Abstract:
A method of converting a liquid into a vapor includes directing liquid into one or more solar collectors through a manifold that supports the one or more solar collectors. Each of the one or more solar collectors includes a transparent outer cylinder having a closed end and an open end, an inner cylinder having a closed end and an open end, the inner cylinder being concentric with and disposed within the transparent outer cylinder so that the closed end of the inner cylinder is located proximate to the closed end of the transparent outer cylinder, an outer surface of the inner cylinder being made of a material that absorbs solar radiation to generate heat, the longitudinal axes of the transparent outer cylinder and the inner cylinder being substantially horizontal, and an enclosed and evacuated space formed between the transparent outer cylinder and the inner cylinder. A maximum value is determined for the amount of liquid to be converted to vapor during a daylight portion of a day as a result of the heat generated in the inner cylinders of the one or more solar collectors. An amount of the liquid is directed into the one or more solar collectors, where the amount is a value that is at least the maximum amount value.

Description:
FIELD OF THE INVENTION 
     The present invention generally concerns solar energy collection. More specifically, the present invention concerns increasing the efficiency of a tubular solar collector apparatus. 
     DESCRIPTION OF THE RELATED ART 
     The application of solar energy to heating generally requires a collector that efficiently absorbs solar radiation. The collector transfers the radiated energy to a fluid, which transports the energy to a final application in the form of heat. This final application may include a domestic water or space heating apparatus. An effective collector must absorb a high percentage of incident solar radiation, while losing only a small amount of the absorbed energy to the ambient through either heat conduction or radiation. 
     Solar collectors including two concentric glass tubes with an evacuated space there between have generally been recognized as an effective configuration for absorbing a high percentage of incident radiation and minimizing heat loss by conduction. A solar collector configured in such a manner is similar to the configuration of a Dewar flask, which may be used as an insulated storage vessel, and are sometimes appropriately referred to as Dewar-type evacuated-tube collectors. The need to minimize radiation heat loss has been addressed by coating the vacuum-side of the inner glass tube with a selective surface that has a high absorptivity for visible radiation and low emissivity for infrared radiation. 
     The primary developmental effort relating to evacuated tube solar concerns removal of the thermal energy absorbed by the elongated glass tubes. One method of removing the thermal energy from the elongated inner glass tube of evacuated tube solar collectors is to circulate water or other working fluid into and out of the interior of the glass tube. The working fluid circulated through the glass tube absorbs the solar energy and carries that energy to a location where the energy can be stored or put to practical use. An alternative methodology circulates the water or working fluid through the elongated glass tube via pipes or circulation tubes positioned inside the glass tube so that the water or other working fluid does not actually come in contact with the glass tube. 
     A further technique uses heat pipes to transfer the absorbed solar energy to a working fluid medium that functions as a heat sink. The heat sink “stores” the collected thermal energy and/or transfers the energy to a location where the stored energy can be put to practical use. In such an embodiment, the heat pipe may include an evaporator portion that absorbs the solar energy and causes a volatile thermal transfer fluid in the heat pipe—not the working fluid medium—to vaporize. The vapor pressure drives the vapor toward the cooler condenser section of the heat pipe, which is placed in contact with the working fluid medium or heat sink. 
     The thermal energy absorbed from the sun in the evaporator portion is conducted from the vapor of the thermal transfer fluid inside the heat pipe to the working fluid or heat sink outside the heat pipe by way of the condenser. The lower temperature of the thermal transfer fluid vapor, which is due to conduction of the heat from the vapor to the working fluid, results in condensation of the thermal transfer fluid in the heat pipe. The condensed thermal transfer fluid then flows from the condenser portion back to the evaporator portion of the heat pipe where solar energy is absorbed to continue the cycle. 
