Abstract:
A cooling system for PV cells includes an evaporator configured to thermally contact the PV cells and transfer heat generated thereby to coolant in the evaporator, a condenser for receiving vaporized coolant from the evaporator and condensing the coolant to a liquid state, tubing connecting the evaporator, and the condenser in a circuit, a compressor arranged in the circuit for pumping coolant therethrough in a coolant flow direction, an active charge control apparatus arranged in the circuit between, in the coolant flow direction, the evaporator and the condenser, and a liquid flow control apparatus arranged in the circuit between, in the coolant flow direction, the condenser and the evaporator. The active charge control apparatus and the liquid flow apparatus cooperate to maintain the evaporator completely wetted by coolant and prevent coolant in the liquid state from leaving the evaporator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/521,071, filed on Aug. 8, 2011, the contents of which are herein incorporated by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the cooling of photovoltaic (PV) cells, and more particularly, to systems and methods for circulating a coolant for this purpose. 
       BACKGROUND OF THE INVENTION 
       [0003]    Various systems and methods are known for cooling PV cells during operation. In some instances—for example in systems using water or glycol or other liquids as a coolant—the coolant will warm up as it traverses the panels of PV cells, therefore providing greater cooling near the entrance into the panel assembly, and less cooling near the exit of the panel assembly. 
       SUMMARY OF THE INVENTION 
       [0004]    In view of the foregoing, it is an object of the present invention to provide improved systems and methods for cooling PV cells. According to an embodiment of the present invention, a cooling system for PV cells includes an evaporator configured to thermally contact the PV cells and transfer heat generated thereby to coolant in the evaporator, a condenser for receiving vaporized coolant from the evaporator and condensing the coolant to a liquid state, tubing connecting the evaporator, and the condenser in a circuit, a compressor arranged in the circuit for pumping coolant therethrough in a coolant flow direction, an active charge control apparatus arranged in the circuit between, in the coolant flow direction, the evaporator and the condenser, and a liquid flow control apparatus arranged in the circuit between, in the coolant flow direction, the condenser and the evaporator. The active charge control apparatus and the liquid flow apparatus cooperate to maintain the evaporator completely wetted by coolant and prevent coolant in the liquid state from leaving the evaporator. 
         [0005]    According to a method aspect, a method for cooling PV cells to increase their efficiency, and for capturing the waste heat from the PV cells, includes placing an evaporator in thermal contact with the PV cells and circulating coolant through the evaporator to remove waste heat therefrom. Circulating coolant through the evaporator includes maintaining the evaporator in a wetted state with substantially no coolant superheating. 
         [0006]    These and other objects, aspects and advantages of the present invention will be better appreciated in view of the drawings and following detailed description of preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram of a photovoltaic (PV) cell cooling system, according to an embodiment of the present invention, including an active charge control (ACC) apparatus, a liquid flow control (LFC) apparatus, and an evaporator; 
           [0008]      FIG. 2  is a sectional side view of the ACC apparatus of  FIG. 1 ; 
           [0009]      FIG. 3  is a sectional side view of the LFC apparatus of  FIG. 1 ; 
           [0010]      FIG. 4  is a sectional side view of an alternate to the evaporator of  FIG. 1 , according to another embodiment of the present invention; 
           [0011]      FIG. 5  is a partially sectioned bottom view of another alternate to the evaporator of  FIG. 1 , according to a further embodiment of the present invention; 
           [0012]      FIG. 6  is a sectional view taken along line C-C of  FIG. 5 ; 
           [0013]      FIG. 7  is a partially sectioned bottom view of an additional alternate to the evaporator of  FIG. 1 , according to an additional embodiment of the present invention; 
           [0014]      FIG. 8  is a sectional view taken along line A-A of  FIG. 5 ; 
           [0015]      FIG. 9  is a partially sectioned bottom view of a further alternate to the evaporator of  FIG. 1 , according to a further embodiment of the present invention; 
           [0016]      FIG. 10  is a sectional view taken along line B-B of  FIG. 5 ; 
           [0017]      FIG. 11  is a partially sectioned side view of an alternate to the LFC apparatus of  FIG. 1 , including a cylinder  103 ; and 
