Abstract:
A solar co-generator for producing both heat energy and electricity is disclosed. A solar concentrator directs sunlight into a container lined with solar cells and filled with a thermal transfer fluid. The fluid is transparent with respect to certain wavelengths of light that may be converted to electricity by the solar cell, but is opaque with respect to longer wavelengths, particular the infrared band. The infrared portion of the sunlight heats the thermal transfer fluid, which then transfer that heat through a storage facility using a heat exchange mechanism. The thermal transfer fluid increases the efficiency of photovoltaic generation by preventing heating of the solar cells due to infrared radiation. The thermal transfer fluid may be a mixture containing barium sulfate and a suspension of zinc sulfide phosphors. A fluorescing anti-reflective coating may be applied to the solar cells to further increase efficiency.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a generator that produces both heat and electricity from solar energy, and in particular to a solar energy co-generator that utilizes a transfer fluid to capture heat energy and increase the efficiency of photovoltaic generation.  
           [0002]    Solar energy is a renewable energy source with many well-known advantages over traditional fossil and nuclear energy resources. Solar energy has remained a largely untapped field, however, because the efficiency of existing solar power collection systems is generally too low to justify the required capital investment for these systems. While most of the electromagnetic radiation received from the sun falls in the visible spectrum (a wavelength range of approximately 430 to 690 nm), the sun&#39;s radiation also includes other components. In particular, solar energy incident upon the earth includes infrared radiation, a band that covers a wavelength range of approximately 1 μm to 1 mm. Typical solar cells, that is, devices designed to convert electromagnetic radiation from the sun into electricity, are only able to utilize energy within a relatively narrow wavelength band in the visible spectrum. Infrared radiation is not only unusable by such solar cells, but the heat caused by incident infrared radiation actually lowers the efficiency of typical semiconductor-based solar cells. Existing thermophotovoltaic cells, which are designed to capture infrared radiation and convert it to electricity, have very low efficiencies and are thus limited to a small number of specialized applications, such as recapturing infrared radiation emitted by heated surfaces. In addition, existing solar cells generally convert longer wavelength visible light into electricity more efficiently than they do shorter wavelength radiation. As a result of these limitations, much of the electromagnetic energy incident on a typical solar cell cannot be converted into electricity. It would be desirable therefore to develop a method of efficiently converting a broader band of the sun&#39;s radiation to usable energy, thereby increasing the efficiency of a solar energy collector and making such systems practical solutions for a greater number of energy applications.  
           [0003]    Prior art systems have been developed to capture heat energy (that is, energy resulting from incident infrared radiation) from the sun. For example, U.S. Pat. No. 6,336,542 to Tirey, Jr. discloses a water heating system utilizing a reflective parabolic dish as a solar concentrator. A fluid line extends through a support pole and is coiled at the focal point of the dish. Solar energy, which is concentrated at the fluid line coil by the reflective dish, heats water that is pumped through the system. Another example, disclosed in U.S. Pat. No. 5,685,151 to Ross, is a system that utilizes a solar concentrator to capture solar energy and focus it onto a cavity-type solar boiler. The interior walls of the boiler are formed by pipes that carry liquid sodium, which acts as a heat transfer medium. The heat causes the sodium to boil, and the sodium vapor is pumped to a heat exchange system in communication with a molten sodium chloride tank. The sodium returns to a liquid state as a result of the heat exchange, and is pumped in a circuit back to the boiler. The heat now stored in the sodium chloride tank is then transferred again by steam pipes as needed and used to power electrical generators. Similarly, U.S. Pat. No. 4,586,334 to Nilsson, Sr. et al. discloses a system for collecting solar power with a concentrator and storing it in a phase change medium, whereby energy may be withdrawn on demand using a variable heat exchange system. U.S. Pat. No. 5,228,293 to Vitale discloses a system that captures solar energy in a cavity receiver and transfers it through a thermal transport fluid to an engine that may produce both electrical power and hot water. The system also includes an auxiliary fossil or biomass heater that may supplement or completely replace the cavity receiver.  
