Abstract:
An integrated solar cell system applies energy created by a solar cell module. The integration system includes a solar cell module, a low-grade heat recovery means, and a process system. The low-grade heat recovery means recovers waste heat from the solar cell module and connects the solar cell module to the process system. The process system is powered at least partially by thermal energy derived from waste heat generated by the solar cell module.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Solar cells, or photovoltaic cells, have the ability to convert sunlight directly into electricity. Conventional solar cells are approximately 15 percent efficient in converting absorbed light into electricity. Concentrated photovoltaic cells have the ability to capture more of the electromagnetic spectrum and are thus more efficient, converting absorbed light into electricity at about 30 percent efficiency. The solar energy that is not converted to electricity is converted to heat that is subsequently discarded. Thus, more than 60 percent of the solar energy captured, in the form of heat, is wasted. Due to the small size and the high-energy absorption of the photovoltaic cells, the heat must be efficiently dissipated from the cells to prevent degradation or damage of the cells. One method of removing heat for both conventional and concentrated photovoltaic cells is to use liquid and air heat exchange devices or heat sinks. An integrated solar cell system that can capture the dissipated heat to use as thermal energy to power a process system connected to the solar cell system would be an environmentally friendly power alternative and greatly increase the overall efficiency of the integrated system.  
       BRIEF SUMMARY OF THE INVENTION  
       [0002]     An integrated solar cell system applies energy created by a solar cell module. The integration system includes a solar cell module, a low-grade heat recovery means, and a process system. The low-grade heat recovery means recovers waste heat from the solar cell module and connects the solar cell module to the process system. The process system is powered at least partially by thermal energy derived from waste heat generated by the solar cell module. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]      FIG. 1  is a schematic diagram of an integrated electrical and thermal energy solar cell system.  
         [0004]      FIG. 2A  is a schematic diagram of a first example of the integrated electrical and thermal energy solar cell system.  
         [0005]      FIG. 2B  is a schematic diagram of a second example of the integrated electrical and thermal energy solar cell system.  
         [0006]      FIG. 3  is a graph showing an example load control strategy of the integrated electrical and thermal energy solar cell system. 
     
    
     DETAILED DESCRIPTION  
       [0007]      FIG. 1  shows a first embodiment of an integrated electrical and thermal energy solar cell system  10  that generally includes concentrator  12 , concentrated photovoltaic cell  14 , electrical energy stream  16 A, waste heat energy stream  16 B, waste heat recover system  18 , and process system  20 . Process system  20  uses low-grade waste heat  16 B generated from cell  14  to provide at least a portion of the energy needed to operate process system  20 . Waste heat  16 B is collected by waste heat recovery system  18 , which transports waste heat  16 B to process system  20  for use as thermal energy. Integrated system  10  increases the overall efficiency of concentrated photovoltaic cell  14  and process system  20  by combining the photovoltaic power generated by cell  14  with waste heat recovery. In addition, integrated system  10  produces less pollution by using solar energy as its primary energy source.  
         [0008]     In operation, concentrator  12  is aligned with respect to the sun so that it collects and focuses a maximum amount of solar energy for the dimensions of concentrator  12 . The solar energy is then directed by concentrator  12  to cell  14 , where the solar energy is converted to either electricity or heat. The electricity generated by cell  14  is transported by energy stream  16 A to power an external device of system. Typically in the past, the heat generated by cell  14  discarded into the atmosphere and is thus wasted heat. Integrated system  10  recovers wasted heat  16 B by waste heat recovery system  18  and uses it as thermal energy. Waste heat recovery system  18  recovers the heat by a heat transfer fluid that is pumped through waste heat recovery system  18 . As the heat transfer fluid flows proximate cell  14 , the heat generated from cell  14  is transferred to the heat transfer fluid. Upon recovering the heat from cell  14 , the heat transfer fluid is sent to process system  20  for use. The heat transfer fluid can also optionally be heated to the operating temperature of process system  20  prior to reaching process system  20 .  
         [0009]     Process system  20  of solar cell integrated system  10  requires only thermal energy from cell  14  for power. In one embodiment, process system  18  can be a renewable cooling, heating, and power generating (CHP) system. A combination of thermal energy from waste heat  16 B of cell  14  and biomass is used to generate the power needed for cooling, heating, or other uses, as needed for a specific site. For example, after recovering waste heat  16 B, the heat transfer fluid can be sent to a hybrid cooling system of the renewable CHP system. The burning biomass can then be used to generate electricity with waste heat  16 B from cell  14  after being used for the cooling process. Alternatively, the biomass can be used to create hydrogen fuel. Any spare waste heat from the process can then be used for cooling. Integrated system  10  provides a lower cost alternative to a CHP system that uses natural gas for the cooling/heating system.  
