Patent Publication Number: US-2009223511-A1

Title: Unglazed photovoltaic and thermal apparatus and method

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of solar energy panels. More specifically, the present invention relates to an improved hybrid thermal and photovoltaic panel that can be used for nighttime thermal dissipation as well as a method for converting photovoltaic panels into a hybrid system. 
     BACKGROUND OF THE INVENTION 
     Devices for collecting and converting solar energy are very useful because they are capable of generating energy without consuming costly fuel or creating noxious emissions. Photons in the visible range constituting the greater portion of solar energy are a high quality energy source, in that they can be readily converted into other forms of energy at useful levels of efficiency. Two well-known devices for effectively converting solar energy are photovoltaic or “PV” arrays and solar thermal collectors. Photovoltaic arrays consist of a plurality of photovoltaic cells that convert solar energy directly into electricity. Solar thermal collectors convert solar energy into thermal energy, and are primarily used for water heating or as part of a building&#39;s heating system. One of the primary advantages of both of these devices is that they can be successfully implemented in a small scale, allowing an individual homeowner to implement one or both types of solar energy conversion. 
     A photovoltaic array typically works at no more than about 20 percent efficiency in generating electricity, with most commercially available PV arrays operating at efficiencies between 14 and 17 percent. The majority of the solar energy incident upon a PV array goes to waste in the form of heat. This heat is radiated back into the atmosphere, removed by convection into ambient air, or conducted away by the supporting structure. Therefore, numerous attempts have been made to harvest the heat and convey it elsewhere for use rather than have it go to waste. When thermal capabilities are combined with photovoltaic capabilities, the collector units are known as hybrid PV/thermal collectors or “PV/T” collectors. These types of collectors are well known in the art but have heretofore not been able to match the performance of PV and thermal collectors separately optimized for their applications. Thus, most installations where both thermal energy and electricity are needed at a single site employ separate collectors. 
     Most existing PV/T systems share the characteristic that they have been designed for heat collection at relatively high temperature heat (e.g., 120-160 degrees F.). To reach this temperature, they employ an insulative glazing. Glazing in this context refers to a sheet of glass or plastic or other transparent sheet that is highly transmissive of visible light, and is non-transmissive to longwave infrared (IR). A glazing typically forms the outer, skyward facing surface of the sealed enclosure containing the PV array. A glazing that is merely protective may be in thermal contact with the PV array. However, where there is an insulative glazing, a gap is left between the PV array and the transparent sheet. This gap is preferably filled with a gas, such as one of the inert gases, but could potentially be a vacuum. The insulative glazing allows solar energy to enter the enclosure and fall upon the PV array where it is converted to either electricity or heat. At the same time the insulative glazing materially reduces the escape of the generated heat into the atmosphere. This, of course, raises the temperature inside the enclosure, which in turn improves the ability of the PV/T panel to convert the heat into usable thermal energy. 
     U.S. Pat. Nos. 4,392,008, 4,493,940, 4,587,376, and 6,018,123 represent various embodiments of PV/T collectors that capture thermal energy by creating a thermal path from a PV cell to a heat transfer fluid. They are remarkably similar in objective, if modestly diverse in detail. All incorporate a sealed enclosure with an insulative glazing into their design. 
     Two PV/T systems without insulative glazing are described in prior art. U.S. Pat. No. 6,630,622 B2 describes the use of a copper plate, a copper-filled epoxy and copper tubing to create a thermal pathway between the PV array and heat transfer fluid circulating in the tubing. The specified use of copper exacts a substantial cost in price of the material, weight added to the system and complexity of fabrication (soldering, brazing, etc). In addition, the Fresnel lens used in this system is separated from the PV array by a gap. Since this gap is not part of a sealed system, the lens is not technically an insulative glazing as outside air is allowed to flow between the lens and the PV array. Nonetheless, this transparent layer is not in thermal contact with the PV array, and therefore this layer substantially hinders the outward flow of long-wave infrared radiation from the PV array. U.S. Pat. Application Publication 2004/0025931 describes a system consisting of a fluid-filled chamber behind the PV array and in thermal contact with it by means of a steel heat exchanger. Fluid is guided by partitions through the chamber in a serpentine path from an inlet to an outlet. The fluid partitions make this type of array difficult to construct and maintain when compared to a tubing system. 
