Patent Application
20080083450
Thermal management system 10 provides enhanced thermal contact between CPV cell 12 and heat removal system 14 by minimizing the thermal resistance across interface structure 16, and by minimizing the effects of thermal mismatch between CPV cell 12 and heat removal system 14 to ensure robustness under thermal cycling. Thermal management system 10 preferably functions to facilitate electrical insulation of CPV cell 12 from heat removal system 14 while providing sufficient thermal conductivity and/or minimizing individual layer thickness to ensure adequate heat removal from CPV cell 12. This maintains the temperature of CPV cell 12 within its specified operating limits. Thermal management system 10 also acts to deliver the maximum amount of heat energy to heat removal system 14 for possible use in heat utilization elements such as, for example, domestic hot water systems or absorption chillers.
CPV cell 12 is positioned directly beneath optical concentrator 20. Optical concentrator 20 is aligned with respect to the sun so that it optimally collects and focuses a maximum amount of solar energy for the dimensions of CPV cell 12. Optical concentrator 20 includes one or more optical elements for the purpose of collecting and concentrating the solar energy and providing this energy to CPV cell 12. Optical elements of optical concentrator 20 may include, but are not limited to, one or more of the following in various combinations: mirrors, lenses, fresnel lenses, prisms, and optical fibers. Optical concentrator 20 may optionally include tertiary optics 18 that refract and further direct light to CPV cell 12, with a portion of the internally glancing rays reflected internally off of the side walls of tertiary optics 18 to CPV cell 12. The solar energy is then absorbed into CPV cell 12 and a portion of the solar energy is subsequently converted into electrical energy. The fraction of solar energy that is not converted into electricity turns into heat. It will be appreciated that because CPV cell 12 is between approximately 10% and approximately 40% efficient, between approximately 60% and approximately 90% of the energy absorbed by CPV cell 12 is correspondingly converted to heat. The heat must be dissipated from CPV cell 12 to prevent damage and decreased performance of CPV cell 12. Optionally, the dissipated heat may be recovered and used as thermal energy to beneficially drive other processes. Although
The methods of thermally managing CPV cell 12 disclosed herein are particularly effective when heat removal system 14 is a heat sink formed of a thermally conductive material, such as aluminum or copper.
Thermal management system 10 includes an interface structure 16 that is positioned between CPV cell 12 and heat removal system 14 and is used to thermally manage the temperature of CPV cell 12 in a concentrating environment. Interface structure 16 is formed of thermally conductive layers, some of which are electrically insulating and some of which are electrically conductive, that align and attach heat removal system 14 with CPV cell 12 in order to prevent overheating of CPV cell 12 and to increase the rate of heat transfer from CPV cell 12 to heat removal system 14. This attachment is most reliably accomplished by minimizing the difference in the coefficients of thermal expansion in the layers of CPV cell 12, heat removal system 14, and interface structure 16. Large differences between the coefficients of thermal expansion of CPV cell 12, heat removal system 14, and interface structure 16 make CPV cell 12 susceptible to cracking or fracture due to thermally induced stress. Interface structure 16 must therefore minimize the difference in the coefficients of thermal expansion between the layers and provide sufficient thermal conductivity to pass heat from CPV cell 12 to heat removal system 14 such that CPV cell 12 stays within its specified operating limits.
An exemplary embodiment of sufficient thermal conductivity from CPV cell 12 to heat removal system 14 is an effective thermal conductivity per unit length between CPV cell 12 and heat removal system 14 of greater than approximately 50 kilowatts per square meter per degree Celsius (kW/m2/° C.). In another exemplary embodiment, sufficient thermal conductivity from CPV cell 12 to heat removal system 14 is an effective thermal conductivity per unit length between CPV cell 12 and heat removal system 14 of greater than approximately 100 kW/m2/° C. In yet another exemplary embodiment, sufficient thermal conductivity from CPV cell 12 to heat removal system 14 is an effective thermal conductivity per unit length between CPV cell 12 and heat removal system 14 of greater than approximately 200 kW/m2/° C. In yet another exemplary embodiment, sufficient thermal conductivity from CPV cell 12 to heat removal system 14 is an effective thermal conductivity per unit length between CPV cell 12 and heat removal system 14 of greater than approximately 300 kW/m2/° C. While specific thermal conductivities per unit length between CPV cell 12 and heat removal system 14 are given, it will be apparent to one skilled in the art that any thermal conductivities per unit length between 50 kW/m2/° C. and 300 kW/m2/° C. is acceptable.
