IMPROVEMENTS IN RADIATION COLLECTORS

Abstract: A solar panel is formed with a flow path (11) for working fluid in the form of a thin film, having a thickness of about 5 mm or less. The flow path is defined between an outer transparent lamella (10) having a thickness of about 1 mm or less and an inner backing member which may comprise either a further thin transparent lamella (12) with a radiation absorbent coating (13) in its rear face, or a non-transparent platten (17) made of a radiation absorbing material, in both cases backed by a reflective metal foil (14) and insulating material (16).

Description: Title: Improvements in Radiation Collectors Description of Invention This invention relates to a solar panel of the type which particularly but not exclusively may be utilised for the heating of water for domestic purposes. Solar panels which collect and utilise the energy present in solar radiation are well known. A typical known construction of solar panel comprises an array of parallel metal conduits through which the working fluid flows, said conduits commonly being in thermal contact with a metal collecting plate which is coated with a radiation absorbing medium. Alternatively the collecting plate may be absent and the conduits may have surfaces coated with an absorbing medium and may also be partly surrounded by radiation reflectors. Solar panels of this type are generally fitted with one or more front cover plates having suitable radiation transmission characteristics which reduce heat loss back into the environment, and are also provided with thermal insulation to minimise heat losses from their rear and side surfaces. Applicants US Patent No. 5411015 describes a form of lightweight solar panel in which the working fluid is conveyed in a plurality of parallel compartments formed in a matrix of plastics material, each compartment having therein a filler member, coated on one surface with a radiation absorbing medium, and a transverse membrane spaced from the coated surface of the filler member to define a respective shallow channel for the working fluid. It is one object of this invention to provide improved high efficiency lightweight solar heating panels which do not require the employment of a metal plate or of metal conduits or reflecting elements for radiation collection purposes. It is a second objective to provide such solar panels in a form which permits them to be incorporated into buildings as elements of roof or wall construction and to successfully discharge the functions of more conventional components in all respects. According to one aspect of the present invention there is provided a solar panel having a fluid flow path defined therein, the thickness of which normal to the direction of flow is defined between an outer transparent lamella and an inner backing member and is of the order of 5 mm or less. The thickness of the outer lamella may typically be of the order of 1 mm or less, preferably in the region of 0.5 mm. The inner backing member may comprise a further lamella of substantially the same dimensions as the outer lamella, and in this case there is preferably provided a coating or layer of radiation absorbing material in optical and thermal contact with the back of the lamella remote from the front of the solar panel, said absorbing surface having no physical contact with the fluid flowing along said flow path. In an alternative arrangement, the inner backing member may formed as a platten of substantially greater thickness than the outer lamella, preferably formed from a material which incorporates a radiation absorbing medium, such as carbon black. To maintain the desired thickness of the fluid layer a number of conveniently spaced longitudinal webs may be provided to support the outer lamella in desired spaced relation relative to the backing member. Such webs need not be continuous and are not required to define a series of separate flow conduits. The webs may run essentially parallel to the direction of fluid flow, but not necessarily defining individual flow conduits. Such webs may be formed integrally with the platten, or as structurally separate members secured thereto. The platten may be formed to define inlet and outlet connections for the working fluid, or alternatively inlet and outlet manifolds may be secured thereto. According to a second aspect of the invention there is provided a solar panel having a fluid flow path defined therein, the thickness of which normal to the direction of flow is defined by two transparent lamellae and is of the order of 5mm or less. Preferably there is provided a coating or layer of radiation absorbing material in optical and thermal contact with the back of the lamella remote from the front of the solar panel, said absorbing surface having no physical contact with the flowing fluid. The thickness of the lamella adjacent to the radiation absorbing material is preferably not more than 0.5 mm. The rear surface of