     There remains an unmet need in the art for a robust apparatus that can be easily assembled from inexpensive, readily available components, and that efficiently transfers thermal energy to and from a heat transfer fluid. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a combination photovoltaic and thermal solar energy panel. This PV/T panel improves upon the prior art by recognizing the need to eliminate insulative glazing from the panel. The absence of insulative glazing allows the photovoltaic cells to operate at a lower temperature and therefore at a higher efficiency. In addition, the absence of insulative glazing or any other transparent layer that is not in thermal contact with the PV array allows the present invention PV/T panel to be used to provide nighttime cooling for a building. 
     The absence of the insulative glazing is efficiently combined in the present invention with a unique construction process. A standard PV panel is converted into a PV/T panel by adhering an aluminum heat transfer plate to the rear of the PV array using a silicone adhesive. This heat-conductive adhesive assures effective heat transfer between the PV array and the heat transfer plate. PEX (cross-linked polyethylene) tubing is then inserted into channels integrally formed into the back of the heat transfer plate. By again using silicone or other heat-conductive compound between the PEX tubing and the heat transfer plate channels, the present invention assures low heat resistance as heat is effectively transferred from the PV array through the heat transfer plate and the PEX tubing into the heat transfer fluid running through the PEX tubing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a simplified water heating application using a PV/T panel of the present invention. 
         FIG. 2  is a schematic diagram of a simplified space heating application using the PV/T panel of  FIG. 1 . 
         FIG. 3  is a flow chart of the process for converting a non-glazed PV panel into a non-glazed PV/T panel of the present invention. 
         FIG. 4  is a bottom plan view of the combined PV/T panel of the present invention with the bottom panel and insulation removed to show the heat transfer plates and tubing in place constructed according to the present invention. 
         FIG. 5  is an enlarged fragmentary cross-sectional view of the apparatus taken substantially on line  5  shown in  FIG. 1 . 
         FIG. 6  is a schematic diagram of energy flows using the present invention to both receive and radiate solar thermal energy. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Recognition of the Problem 
     Prior art PV/T systems work by using an insulative glazing to retain heat within the panel. This heat retention allows the heat transfer fluid that removes heat from the panel to reach a higher temperature. This in turn extends the range of potential applications and allows heat to be carried away from the panel at lower fluid flow rates. However, as recognized by the applicant, the use of higher working temperatures in the PV/T panel comes at a significant cost. The differential coefficients of thermal expansion of insulative glazing and frame require intricate provisions to maintain sealing integrity under the wide temperature variations encountered. Evacuation of gas from the enclosure or instillation of an inert gas is also usually required to prevent PV deterioration. These measures add to the weight, complexity, cost and maintenance requirements of glazed PV/T systems, thus decreasing their practical application while also compromising the electric conversion efficiency. Finally, the presence of an insulative glazing eliminates the opportunity to efficiently radiate heat outward from the PV array for cooling purposes, since glazing is non-transmissive to longwave IR. 
     An additional benefit to eliminating the insulative glazing from PV/T panels is that the temperature of the panel during electrical generation is reduced. This reduced temperature materially increases the efficiency of the photovoltaic cells that are converting sunlight into electricity. Prior art PV/T panels use insulative glazing to increase the efficiency of the thermal collection, but this improved efficiency comes at the cost of reducing the efficiency of the photovoltaic cells. 
     Overview of the System 