For a thermal flux of approximately 500 kilowatts per square meter, the effective thermal conductivity per unit length of 50 kW/m2/° C. will yield a temperature difference between CPV cell 12 and heat removal system 14 of approximately 10 degrees C. (° C.). For a thermal flux of approximately 500 kilowatts per square meter and an effective thermal conductivity per unit length between CPV cell 12 and heat removal system 14 of greater than approximately 300 kW/m2/° C., there is an effective temperature difference between CPV cell 12 and heat removal system 14 of less than approximately 2° C. Another exemplary embodiment of sufficient thermal conductivity from CPV cell 12 to heat removal system 14 is when at least 75% of the heat generated by CPV cell 12 is transferred to heat removal system 14. More particularly, there is sufficient thermal conductivity from CPV cell 12 to heat removal system 14 when at least 90% of the heat generated by CPV cell 12 is transferred to heat removal system 14.
Interface structure 16 must also provide mechanical holding strength in order to maintain the physical attachment and thermal contact of CPV cell 12 to heat removal system 14. In some embodiments, heat removal system 14 is maintained in position relative to optical concentrator 20 by other structures (not shown). In these cases, maintaining the position of CPV cell 12 relative to heat removal system 14 will also maintain the needed alignment of CPV cell 12 relative to optical concentrator 20.
Heat removal system 14 contacts interface structure 16 and either passively or actively dissipates heat from CPV cell 12. Passive dissipation of heat involves no forced fluid flow past or through heat removal system 14, whereas active dissipation of heat involves forced fluid flow past or through heat removal system 14. In order to ensure high heat transfer from CPV cell 12 to heat removal system 14, interface structure 16 acts to provide increased thermal contact between heat removal system 14 and CPV cell 12. The heat removed from CPV cell 12 may subsequently either be dissipated into the environment or recovered and transported for use in an adjoining process system that is heat driven. Heat removal system 14 may include, but is not limited to: a heat sink, a heat spreader, or a heat exchanger having fluid passages 31 through which a heat transfer fluid is circulated to efficiently remove heat and maintain the temperature of CPV cell 12 within specified bounds. The heat removed by the fluid circulating through passages 27 and heat removal system 14 may be beneficially used to heat a building space or to drive a process that employs heat.
As mentioned above, in an exemplary embodiment, thermal management system 10a includes aluminum nitride (AlN) substrate layer 22 as a part of interface structure 16. AlN substrate layer 22 is connected to heat removal system 14 by bonding material layer 24, which is thermally conductive and may be either electrically insulating or electrically conducting. The purpose of AlN substrate layer 22 is to electrically insulate CPV cell 12 from heat removal system 14 while also physically holding the position of CPV cell 12 relative to heat removal system 14 and providing a high thermal conductivity path from CPV cell 12 to heat removal system 14. The purpose of bonding material layer 24 is to provide mechanical and thermal attachment of AlN substrate layer 22 to heat removal system 14. Bonding material layer 24 may be comprised of any suitable material, including, but not limited to: a high thermal conductivity epoxy, thermoplastic resin, glass, or solder. Exemplary embodiments of layer thicknesses for AlN substrate layer 22 and: bonding material layer 24 are approximately 0.1 millimeters (mm) for bonding material layer 24 and bonding material 30, between approximately 0.6 mm and approximately 1.0 mm for AlN substrate layer 22, and between approximately 0.01 mm and approximately 0.1 mm for patterned metal layers 26a and 26b.
AlN substrate layer 22 may also be made of other electrically insulating materials such as ceramic, glass, epoxy, polyimide, plastic, or the like. As shown in
A suitable CPV die backside metal layer 28, such as gold, is deposited on the backside of CPV cell 12 as employed in semiconductor die processing. Backside metal layer 28 provides increased thermal conductivity and improved solder wetting capability to enable die attach of CPV cell 12 to patterned metal layer 26b using solder for bonding material 30. Electrically conductive epoxy, or other suitable die attach adhesive may also be used as bonding material 30. CPV cell 12 with backside metal layer 28 is positioned on top of bonding material 30 to provide a mechanical connection of CPV cell 12 to the rest of the structure as well as the needed backside electrical contact to CPV cell 12. CPV cell 12 is positioned on top of bonding material 30. Patterned metal layers 26a and 26b are positioned on AlN substrate layer 22 and may provide the means for electrical connection of CPV cell 12 to other CPV cells.
