Patent Application
20070215198
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 cooling the cell is to use a heat spreader to spread the heat generated in the cell, and then either passively or actively cool the cell by a heat sink or a heat exchanger, respectively. However, because active and passive cooling methods often require different constructions of the cell module assembly and are typically constructed with the cell module assembly, various constraints are imposed on the manufacturer regarding fixtures, tools, and equipment.
A thermally managed solar cell system includes a photovoltaic cell for generating electricity and heat. The system includes a housing, a base, and a heat removal device. The housing surrounds the solar cell system and has an open, rear portion. The base is positionable in the open portion of the housing and supports the photovoltaic cell. The base is also thermally conductive and spreads heat generated from the photovoltaic cell. The heat removal device and the base act as a single unit with the heat removal device being coupled to the base to remove the heat from the base.
Housing 16 surrounds solar cell system 10a and supports concentrator 14. Housing 16 generally includes side frame 22, window 24, and base plate 26. Side frame 22 is positioned along the outer side perimeter of photovoltaic cell 12 and concentrator 14 and protects photovoltaic cell 12 and concentrator 14 from external elements. Window 24 is formed of a transparent glass and is connected to side frame 22 at top edge 28 of side frame 22. Window 24 is positioned above concentrator 14 and provides an enclosure to evacuate space for the optics of concentrator 14 as well as to protect photovoltaic cell 12 from damage from external sources. Base plate 26 provides the foundation of housing 16 and is attached to side frame 22 at bottom edge 30 of side frame 22 by fasteners 32a and 32b, allowing for quick and easy access to photovoltaic cell 12 if needed. Base plate 26 also includes aperture 34 in the center of base plate 26 to receive removable base 18 of modular thermal management structure 11a.
Modular thermal management structure 11a is connected to solar cell system 10a at housing 16. Removable base 18 is positioned directly beneath photovoltaic cell 12 and is formed from a lightweight sheet of highly thermally conductive material. Because removable base 18 is thermally conductive, removable base 18 also functions as a heat spreader for photovoltaic cell 12. Heat removal device 20 is connected to photovoltaic cell 12 by removable base 18. Thus, removable base 18 spreads the high heat flux (heat transfer rate per unit area) of photovoltaic cell 12 created by the high absorption of energy into the relatively small surface area of photovoltaic cell 12 by increasing the heat transfer area between photovoltaic cell 12 and heat removal device 20. By increasing the heat transfer area between photovoltaic cell 12 and heat removal device 20, the heat flux from photovoltaic cell 12 decreases. In one embodiment, removable base 18 is formed of aluminum.
Heat removal device 20 is directly attached to removable base 18 and passively dissipates the heat generated by photovoltaic cell 12 after the heat has spread through removable base 18. In one embodiment, heat removal device 20 is a heat sink. Heat sinks are typically used in combination with solar cell systems that are passively cooled. In passive cooling, ambient air is used as the heat transfer source, which cools the solar cell system by natural convection. Because the objective of a heat sink is to simply dissipate the excess heat, rather than capture the heat for subsequent use, no insulation is needed. Heat removal device 20 can be connected to housing 16 by removable base 18 by any means known in the art, including, but not limited to: brazing, welding, or mechanical means.
Because the heat from photovoltaic cell 12 is captured for subsequent use, modular thermal management structure 11c includes insulator 42 positioned between base plate 26, removable base 18, and heat removal device 40. Insulator 42 prevents heat generated from photovoltaic cell 12 from escaping into the environment, maximizing the heat transfer from photovoltaic cell 12 to the coolant and thus any heat supply to an adjoining process system. In one embodiment, heat removal device 40 is a heat exchanger.
Although
In operation, photovoltaic cell 12, base 26, and modular thermal management structures 11a-11d can be separated from housing 16 of solar cell systems 10a-10d, respectively, by removing fasteners 32a and 32b. Depending on the desired function of the heat collected from solar cell systems 10a-10d, the heat removal device can be designed to perform passive or active cooling. However, solar cell system 10a-10d will remain the same, allowing for easy installation and replacement of modular thermal management structures 11a-11d, depending on the particular needs and expectations of solar cell systems 10a-10d. For example, various heat removal device embodiments can be utilized to actively cool photovoltaic cell 12, as described below. One type of heat removal device includes a plurality of hemispherical blocks positioned below the photovoltaic cells to reduce the local heat flux of the photovoltaic cells. Another type of heat removal device includes a plurality of microchannels that extend beneath the photovoltaic cells to increase the surface area between the photovoltaic cells and the heat transfer fluid. Yet another type of heat removal device includes positioning a vapor compression system below the solar system. All of these active heat removal devices use coolants to dissipate the heat from the photovoltaic cells.