       FIG. 1  show a schematic view of a residential system using the PV/T panel or collector  100  of the present invention for electrical generation and water heating.  FIG. 1  does not show several elements that would be required for practical implementation of the present invention, such as the valves that switch among the various modes of operation or the backup boiler that provides heat on cloudy days. These elements are well known in the art, and are not directly relevant to the inventive aspect of the present invention. The PV/T panel  100  receives solar energy from the sun (not shown). Photovoltaic cells  110  which form part of the photovoltaic array  140  (not shown in  FIG. 1 ) convert light energy into electricity, which is then extracted from the panel  100  through wires  112 . The wires are preferably attached to an electric junction secured within the panel  100 . The electricity can be used immediately at the location of the panel  100 , stored in an electric storage system, or sent into an electrical power grid. These options are jointly represented in  FIG. 1  as electrical load or storage  200 . 
     In addition to the generation of electricity, the PV/T panel  100  is capable of generating useful heat energy. As explained in further detail below, this is accomplished by passing a heat transfer fluid through a fluid system that transfers heat energy in the panel to the transfer fluid. This transfer fluid is preferably of a chemical composition that will not freeze within the expected temperature range of the environment of the panel  100 . The anti-freeze heat transfer fluid in the system of  FIG. 1  passes through the transfer fluid tube system  300 . The transfer fluid tube system  300  includes an internal heat exchange system or heat collector portion  130  within the PV/T panel  100 , which is described in more detail below. Cool transfer fluid enters this system  130  at input  132 , receives heat from the panel  100  in serpentine section  134 , and exits the panel heated at exit  136 . In the preferred embodiment, the input  132  and output  136  are fitted with appropriate plumbing fixtures to allow secure attachment of tubing or piping to the heat exchange system  130  of the panel  100 . 
     Although section  134  is shown as a serpentine in shape in  FIG. 1 , other configurations of the heat collector portion  130  within the PV/T panel  100  are also possible. For example, it would be possible to lay multiple sections of tubing in parallel, with each of these sections connected to a source tube on one end and a discharge tube on the other end. Heat transfer fluid would then enter the heat collector portion  130  at input  132 , flow from the input tube simultaneously through the parallel tubing sections, and be collected at the output tube to leave the panel  100  through output  136 . 
     A pump that forms part of the circulator  310  effects the movement of fluid through system  300 . Heat transfer fluid passes in a closed loop from the circulator  310 , through the internal heat exchange system  130  in the panel  100 , on to an internal heat exchanger  410  within a water tank  400 , and back to the circulator  310 . The circulator  310  operates when the temperature of the panel  100  exceeds the temperature of the water in the hot water tank  400 . The temperature of the heat transfer fluid rises as it passes through the panel  100  and falls as it gives up its heat to the tank  400 . A differential temperature controller  500  uses signals from temperature probes in the panel  502  and the tank  504  to determine when to send a signal to the circulator controller  506  informing the circulator  310  when to start and stop the pump. If the hot water in the tank reaches a maximum desired temperature but the panel is still yielding heat, the heat transfer fluid is diverted to a heat disposal mechanism, such as an outdoor fountain. 
       FIG. 2  is similar to  FIG. 1 , except that it shows a residential system that uses the present invention for electric generation and space heating and cooling. In this Figure, the PV cells  110  provide electricity to the electrical load or storage  200  just as in  FIG. 1 . The difference from  FIG. 1  is that the heating capabilities of the panel  100  are being used to heat a concrete slab  600  or other thermal mass within a dwelling or other building. The circulator  310  operates to bring heat to the slab  600  whenever the temperature controller  500  indicates that the temperature of the panel  100  exceeds the effective temperature of the slab  600 . Heat transfer fluid passes in a closed loop from the circulator  310  through the tubing in the collector  130 , on through the tubing loops in the slab  600 , and back to the circulator  310 . The temperature of the heat transfer fluid rises as it passes through the heat collector  130  and falls as it gives up its heat to the slab  600 . As described above, the temperature controller  500  uses signals from temperature probes in the slab  600  and the collector  130  to determine when to start and stop the motor in circulator  310 . In addition, a thermostat  508  on the temperature controller  500  can be located in the dwelling unit to control the amount of heat that is provided to the slab  600 . Although not shown in  FIG. 2 , many heating systems using this type of a layout will use a heat exchanger to separate the heat transfer fluid passing through the present invention PV/T panel  100  from ordinary water that is used to actually heat the slab  600 . 
