US20130276777A1

US20130276777A1 - Solar Absorber

Info

Publication number: US20130276777A1
Application number: US13/451,855
Authority: US
Inventors: Bradley E. Reis; Robert Anderson Reynolds, III; G. M. Fazley Elahee; Gregory P. Kramer; Mark Pollock
Original assignee: Graftech International Holdings Inc
Current assignee: Graftech International Holdings Inc
Priority date: 2012-04-20
Filing date: 2012-04-20
Publication date: 2013-10-24
Also published as: KR20130006402U; DE202013003733U1; JP3184471U; CN203771748U

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

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

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.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a solar absorber system.

FIG. 2 is an elevated top view of a solar absorber including a plurality of columns.

FIG. 3 is a cross-section view taken along A-A of FIG. 2.

FIG. 4 is an alternate cross-section view taken along A-A of FIG. 2.

FIG. 5 is a second alternate cross-section view taken along A-A of FIG. 2.

DETAILED DESCRIPTION

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.
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 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.
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.
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 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.
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 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.
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 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.
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.

Claims

What is claimed:

1. A heat absorber assembly comprising:

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 positioned proximate to said bottom major surface and for receiving a fluid therethrough;

a heat spreader in contact with at least a portion of the bottom major surface of said solar absorber layer, said conduit being positioned between said solar absorber layer and said heat spreader; and

wherein said heat spreader is a thermally anisotropic material having an in-plane thermal conductivity of at least about 250 W/mK.

2. The heat absorber assembly according to claim 1 wherein said thermally anisotropic material comprises a sheet of a compressed mass of exfoliated graphite particles.

3. The heat absorber assembly according to claim 1 wherein said thermally anisotropic material comprises a sheet of graphitized polyimide.

4. The heat absorber assembly according to claim 1 wherein said heat spreader is in contact with at least about 60 percent of the surface area of said bottom major surface of said heat absorber.

5. The heat absorber assembly according to claim 1 wherein said heat spreader is in contact with at least about 80 percent of the surface area of said bottom major surface of said heat absorber.

6. The heat absorber assembly according to claim 1 wherein said conduit includes a circumference and said heat spreader is in contact with at least about 30 percent of said conduit circumference.

7. The heat absorber assembly according to claim 1 wherein said conduit includes a circumference and said heat spreader is in contact with at least about 50 percent of said conduit circumference.

8. The heat absorber assembly according to claim 1 wherein said conduit includes a circumference and said heat spreader is in contact with at least about 75 percent of said conduit circumference.

9. The heat absorber assembly according to claim 1 wherein said heat spreader has a thickness of less than about 2 mm.

10. The heat absorber assembly according to claim 1 wherein said heat spreader has a thickness of less than about 1 mm.

11. The heat absorber assembly according to claim 1 wherein said heat spreader has a thickness of less than about 0.1 mm.

12. A heat absorber assembly comprising:

a conduit in contact with said bottom major surface and for receiving a fluid therethrough; and

wherein said absorber is a thermally anisotropic material having an in-plane thermal conductivity of at least 250 W/mK.

13. The heat absorber assembly according to claim 12 wherein said thermally anisotropic material comprises a sheet of a compressed mass of exfoliated graphite particles.

14. The heat absorber assembly according to claim 12 wherein said thermally anisotropic material comprises a sheet of graphitized polyimide.

15. The heat absorber assembly according to claim 13 wherein said solar absorber layer includes a center enveloping section and opposed outwardly extending sections.

16. The heat absorber assembly according to claim 15 wherein said conduit has a circumference and said center enveloping section is in contact with at least about 30 percent of said circumference

17. The heat absorber assembly according to claim 15 wherein said conduit has a circumference and said center enveloping section is in contact with at least about 50 percent of said circumference

18. The heat absorber assembly according to claim 15 wherein said conduit has a circumference and said center enveloping section is in contact with at least about 75 percent of said circumference

19. The heat absorber assembly according to claim 12 wherein a graphite powder is adhered to said top major surface of said solar absorber layer.

20. The heat absorber assembly according to claim 19 wherein said graphite powder comprises d90% less than about 500 μm.

21. The heat absorber assembly according to claim 19 wherein said graphite powder comprises d90% is less than about 200 μm.

22. The heat absorber assembly according to claim 19 wherein said graphite powder comprises d90% is less than about 100 μm.