PASSIVE RADIATIVE COOLING OF WINDOW STRUCTURES

Abstract

A system, apparatus, and method for passive cooling via selective radiative emission are described. The disclosed apparatus uses an optically transparent chemical coating that can cool a transparent substrate, such as a window, even while exposed to sunshine, and without being coupled to a liquid coolant or electrical source. The apparatus can include a transparent substrate and an optically transparent chemical coating on the substrate for radiating heat away from the substrate.

Claims

1. An apparatus for passive cooling via selective radiative emission, comprising: a transparent substrate; and a coating positioned on the transparent substrate, wherein the coating is configured to emit more thermal infrared radiation to the atmosphere than an amount of infrared radiation that can be received by the coating.

2. The apparatus of claim 1, wherein the coating has a thermal blackbody emissivity coefficient of at least 0.9 corresponding to an infrared radiation wavelength range.

3. The apparatus of claim 2, wherein the infrared radiation wavelength range comprises wavelengths in an atmospheric transparency window of 8 micrometers to 13 micrometers.

4. The apparatus of claim 1, wherein the coating includes a polymer.

5. The apparatus of claim 1, wherein the chemical coating includes one or more polymers selected from the group consisting of: ethyl cellulose, poly ethyl methacrylate (PEMA), poly methyl methacrylate (PMMA), polyvinyl butyral (PVB), cellulose acetate, polyethylene, polypropylene, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyesters, polyacrylic acid, polycarbonates, and a copolymer mixture.

6. The apparatus of claim 1, wherein the transparent substrate comprises a piece of glass, and wherein the coating is coated on the piece of glass.

7. The apparatus of claim 6, further comprising: a second piece of glass that is at least partially transparent in the infrared wavelength range; and a second coating on the second piece of glass for radiating additional heat; wherein the second piece of glass is stacked with the first piece of glass.

8. The apparatus of claim 6, further comprising one or more layers of reflective coating for reflecting incident sunlight.

9. The apparatus of claim 1, wherein the transparent substrate is part of a window, an automotive window, or an automotive sunroof.

10. The apparatus of claim 1, wherein the coating is not coupled to a liquid coolant source or an electrical source.

11. A method for passive cooling via selective radiative emission, the method comprising: applying a coating on a transparent substrate, wherein the coating is configured to emit more thermal infrared radiation to the atmosphere than an amount of infrared radiation that can be received by the coating.

12. The method of claim 11, wherein the coating has a thermal blackbody emissivity coefficient of at least 0.9 corresponding to an infrared radiation wavelength range.

13. The method of claim 12, wherein the infrared radiation wavelength range comprises wavelengths in an atmospheric transparency window of 8 micrometers to 13 micrometers.

14. The method of claim 11, wherein the coating includes a polymer.

15. The method of claim 11, wherein the coating includes one or more polymers selected from the group consisting of: ethyl cellulose, poly ethyl methacrylate (PEMA), poly methyl methacrylate (PMMA), polyvinyl butyral (PVB), cellulose acetate, polyethylene, polypropylene, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyesters, polyacrylic acid, polycarbonates, and a copolymer mixture.

16. The method of claim 11, wherein the transparent substrate comprises a piece of glass.

17. The method of claim 16, further comprising applying a second optically transparent chemical coating on a second piece of glass to radiate additional heat, wherein the second piece of glass is stacked with the first piece of glass and is at least partially transparent in the infrared wavelength range.

18. The method of claim 16, further comprising applying one or more layers of reflective coating to reflect incident sunlight.

19. The method of claim 11, wherein the transparent substrate is part of a window, an automotive window, or an automotive sunroof.

20. The method of claim 11, wherein the coating is not coupled to a liquid coolant source or an electrical source.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0016] FIG. 1 shows contributions to heating and cooling in an automotive sunroof.

[0017] FIG. 2 shows an operational principle of passive radiative cooling, in accordance with embodiments of the present invention.

[0018] FIG. 3 illustrates exemplary polymers and presents a schematic diagram of their vibrational excitations.

[0019] FIG. 4A shows an exemplary layered structure of a passive radiative cooling system, according to one embodiment of the present invention.

[0020] FIG. 4B shows an exemplary layered structure of a passive radiative cooling system, according to one embodiment of the present invention.

[0021] FIG. 4C shows an exemplary layered structure of a passive radiative cooling system, according to one embodiment of the present invention.

[0022] FIGS. 4D and 4E show exemplary layered structures of a passive radiative cooling system, according to embodiments of the present invention.

[0023] FIG. 5A shows an exemplary emissivity measurement of the passive radiative cooling system, according to one embodiment of the present invention.

