a reduced pressure. In some examples this may be achieved by filling the tube  203  with the inert gas and then evacuating the tube  203 . In particular, the inert gas may be nitrogen. 
     In the illustrated third embodiment the tube  203  is cylindrical having a circular cross section. Similarly to the first and second embodiments, in alternative examples the tube  203  may have other shapes. In some examples the cross sectional size and/or shape of the tube  203  may vary at different positions along its length. In an alternative example the tube  203  may have an elliptical cross section. In particular, the tube  203  may have an elliptical cross section with the long axis of the ellipse aligned with the plane of the solar energy collector assembly  202 . 
     In the illustrated second embodiment the tube  203  is formed of glass. In alternative examples suitable transparent plastics materials or laminated structures may be used to form the tube  203 . 
     In the illustrated second embodiment the tube  203  is transparent. In alternative examples the tube may be only partially transparent. 
     In the illustrated second embodiment the metal end cap  220  may be bonded to the glass tube  203  by adhesive. In other embodiments alternative glass to metal bonding techniques may be used, for example welding, brazing or soldering. 
     Similarly to the first embodiment the tube  203  has a metal end cap  220  at one end. In alternative examples the end cap  220  may be made of other materials. In some examples the end cap  220  may be made of glass. This may reduce conductive heat losses from the collector assembly  202 . 
     Collector Assembly 
     In the third embodiment, the solar energy collector assembly  202  includes a heat transport element  204  and an array of photovoltaic elements  205  mounted on one surface of the heat transport element  204 . In order to allow radiant solar energy to be incident on the photovoltaic elements  205  the array of photovoltaic elements  205  are mounted on the surface of the heat transport element  204  which is exposed to the incident radiant solar energy in operation of the hybrid solar energy converter  201 . In the third embodiment the heat transport element  204  may be mounted vertically. In examples where the heat transport element  204  is not mounted vertically the surface which is exposed to the incident radiant solar energy in operation will usually be the upper surface of the heat transport element  204 . 
     In some arrangements the surface of the heat transport element  204  exposed to the incident radiant solar energy may not be the upper surface. In particular, this would be the case if the incident solar radiant energy was incident horizontally or from below, for example after redirection by an optical system such as a mirror. 
     In the illustrated example of the third embodiment, the solar energy collector assembly  202  is supported by cylindrical tubes  219  of the heat transport element  204 . The cylindrical tubes  219  pass through the end cap  220  and into the heat exchange assembly  206 , as will be explained in more detail below. Where the cylindrical tube  219  passes through the end cap  220  the cylindrical tube  119  is soldered to the end cap  220  to retain the cylindrical tube  119  in place and support the solar energy collector assembly  102 . 
     In alternative examples the cylindrical tube  219  may be secured to the end cap  220  in other ways. In one example the cylindrical tube  119  may be welded to the end cap  220 . 
     The supporting of the solar energy collector assembly  202  by physical connections through the cylindrical tubes  219  may increase the efficiency with which heat can be collected from incident solar energy by the solar energy collector assembly  202 . Having the solar energy collector assembly  202  supported by physical connections only through the cylindrical tubes  219  may reduce conductive heat loss from the solar energy collector assembly  202  into the supporting structure outside the transparent tube. 
     In the illustrated example of the third embodiment the heat transport element  204  has a substantially flat front surface  204   a . Each of the photovoltaic elements  205  is square, and the width of the heat transport element  204  is the same as the width of each square photovoltaic element  205 . Six square photovoltaic elements  105  are mounted side by side to one another along the length of the heat transport element  204 . Substantially the entire front face of the heat transport element  204  is covered by the photovoltaic elements  205 . Covering a large proportion of the upper surface  204   a  of the heat transport element  204  with photovoltaic elements  205  may increase the efficiency of the hybrid solar energy converter  201 . 
     In one example the square photovoltaic elements  205  may each be a 125 mm by 125 mm square and 0.2 mm thick. In another example the square photovoltaic elements may each be a 156 mm by 156 mm square. In other examples, photovoltaic elements having other sizes or shapes may be used. 
     The photovoltaic elements  205  are bonded to the substantially flat upper surface  204   a  of the heat transport element  204  using a layer of heat conducting adhesive in a similar manner to the first and second embodiments. The adhesive bonding layer is electrically insulating. The adhesive bonding layer between the photovoltaic elements  205  and the heat transport element  204  is arranged to be thin. This may improve the degree of thermal conduction between the photovoltaic elements  205  and the heat transport element  204 . This may increase the rate of heat transfer laterally across the photovoltaic elements  205 . An adhesive material loaded with solid spheres of a predetermined size may be used to form the adhesive bonding layer. This may allow a thin adhesive layer to be consistently and reliably formed. The adhesive bonding layer is formed of a flexible or “forgiving” adhesive material. This may relieve stresses in the assembled solar energy collector assembly  202  and reduce any stress applied to the photovoltaic elements  205 . 
     The photovoltaic elements  205  are semiconductor photovoltaic elements formed of silicon. In one embodiment the photovoltaic elements are formed of single-crystal silicon. In one embodiment the photovoltaic elements are formed of amorphous silicon. In one embodiment the photovoltaic elements are formed of polycrystalline silicon, or polysilicon. In other embodiments alternative types of semiconductor photovoltaic elements may be used. 
     Similarly to the first and second embodiments, in operation of the hybrid solar energy converter  201  the photovoltaic elements  205  are cooled by the heat transport element  204 , which may provide similar advantages to those discussed above. This cooling may allow the temperature of the photovoltaic elements  5  to be maintained at a desired value. 
     This cooling may provide the advantage that the appearance of hot spots or regions in the photovoltaic elements  205  can be reduced or eliminated, and the temperature of the photovoltaic elements  205  maintained at a uniform desired value. Such hot spots or regions may for example be produced by heating by incident solar radiation, by inhomogeneities or faults in the photovoltaic elements  205 , or by a combination of, or interaction between, these causes. 
     As discussed above regarding the first and second embodiments, such hot spots or regions can reduce the efficiency of the photovoltaic elements  205  in the short term, and may also degrade the performance of the photovoltaic elements  205  in the longer term. 
     Accordingly, maintaining the photovoltaic elements  205  at a more uniform temperature value and reducing, or eliminating, hot spots or regions may improve the efficiency of the photovoltaic elements  205  at a specific temperature, and may reduce the amount of degradation of the photovoltaic elements  205  caused by higher temperatures. 
     This may allow the photovoltaic elements  205  to operate at a higher overall temperature than would otherwise be the case, for the same reasons as discussed regarding the first and second embodiments. 
     The illustrated example of the third embodiment has a solar energy collector assembly  202  supported only by physical connections through the cylindrical tubes  219 . In other examples alternative supporting arrangements may be used. In some examples the solar energy collector assembly  202  may be supported by a physical connections both ends of the solar energy collector assembly  202 . In some examples, the physical connections at one end of the solar energy collector assembly may be the through the cylindrical tubes  219 . In general, it is advantageous to minimize the number of physical supports in order to minimize the escape of heat from the solar energy collector assembly by conduction through the physical supports. 
     In other examples the number of photovoltaic elements  205  mounted on the heat transport element  204  may be different. In other examples the relative sizes of the photovoltaic elements  205  and the heat transport element  204  may be different. 
     In some examples the adhesive layer may comprise an epoxy resin which remains non-brittle after curing. 
     In other examples the adhesive layer may be formed by a double sided adhesive tape. 
     Heat Transport Element 
     The heat transport element  204  according to the third embodiment is shown in more detail in a cut away view in  FIG. 21 . 
     In the third embodiment, the heat transport element  204  is generally rectangular. The heat transport element  204  has a flat front surface  204   a  and a rear surface  204   b  which is flat across most of its area, and has three outwardly projecting sections  210  spaced out along its length, with a first outwardly projecting section  210  at an upper end of the heat transport element  204 , a second outwardly projecting section  210  located one third of the way along the length of the heat transport element  204 , and a third outwardly projecting section  210  located two thirds of the way along the length of the heat transport element  204 . 
     The heat transport element  204  is divided into three sections, an upper section  204   c , a central section  204   d , and a lower section  204   e . Each section  204   c  to  204   e  is cooled by a separate density driven circulation acting as a heat transport mechanism similar to the mechanism of the second embodiment and comprising a respective one of the three outwardly projecting sections  210 . Each of the three sections  204   c  to  204   e  supports and cools two of the six photovoltaic elements  205 . 
     Each outwardly projecting section  210  contains and defines a vapor manifold  211 . In operation the heat transport element  204  is arranged to be longitudinally sloping, so that the heat transport element  204  has an upper end and a lower end. The heat transport element  204  may be arranged longitudinally vertically, or at an angle to the vertical. 