           [0004]    There have also been attempts to develop solar energy co-generators, that is, solar energy collection systems that capture energy from both the visible and infrared spectrums, utilizing a combination of solar cells and heat transfer fluids, to produce both electricity and heat energy. Such systems should in theory operate with greater efficiency than systems designed to produce heat or electricity alone since a wider portion of the sun&#39;s spectral band is utilized. Such systems would have important practical applications, such as using electricity to power electrical appliances, and heat for home heating or hot water production. An example of an attempt to design such a system is disclosed in U.S. Patent No. 6,080,927 to Johnson. The Johnson system includes a solar concentrator that focuses sunlight onto the solar cells. A heat transfer system is placed in contact with the solar cells in order to carry away heat resulting from the infrared radiation incident upon the solar cells. The heat transfer system is simply water or the like pumped through pipes in contact with the solar cells, with a heat exchanger that transfers heat from the heat transfer system to some useful work, such as heating a swimming pool. By carrying heat away from the solar cells, this system allows the solar cells to operate more efficiently. A further example is provided by U.S. Pat. No. 4,191,164 to Kelly, which teaches a system with a solar concentrator that provides solar energy to pre-heat a flash boiler pipe for steam production. The system also includes an array of solar cells mounted at the cold water entrance end of the pipe. The cold water serves as a thermal transfer fluid that dissipates heat from the solar cells as in Johnson, while the infrared radiation is used for pre-heating the boiler pipe water.  
           [0005]    Prior art co-generation systems such as Johnson and Kelly suffer from some significant limitations and disadvantages. One critical problem with these systems is that while they may include a heat transfer medium to draw heat away from the solar cells, they do not prevent the heating of the solar cells by infrared radiation. Since these systems typically include a solar concentrator, the heat generated at the solar cells may be quite intense. Although the use of a heat transfer system in contact with the cells does provide some cooling, the efficiency of the solar cells is still reduced simply by the fact that the infrared radiation reaches the cells at all. A system whereby a significant part of the sun&#39;s infrared radiation never reaches the cells, but is instead directly converted to heat energy before it reaches the cell surface, would be inherently more efficient.  
           [0006]    One system that attempts to address this problem is taught by U.S. Pat. No. 6,407,328 to Kleinwacchter. Kleinwachter teaches that a liquid medium that permits passage of radiation energy useful for photovoltaic effects, but absorbs the remaining radiation energy, would be desirable. Kleinwachter attempts to achieve this by the use of water in combination with a fluoropolymer film. Kleinwachter teaches that water absorbs only the longer-wave radiation and allows passage of wavelengths that may be useful for photoelectric production, while the fluoropolymer film blocks only a portion of the short-wave range. As a result, most radiation in the band that is useful to solar cell electricity production passes through while the longer-wave radiation heats the water passing over the solar cells. Kleinwachter further teaches a heat exchange system for cooling of the water.  
           [0007]    A significant limitation of the system taught by Kleinwachter is that it provides no means by which to disperse radiation uniformly across the solar cell. Solar cells reach their maximum efficiency only when radiation is evenly distributed across their functioning surface. Providing a uniform distribution of light across a solar cell is particularly difficult where a solar concentrator is employed as part of the collection system. One method that has been utilized to alleviate this problem is to employ a luminescent material between the solar cell surface and the sun. Luminescent materials have the property that they may absorb light falling upon them from one direction and reemit light that is isotropic. As a result, a luminescent material may be constructed that will “smooth” the intensity distribution of incident sunlight across a surface, such as a solar cell. An example of a system that attempts to utilize this principle is taught in U.S. Pat. No. 4,175,980 to Davis et al. Davis et al. teaches that luminescent materials may be suspended in a sheet of glass or plastic positioned above a solar cell in order to smoothly distribute light across the solar cell.  
           [0008]    The use of a luminescent material as described above may also increase the efficiency of a solar cell by producing light of a frequency that is well matched to the performance characteristics of the solar cell. Roughly ten percent of the sun&#39;s electromagnetic energy falls in a wavelength range of about 305 to 450 nm (that is, from near ultraviolet to blue). Typical semiconductor solar cells, however, operate more efficiently at somewhat longer wavelengths, toward the red end of the visible spectrum. In addition to their isotropic scattering property, certain luminescent materials also act to absorb visible light at the higher frequencies and emit light at the lower, more desirable frequencies. Thus the deployment of a luminescent material between a solar cell and the sun (or a concentrator) may serve to increase efficiency not only through more efficient distribution of incident light upon the solar cell but also through frequency conversion. U.S. Pat. No. 4,135,537 to Blieden et al. teaches a light collector that attempts to take advantage of this idea. Blieden et al. teaches that a luminescing fluid is deployed over the solar cell surface. This luminescing fluid serves to isotropically distribute light across the solar cell surface and also acts as a heat dissipation medium. Blieden et al. also teaches, however, that the luminescing agent may be chosen for its ability to emit light in an energy level range that suits the conversion characteristics of the particular solar cell being used.  