[0024] FIG. 5B illustrates an exemplary transmittance measurement of the passive radiative cooling system, according to embodiments of the present invention.

[0025] FIG. 6 shows exemplary cooling effects, based on experimental results using the passive radiative cooling system.

[0026] In the figures, like reference numerals refer to the same figure elements.

DETAILED DESCRIPTION

[0027] The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

Overview

[0028] Embodiments of the present invention provide a system, apparatus, and method for passive cooling of a transparent substrate via selective radiative emission. The disclosed apparatus uses an optically transparent chemical coating that can cool a transparent substrate, such as a window, even while exposed to sunshine, thereby reducing the need for a liquid coolant or electrical source. To do so, embodiments of the present invention use a material that can emit more thermal infrared (IR) radiation than it receives from the atmosphere, even at ambient temperature. In the absence of sunlight, the system can reach a temperature below the ambient one.

[0029] By providing passive cooling in glass windows, the system can decrease the interior temperature in an enclosure (such as a room or car interior) without the need for any electricity or liquid coolant. It is estimated that the present system can provide additional cooling capacity compared with conventional window glazing. This cooling effect can be obtained without alteration of the glass' visible-wavelength transmittance or its color. Moreover, the disclosed solution can use thin, inexpensive polymer films, and hence is cost-effective and scalable.

Contributions to Heating and Cooling

[0030] FIG. 1 shows contributions to heating and cooling in an interior space enclosed by an optically transparent window, such as the interior of an automobile with a sunroof 102. Table 1 further summarizes these contributions to heating and cooling in the sunroof 102, and lists their net effect on the temperature inside the car, T.sub.car. As shown in FIG. 1, sunlight 104 shines on sunroof 102 and can be absorbed by sunroof 102. Sunlight 106 can also pass through the sunroof to the interior of the car and be absorbed there.

[0031] The car, the sunroof, and the car's environment can all radiate thermally (i.e., corresponding to a blackbody radiation spectrum at their current temperatures, typically peaking in the IR range). Thus, thermal radiation 108 from the atmosphere and the car's external surroundings can arrive at sunroof 102, and likewise thermal radiation 110 from the car's interior can arrive at sunroof 102. When sunroof 102 absorbs radiation 108 and 110, it contributes to raising T.sub.car. Sunroof 102 can also emit external thermal radiation 112 to the car's environment and internal thermal radiation 114 to the car's interior. Finally, convection currents outside the car 116 and inside the car 118 contribute to lowering T.sub.car by carrying heat from the car to the sunroof when T.sub.car>T.sub.glass, and from the sunroof to the environment when T.sub.glass>T.sub.ambient.

[0032] Table 1 also lists properties of the sunroof that could be engineered to tune each heating and cooling contribution in order to keep T.sub.car low. Thus, for the thermal radiation contributions, the thermal emissivity coefficient of the sunroof in the IR wavelength region can affect T.sub.car. For sunlight 104 and 106, the sunroof's absorbance and transmittance in the solar wavelengths affects T.sub.car. For both the convection and thermal emission contributions, the sunroof's temperature T.sub.glass would affect T.sub.car. Although T.sub.glass is not a property of the sunroof that can be directly engineered, it is determined by a balance between heating and cooling contributions similar to the ones discussed above, and therefore can be influenced by properties like the sunroof's thermal emissivity coefficient and solar absorbance and transmittance.

TABLE-US-00001 TABLE 1 Contributions to heating and cooling of a car equipped with an automotive sunroof. Sunroof glazing Net effect on variables to tune Source Contribution temperature in car effect Thermal From sunroof Lower Emissivity, T.sub.glass radiation 112, 114 From sky 108 Raise Emissivity From car 110 Raise Emissivity Sunlight Absorbed by Lower Solar absorbance glass 104 Absorbed by Raise Solar transmittance interior 106 Convection Outside 116 Lower (T.sub.glass > T.sub.glass T.sub.ambient) Inside 118 Lower (T.sub.glass < T.sub.car) T.sub.glass

[0033] In particular, notice that thermal radiation 112 and 114 from the sunroof carries heat away, and therefore tuning the emissivity could help lower T.sub.car, as shown in Table 1. A body's thermal emissivity and absorptivity (also known as absorptance) coefficients are typically equal. Under normal circumstances, thermal emission is balanced by equal thermal absorption, which would bring the sunroof into thermal equilibrium with the radiation from its environment. However, the Earth's atmosphere has a transparency range of wavelengths (also called the atmospheric or sky transparency window), in which the atmosphere has low thermal emissivity and absorptivity. Therefore, by selectively emitting in this wavelength range, the present invention can emit heat that will not be absorbed by its surroundings, engendering a net outflow of heat, as will be described in more detail below.