Examples of patterned metal layers 26a and 26b include, but are not limited to: thick film metal, thin film metal, and direct bond copper (DBC). The thick film metal may be deposited onto AlN substrate layer 22 to make a continuous conductive material by any suitable method known in the art, including, but not limited to: screen-printing in paste form and pattern dispensing with a syringe and subsequently drying to remove solvents and heat treating to sinter metal particles. The thin film metal may be deposited onto AlN substrate layer 22 by any suitable method known in the art, including, but not limited to: sputter deposition, metal vapor deposition, and chemical vapor deposition. Thin film metal layers may be deposited as a continuous layer and then patterned using photolithography and etching processes. DBC may be formed by oxidizing a copper sheet and then pressing it against a pre-oxidized AlN substrate layer 22 at a temperature of approximately 1070° C. At this temperature, the copper contains approximately 1.6 atomic percent oxygen at the interface that forms a eutectic copper-oxygen melt zone while the AlN substrate layer 22 and the copper remain solid. This melt zone chemically bonds to both the copper layer and AlN ceramic substrate layer 22, resulting in the direct bond copper metallization. This DBC may be etched to the desired pattern after bonding by suitable mask and etch techniques known in the art. Alternatively, the copper may be etched or stamped to the desired shape prior to the bonding process by suitable methods known in the art.
Wire bonds, beam leads, ribbon bonds, or flexible circuits 32 connect patterned metal layers 26a to CPV cell 12 and form electrical attachments to a top side of CPV cell 12. Wire bonds and beam leads may be formed of materials including, but not limited to: aluminum, gold, and copper. Wire bonds and beam leads may then be attached to CPV cell 12 and patterned metal layers 26a by ultrasonic or thermosonic metal-to-metal bonding methods. If the beam lead is formed of copper, the beam lead can be attached through soldering. Flexible circuits 32 may be formed of copper or other patterned metal that was previously attached to a flexible dielectric material, such as polyimide. Both ends of flexible circuit 32 have exposed bare metal so that flexible circuit 32 may be soldered to CPV cell 12 and patterned metal layers 26a. Also, as previously mentioned, CPV cell 12 may also have a backside electrical connection through backside metal layer 28 to conductive patterned metal layer 26b.
Another exemplary embodiment of thin dielectric layer 34 is aluminum phosphate. The aluminum phosphate facilitates bonding between patterned metal layer 26b and heat removal system 14 and therefore provides increased adhesion of CPV cell 12 to heat removal system 14. The increased adhesion results in an improved rate of heat transfer between CPV cell 12 and heat removal system 14 by helping to minimize poor conductive voids in the bond between CPV cell 12 and heat removal system 14. By forming the dielectric layer containing aluminum directly on heat removal system 14, the need for a carrier substrate, such as AlN substrate layer 22 (shown in
Other suitable examples of thin dielectric layers 34 include, but are not limited to: polyimide, polybenzyl imidizole, and mixtures thereof. Thin dielectric layer 34 may also act as a crack-resistant adhesive layer with appropriate functionalization of the monomer and addition of higher thermal conductivity, electrically insulating nano-fibrils.
Suitable nano-fibrils are formed of thermally stable materials, including, but not limited to: boron nitride, aluminum nitride, silicon carbide, and formulated polymer resins such as thermopolymers. Examples of thermopolymers include, but are not limited to: polyamide, polyamide-imides, polybenzyl imidazole, polyphthalamide, polyethylene naphthalate (PEN), polyamideimide, polyphenylene oxide, polysulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone (PEEK), polyetherimide, polyarylates, and appropriate mixtures, copolymers, and the like.
Additionally, patterned metal layers 26a and 26b on thin dielectric layer 34 may be used. Patterned metal layers 26a and 26b enable the electrical circuitry of CPV cell 12 that connects CPV cell 12 to other cells in series or parallel to obtain electricity from CPV cell 12, reducing material volume and cost. Conductive patterned metal layers 26a and 26b may include, but are not limited to: DBC, thick film metal, or thin film metal.