     The heat transport element  204  has a front surface  204   a  formed by a front sheet  214  and a rear surface  204   b  formed by a rear sheet  215 . Three central sheets  216  are located between the front sheet  214  and the rear sheet  215 , with one of the central sheets  216  in each of the sections  204   a  to  204   c , so that fluid flow passages  217  and  218  running longitudinally along the heat transport element  204  are defined between each central sheet  216  and each of the front sheet  214  and the rear sheet  215 . Since the heat transport element  204  is longitudinally sloping the fluid flow passages  217  and  218  running longitudinally along the heat transport element  204  will be sloped along their lengths. 
     Each central sheet  216  has a similar profile to the central sheet  116  of the second embodiment, except that, compared to the second embodiment, the profile of the central sheets  216  of the third embodiment is rotated through 90° to define flow channels running longitudinally along the heat transport element  204 . The cross-sectional profile of the corrugated central sheets  216  can be understood as a zig-zag profile with the points of the zig-zag forming the peaks and troughs being flattened. 
     To be more specific, in the illustrated example of the third embodiment the central sheets  216  each comprise a plurality of flat surfaces connected by folds running longitudinally along the heat transport element  204 . Accordingly, the front, rear, and central sheets  214 ,  215 ,  216  define a plurality of trapezoid cross-section front fluid flow channels  217  and rear fluid flow channels  218  between them. The front fluid flow channels  217  are defined between the front sheet  214  and the central sheets  216 . The rear fluid flow channels  218  are defined between the rear sheet  215  and the central sheets  216 . The trapezoid front fluid flow channels  271  are arranged so that the larger one of the two parallel faces of each trapezoid channel  217  is formed by the upper sheet  214 . 
     The front and rear fluid flow channels  217  and  218  of the third embodiment respectively correspond in function to the upper and lower fluid flow channels  117  and  118  of the second embodiment. 
     The edges of the heat transport element  204  are formed by bent parts of the rear sheet  215 , which are bonded to the front sheet  214 . The photovoltaic elements  205  are bonded to the front sheet  214 . At the edges of the heat transport element  204 , the front sheet  214  is bonded directly to the rear sheet  215 , the central sheets  216  are not located between the front and rear sheets  214  and  215  at their edges. 
     In some examples the central sheets  216  may extend at least partially between the front and rear sheets  214  and  215  at the side edges of the heat transport element  204  so that the front and rear sheets  214  and  215  are both bonded to the central sheets  216 . This may assist in locating and securing the central sheets  216  relative to the front and rear sheets  214  and  215 . 
     As discussed above, the heat transport element  204  has three outwardly projecting sections  210  each running transversely across the rear surface  204   b  of the heat transport element  204 . Each outwardly projecting section  210  is substantially semi-cylindrical and is formed by an outwardly projecting part of the rear sheet  215 . Each outwardly projecting section  210  defines a vapor manifold  211 . The fluid flow channels  217  and  218  connect to the vapor manifolds  211 . It should be noted that the central sheets  216  extend across most of the width of the vapor manifolds  211 . Accordingly, the front fluid flow channels  217  defined between the front sheet  214  and the central sheets  216  connect to the vapor manifolds  211  towards the top of each vapor manifold  211 , while the rear fluid flow channels  218  defined between the rear sheet  215  and the central sheets  216  connect to the vapor manifolds  211  towards the bottom of each vapor manifold  211 . 
     The front and rear fluid flow channels  217  and  218  are formed into three groups with the front and rear fluid flow channels  217  and  218  of each group interconnected by one of the vapor manifolds  211 . Each group of fluid flow channels  217  and  218  extends along one of the sections  204   c  to  204   e  of the heat transport element  204  and, together with the vapor manifold with which they are connected, forms a separate heat transport mechanism cooling the respective section  204   c  to  204   e  of the heat transport element  204 . 
       FIG. 21  is an explanatory diagram showing a longitudinal cross section of a part of the heat transport element  204  along the line D-D in  FIG. 20 .  FIG. 21  shows the section of the heat transport element  204  around the boundary between the central section  204   d  and the lower section  204   e . The boundary between the central section  204   d  and the upper section  204   c  is identical. 
     At the top of the lower section  204   e  of the heat transport element  204 , at the top of the outwardly projecting section  110 , there is a wall  231  extending transversely across the interior of the heat transport element  204 . The wall  231  contacts and is bonded to the front and rear sheets  214  and  215  and forms a fluid tight seal between the fluid flow channels  217  and  218  of the central section  204   d  of the heat transport element  204  and the vapor manifold  211  of the lower section  204   e  of the heat transport element  204 . The walls  131  divide the interior of the heat transport element  204  into three separate fluid circulation regions corresponding to the sections  204   c  to  204   e  of the heat transport element  204 . 
     There is a gap  223  between the edge of the central sheet  216  of the central section  204   d  of the heat transport element  204  and the wall  231 . This gap  223  allows water to flow between different ones of the fluid flow channels  217  and  218 . The gap  223  extends along the side wall  231 , and forms a fluid manifold  224  interconnecting all of the front and rear fluid flow channels  217  and  218  of the central section  204   d.    
     At one edge of the heat transport element  204  each of the substantially semi-cylindrical outwardly projecting sections  210  transitions to a projecting cylindrical tube  219 . The front and rear sheets  214  and  215  are sealed to the cylindrical tubes  219  so that the interior of the heat transport element  204  is sealed. The cylindrical tubes  219  pass through the end cap  12  and into the heat exchange assembly  206 . The central bore of each of the cylindrical tubes  219  is connected to one of the vapor manifolds  111  and acts to carry heat energy from the heat transport element  204  to the heat exchange assembly  206 , as will be explained below. 
     The cylindrical tubes  219  physically support the solar energy collector assembly  202  within the sealed transparent tube  203 . There is no other physical support of the solar energy collector assembly  202 . As in the previous embodiments this may reduce conductive heat losses from the solar energy collector assembly  202 , which may increase the amount of useful heat energy produced by the hybrid solar energy converter  201 . 
     The fluid flow channels  217  and  218  are at least partially filled with degassed distilled water  221  as a working fluid and the interior of the heat transport element  204  including the fluid flow channels  217  and  218 , the vapor manifolds  211 , and the tubes  219  are at least partially evacuated. That is the interior of the heat transport element  204  is below normal atmospheric pressure. the interior of the heat transport element  104  may be under a vacuum at a pressure of 10 −3  mbar. 
     In the third embodiment the amount of water  221  in the fluid flow channels  217  and  218  is similar to the second embodiment except that the interior of each of the sections  204   c  to  204   e  is sealed off from the others so that the level of the water  221  is independent in each of the sections  204   c  to  204   e  of the heat transfer element  204 . 
     In each of the three sections  204   c  to  204   e  the level of the water  221  in the fluid flow channels  217  and  218  is such that the upper surface of the water  221  in the rear fluid flow channels  218  is level with the ends of the rear fluid flow channels  218  where they connect to the vapor manifold  211 . In the illustrated third embodiment the level of the surface of the water  221  in the front fluid flow channels  217  and rear fluid flow channels  218  is the same. Accordingly, in the illustrated third embodiment the rear fluid flow channels  218  are filled with liquid water, while the front fluid flow channels  217  are only partially filled with liquid water. 
     Similarly to the second embodiment, in other examples the level of the water  221  may be different. In some examples the upper surface of the water  221  in the rear fluid flow channels  218  may be below the vapor manifold  211 . In some examples the upper surface of the water  221  in the rear fluid flow channels  218  may be above the bottom of the vapor manifold  211 , with some water being present in the bottom of the vapor manifold  211 . 
     It is expected that in practice the heat transport element  204  will operate most efficiently with the upper surface of the water being at, or close to, the point where the lower fluid flow channels  218  contact the vapor manifold  211 . If the level of the water in the heat transport element  204  is too high, so that the upper surface of the water is too high within the vapor manifold  211 , the efficiency of operation of the heat transport element  204  may be reduced, for the same reasons as are discussed regarding the second embodiment. 
     The upper surface of the water  221  in the front fluid flow channels  217  may be higher than in the rear fluid flow channels  218  as a result of capillary action. The extent of this capillary effect in any specific example will depend upon the dimensions of the front fluid flow channels  217 . In the illustrated second embodiment some of the inner surface of the upper sheet  214 , that is, the surface forming a part of the upper fluid flow channels  217 , is above the surface of the water  221 . In some examples the front fluid flow channels  217  may have a small enough cross-sectional area that the upper surface of the water  221  in the front fluid flow channels  217  is at the ends of the front fluid flow channels  217  due to capillary action. 
     Similarly to the second embodiment, and unlike the first embodiment, it is not necessary that the inner surface of the front sheet  214 , that is, the surface forming a part of the front fluid flow channels  217 , is below the surface of the water  221  at a position corresponding to the location of the uppermost parts of the photovoltaic elements  205  for each of the sections  204   c  to  204   e  of the heat transport element  204 . However, in some embodiments this may be the case. 