     An additional method for transferring heat out of a Dewar-type evacuated-tube solar collector involves absorbed solar energy boiling water within the collector. The steam generated from the boiling water transports heat out of the collector through a process called vapor-phase pumping. A solar collector utilizing vapor-phase-pumping involves a tubular absorber filled almost to the top with a heating liquid, such as water, to provide a relatively small vapor-phase zone at the upper end of the absorber; a boiler mounted at a higher elevation than the solar collector; a tube through which liquid flows from the boiler into the tubular absorber and that extends into the interior of the tubular absorber for substantially the full length of the absorber; and a tube that connects the upper vapor-phase zone in the boiler with the vapor-phase zone in the tubular absorber. Extracting heat from an evacuated-tube solar collection using vapor-phase-pumping is easier than with a heat pipe and also avoids the need for a mechanical pump. 
     It has generally been viewed as disadvantageous not to fill the tubular absorbers such that the top half is in contact with the liquid. Vapor-phase pumping devices have intentionally avoided such a configuration by filling the inner absorber cylinder with liquid or converting the inner absorber cylinder into a heat pipe. Another option has involved inserting a separate heat pipe or U-tube into the inner absorber cylinder and using a metal, thermally conductive fin to thermally couple the evaporator of the heat pipe or the U-tube to the inner absorber cylinder. 
     The vapor-phase-pumping arrangement described above, however, has several limitations. By operating with the tubular absorber filled almost to the top, the hot fluid within the tubular absorber stores a significant amount of thermal energy. Most of this thermal energy will be lost to the ambient during the night. Furthermore, if the fluid within the tubular absorber is water, the absorber is likely to be damaged by freezing in cold climates since water expands when it freezes. If the vapor that is produced within the absorber is to flow to the boiler without interfering with the in-flowing liquid, an inlet tube that extends into the absorber must be used. This tube, which is often metallic as to withstand possible stagnation conditions within the evacuated-tube collector, increases the cost for the solar collector, especially for more expensive metals such as copper. 
     SUMMARY OF THE CLAIMED INVENTION 
     A first claimed embodiment sets forth an apparatus for converting liquid to vapor. The apparatus includes a tubular solar collector having a transparent outer cylinder with one closed end and a concentric inner cylinder with one closed end. The inner cylinder includes a surface coating that absorbs solar radiation. The longitudinal axes of both cylinders are substantially horizontal and the inner cylinder is oriented within the outer cylinder so that the closed ends of the two cylinders are proximate to each other whereby an evacuated space is formed between the two cylinders. The apparatus also includes a manifold that maintains the level of a volatile liquid flowing into the tubular solar collector so that no more than 80% of the volume of the inner cylinder is filled with liquid. The manifold also collects the vapor produced when heat is transferred from the surface of the inner glass cylinder that absorbs solar radiation to the volatile liquid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perspective view of a multi-tube solar collector with tubes in the same horizontal plane. 
         FIG. 2  illustrates a longitudinal sectional view through one horizontal tube of the multi-tube solar collector of  FIG. 1 . 
         FIG. 3  illustrates a side view of the multi-tube solar collector of  FIG. 1  including a reflecting back plane. 
         FIG. 4  illustrates a perspective view of a multi-tube solar collector composed of three separate manifold-tube sub-assemblies. 
         FIG. 5  is a longitudinal sectional view through the end-caps and horizontal tubes of a multi-tube solar collector for which the horizontal tubes lie in a plane that is not horizontal. 
         FIG. 6  is a perspective view of a multi-tube solar collector with horizontal tubes used to evaporate a volatile component of a multi-component liquid. 
         FIG. 7  illustrates a cut-away perspective view of a multi-tube solar collector with horizontal tubes used to evaporate a volatile component of a multi-component liquid and including an internal artery to deliver the multi-component liquid to one of the tubes. 
         FIG. 8  illustrates a longitudinal sectional view through the center plane of a manifold that uses a partition with an orifice to allow concentrated liquid to be withdrawn from the manifold through an end wall fitting. 
         FIG. 9  illustrates performance data corresponding to a multi-tube solar collector that produces steam. 
         FIG. 10  illustrates a water level control system for a solar collector according to an exemplary embodiment of the present invention. 
