Abstract:
A device for dissipating heat from a photovoltaic cell is disclosed. A first thermally conductive layer receives heat from the photovoltaic cell and reduces a density of the received heat. A second thermally conductive layer conducts heat from the first thermally conductive layer to a surrounding environment. An electrically isolating layer thermally couples the first thermally conductive layer and the second thermally conductive layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 13/716,685, entitled “THERMAL RECEIVER FOR HIGH POWER SOLAR CONCENTRATORS AND METHOD OF ASSEMBLY”, filed on Dec. 17, 2012, which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to solar energy, and more specifically, to dissipating heat from photovoltaic cells illuminated by concentrated solar rays. 
         [0003]    Solar power concentrators are often used in photovoltaic systems to increase an output of the photovoltaic cells. New solar concentrators are able to increase a concentration of incident solar energy by up to and beyond 2000 times. A consequence of this concentration is the production of high levels of heat which raises the temperatures of solar cells. However, solar cells must be operated at temperatures that are typically less than about 110° C. in order to prevent heat damage. Another consequence of the solar concentration is a large current density. It is desired to couple this current to a load in a manner that offers as little electrical resistance as possible to avoid dissipating electrical energy as heat. 
       SUMMARY 
       [0004]    According to one embodiment of the present disclosure, a device for dissipating heat from a photovoltaic cell includes: a first thermally conductive layer configured to receive heat from the photovoltaic cell and reduce a density of the received heat; a second thermally conductive layer configured to conduct heat from the first thermally conductive layer to a surrounding environment; and an electrically isolating layer configured to thermally couple the first thermally conductive layer and the second thermally conductive layer. 
         [0005]    According to another embodiment of the present disclosure, a photovoltaic cell assembly includes: a photovoltaic cell; a first thermally conductive layer configured to receive heat from the photovoltaic cell and reduce a density of the received heat; a second thermally conductive layer configured to conduct heat from the first thermally conductive layer to a surrounding environment; and an electrically isolating layer configured to thermally couple the first thermally conductive layer and the second thermally conductive layer. 
         [0006]    According to another embodiment of the present disclosure, a solar panel includes: a plurality of photovoltaic cell assemblies; at least one wire coupling the photovoltaic cell assemblies; wherein a photovoltaic cell assembly selected from the plurality of photovoltaic cell assemblies includes: a photovoltaic cell, a first thermally conductive layer configured to receive heat from the photovoltaic cell and reduce a density of the received heat, a second thermally conductive layer configured to conduct heat from the first thermally conductive layer to a surrounding environment, and an electrically isolating layer configured to thermally couple the first thermally conductive layer and the second thermally conductive layer. 
         [0007]    Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure. For a better understanding of the disclosure with the advantages and the features, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    The subject matter which is regarded as the disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0009]      FIG. 1  shows a photovoltaic assembly according to an exemplary embodiment of the present disclosure; 
           [0010]      FIG. 2  shows an exploded view of an exemplary cell package of the photovoltaic assembly  FIG. 1  in one embodiment of the present disclosure; 
           [0011]      FIG. 3  shows an alternate embodiment of a cell package of the photovoltaic assembly of  FIG. 1 ; 
           [0012]      FIG. 4  shows a cross-sectional view of an exemplary photovoltaic assembly coupled to a backplane; 
           [0013]      FIG. 5  shows an exemplary assembly for mounting a plurality of photovoltaic assemblies; 
           [0014]      FIG. 6  shows a top view of a solar panel assembly in an exemplary embodiment; 
           [0015]      FIG. 7  shows an exemplary solar panel package of the present disclosure; and 
           [0016]      FIG. 8  shows an exploded view of the solar panel package of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  shows a photovoltaic assembly  100  according to an exemplary embodiment of the present disclosure. The exemplary photovoltaic assembly  100  includes an optical assembly  106  that is affixed to a front surface of a photovoltaic cell  102  for concentrating solar rays. In an exemplary embodiment, the optical assembly  106  provides a concentration of about 1600 times or greater than the incident solar concentration. In an exemplary embodiment, this level of solar concentration produces a large amount of heating at the photovoltaic cell  102 . The photovoltaic cell  102  is coupled to a cell package  104  for dissipating heat from the photovoltaic cell  102 . In an exemplary embodiment, a back surface of the photovoltaic cell  102  is directly soldered to the cell package  104  using a solder. The solder may include an 80/20 lead/tin solder or a low melt solder. Alternate solders usable for soldering the photovoltaic cell  102  and cell package  104  may include alloys containing at least one of lead, tin, copper, gallium, silver, manganese, magnesium, bismuth, indium, zinc and antimony, such as for example Sn—Ag—Cu, Sn—Ag—Cu—Zn and Sn—Ag—Cu—Mn. Alternately, the photovoltaic cell  102  may be coupled to the cell assembly  104  using a conductive particle infused polymer adhesive, such as silver paste. Wires  108  are bonded between the photovoltaic cell  102  and the cell package  104  to provide an electrical path for current produced at the photovoltaic cell  102 . In an exemplary embodiment, the wires  108  may be bonded using a solder such as the exemplary solders listed above. In an alternate embodiment the cell is connected to the top surface conductors on the cell package using on of wire bonding and ribbon bonding methods where the wire and ribbon may comprise one of gold, silver, platinum, palladium, aluminum, silicon and copper. 