     In operation of the third embodiment, when the solar energy collector assembly  202  is exposed to incident solar radiative energy, the photovoltaic elements  205  absorb some of this energy, converting a part of the absorbed energy into electrical energy. The remainder of the absorbed energy is converted into heat energy, raising the temperature of the photovoltaic elements  205 . The absorbed heat energy flows from the photovoltaic elements  205  into the heat transport element  204 , being transmitted through the front sheet  214  and into the water  221  inside the front fluid flow channels  217 , which water is in contact with the inner surface of the front metal sheet  214  across the larger parallel faces of the trapezoid front fluid flow channels  217 . 
     The liquid water  221  inside the front fluid flow channels  217  absorbs the heat energy from the photovoltaic elements  205  passing through the front sheet  214  and vaporizes, producing bubbles  222  of steam or water vapor. At the vacuum pressure of 10 −3  mbar inside the front fluid flow channels  217  water boils from around 0° C., so that the water  221  vaporizes readily at the normal operating temperatures of the hybrid solar energy converter  201 . 
     As discussed above regarding the second embodiment, the bubbles  222  of water vapor are less dense than the liquid water  221 . Further, as explained above the front fluid flow channels  117  are sloping along their lengths. Accordingly, as a result of this density difference the water vapor bubbles  222  travel upwards along the front fluid flow channels  217  towards the top of the heat transport element  204  and the surface of the water  221 . When a bubble of water vapor  222  reaches the surface of the water  221  the vapor is released into the vacuum above the water  221  in the respective vapor manifold  211 . Further, the bubbles  222  will give rise to pistonic driving in a similar manner to the second embodiment. In the illustrated third embodiment, where some of the inner surface of the upper sheet  214  is above the surface of the water  221 , this pumping of liquid water upwards along the upper flow channels  217  ensures that the part of the inner surface of the upper sheet  214  above the surface of the water  221  is in contact with a flow of water so that it can be cooled. 
     The bursting of the bubbles of water vapor at the water surface and any pistonic pumping of liquid water out of the ends of the front fluid flow channels  217  may generate droplets of liquid water, and may project at least some of these water droplets into the vacuum within the respective vapor manifold  211  above the water surface. As a result, the heat transfer mechanism may be a multi-phase system comprising liquid water, water vapor and droplets of liquid water, and not just a two-phase system comprising liquid water and water vapor only. The presence of such droplets of water in the vacuum, and any pumping of liquid water out of the ends of the front fluid flow channels  217 , may enhance the rate of vaporization by increasing the surface area of the water exposed to the vacuum. 
     Similarly to the first and second embodiment, the water vapor in the vacuum within each vapor manifold  211  travels at a very high speed through the vacuum along the vapor manifold  211 , along the respective tube  219  and into the heat exchange assembly  206 . The travel speed of the hot water vapor in the vacuum is very fast, approximating to the thermal speed of the water vapor molecules. Inside the heat exchange assembly  206  the water vapor from each tube  219  condenses on a respective heat exchange surface of one of the primary and secondary heat exchange assemblies  207  and  208 . The condensed water flows back out of the heat exchange assembly  206  down the same respective tube  219  to the respective vapor manifold, along the bottom of the vapor manifold  211 , and is returned back into the water  121  within the rear fluid flow channels  218  associated with that vapor manifold  211 . This generating of hot water vapor within the front fluid flow channels  217  and the vapor manifolds  211 , and subsequent travel of hot water vapor from the vapor manifolds  211  to the heat exchange assembly  206  where it condenses, followed by return of the condensed water, transfers heat energy from the heat transfer element  204  to the operating fluids in the heat exchange assembly  206 . 
     The tubes  219  are not interconnected within the heat exchange assembly  206 . The respective heat exchange surfaces of the primary and secondary heat exchange assemblies  207  and  208  connected to each of the tubes  219  are separate from one another so that liquid water and water vapor cannot be transferred between different ones of the separate heat transport mechanisms cooling the respective sections  204   c  to  204   e  of the heat transport element  204 . 
     Any liquid water ejected from the front fluid flow channels  217  into a vapor manifold  211  which does not vaporize will also fall to the bottom of the respective vapor manifold  211 , and is returned back into the water  221  within the rear fluid flow channels  218  associated with that vapor manifold  211 . 
     As is explained above all of the front and rear fluid flow channels  217  and  218  in each section  204   c  to  204   e  of the heat transfer element  204  are interconnected by the respective fluid manifold  224  formed by the respective gap  223 . Accordingly, within each section  204   c  to  204   e  of the heat transfer element  204 , it is not important which of the rear fluid flow channels  218  is entered by any liquid water returning from the respective vapor manifold  211 . 
     As is clear from the description above, each vapor manifold  211  generally includes liquid water in addition to water vapor when the hybrid solar energy converter  201  is operating. However, as is also discussed above, if the level of the water in a section  204   c  to  204   e  of the heat transport element  204  is too high, so that the upper surface of the water is too high within the respective vapor manifold  211 , the efficiency of operation of the heat transport element  204  may be reduced. This reduction in efficiency of operation may occur because there is insufficient space within the vapor manifold  211  above the surface of the water for the movement and evaporation of the droplets of liquid water. This reduction in efficiency of operation may occur because the droplets of liquid water and waves and splashing upwardly of the liquid water surface may reduce the open, or water free; cross sectional area of the vapor manifold at some locations to a relatively small amount, or even to zero, momentarily closing the vapor manifold. This reduction in the open, or water free, cross sectional area of the vapor manifold may interfere with the movement of the water vapor in the vacuum within the vapor manifold  211 . 
     In a similar manner to the second embodiment the bubbles  222  of water vapor will tend to move upwardly through the liquid water in the front fluid flow channel  217  because of the lower density of the water vapor compared to the liquid water  221 , which will result in an upward buoyancy force on each bubble  222 . Further, the movement of the bubbles  222  of water vapor will tend to drive the liquid water  221  in the front fluid flow channel  217  upwardly, particularly in examples where pistonic driving takes place. As a result, the bubbles  222  of water vapor cause the water  221  in the front and rear fluid flow channels  217  and  218  in each section  204   c  to  204   e  to circulate, with relatively hot liquid water and bubbles  222  of water vapor flowing upwards along the front fluid flow channels  217 , and relatively cool liquid water flowing downwards along the rear fluid flow channels  218 . The front and rear fluid flow channels  217  and  218  are interconnected by the vapor manifold  211  and the fluid manifold  224 , as explained above. Accordingly, the relatively hot liquid water flowing upwards along the front fluid flow channels is continuously replaced by relatively cool liquid water from the rear fluid flow channels  218 . This circulation is driven primarily by the difference in density between the water vapor and the liquid water. However, this circulation may also be driven by convection as a result of the difference in density between the relatively hot liquid water in the front fluid flow channels  217  and the relatively cool liquid water in the rear fluid flow channels  218 , in a similar manner to a thermosiphon. Accordingly, the front fluid flow channels  217  may be regarded as riser channels, while the rear fluid flow channels  218  may be regarded as sinker channels or return channels. 
     As the bubbles  222  of water vapor travel upwardly along the front fluid flow channels  217  the pressure head acting on the bubbles  222  decreases, so that the bubbles  222  tend to expand. As a result, the tendency of the vapor bubbles  222  to collapse and implode is reduced by the effects of the expansion and decreasing pressure as the bubbles  222  move upwardly. When considering this point, it should be remembered that when the heat transport element  204  is operating the bubbles  222  will be forming within established density driven circulation fluid flows and will move upwardly carried by these flows in addition to the bubbles movement due to their own buoyancy relative to the liquid water. Further, it is believed that expansion of the bubbles  222  as they move upwardly will further increase the speed of the density driven circulation flow by increasing the buoyancy of the expanding bubbles  222 . In some examples expansion of the bubbles as they move upwardly may also increase the degree of pistonic driving. 
     This density driven circulation may form a highly effective heat transport mechanism because water has a relatively high enthalpy of vaporization, so that the movement of the bubbles  222  of water vapor may carry a large amount of heat energy, in addition to the heat energy carried by the movement of relatively hot water out of the front fluid flow channels  217 , and its replacement by cooler water. In arrangements where pistonic driving of the flow of the liquid water by the water vapor bubbles takes place the effectiveness of the heat transport mechanism may be further increased by the increase in the flow rate of the liquid water caused by the pistonic driving. This pistonic driving is a component of the overall density driving producing the density driven circulation. The pistonic driving is caused by the density difference between the liquid water and the bubbles of water vapor. 
     In general, the speed of the density driven circulation increases and the effectiveness of the heat transport mechanism increases as the temperature of the upper sheet  214  of the heat transport element  204  increases. 
     The density driven circulation of the water  221  within the fluid flow channels  217  and  218  is a vapor driven circulating or rolling flow. 
