Abstract:
A solar absorber system includes a solar absorbing layer that receives and converts solar radiation to thermal energy. The solar absorber assembly includes an anisotropic material to more effectively move the absorbed energy to a fluid conduit for capture and storage.

Description:
BACKGROUND 
       [0001]    The energy in solar radiation is in the form of electromagnetic radiation from infrared to ultraviolet wavelengths and can average approximately 1,000 watts per square meter in optimal conditions. A solar absorber or collector is a device that converts the energy in solar radiation to a usable or storable form, and in particular, converts the radiant energy to thermal energy. A solar absorber or collector may be used in a variety of applications. For example, absorbers may be used for supplemental space or water heating in residential and commercial buildings. 
       BRIEF DESCRIPTION 
       [0002]    According to one aspect of the present disclosure, a heat absorber assembly includes a solar absorber layer having a top major surface adapted to absorb solar radiation and a bottom major surface opposed from said top major surface. A conduit is positioned proximate to the bottom major surface and for receiving a fluid therethrough. A heat spreader is in contact with at least a portion of the bottom major surface of the solar absorber layer. The conduit is positioned between the solar absorber layer and the heat spreader. The heat spreader is a thermally anisotropic material having an in-plane thermal conductivity of at least about 250 W/mK. 
         [0003]    According to another aspect of the present disclosure, a heat absorber assembly includes a solar absorber layer having a top major surface adapted to absorb solar radiation and a bottom major surface opposed from the top major surface. A conduit is in contact with the bottom major surface for receiving a fluid therethrough. The absorber is a thermally anisotropic material having an in-plane thermal conductivity of at least about 250 W/mK. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is a schematic of a solar absorber system. 
           [0005]      FIG. 2  is an elevated top view of a solar absorber including a plurality of columns. 
           [0006]      FIG. 3  is a cross-section view taken along A-A of  FIG. 2 . 
           [0007]      FIG. 4  is an alternate cross-section view taken along A-A of  FIG. 2 . 
           [0008]      FIG. 5  is a second alternate cross-section view taken along A-A of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    With reference now to  FIG. 1 , a solar absorber system is shown and generally indicated by the numeral  10 . Generally, system  10  includes a solar absorber  12  which absorbs radiant energy from the sun, converts that energy to thermal energy, and transfers that thermal energy to a working fluid. The working fluid is directed to a heat exchanger  14  where the thermal energy is transferred to a heating or energy storage unit  16 . After traveling through the heat exchanger, the working fluid may thereafter be recirculated to the absorber  12 . Unit  16  may, for example, be a residential or commercial hot water heater or a residential or commercial space or floor heating system. 
         [0010]    Though the above described system  10  is closed-loop (i.e. the working fluid is continuously recycled back through the heat absorber), it should be appreciated that an open-loop system may also be employed, in particular in a water heater application. In one embodiment of an open-loop configuration the heat exchanger is eliminated and water is drawn through the heat absorber and piped directly to a water storage tank for eventual use. 
         [0011]    With reference now to  FIGS. 2 and 3 , an exemplary heat absorber  12  is shown in greater detail. Heat absorber  12  includes an input conduit  18  and an output conduit  20  for receipt and output of working fluid. A plurality of column assemblies  22  interconnect the input conduit  18  and the output conduit  20 . The columns  22  receive and absorb sunlight and transmit the thermal energy to the working fluid flowing therein. In this manner the temperature of the working fluid exiting the output conduit  20  is raised relative to the temperature of the working fluid entering via the input conduit  18 . 
         [0012]    Though the configuration shown in  FIG. 2  shows a plurality of columns  22 , it should be appreciated that more or fewer columns  22  may be employed. Further, though a manifold configuration is shown, with working fluid traveling through a plurality of parallel pathways, other conduit configurations may be employed such as, for example, a series configuration wherein the working fluid travels through a plurality of columns. 
         [0013]    Each column  22  includes a working fluid conduit  24  through which working fluid flows. Working fluid conduit  24  is advantageously made from a thermally conductive material having a thermal conductivity greater than about 100 W/m-K, more advantageously greater than 250 W/m-K, and still more advantageously greater than about 400 W/m-K. Exemplary materials may include many metals, such as for example, aluminum, copper, or alloys thereof. 
         [0014]    A heat absorbing layer  26  includes a top major surface  28  and an opposed bottom major surface  30 . Heat absorbing layer  26  is provided to receive the electromagnetic energy of the solar rays contacting top major surface  28  and convert that energy to thermal energy. Thus, the heat absorbing layer  26  advantageously has a larger surface area facing the incoming solar radiation than the cylindrical conduit  24  alone. As can be seen in  FIG. 2 , heat absorbing layer  26  may be generally in the form of an elongated rectangle. In one embodiment, heat absorbing layer  26  may be generally planar. In other embodiments, the heat absorbing layer  26  may include a radius or a convex or concave shape in cross-section. In still further embodiments, the heat absorbing layer may include a generally curved or radiused central portion adapted to at least partially receive the conduit  24  therein. 