         [0010]     In another embodiment, process system  20  can be a rankine cycle. After recovering waste heat  16 B, the heat transfer fluid is transported to a rankine cycle to produce power. Optionally, the rankine cycle can also be fed waste heat from an alternate source and a control system can be used to optimize the heat input to the rankine cycle from either the heat transfer fluid from cell  14  or from the alternate heat source.  
         [0011]     In another embodiment, process system  20  can be a solid-state energy conversion system that converts waste heat directly to electricity. A solid-state energy conversion system would require no moving parts for converting the waste heat to electricity.  
         [0012]     In another embodiment, process system  20  can be a hydronic heating system. The heat transfer fluid carrying waste heat  16 B is passed through, a heat exchanger that transfers the heat from the heat transfer fluid to a secondary fluid, such as water. The heated water is then used to provide hydronic heat.  
         [0013]     In yet another embodiment of integrated system  10 , process system  20  can be an absorption chiller. When used with an absorption chiller, cell  14  has an operating temperature of at least 80° Celsius (° C.). The heat transfer fluid is pumped to cell  14  at approximately 25° C. The heat transfer fluid can also be heated to a temperature higher than 25° C. as long as it does not exceed the temperature of cell  14 . Upon recovering the heat from cell  14 , the heat transfer fluid is heated to approximately 80° C. or higher. At this temperature, the heat transfer fluid can be used in the regeneration step of a cooling subsystem of an absorption chiller having a back-up burner. A back-up burner is necessary to operate the absorption chiller when the waste heat is not of an adequate grade, or temperature, or when the amount of solar energy absorbed by cell  14  is insufficient to run the absorption chiller. The quality of the waste heat can optionally be improved by a thermoelectric device having a high coefficient of performance to cool cell  14  while rejecting the adequate amount of waste heat.  
         [0014]     In another embodiment of the first embodiment of integrated system  10 , cell  14  and process system  20  have an operating temperature of approximately 50° C. of higher. Due to the lower operating temperature of process system  20 , the quality of waste heat  16 B collected from cell  14  does not have to be as high to run process system  20 . In one embodiment, process system  18  can be an adsorption cooling system. The use of an adsorption cooling system can avoid the use of conventional refrigerants that are currently used in vapor compression systems. These systems also operate at temperatures of between 60° C. and 90° C., making them highly compatible with low grade waste heat sources.  
         [0015]     Optionally, process system  20  can also receive electricity from energy stream  16 A in addition to thermal energy from waste heat energy stream  16 B if process system  20  also requires electricity for power. In one embodiment, process system  20  can be a membrane water purification system. In operation, cell  14  operates at a temperature of at least 50° C. The thermal energy carried by the heat transfer fluid can be used to power a membrane water purification bed of the membrane water purification system. The electricity carried by energy stream  16 A can be used to power water pumps of the membrane water purification system. The overall energy consumption required to run a membrane water purification system is significantly less than the overall energy consumption required to run a conventional flash distillation system. In addition, integrated system  10  can easily be scaled up or down depending on the energy and water requirements of the specific application.  
         [0016]     In another embodiment of integrated system  10 , process system  20  is a solid state cooling system having a cold zone and a hot zone that enables water desalination and/or purification. The thermal energy from cell  14  collected by the heat transfer fluid can be used in a membrane distillation water purification subsystem to evaporate the water and separate it from impure water. Alternatively, solid state cooling devices that use electricity to create cold zones can be used to freeze desalinate water for purification. The heat from the hot zone of the solid state cooling device can then be coupled with a waste heat driven cooling or heating process to create a fully integrated comfort system that provides power, cooling, heating, and water purification functions. Integrated system  10  with a solid state energy conversion system does not require moving parts for either generating power or for the cooling system and additionally does not require refrigerants in either the cooling or heating systems.  