           [0018]      FIG. 12  is an end view of the cylinder  103  of  FIG. 11 , showing an inlet thereinto. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    Referring to  FIG. 1 , according to an embodiment of the present invention, a PV cell cooling system  10  includes a compressor  11  that pumps refrigerant vapor from outlet  12  to a condenser  13  (flow direction of coolant represented by unlabeled arrows), where the hot refrigerant is cooled by the condenser  13 , thereby delivering heat energy into the fluid  14  in tank  28 . The refrigerant, being cooled by the fluid in tank  28 , condenses to a liquid state within Condenser  13 , and exits the condenser at outlet  15 . Conduits connect the system  10  components into a complete circuit. The refrigerant is delivered to a liquid flow control (LFC) apparatus  17  via conduit  16 . The refrigerant leaving the LFC apparatus is delivered to the inlet  19  of the evaporator  20  by way of conduit  18 . The liquid refrigerant then contacts an evaporator wall  21  as it moves upward through an evaporator space  22 . As the refrigerant evaporates in space  22 , it absorbs heat from the wall  21  that is in physical and thermal contact with PV support member  27 , and thereby cools the PV cells  26 , which are bonded to, and in thermal contact with, support member  27 . The refrigerant moving up through the evaporator space  22  is forced by an active charge control (ACC) apparatus  24 , working in conjunction with LFC apparatus  17 , to finish evaporating and be in vapor form only as it leaves the evaporator space  22  and proceeds via conduit  23  to the ACC apparatus  24 . This is accomplished by the LFC  17  holding a fixed amount of sub-cooling in the condenser  13  while the ACC apparatus allows only vapor to leave the ACC apparatus, and more preferably only vapor that is not superheated. 
         [0020]    Referring to  FIG. 2 , the ACC apparatus  24  receives refrigerant vapor at entrance  81 . The refrigerant proceeds past venturi  85  which entrains liquid refrigerant into evaporator tube  82 . When the vapor/liquid exits tube  82  and impinges on deflector disc  86 , the liquid falls back into the liquid pool at liquid level  84 , while the lighter vapor flows past the deflector disc to leave the ACC apparatus  24  at outlet  87 . Thus the ACC apparatus  24  effectively separates the vaporized refrigerant from the liquid refrigerant, and only saturated vapor (not superheated) leaves the ACC apparatus  24 . The temperature of the contents of the ACC tank  88  is determined by the amount of suction pressure at the compressor inlet. Therefore if superheating of the refrigerant starts to occur in the evaporator  20 , the superheated vapor going through the evaporator tube  82  will be warmer than the liquid reserve in the ACC apparatus  24 , and will evaporate refrigerant from the reserve of liquid in ACC apparatus  24 , and place more refrigerant into active circulation through the whole refrigerant circuit. This additional refrigerant will further “flood” or “wet” the evaporator  20  until all superheating is eliminated. Conversely, if too much refrigerant is in circulation, some amount of liquid will pass through and out of evaporator space  22 , but when any liquid reaches the ACC apparatus  24 , it simply falls back into the reserve pool to eliminate the excess of refrigerant in active circulation. Thus the ACC apparatus ensures that the space  22  in the evaporator  20  is constantly “flooded” or “wetted,” and therefore superheating does not occur in the evaporator  20 . 
         [0021]    The ACC apparatus  24  and LFC apparatus  17  operate together to ensure the entire evaporator space  22  has liquid refrigerant present throughout the evaporator  20 , from entrance  19  to the outlet  23 , and therefore heat is uniformly absorbed from wall  21  of the evaporator  20 , with the result that the cooling of the PV support member  27 , and in turn the cooling of the PV cells  26 , is uniform throughout the panel of PV cells. The refrigerant passes from the ACC apparatus  24  and on to the compressor  11  to repeat the cycle again. 
         [0022]    Referring to  FIG. 3 , extensive testing has shown that a refrigerant circuit is more efficient if subcooling in the condenser is held to a low value, preferably in the range of 1 to 8 degrees Fahrenheit, with the optimum amount of subcooling being about 4 degrees Fahrenheit. The LFC apparatus  17  (a subcool control valve (SCV), in the  FIG. 3  embodiment) is designed to achieve that purpose when used in concert with ACC apparatus  24 . In the SCV  17 , liquid refrigerant from the condenser  13  flows in at inlet tube  61 , and on and upward between outer tube  62  and outlet tube  63 , and onward to main orifice  65  and side orifice  66  in orifice plug  68 . The liquid is metered at the orifices  65  and  66 , and after metering and expanding, leaves the valve via outlet tube  63 . Dome  67  and diaphragm  66  form a sealed cavity  72  which contains a controlling liquid refrigerant  70 . The controlled fluid, represented by the flow arrows, makes physical and thermal contact with diaphragm  66 , which in turn requires the fluid  70  to assume approximately the same temperature and pressure as controlled fluid flowing to the orifices. 