Channel 104 acts as a coolant flow passage and is formed from contact plate 108 and bottom plate 110. As can be seen in
Contact plate 108 and bottom plate 110 are connected together to form channel 104. First side 112a of contact plate 108 is connected to first side 118a of bottom plate 110, and second side 112b of contact plate 108 is connected to second side 118b of bottom plate 110. Although
Blocks 106a and 106b have a hemispherical shape and are sized to rest within hemispherical recesses 116 of contact plate 108. Photovoltaic cells 102a and 102b are then positioned directly on blocks 106a and 106b, respectively, which act to reduce the local heat flux of photovoltaic cells 102a and 102b. Blocks 106a and 106b are formed from highly thermally conductive material and significantly increase the contact surface area between from photovoltaic cells 102a and 102b and the coolant flowing through channel 104. As the contact surface area between photovoltaic cells 102a and 102b and the coolant increases, the local heat flux decreases, minimizing potential damage to photovoltaic cells 102a and 102b. The hemispherical shape of blocks 106a and 106b cause the heat from photovoltaic cells 102a and 102b to be dissipated in a radial direction, evenly spreading the heat to a larger surface area and thus reducing the heat flux. Because blocks 106a and 106b and channel 104 are both formed of highly conductive material, any temperature difference between photovoltaic cells 102a and 102b and blocks 106a and 106b will be minimal. Although blocks 106a and 106b are depicted in
In operation, a coolant passes through channel 104 of active heat removal device 100 and acts as a heat transfer fluid for the heat being dissipated from photovoltaic cells 102a and 102b. The heat from photovoltaic cells 102a and 102b is first dissipated into blocks 106a and 106b, respectively, and then radiates in a radial direction through blocks 106a and 106b to contact plate 108. This increased contact surface area created by blocks 106a and 106b and recesses 116 of contact plate 108 allows heat transfer from photovoltaic cells 102a and 102b to the coolant flowing through channel 104 with significantly reduced heat flux, thus avoiding localized boiling of the coolant. This increased heat transfer contact surface area also allows heat to be dissipated from photovoltaic cells 102a and 102b without a large temperature drop. As a result of the small temperature difference between photovoltaic cells 102a and 102b and the coolant, useful heat can be generated from photovoltaic cells 102a and 102b, such as heated water.
To integrate heat removal device 100 with solar cell systems 10c or 10d, contact plate 108 of heat removal device 100 acts as removable base 18. Contact plate 108 is attached to housing 16 by fasteners 32a and 32b with channel 104 and blocks 106a and 106b removing the heat from photovoltaic cells 102a and 102b.
By forming groove 220 along the entire length of contact plate 208 and positioning block 206 within the entire length of groove 212, the cross-sectional area of channel 204 remains constant along the entire length of channel 204. This results in an more constant rate of heat transfer along channel 204 of active heat removal device 200 compared to the rate of heat transfer in channel 104 of active heat removal device 100. The rate of heat transfer in channel 104 is smaller and less consistent due to the intermittent contact surface areas between blocks 106a and 106b and the coolant. Because block 206 provides heat transfer along the entire length of channel 204, the heat transfer of active heat removal device 200 is more uniform and can be more easily controlled.
To integrate heat removal device 200 with solar cell systems 10c or 10d, contact plate 208 of heat removal device 200 acts as removable base 18. Contact plate 208 is attached to housing 16 by fasteners 32a and 32b with channel 204 and block 206 removing the heat from photovoltaic cells 202a and 202b.
Base 304 is an insulated structural base that supports photovoltaic cells 302a, 302b, and 302c, substrate 308, and heat exchanger 314. Substrate 308 is a thin film and forms the foundation at which the electrical circuit is laid out. Apertures must first be cut out from substrate 308 such that photovoltaic cells 302a, 302b, and 302c can be mounted directly on base 304 without overlapping substrate 308 once photovoltaic cells 302a, 302b, and 302c are ready to be mounted. In one embodiment, the apertures are cut from substrate 308 such that portions of substrate 308 will overlap edges of photovoltaic cells 302a, 302b, and 302c when photovoltaic cells 302a, 302b, and 302c are mounted to base 304. After the apertures have been cut from substrate 308, substrate 308 is mounted on base 304.
Once substrate 308 is in place, photovoltaic cells 302a, 302b, and 302c are mounted and mechanically attached to base 304. As shown in
Substrate 308 is electrically insulated and has a power bus imprinted with two terminals 308a and 308b to connect each of photovoltaic cells 302a, 302b, and 302c to substrate 308 and to transfer power from photovoltaic cells 302a, 302b, and 302c to a connector. Because substrate 308 is electrically insulating, substrate 308 typically has low thermal conductivity, resulting in high heat transfer resistance across substrate 308. Low temperature coolants are thus needed to effectively remove heat from photovoltaic cells 302a, 302b, and 302c. Once photovoltaic cells 302a, 302b, and 302c have been mounted to base 304, covercoat 312 is coated over photovoltaic cells 302a, 302b, and 302c to protect photovoltaic cells 302a, 302b, and 302c from exposure. In one embodiment, covercoat 312 is silica gel.