         FIG. 11  illustrates a water level control system for a solar collector according to another exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A solar collector apparatus with Dewar-type evacuated tubes is generally described. The tubes may be oriented essentially horizontally and partially filled with liquid unlike a prior art solar collection apparatus that operates with liquid filling the tube almost to the top. Instead, the tubes of embodiments of the present invention may be filled only partially with liquid so that the space for vapor above the liquid extends more than three-quarters the length of the tube. With the tubes oriented close to horizontal and partially filled with liquid, the vapor produced within the tube can leave the tube without interfering with the entering liquid. Such a configuration, specifically the relatively low amount of liquid within the tube, reduces the heat lost during the night and, in applications where the tubes are less than half filled with liquid, a liquid that expands during freezing can do so within the tube without creating high stresses that might damage the tube. 
     An embodiment of the invention as described herein may heat a liquid within an absorber tube, the liquid having a volatile and a non-volatile component. As heat is transferred to the liquid, a fraction of the volatile component is converted to vapor, leaving the liquid more concentrated in the non-volatile component. Both the vapor and the more concentrated liquid leave the absorber tube through its open end.  FIG. 1  illustrates a perspective view of a multi-tube solar collector  10  with tubes  20  in the same horizontal plane. Each of the tubes  20  of solar collector  10  may be of a Dewar-type evacuated tube configuration as is reflected in the context of  FIG. 2 .  FIG. 2  illustrates a longitudinal sectional view through one horizontal tube  20  of the multi-tube solar collector  10  of  FIG. 1 . Each evacuated tube  20  has, as illustrated in  FIG. 2 , an outer transparent cylinder  22  and an inner absorber cylinder  24 . 
     As shown in  FIG. 1 , the open end of each evacuated tube  20  is inserted into a grommet  36  (also illustrated in  FIG. 2 ) that fits into a circular opening  32  in the sidewall  31  of a central manifold  30 . The central manifold  30  has a layer of insulation  50  ( FIG. 2 ) to reduce heat loss to the ambient. Insulation  50  is not shown in  FIG. 1  so that the underlying features of the solar collector  10  may more easily be understood. 
     In  FIGS. 1 and 2 , the manifold  30  has a square cross section, but may be circular, rectangular, or of any other shape that corresponds to the particular requirements of the application or associated manufacturing costs. The grommet  36  is preferably made from an elastomer that is compatible with liquid  40  that is being heated within the collector  10 . The grommet  36  is designed to form a fluid seal between the evacuated tube  20  and the manifold  30 . 
     Use of grommet  36  allows for damaged evacuated tubes  20  to be more easily replaced. Notwithstanding, grommet  36  may be interchangeable with other means to seal the evacuated tubes  20  in the manifold  30 . For example, the manifold  30  may be made from a material that creates a fluid seal when the evacuated tubes  20  are inserted into the circular openings  32 . Beads of sealant such as a silicone RTV can alternatively be laid around the joints between the evacuated tubes  20  and the manifold  30  to achieve the same sealing effect. 
     Liquid  40  enters the manifold  30  through an inlet fitting  35  ( FIG. 1 ), which may be located in an end wall  33  of the manifold  30 . The flow of liquid  40  may be controlled such that liquid  40  only partially fills the tubes  20 . With the tubes  20  horizontally configured, the vapor space  44  above the liquid will extend the entire length of each tube  20 . Though shown in  FIGS. 1 and 2  as being round, the tubes  20  could be designed in other shapes including oval, pear, or reverse pear shaped, as well as other shapes that may be used to maximize solar energy collection or optimize fluid flow in the collector  10 . 
     While a preferred embodiment of collector  10  may implement the tubes  20  in a horizontal configuration, if the tubes  20  are arranged such that the open end is higher than the closed end, the length of the vapor space  44  might be less than the length of the tube  20 . The length of the vapor space  44  as a fraction of the length of the tube  20  may contribute to the efficient operation of solar collector  10 . For example, if liquid  40  is water, the collector  10  may be more vulnerable to damage by freezing as the amount of water (liquid  40 ) in the tubes  20  increases. Heat loss at night may likewise increase as the amount of liquid  40  in the tubes  20  increases. 
