COMPOSITE MATERIAL FOR PASSIVE RADIATIVE COOLING

Abstract

A composite material for passive radiative cooling including a base layer, and at least one emissive layer located adjacent to a surface of the base layer, wherein the at least one emissive layer is affixed to the surface of the base layer via a binding agent. Also disclosed are methods of applying passive coolers to articles and surfaces to be adapted for passive radiative cooling.

Claims

1-52. (canceled)

53. A method of providing a composite material for passive radiative cooling to a roof surface, said method comprising: applying to said roof surface a liquid suspension of microparticles in a liquid binding agent and curing said binding agent so as to form a thermally-emissive layer on said roof surface, whereby said thermally-emissive layer is affixed to said roof surface via said binding agent.

54. The method of claim 53, wherein said object comprises a base layer to which said liquid suspension is applied, said base layer comprising a reflective substrate.

55. The method of claim 54, wherein said reflective substrate is composed of at least one of aluminum, silver, glass, polyurethane, nylon, and polyethylene fibers.

56. The method of claim 54, wherein said reflective substrate comprises paint.

57. The method of claim 53, wherein said binding agent is composed of a polymer material.

58. The method of claim 53, wherein said binding agent is transparent.

59. The method of claim 53, wherein said binding agent includes a characteristic thickness less than or equal to approximately 50 μm.

60. The method of claim 53, wherein said at least one emissive layer is composed of silica material.

61. The method of claim 53, wherein said at least one emissive layer comprises a plurality of microparticles.

62. The method of claim 61, wherein each of said plurality of microparticles is composed of silica material.

63. The method of claim 61, wherein each of said plurality of microparticles includes a characteristic dimension between about 5 to about 50 μm.

64. The method of claim 61, wherein each of said plurality of microparticles includes a characteristic dimension less than or equal to 30 μm.

65. The method of claim 53, wherein said liquid suspension is applied in the form of a spray.

66. A liquid suspension composition adapted to form a passive radiative cooling coating on a surface, said composition comprising: a liquid suspension of microparticles in a liquid binding agent, said binding agent adapted to be disposed upon a surface and cured so as to form a thermally-emissive layer on said surface, whereby said thermally-emissive layer is affixed to said surface via said binding agent.

67. The method of claim 66, wherein said binding agent is composed of a polymer material.

68. The method of claim 66, wherein said binding agent is transparent.

69. The method of claim 66, wherein said microparticles are composed of silica material.

70. The method of claim 69, wherein each of said plurality of microparticles includes a characteristic dimension between about 5 to about 50 μm.

71. The method of claim 69, wherein each of said plurality of microparticles includes a characteristic dimension less than or equal to 30 μm.

72. The method of claim 66, wherein said liquid suspension is of sufficiently reduced viscosity to be applied to a surface in the form of a spray.

73. A liquid suspension composition adapted to form a passive radiative cooling coating on a surface, said composition comprising: a liquid suspension of a microparticles composed of silica material in a liquid binding agent composed of a transparent polymer material, said binding agent adapted to be disposed upon a surface and cured so as to form a thermally-emissive layer on said surface, whereby said thermally-emissive layer is affixed to said surface via said binding agent.

74. The method of claim 73, wherein each of said plurality of microparticles includes a characteristic dimension between about 5 to about 50 μm.

75. The method of claim 73, wherein each of said plurality of microparticles includes a characteristic dimension less than or equal to 30 μm.

76. The method of claim 73, wherein said liquid suspension is of sufficiently reduced viscosity to be applied to a surface in the form of a spray.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0029] FIG. 1 illustrates a schematic cross-sectional view of a composite material for passive radiative cooling according to one embodiment of the present disclosure;

[0030] FIG. 2 illustrates a cross-sectional view of a microparticle according to one embodiment of the present disclosure;

[0031] FIG. 3 illustrates a graph of a cooling potential of composite materials against ambient daytime temperatures;

[0032] FIG. 4 illustrates a graph of a cooling potential of composite materials against ambient nighttime temperatures;

[0033] FIG. 5 illustrates a schematic cross-sectional view of a roof surface bearing a composite material for passive radiative cooling according to another embodiment of the present disclosure; and

[0034] FIG. 6 illustrates a schematic cross-sectional view of a roof surface bearing a composite material for passive radiative cooling according to still another embodiment of the present disclosure.