     One of the primary benefits of the present invention is that, in addition to winter-time heating, the present invention panel  100  is ideally designed to allow for night sky radiant cooling in the summer. When used in this capacity, the circulator  310  operates when the temperature of the panel  100  falls below the effective temperature of the concrete slab  600 . Heat transfer fluid passes in a closed loop from the circulator  310  through the tubing in the collector  130 , on through the tubing path within the slab  600 , and back to the circulator  310 . The temperature of the heat transfer fluid falls as it passes through the collector and rises as it accepts heat from the slab. The thermostat  508  provides overall control over the circulation system  300  and the controller  310  to ensure that the dwelling is maintained at a comfortable temperature. 
     Construction 
     Another benefit of the present invention is that it can be implemented through a simple conversion process where a standard PV panel is converted into PV/T panel  100 . This process  700  is set forth in the flow chart of  FIG. 3 , which can be read in conjunction with the rear plan view of panel  100  in  FIG. 4  and the fragmentary cross-sectional view of  FIG. 5 .  FIG. 4  shows the panel  100  with the rear panel  170  and insulation  160  removed.  FIG. 5  is a cross sectional view of the panel  100  showing all of the layers of the panel  100  including the tubing  138 . One potential location for this cross-sectional view is along line  5  as indicated in the schematic diagram of  FIG. 1 . 
     A suitable PV panel for conversion is the Uni-Solar ES-62T framed solar module manufactured by United Solar Ovonic LLC, Auburn Hills, Mich. This PV module is 49.5 inches long by 31.2 inches wide and contains a photovoltaic layer or array  140  that is responsible for converting solar to electrical power. 
     The ES-62T panel has a rectangular black-anodized aluminum frame  102  that is 1.25 inches deep and open on the back side. Securely attached horizontally within the frame  102  on its front side is the PV array  140 . This array  140  forms the top cover of the panel  100 , with the light-accepting surface of the array  140  facing outward. The PV array  140  in the ES-62T PV panel consists of PV cells encapsulated in ETFE high-light-transmissive polymer (sold by Dupont under the trademark Tefzel®) that are in turn mounted on an 0.024 inch aluminum-zinc alloy coated steel (sold by BIEC International Inc. under the trademark Galvalume®) sheet backing. This transparent ETFE glazing is a protective glazing but does not constitute an insulative glazing because it is not separated from the PV array  140  by an air or vacuum gap. In consequence, heat is efficiently transmitted from the PV cells to the surface of the glazing, from where it is released by radiation or convection. Conversely, IR or convective heat incident upon the ETFE glazing is readily transmitted to the PV cells. 
     The first step  702  in the process  700  of converting a standard PV panel into the PV/T panel  100  of the present invention is to place the PV panel face down. This exposes the back surface of the PV layer  140 , which, in the context of the Uni-Solar ES-62T, consists of the Galvalume sheet. The next step  704  is to mount heat transfer plates  150  directly to the back of the PV layer  140 . In  FIG. 4 , four heat transfer plates  150  are shown. However, in the preferred embodiment, eight aluminum heat transfer plates  150  are used, with each plate  150  being 48 inches long and 3.5 inches wide. These transfer plates  150  are constructed with channels  152  running their entire lengths, with the channels  152  being specially constructed in order to fit ½ inch I.D. PEX (cross-linked polyethylene) tubing. This type of heat transfer plate is available commercially, with the preferred embodiment using Wirsbo-Uponor Joist-Trak® heat transfer plates. An approximately 0.002 inch layer of silicone adhesive  154  (e.g., GE Silicone II Clear®) is spread between the plates  150  and the PV layer  140  to bond the two components. The plates  150  are held in place on the Galvalume sheet  140  with clamping or weights until the bond matures. Small portions of heat transfer plate may need to be cut out to clear electrical connection terminals or allow bends in the tubing. 
     At step  706 , sections of ½ inch I.D. PEX tubing  138  (such as the commercially available Zurn PEX tubing) are cut to length to fit in heat transfer plate channels  152 . Tubing ends are connected with short lengths of PEX tubing and right angle connectors, to form a serpentine path  134 . The unconnected ends of each outermost tubing section exit the frame through holes drilled in the frame to the appropriate diameter and caulked with silicone. Alternatively tubing ends may exit the back panel  170 . The assembly of tubes is then snapped into the channels of the heat transfer plates after wetting them with a thin layer of silicone or other heat-conductive compound. Although PEX is the material of choice of tubing in the preferred embodiment, it would be possible to use other plastic tubing, especially as new and improved plastics appear in the marketplace. 