           [0009]    The device taught by Blieden et al. also has significant limitations. Importantly, this device depends entirely upon luminescent materials to isotropically distribute light. As Blieden et al. discloses, this is not truly isotropic scattering, but is rather isotropic re-emission. In other words, the fluid taught by Blieden et al. does not simply scatter incident light in all directions, but instead absorbs the incident light (by means of luminescent materials in the fluid) and re-emits longer wavelength light isotropically. Although Blieden et al. claims that the invention is directed to the use of any luminescent material, in fact it teaches the use of only one class of such materials, namely, organic dyes. Organic dyes degrade with use, and this degradation is hastened by the absorption and re-emission as taught by Blieden et al. Furthermore, this degradation would be even further hastened by a higher concentration of incident light reaching the luminescent material, such as would be the case if one attempted to combine the Blieden et al. device with a solar concentrator. The constant replacement of transfer fluid material would render the device of Blieden et al. impractical in many-real-world applications, particularly with respect to applications that make use of a solar concentrator.  
           [0010]    Another limitation on the use of organic dyes in such applications is self-absorption. These dyes not only absorb light in the shorter wavelength bands but in the longer wavelength bands as well. Thus the dye material could, for example, absorb incident light, re-emit light at a longer wavelength, and then re-absorb the emitted light. This re-absorption lowers the efficiency of the system and would hasten the degradation of the organic dyes.  
           [0011]    Finally, the prior art includes the use of anti-reflective coatings to prevent the loss of efficiency due to reflective losses at the point of light entry into a solar cell. Materials such as zinc sulfide, an inorganic phosphor, have been used for this purpose, due to this material&#39;s high index of refraction. In a co-generator where a fluid is disposed above a solar cell, a protective coating must be provided between the solar cell and fluid in order to protect the solar cell from damage. This coating material is typically glass. The anti-reflective coating on the solar cell is typically matched to the refractive index of the protective coating in order to minimize losses. One important limitation of such prior art anti-reflective coatings, however, is that they provide no means by which to modify the wavelength of light passing into the solar cell in order to increase the solar cell&#39;s efficiency.  
           [0012]    It would therefore be desirable to develop a solar co-generator with a long-life transfer fluid that blocks longer-wave radiation from reaching the solar cells and also converts that radiation to heat energy, while simultaneously providing isotropic scattering (rather than simply isotropic re-emission) and selected wavelength modulation with respect to the light reaching the solar cell. Further, it would be desirable to employ with such a co-generator an anti-reflective coating on the solar cells that serves to further provide wavelength modulation to increase the efficiency of the solar cells. Finally, it would be desirable to create such a solar co-generator that could be used with a light concentrator without significantly reducing the life of the transfer fluid used in the system.  
         BRIEF SUMMARY OF THE INVENTION  
         [0013]    The present invention is directed to a co-generator system with improved energy conversion efficiency using a transfer fluid that acts to block longer-wavelength radiation from reaching the solar cells, thereby increasing photovoltaic efficiency, and also to diffuse the light incident upon the solar cells to further improve conversion efficiency. According to the present invention, a container houses a thermal transfer fluid that includes a suspension of an isotropic scattering agent and luminescent material. In certain preferred embodiments, the isotropic scattering agent is barium sulfate. A solar concentrator is used in certain embodiments to collect and concentrate light onto a window on the bottom of the container. Solar cells lining the container walls and back collect light passing from the concentrator, through the window, diffused in the transfer fluid, and finally isotropically scattered onto the solar cell surfaces. The particular thermal transfer fluid chosen is transparent to those radiation wavelengths within the solar cell operating range, but is opaque with respect to certain of the longer (infrared) wavelengths. Because the thermal transfer fluid is opaque with respect to these infrared frequencies, it prevents them from every reaching the solar cells. In this way, the solar cells are kept cooler than the solar cells in prior art systems that employ diffusion. Although the infrared radiation cannot penetrate through the container to the solar cells, it is absorbed by the transfer fluid. This results in a heating of the transfer fluid at a point removed from the solar cells. This heat is drawn away from the container and stored in a heat storage facility through the use of a pump and heat exchange system.  
           [0014]    The present invention primarily uses barium sulfate or other isotropic scattering agents rather than a luminescent material as a medium for isotropic scattering of incident light. Because the scattering is direct, rather than indirect scattering through luminescent re-emission, the transfer fluid is less susceptible to degradation due to incident light. This is a particularly important advantage when a solar concentrator is employed as in certain embodiments of the present invention. Luminescent material is still used in the present invention for its property of absorbing shorter wavelength light that cannot be efficiently converted by solar cells, and re-emitting light in a band that is more usable by photovoltaic cells. In a preferred embodiment, this luminescent material is of an inorganic type that is less susceptible to degradation in any event. These materials also have the important advantage that they do not create the re-absorption problems experienced with organic dyes. Since materials such as barium sulfate simply scatter rather than absorb incident radiation, they cannot create re-absorption efficiency losses. The inorganic dyes described above absorb in the ultraviolet band only and emit in the green band only, thereby also avoiding any re-absorption problems.  
