RADIATIVE COOLING STRUCTURES AND SYSTEMS

Abstract

Polymer-based selective radiative cooling structures are provided which include a selectively emissive layer of a polymer or a polymer matrix composite material. Exemplary selective radiative cooling structures are in the form of a sheet, film or coating. Also provided are methods for removing heat from a body by selective thermal radiation using polymer-based selective radiative cooling structures.

Claims

1. A selective radiative cooling structure, the structure comprising a selectively emissive layer comprising a polymer and a plurality of dielectric particles dispersed in the polymer, the volume percentage of the dielectric particles in the selectively emissive layer ranging from 2% to 25% and the particles characterized by an average size ranging from 3 μm to 30 μm wherein the selective radiative cooling structure is characterized by an average emissivity ranging from 0.6 to 1.0 over the wavelength range 7 μm to 13 μm.

2. The selective radiative cooling structure of claim 1, wherein the structure is in the form of a sheet.

3. The selective radiative cooling structure of claim 1, wherein the selective radiative cooling structure provides a radiative heat flux from 50 W/m.sup.2 to 150 W/m.sup.2 at a working temperature of 15° C.

4. The selective radiative cooling structure of claim 1, wherein dielectric particles are characterized by an average size ranging from 4 μm to 10 μm.

5. The selective radiative cooling structure of claim 1, wherein the dielectric particles are selected from the group consisting of silicon dioxide (SiO.sub.2), calcium carbonate (CaCO.sub.3), silicon carbide (SiC), zinc oxide (ZnO), titanium dioxide (TiO.sub.2), and alumina (Al.sub.2O.sub.3).

6. The selective radiative cooling structure of claim 1, wherein the polymer is selected from the group consisting of a 4-methyl-1-pentene polymer, a 4-methyl-1-pentene copolymer and polyvinyl fluoride.

7. The selective radiative cooling structure of claim 1, wherein the selectively emissive layer is characterized by an average thickness from 10 μm to 3 mm.

8. The selective radiative cooling structure of claim 1, wherein the selective radiative cooling structure further comprises a protective film that is a solar transparent and weather-resistant polymer.

9. The selective radiative cooling structure of claim 1, wherein selective radiative cooling structure is characterized by a solar absorptivity from 0 to 0.2 over wavelength range 0.3 μm to 3 μm.

10. The selective radiative cooling structure of claim 1, further comprising a solar reflective layer in contact with the selectively emissive layer, the solar reflecting layer comprising a metal film or metal substrate, wherein the selectively emissive layer is characterized by an emissivity ranging from 0.6 to 1.0 over the wavelength range 7 μm to 13 μm and the selective radiative cooling structure is characterized by a solar reflectivity ranging from 0.8 to 1 over the wavelength range 0.3 μm to 3 μm.

11. The selective radiative cooling structure of claim 10, wherein the metal film is characterized by an average thickness from 30 nanometers to 1000 nanometers.

12. A method for removing heat from a body by selective thermal radiation, the method comprising the steps of: a. placing a selective radiative cooling structure in thermal communication with a surface of the body, the selective radiative cooling structure comprising a selectively emissive layer comprising a polymer, wherein the selectively emissive layer is in thermal communication with the body and the selective radiative cooling structure is characterized by an average emissivity ranging from 0.6 to 1.0 over the wavelength range 7 μm to 13 μm; b. transferring heat from the body to the selective radiative cooling structure; and c. radiating heat from selectively emissive layer of the selective radiative cooling structure.

13. The method of claim 12 wherein the selective radiative cooling structure is characterized by a solar absorptivity from 0 to 0.20 over the wavelength 0.3 μm to 3 μm.

14. The method of claim 12, wherein the selectively emissive layer further comprises a plurality of dielectric particles dispersed in the polymer, the volume percentage of the dielectric particles in the selectively emissive layer ranging from 2% to 25% and the particles characterized by an average size ranging from 3 μm to 30 μm.

15. The method of 12, wherein the body is a solar panel, the roof of an automobile, or a storage structure for cold energy, food or oil.

16. The method of claim 12, wherein the selective radiative cooling structure further comprising a solar reflecting layer comprising a metal film or substrate and is characterized by a solar absorptivity from 0 to 0.2 over the wavelength 0.3 μm to 3 μm.

17. The method of claim 16, wherein the selectively emissive layer further comprises a plurality of dielectric particles dispersed in the polymer, the volume percentage of the dielectric particles in the selectively emissive layer ranging from 2% to 25% and the particles characterized by an average size ranging from 3 μm to 30 μm.

18. The method of claim 16, wherein the body is a portion of a building or the roof of a structure.

19. The method of claim 16, wherein the body is a passive thermosiphon or an active microchannel array and wherein a heat transfer fluid circulates inside the body.