         [0017]      FIGS. 2A and 2B  depict integrated systems  10   a  and  10   b , respectively, that generally includes concentrated photovoltaic cell  14  having concentrator  12 , process system  20 , waste heat recovery stream line  22 , and large capacity water-cooled vapor compression chiller (VCC)  24 . As discussed in  FIG. 2A , process system  20  is a small capacity single-effect absorption chiller  20   a . As discussed in  FIG. 2B , process system  20  is a small capacity double-effect absorption chiller  20   b . Small capacity absorption chiller  20  (ABS) is placed in parallel with vapor compression chiller  24  so that the solar energy absorbed by cell  14  can be used to supply a relatively small fraction of the energy required to power a structure connected to integrated system  10   a  or  10   b , with the remaining power supplied by vapor compression chiller  24 . The initial capital cost of integrated systems  10   a  and  10   b  are thus significantly reduced while significantly improving economic competitiveness.  
         [0018]     Concentrated photovoltaic cell  14  collects solar energy from sunlight in order to provide part of the power required to run integrated system  10   a . In one embodiment, cell  14  is an evacuated tube solar collector with a compound parabolic concentrator that is set up in parallel with back-up gas-fired boiler  26 . Together, cell  14  and gas-fired boiler  26  generate hot water for integrated system  10   a  either with solar energy collected from cell  14  or from gas-fired boiler  26  when there is insufficient solar energy. Input valve  28  is switchable between a first position and a second position and controls whether the hot water is received from cell  14  or gas-fired boiler  26 . Although  FIG. 2A  depicts only one cell  14 , any number of cells can be used to form an array of cells as needed to generate sufficient power to run integrated system  10   a.    
         [0019]     A working fluid refrigerant and absorbent flow through integrated system  10   a . The refrigerant has a high affinity for the absorbent and will boil at a lower temperature and pressure than under normal conditions. Water is typically used as the refrigerant and flows through waste heat recovery line  22  to transport the thermal energy collected from cell  14  to storage tank  30  and subsequently to absorption chiller  20   a  and vapor compression chiller  24 . Storage tank  30  is a well-insulated hot water storage tank for storing the hot water until it is needed for use. Although  FIG. 2A  discusses the working fluid as being water, the working fluid can be any thermally conductive fluid, including, but not limited to: water, a water/glycol mixture, steam, oil, or any combination thereof.  
         [0020]     When needed, hot water from storage tank  30  is passed through hot water line  32  to hot water valve  34 . Hot water valve  34  is switchable between a first position and a second position. When hot water valve  34  is in the first position, water from hot water line  32  is directed through first intermediate line  34   a  to absorption chiller  20   a , which is driven by thermal energy. The hot water is used as the driving heat source for absorption chiller  20   a  to produce chilled water. The chilled water is then passed through output line  36  to chilled water line  38  for space cooling. When hot water valve  34  is in the second position, water from hot water line  32  is directed through second intermediate line  34   b  to be used in other applications, including, but not limited to: space heating and providing domestic hot water. The thermal energy can also be used as fuel to generate exhaust gas, hot water, or steam to ensure the operability of integrated system  10   a  when the solar energy collected by cell  14  is insufficient to meet the building load of the structure connected to integrated system  10   a . Once the hot water and the chilled water has been used for their respective purposes, they are returned to integrated system  10  through return water lines  40   a  and  40   b , respectively, for reuse. Return valve  42  is switchable between a first position and a second position and controls whether the water returning through water line  40   b  is fed to absorption chiller  20   a  or vapor compression chiller  24 .  
         [0021]     Vapor compression chiller  24 , which is in parallel with absorption chiller  20   a , serves to meet the cooling load that exceeds the capacity of absorption chiller  20   a  or as a back-up cooling equipment, when the amount of solar energy absorbed by cell  14  is insufficient to meet the building load. When needed, chilled water from vapor compression chiller  24  is sent through chilled water line  38  to provide any additional space cooling. Due to its high fuel to electricity conversion efficiency, vapor compression chiller  24  can be effectively used as the primary source of cooling equipment for commercial buildings. Additionally, because vapor compression chiller  24  is powered by electricity, cell  14  can also provide electricity to run vapor compression chiller  24 .  
         [0022]     Depending on system specifications, process systems in addition to absorption chiller  20   a  and vapor compression  24  can also be connected to integrated system  10   a . As an example, cooling tower  43  could be connected to integrated system  10   a , as shown by the dotted lines in  FIG. 2A . The dotted lines represent additional potential integration opportunities that could be accomplished given additional energy collection and recovery within integrated system  10   a.    