           [0015]    Another feature of certain embodiments of the present invention is the application of an anti-reflective solar cell coating with fluorescent properties. Such a coating would increase the efficiency of a solar cell by evolving longer wavelengths from shorter incident wavelengths in the same manner as the luminescent materials described above. In the preferred embodiment, the fluorescing anti-reflective coating is formed using a thin film of ZnS:Cu, Al. The particular combination of scattering material, luminescent material, and a fluorescing anti-reflective coating results in a system with efficiency not reached by prior art devices using similarly efficient solar cells.  
           [0016]    The present invention need not be used with a solar concentrator, but is particularly well suited for such use. Unlike prior art systems, the application of a solar concentrator to the present system requires only that the fluid mixture be adjusted to optimize performance with the particular concentrator employed. For example, as a more high-power solar concentrator is used, the concentration of barium sulfate or other scattering agent in the transfer fluid is increased proportionally. The proper concentration of such material for peak performance with any particular light concentration arrangement may be mathematically derived.  
           [0017]    It is therefore an object of the present invention to provide for a solar co-generator of increased efficiency.  
           [0018]    It is a further object of the present invention to provide for a solar collection system using a transfer fluid that is transparent to certain wavelengths of light utilized by the solar cells but is opaque with respect to certain longer frequencies of light.  
           [0019]    It is a still further object of the present invention to provide for a solar collection system using a transfer fluid suspension that isotropically scatters incident light prior to striking the solar cells.  
           [0020]    It is also an object of the present invention to provide a solar co-generator that utilizes a transfer fluid with luminescent material to increase the photovoltaic efficiency of the solar cells.  
           [0021]    It is also an object of the present invention to provide a solar cell with a fluorescing anti-reflective coating to increase the efficiency of the cell.  
           [0022]    It is also an object of the present invention to provide a solar co-generator that is particularly well suited for use with solar concentrators.  
           [0023]    These and other features, objects and advantages of the present invention will become better understood from a consideration of the following detailed description of the preferred embodiments and appended claims in conjunction with the drawings as described following: 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a schematic view of a preferred embodiment of the present invention.  
         [0025]    [0025]FIG. 2 is a schematic, partial cut-away view of the collection chamber of a preferred embodiment of the present invention.  
         [0026]    [0026]FIG. 3 is a schematic, partial cut-away view of a heat storage unit of a preferred embodiment of the present invention.  
         [0027]    [0027]FIG. 4 is a graph showing the spectral irradiance of the sun across a defined wavelength band.  
         [0028]    [0028]FIG. 5 is a graph showing the relative energy of excitation/emission for a particular inorganic phosphor across the wavelength band of FIG. 4.  
         [0029]    [0029]FIG. 6 is a graph showing the modified spectral irradiance of the sun after application of the light through the inorganic phosphor of FIG. 5 across the wavelength band of FIG. 4.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    With reference to FIG. 1, a preferred embodiment of the present invention may be described. A solar concentrator  10  is mounted on tracking system  12  in a suitable location to receive sunlight. Solar concentrator  10  in the illustrated embodiment is of the parabolic dish type. The interior of solar concentrator  10  is coated with a reflective material, such as aluminum foil, or the concentrator  10  may itself be formed of a reflective material. Alternative shapes for solar concentrator  10  may include other three-dimensional (or point-type) systems, two-dimensional (or linear-type) systems, such as a Fresnel lens, or any of the various other concentrator types that are known in the art.  
         [0031]    The embodiment is ideally placed in a location that has a direct line of sight to the sun during a significant portion of the day. Tracking system  12  makes it possible for concentrator  10  to follow the sun as it progresses across the sky during the day. Such tracking systems may take many different forms as are well known by those skilled in the art.  
         [0032]    At the focal point of solar concentrator  10  lies airtight collection chamber  14 , as will be described more fully below. Collection chamber  14  is filled with, transfer fluid  44 . Fluid conduit  20  connects the interior of collection chamber  14  to heat exchanger  18 . Preferably, fluid conduit  20  comprises an inlet and outlet pipe, each of which are insulated by foam pipe insulation or other insulation materials as are known in the art. Fluid conduit  20  allows transfer fluid  44  to flow from collection chamber  14 , through fluid conduit  20 , through heat exchanger  18  located in heat storage unit  22 , back through fluid conduit  20 , and then once again into collection chamber  14 . Fluid  44  is directed through this circuit by a fluid pump (not shown) of a conventional type that is capable of withstanding the heat generated during operation.  