20. A cold collection system comprising a. a cold storage tank comprising a heat exchanger; b. a plurality of cold collection devices, each cold collection device configured to be in fluidic communication with the heat exchanger of the cold storage tank; and c. a plurality of selective radiative cooling structures, each selective radiative cooling structure being in thermal communication with the surface of one of the plurality of cold collection devices and each selective radiative cooling structure comprising a selectively emissive layer comprising a polymer, wherein the selectively emissive layer of each selective radiative cooling device is in thermal communication with one of the plurality of cold collection devices and each selective radiative cooling structure is characterized by an average emissivity ranging from 0.6 to 1.0 over the wavelength range 7 μm to 13 μm.

21. The cold collection system of claim 20, wherein the cold collection device is a passive thermosiphon or active microchannel array.

22. A method for making a selective radiative cooling structure, the method comprising the steps of: a. extruding a feed material comprising polymer through a die to form a film or sheet, wherein the polymer is characterized by an absorptivity of 0.6 to 1 in the range 5 μm to 50 μm; and b. cooling the film or sheet.

23. The method of claim 22, wherein the feed material further comprises a plurality of dielectric particles, the volume percentage of the dielectric particles ranging from 2% to 25% and the particles characterized by an average size ranging from 3 μm to 30 μm and the film or sheet is a composite film.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] FIG. 1: spectral irradiance of AM1.5 on earth surface and radiation spectrum of a black body surface at 15° C. through the atmospheric transparent window;

[0052] FIG. 2: schematic illustration of an emissive layer 5 of polymer 20.

[0053] FIG. 3A: schematic illustration of an emissive layer 5 including polymer 20 including particles dispersed in polymer matrix 30.

[0054] FIG. 3B shows the absorptivity/emissivity as a function of wavelength for a 50-μm-thick polymethylpentene film with 5% 8-μm-diameter solid silica microspheres.

[0055] FIG. 4A: schematic illustration of an emissive layer 5 including polymer 20 and particles 30 in contact with metal reflective layer 40.

[0056] FIG. 4B: schematic illustration of an emissive layer 5 including polymer 20 and particles 30; one face of the emissive layer is in contact with metal reflective layer 40 and the other face of the emissive layer is in contact with an anti-reflection layer 50, which in turn is in contact with a protection layer 60.

[0057] FIG. 4C: schematic illustration of an emissive layer 5 including polymer 20 and particles 30; one face of the emissive layer is in contact with a barrier layer 70, which in turn is in contact with a metal reflective layer 40 and the other face of the emissive layer is in contact with a protection layer 60.

[0058] FIG. 4D: schematic illustration of an emissive layer 5 including polymer 20 and particles 30; one face of the emissive layer is in contact with a barrier layer 70, which in turn is in contact with a metal reflective layer 40, which in turn is in contact with a thermal-conduction layer and the other face of the emissive layer is in contact with an anti-reflection layer 50, which in turn is in contact with a protection layer 60.

[0059] FIG. 4E: schematic illustration of an emissive layer 5 including polymer 20 and particles 30; one face of the emissive layer is in contact with a barrier layer 70, which in turn is in contact with a metal reflective layer 40 and the other face of the emissive layer is in contact with an anti-reflection layer 50, which in turn is in contact with a protection layer 60.

[0060] FIG. 5: schematic illustration of a radiative cooling sheet directly attached to the top of a solar cell.

[0061] FIG. 6: schematic illustration of a radiative cooling sheet directly attached to a radiant ceiling panel.

[0062] FIG. 7A: schematic illustration of a radiative cooling sheet with reflective layer directly attached to the roof of a built structure.

[0063] FIG. 7B: schematic illustration of a radiative cooling sheet with reflective layer directly attached to the roof of a car.

[0064] FIGS. 8A and 8B: FIG. 8A: schematic illustration of a radiative cooling structure in contact with multiple cold collection devices and FIG. 8B: cross section through FIG. 8A along A-A.

[0065] FIGS. 8C and 8D: FIG. 8C: schematic illustration of a thermosiphon that can be used in contact with the radiative cooling structure to collect cold enabled by radiative cooling structure and FIG. 8D: side view of the thermosiphon.

[0066] FIG. 9: schematic illustration of an active cooling system for buildings where radiative cooling structures is used to cool the cold storage system.

[0067] FIG. 10A: schematic illustration of multiple cold collection devices for supplemental cooling for a power plant condenser.

[0068] FIG. 10B: An exemplary 12-hour operating schedule to enable cold collection and storage using a passive thermosiphon to collect the cold achieved using selective radiative cooling structure.

[0069] FIG. 11: schematic illustration of an apparatus for extrusion of a polymer-based film.

[0070] FIG. 12: schematic illustration of an apparatus for coating a film.