     The density driven circulation of the water  221  within the fluid flow channels  217  and  218  becomes particularly vigorous, and becomes particularly effective as a heat transport mechanism, when the temperature of the upper sheet  214  of the heat transport element  204  becomes sufficiently high that the water  221  within the fluid flow channels  217  and  218  enters a rolling boil state. The effectiveness of the heat transport mechanism significantly increases when rolling boiling of the water  221  commences. In general, when other parameters of the system remain constant, entry into the rolling boil state will take place when the temperature of the front sheet  214  of the heat transport element  204  reaches a specific temperature. 
     In the illustrated example using water, the water  221  within fluid flow channels  217  and  218  may enter a rolling boil state at a temperature of about 40° C. 
     The arrangement of the heat transfer element  204  into sections  204   c  to  204   e  with separate fluid flow channels  217  extending along the heat transport element  104  may allow the vertical height of the liquid water in each section  204   c  to  204   e  of the heat transport element  204  to be reduced compared to embodiments in which the density driven flow extends along the length of a heat transport element, and so reduce the pressure head acting on the liquid water at the bottom of the heat transport element  204 . In general, increased pressure reduces the tendency of liquids to vaporize and so increases the boiling point of liquids. Accordingly, reducing the pressure head acting on the liquid water at the bottom of the heat transport element  204  may increase the tendency of the liquid water  221  in the front fluid flow channels  217  to vaporize and produce bubbles  222 , and so may improve the efficiency and effectiveness of the heat transport element  204 . 
     In particular, the reduction of the pressure head acting on the liquid water at the bottom of the front fluid flow channels  217  may reduce any temperature differential along the lengths of the front fluid flow channels between their the top and bottom ends by reducing any difference in the tendency of the liquid water to vaporize due to differences in pressure. This may reduce temperature differentials between the different points on the heat transport element  204  and may avoid the formation of hot spots in the photovoltaic elements  205 . Accordingly, reducing the pressure head acting on the liquid water at the bottom of the heat transport element  204  may make the temperature of the front sheet  214  of the heat transport element  204  more isothermal. 
     The arrangement of fluid flow channels  217  extending longitudinally along the heat transport element  204  and interconnected by vapor manifolds  211  extending laterally across the heat transport element  204  may allow a very rapid flow of heat energy along the heat transport element  204  away from any fluid flow channel  217  having a higher temperature. This may reduce temperature differentials between the different points on the heat transport element  204  and may reduce, or avoid, the formation of hot spots in the photovoltaic elements  205 . 
     The provision of the two separate heat transport mechanisms of the movement of water vapor along the vapor manifold  211  and the density driven flow of liquid water and water vapor along each of the front fluid flow channels  217 , respectively acting longitudinally and transverse the length of the heat transport element  204  may tend to equalize the temperature across the entire upper surface of the heat transport element, and thus tend to equalize the temperature across the photovoltaic elements  205  and reduce, or avoid, the formation of hot spots. 
     The movement of water vapor along the vapor manifold  211  provides a very rapid heat transport mechanism that tends, by the vaporization and condensation of water, to move heat energy from relatively hot locations to relatively cold locations. As a result, the movement of water vapor along the vapor manifold  211  may tend to equalize the temperature of the liquid water surface at different positions across the heat transfer element  204 , in addition to transporting heat energy from the heat transport element  204  to the heat exchange assembly  206 . This temperature equalization may have the effect of removing more heat energy from hotter parts of the heat transport element  204 , and so tending to equalize the temperature across the front surface of the heat transport element  204 . It is clear that such isothermal cooling will tend to reduce, or avoid, the formation of hot spots, for example in any photovoltaic element attached to the front surface of the heat transport element  204 . 
     Similarly to the second embodiment, the rear sheet  215  of the heat transport element  204  has a plurality of hollow ridges  225  extending between the flat part of the rear surface  204   b  and the semi-cylindrical surface of each outwardly projecting section  210 . Each hollow ridge  225  has a ‘V’ profile, and the hollow ridges  225  are located spaced apart at regular intervals along the length of each outwardly projecting section. The hollow ridges  225  act as supports for the outwardly projecting sections  210 , and also act as drains to return liquid water from the vapor manifolds  211  into the rear fluid flow channels  218  in a similar manner to the hollow ridges  125  of the second embodiment. 
     The hollow ridges  225  may extend the range of angles of inclination at which the heat transport element  204  can be used, as explained above regarding the second embodiment. 
     Depending upon the geometry of the different parts of the heat transport element  204  in any specific design, even when the hollow ridges  225  are used there may still be a minimum angle of inclination at which the heat transport element  204  can operate without the retention of liquid water in the vapor manifolds  211  having adverse effects on operation of the heat transport element  204 . 
     The corrugated profile of the central sheet  216  and the bonding of the central sheets  216  to the front sheet  214  and the rear sheet  215  increases the strength and rigidity of the heat transport element  204 , and may reduce or prevent ballooning for the reasons discussed regarding the second embodiment. This may make the heat transport element  204  a more rigid structure. This may tend to reduce the amount of flexing of the heat transport element  204  in use. This may prevent damage to the photovoltaic elements  205  by reducing the amount of mechanical stress applied to the photovoltaic elements  105 . This may allow the front, rear, and/or central sheets  214 ,  215 ,  216 , to be thinner, which may reduce weight and costs. This may allow the front sheet  214  to be thinner, which may improve the transfer of heat from the photovoltaic elements  205  into the liquid water within the front fluid flow channels  217 . 
     The heat transport element  204  is a substantially rigid structure. This may minimize changes in the level of the upper surface  232  of the water  221  due to flexing of the components of the heat transport element  204 , such as the upper and lower sheets  214  and  215 . Such changes in the level of the upper surface  232  of the water  221  may affect the efficiency of the cooling of the photovoltaic elements  205 . 
     As is explained above, the interior of the heat transport element  204  is evacuated, and the heat transport element  104  is located within an evacuated tube  203 . Usually the heat transport element  204  and the evacuated tube  203  are evacuated to the same pressure. In the illustrated example of the second embodiment described above this pressure may be 10 −3  mbar. 
     The interconnection of the front and rear sheets  214  and  215  by the linking surfaces of the central sheet  216  may resist ballooning of the front and rear sheets  214  and  215  and reduce or prevent ballooning. Arranging for the linking surfaces of the central sheet  216  to be straight may increase the resistance to ballooning. Reducing or preventing ballooning may prevent damage to the photovoltaic elements  205  by reducing the amount of mechanical stress applied to the photovoltaic elements  205 . This may allow the front sheet  214  to be thinner, which may reduce weight and costs and/or may improve the transfer of heat from the photovoltaic elements  205  into the liquid water within the front fluid flow channels  217 . 
     For the same reasons as explained with regard to the second embodiment it is preferred for the sizes of the surfaces of the central sheets  216  in contact with the front sheet  214  to be as small as possible, subject to the contact area between the central sheets  216  and the upper sheet  214  being sufficiently large to form a reliable bond of the required strength. 
     In the illustrated example of the third embodiment 0.2 mm thick tin coated mild steel sheets are used to form the different sheets of the heat transport element. In alternative examples other thicknesses may be used, in particular 0.1 mm thick tin coated mild steel sheets may be used. 
     In the illustrated example of the third embodiment the spacing between the front sheet  214  and the parallel parts of the rear sheet  215  is 1.8 mm at the locations of the recesses. Accordingly, the thickness of the fluid flow channels  217  and  218  at the locations of the recesses is 1.6 mm, since the thickness of the central sheet is 0.2 mm. 
     The sheets used to form the heat transport element may be shaped by pressing. 
     In the illustrated third embodiment the heat transport element  204  is arranged to be horizontal transversely to longitudinal axis. That is, the vapor manifolds  211  should be horizontal. However, in practice some deviation from the horizontal may be tolerated without significant impact on the operation of the heat transport element  204 . Such deviation from the horizontal will result in differences in the level of the liquid water surface relative to the structure of the heat transport element  204  at different positions along the length of each vapor manifold  211 . As is explained above, the level of the liquid water surface may be varied. Accordingly, the minor differences in level caused by small deviations from the horizontal may be accommodated. 
     In some examples the hybrid solar energy converter  201  may be arranged so that the tubes  219  and the internal passages of the heat exchanger assembly  206  are inclined at an angle to the horizontal downwardly from the heat exchanger assembly  206  towards the heat transport element  204  in order to assist the return flow of condensed liquid water from the primary and secondary heat exchangers  208  and  209  to the vapor manifold  211  of the heat transport element  204 . 
     The front and rear sheets  214  and  215  of the third embodiment have a dimpled profile similarly to the upper and lower metal sheets  114  and  115  of the second embodiment. 
     As discussed above the heat transport element  204  has a flat front surface  204   a  formed by a front sheet  214  with a dimpled profile. In addition, the front sheet  214  is has two longitudinal recesses running across in its front surface  204   a  which form two parallel troughs running along the upper surface  204   a  of the heat transport element  204  behind the photovoltaic elements  205 . Similarly to the preceding embodiments electrically conductive ribbons or wires run along the longitudinal recesses between the heat transport element  204  and the photovoltaic elements  205 . The wires are electrically connected to the photovoltaic elements  205  and to the conductors  21  which pass through the cap  12  to provide a conductive path to carry the electrical power generated by the photovoltaic elements  205  out of the sealed transparent tube  203 . This electrical power may be supplied to an inverter for voltage conversion and/or for conversion to alternating current for supply to a domestic or mains electrical system. 