         [0033]    Mounted above collection chamber  14  is intensity sensor assembly  17 . In the preferred embodiment, intensity sensor assembly  17  may comprise a metal disk covered with reflecting tape or other reflecting material. Intensity sensor assembly  17  mounts to solar concentrator  10  through support poles  18 . Intensity sensor assembly  17  is attached to support poles  18  by screws, bolts, or other fastening means. Intensity sensor  16  is mounted on top of intensity sensor assembly  17  such that intensity sensor  16  is directed toward the sun as solar concentrator  10  moves in response to tracking system  12 . Intensity sensor  16  functions to measure the intensity of solar radiation incident upon the system and controls the rate of flow of transfer fluid  44  through fluid conduit  20  based on the solar intensity. As solar radiation intensity increases, the rate of flow through the pump (not shown) is increased, and as the solar radiation intensity decreases (as, for example, while cloud cover persists), the rate of flow through the pump is decreased. In this way, excessive energy is not wasted in pumping transfer fluid  44  through the system when a lower rate of pumping would be sufficient for heat transfer and dissipation. Intensity sensor  16  is electrically coupled with the power to the pump in a means as would be well understood by those skilled in the art.  
         [0034]    Referring now to FIG. 2, the structure of collection chamber  14  may be described. Collection chamber housing  32  is an airtight, box-shaped chamber in the illustrated embodiment but may be of any convenient shape. Fluid conduit  18  provides means for transfer fluid  44  to enter and leave the interior of collection chamber  14 . On the bottom side of collection chamber housing  32  is window  42 , which may be constructed of glass, plastic, or other material that is transparent within the radiation band of interest. Lining the sides and top of the interior of collection chamber housing  32  is solar cell array  36 . Preferably, solar cell array  36  is composed of high-efficiency semiconductor-based crystalline solar cells as are known in the art, but other types of solar cells may be used. Sandwiched between solar cell array  36  and the interior of collection chamber housing  32  is insulation  34 , which serves to protect solar cell array  36  from conductive heating from the outside through collection chamber housing  32 .  
         [0035]    Referring now to FIG. 3, the structure of heat storage unit  22  in a preferred embodiment of the invention may be described. Heat storage unit housing  24  is constructed of concrete, which has heat storage capacities well known to those skilled in the art. In a preferred embodiment, heat storage unit housing  24  is surrounded by heat storage insulation  46  to provide further thermal isolation from the environment. Also in the preferred embodiment, heat storage unit housing  24  may be coated on its interior side with heat storage lining  48 , which may be constructed of vinyl or any other appropriately non-reactive material. Extending through heat storage unit housing  24  is fluid conduit  18  (which, as already described, may preferably consist of two insulated pipes), and also hot water distribution duct  26 .  
         [0036]    Connected to fluid conduit  20  is heat exchanger  28 , which is preferably shaped as one or more coils. Wrapped around the coils of heat exchanger  28  is flexible tubing  30 . In the preferred embodiment, flexible tubing  30  is {fraction (1/16)}th inch plastic tubing filled with a eutectic salt such as sodium sulfate. It is believed that the addition of flexible tubing  30  filled with a eutectic salt will reduce incomplete recrystalization in heat transfer fluid  44 ; this is a process whereby heat is “trapped” in liquid phase after a large number of melting-crystallization cycles. In addition, sodium sulfate is an excellent heat storage medium, and the heat storage of the system is thus increased by the use of flexible tubing  30  filled with a eutectic salt.  
         [0037]    Heat storage unit  22  is filled with a liquid that has good heat storage capabilities, such as water. Water flows in and out of heat storage unit  22  through hot water distribution duct  26 , which preferably consists of two or more pipes. The hot water that leaves heat storage unit  22  may be used for heating or any other application to which hot water may be applied as are known in the art. A water pump (not shown) may be used to pump hot water through heat storage unit  22  for heating as hot water is needed.  
         [0038]    The transfer fluid  44  of a preferred embodiment of the invention comprises a suspension of barium sulfate and inorganic phosphors in a 1:5 to 1:25 solution of potassium silicate and water. Specifically, transfer fluid  44  for one preferred embodiment of the invention has been created using 9 grams of barium sulfate added to 1.5 liters of a 1:16 solution of potassium silicate to water solution. The potassium silicate has three purposes. First, it acts as an anti-freeze agent. Second, it has an infrared absorption band and will therefore increase infrared absorption. Third, it is a dispersant for inorganic phosphors to prevent clumping. Potassium silicate solutions are manufactured by the PQ Corporation of Valley Forge, Pa. under the tradename Kasil. Other components may include sodium silicate, which can be used along with potassium silicate to control the viscosity of heat transfer fluid  44 . Sodium silicate solutions are manufactured by the PQ Corporation under the tradename N Clear.  