[0035] FIG. 7 is a stylized representation of a layer of a composite material for passive radiative cooling according to one embodiment of the present disclosure.

DESCRIPTION

[0036] For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended.

[0037] To enhance the emissivity in the 8-13 μm wavelength range or in the wavelength range supported by a blackbody with temperatures in the range of 250-350° K, a composite material, generally indicated at 10 is applied to the surface of an object. This leads to the preferential emission of light in the 8-13 μm range or in the wavelength range supported by a blackbody with temperatures in the range of 250-350° K. The preferential emission of light is embodied in the emissivity spectrum.

[0038] In some embodiments, the composite material 10, as shown in FIG. 1, includes a base layer 12 composed of a reflective substrate composed of at least one of aluminum, silver, glass, polyurethane, nylon, and polyethylene fibers. In some embodiments, the reflective substrate comprises paint. In some embodiments, the paint comprises white paint. In some embodiments, the reflective substrate comprises glue. The base layer 12 may be placed in thermal contact with an object to be cooled.

[0039] Immediately above the base layer 12 is at least one emissive layer 14 in some embodiments. The at least one emissive layer 14 may be arranged in a hexagonal monolayer, square monolayer, irregular monolayer, or irregular combination of between one and ten layers; exposed to sunlight and also to the atmosphere and paths for radiating thermal energy. In an embodiment, the at least one emissive layer 14 is composed of a plurality of microparticles 16. In one embodiment, each of the plurality of microparticles 16 may be formed in a geometric shape, and composed of a silica material. For example, the at least one emissive layer 14 may include a plurality of microspheres. The plurality of microparticles 16 may also be formed in square, cylindrical, or an irregular geometric shape to name a few non-limiting examples.

[0040] In an embodiment, with reference to FIG. 2, each of the plurality of microparticles 16 includes a characteristic dimension 18. In one embodiment, the characteristic dimension 18 is between about 5 to about 50 microns. In another embodiment, the characteristic dimension 18 is less than 30 microns. In still another embodiment, the characteristic dimension 18 is between about 10 and about 20 microns.

[0041] In an embodiment, with reference to FIGS. 1 and 2, the at least one emissive layer 14 is bonded to the base later 12 via a binding agent 20. The binding agent 20 may be composed of a transparent, polymer material. In some embodiments, the binding agent 20 may be uniformly applied to the at least one emissive layer 14 or applied to each of the plurality of microparticles 16, as shown in FIG. 2, via an electromagnetic brush (EMB) process or other suitable process. In some embodiments, the EMB process of the present disclosure utilizes a rotating wire brush in conjunction with static electromagnetic fields to deposit the plurality of microparticles 16 to the base layer 12. In some embodiments, the EMB process requires the use of the binding agent 20 coating each of the plurality of microparticles 16 to achieve adhesion under subsequent heating. When applied to each of the plurality of microparticles 16 via an electromagnetic brush (EMB) process, the binding agent 20 may have a characteristic thickness 22 between about 1 and about 50 microns in one embodiment. The binding agent 20 is melted after application of the plurality of microparticles 16 to the emissive layer 14 to form the binding agent 20 layer illustrated in FIG. 1.

[0042] In some embodiments, the polymer binder may be in the form of a polyacrylic base, such as those sold commercially under the MINWAX® trademark.

[0043] In one embodiment, a liquid suspension of silica microspheres may be sprayed and cured to create the emissive layer. This liquid will contain the polymer that, after spray deposition, cures to form a film containing the microspheres, the film in some embodiments the film resembling a dried, glue-like layer. Referring to FIG. 1, this Figure may be viewed as an application to an article or surface (with or without an intervening base layer 12) onto which a liquid suspension of silica microspheres 16 may be sprayed so as to be cured to form a cured binding agent 20 so as to form the emissive layer 14 on the article or surface as desired. Accordingly, this embodiment also includes the described deposition method of creating the described film structure.