     In examples where adhesive is used to attach the photovoltaic elements  205  to the heat transport element  204 , an electrically insulating adhesive can be used in a similar manner to the second embodiment. 
     In the third embodiment the longitudinal recesses run parallel to the fluid flow channels  217  and  218 . Accordingly, each of the longitudinal recesses can be accommodated by reducing the thickness of one of the front fluid flow channels  217  in each section  204   c  to  204   e  of the heat transfer element  204 . 
     In the illustrated example of the third embodiment the spacing between the front sheet  214  and the parallel rear sheet  215  is 1.8 mm at the locations of the longitudinal recesses  129 . Accordingly, the thickness of the front fluid flow channels  217  at the locations of the longitudinal recesses is 1.6 mm, since the thickness of the central sheet is 0.2 mm. 
     The heat transport element of the third embodiment may be formed using the same materials and bonding techniques as in the second embodiment. 
     In the illustrated example of the third embodiment the flow of water vapor and liquid water through the heat transport element  204  tends to keep the cooled front surface of the heat transport element  204  at a uniform operating temperature during operation. That is, the cooled upper surface of the heat transport element  104  tends to be kept isothermal. The isothermal nature of the cooled upper surface of the heat transport element  104  tends to give rise to isothermal cooling of the photovoltaic elements  105 , where hotter parts of the photovoltaic elements  105  tend to be preferentially cooled so that the photovoltaic elements  105  themselves tend to become isothermal 
     Such isothermal cooling provides further advantages in addition to those provided by cooling. 
     Isothermal cooling may provide the advantage that the appearance of hot spots or regions in the photovoltaic elements  205  produced by heating by incident solar radiation can be reduced or eliminated. Such hot spots or regions can reduce the efficiency of the photovoltaic elements  205 . 
     Isothermal cooling may simplify the control and wiring arrangements of the photovoltaic elements  205  by reducing or eliminating any requirement for compensation for differences in the performance of the different parts of the photovoltaic elements  205  that are at different temperatures. 
     Isothermal cooling tends to reduce, or prevent, the formation of hot spots or regions in the photovoltaic elements  205 . As is explained above, this may allow the efficiency of the photovoltaic elements  205  to be improved at a specific temperature. Further, this may reduce the amount of degradation of the photovoltaic elements  205  caused by higher temperatures. 
     Still further, this may allow the photovoltaic elements  205  to operate with a given degree of efficiency at a higher temperature than would otherwise be the case. This may allow the solar energy collector assembly  202  including the photovoltaic elements  205  to be operated at a higher temperature without reducing the efficiency with which the photovoltaic elements  205  produce electrical energy. 
     One example of this effect of isothermal cooling is that the general figure quoted above for silicon photovoltaic elements that the efficiency of electrical energy generation generally drops by about 0.35% to 0.5% for each degree centigrade of temperature increase above 25° C. may not apply to silicon photovoltaic elements that are isothermally cooled. Such isothermally cooled silicon photovoltaic elements having hotspots eliminated or reduced may have a higher threshold temperature at which the efficiency of electrical energy generation begins to drop and/or may have a reduced rate of reduction in efficiency for each degree centigrade of temperature increase above the threshold temperature. Further, the temperature at which there is a risk of permanent degradation of the silicon photovoltaic elements may also be increased for isothermally cooled silicon photovoltaic elements. Similar effects may be found in photovoltaic elements formed of other semiconductor materials. 
     In some examples, one or more layers of heat conductive material may be located between the upper sheet  214  and the photovoltaic elements  205 . Such layers of heat conductive material may increase the rate of heat transfer between the photovoltaic elements  205  and the front sheet  214 , and thus the rate of heat transfer between the photovoltaic elements  205  and the liquid within the front fluid flow channels  217 . Such layers of heat conductive material may also increase the rate of heat transfer laterally across the photovoltaic elements  205 . 
     Accordingly, providing a layer of heat conductive material may increase the degree of isothermal cooling and further tend to reduce, or eliminate, the formation of hot spots or regions in the photovoltaic elements  205 . 
     The heat transport element may be used in other applications separately from the rest of the solar energy converter. 
     Heat Exchange Assembly 
     The heat exchange assembly  206  of the third embodiment may be the similar to the heat exchange assemblies of the first and second embodiments. 
     In the third embodiment the general arrangement and operation of the heat exchange assembly  206  is similar to that in the first and second embodiments. As explained above, in the third embodiment the heat exchange assembly  206  includes a primary heat exchange assembly  207  and a secondary heat exchange assembly  208  separated by a heat transfer control valve assembly  209 . These are similar to, and operate similarly to, the heat exchange assembly  6  including a primary heat exchanger  7  and a secondary heat exchanger  8  separated by a heat transfer control valve  9  according to the first embodiment. 
     In the third embodiment there are three separate pipes  219  respectively connecting the respective vapor manifolds  211  of the three separate heat transfer mechanisms to the heat exchange assembly  206 . Each of the three heat transfer mechanisms is connected by a respective pipe  219  to a respective fluid flow passage through the primary heat exchange assembly  207 , secondary heat exchange assembly  208  and valve assembly  209 . These fluid flow passages are kept separate within the heat exchange assembly  206  by gas tight barriers so that no exchange of material, and in particular no exchange of liquid water or water vapor, can occur between the different heat transfer mechanisms. 
     If an exchange of liquid water or water vapor between the different heat transfer mechanisms was possible, this transfer of water could result in the liquid water level in one or more of the heat transfer systems becoming too high or too low for efficient operation. This could result in the different sections  204   c  to  204   e  of the heat transport element  204  being at different temperatures. 
     In the third embodiment the primary heat exchange assembly  206  comprises three primary heat exchangers each having a plurality of heat transfer fins extending into a flow channel, or channels, carrying a first operating fluid. In the illustrated example of the third embodiment the first operating fluid is a pumped flow of water forming part of a domestic hot water and/or heating system. The secondary heat exchange assembly comprises three secondary heat exchangers each having a plurality of heat transfer fins extending into a second operating fluid. In the illustrated example of the third embodiment the second operating fluid is ambient air. 
     Each of the pipes  219  is connected to a respective one of the primary heat exchangers and a respective one of the secondary heat exchangers by a fluid flow passage. Each of these fluid flow passages is selectively blockable between the primary and secondary heat exchangers by a respective heat transfer control valve of the heat transfer control valve assembly  209 . The three heat control valves are all operated simultaneously by the heat control valve assembly to ensure that the different sections  204   c  to  204   e  of the heat transport element  204  are maintained at the same temperature. 
     In some examples the three primary heat exchangers may be physically combined together. In some examples the three secondary heat exchangers may be physically combined together. 
     In other examples the three pipes  219  may be connected to a single fluid flow passage through the primary and secondary heat exchangers and heat transfer control valve so that the exchange of water between the different heat transfer mechanisms is possible. In such examples means for equalizing the water levels in the different heat transfer mechanisms may be provided. 
     In the third embodiment the trigger temperature of the heat transfer control valve assembly  209  may be predetermined. In some examples the trigger temperature may be settable in use, or on installation or manufacture of the hybrid solar energy converter  201 . In some examples the trigger temperature may be settable to different values depending on the intended maximum water temperature of the water to be heated. In particular, in some examples the trigger temperature may be settable to 65° C. when the hybrid solar energy converter is to be used to heat water for a domestic hot water system and may be settable to 135° C. when the hybrid solar energy converter is to be used to heat water for an industrial hot water system. 
     In some examples the trigger temperature of the heat transfer control valve may be selected to maximize the generation of electrical energy by the photovoltaic elements  205 . In some examples the trigger temperature value may be selected to increase the amount of heat energy transferred to the first operating fluid. In some examples the trigger temperature may be selected to optimize the overall production of energy, taking into account both the amount of electrical energy produced by the photovoltaic elements  205  and the amount of heat energy transferred to the first operating fluid. In some examples the optimizing may maximize the total production of energy. In some examples the optimum overall production of energy may take into account the relative demand for, or value of, the different types of energy, rather than simply maximizing the total amount of energy produced. 
     As explained above, the isothermal cooling tends to reduce, or prevent, the formation of hot spots or regions in the photovoltaic elements  205 . This may allow the solar energy collector assembly  202  including the photovoltaic elements  205  to be operated at a higher temperature without reducing the efficiency with which the photovoltaic elements  205  produce electrical energy. This may allow the temperature of the collector assembly to be increased to produce more useable heat energy without the increase in temperature reducing the efficiency with which the photovoltaic elements  205  produce electrical energy. This may allow the trigger temperature to be increased. 