     The next step  708  is to install insulation  160  behind and between the PEX tubing  138 . In the preferred embodiment, half-inch thick polyisocyanurate insulation  162  is cut to size to fit between the channels  152  of adjacent heat transfer plates  150  and press-fit into place. A large sheet  164  of the same insulation is then placed over the previous layer of insulation  162 , tubing  138 , and channels  152 . The large sheet of insulation  164  is preferably flush with the edge of the frame  102 . An alternative method is to use spray-on polyurethane foam insulation (Froth-Pak from Dow Chemical) instead of the polyisocyanurate panels. 
     At step  710 , the back sheet metal panel  170  is placed over the frame and insulation. When installing this panel  170  to the back of the PV/T panel  100 , it is preferred that a weatherproofing gasket or seal is placed in place between the panel  170  and the frame  102  of the PV/T panel  100 . The panel  170  is preferably attached with a removable attachment mechanism such as screws or their equivalent. Alternatively, the aluminum foil covering of the large polyisocyanurate sheet can serve as the back panel  170 , and to employ a suitable caulking such as polyurethane foam (Great Stuff from Dow Chemical) or silicone (Silicone II from GE) for weatherproofing. When spray-on polyurethane foam is used for insulation, no back cover is required, since its closed-cell characteristic provides weatherproofing and rigidity. 
     Finally, at step  712 , it is necessary to ensure that the wires  112  that carry the electric current from the PV cells  110  in the PV layer  140  are routed out of the enclosure through holes in the back panel  170 . These holes are then protected with a grommet and sealed with silicone. When spray-in polyurethane is employed, the wires simply exit the polyurethane and may be anchored on the polyurethane surface with small plastic pads. 
     Thermal Radiant Cooling and Relevant Energy Fluxes 
     One of the primary distinctions of the present invention PV/T panel  100  is its ability to be used to not only allow thermal heating but also thermal cooling of dwelling spaces. To understand the operation of the PV/T panel  100  while cooling, it is useful to understand the relevant energy fluxes. The term flux refers to the amount of thermal or electromagnetic energy that flows through a unit area per unit time, and is represented by a vector. The energy fluxes involving the PV array  100  are depicted in the schematic flux representation  800  in  FIG. 6 . 
     Energy from the sun  810  is represented by solar energy flux  820 , which consists of UV, visible light and shortwave infrared radiation. When the sun  810  is shining, the solar energy  820  is absorbed by the PV array  140 , where it is transformed into thermal energy, which is to say kinetic energy of vibration of the molecules in the panel  100 . Some thermal energy goes along the fluid convection path  822  in the heat transfer fluid found in the heat transfer fluid system  300 , where it is put to use by heating water or heating a dwelling. The remainder of the solar energy  820  is lost from the surface of the PV array  140  as longwave infrared (IR) radiation  824  or by convection as increased kinetic energy of nearby air molecules  826 . Flux  828  is downwelling longwave IR radiation from the atmosphere, as will be subsequently explained. 
     In order to heat the thermal fluid required for the fluid convection path flux  822 , three thermal resistances  830 - 834  in series must be overcome. The first resistance R PV    830  is the resistance of the photovoltaic array  140 . The aluminum heat transfer plate is the second resistance, R PL    832 . The wall of the PEX tubing is the final resistance, R PEX    834 . Contact resistances due to microgaps between the tubing  138  and heat transfer plate channel  152  and between the heat transfer plate  150  and the PV array  140  are minimized by a very thin layer of silicone  154  used as an adhesive and heat transfer medium. Thermal resistance due to stagnant layers within the fluid itself is avoided by establishing a turbulent flow regime, achieved by sufficient fluid velocity to exceed the critical Reynolds number of ˜4000. 
     Conduction to the heat transfer fluid is expressed as 
         Q   cond   =U   cond ( T   r   −T   fl ), 
     where T r    840  is the temperature of the PV array  140 , and T fl    842  is the fluid temperature. T fl    842  is not constant but varies from T i  at the inlet to T o  at the outlet. For practical purposes, 
     