[0071] FIG. 13A shows the emissivity/absorptivity as a function of wavelength for a 50-μm-thick bare polymethylpentene (TPX™) films and a 50 μm thick film with 5% 8-μm-diameter solid silica microspheres.

[0072] FIG. 13B shows the emissivity/absorptivity as a function of wavelength for polymethylpentene films of different thickness With 5% 8-μm-diameter solid silica microspheres.

[0073] FIG. 13C shows the absorptivity/emissivity as a function of wavelength for of 55-μm-thick polymethylpentene film with 5% 8-μm-diameter solid silica microspheres.

[0074] FIG. 14: illustrates the emissivity as a function of glass bead concentration for different size beads.

DETAILED DESCRIPTION OF THE INVENTION

[0075] In the drawings, like reference numbers refer to like elements.

[0076] The electromagnetic spectrum can be classified into several regions. Regions referred to herein are the infrared region (wavelength approximately 1 mm to 750 nm), visible region (wavelength approximately 750 nm to 400 nm) and the ultraviolet region (wavelength approximately 400 nm to 40 nm). The infrared region has also been classified into sub-regions using various classification schemes; in the ISO classification scheme the mid-infrared is approximately 3 μm to 50 μm. As used herein the radiant flux is the radiant energy per unit time (e.g. W), the irradiance is the radiant flux received by a surface per unit area (e.g. Wm.sup.−2) and the spectral irradiance is the irradiance of a surface per unit wavelength (e.g. Wm.sup.−2nm.sup.−1).

[0077] Electromagnetic radiation emitted from matter at a temperature above absolute zero may be referred to as thermal radiation. The solar spectrum refers to the distribution of electromagnetic radiation emitted by the sun. Most of the energy in the solar spectrum is concentrated from about 0.3 μm to about 3 μm, as can be seen from FIG. 1.

[0078] Emissivity specifies how well a real surface radiates energy as compared with a black-body and can range between 0 and 1. The directional spectral emissivity is the ratio of the emissive ability of the real surface to that of a black body. A total emissivity is averaged with respect to all wavelengths; a hemispherical emissivity is averaged with respect to all directions. As used herein, a selectively emissive layer is configured to emit electromagnetic radiation with an emissivity greater than zero at a temperature above absolute zero.

[0079] As used herein, a selectively emissive layer has an emissivity that is wavelength-selective. A selectively emissive layer is configured to thermally-generating electromagnetic emissions at temperatures other than absolute zero and is not a blackbody. Since emissivity correlates with absorptivity, a selectively emissive layer is also a selectively absorptive layer. In embodiments, the selectively emissive layer has high emissivity in at least some portions of the infrared portion of the spectrum, but has limited emission in at least some portions of the solar spectrum. Such a selectively emissive layer is also selectively absorptive, having high absorption in at least some portions of the infrared portion of the spectrum, but limited absorption in at least some portions of the solar spectrum.

[0080] As used herein, absorptivity is defined as the fraction of radiation energy incident on a surface of a body that is absorbed by the body. The incident radiation depends on the radiative conditions at the source of the incident energy. In an embodiment, the average absorptivity is a hemispherical absorptivity averaged over the wavelength range of interest.

[0081] As used herein, transmissivity is defined as the fraction of radiation energy incident on the surface of a body that is transmitted by the body. As used herein, transmissive material has a transmissivity on average greater than zero for radiation in the specified wavelength range. y. In an embodiment, the average transmissivity is a hemispherical transmissivity averaged over the wavelength range of interest. In some embodiments a transparent material has a transmittivity greater than 0.9 for the specified wavelength range.

[0082] As used herein reflectivity is defined as the fraction of radiation energy incident on a body that is reflected by the body. Solar reflectivity is defined as the fraction of radiation energy incident on a body that is reflected by the body in a specified region of the solar spectrum (e.g. 0.3 μm to 3 μm). In an embodiment, the solar reflectivity is averaged over the specified region of the spectrum. In an embodiment, the average reflectivity is a hemispherical reflectivity averaged over the wavelength range of interest.

[0083] As used herein, room temperature is approximately 20° C. to 25° C.

[0084] Embodiments of the present invention also pertain to methods for manufacturing radiative cooling structures on size scales useful for relevant cooling applications. In some embodiments of the present invention, dry polymer or polymer-based material is fed into an extruder, optionally an industrial extruder or die-caster, and is melted and extruded into thin sheets. As examples, the polymer fed to the extruder is in pellets, powdered, or any other dry form. In embodiments for producing composite emissive layers, the non-polymer materials, e.g., the dielectric or glass particles discussed above, are mixed into the polymer prior to, during or after melting of the polymer, and before extrusion. The non-polymer materials can be mixed in any manner and may be mixed to a uniform or near-uniform blend of the polymer and non-polymer materials. As discussed above, such polymer-based sheets can be 3 μm to several millimeters in thickness. The extruded sheets can be cast onto solid substrates or, in one embodiment, formed onto chilled rollers, forming standalone thin films.