this absorbing layer is preferably covered with a sheet of reflective metal foil so as to minimise radiative heat loss backwards into the panel. In order that the two laminae which define the film thickness may be maintained at the desired separation there are provided between them a number of conveniently spaced longitudinal webs. Said webs need not be continuous and are not required to define a series of separate fluid flow conduits. The webs may run essentially parallel to the direction of fluid flow, but not necessarily defining individual fluid flow conduits. Additionally there are provided inlet and an outlet manifolds which cooperate with the aforementioned lamellae to form watertight conduits whereby fluid may be introduced into and withdrawn from the space between the lamellae; said inlet and outlet manifolds being disposed so that the length of the fluid flow path contained within the lamellae is generally in the range 1500mm to 2000mm. The width of the flow path is usually in the range 400mm to 1000mm and may be defined by side pieces attached to said lamellae in a fluid tight manner. The flow path as defined by the outer lamella and the backing member, or between the respective lamellae, is disposed within the solar panel so as to ensure that the fluid film contained within it is effectively irradiated by incoming solar radiation entering the front of said panel and also by secondary heat radiation re-emitted from the aforementioned absorbing layer. It will be understood that the lamellae are formed from a suitable material which combines desirable optical transmission characteristics with the necessary physical properties. One such material is extruded polycarbonate resin which exhibits high strength and durability and suitable optical characteristics. An alternative material for at least the outer lamella is a micro-cystalline nylon material. Suitable materials from which the platten may be formed include nylon 6 or polypropylene, suitably pigmented. According to a third aspect of the invention there is provided a solar panel having a fluid flow path defined therein, the thickness of which normal to the direction of flow is defined by a transparent lamella and a radiation absorbing backing member, and is of the order of 5 mm or less. To maintain the desired thickness of the fluid layer a number of conveniently spaced longitudinal webs may be provided to support the outer lamella in desired spaced relation relative to the backing member. Such webs need not be continuous and are not required to define a series of separate flow conduits. The webs may run essentially parallel to the direction of fluid flow, but not necessarily defining individual flow conduits. Such webs may be formed integrally with the platten, or as structurally separate members secured thereto. The platten may be formed to define inlet and outlet connections for the working fluid, or alternatively inlet and outlet manifolds may be secured thereto. A suitable material for at least the outer lamella is a micro-crystalline nylon material. Suitable materials from which the platten may be formed include nylon 6 or polypropylene, suitably pigmented. According to a further feature of the invention there is provided an outer window which is sealed into a rigid frame and which is of twin or multi- walled construction with adjacent walls connected by transverse webs dividing the space between them into non-intercommunicating channels so as to restrict heat loss by diffusion. Said outer window is preferably located 15mm to 20mm above the fluid flow path of the solar panel which is also mounted in said frame. The outer window is conveniently formed from a mechanically strong plastic material with appropriate optical transmission characteristics such as extruded polycarbonate. It may be provided with a protective coating to prevent damage to the plastic arising from the effects of ultraviolet radiation. According to a further feature of the invention the inlet and outlet manifolds and the irradiation flow path are housed within a rigid protective casing and are surrounded on three sides by thermally insulating material to reduce heat loss from the panel. Said rigid casing cooperates with the aforementioned rigid window frame and is fixed to it in such manner that the panel constitutes a fully sealed weatherproof unit; the manifolds being in communication with outlet ports fitted into the rigid casing by flexible flow conduit means so as to permit such differential movement as may be caused by temperature effects. The design dimensions and materials of construction of panels manufactured in accordance with it may be such as to meet standard building requirements for roof or walling elements. Suitable materials for the panel frames and casings are UPVC plastic and aluminium alloys which may conveniently be formed into appropriate shapes and are of proven performance in building applications. In order that the invention