US20070157922A1

US20070157922A1 - Integrated electrical and thermal energy solar cell system

Info

Publication number: US20070157922A1
Application number: US11/324,000
Authority: US
Inventors: Rakesh Radhakrishnan; Zidu Ma; Craig Walker; Yu Chen; Yuanguang Li; Yuhui Kuang
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 2005-12-29
Filing date: 2005-12-29
Publication date: 2007-07-12
Also published as: WO2007079235A3; CN101351896A; WO2007079235A2

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

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

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an integrated electrical and thermal energy solar cell system.
FIG. 2A is a schematic diagram of a first example of the integrated electrical and thermal energy solar cell system.
FIG. 2B is a schematic diagram of a second example of the integrated electrical and thermal energy solar cell system.
FIG. 3 is a graph showing an example load control strategy of the integrated electrical and thermal energy solar cell system.

DETAILED DESCRIPTION

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 16A, waste heat energy stream 16B, waste heat recover system 18, and process system 20. Process system 20 uses low-grade waste heat 16B generated from cell 14 to provide at least a portion of the energy needed to operate process system 20. Waste heat 16B is collected by waste heat recovery system 18, which transports waste heat 16B 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.
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 16A 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 16B 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.
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 16B 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 16B, 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 16B 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.
In another embodiment, process system 20 can be a rankine cycle. After recovering waste heat 16B, 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.
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.
In another embodiment, process system 20 can be a hydronic heating system. The heat transfer fluid carrying waste heat 16B 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.
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.
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 16B 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.
Optionally, process system 20 can also receive electricity from energy stream 16A in addition to thermal energy from waste heat energy stream 16B 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 16A 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.

Claims

1. An integrated solar cell system comprising:

a solar cell module that converts solar energy to electrical energy and waste heat;

a low-grade heat recovery means for recovering the waste heat from the solar cell module; and

a process system connected to the solar cell module by the low-grade heat recovery means, wherein the process system is powered at least in part by thermal energy derived from the waste heat recovered from the solar cell module.

2. The integrated system of claim 1, wherein the solar cell module is a concentrated photovoltaic cell.

3. The integrated system of claim 1, wherein the low-grade heat recovery means comprises a heat transfer fluid.

4. The integrated system of claim 1, wherein the process system comprises a hydronic heating system.

5. The integrated system of claim 1, wherein the process system comprises a solid-state energy conversion system.

6. The integrated system of claim 1, wherein the solar cell system has an operating temperature of at least 80 degrees Celsius.

7. The integrated system of claim 6, wherein the process system comprises an absorption chiller.

8. The integrated system of claim 1, wherein the solar cell system has an operating temperature of at least 50 degrees Celsius.

9. The integrated system of claim 8, wherein the process system comprises an adsorption cooling system.

10. The integrated system of claim 1, wherein the process system comprises a rankine cycle.

11. The integrated system of claim 1, wherein the process system comprises a renewable cooling, heating, and power generating system.

12. The integrated system of claim 1, wherein the process system comprises a membrane water purification system.

13. The integrated system of claim 1, wherein the process system is also powered at least in part by electricity generated from the solar cell system.

14. The integrated system of claim 13, wherein the process system comprises a membrane water purification system.

15. The integrated system of claim 13, wherein the process system comprises a solid state cooling system.

16. The integrated system of claim 15, wherein the solid state cooling system comprises a water desalination system.

17. The integrated system of claim 13, wherein the process system comprises an absorption chiller system integrated with a vapor compression chiller.

18. A method for using energy created by a solar cell system, the method comprising:

capturing energy from sunlight directed at the solar cell system;

generating electricity from the sunlight captured by the solar cell system;

recovering heat dissipating from the solar cell system; and

transporting the recovered heat to a process system for use.

19. The method of claim 18, wherein transporting the recovered heat to a process system for use comprises transporting the recovered heat to a hydronic heating system, a solid-state energy conversion system, an absorption chiller, an adsorption cooling system, a solid state cooling system, a rankine cycle, a membrane water purification system, an absorption chiller system integrated with a vapor compression chiller, or a renewable cooling, heating, and power generating system.

20. The method of claim 18, wherein generating electricity comprises sending the generated electricity to a membrane water purification system, a solid state cooling system, or an absorption chiller system integrated with a vapor compression chiller.