         [0018]      FIG. 2  shows an exploded view of an exemplary cell package  104  of  FIG. 1  in one embodiment of the present disclosure. The exemplary cell package  104  includes a coating  202 , a solder mask  204 , a copper layer  206 , a dielectric layer  208  and a substrate layer  210 . In an exemplary embodiment, copper layer  206  and substrate layer  210  form electrodes coupled to the photovoltaic cell  102 . In an exemplary embodiment, copper layer  206  forms an electrical circuit with the photovoltaic cell  102 . 
         [0019]    Coating  202  provides a top surface of the cell package and is in contact with the photovoltaic cell  102 . Coating  202  provides a substantially corrosion resistant surface for wire bonding and may include at least one of gold, silver, nickel, zinc and tin. Solder mask  204  provides a second layer of the cell package  104  and includes an insulating material. The solder mask may include standard solder mask material such as, for example, an epoxy paint. The solder mask  204  may be applied via screen printing in an exemplary embodiment. The solder mask may have windows for wire bonding, strap bonding or strap welding of connections between the photovoltaic cell  102  and package electrodes (not shown) of the cell package  104 . The solder mask  204  further includes windows allowing interconnecting wires (see  FIG. 5 ) to be soldered to the package electrodes, thereby interconnecting a plurality of photovoltaic assemblies  100  to each other and/or to an external device. In exemplary embodiments, the interconnecting wires may be bonded between the photovoltaic cell  102  and the electrodes of the cell package  104  using gold wire bonding, ribbon bonding or strap welding, for example. Connective bonding material may include at least one of gold, silver, Invar, iron, copper and tin. 
         [0020]    Copper layer  206  provides a third layer of the cell package  104  and is an electrically conductive layer that forms a second electrode of the photovoltaic cell  104 . In alternate embodiments, the copper layer  206  may be made of any material that is electrically conductive. Copper layer  206  may be patterned using exemplary lithographic methods such as photolithography, screen printing, and ink jet printing, for example. After patterning, an etch process may be used to remove unwanted copper from the copper layer  206  to form a selected shape. In addition, the copper layer is selected to provide an electrically conductive channel for conducting current generated from the photovoltaic cell, for example, to the interconnecting wires. In various embodiments, the current densities are in a range from about 6.3 amps per square centimeter (amps/cm 2 ) to about 25.2 amp/cm 2 . The copper layer  206  may be electrically coupled to an electrode of the photovoltaic cell  102  using a ribbon bond, a wire bond, etc. 
         [0021]    Dielectric layer  208  provides a fourth layer of the cell package  104  that provides electrical isolation of the copper layer  206  from the underlying substrate layer  210 . The dielectric layer  208  further allows heat transfer from the copper layer  206  to the substrate layer  210 . In an exemplary embodiment, the dielectric layer  208  may include an FR4 matrix of glass fiber and epoxy that may be cured by thermal, chemical or ultraviolet methods. 
         [0022]    Substrate layer  210  provides a fifth and bottom layer of the cell package  104  and may be a thick layer in comparison to layers  202 - 208 . The substrate layer  210  may serve as both an electrode and a thermal conductor. Increasing the thickness d of the substrate layer  210  relative to the thicknesses of layers  202 - 208  increases an ability of the substrate layer  210  to spread the heat conducted to the substrate layer from the photovoltaic cell  104  via the layers  202 - 208 . The substrate layer may have lateral dimensions of length and width. Increasing the size of the lateral dimensions may improve a thermal coupling of the substrate layer  210  to a backplane (see  FIG. 4 ). The lateral dimension and thickness of the substrate layer may be selected to achieve a selected thermal performance (i.e., solar heat dissipation) of the cell package  104  and a selected operating temperature of the related photovoltaic assembly  100 . In an exemplary embodiment, the substrate layer  210  is made of copper or other material selected to achieve high thermal conductivity. In alternate embodiments, layer  210  may include aluminum or at least one of copper, aluminum, iron, chrome, nickel, molybdenum, zinc and tin. In an embodiment in which a photovoltaic cell has a length and width of about 3.75 millimeters (mm), a length and width of the substrate layer  210  may be about 15 mm and the thickness may be about 1.5 mm. 