[0044] In one such embodiment, the liquid suspension of silica microspheres may be sprayed and cured to create the emissive layer on an article or surface to be adapted for passive cooling, such as a building surface or roof surface. In such an embodiment, again referring to FIG. 1 analogously, this Figure may be viewed as an application to a building surface or roof surface (with or without an intervening base layer 12) onto which a liquid suspension of silica microspheres 16 may be sprayed so as to be cured to form a cured binding agent 20 so as to form the emissive layer 14 on the article or surface as desired. In such an embodiment, an existing roof surface may be considered a reflective surface as described herein onto which the liquid suspension of silica microspheres 16 may be sprayed so as to be cured to form a cured binding agent 20 so as to form the emissive layer 14 on the roof surface.

[0045] FIG. 5 shows an example of such an embodiment showing an application to a building surface or roof surface bearing a composite coating 110 (without an intervening base layer) formed by a liquid suspension of silica micro spheres 116 being sprayed so as to be cured to form a cured binding agent 120 so as to form the emissive layer 114 on the roofing surface 112. In such an embodiment, an existing roof surface may be considered a reflective surface as described herein onto which the liquid suspension of silica micro spheres 116 may be sprayed so as to be cured to form a cured binding agent 120 so as to form the emissive layer 114 on the roof surface, the characteristic thickness 123 of the binding agent 120 between about 1 and about 50 microns in one embodiment.

[0046] In another embodiment, the liquid suspension of silica microspheres may be sprayed and cured to create the emissive layer on an article or surface to be adapted for passive cooling, such as a building surface or roof surface. In such an embodiment, again referring to FIG. 1 analogously, this Figure may be viewed as an application to a building surface or roof surface (with or without an intervening base layer 12, or alternatively wherein the building surface or roof surface serves as the base layer) onto which a liquid suspension of silica microspheres 16 may be sprayed so as to be cured to form a cured binding agent 20 so as to form the emissive layer 14 on the article or surface as desired. In such an embodiment, an existing roof surface may be considered a reflective surface as described herein onto which the liquid suspension of silica microspheres 16 may be sprayed so as to be cured to form a cured binding agent 20 so as to form the emissive layer 14 on the roof surface. Thus, an embodiment of the invention is a composite material for passive radiative cooling may include a base layer (such as a building surface or roof surface), and at least one emissive layer located adjacent to a surface of the base layer, wherein the at least one emissive layer is affixed to the surface of the base layer via a binding agent.

[0047] One such embodiment is shown in FIG. 6 showing an application to a roof surface 213 (with an intervening base layer 212) to form a roof surface bearing a composite coating 210 formed by a liquid suspension of silica microspheres 216 sprayed so as to be cured to form a cured binding agent 220 so as to form the emissive layer 214 on the roof surface 213. In such an embodiment, the intervening base layer 212 may be considered a reflective surface as described herein onto which the liquid suspension of silica microspheres 216 may be sprayed so as to be cured to form a cured binding agent 220 so as to form the emissive layer 214 on the roof surface. Thus, an embodiment of the invention is a composite material for passive radiative cooling may include a base layer (such as a building surface or roof surface), and at least one emissive layer located adjacent to a surface of the base layer, wherein the at least one emissive layer is affixed to the surface of the base layer via a binding agent having a characteristic thickness 223 of the binding agent 220 between about 1 and about 50 microns in one embodiment.

[0048] FIG. 7 is a stylized representation of a layer of a composite material for passive radiative cooling according to one embodiment of the present disclosure. FIG. 7 shows one embodiment of the composite material for passive radiative cooling that include an emissive layer that may include up to approximately ten layers of irregular spaced silica particles embedded in a polymer medium, with a reflective base layer, though more layers may be incorporated as may be beneficial to a given application. The reflective base layer reflects a majority of solar energy, which is transmitted through the transparent emissive layer. The emissive layer emits radiation in the infrared radiation band including the 8-13 micron atmospheric window.