       
         
           
             
               
                 T 
                 fl 
               
               = 
               
                 
                   
                     T 
                     i 
                   
                   + 
                   
                     T 
                     o 
                   
                 
                 2 
               
             
             , 
           
         
       
     
     the average of T i  and T o . U cond  is the reciprocal of the sum of series resistances, 
     
       
         
           
             
               U 
               cond 
             
             = 
             
               
                 1 
                 
                   ( 
                   
                     
                       R 
                       PV 
                     
                     + 
                     
                       R 
                       PL 
                     
                     + 
                     
                       R 
                       PEX 
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
     Convection is expressed by the equation 
         Q   conv   =U   conv ( T   r   −T   a ), 
       where 
         U   Conv   =A   c   +B·V   z , 
     with V z  being wind velocity at the panel. Parameter A c  represents natural convection, that is, the component occurring in the absence of wind, while B reflects the forced convection component due to the wind. When air temperature  846  is below PV array temperature  840 , as is usual, convection cools the array. If the air temperature  846  is higher than the PV array temperature  840 , convection heats the PV array  140 . 
     Radiant energy leaving the PV array (flux  824 ) is expressed as 
       Q rad+ =σε r T r   4 , 
     where σ is the Stefan-Boltzmann constant, ε r  is the surface emissivity and T r    840  is absolute temperature of the array. A range of wavelengths is emitted as described by Planck&#39;s law with a peak temperature given by the Wein displacement law. At typical ambient temperatures, for example 70° F., terrestrial objects with high emissivity emit approximately 135 Btu per hour per ft 2  concentrated in the far infrared (IR) spectrum with a peak wavelength of around 10 microns. 
     When exposed to the open sky, the IR energy  824  emitted by the PV array  140  travels upward through the atmosphere, where it meets a variety of fates depending on its wavelength. The atmosphere is transparent to certain wavelengths, which pass through the atmosphere into space unattenuated. Other wavelengths are absorbed by molecules in the atmosphere, including especially CO 2  and H 2 O. The absorbed energy is eventually reradiated, some outward and ultimately into space. 
     The energy reradiated earthward from the atmosphere and absorbed by the PV array (flux  828 ) is expressed as 
       Q rad− =σε r T a   4 , 
     T sky    844  is a calculated equivalent sky temperature, defined by 
       T sky   4 =ε sky T a   4 , 
     where T a    846  is the absolute outdoor air temperature and ε sky  is an empirically determined emissivity factor found to depend strongly on atmospheric water content. A number of correlations for clear night sky emissivity have been reported, for example, one by Chen et al (1995) based on dew point temperature in degrees Celsius, T dp , 
       ε sky,clear =0.736+0.00577 *T   dp . 
     The presence of clouds is addressed with the cloudiness factor of Clark and Blanplied (1979), C a , where n is cloud cover expressed as a fraction from 0 (clear) to 1.0 (overcast), 
         C   a =1.000+0.0224 *n+ 0.0035 *n   2 +0.00028 *n   3 , 
     leading to the final formulation 
       ε sky =ε sky,clear C a . 
     Energy lost as infrared radiation and convection from the underside of the panel must be considered. In practical use, sufficient insulation is applied to the bottom so that its exchange with its environment is small compared to that between the top and the atmosphere, so losses from the back may be ignored. 
     The energy balance on the PV array is given by 
         Q   net   =Q   sun −σε r ( T   r   4   −T   sky   4 )− U   conv ( T   r   −T   a )− U   cond ( T   r   −T   fl ). 
     When Q net =0, the system is at equilibrium. When Q net  is positive, T r  rises, and when Q net  is negative, T r  falls, until equilibrium is reached. During disequilibrium, in addition to changing fluxes defined in the equation, heat is also stored in the thermal mass of the PV array as T r  rises or is withdrawn from it as T r  falls, 