     When tubes  20  are arranged such that the open end is lower than the closed end, the level of liquid  40  within the tube  20  may be such that liquid  40  does not extend the full length of the tube  20 . In such a configuration, the surface of the inner absorber cylinder  24  may be modified to act as a wick so that liquid is drawn by capillary forces either axially towards the closed end of the inner absorber cylinder  24  or circumferentially around the inner absorber cylinder. This wick can be of granulated glass particles that are bonded to the inner absorber cylinder like that illustrated in U.S. Pat. No. 4,474,170. Other refractory particles that can withstand high temperatures may be used such as sand or particles of aluminum oxide. 
     The wick can also be a thin woven or non-woven layer of glass fibers that are inserted into the inner absorber cylinder  24 . Other wicks are possible so long as they can be wetted by liquid  40  and do not degrade when exposed to the highest temperatures that could be produced within an evacuated-tube solar collector  10 . In one embodiment, a wick might draw liquid onto the hotter upper portion of the inner absorber cylinder  24  thereby improving performance of the solar collector  10 . 
     During the operation of solar collector  10 , solar radiation passes through the outer transparent glass cylinders  22  of the evacuated tubes  20  and impinges on the inner absorber cylinder  24 . The volume  23  between the outer glass cylinder  22  and the inner absorber cylinder  24  is evacuated to eliminate heat loss by conduction from the inner absorber cylinder to the surroundings. 
     The surface of the absorber cylinder  24  that faces the vacuum may have high absorptivity for solar radiation and low emissivity for infrared (i.e., thermal) radiation. The absorptivity and emissivity of the absorber cylinder  24  may be controlled through a galvanically applied selective coating such as black chrome, black nickel, or aluminum oxide with nickel. A titanium-nitride-oxide layer may alternatively be applied via steam in a vacuum process. This titanium-nitride-oxide coating has low emission rates and can be produced by an emission-free, energy-efficient process. 
     The solar radiation that impinges on the absorber cylinder  24  is converted to thermal energy and raises the temperature of the upper portion of the absorber cylinder  24 . A combination of radiation from the inner wall of the absorber cylinder  24 , heat conduction, and heat convection transfers thermal energy from the absorber cylinder  24  to liquid  40  within the absorber cylinder  24 . If the incident solar radiation is sufficiently intense, liquid  40  will be heated to a temperature at which its vapor pressure is above the ambient pressure. At this temperature, the evolving vapor will flow out of the absorber cylinder  40  and into the manifold  30 . Said flow pushes out any air that might be in the cylinder. The vapor leaves the manifold  30  through the vapor outlet fitting  37 . The vapor can be used as a heat source for desiccant regeneration, water heating, space heating or similar operations. 
     During normal operation of the invention, steam is produced within the tubes during the day. As discussed, this steam flows to a point-of-use, which might include a desiccant regenerator, a water heater, a space heater or similar thermal device, where it condenses, providing thermal energy to the point-of-use. According to an exemplary embodiment of the present invention, the condensed steam is returned to the solar collector to maintain an approximately constant level of water in the collector. 
     As previously noted, a solar collector apparatus with Dewar-type evacuated tubes becomes more efficient as the amount of water stored in the tubes decreases since the lower mass of water reduces heat loss that will occur at night. Since the critical parameter affecting efficiency is the amount of water within the solar collector at night, a more efficient mode of operation would not immediately return the condensed steam to the solar collector. In this more efficient mode of operation, the condensed steam may be stored in an insulated tank and returned to the solar collector shortly before the sun begins to illuminate the solar collector in the morning. Since the volume of water in the solar collector during the night is further reduced by storing the condensed steam, night-time heat loss is also reduced. 
     A common Dewar-type evacuated tube has an inner tube with a diameter of 47 mm. Under clear sky conditions, this inner tube will absorb solar radiation in one day that at most could convert an amount of water into steam that was equivalent to between 25% and 30% of the total tube inner volume. Thus, a Dewar-type evacuated tube that was more than 30% full of water at the start of the day, would never convert all of the water into steam before the end of the day. 