[0085] In other embodiments, the polymer or polymer-based sheet can be manufactured by any one or combination of a variety of polymer production methods, including without limitation liquid or solution casting, blowing or blow molding, spinning, compression molding, spraying methods, and injection molding. For example, dry initial polymer material may be mixed with non-polymer particles, melted, and the melted mixture blown, compressed, or otherwise molded into any thickness sheets.

[0086] All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. All references cited herein are hereby incorporated by reference to the extent not inconsistent with the disclosure herewith. All references throughout this application, for example patent documents including issued or granted patents or equivalents; patent application publications; and non-patent literature documents or other source material; are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in this application (for example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference).

[0087] All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. References cited herein are incorporated by reference herein in their entirety to indicate the state of the art, in some cases as of their filing date, and it is intended that this information can be employed herein, if needed, to exclude (for example, to disclaim) specific embodiments that are in the prior art. For example, when a compound is claimed, it should be understood that compounds known in the prior art, including certain compounds disclosed in the references disclosed herein (particularly in referenced patent documents), are not intended to be included in the claim.

[0088] When a group of substituents is disclosed herein, it is understood that all individual members of those groups and all subgroups, including any isomers and enantiomers of the group members, and classes of compounds that can be formed using the substituents are disclosed separately. When a compound is claimed, it should be understood that compounds known in the art including the compounds disclosed in the references disclosed herein are not intended to be included. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure.

[0089] Every formulation or combination of components described or exemplified can be used to practice the invention, unless otherwise stated. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently. When a compound is described herein such that a particular isomer or enantiomer of the compound is not specified, for example, in a formula or in a chemical name, that description is intended to include each isomers and enantiomer of the compound described individual or in any combination. One of ordinary skill in the art will appreciate that methods, device elements, starting materials and synthetic methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such methods, device elements, starting materials and synthetic methods are intended to be included in this invention. Whenever a range is given in the specification, for example, a temperature range, a time range, or a composition range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure.

[0090] As used herein, “comprising” is synonymous with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, “consisting of” excludes any element, step, or ingredient not specified in the claim element. As used herein, “consisting essentially of” does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term “comprising”, particularly in a description of components of a composition or in a description of elements of a device, is understood to encompass those compositions and methods consisting essentially of and consisting of the recited components or elements. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.

[0091] The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.

[0092] In general the terms and phrases used herein have their art-recognized meaning, which can be found by reference to standard texts, journal references and contexts known to those skilled in the art. The preceding definitions are provided to clarify their specific use in the context of the invention.

[0093] Although the description herein contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of the invention. For example, thus the scope of the invention should be determined by the appended claims and their equivalents, rather than by the examples given.

[0094] The invention may be further understood by the following non-limiting examples.

EXAMPLE

Polymethylpentene-Based Radiative Cooling Films

[0095] FIG. 13A shows the emissivity/absorptivity as a function of wavelength for a 50-μm-thick bare polymethylpentene (TPX™) films and a 50 μm thick film with 5% 8-μm-diameter solid silica microspheres. The trace for the TPX™ film is lighter gray than the trace for the composite film. Polymethylpentene is index-matched with silica in the solar spectrum, but not in the infrared spectrum. The polymethylpentene film with silica microspheres significantly reduces the solar absorption but maintains a high emissivity at IR wavelengths of 7-13 μm. Without wishing to be bound by any particular belief, the silica spheres are believed to act as infrared scatterers and resonantly interact with infrared radiation, contributing to improved emissivity of the films.

[0096] FIG. 13B shows the emissivity/absorptivity as a function of wavelength for polymethylpentene films of different thickness with 5% 8-μm-diameter solid silica microspheres. (key: 50 μm film darker gray solid line, 80 μm film dashed line, 120 μm film lighter gray solid line). The thicker polymethylpentene film with silica microspheres increases the emissivity at IR wavelength of 7-13 μm without significant adsorption effect in solar spectrum.

[0097] FIG. 13C shows the absorptivity/emissivity as a function of wavelength for a 55-μm-thick polymethylpentene film with 5% 8-μm-diameter solid silica microspheres. The net day-time cooling power is 113 W/m.sup.2 with <4% averaged solar absorptivity and >0.8 IR emissivity on average. Percentages of fillers given in FIGS. 13A-13C are by volume.

[0098] FIG. 14 shows the projected radiative cooling power of an emissive layer with a reflective layer. The emissive layer includes a polymer sheet with embedded dielectric spheres according to different volume ratios of embedded spheres and film thicknesses.