may be more fully understood one embodiment will now be described with reference to the accompanying drawings, in which :- FIGURE 1 is an enlarged diagrammatic section through a fluid flow channel forming part of a solar panel in accordance with one aspect of the invention; FIGURE 2 illustrates the optical transmission characteristics of a preferred material for use in the construction of the lamellae as herein described. FIGURE 3 is a graph illustrating input and output temperatures achieved during daylight hours by a solar panel employing the principles of the construction shown in Figure 1; FIGURE 4 is a diagrammatic section similar to Figure 1 showing a solar panel in accordance with the second aspect of the invention; FIGURE 5 is a diagrammatic transverse section through part of a roof which incorporates solar panels designed and manufactured according to the first aspect of the invention; FIGURE 6 is a view similar to Figure 5 of a roof which incorporates solar panels designed and manufactured in accordance with the second aspect of the invention; and FIGURE 7 is a diagrammatic longitudinal section through part of a roof as shown in Figure 6 or 7. Referring firstly to Figure 1, which shows diagrammatically a section through a first embodiment of solar collector panel comprising an outer thin lamella 10 spaced from a rear thin lamella 12, incoming solar radiation, principally in the form of light (L), passes through the outer thin lamella 10 and thence through the water or other working fluid F in the channel 11 to the rear lamella 12. The radiation energy is transmitted through the rear lamella 12 to a radiation-absorbing coating 13 on the rear face of the rear lamella 12, which in turn is backed by a polished reflecting metal foil 14. The spacing between the lamella is defined by a plurality of webs 16 (Figure 5). The panel assembly comprising lamellae 10,12 and the webs 16 may be formed integrally as an extrusion of a suitable plastics material or as a fabrication from separate components. The incoming radiation is absorbed by said absorbing coating 13 with a consequent rise in temperature of the latter. Part of the energy so gained is transferred to the working fluid F by conduction across the rear lamella 12 and part by re-radiation forward as heat radiation (h) which is readily absorbed by said working fluid. Heat loss from the back of the rear lamella 12 is limited by the polished reflecting foil 14 and the provision of thermal insulation material 15. In order to achieve maximum efficiency of heat transfer from the absorbing coating 13 to the working fluid 11 it is necessary to restrict the thickness of the lamella 12 which separates the two, and this dimension should preferably not exceed 0.5 mm (dimension "X" in Figure 1). There is also an optimum spacing (dimension "Y" in Figure 1) between the lamellae which define the depth of the flow channel taking into account the requirement that as much as possible of the available heat energy is required to be transferred to the working fluid, while at the same time maintaining the volume flow rate and hold-up volume in the panel within practical limits. In this context, it will be understood that according to known principles relating to boundary layers direct heat transfer from the lamella 10,12 into the working fluid F will be favoured by a high flow velocity such as will occur, all things being equal, with a shallow channel; while re-emitted radiation (h) will be more effectively absorbed with a deeper flow path. In practice we have found a flow path depth of the order of 5 mm provides highly efficient energy collection with volume flow rates within the range 0.20-1.0 litres per minute per square metre of active surface. This range corresponds with the levels of flow which can be conveniently induced in installed systems by known means in practice. The lamellae 10,12 and webs 16 are preferably formed from extruded polycarbonate resin, which has the optical transmission characteristics illustrated in Figure 2, or an other material having appropriate optical and physical properties which is suitable for use in contact with water for other working fluid at elevated temperatures. It will be seen from Figure 2 that polycarbonate materials are substantially transparent to radiation in the wavelength range 0.3 to 2.5 micrometers which encompass the main part of the spectrum or solar energy incident at ground level. The secondary radiation re-emitted by the absorber will have a spectrum determined by its temperature in accordance with Plancks Law. In practice this will result in the major part of the energy being re-emitted at wavelengths greater than 4 micrometers. As will be seen from Figure 2, only that part of the energy re-emitted at wavelengths close to 5 micrometres will be directly transmitted through the lamella to any extent, the remainder being largely absorbed by the latter and transferred across it by conduction. This