Patent Application
20130276777
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.
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.
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.
With reference now to
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.
With reference now to
Though the configuration shown in
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
With reference now to
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.
With reference now to
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.