[0049] Other embodiments include the application of passive cooling compositions to fabrics and garments, and the fabrics and garments so treated. These embodiments include the application of a liquid suspension of silica microspheres 16 may be sprayed so as to be cured to form a cured binding agent 20 so as to form the emissive layer 14 on the fabric and garment as desired.

[0050] In some embodiments, a dry dusting process, rather than the EMB process, is utilized by dry dusting the plurality of microparticles 16 over the base layer 12. The dry dusting process achieves a rough approximation of the uniform thin layer using a standard powder duster or squeeze bottle filled with the plurality of microparticles 16. In some embodiments, the dry dusting process is used with the binding agent 20 coating each of the plurality of microparticles 16 to achieve adhesion under subsequent heating. In some embodiments, the dry dusting process is used when the surface of base layer 12 comprises a reflective substrate comprising an adhesive layer that will dry, creating an adhesion and surface morphology. In some embodiments, the reflective substrate is glue or paint, to name a couple of non-limiting examples. It is envisioned that any suitable reflective substrate may be employed in the dry dusting process utilized in accordance with the embodiments of the present disclosure. In some embodiments, when base layer 12 comprising an adhesive layer is utilized in the dry dusting process, use of binding agent 20 is optional.

[0051] In some embodiments, wet printing and fusing is utilized in lieu of the EMB process and dry dusting process. In some embodiments, the wet printing and fusing process uses a printer to print a liquid suspension of the plurality of microparticles 16 directly on the base layer 12. In some embodiments, the wet printing and fusing process requires subsequent heating through infrared heating or a hot roller process to cure the binding agent 20 coating each of the plurality of microparticles 16 to achieve adhesion.

[0052] In one embodiment, the binding agent 20 layer shown in FIG. 1 includes a characteristic thickness 23 between about 1 and about 50 microns in one embodiment. It will be appreciated that the characteristic thickness 23 may be greater than approximately 50 μm in other embodiments. It will further be appreciated that when the characteristic thickness 22 is larger than the characteristic dimension 18 of the plurality of microparticles 16, a smooth surface morphology is created. Additionally, a rough surface morphology is created when the characteristic thickness 23 is less than the characteristic dimension 18 of the plurality of microparticles 16.

[0053] As shown in FIGS. 3 and 4, a computational study was performed to study the cooling power of a composite material 10 in one embodiment composed of an aluminum-silver-silica (SiO.sub.2) combination. The computational study of composite material 10 (shown by line 24), yielded a cooling potential of approximately 113 W/m.sup.2 at an ambient temperature of approximately 300° K when exposed to direct sunlight. The computational study also analyzed an aluminum-silica combination, shown by line 26, and yielded a cooling potential of approximately 82 W/m.sup.2 at an ambient temperature of approximately 300° K when exposed to direct sunlight. For the ideal case, the computational study assumed a fictional ideal material that is purely reflective in all bands other than 8-13 microns. In the 8-13 micron band, the ideal material is purely emitting (i.e., emissivity=1). As shown, the aluminum-silver-silica combination outperforms the ideal case, shown by line 28, above an ambient temperature of approximately 310° K due to a broader emission spectrum above approximately 13 μm thereby accessing narrower atmospheric window bands in the 20 to 25 micron range.

[0054] As shown in FIG. 4, the computational study of the aluminum-silver-silica combination, shown by line 30, yields a cooling potential exceeding approximately 250 W/m.sup.2 when exposed to the nighttime sky and outperforms the ideal case, line 32, above an ambient nighttime temperature of 255° K. It will therefore be appreciated that the composite material 10 includes at least one thermally-emissive layer 14 bonded to a base layer 12, via a binding agent 20, where the composite material 10 produces a positive cooling potential in both daytime and nighttime ambient temperatures.

[0055] In still other embodiments, compositions of the present invention may be applied to fabric where passive cooling may be beneficially applied, such to as a tent, awning or even an article of clothing.

[0056] In yet another embodiment, the compositions of the present invention may be included in sunscreen formulation in a base or carrier material otherwise suitable to topical applications, as are known and used in the art. Such sunscreen formulation embodiments may include a suspension of the plurality of microparticles in a base other than a transparent polymer.

[0057] While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.