conducted energy will then be transferred from the lamella to the working fluid mainly by conduction across the boundary layer between them but also partly by re-radiation into the latter. The re-radiation energy will effectively increase in wavelength as it passes through the working fluid so that little if any will be transmitted directly through the outer lamella and the larger part will be absorbed within the said working fluid. The geometry of the flow path and the thickness of the inner lamella are important factors in ensuring efficient energy transfer by the mechanisms described above. Figure 3 illustrates the level of performance obtained during tests with a panel incorporating a flow channel of the form herein disclosed. In this Figure Trace 1 relates to the temperature of water entering the panel and Trace 2 to that of water leaving. The flow rate during this test was in the order of 0.20 litres per minute per square metre of panel area. Referring now to Figure 4, this illustrates a second embodiment of solar collector panel which is generally similar to that described above, but in which the rear lamella 12, with its radiation absorbing coating 13, is replaced by a thicker platten 17 which is substantially rigid and non-transparent, being made for example of a material such a nylon 6 or a polypropylene, in each case pigmented with carbon black or other suitable radiation absorbing material. Thus, in this embodiment incoming solar radiation is absorbed directly by the platten 17, but because the radiation absorbing material is incorporated into the material of which the platten is formed it is not liable to erosion by the working fluid, as would be the case with a coating of radiation absorbing material disposed internally of the flow channel. The webs 16 whereby the outer lamella 10 is in this case spaced from the platten 17 may be formed integrally with the platten or as structurally separate members secured thereto. The outer lamella 10 may be secured to the ribs if required in any appropriate manner compatible with the choice of materials used, for example by a mechanical means, such as rivetting, by chemical bonding, or by welding, and it will be understood that the outer lamella 10 will be similarly connected in fluid-tight manner at the edges of the platten 17. Referring now to Figure 5, which illustrates a practical embodiment in which a solar panel in accordance with the first aspect of the invention forms part of a roof-structure, a timber rafter 20 which forms part of a conventional roof framework carries a load-spreading cap 21 covered by a waterproof membrane 23. Said load-spreading cap 21 supports one edge of a solar panel assembly which includes a flange 24 formed on a rigid panel casing 25, and rigid window frame 26 fixed to said flange 24 by screws 27, in such manner as to compress a hard elastomer seal 28 against an upturn 29 at the outer edge of the flange 24. The frame 26 also seals a twin-walled outer window 30 between closed-pore expanded elastomer seals 31, 32, 33 and 34. A fillet of silicone elastomer 35 further seals and bonds the outer window 30 to the rigid frame 26. A channel section member 36 carried by the cap 21 cooperates with the window frames 26 of adjacent panels as shown so as to locate and support same whilst providing a weatherproof seal. A solar panel is located within the casing 25, and as previously described comprises, in this embodiment lamellae 10 and 12 defining a fluid flow channel 11 which is in communication with an inlet manifold 40 which in turn communicates with an inlet port 41 via a flexible conduit 42. A similar outlet manifold with its associated flexible conduit and outlet port (not shown) is provided at the opposite end of the panel. The radiation absorbing layer 13 on the lower face of the rear lamella 12 is backed by the sheet of reflective metal foil 14 which is supported by the thermal insulation material 15 which fills the remainder of the casing 25. Additional thermal insulation is provided to the roof by the ceiling lining board 46 and the packing piece 47. Referring to Figure 7, transverse bearers 48, 49, 50 and 51 span parallel rafters 20 with the waterproof membrane 23 overlaying these elements. The panel outer window frame 26 is weathered at the top by the metal flashing 52 which passes under the conventional roofing slate 53 which forms part of the upper section of the roof covering. The bottom of the panel rests upon the metal flashing 54 which is fixed to the bearer 50 and passes over the conventional roofing slate 55 which forms part of the lower section of the roof covering. Waterproof underlays 56,57 such as are conventionally used in slated or tiled roof construction are secured between the rafters 20 and transverse bearers 48-57 as shown. The solar panel is held in position by a number of screws, not shown, which pass through the window frame 26 and the support flange 24 into the rafter 20 and are