         [0015]    Heat absorbing layer  26  is preferably a thin element relative to the length and width of the major surfaces  28  and  30 . In certain embodiments, heat absorbing layer  26  may have a thickness of from about 0.25 mm to about 5 mm. Heat absorbing layer  26  is advantageously a metallic material. The metallic material may be, for example, aluminum, copper or alloys thereof. 
         [0016]    The bottom major surface  30  of heat absorbing layer  26  may be attached to a portion of the outer radial surface of conduit  24  using, for example, adhesives, welding, or mechanical fasteners. In other embodiments, bottom major surface  30  may be in contact with, but not fastened to the conduit  24 . In one embodiment, the heat absorbing layer  26  contacts the conduit  24  at a location generally bisecting the lateral width of the heat absorbing layer  26 . 
         [0017]    In order to improve absorption, the top major surface  28  may be coated with an emissive material. Emissive material  28  improves absorption and conversion of the solar energy to thermal energy. In one embodiment the coating results in an emissivity of greater than about ε=0.90. In further embodiments the coating may provide an emissivity of greater than about ε=0.95. In still further embodiments, the coating may provide an emissivity greater than about ε=0.98. 
         [0018]    Though individual absorbing layers  26  are shown for each column  22 , it should be appreciated that a single contiguous heat absorbing layer  26  may be provided for a plurality of conduits  24 . In other words, a single absorbing layer  26  may span a plurality of conduits  24 . 
         [0019]    A heat-spreader  32  is in thermal contact with at least a portion of bottom major surface  30  and to at least a portion of the outer radial surface of conduit  24 . As used herein, thermal conduct means physical contact sufficient to allow conductive heat transfer therebetween. As can be seen, in this manner, the conduit  24  is positioned between, and encompassed by, the heat absorbing layer  26  and heat-spreader  32 . In one embodiment, the heat-spreader layer  32  is flexible and conformable to follow the curvature of the conduit  24 . Thus, the heat-spreader  32  is advantageously in thermal contact with at least about 30% of the circumference of conduit  24 , more advantageously at least about 50% of the circumference, and still more advantageously at least about 75% of the circumference. 
         [0020]    In one embodiment, the heat-spreader  32  is in thermal contact with at least about 40% of the surface area of the bottom major surface  20  of heat absorber  26 . In other embodiments, the heat-spreader  32  is in thermal contact with at least about 60% of the surface area of the bottom major surface  20  of heat absorber  26 . In still further embodiments, the heat-spreader  32  is in thermal contact with at least about 80% of the surface area of the bottom major surface  20  of heat absorber  26 . 
         [0021]    Each heat-spreader  32  is optionally thin and sheet-like, having a top major surface  34  and a bottom major surface  36 . In one embodiment, the heat spreader  32  is between about 2 mm and about 0.05 mm thick. In this or other embodiments, the heat-spreader may be less than about 2 mm thick. In other embodiments the heat-spreader  32  may be less than about 1 mm thick. In still other embodiments, the heat-spreader may be less than about 0.5 mm thick. In still further embodiments, the heat-spreader may be less than about 0.1 mm thick. 
         [0022]    According to one or more embodiments, heat spreader  32  may be a sheet of a compressed mass of exfoliated graphite particles, a sheet of graphitized polyimide or combinations thereof. Such materials are highly anisotropic having greater thermal conductivity in the in-plane direction relative to the thru-plane conductivity. Advantageously, the anisotropic ratio is at least about 10, more advantageously at least about 20 and still more advantageously at least about 50. 
         [0023]    Where a heat spreader  32  includes multiple portions (i.e. curved and straight portions) it should be appreciated that the heat spreader  32  is advantageously a single contiguous sheet. In other embodiments the heat spreader may be multiple sheets joined together as by, for example, thermal adhesive, mechanical fasteners or other means. 