     In some examples the trigger temperature may be set to different temperatures during use of the hybrid solar energy converter  201 . This may allow the temperature of the collector assembly to be controlled to produce different amounts of useable heat energy or electricity depending upon which type of energy is most in demand at a specific time. 
     For example, when hot water is more in demand than electricity the valve assembly  209  may be closed to pass hot water vapor from the heat transport element  204  only to the primary heat exchanger assembly  207  to maximize the amount of heat applied to the water acting as the first operating fluid regardless of any temporary reduction in efficiency of the photovoltaic elements  205  as a result of any resulting increase in temperature of the collector assembly. Further, when hot water is less in demand than electricity, the valve assembly  209  may be opened in order to pass hot water vapor from the heat transport element  204  to both of the primary and secondary heat exchanger assemblies  207  and  208  in order to cool the photovoltaic elements  205  as much as possible and maximize the efficiency of electricity generation regardless of the effects on the temperature of the water acting as the first operating fluid. 
     In the illustrated example of the third embodiment the temperature of the solar energy collector assembly  202 , and thus the temperature of the photovoltaic elements  205 , is controlled by operating the heat transfer control valve assembly  209  to selectively enable or disable the transfer of heat energy from the solar energy collector assembly  202  to the secondary heat exchanger  208 . 
     In other examples other control methods can be used additionally or alternatively to control the temperature of the solar energy collector assembly  202 . In some examples the temperature of the solar energy collector assembly  202  may be controlled by changing the rate of removal of heat energy from the solar energy collector assembly  202 . 
     In some examples the rate of removal of heat energy from the solar energy collector assembly  202  can be controlled by altering the flow rate of the first operating fluid passing through the primary heat exchanger assembly  207 . In some examples the rate of removal of heat energy from the solar energy collector assembly  202  can be controlled by altering the surface area over which the first operating fluid is in contact with the primary heat exchanger assembly  207 , for example by selectively opening or closing fluid flow passages of the first operating fluid within the primary heat exchanger assembly  202 . 
     In some examples the rate of removal of heat energy from the solar energy collector assembly  202  can be controlled by altering the vacuum pressure within the tube  203 . This may change the rate of convective heat loss from the solar energy collector assembly  202  to the tube  203 . In general, heat transferred to the tube  203  will be rapidly lost to the outside environment by convection and/or conduction. 
     In some examples the rate of removal of heat energy from the solar energy collector assembly  202  can be controlled by altering the vacuum pressure within sections  204   c  to  204   e  of the heat transport element  204 . In general, the tendency of the liquid water within the front fluid flow channel  217  to vaporize and form bubbles of vapor  222  will increase as the vacuum pressure is reduced, and the tendency of the liquid water within the front fluid flow channel  217  to vaporize and form bubbles of vapor  222  will decrease as the vacuum pressure is increased. As is explained above, the density driven circulation of water around the front and rear fluid flow channels  217  and  218  and the transport of heat energy along the vapor manifolds  211  and the tubes  219  are both driven by water vapor. Accordingly, altering the tendency of the liquid water to vaporize by altering the vacuum pressure may allow the rate of removal of heat energy from the solar energy collector assembly  202 , and the rate of removal of heat energy from the photovoltaic elements  205  to be controlled, and so allow the temperature of the solar energy collector assembly  202  and photovoltaic elements  205  to be controlled. 
     Further, the temperature at which rolling boiling of the water  221  within the front fluid flow channels  217  commences will tend to increase as the vacuum pressure is increased, and will tend to decrease as the vacuum pressure is decreased. Accordingly, in examples where the vacuum pressure within the heat transport element  204  is altered the temperature at which the water  221  within the front fluid flow channels  217  commences rolling boiling can be changed. 
     As is explained above, the density driven circulation of water around the front and rear fluid flow channels  217  and  218  becomes particularly vigorous, and becomes particularly effective as a heat transport mechanism, when the water  221  within the front fluid flow channels  217  enters a rolling boil state. Accordingly, altering the temperature at which the water  221  within the front fluid flow channels  217  commences rolling boiling by altering the vacuum pressure may allow the rate of removal of heat energy from the solar energy collector assembly  202  and photovoltaic elements  205  to be controlled, and so allow the temperature of the solar energy collector assembly  202  and photovoltaic elements  205  to be controlled. 
     In some examples the temperature of the solar energy collector assembly  202  may be controlled by changing the amount of solar energy incident on the solar energy collector assembly  202 , and so changing the rate of absorption of heat energy by the solar energy collector assembly  202 . 
     In some examples the amount of incident solar energy may be controlled by changing the orientation of the solar energy collector assembly relative to the direction of the incident solar energy. This can be carried out using a drive mechanism able to rotate the solar energy collector assembly about one or more axes. 
     In some examples the amount of incident solar energy may be controlled using adjustable light intercepting or blocking mechanisms in the path of the incident solar energy. In some examples variable filters, shutters, stops, or the like may be used. In some examples these adjustable light intercepting or blocking mechanisms may comprise physical devices. In some examples these adjustable light intercepting or blocking mechanisms may comprise devices having electronically controlled optical characteristics, such as liquid crystals. 
     In examples where the temperature of the solar energy collector assembly and/or the photovoltaic elements are to be controlled, a temperature sensor and a temperature controller may be provided, together with a temperature control mechanism arranged to carry out one, some, or all, of the methods of controlling temperature described above. 
     The temperature sensor is arranged to measure the temperature of the solar energy collector assembly and provide this temperature value to the temperature controller. The temperature controller can then operate the temperature control mechanism in a suitable manner to control the temperature of the solar energy collector assembly to the desired value. 
     Examples where the temperature of the photovoltaic elements is to be controlled a temperature sensor arranged to measure the temperature of a photovoltaic element or elements and provide this temperature value to the temperature controller may be provided. This may be additional to, or instead of, the temperature sensor arranged to measure the temperature of the solar energy collector assembly. The temperature controller can then operate the temperature control mechanism in a suitable manner to control the temperature of the photovoltaic element or elements to the desired value. 
     In some examples the temperature sensor can be provided on the upper surface of the solar energy collector assembly. In some examples the temperature sensor can be formed on the same semiconductor wafer as a photovoltaic element. 
     Conveniently, the temperature controller may be a suitably programmed general purpose computer. 
     In the illustrated third embodiment, the heat transport element  204  is divided into three sections  204   c  to  204   e , each of which has a separate heat transfer system comprising a number of front and rear fluid flow channels  217  and  218 , a vapor manifold  211 , and a tube  219 . Each of these separate heat transfer systems operates in a similar manner to the second embodiment described above. In other examples the heat transport element  204  may be divided into a different number of sections, each having a separate heat transfer system. 
     In the illustrated third embodiment the tubes  219  each extend outwardly from the side of the heat transport element  204 , then turn through a right angle and extend parallel to the axis of the tube  203  to pass through the end cap  220  of the tube  203 . 
     In other examples, the tubes  219  may be arranged differently. In some examples the tubes  219  may be interconnected for mutual support. This may improve the support provided to the heat transport element  204 . 
     In the illustrated third embodiment the tubes  219  each extend outwardly from the end of a respective vapor manifold  211 . In some examples the tubes  219  may extend from a different part of the respective vapor manifolds  211 . In some examples the tubes  219  may extend from different parts of the respective vapor manifolds  211  from one another. 
     In the illustrated third embodiment the different sections  204   c  to  204   e  of the heat transport element  204  are each divided by a wall  231  extending between the front and rear sheets  214  and  215  to form a fluid tight seal between the fluid flow channels of the different sections. In other examples a different sealing structure could be used. In some examples the front and rear sheets  214  and  215  could be brought into contact to form the fluid tight seal. In some examples the rear sheet  215  could be bent towards the flat front sheet  214  to contact the front sheet  214  and form the fluid tight seal. In some examples the rear sheet  215  may be shaped by pressing. 
     The illustrated third embodiment is a hybrid solar energy converter comprising photovoltaic elements and arranged to convert incident solar radiation into outputs of both electrical energy and hot water. In other examples the photovoltaic elements may be omitted to provide a solar energy converter arranged to convert incident solar radiation into an output of hot water. 
     Alternative Collector Arrangements 
     The illustrated embodiments all employ a single substantially flat collector assembly within a tube. Other arrangements may be used. 
     In some examples the collector assembly may be curved. The curved collector assembly may be arranged to have a curved outer surface concentric with a cylindrical tube within which the collector assembly is mounted. This may allow a collector assembly having a greater surface area to be fitted within a cylindrical tube of a particular size. The curved collector assembly may have curved photovoltaic elements mounted on it. 
     Some examples may mount multiple collector assemblies within a single tube. 
     Some examples may mount multiple collector assemblies at different angles within a single tube. In examples where the collector assemblies and the tube are fixed this may allow the efficiency of the collector to be increased by arranging the different collector assemblies at angles adapted to more efficiently collect energy at different times of day. 