provided with suitable sealing means. Figure 6, which is generally similar to Figure 5, illustrates a practical embodiment in which a solar panel in accordance with the second aspect of the invention forms part of a roof-structure as described in relation to Figure 5. In use an array of solar panels of the form described is assembled to form that part of the roof of a building which receives the maximum sunlight according to said building's location and orientation; the area of the array being typically in the range 3 to 10 square metres. The inlet and outlet ports of the panels are connected together by conventional plumbing means to form a continuous flow path through which water may be circulated by convection or by means of a pump. The continuous flow path may include a reservoir for containing a volume of heated water which is provided with known means for drawing off hot water as required and automatically replacing this with cold water so as to maintain a substantially constant volume within the reservoir. Alternatively the circulating liquid may flow through a heat exchange element of known form within a reservoir which is kept filled with unadulterated water to be heated and drawn off for use in known manner. In this case the circulating liquid may consist of an aqueous solution of a known anti-freezing agent such as ethylene glycol. The plumbing includes known means of controlling flow and draining down the system as required together with provision for liquid expansion. The embodiment heretofore described provides a form of roof construction which is characterised by its light weight and high thermal insulation while at the same time it performs as a very efficient collector and converter of solar energy as is demonstrated in Figure 3. Typically a panel incorporating a flow channel of the form described by reference to Figures 5,6, and 7 will have a hold-up volume in the order of 5 litres of working fluid per square metre of effective area. This relatively small volume is of considerable practical benefit in helping to restrict both the thermal inertia and weight of the panel during operation within desirable limits. There have been earlier descriptions of forms of energy collectors of heat exchangers which incorporate flow conduits comprised of extruded plastic mouldings having two parallel lamellae which define the width of said flow conduits. But the particulars illustrated by Figure 1 and 4 as hereinbefore described differ significantly from those disclosed in the prior art and may be clearly distinguished from the latter by consideration of the following. (1) The prior art does not define the width of the flow channel from front- to-back or discuss the importance of choosing this dimension so as to obtain optimum performance as hereinbefore described. As far as can be deduced from the information provided the depth of the flow path is in all cases significantly greater than the optimum value herein specified. (2) The prior art does not deal with the importance of restricting the thickness of a transparent lamella separating the flow path from the absorber to the practical minimum so as to facilitate heat transfer and the information available in all cases suggests a dimension at least twice the upper limit herein proposed. (3) In some cases of prior art it is stated or implied that both lamellae should be coated or pigmented to promote energy absorption. In such cases heat loss by forward re-radiation from the outer lamella will be very significantly greater than with the configuration disclosed in the present invention. A blackened outer lamella will act as the primary absorber of incoming radiation and will assume the maximum temperature within the array. Since there is no flow channel outboard of it, the consequent radiative heat loss will not be mitigated by absorption by the working fluid in the manner characteristic of the present invention. (4) In no case does the prior art disclose the provision of a polished metal foil to minimise radiative loss from the back of the rear member of the flow channel. It will be understood that the embodiments described do not encompass all possible versions of the invention and should not be held to delimit the latter. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims: CLAIMS:- 1. A solar panel having a fluid flow path (11) defined therein, the thickness of which normal to the direction of flow is defined between an outer transparent lamella (10) and an inner backing member (12; 17) and is of the order of 5 mm or less. 2. A solar panel according to Claim 1 wherein the thickness of the outer lamella is of the order of 1 mm or less. 3. A solar panel according to Claim 2 wherein the thickness of the outer lamella is in the region of 0.5 mm. 4. A solar panel according to Claim 3 wherein the inner backing member comprises a further lamella (12) of substantially the same dimensions as the outer lamella (10). 