         [0023]    Layers  206 ,  208  and  210  may be coupled to each other by pressure and heat to cure layer  208  to form a bond. In an exemplary embodiment, layers  206 ,  208  and  210  may be bonded to form a sheet that is then separated into individual substrates suitable for use in a selected cell package  104 . The separated substrates may be patterned into individual substrate layers using printed circuit methods. 
         [0024]      FIG. 3  shows a cell package  104  in an alternate embodiment. The alternate cell package  104  includes a coating  302 , a solder mask  304 , a copper layer  306 , a dielectric layer  308  and a substrate layer  310 . In the alternate embodiment, layer  306  includes both cell electrodes  306   a  and  306   b  coupled to the photovoltaic cell  102  and is made of thick copper that has a thickness that is substantially between about 20 microns and about 400 microns. The dimensions of the layer  306  are selected so as to be conducive to spreading heat laterally. Layer  308  is made thin in comparison to layer  208  of  FIG. 2  to increase heat transfer between layer  306  and the substrate  310 . 
         [0025]      FIG. 4  shows a cross-sectional view  400  of an exemplary photovoltaic assembly coupled to a backplane  410 . The photovoltaic assembly includes photovoltaic cell  402  mounted on an exemplary cell package  404 . The cell package  404  is coupled to an insulation layer or layers  408  via a thermal adhesive  406  that allows heat transfer between the cell package  404  and the insulation layer or layers  408 . The insulation layer or layers  408  may be a printable layer. In one embodiment, the insulation layer or layers  408  may be multi-layer insulators. The insulation layer or layer  408  may also include aluminum oxide, polymers, or other electrically resistive particles, etc. 
         [0026]    In an exemplary embodiment, the thermal adhesive  406  may include a material having at least one of a high thermal conductivity, a high mechanical flexibility, an ability to cure at low temperatures, an ability to withstand operating temperatures in a range from about −40° C. to about 120° C., and an ability to adhere to the contacting faces of the cell package  404  and of the insulation layer or layers  408  providing electrical insulation. The thermal adhesive  406  may include, but is not limited to, SilCool™ TIA-0220 of Momentive Performance Materials, Inc. In an exemplary embodiment, the thermal adhesive  406  includes an insulating silicone adhesive. In alternate embodiments, the thermal adhesive  406  may include epoxy and acrylic adhesives. In another embodiment, the thermal adhesive  406  includes a polymer with thermally conductive particles embedded therein. Exemplary polymers may include at least one of silicone, acrylic and epoxy. Exemplary particles may include at least one of aluminum oxide, aluminum nitride and silicon dioxide. In an exemplary embodiment, the thermal adhesive  406  is compressed to a thin bond line of approximately 50 microns or less and is allowed to slightly extrude beyond the edges of the cell package  404 . 
         [0027]    The insulation layer  408  or layers reduces electrical conduction between the cell package  404  and the backplane  410 , while allowing heat transfer therebetween. The insulation layer or layers  408  may be bonded to an aluminum backplane  410  prior to bonding the insulation layer or layers  408  to the cell package  404 . The insulation layer or layers  408  may include an epoxy-based screen-printable material. In various embodiments, the insulation layer or layers  408  may include any electrically-insulating material with high dielectric strength, strong adhesion to the anodized aluminum of the backplane  410  and an ability to resist heat damage at temperatures in an operating range from about 85° C. to about 120° C. In an exemplary embodiment, the insulation layer or layers  408  may include TechniFlex by Technic Corp. and may be applied using screen printing methods on the backplane  410  to a thickness of about 15 microns. In alternative embodiments, the insulation layer or layers  408  may include, but is not limited to, paints, lacquers, powder coats, etc. Such materials in the alternative embodiment of the insulation layer or layers  408  may include at least one of polyester, polyurethane, polyester-epoxy, epoxy, acrylic and silicone. 
         [0028]    The aluminum backplane  410  may include a sheet of anodized aluminum. In an exemplary embodiment, the backplane  410  includes a sheet of about 1.5 mm in thickness and an anodized layer thickness of about 10 microns. In various embodiments, the anodized layer may have a thickness that provides a protective layer to the aluminum surface as well as an electrical breakdown resistance. Electrical breakdown resistance is provided by the thermal adhesive  406 , the insulation layer or layers  408  and the anodization of the backplane  410 . 
         [0029]    In an exemplary operation of the photovoltaic assembly, heat concentrated at the photovoltaic cell is transferred to the cell package  404 . At the cell package  404 , the heat is distributed in along lateral dimensions of the cell package at the copper substrate, such as substrate  210  in  FIGS. 2  or alternately substrate  310  in  FIG. 3  in order to reduce an areal density of the heat by spreading the heat, in general along a lateral dimension of the substrate. Heat from the substrate  210  is transferred to the aluminum backplane  410  through the insulation layer  408 . In various embodiments, the insulation layer  408  prevents current transfer between substrate  310  and the aluminum backplane  410  by providing a breakdown resistance to about 1700 volts or more. 