     In some examples mirrors and/or lenses may be associated with the hybrid solar energy converter to direct or focus incident solar energy onto the collector assembly. Such mirrors may be flat or curved. Such mirrors and/or lenses may be fixed or moveable. In some examples moveable mirrors or lenses may be arranged to track the sun. 
     In some examples the transparent tube may incorporate a lens to direct or focus incident solar energy onto the collector assembly. In some examples the transparent tube may incorporate a Fresnel lens. 
     Alternative Tube Arrangement 
       FIG. 22  illustrates an alternative arrangement of the transparent tube. In this arrangement, the transparent tube is formed by a cylindrical glass tube  40  having a first glass end cap  41  and a second glass end cap  42 . 
     The first and second end caps  41  and  42  each have a respective central spigot  43  and  44  extending inwardly along the axis of the cylindrical glass tube  40 . In this arrangement the heat transport element  4  has a bearing  45  at each end. Each of the bearings  45  fits over one of the spigots  43  and  44  to rotatably support the solar energy collector assembly  2  within the transparent tube  3 . In the illustrated arrangement the solar energy collector assembly  2  is supported at both ends by the bearings  45 , and is not supported, or is not wholly supported, through the cylindrical tube  13 . 
     The spigot  44  of the end cap  42  at the end of the tube  3  adjacent the heat exchange assembly  6  has a central through bore  46  allowing the tube  13  of the heat transport element  4  to reach the heat exchange assembly  6 . The end cap  42  adjacent the heat exchange assembly  6  also has electrical conductors  47  passing through the end cap  42  to carry the electrical energy generated by the photovoltaic elements  5  away from the solar energy collector assembly  2 . 
     In this arrangement the solar energy collector assembly  2  may be rotated within the cylindrical glass tube  40 , and independently of the cylindrical glass tube  40 , to suit the geography of the location at which it is mounted in order to maximize exposure of the solar energy collector assembly  2  to incident solar radiation. 
     In some examples the cylindrical tube  13  may be connected to the heat exchange assembly  6  through a rotating seal or joint to allow the solar energy collector assembly  2  to rotate within the cylindrical glass tube  40  independently of the heat exchange assembly  6 . 
     In the illustrated arrangement shown in  FIG. 22  the alternative transparent tube is shown in combination with the solar energy collector assembly  2  according to the first embodiment. The illustrated alternative transparent tube can also be combined with the solar energy collector assemblies according to the other embodiments. 
     Sun Tracking 
     The embodiments described above are solar energy converters which convert incident solar radiation into useable electrical and/or heat energy. 
     In some examples the collector assemblies of the solar energy converters may be arranged to change their orientation to follow the apparent movement of the sun across the sky, or track the sun. This may increase the amount of solar radiation energy incident on the collector assemblies, for well-known geometric reasons, and so may increase the amount of useable electrical and/or heat energy produced. 
       FIG. 23  shows a general view of a solar energy converter  300  arranged to be able to change orientation to track the sun. 
     The solar energy converter  300  comprises a sealed transparent tube  301  containing a solar energy collector assembly  302  and mounted to a heat exchange assembly  303 . The solar energy converter  300  may be a solar energy converter according to any of the embodiments disclosed herein. Sun tracking arrangements may be added to any of the embodiments. 
     In the example of  FIG. 23  the sealed transparent tube  301  is cylindrical and has an axis  304 . The sealed transparent tube  301  is mounted for rotation about the axis  304  together with the solar energy collector assembly  302  mounted within the tube  301 . A drive motor  305  is arranged to rotationally drive the tube  301  through a transmission mechanism  306 . In the illustrated example the transmission mechanism  306  is a cog and chain transmission mechanism. 
     By selectively operating the drive motor  305  based on the time and date, the sealed transparent tube  301  and solar energy collector assembly  302  can be rotated to follow the sun as the apparent position of the sun changes as a result of the rotation of the earth. 
     Adding such a solar tracking drive system may increase the amount of energy gathered by the solar energy collector assembly by about 20%. 
     In the example of  FIG. 24 , a plurality of solar energy converters  300  are mounted to form an array  307 . Each of the solar energy converters  300  comprises a sealed transparent tube  301  containing a solar energy collector assembly  302  and mounted to a heat exchange assembly  303 . Each sealed transparent tube  301  is mounted for rotation about an axis  304  together with the solar energy collector assembly  302  mounted within the tube  301 . The transparent tubes  302  are mounted on the array  310  so that their respective axes of rotation  304  are parallel. 
     A drive motor  311  is arranged to rotationally drive the tubes  301  of the array  310  in synchrony through a transmission mechanism  312 . In the illustrated example the transmission mechanism  312  is a cog and chain transmission mechanism. 
     The array  310  is mounted on a turntable  313  for rotation about an axis  314  perpendicular to the axes  304 . A drive motor  315  is arranged to rotationally drive the turntable  313  through a transmission mechanism  316 . In the illustrated example the transmission mechanism  316  is a geared transmission mechanism. 
     By selectively operating the drive motors  305  and  315  based on the time and date, the sealed transparent tubes  301  and solar energy collector assemblies  302  of the array  310  can be rotated to follow the sun as the apparent position of the sun changes as a result of the rotation of the earth. 
     Adding such a dual axis solar tracking drive system may increase the amount of energy gathered by the solar energy collector assemblies  302  by up to about 48%. 
     In the examples of  FIGS. 23 and 24 , the operating of the drive motor or motors should take into account the location of the solar energy converter or converters  300 . 
     In other examples the array  310  can be rotated about one or two axes to follow the sun. Rotation about a single axis may increase the amount of energy gathered by up to about 20%, while rotation about two axes may increase the amount of energy gathered by up to about 48%. 
     In other examples the solar energy collector assembly may be mounted within the tube for rotation relative to the tube and a drive motor arranged to rotationally drive the solar energy collector assembly only. In such examples a drive mechanism which will not allow air leakage, which would destroy the vacuum within the tube, should be used. 
     In other examples the solar energy collector assembly, or the solar energy collector assembly together with the tube, may be rotated about an axis other than the axis of the tube. 
     General 
     In the description above the level of water within the heat transport elements of the different embodiments is referred to. The references to the level of water refer to the level of water when the heat transport element is cold and the liquid water contains essentially no bubbles of water vapor. It will be understood from the above description that the level of the water will vary during operation of the heat transport elements as water vapor bubbles are formed in the liquid water and burst, and as the liquid water is vaporized and the water vapor condenses. 
     In the illustrated embodiments primary and secondary heat exchangers separated by a heat transfer control valve are used. As is explained above, this arrangement may provide advantages in preventing stagnation, limiting the maximum temperature of the solar energy collector assembly and any attached components such as photovoltaic elements, and controlling a hybrid solar energy collector to selectively maximize production of electricity or useable heat energy. In other examples a primary heat exchanger or exchangers only may be used, and the arrangement of a secondary heat exchanger switched by a heat transfer control valve may be omitted. 
     In some examples, one or more of the arrangements described above for controlling the temperature of the solar energy collector may be used instead of, or in addition to, the provision of a secondary heat exchanger and a heat transfer control valve. 
     In the illustrated embodiments the heat transport elements may have an operating temperature range from just over 0° C. to about 270° C. In practice, the operating temperature range for domestic instillations may be limited to a maximum temperature of 95° C., or of 65° C., for safety, and to comply with legal requirements in some jurisdictions. Where silicon photovoltaic elements are used the optimum temperature range to maximize the generation of electricity may be in the range 20° C. to 65° C., or in the range 20° C. to 30° C., or in the range 25° C. to 30° C. 
     In the illustrated embodiments the heat exchangers are connected to the vapor manifold or liquid passage by a tube or channel so that only water vapor contacts the heat exchanger surfaces and is condensed to transfer heat to the heat exchanger. In other examples the, or each, heat exchanger may be located so that some liquid water contacts the heat exchanger. The, or each, heat exchanger may be partially immersed in the liquid water. This would also apply is other working fluids were used instead of water. 
     The heat transfer rate of the primary and secondary heat exchangers, that is the rate at which the heat exchangers can transfer heat energy from the heat transfer element to their respective operating fluids, may be matched to the heat transfer rate of the heat transfer element, that is the rate at which the heat transfer element can transfer heat from the isothermally cooled face of the collector assembly to the heat exchanger assembly, at the expected operating temperature, or over the expected operating temperature range, of the system. This may improve efficiency. 
     In the illustrated embodiments the primary operating fluid is water to be heated and the secondary operating fluid is ambient (free) air. In other examples the secondary operating fluid may be ducted air. This may allow the secondary operating fluid air to be used for low level heating such as space heating, and may allow the secondary operating fluid air to be blown past the secondary heat exchanger, which may increase the rate of heat loss from the secondary heat exchanger. In other examples the primary operating fluid may be air. In other examples the secondary operating fluid may be water. 
     In other examples the primary and/or secondary operating fluids may be fluids other than water and air. 
     In the illustrated embodiments a transparent tube or envelope is used. In other examples this may be replaced by a translucent or partially opaque tube or envelope. 