     Since most days do not exhibit perfect, clear sky conditions, the amount of water in the Dewar-type evacuated tube at the start of the daylight portion of the day can be less than 30% without the tube drying out before the end of the daylight portion of the day. A control algorithm that predicts the maximum amount of water that could be converted to steam during one day and adjusts the amount of water in the solar collector at the start of the daylight portion of the day to be greater than this amount would reduce night-time heat loss and improve the efficiency of the solar collector. In an exemplary embodiment, an object of the control algorithm may be to control the amount of water to be just slightly greater than the maximum amount that could be converted to steam. In any case, the amount of water delivered to the solar collector at the start of the daylight portion of the day may be controlled so that no additional water needs to be delivered until the next day, which improves the overall efficiency of the solar collector apparatus. The control algorithm could account for several factors including (1) time of year, (2) latitude of location (3) site-specific shading, (4) historical weather conditions, and (5) forecasted weather conditions. 
     According to an exemplary embodiment, the preferred approach to reducing night-time heat loss may be to drain the water from the solar collector into an insulated tank at the end of the day. This approach may require at least one pump to transfer the hot water either from the collectors to the insulated tank or from the insulated tank to the collectors. When water is drained from the solar collector at the end of the daylight portion of the day and stored in an insulated tank during the night, then heat loss from the solar collector depends only slightly on the amount of water in the solar collector during the daylight portion of the day. While efficient operation of the solar collector still requires that it not be empty of water during the daylight portion of the day, the amount of water within the collector both at the start of and throughout the daylight portion of the day can be any amount that does not restrict the flow of vapor out of the inner tubes of the collector. Furthermore, vapor produced within the collector that condenses at the point-of-use may or may not be immediately returned to the collector. 
     For installations with one or more manifolds that lie in a common horizontal plane and which are fluidly coupled together as shown in  FIG. 4 , a common water level will exist within the coupled manifolds and the tubes connected to these manifolds. For installations where all water is transferred to an insulated tank at the end of the daylight portion of the day, the coupled manifolds and associated tubes can be filled to a desired level by delivering a predetermined quantity of water from the tank to the empty solar collectors. According to an exemplary embodiment, a method for determining the quantity of water that is delivered to the collectors involves monitoring the level of water in the storage tank and delivering a quantity of water that produces a change in level that corresponds to the desired quantity of water. Conventional methods for measuring the level of water in a tank could be used, including but not limited to ultrasonic sensors, radar sensors, laser sensors, pressure sensors and mechanical float-type gauges.  FIG. 10  shows one possible configuration in which a pressure sensor  202  at the bottom of the storage tank  204  measures the level of water  206  within the storage tank. A controller  210  receives a signal from the pressure sensor and energizes a transfer pump  212  and opens a flow valve  216   b  so that the desired quantity of water is delivered to the collectors  214 . The pressure sensor  202  may also be used to identify a condition in which insufficient water is in the storage tank following the transfer of water from the collectors to the tank. If a low level is detected, the controller  210  may open a flow valve  216   a  that introduces additional water into the tank  204 . At the end of the daylight portion of the day, the control valve  216   b  in the line that connects the collector  214  to the storage tank  204  is opened by the controller  210  so that water in the collector can drain by gravity through the idle transfer pump  212  into the storage tank. It should be appreciated that the control algorithms discussed herein may be implemented via one or more processors based on instructions received from one or more non-transitory, computer readable media. 
     For installations where water is not drained from the manifolds and associated tubes at the end of the daylight portion of the day, it may be necessary to directly measure the level of water within the manifolds and associated tubes. In such an embodiment, the level sensor may be mounted in a convenient location where it can sense the level of water in one of the one or more coupled manifolds.  FIG. 11  shows one possible configuration in which an ultrasonic level sensor  222  is mounted in a fitting on the top surface of the first of two coupled manifolds  224 . A controller  210  receives a signal from the ultrasonic level sensor  222  and energizes the tank outlet valve  226   a  when water is to be gravity fed from the storage tank to the manifolds and associated tubes and, in the event that a low water level is detected by the ultrasonic level sensor, energizes the make-up water valve  226   b.    