         [0030]      FIG. 5  shows an exemplary assembly  500  for mounting a plurality of photovoltaic assemblies. Cell package  502   a  is shown having an associated photovoltaic cell  504   a  and an associated secondary optic  506   a.  Cell package  502   b  is shown having an associated photovoltaic cell  504   b  and an associated secondary optic  506   b.  Cell packages  502   a  and  502   b  are coupled to the aluminum backplane  510  via exemplary screen-printed dielectric  520 . Cell packages  502   a  and  502   b  are disposed on the backplane  510  at a location that corresponds to a focal point of their respective secondary optics  506   a  and  506   b  when the backplane  501  is perpendicular to solar radiation. In one embodiment, a protection diode  512  is packaged to the backplane in a manner similar to the packaging of cell packages  502   a  and  502   b.  The protection diode  512  includes a heat shield  514  that protects the protection diode  512  from heat or dissipates heat from the protection diode  512 . In an exemplary embodiment, the heat shield  514  includes a copper strip that covers the protection diode  512  and is soldered to contact pads of the diode package. 
         [0031]    Interconnecting wiring  516  provides an electrical connection between cell packages  502   a  and  502   b.  In one embodiment, the interconnecting wiring  516  connects printed circuit layers of the cell packages  502   a  and  502   b.  The interconnecting wiring  516  includes copper wire that is soldered to electrodes of the cell packages  502   a  and  502   b.  The diameter of the wire is selected to handle a current provided by the exemplary cell packages and to reduce internal resistance losses. Kinks  518  are introduced into the interconnecting wiring  516  to avoid mechanical stress due to thermal expansion of the interconnecting wiring  516 . The interconnecting wiring  516  may be sufficiently rigid to be self-supporting. The interconnecting wiring  516  may be affixed to the backplane  510  separated by a separation distance in a range of about 1 millimeter to about 2 millimeters above the surface of the backplane  510 . Such a configuration avoids physical contact with the insulating dielectric or with the aluminum anodized surface. 
         [0032]      FIG. 6  shows a top view of a solar panel assembly  600  in an exemplary embodiment. The exemplary solar panel assembly  600  includes five tiers  602   a - 602   e  of cell packages. In each tier, four cell packages, such as exemplary cell packages  604   a - 604   d,  and a protection diode  606  are connected in parallel using the interconnecting wiring  608 . The cell packages are coupled to an aluminum backplane  610  via dielectric layer  612 . The tiers  602   a - 602   e  are connected in series to form a solar panel assembly  600  having 20 cell packages. In various embodiments, the number of cells in parallel vs. the number of cells in series may be selected to achieve a selected current-voltage ratio of the solar panel assembly  600 . Having cells wired in parallel (in the tiers) allows one or more cells to fail while maintaining the function of the panel at a reduced power, thereby improving an overall reliability of the solar panel assembly  600 . Terminal connections  614  provide electrical coupling from the interconnecting wiring  608  to external circuitry. The interconnect wiring  608  may be soldered to an insulated multi-strand copper external connection wire (not shown) that penetrates the aluminum backplane  610  to an exterior of the solar panel assembly  600  via a strain relief cord grip. 
         [0033]      FIG. 7  shows an exemplary solar panel package  700  of the present disclosure. The exemplary solar panel package  700  includes an enclosure  702  that encloses a solar panel assembly (not shown) having one or more cell packages according to the present disclosure. A lens  704  such as a Fresnel lens is coupled to a top of the enclosure  702  using an adhesive, to enclose the solar panel assembly. In various embodiments, the adhesive includes a silicone adhesive. Filtered vents are provided in the enclosure  702  to equalize pressures between an interior and an exterior of the enclosure  702  and to allow moisture within the enclosure to escape to an exterior of the enclosure  702 . 
         [0034]      FIG. 8  shows an exploded view  800  of the solar panel package  700  of  FIG. 7 . The exploded view  800  shows the enclosure  702  and the Fresnel lens  804 . Additionally, the exploded view  800  shows the solar panel assembly  802  that includes a number of cell packages and resides in a chamber formed by the enclosure  702  and the Fresnel lens  704 . The enclosure  702  may further include one or more cooling fins  804  to aid in the dispersion of heat from the enclosure  802  and thus from the solar panel  802 . 
         [0035]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one more other features, integers, steps, operations, element components, and/or groups thereof. 
         [0036]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The embodiment was chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated 
         [0037]    The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the disclosure. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed disclosure. 
         [0038]    While the exemplary embodiment to the disclosure had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the disclosure first described.