       ΔQ PV =m PV c p ΔT r , 
     where m PV  is the mass of the array and c p  is its specific heat capacity. 
     Lack of Glazing 
     As described above, a glazing layer is a layer that exists above the PV array  140  that is effectively transparent to visible light and which blocks a significant portion of infrared radiation. This transparency need not be complete, but it should not significantly block the amount of useable light that is received by the PV array to be converted into electricity. Since the useful wavelengths for generating electricity in a PV array  140  are confined to the visible spectrum, useful glazings are at least eighty-five percent transmissive of visible light. Glazings that are less transparent would negatively affect the efficiency of the array. Consequently, for the purposes of this invention, a glazing is transparent if it allows eighty-five percent of visible light to pass through. All known transparent glazings that are sufficiently rigid for the structure of the PV panel  100  will block a significant portion of infrared radiation emanating from the PV array  140 . 
     The PV panel  100  of the present invention is purposefully designed without a glazing layer between the PV array  140  and the sun  810 . If a glazing layer were present, especially an insulative glazing with a sealed enclosure, the energy flux situation would be dramatically altered. The quantitative description with an insulative glazing present is considerably more complex but need not be considered here. Qualitatively, the glazing passes solar energy  820  but blocks and absorbs outgoing longwave IR  824  emitted from the PV array  140  and prevents convection  826  from directly interacting with the PV array  140 . The temperature of the PV array  140  rises until the temperature of the insulative glazing is high enough to give off as much by IR radiation and convection as is being received from the sun, less the amount  822  transferred to the heat transfer fluid. 
     These effects of an insulative glazing and sealed enclosure can be very beneficial in a purely thermal solar collector. However, in combined PV/T collector such as panel  100 , the higher temperature of the PV array  140  decreases the efficiency of electricity generation. Another undesired effect of the insulative glazing is that, at night, longwave IR radiation  824  from the array would be blocked and absorbed by the glazing. In addition, the heat loss through convection  826  is drastically altered. 
     Heating and Cooling without Glazing 
     During heating applications, significant amounts of thermal energy will flow to the heat transfer fluid only if the conductance along that pathway  830 - 834  is high relative to losses by radiation and convection. Meticulous attention has been paid to maximizing the conductance of the path to the heat transfer fluid (i.e., reducing resistances  830 - 834 ) in order that sufficient thermal energy is collected to justify the expense and effort of doing so. Highlighting that necessity is a key aspect of this invention. 
     Thermal analysis of the circuit indicated that a total series resistance R cond  of approximately 0.15 would be achievable, equivalent to a conductance U cond  of 7 Btu/ft 2 -° F. This value is compatible with reasonable efficiency for both heating and cooling applications. Prototypes have achieved values in this range. 
     The potential for space cooling achieved by radiating heat to the night sky has long been recognized. Cooling is accomplished by converting thermal energy into IR energy  824 , which is radiated away from the PV array. In operation, heat is brought to the PV/T module in the heat transfer fluid in system  300  from the space to be cooled. The heat then passes through the conductance pathway previously described to the PV array  140 . 
     Although there is downwelling IR  828  from the atmosphere absorbed by the PV array  140 , on balance, under most circumstances, more IR is radiated away  824  from the array than is received by it in return  828  from the atmosphere. Thus, the atmosphere can serve as a heat sink for night cooling using the PV/T panel  100 . 