       FIG. 3  illustrates a side view of the multi-tube solar collector of  FIG. 1  including a reflecting back plane. As shown in  FIG. 3 , a reflecting surface  52  positioned below the tubes  20  can be used to increase the amount of solar radiation collected by each tube  20 . In  FIG. 3 , the reflecting surface  52  is illustrated as flat and the incident solar radiation  53  that is not absorbed by the evacuated tube  20  reflects as diffuse radiation  55  upward towards the evacuated tubes  20 . 
     Reflecting surface  52  can also specularly reflect the solar radiation; surface  52  may thus have a compound parabolic shape. Other shapes may be implemented in the context of reflecting surface  52 . Alternatively, mirror image pairs of manifolds  30  can be interspersed to maximize the amount of surface areas available for solar absorption. This interleaved or interspersed tube arrangement would result in two parallel manifolds and a smaller space requirement for implementation. Such an arrangement may sacrifice some reflected energy potential provided by a mirror placed behind the tubes of the collector  10 . 
     In applications that require large amounts of thermal energy, it may not be practical to couple all the evacuated tubes  20  to a single manifold  30 . As shown in FIG.  4 , which illustrates a perspective view of a multi-tube solar collector composed of three separate manifold-tube sub-assemblies, coupling sleeves  38  can be used to join two or more manifolds  30  end-to-end so that they function as a single manifold. A toric joint  39 , sometimes referred to as a mechanical gasket or O-ring, may be utilized to properly seal manifolds  30  and coupling sleeves  38 . When multiple manifolds are joined together, end walls  33  may be applied only to the outer ends of the first and last manifold  30  in the series. Such end walls  33  may sealed with the manifold  30  using the aforementioned toric joint  39 . 
     The evacuated tubes  20  of the solar collector  10  illustrated in  FIGS. 1 and 2  all lie in essentially the same flat, horizontal plane. With the evacuated tubes  20  all in the same horizontal plane and no obstructions to the flow of liquid between the evacuated tubes  20 , the level of liquid  40  will generally be the same in all the evacuated tubes  20 . In some embodiments, however, the evacuated tubes may lie in a plane that is not horizontal.  FIG. 5  is just such an embodiment and illustrates a longitudinal sectional view through the end-caps and horizontal tubes of a multi-tube solar collector for which the horizontal tubes lie in a plane that is not horizontal. The embodiment illustrated in  FIG. 5  will allow the solar collector  10  to operate with each tube  20  substantially horizontal but the tubes no longer lying in the same horizontal plane. 
     In  FIG. 5 , a levelizing end cap  64  is added to the open end of each evacuated tube  20 . During the operation of the solar collector  10  of  FIG. 5 , liquid  40  is pumped into the uppermost levelizing end cap  64   a  through an inlet tube  66 . Upon reaching a predetermined level within the evacuated tube  20 , the liquid flows over weir  65 . The overflow liquid flows by gravity through transfer tube  67  and into the next lower levelizing end cap  64   b . This cascading of liquid  40  continues to the lowest levelizing end cap  64   c . From the lowest levelizing end cap  64   c , liquid  40  flows out of the solar collector  10  via the outlet tube  68 . Liquid  40  that flows from outlet tube  68  can be re-circulated to inlet tube  66  using a pump (not shown). Aside from the use of levelizing end caps  64  and the possible recirculation of liquid  40 , the operation of the solar collector  10  as shown in  FIG. 5  is substantially similar to that of the solar collector  10  of  FIGS. 1 and 2 . 
       FIG. 6  is a perspective view of a multi-tube solar collector  70  with horizontal tubes used to evaporate a volatile component of a multi-component liquid. The solar collector  70  shown in  FIG. 6  is used to partially separate a liquid mixture into a volatile component and a non-volatile component. Although not limited to this application, the solar collector  70  can be used to remove water (i.e., the volatile component) from an aqueous solution of an ionic salt such as calcium chloride (i.e., the non-volatile component). 