In an exemplary embodiment of thermal management system 10c, the electrical connection of flexible circuits 32 from the top of CPV cell 12 to patterned metal layers 26a may be provided by wire bonds, beam leads, ribbon bonds, or flexible circuits. Patterned metal layers 26a are deposited on dielectric layer 24, which is deposited on top of heat spreader 36. Similar to thermal management system 10a shown in
In another exemplary embodiment shown in
Bonding material 30 may also include filler materials, including, but not limited to: silicon carbide and other metals, ceramics, and polymeric materials. If particulate filler materials are used, they may be of any single geometry including, but not limited to: spheres, rods, fibers, and other regular shapes, irregular shapes, or combinations of geometries. The composition and morphology of the filler(s) are chosen to control the thermal, mechanical, and electrical properties of bonding material 30. For example, if silicon carbide fibers are used, the fibers are preferably aligned parallel to the direction of desired heat flow so that the fibers can guide the heat flow in a direction parallel to the axis of the fiber. In compression, the incorporated silicon carbide fibers then act to stiffen the die attach material relative to a direction perpendicular to the axis of the orientation of the fibers.
Thermal management systems 10f, 10g, and 10h are very similar to each other and will be discussed in conjunction with one another. The difference between thermal management systems 10f, 10g, and 10h is the configurations of clips 52a and 52b. Thermal management systems 10f, 10g, and 10h include a cavity 58 within heat removal system 14 in which CPV cell 12 is positioned. Cavity 58 includes a layer of thermally conducting, dielectric material 54 between CPV cell 12 and heat removal system 14. Springs 60a and 60b are positioned in cavity 58 between CPV cell 12 and heat removal system 14 and hold CPV cell 12 in firm contact with heat removal system 14.
As can be seen in
Thermal management system 10i, shown in
Although the above exemplary embodiments are discussed independently of each other, in some cases the various embodiments may be used in combination with each other. In addition, other methods may also be used in combination with the embodiments previously mentioned. For example, the enclosed space above CPV cell 12 may be backfilled with an overpressure of inert gas. The inert gas prevents ambient ingress, condensation, and corrosion problems, which decrease the thermal contact between CPV cell 12 and heat removal system 14 as well as result in the potential loss of desired electrical connections, resulting in the loss of the electrical power output of CPV cell 12. Because backfilling the enclosed space above CPV cell 12 with inert gas increases the tolerance of the system to small leaks, backfilling CPV cell 12 is more effective than using a vacuum in this space.
In another exemplary embodiment, a metal collar may be positioned on tertiary optics 18. If the metal collar is also in contact with the electrical connection on the top surface of CPV cell 12, the metal collar will also provide an electrical contact to the top of CPV cell 12 in order to connect CPV cell 12 in series or in parallel with other cells as well as provide a means for extracting electricity from the system. Each CPV cell 12 has two electrical connections and essentially acts as a photodiode that generates electrical power when solar energy is incident upon the top surface of CPV cell 12. Use of the metal collar acting as the top surface electrical contact for CPV cell 12 eliminates the need for other means of making this electrical connection. Other means for making this CPV cell 12 top surface electrical connection previously noted include, but are not limited to: wire bonds, beam lead bonds, and flex circuits. Additionally, the metal collar may be designed as a spring to account for thermal effects.
In yet another exemplary embodiment, the surface of heat removal system 14 may be roughened or anodized to increase the adhesion strength and thermal conductivity of interface structure 16. By anodizing heat removal system 14, there is increased bonding between CPV cell 12 and heat removal system 14. In addition, the contact surface area between CPV cell 12 and heat removal system 14 may be increased by using a bonding material or thermal grease as interface structure 16.
The thermal management system thermally manages the temperature of a CPV cell by ensuring increased thermal contact and decreased thermal resistance between the CPV cell and a heat removal system. By adding an interface structure between the CPV cell and the heat removal system, the mechanical stresses on CPV cell 12 due to differences in the coefficients of thermal expansion of the layers of the solar cell system and the heat removal system are minimized. The interface structure and the heat removal system are also arranged in such a way as to ensure that the electrical contacts of the CPV cell are insulated from each other and from the thermal management system while providing sufficient thermal conductivity to remove heat from the CPV cell.
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. Throughout the specification and the claims, the use of the term “a” should not be interpreted to mean “only one”, but rather should be interpreted broadly as meaning “one or more.” Furthermore, the use of the term “or” should be interpreted as being inclusive unless otherwise stated.