5. A solar panel according to Claim 4 wherein a coating or layer of radiation absorbing material (13) is provided in optical and thermal contact with the back of the lamella (12) remote from the front of the solar panel to form an absorbing surface having no physical contact with the fluid flowing along said flow path (11). 6. A solar panel according to Claim 5 wherein the inner backing member is formed as a platten (17) of substantially greater thickness than the outer lamella (10) 7. A solar panel according to Claim 6 wherein the platten (17) is formed from a material which incorporates a radiation absorbing medium. 8. A solar panel according to Claim 7 wherein the radiation absorbing material comprises carbon black. 9. A solar panel according to Claim 1 wherein a plurality of spaced longitudinal webs (16) is provided to support the outer lamella (10) in desired spaced relation relative to the backing member (12; 17). 10. A solar panel according to Claim 9 wherein said webs (16) are formed integrally with the inner backing member (17). 11. A solar panel having a fluid flow path (11) defined therein, the thickness of which normal to the direction of flow is defined by two transparent lamellae (10,12) and is of the order of 5mm or less. 12. A solar panel according to Claim 11 wherein the thickness of the outer lamella is of the order of 1 mm or less. 13. A solar panel according to Claim 12 wherein the thickness of the outer lamella is in the region of 0.5 mm. 14. A solar panel according to Claim 11 wherein a coating or layer of radiation absorbing material (13) is provided in optical and thermal contact with the back of the lamella (12) remote from the front of the solar panel to form an absorbing surface having no physical contact with the fluid flowing along said flow path (11). 15. A solar panel according to Claim 14 wherein the thickness of the lamella (12) adjacent to the radiation absorbing material (13) is not more than 0.5 mm. 16. A solar panel according to Claim 15 wherein the rear surface of the absorbing material (13) is covered with a sheet of reflective metal foil (14) so as to minimise radiative heat loss backwards from the panel. 17. A solar panel according to Claim 11 wherein a plurality of spaced longitudinal webs (18) is provided to maintain the lamellae in the desired spaced apart relation. 18. A solar panel according to Claim 11 wherein inlet and an outlet manifolds (40) are provided which co-operate with said lamellae (10,12) to form watertight conduits whereby fluid may be introduced into and withdrawn from the space between the lamellae (10,12). 19. A solar panel according to Claim 18 wherein said inlet and outlet manifolds are disposed so that the length of the fluid flow path contained within the lamellae (10,12) is in the range of 1500 mm to 2000 mm. 20. A solar panel according to Claim 11 wherein the lamellae are formed from extruded polycarbonate resin. 21. A solar panel having a fluid flow path (11) defined therein, the thickness of which normal to the direction of flow is defined by a transparent lamella (10) and a radiation absorbing backing member (17) and is of the order of 5 mm or less. 22. A solar panel according to Claim 21 wherein the outer lamella is formed from a micro-crystalline nylon material. 23. A solar panel according to Claim 21 wherein the platten is formed from nylon 6 suitably pigmented. 24. A solar panel according to Claim 21 wherein the platten is formed from polypropylene suitably pigmented. 25. A solar panel according to Claim 23 or 24 wherein the material of which the platten is formed is pigmented with carbon black. 26. A solar panel according to Claim 21 wherein a plurality of spaced longitudinal webs (16) is provided to support the outer lamella (10) in desired spaced relation relative to the platten (17). 27. A solar panel according to Claim 26 wherein said webs (16) are formed integrally with the platten (17). 28. A solar panel according to any one of the preceding claims wherein an outer window (30) is provided and is sealed into a rigid frame (26), the window being of twin or multi-walled construction with adjacent walls thereof connected by transverse webs dividing the space between them into non- intercommunicating channels. 29. A solar panel according to Claim 28 wherein said outer window is spaced 15 mm to 20 mm above the fluid flow path (11) of the solar panel. 30. A solar panel according to Claim 28 wherein the solar panel is housed within a rigid protective casing (25) and is surrounded on three sides by thermally insulating material (15). 31. A solar panel according to Claim 30 wherein said rigid casing (25) co¬ operates with said rigid window frame (26) and is fixed to it in such manner as to form a fully sealed weatherproof unit.

Inventors: STARNES PETER, STARNES, PETER
Applicants: COLLINS STARNES ASS, STARNES PETER, COLLINS STARNES ASSOCIATES LIMITED, STARNES, PETER
Application year: 1996
Main classification: F24J2
IPC codes: F24J2