     Other ionic salts that are soluble in water include lithium chloride, calcium bromide, lithium bromide, sodium chloride, potassium sulfate, sodium sulfate, as well as solutions in which the vapor produced when the solution is heated has only one component (i.e., water in the case of an aqueous salt solution). The liquid mixture with the higher fraction of the non-volatile component will be called the concentrated liquid, and the liquid mixture with the lower fraction, the dilute liquid. Thus, dilute liquid  72  is supplied to the solar collector  70  and concentrated liquid  74  and vapor  75  are returned from the solar collector. 
     As the concentration of the non-volatile component increases and as its temperature decreases, the density of the liquid mixture may increase. In this context, consider an application where the dilute liquid  72  that is supplied to the solar collector  70  is heated to a sufficiently high temperature to ensure that its density is lower than that of the hot, concentrated liquid  74  that is returned from the solar collector. In this case, the inlet fitting  76  for the dilute liquid  72  may be located on the front end wall  82  of the central manifold  84  at an elevation that is close to the level of the liquid within the evacuated-tubes  20 . Such a configuration is illustrated in  FIG. 6 . 
     The concentrated liquid  74  is withdrawn from the central manifold  84  through an outlet fitting  78  that may be located on the rear-end wall  83  at an elevation near the bottom of the central manifold  84  as is also illustrated in  FIG. 6 . With this arrangement of inlet fitting  76  and outlet fitting  78 , the dilute liquid  72  will tend to spread out over the surface of the liquid within the solar collector  70  and be delivered to each evacuated tube  20  in a relatively uniform manner. 
     As the dilute liquid  72  is heated by the solar radiation that is absorbed by the evacuated tube  20 , some of the volatile component of dilute liquid  72  is converted to vapor. As this happens, the dilute liquid  72  becomes more concentrated and the density of dilute liquid  72  increases thereby causing dilute liquid  72  to sink to a lower level within the evacuated tube  20 . During the operation of the solar collector  70 , there may be a continuous flow of dilute liquid to the tubes and return of concentrated liquid from the tubes, the dilute liquid flowing above the concentrated liquid. The vapor  75  flows out of manifold  84  through vapor outlet fitting  77  that is located above the liquid level within the manifold as illustrated in  FIG. 6 . 
     In some applications, it may be convenient to locate the inlet fitting  76  for the dilute liquid and the outlet fitting  78  for the concentrated liquid on the same end wall of the manifold  84 . In these applications, the inlet fitting  76  can be extended within the manifold so that the dilute liquid is delivered to a location within the manifold that is away from location where the concentrated liquid flows out of the manifold. This design would prevent “short-circuiting” of the weak liquid directly to the outlet fitting. 
     In some applications it may not be practical to preheat the dilute liquid  72  to a sufficiently high temperature to ensure that its density is lower than the concentrated liquid  74 . In these applications, an internal artery  86  can be added to the solar collector  70  as shown in  FIG. 7 .  FIG. 7  illustrates a cut-away perspective view of a multi-tube solar collector with horizontal tubes used to evaporate a volatile component of a multi-component liquid and including an internal artery to deliver the multi-component liquid to one of the tubes. 
     Internal artery  86  delivers the dilute liquid  72  to the closed end of one evacuated tube  20   a  of the many tubes  20  that are a party of solar collector  70 . The dilute liquid will be heated within this evacuated tube  20   a  to a temperature at which its density is less than that of the concentrated liquid  74 . The heated dilute liquid that leaves the evacuated tube  20   a  will then flow to the other evacuated tubes  20  along the surface of the liquid that fills these tubes  20 . Once the dilute liquid  72  has flowed to the other evacuated tubes  20 , the process of creating vapor and concentrated liquid within these tubes  20  will be the same as that described for the operation of the solar collector  70  in  FIG. 6 . 
     Depending on the temperature of the dilute liquid  72  that is supplied to the solar collector  70 , it may be necessary to use more than one internal artery  86  and more than one evacuated tube  20   a  for preheating. The number of internal arteries  86  will nevertheless be less than the number of evacuated tubes  20 ; this configuration remains a simplification over those prior designs that require one internal artery for each evacuated tube. 