         [0024]    Each heat spreader  32  may have an in-plane thermal conductivity of greater than about 250 W/mK at about room temperature (using the Angstrom method to test at room temperature being approximately 25° C.). In another embodiment the in-plane thermal conductivity of spreader  32  is at least about 400 W/mK. In yet a further embodiment, the in-plane thermal conductivity of spreader  32  may be at least about 600 W/mK. In additional embodiments, the in-plane thermal conductivity may range from at least 250 W/mK to at least about 1500 W/mK. In these or other embodiments, the thru-plane thermal conductivity of spreader  32  may be less than about 10 W/mK. In other embodiments the thru-plane thermal conductivity is less than about 5 W/mK. In one embodiment, heat-spreader  32  has an in-plane thermal conductivity of at least about 1 times the in-plane thermal conductivity of the material of the heat absorbing layer  26 . In other embodiments, the heat-spreader  32  has an in-plane thermal conductivity of at least about 1.5 times the in-plane thermal conductivity of the material of the heat absorbing layer  26 . In still further embodiments, the heat-spreader  32  has an in-plane thermal conductivity of at least about 2 times the in-plane thermal conductivity of the material of the heat absorbing layer  26 . Any combination of the above in-plane thermal conductivities may be practiced. Suitable graphite sheets and sheet making processes are disclosed in, for example, U.S. Pat. Nos. 5,091,025 and 3,404,061, the contents of which are incorporated herein by reference. In one embodiment, the heat spreader may be made from, for example, eGRAF® Spreadershield™ sold by GrafTech International Holdings, Inc, the assignee of the instant application. 
         [0025]    In an optional embodiment, one or more heat-spreaders  32  may be resin reinforced. The resin may be used, for example, to improve the rigidity, strength and/or impermeability of spreader  32 . In combination with resin reinforcement, or in the alternative, one or more spreaders  32  may include carbon and/or graphite fiber reinforcement. 
         [0026]    Heat spreader  32  may be advantageously a relatively more conformable material than conventional materials that might be used in typical heat spreading applications (ex. aluminum). Use of heat spreader  32  may offer a reduction in interfacial thermal heat transfer resistance between spreader  32  and conduit  24  as compared to conduit  24  and heat absorbing layer  26 . Further, as discussed above, the surface area of the spreader  32  in contact with the conduit  24  is greater than the surface area of the heat absorbing layer  26  in contact with the conduit  24 . This enables greater heat transfer to the conduit  24  when compared to heat absorber systems lacking the heat spreader as disclosed herein. 
         [0027]    Heat spreader  32  may optionally be coated with a film adhesive to enable or improve attachment to conduit  24  and/or heat absorbing layer  26 . The adhesive layer should be advantageously thin enough not to appreciably impede heat transfer to the spreader  32 . The use of spreaders  32  incorporating an adhesive layer and supplied on a release liner can simplify the assembly of the heat absorber  12  by enabling “peel and stick” application to individual columns  22 . 
         [0028]    With reference now to  FIG. 4 , an alternate embodiment is disclosed wherein like numbers indicate like elements. As can be seen, the column  22  is substantially similar in cross-section, except that a second spreader  32   b  is provided between a first spreader  32   a  and the heat absorbing layer  26 . According to this embodiment, the first spreader  32   a  and second spreader  32   b  arranged to encompass the conduit  24  therebetween. Further, the second spreader  36   b  is positioned between the conduit  24  and the heat absorbing layer  26 . 
         [0029]    According to any one of the above embodiments the solar absorbing layer  26  may, instead of being a metal, be a graphite sheet material as described and disclosed hereinabove. In this or other embodiments, the top major surface  28  of the graphite solar absorbing layer may be knurled or otherwise roughened to improve surface emissivity. In embodiments wherein heat absorbing layer is a sheet of compressed expanded natural graphite, the top major surface  28  may be roughened by adhering a tape to the surface and then pulling off the tape to remove the top smooth surface and reveal a textured surface below. In still other embodiments, a graphite powder may be adhered to the top major surface  28 . In one embodiment, the graphite powder may be d90% less than about 500 μm. In other embodiments, d90% is less than about 200 μm. In still other embodiments, d90% is less than about 100 μm. In still further embodiments, d90% is less than about 55 μm. 
         [0030]    With reference now to  FIG. 5 , another embodiment is disclosed wherein like numbers indicate like elements. According to this embodiment, a solar absorbing layer  26  is provided without a spreader  32 . Solar absorbing layer  26  in accordance with this embodiment is a graphite material as described herein above, and includes one or more surface treatments described herein above. As can be seen, because the graphite material is relatively flexible, it may be conformed around conduit  24  in a manner so the solar absorbing layer  26  includes a center enveloping section  40  and opposed outwardly extending sections  42 . The center section  40  is advantageously in thermal contact with at least about 30% of the circumference of conduit  24 , more advantageously at least about 50% of the circumference, and still more advantageously at least about 75% of the circumference. 
         [0031]    The disclosures of all cited patents and publications referred to in this application are incorporated herein by reference in their entirety. The various embodiments disclosed herein may be practiced in any combination thereof. The above description is intended to enable the person skilled in the art to practice the invention. It is not intended to detail all of the possible variations and modifications that will become apparent to the skilled worker upon reading the description. It is intended, however, that all such modifications and variations be included within the scope of the invention that is defined by the following claims. The claims are intended to cover the indicated elements and steps in any arrangement or sequence that is effective to meet the objectives intended for the invention, unless the context specifically indicates the contrary.