     A number of regimes using night sky cooling to cool interior spaces have been designed and tested. Early work used the surface of roof-mounted ponds as the heat emitter. These methods often required elaborate systems to open shutters at night and close them during the day, limiting their practicality. Parker (2005) described a concept called Night Cool in which the metal roof of a house serves as a nocturnal thermal radiator. Air from the living space is circulated into the attic at night to thermally couple the roof to the living space by convection. Computer simulation suggested that quantitatively significant cooling could be obtained in this manner. A representative calculated cooling rate is 25 Btu/ft 2 -hr (75 W/m 2 ) for approximately 10 hours per night in the Southeastern U.S. A comparable analysis performed for the present invention PV/T panel  100  reveals that, under most nighttime conditions, the associated T sky    844  is sufficiently below the target minimum indoor temperature to yield quantitatively useful cooling, assuming 1) there is sufficiently high thermal conductance between the PV/T panel  100  and the indoor environment and 2) the area of the panel  100  is sufficient. These requirements can be met in practice. 
     In operation, heat is supplied to the array from the conditioned space through the heat transfer fluid system  300 , and an equilibrium temperature T e  is reached at which heat gain to the array  140  is just equal to heat loss to the atmosphere by radiation  824  and convection  826 . If T i  is the inlet fluid temperature, T o  is the outlet fluid temperature, T e  is the equilibrium array temperature, c p  is specific heat capacity of the fluid, A r  is the total array area and q s  is fluid mass flow rate, then the heat balance equation is 
       ( T   i   −T   o ) q   s   c   p =σε r ( T   e   4   −T   sky   4 ) A   r   +U   conv ( T   e   −T   a ) A   r . 
     The possibility of condensation on the PV array  140  points to a potential problem. The temperature of the panel  840  free in still air would be expected to drop below ambient air temperature  846  and could approach T sky    844 , limited only by convection  826 . Once T r    840  falls to T dp , the dew point temperature, condensation begins, and cooling by radiation  824  is then matched by heat gain from condensation of water on the surface of the array, at which point there is no further drop in T r    840 . For efficient operation, the equilibrium temperature T e  must remain at or above T dp , because otherwise the condensation could divert much of the cooling capacity. Under practical operating conditions, T e  is almost always above T dp , and thus condensation is not a practical concern. 
     The most efficient use of night cooling occurs when there is sufficient thermal mass to carry the cooling effect over into the daytime. One very practical thermal mass is a concrete slab foundation  600  (shown in  FIG. 2 ) into which tubing has been installed for radiant heating. The slab  600  gets cooled simply by reversing the direction of heat flow, so that the slab  600  supplies heat that is carried via the heat transfer fluid to the PV/T panel  100  to be emitted  824  to the sky. As the slab  600  cools below the temperature of the interior thermal masses such as the gypsum board and framing, they give up heat to the slab by radiation, and that heat is also transferred through fluid convention path  822  to the panel  100 . The temperature of the slab  600  and internal thermal masses thus lowered, they are capable of absorbing heat gain into the building for many hours of the following day, delaying—or on many days avoiding—the need for vapor compression cooling. 
     If there is insufficient thermal mass in the slab  600  to bank the cooling capacity, then its use is limited to cooling the indoor air in lieu of vapor compression cooling for the period during which T sky    824  is below the indoor temperature. If it is acceptable to set the thermostat below normal during the night, then that strategy may allow a modest degree of thermal storage in the house infrastructure—gypsum board, framing, etc.—in excess of the direct cooling. 
     The many features and advantages of the invention are apparent from the above description. Numerous modifications and variations will readily occur to those skilled in the art. Since such modifications are possible, the invention is not to be limited to the exact construction and operation illustrated and described. Rather, the present invention should be limited only by the following claims.