Heat exchangers 314 have microchannels 316 and are housed within base 304. Heat exchangers 314 extend through the length of base 304 beneath photovoltaic cells 302a, 302b, and 302c. Microchannels 316 are extruded tubes designed to ensure high heat spreading along the wall of heat exchanger 314. The coolant flows through microchannels 314 and captures the heat generated from photovoltaic cells 302a, 302b, and 302c. Although
In operation, microchannels 316 of heat exchanger 314 and highly thermally conductive coating 306 provide high convective heat transfer of heat generated by photovoltaic cells 302a, 302b, and 302c to the coolant flowing through microchannels 316. The high convective heat transfer results in efficient heat removal from photovoltaic cells 302a, 302b, and 302c. Due to the high heat transfer rate, heat is transferred to the coolant with a minimal temperature drop, resulting in a low temperature difference between photovoltaic cells 302a, 302b, and 302c and the coolant. Similar to active heat removal devices 100 and 200, useful heat can be generated from cells 302a, 302b, and 302c with active heat removal device 300. Additionally, due to the size and material of microchannels 316, microchannels 316 provide a low-cost and lightweight thermal management system, allowing for high volume production and reducing the mechanical load of active heat removal device 300.
To integrate heat removal device 300 with solar cell systems 10c or 10d, base 304 of heat removal device 300 acts as removable base 18. Base 304 is attached to housing 16 by fasteners 32a and 32b with microchannels 314 removing the heat from photovoltaic cells 302a, 302b, and 302c.
In a fourth embodiment, active heat removal device 400 is an evaporator of vapor compression system 402. Shown in
Evaporator 406 and condenser 410 are heat exchangers that evaporate and condense the refrigerant, respectively. Evaporator 406 boils the refrigerant to provide cooling. As the refrigerant is boiled and evaporated in evaporator 406, the temperature and pressure are generally low, Tlow, Plow. At this temperature, the refrigerant in evaporator 406 readily absorbs heat rejected from solar cell system 404. In addition, because the temperature of the refrigerant is low, it can act to cool an external source such as a refrigerator or an air conditioner.
Upon leaving evaporator 406, the refrigerant is sent to compressor 408. Compressor 408 takes the refrigerant vapors that were boiled from evaporator 406 and raises the pressure of the refrigerant vapor to a level Phigh sufficient for the refrigerant vapor to condense in condenser 410. As the refrigerant is compressed and the pressure of the refrigerant increases, the temperature of the refrigerant also increases. At this stage, the refrigerant is a high pressure Phigh, high temperature Thigh fluid vapor.
Once the refrigerant has been compressed, it is sent to condenser 410, where the refrigerant is cooled to a liquid state that is still high pressure Phigh and high temperature Thigh. The heat is thus rejected from the refrigerant in condenser 410. Condenser 410 can be any design known in the art, including, but not limited to, a cooling tower or an evaporative condenser.
After leaving condenser 410, the refrigerant enters expansion device 412. Expansion device 412 controls the flow of the condensed refrigerant leaving condenser 410 at increased pressure Phigh and increased temperature Thigh into evaporator 406. Expansion device 412 lowers both the pressure and the temperature of the refrigerant to a low pressure Plow and a low temperature Tlow prior to entering evaporator 406 for heat absorption. At this pressure and temperature, the refrigerant is a two-phase fluid, or a vapor/liquid mixture, which has better heat transfer properties than a single-phase fluid. Furthermore, the refrigerant generally stays at a constant temperature and pressure when boiling/evaporating. Use of evaporator 406 to absorb the heat allows better temperature control of photovoltaic cell 404. The refrigerant is passed continuously through vapor compression system 402 to remove heat from solar cell system 404.
To integrate heat removal device 400 with solar cell systems 10c or 10d, evaporator 406 of heat removal device 400 acts as removable base 18. Evaporator 406, which can be, for example, any of the above the first, second, and third embodiments of heat removal devices 100, 200, and 300, respectively, is attached to housing 16 by fasteners 32a and 32b and removes the heat from photovoltaic cells 302a, 302b, and 302c.
The solar cell systems attached to modular thermal management structures provide passive and active cooling modular configurations for removing heat from a solar cell system. Various modular structures are disclosed that allow connection of either a passive or an active cooling device to a photovoltaic cell subsequent to assembly of the solar cell system. A heat sink can be connected to the solar cell system either after the construction of a solar cell housing or integrally with the modular thermal management structure for a passive thermal management system. Likewise, a heat exchanger or other active cooling heat removal device as described below can be connected to the solar cell system either after construction of the solar cell housing or integrally with the modular thermal management structure for an active thermal management system.
Various active cooling heat removal devices can be used to effectively remove heat from the solar cell system. In one heat removal device, a plurality of blocks are positioned directly below photovoltaic cells of the solar cell system to reduce the local heat flux of the photovoltaic cells. In another heat removal device, a plurality of microchannels extend below the photovoltaic cells to increase the heat transfer from the photovoltaic cells to a heat transfer fluid. In yet another type of heat removal device, a vapor compression system is connected to the solar cell system. The active heat removal devices use a coolant as a heat transfer means to dissipate the heat from the photovoltaic cells.
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.