         [0023]     Small capacity absorption chiller  20  can also be a double-effect absorption chiller  20   b , as shown in  FIG. 2B . Double-effect absorption chiller  20   b  differs from single-effect absorption chiller  20   a  in that double-effect absorption chiller  20   b  includes a two-stage heater that regenerates the working fluid. A portion of the internal heat in absorption chiller  20   b  is also recycled to provide part of the energy needed in the generator of absorption chiller  20   b  to create a high-pressure refrigerant vapor. While double-effect absorption chiller  20   b  outputs more cooling per power input, the system is also more complicated than single-effect absorption cooler  20   a.    
         [0024]     In integrated system  10   b , cell  14  is used to generate steam. Feed valve  44  is positioned proximate the outlet of cell  14  and controls steam input. When steam is needed, feed valve  44  allows steam to pass through feed line  46  to steam valve  48 , which splits feed line  46  into primary line  48   a  and secondary line  48   b . Steam valve  48  is switchable between a first position and a second position. When steam valve  48  is in the first position, the steam passes through primary line  48   a  to absorption chiller  20 . Depending on the temperature of the steam, the steam can be fed to a low temperature generator of double-effect absorption chiller  20   b , which is steam/gas-fired dual source driven, to produce chilled water. Alternatively, the steam can also be sent to a high temperature generator of absorption chiller  20   b  if the quality of the waste heat is high enough. When steam valve  48  is in the second position, the steam passes through secondary line  48   b  to first valve  50 .  
         [0025]     Depending on the position of first valve  50 , the steam can be passed through first intermediate line  50   a  to storage tank  30  or through second intermediate line  50   b  to second valve  52 . The steam in first intermediate line  50   a  is stored in storage tank  30  until needed. When needed, the steam can be sent from storage tank  30  through hot water line  54  as hot water to provide space heating or through chilled water line  56  as chilled water to provide space cooling. Second valve  52  is also switchable between a first position and a second position and feeds the steam from cell  14  to either vapor compression chiller  24  through third intermediate line  52   a  or absorption chiller  20   b  through fourth intermediate line  52   b.    
         [0026]     Storage tank  30  receives chilled water from vapor compression chiller  24  through VCC line  58 , which passes chilled water from vapor compression chiller  24  and chilled water from absorption chiller  20   a  through ABS line  60 , which is connected to VCC line  58  upstream of storage tank  30 , to chilled water valve  62 . Chilled water valve  62  is switchable between two positions and sends the chilled water from vapor compression chiller  24  to storage tank  30  through fifth intermediate line  62   a  and to cell  14  and valve  44  through sixth intermediate line  62   b . Similar to  FIG. 2A , although  FIG. 2B  depicts only one cell  14 , any number of cells can be used to form an array as needed to generate sufficient power to run integrated system  10   b . Also similar to integrated system  1   a , additional process systems in addition to absorption chiller  20   b  and vapor compression  24  can be connected to integrated system  10   b , such as cooling tower  43 .  
         [0027]      FIG. 3  is a graph showing an example load control strategy of integrated system  10   a  or  10   b . Small capacity absorption chiller  20  is placed in parallel with vapor compression chiller  24  so that the solar energy absorbed by cell  14  can be used to meet a relatively small fraction of the building load of a structure connected to integrated system  10   a  or  10   b  as a baseload. The building load is the total energy consumed by a structure for electricity, cooling, and heating. The baseload is the typical average energy consumed by the structure for an 8760-hour period (i.e., averaged over a year). Vapor compression chiller  24 , which is low-cost compared to cell  14 , can be used to meet the remaining building load, as shown in  FIG. 3 .  
         [0028]     The integrated electrical and thermal energy solar cell system utilizes a renewable and environmentally-friendly source of energy to run process systems in parallel with a solar cell system. Use of a solar cell system in parallel with a process system enhances the overall efficiency of the system and makes use of thermal energy that is typically wasted. The integrated system captures the low-grade waste heat generated by photovoltaic cells during the absorption of solar energy and the production of electricity. The low-grade waste heat is transported by a heat transfer fluid flowing through a waste heat recovery line to a process system in parallel with the solar cell system. The process system can include, but is not limited to: an absorption chiller, an adsorption cooling system, a hydronic heating system, a rankine cycle, or a renewable cooling, heating and power generating system. In some systems, both electricity and thermal energy are needed to run the process system. These process system can include, but is not limited to: a membrane water purification system, a solid state cooling system, or an absorption chiller in combination with a vapor compression chiller.  
         [0029]     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.