         [0039]    The isotropic scattering material used should preferably have the property that it absorbs radiation of a wavelength of about 650 nm or greater, and is transparent with respect to radiation of a wavelength of less than 650 nm. Since transfer fluid  44  is used to deliver thermal energy by the action of pumping it through heat storage unit  22 , it should have the properties of low viscosity, high heat capacity, and a high heat transfer coefficient. Thus transfer fluid  44  essentially captures the IR portion of the solar spectrum as heat, delivers it to heat storage, and leaves the visible portion of the solar spectrum available for photovoltaic conversion.  
         [0040]    One class of inorganic phosphors that may be used in a preferred embodiment is the zinc sulfide type, such as phosphor numbers 2312, 2363, and 1260 manufactured by Osram Sylvania of Danvers, Mass. In operation, the inorganic phosphors absorb solar energy in a broad band of about 305 nm to 450 nm (corresponding to a band from near UV to blue), and re-emit radiation in narrow bands centered at 526 nm and 623 nm (green and red, respectively). This longer-wavelength radiation is emitted isotropically, so the phosphors also provide some degree of a light-trapping function, although the bulk of this function is preferably performed by the isotropic scattering component of transfer fluid  44 . These phosphors are poor choices as the sole means of isotropic scattering in transfer fluid  44  because they have a narrow absorption peak, and thus only light absorbed about that peak turns to fluorescence. The barium sulfate or other isotropic scattering agent of the preferred embodiment of the invention lacks this significant limitation. In addition, barium sulfate is chemically stable, is heat resistant (unlike organic phosphors that degrade quickly), and is very inexpensive when compared to prior art methods. It should be noted that the churning action of the pump may function to maintain the phosphors in suspension within transfer fluid  44 ; this is important with respect to the preferred class of phosphors, since unlike prior art methods that use liquid phosphors, these phosphors are solids that are simply suspended in the water-silicate solution along with barium sulfate.  
         [0041]    The increase in photo-generated current J L  from longer-wavelength phosphor re-emission may be estimated by assuming that a fraction of the energy in the AM0 spectral distribution, corresponding to the excitation band of a commercially available phosphor, was shifted to wavelengths corresponding to the emission band of the phosphor. The net effect of the phosphor may therefore be modeled as increasing the area under the AM0 distribution at longer wavelengths while decreasing area at shorter wavelengths. The photo-generated current could then be found from  
         J   L     ≈       q   hc            ∫   0     λ   G                λφ   m          (   λ   )                          λ                                 
 
         [0042]    as developed in Partain, L. D., Solar Cells and their Applications, John Wiley and Sons, Inc., (  1995  ), which is incorporated herein by reference. The value  φ     m     (λ)  is the modified AM0 spectral distribution shown in FIG. 6, which is obtained by shifting area from regions of short wavelength to regions of long wavelength. FIG. 4 shows the AM0 spectral irradiance values  φ(λ)  from Wehrli, C., Extraterrestrial Solar Spectrum, Pub. No. 615, Physikalisch-Meteorologisches Observatorium and World Radiation Center, Davos, Switzerland, which is incorporated by reference herein. FIG. 5 shows the percent energy absorbed, RA, and the percent energy emitted, RE, on the excitation and emission bands of a particular ZnS type phosphor, Osram Sylvania 1260, used in a preferred embodiment of the present invention. Since only the wavelengths corresponding to the excitation and emission bands of 1260 are assumed to contribute to the change in photo-generated current, one may numerically integrate  
         Δ                   J   L       ≈       q   hc          (         ∫   200   450            λ        (         φ   m          (   λ   )       -     φ        (   λ   )         )                          λ         +       ∫   470   640            λ        (         φ   m          (   λ   )       -     φ        (   λ   )         )                          λ           )                             
 
         [0043]    where (200 nm, 450 nm) and (470 nm, 640 nm) are the excitation and emission bands of phosphor 1260. The value of  φ     m     λ)  may be obtained by first assuming that AM0 radiation was direct-normally incident on a volume with zero surface reflectivity and containing enough phosphor 1260 to absorb 100% of the incident radiation at 200 nm, which is the peak excitation wavelength for this particular phosphor. Assuming that 100% of incident radiation was absorbed at 200 nm, one may estimate what percent of incident radiation was absorbed on the entire excitation band. If the excitation band in FIG. 5 is then partitioned into 49 sub-bands, each 5 nm long, then the midpoint formula may be used to find the average percent radiation absorbed, (RA) iav , of RA on the i th  sub-band of the (200 nm, 450 nm) band. One may likewise partitioned the (200 nm, 450 nm) band of the AM0 distribution in FIG. 5 into 49 sub-bands of 5 nm width; each sub-band defined a rectangular cross section on the figure one curve of area A ia =(5 nm) φ     avix   , where  φ     avis    is the average value of  φ(λ)  on the i th  sub-band of (200 nm, 450 nm). One may then multiply each A ia  by the corresponding (RA) iav  to determine how much area to subtract from the i th  cross section in FIG. 4. This area may be subtracted as a rectangle 5 nm long and h ix W/m 2 nm high. One may determine h ix  from the condition 5h ix =(RA) iav  A ia . The total area subtracted from the 49 sub-bands on (200 nm, 450 nm) in FIG. 4 is 131W/m 2 , which is about ten percent of the total AM0 direct-normal spectral irradiance as explained in Wehrli.  