     In the solar collector  70  illustrated in  FIGS. 6 and 7 , if the diameter of the outlet fitting  78  is large and the concentrated liquid  74  freely flows out of the central manifold  84 , then the elevation of this fitting may set the level of the liquid within the manifold  84  and the evacuated tubes  20 . The denser concentrated liquid that is produced within the solar collector  70  will, however, flow below the less dense dilute liquid. As such, it may be preferable for the outlet fitting  78  to be located near the bottom of the central manifold  84  so that only the concentrated liquid is withdrawn from the manifold. 
       FIG. 8  illustrates a longitudinal sectional view through the center plane of a manifold that uses a partition with an orifice to allow concentrated liquid to be withdrawn from the manifold through an end wall fitting. As shown in  FIG. 8 , a partition  89  with an orifice  91  near the bottom of the partition  89  can be used to form a pool  93  of liquid at the end of the central manifold  84 . The liquid that flows into this pool  93  through the orifice  91  will be concentrated liquid. The level of the liquid in the pool  93  will be nearly the same as the level of liquid within the remainder of the central manifold  84  and the evacuated tubes  20 . If the concentrated liquid freely flows out of the outlet fitting  78 , then the elevation of this fitting can be used to establish the level of liquid within the pool  93 , the remainder of the central manifold  84 , and the evacuated tubes  20 . 
     In the embodiments illustrated in  FIGS. 1 through 8 , only the lower portion of the inner absorber cylinder  24  is wetted by liquid. A significant fraction of the solar radiation that impinges on the solar collector is absorbed by the upper portion of the inner absorber cylinder  24 , which faces the sky. This portion of the inner absorber cylinder is not wetted by liquid. Nor if there is a thermally conductive element within the evacuated tubes that would facilitate the transfer of heat from the hot, upper portion of the inner absorber cylinder  24  and the liquid  40  that is within the tube. 
       FIG. 9  illustrates performance data corresponding to a multi-tube solar collector that produces steam. The performance data of  FIG. 9  corresponds, specifically, to a 24-tube solar collector implemented in the context of the embodiment illustrated in  FIG. 3 . The diameter of the transparent outer cylinder of the evacuated tubes resulting in the data of  FIG. 9  was 58 mm and the diameter of the inner absorber cylinder was 47 mm; the length of each tube was 1.8 m. The tubes were spaced apart along the central manifold so that the distance between their axial centerlines was equal to 94 mm. The backplane was a white, diffusely reflecting surface located 94 mm below the axial centerlines of the evacuated tubes. The 24-tube solar collector operated with water entering the collector at close to ambient temperature and pressure, and steam leaving the collector slightly superheated to approximately 105.degree. C. 
     The curve labeled A in  FIG. 9  shows the solar energy that was productively used to produce steam in the 24-tube solar collector expressed as a percentage of the non-reflected solar energy that was incident on the inner absorber cylinders of the collector when the tubes were approximately one-half full of water. The curve labeled B in  FIG. 9  shows the performance of the collector when the tubes were approximately one-third full of water. 
     As shown in  FIG. 9 , following an initial three-hour period of heating for the collector that was one-half full (i.e., Curve A) and a two and one-half hour period of heating for a collector that was one-third full (i.e., Curve B), both collectors produced steam at a rate that corresponds to approximately 80% to 100% of the non-reflected solar radiation incident on the inner absorber cylinders of the related tube. It should be noted that the tubes also receive reflected radiation from the backplane, so the 100% conversion rate does not imply that all the incident radiation is converted to steam. Otherwise, once at operating temperature, both collectors—regardless of fill levels—produce steam at about the same efficiency. 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. The descriptions are not intended to limit the scope of the invention to the particular forms set forth herein. To the contrary, the present descriptions are intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims and otherwise appreciated by one of ordinary skill in the art. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments. For example, the solar absorber of the present invention can also be used to heat a multi-component liquid like glycol and water, thereby producing vapor that has a much higher fraction of water than the initial liquid. In such case the present invention could be used as the thermal source for a distillation column.