         [0023]    If the incoming controlled fluid is overly subcooled, the controlling fluid  70  will also become more subcooled which reduces the pressure on top of the diaphragm  66 , causing the diaphragm to flex upward, thereby opening the main orifice, and increasing the flow of the controlled fluid from the condenser  13  and reducing the amount of subcooling. Conversely, if the subcooling becomes too little, the pressure above the diaphragm will decrease and increase the amount of subcooling, all with the result that a predetermined amount of subcooling is maintained in the condenser. The thickness and flexibility of the diaphragm  66  are the primary factors that determine the amount of subcooling that results. The side orifice  73  provides a minimum flow and prevents instability and a possible unintentional shutdown of the system. Note that the diaphragm stop  71  serves to prevent overstressing the diaphragm under various unusual conditions. The LFC apparatus of  FIG. 3  has an additional feature and advantage, in that a “built in” heat exchanger is formed by outer tube  62  and outlet tube  63 , wherein the metered, expanded, and chilled refrigerant leaving through outlet tube  63  makes thermal contact with the incoming and un-metered liquid inside outer tube  62  via the wall of outlet tube  63 , to thereby provide inverse thermal feedback to the incoming liquid, for stabilizing the LFC apparatus. 
         [0024]    Advantageously, the present method uses a refrigerant in connection with PV cell cooling. As used herein, a refrigerant is a specific type of coolant which evaporates as it traverses through an evaporator. Thus, while a “coolant” could be any fluid capable transferring heat away from a heat source, a “refrigerant” is more specifically a coolant that will undergo a phase change while doing so. In other words, as the terms are used herein, all refrigerants are coolants, but all coolants are not necessarily refrigerants. By keeping the evaporator wetted with refrigerant at its vaporization point, an even temperature profile will be experienced by the PV cells being cooled. With this in mind, water and glycol are not preferred coolants for many PV cell applications as they will remain subcooled liquids under expected operational conditions. 
         [0025]    As will be appreciated from the foregoing, the refrigerant circuit which drives the refrigerant through the evaporator is designed to continuously supply the required amount of refrigerant to the evaporator that will result in the “wetting” or “flooding” of the entire evaporator. When an evaporator is flooded, there is minimal difference in temperature of the evaporator from the entrance end to the exit end of the evaporator. Flooding the evaporator eliminates any superheating of the refrigerant within the evaporator. Eliminating superheating results in a more uniform cooling of the PV panel. Overheating of even one PV module in multiple modules connected in series can result in limiting the output power of the series of modules, and could result in damage to the overheated module. The uniform cooling provided by the present invention prevents such results. Eliminating superheating also results in a higher “suction pressure” and a cooler, more dense refrigerant vapor at the compressor entrance. These two factors result in more refrigerant pumped per stroke of the compressor, and improvement of efficiency of the refrigerant circuit. 
         [0026]    The heat removed from the PV panel(s) may typically be delivered to a condenser for useful heating of water, or to the condenser of an air handler for the heating of air. The condenser may be used in other heating applications. This arrangement allows for the more effective recapture of what would otherwise be waste heat generated by the PV panels. Thus, the PV cell cooling system is not only able to enhance efficiency of the PV cells—thereby reducing electricity that needs to be supplied by other (potentially less “green”) sources, it can also simultaneously recapture more waste heat for other applications that would otherwise require another (again, potentially less “green”) heat source. 
         [0027]    The above described embodiment is presented for exemplary and illustrative purposes; the present invention is not necessarily limited thereto. For example, referring to  FIG. 4 , the structure therein replaces the structure of  19 ,  20 ,  21 ,  22 ,  26 , and  27  in  FIG. 1 , with like reference numerals (followed by an “A”) referring to analogous elements. Support member  27 A serves multiple purposes. For example, it forms one wall  21 A of the evaporator chamber  22 A, and secondly it serves as the support member  27 A for the PV cells  26 A, thereby eliminating the need for a separate wall member  21 , and reducing the complexity, weight, and cost of the assembly, while improving the thermal conductivity between the evaporating refrigerant and the PV cells  26 A. Thus, as the refrigerant circuit absorbs unwanted heat from the PV cells, it delivers it as useful heat to the condenser in tank  28 , or to any condenser for delivering useful heat. 