         [0044]    The value of  φ     m     (λ)  on the (470 nm, 635 nm) band of FIG. 6 may be generated by first partitioning the emission band of FIG. 5 into 35, 5 nm long sub-bands. Each such sub-band on figure two defines a rectangular cross section of area A csi =(5 nm) (RE) iav  where (RE) iav  is the average percent radiation emitted, RE, on the i th  sub-band of FIG. 4. Therefore, the fraction of the total radiation absorbed by phosphor 1260 that is subsequently fluoresced into the i th  sub-band of (470 nm, 635 nm) is  
         A     %                 i       ≈         A   csi         ∑   1   35                     A   csi         .                           
 
         [0045]    One may partition the (470 nm, 635 nm) band of FIG. 4 into 35, 5 nm long sub-bands and find the average value of  φ(λ)  on each 5 nm sub-band, namely  φ     avim   . As before, each 5 nm sub-band defines a rectangular cross section of area (5 nm)( φ     avim   ) and to each such cross section one may add an area of (131W/m 2 )(A %i ) as a rectangle 5 nm long and h im W/m 2 -nm, with h im  determined from the condition 5 h im =(131)(A %i ).  
         [0046]    The total change in photo generated current is the sum of the decrease ΔJ L −on (200 nm, 450 nm) and the increase ΔJ L +on (470 nm, 640 nm), which is given by the equation provided above. This area can be approximated with the rectangle rule and one may then calculate a ΔJ L −of 13.7 mA/cm 2  and a ΔJ L +of 56.52 mA/cm 2 . This gives a net photo-generated current increase of ΔJ L =4.29 mA/cm 2 . According to Partain, the photo generated current equation given above can be used to calculate the maximum short circuit current of a Shockley n/p diode, and so the 4.29 mA/cm 2  result gives the maximum increase in short circuit current above that predicted for AM0 spectral irradiance assuming a semiconductor with a band gap less then about 1.9 eV. Assuming that the maximum short circuit current for silicon is about 54 mA/cm 2 , a ΔJ L =4.29 mA/cm 2  gives a maximum increase in efficiency of about 8% from the use of the 1260 phosphor.  
         [0047]    The equations above show how the phosphors absorb in UV to blue and emit at green and red wavelengths. The photo-generated current is proportional to the integral of wavelength times a wavelength dependent distribution function. The quantum efficiency (QE) of amorphous silicon is greater in the green and red bands than in UV to blue bands. So for the same total photon energy, there is an increase in photo-generated,current at long wavelengths: for every electron generated by a blue photon, ˜1.1 electrons are generated by a green photon and ˜1.15 electrons are generated by a red photon, neglecting the increase in photo-generated current due to enhanced QE at green and red wavelengths. As already explained, the preferred embodiment includes an inorganic luminescent material in a suspension with an isotropic scattering medium, preferably barium sulfate. Since phosphors in the heat transfer liquid  44  absorb more visible light than just the heat transfer liquid  44  alone, the phosphor concentration in transfer fluid  44  must be optimized in order to absorb all of the available shorter wavelength radiation and re-emit longer wavelength radiation. For example, suppose N ph andN ph   Sat  are the concentration of phosphor given and the concentration of phosphor required to absorb the entire 400 nm light content in solar radiation. Assuming  N     ph     ≦N   ph   Sat , then the phosphor absorbs according to the spectral distribution function  
           φ   ab          (   λ   )       =         φ   amo          (   λ   )            (       N     p                 h         N     p                 h     Sat       )                   -       (     λ   -     λ   ab       )     2       σ       .                             
 
         [0048]    The phosphor fluoresces according to a spectral distribution function such that  
         φ fl   * (λ)=φ fl (λ)−φ w (λ).  