     In general, in all of the embodiments it may be preferred to have the photovoltaic elements as thin as possible to ensure effective cooling of the entire thickness of the photovoltaic elements by the heat transport element. This may assist in preventing localized hot spots of elevated temperature developing within the photovoltaic elements, which hot spots may degrade the performance and reliability of the photovoltaic elements. However, in practice there may be a minimum required thickness of the photovoltaic elements for other reasons, for example physical strength. 
     In the illustrated embodiments degassed distilled water is used. This may provide the advantage that the tendency to vaporize of the water is maximized, increasing the efficiency of the heat transfer by the thermo siphon. Impurities dissolved in the water, including dissolved gasses, will tend to suppress vaporization of the water. 
     In some examples the water may contain vaporization enhancing additives to increase the tendency of the water to vaporize. In some embodiments particles of hydrophobic materials may be used, in particular particles of zinc oxide may be used. The particles of hydrophobic molecules may act as nucleating sites; boosting the formation of bubbles of water vapor, without tending to suppress vaporization. 
     In all of the embodiments, nucleation enhancing structures may be added to the surfaces of the riser channels only, and not the return channels. This may encourage the liquid water to vaporize and form bubbles primarily, or only, in the riser channels even when the water in the riser and return channels are at similar, or the same, temperature. Suitable nucleation enhancing structures may include micropores and/or surface roughening. 
     In all of the embodiments, pores or apertures may be provided in the sheet separating the riser and return channels to allow water to pass from the return channel to the riser channel. This may improve the circulation of the liquid water and improve the efficiency of the heat transfer. 
     In the illustrated embodiments water is used as the working fluid within the heat transport element to provide the density driven circulation. In other embodiments other vaporizable liquids, solutions or mixtures may be used. In particular a mixture of water and glycol may be used, ethanol may be used, and a mixture of ethanol and water may be used. Mixtures of dissimilar fluids where one fluid acts as a nucleating agent for another fluid may be used. 
     In other examples a mixture of 75% water and 25% ethanol may be used as the working fluid within the heat transport element. When a mixture of 75% water and 25% ethanol is used the mixture may enter a rolling boil state at a temperature of about 22° C. In other embodiments the relative proportions of water and ethanol used as the working fluid may be varied in order to set the temperature at which a rolling boil commences to a desired temperature. 
     As discussed above, the effectiveness of the heat transport mechanism significantly increases when rolling boiling of the working fluid commences. Accordingly, it applications where it is desirable to keep the temperature of the cooled face of the collector assembly below a specific temperature, it may be preferred to select a working fluid, or mixture, which commences rolling boiling at a temperature at or below said specific temperature at the intended vacuum pressure conditions within the heat transfer device. 
     In examples where the solar energy collector assembly rotates relative to the evacuated tube a rotating vacuum seal must be provided between them. In some examples a rotating vacuum seal may be provided by a multi-stage seal. In particular a multi-stage o-ring seal may be used. 
     Where a multi-stage o-ring seal is used an advantageous method of manufacture may be to form the o-ring seals of the different stages in order from the interior of the evacuated tube to the exterior while evacuating the tube. This will provide a multi-stage o-ring seal with the regions between the seals initially having the same vacuum pressure as the interior of the tube. Such a multi-stage o-ring seal may support a long lasting vacuum within the tube even when the multi-stage o-ring seal is used as a rotating vacuum seal. 
     The above embodiments illustrate and describe a single solar energy converter. In practice an array made up of a plurality of such units may be used. In such an array each solar energy converter may have a dedicated electrical inverter. Alternatively, a group of a plurality of solar energy converters may share a common inverter. 
     In an array of solar energy converters it may be preferred to have a primary operating fluid channel running through the primary heat exchangers of all of the energy converters of the array as a common manifold. 
     In an array of solar energy converters it may be preferred for adjacent solar energy converters to have their respective inlet opening and outlet opening connected directly together. This may be done by providing a flange around each inlet opening and outlet opening and clamping together the flanges of the adjacent inlet opening and outlet opening of adjacent solar energy converters. 
     In an array of solar energy converters it may be desirable to be able to extract individual solar energy converters from the array for servicing, or to replace faulty converters, without having to drain all of the fluid from the common manifold. Accordingly, fluid cut off valves may be provided in the primary heat exchanger of each solar energy converter in order to seal the appropriate one of the inlet opening or outlet opening when an adjacent solar energy converter is removed from the array. 
     The embodiments described above comprise a collector assembly within an evacuated cylindrical tube. In some examples the collector assembly may be located within an enclosure which is not evacuated. In some examples enclosures which are not cylindrical tubes may be used. 
     The embodiments set out above are described in the context of a hybrid solar energy converter. The different parts of the described hybrid solar energy converter may be useable independently. 
     In particular, the solar energy collector assembly and the heat exchange assembly may be used in a flat panel device without a separate evacuated transparent tube for the solar energy collector assembly. Such a flat panel device may be evacuated, or alternatively may not be evacuated. 
     In particular, the collector assembly may be used as a thermal collector to gather heat energy from incident solar radiation without any photovoltaic elements being mounted on the collector assembly. 
     An array of solar energy converters may comprise both hybrid solar energy converters with photovoltaic elements mounted on the collector assembly and thermal solar energy converters without photovoltaic elements mounted on the collector assembly. Such an array may be used to heat water, with the hybrid solar energy converters heating the water to an intermediate temperature and the thermal solar energy converters heating the water from the intermediate temperature to a high temperature. The thermal solar energy converters without photovoltaic elements may operate at a higher temperature than the hybrid solar energy converters because they do not have any photovoltaic elements to suffer thermal degradation. 
     In some examples the collector assembly may be used as a thermal collector to heat air or water in industrial or domestic applications. In some examples the collector assembly may be used as a thermal collector to heat water in a desalination or water purifying application. 
     In particular, the heat exchange assembly may be used separately in solar energy heat collectors without the photovoltaic elements and/or without the heat transport element. This may allow the problem of stagnation, to be solved. 
     In particular, the heat transport element may provide a density driven heat transport mechanism useable in other heat transport applications. 
     In particular, the heat transport element may provide an isothermal cooled surface useable in other applications. 
     In particular, the isothermal cooled surface may be curved. This may allow curved objects to be cooled more efficiently. 
     In one example the heat transport element may be used to cool electrical circuits, for example in a computer. 
     If the heat transport element is used in other applications, and not in conjunction with photovoltaic elements, the heat transport element may operate at a wider range of temperatures. In one example the heat transport element using water as the working fluid may operate at a temperature of up to 280° C. In other examples other fluids may be used as the working fluid. In one example of a high temperature application sodium may be used as the working fluid within the heat transport element. 
     In some examples the heat transport element may transport heat to one or more electrothermal power generators in place of one or both heat exchangers. This may increase the amount of electrical energy generated. In particular the heat transport element may transport heat to a Stirling engine or engines. 
     In the illustrated embodiment vacuums are used within the heat transport element having a pressure of about 10 −3  mbar. Higher or lower pressures may be used. In general, it is expected that using lower vacuum pressures would improve the performance of the hybrid solar energy converter. In some examples a vacuum pressure of 10 −2  bar or lower may be used. In some examples vacuum pressures of 10 −6  mbar or 10 −8  mbar may be used. 
     A vacuum pressure of 10 −3  mbar is generally the lowest pressure that can be provided by simple vacuum pumps, so that the use of this vacuum pressure is convenient as the necessary vacuum pumps are readily available. The use of this vacuum pressure may be economically advantageous in commercial scale production of hybrid solar energy converters because of the cost of providing a lower vacuum pressure. In other embodiments higher or lower vacuum pressures may be used. 
     In the illustrated embodiments the hybrid solar energy converter has roof and/or wall mounting brackets. In other embodiments different mounting methods and components may be used. 
     The description above describes three embodiments. All of the embodiments are closely related and alternatives, explanations and advantages disclosed in relation to one of the embodiments can generally be applied in an analogous manner to the other embodiments. In particular, elements of one embodiment may be used in the other embodiments, and analogous elements can be exchanged between the embodiments. 
     The above description uses relative location terms such as upper and lower and front and rear. These are used for clarity to refer to the relative locations of the referenced parts in the illustrated figures, and should not be regarded as limiting regarding the orientation and/or location of parts of embodiments of the invention during manufacture or in use. 
     Those skilled in the art will appreciate that while the foregoing has described what are considered to be the best mode and, where appropriate, other modes of performing the invention, the invention should not be limited to specific apparatus configurations or method steps disclosed in this description of the preferred embodiment. It is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that the teachings may be applied in numerous applications, only some of which have been described herein. It is intended by the following claims to claim any and all applications, modifications and variations that fall within the true scope of the present teachings. Those skilled in the art will recognize that the invention has a broad range of applications, and that the embodiments may take a wide range of modifications without departing from the inventive concept as defined in the appended claims.