         [0028]    In other examples,  FIGS. 5-10  show various alternative structures that may be used for PV cooling, as alternatives to the simplified evaporators  20 ,  20 A as shown in the refrigerant circuit of  FIGS. 1 and 1A . 
         [0029]    Referring to  FIGS. 5 and 6 , a cooled panel A includes serpentine tubing  32  attached to tubing support member  30  by thermally conductive bonding material  34 . The tubing support member  30  is in physical and thermal contact with PV cells  31 . The refrigerant evaporates as it flows through the tubing  32  from inlet  36  through passage  35  to outlet  37 , and tubing support member  30  is cooled, which in turn cools PV cells  31 . Tubing support member  30  is made such that it supports the PV cells, and the support member  27  in  FIG. 1  is eliminated. 
         [0030]    Referring to  FIGS. 7 and 8 , a cooled panel B includes a corrugated member  42  joined to panel support member  40  by seam welds  43  thereby forming fluid passages  44  such that a refrigerant flowing through the passages from entrance  45  to exit  46  extracts heat from member  40  and in turn from PV cells  41 . Support member  40  also serves as one wall of the cooled panel B. 
         [0031]    Referring to  FIGS. 9 and 10 , a cooled panel C uses staggered seam-welds  53  to attach corrugated member  52  to panel support member  50  thereby forming a serpentine pathway  54  where the coolant enters at inlet  55  and after traversing the pathway exits the panel at outlet  56 , thereby cooling PV cells  51 . Support member  50  also serves as one wall of the cooled panel C. 
         [0032]    In a further alternative, referring to  FIGS. 11 and 12 , a different LFC apparatus—LFC apparatus  17 A—is used. The LFC apparatus  17 A is used to hold a fixed amount of subcooling in the condenser  13 , thereby cooperating with the ACC  24 , but that amount of subcooling is normally zero, because a small trickle of uncondensed vapor is required to arrive at the LFC  17 A to operate the float  107 . The body of the LFC  17 A in  FIG. 7  is a cylinder  103  which is enclosed with end caps  111 , thus providing a closed chamber for float  107 . Liquid refrigerant enters the chamber at entrance tube  113 , and inlet  112 . Cylinder  103  fills with liquid to a height determined by the amount of vapor refrigerant arriving with the liquid. The float  107  is attached to metering segment  116  by float attachment  110  and attaching rod  109 , thereby making the metering segment  116  swivel on swivel pin  102 . The metering segment  116  being otherwise generally circular, is flat at segment  116 . When little or no vapor arrives at valve  117 , the cylinder  103  fills with liquid, indicating that liquid is backing up in the condenser  113 . As cylinder  103  fills with liquid, float  107  rises. 
         [0033]    Raising the float  107  presents the flat segment  116  to the valve orifice which is centered in valve plug  105 , which in effect opens the LFC apparatus  17 A to release liquid refrigerant from condenser  13 . When all liquid is released from condenser  13 , vapor arrives in cylinder  103  and the float  107  is forced downward by the vapor rising to the top of cylinder  103 , which closes the valve to require more complete condensing of the refrigerant within the condenser. Equilibrium is reached when just a small trickle of bubbles (vapor) arrives from the condenser, and just enough of the flat on the metering segment  116  is presented to the outlet orifice of LFC apparatus  17 A to maintain zero subcooling in condenser  13 . After the refrigerant is metered and expanded in metering plug  105 , it proceeds through outlet tube  101  and  106  which is formed to make thermal contact with cylinder  103  using a silver braze  114 , thus LFC apparatus  17 A. The metered and expanded refrigerant exits the LFC apparatus  17  at outlet  115 . 
         [0034]    The foregoing is not intended to be an exhaustive list of alternatives. Rather, those skilled in the art will appreciate that these and other modification, as well as adaptations to particular circumstances, will fall within the scope of the invention as herein shown and described and of the claims appended hereto.