         [0049]    The term  φ     fl     (λ)  is for 100% QE and is given by the distribution function  
           φ   fl          (   λ   )       =         φ   amo          (   λ   )            (       N     p                 h         N     p                 h     Sat       )                     -       (     λ   -     λ   ab       )     2       σ            (            -       (     λ   -     λ   fl       )     2         σ   ′         )       .                             
 
         [0050]    The term  φ     w     (λ)  is a correction that accounts for the fraction of light energy absorbed that is not fluoresced and is given by the distribution function  
         φ w =φ amo ( l−e   −k(λ)N     ph   ),  
         [0051]    where k(λ) is related to a silicon absorption coefficient. The modified spectral distribution due to the phosphor can therefore be written:  
           φ   mod          (   λ   )       =         φ   amo          (   λ   )       -         φ   amo          (   λ   )            (       N     p                 h         N     p                 h     Sat       )                 -       (     λ   -     λ   ab       )     2       σ         +       φ   amo          (     1   -            -     k        (   λ   )              N     p                 h             )       +         φ   amo          (   λ   )            (       N     p                 h         N     p                 h     Sat       )                     -       (     λ   -     λ   ab       )     2       σ            (            -       (     λ   -     λ   fl       )     2         σ   ′         )       .                               
 
         [0052]    Since  J     PH     ≈q/hc∫λφ(λ)dλ , in order to break even one must have  
         ∫λφ amo (λ) dλ=∫λφ   mod (λ) dλ   
         [0053]    which means that  
         ∫         λφ   amo          (   λ   )            (       N     p                 h         N     p                 h     Sat       )                 -       (     λ   -     λ   ab       )     2       σ               λ         =       ∫         λφ   amo          (   λ   )            (       N     p                 h         N     p                 h     Sat       )                   -       (     λ   -     λ   ab       )     2       σ            (            -       (     λ   -     λ   fl       )     2         σ   ′         )               λ         +     ∫         λφ   amo          (     1   -            -     k        (   λ   )              N     p                 h             )                            λ     .                                 
 
         [0054]    Referring again to FIG. 2, solar cell array  36  may in the preferred embodiment be coated with fluorescent anti-reflective coating (FARC)  38 . FARC  38  has two purposes. The first purpose is to prevent the loss of efficiency due to reflective losses at the point of light entry into solar cell array  36 . For this purpose, the refractive index of FARC  38  is determined by the refractive index of the protecting coating  40  that is deposited over FARC  38 . Protective coating  40  serves to prevent damage to the solar cell array  36  by transfer fluid  44 . Protective coating  40  must therefore have the property that it does not react with transfer fluid  44 . A number of such coating materials are known in the art, but a thin layer of silicon dioxide (glass) is used in the preferred embodiment.  
         [0055]    The second purpose of FARC  38  is to further increase the photo-generated current from solar cell array  36  by evolving longer wavelength radiation from shorter wavelengths. In the preferred embodiment, a thin film of ZnS:Cu,AI is used to form FARC  38 . A 1.33 μm thick layer of this material may be resistively evaporated at 10 −5  torr from a graphite crucible onto glass substrates. It has been experimentally determined that the emission band of this resistively deposited film corresponds to the emission band of the commercial ZnS type phosphor as described above. It has also been determined that ZnS and ZnS:Cu,AI have essentially identical refractive indices. Absorption coefficient (α) vs. wavelength (λ) was obtained for the ZnS and ZnS:Cu,AI samples by dividing the thickness of each film into its spectrophotometer data. The α vs. λ curves were applied as a filter to the AMO spectral distribution to generate modified AMO distributions. On a wavelength band of 325 nm to 625 nm, ∫λφmod(λ)dλ for ZnS:Cu,AI was found to be 2% larger than for ZnS, thus confirming the fluorescing nature of the material. This fluorescence results in a corresponding increase in efficiency generated through the use of ZnS:Cu, Al instead of ZnS in an anti-reflective coating.  
         [0056]    Although the preferred embodiment of the invention has been described for use with a solar concentrator  10 , other embodiments may be used with no concentrator. In these embodiments, window  42  would simply be placed toward the incident light, or toward a mirror, prism, or other means that directs light to the present invention. As explained above, the optimum mix of isotropic scattering medium and phosphor must be selected given the light concentration factor used with respect to the invention; however, the present invention allows for easy adjustment to light concentration by simply modifying this mix.  
         [0057]    The present invention has been described with reference to certain preferred and alternative embodiments that are intended to be exemplary only and not limiting to the full scope of the present invention as set forth in the appended claims.