Operational Principle of Passive Radiative Cooling

[0034] This section describes the operating principles of passive radiative cooling, according to embodiments of the present invention. In general, the system can produce a negative heat flow (resulting in cooling) if the window is engineered to emit more radiation than it absorbs. This can be accomplished if the window emits radiation at wavelengths where the atmosphere is relatively transparent (such as the sky transparency window). Thus, the system operates by virtue of having a selectively enhanced thermal emissivity in an infrared (IR) radiation wavelength range, which may correspond to the Earth's atmospheric transparency window.

[0035] FIG. 2 shows the operational principle of passive radiative cooling, in accordance with embodiments of the present invention. As shown, sunlight 202 can arrive at window or sunroof 200 containing selectively emissive film 204 as part of the passive cooling system. Film 204 can transmit sunlight, which then arrives at low-emissivity reflective coating 206. Low-emissivity coating 206 can reflect broadband sunlight 208, especially in the IR wavelengths. Coating 206 thereby prevents substantial amounts of solar IR radiation from passing through window or sunroof 200 and heating the enclosed room or car. However, visible-wavelength range sunlight 212 can pass through coating 206 and glass substrate 210 into the enclosed space, thereby allowing a user to see through the window.

[0036] Meanwhile, selectively emissive film 204 can absorb thermal radiation from both the enclosed space and the exterior environment. Film 204 can be configured to absorb and emit thermal radiation 214 selectively, as described above, in order to maintain a non-equilibrium, directed heat flow, and cool the system.

[0037] In particular, the system has a high emissivity coefficient over the atmospheric transparency window, which corresponds to wavelengths 8 m<<13 m. Due to these desirable characteristics, the system can emit strong thermal radiation in the atmospheric transparency window, which is a range of wavelengths that is unlikely to be absorbed by the atmosphere or other surroundings. Specifically, since the sky is relatively transparent to radiation in these wavelengths, upward-emitted thermal radiation 214 is likely to continue propagating for great distances, possibly through the atmosphere and on to outer space. As a result, system 200 can maintain a non-equilibrium state, emitting 214 to its environment more thermal radiation than it receives in the atmospheric transparency window.

Optically Transparent Coating for Passive Radiative Cooling

[0038] Implementing the passive radiative cooling principles described above can be based on an appropriate, selectively emitting material for the chemical coating. Ideally, an outer glazing surface with a high emissivity >0.9 in the atmospheric transparency window can result in better cooling. It is frequently assumed that glass, having a thermal emissivity coefficient of 0.84, is effective at passive radiative cooling. Yet despite its high thermal emissivity, glass selectively emits at wavelengths of 6.6-8.2 m. This wavelength range is practically outside the 8-13 m atmospheric transparency window, so glass' passive radiative cooling effect is sub-optimal.

[0039] Embodiments of the present invention improve significantly over existing systems by implementing the passive radiative cooling principles described above with inexpensive materials, without any need for complex techniques such as lithography. Moreover, the present system can achieve superior passive cooling to other systems, can cool effectively even when exposed to intense sunlight, and is optically transparent.

[0040] The system can achieve these results by using a chemical coating containing selectively emissive polymers. FIG. 3 illustrates exemplary polymers and presents a schematic diagram of their vibrational excitations. As shown, polymers such as polycarbonate 304, polyethylene terephthalate (PET) 306, and cellulose acetate 308 have specific vibrational spectra. These spectra correspond to specific frequencies at which the molecular structures of the polymers have vibrational modes. Therefore, when excited thermally, polymers such as 304, 306, and 308 can emit IR radiation at these frequencies.

[0041] For passive radiative cooling, a desirable polymer film can have high visible transmittance, low solar absorbance, and average emissivity >0.9 in the 8-13 m sky transparency window. Examples of polymers satisfying these requirements are PET, cellulose acetate, polycarbonate and ethyl cellulose. (Note that these polymers have high emissivity >0.9 weighted for thermal radiation from inside the room). Specifically, in various embodiments, the selectively emissive chemical coating may include one or more of: polycarbonate, PET, polymethyl methacrylate, polyacrylic acid, polyethylene, polypropylene, cellulose acetate, ethyl cellulose, and a copolymer mixture. Note that the choice of an ideal polymer might depend on the intended temperature of the pane and atmospheric conditions. In some cases, one might want a broadband emitter. If other cases, one might want an emitter that selectively emits in the 8-13 m window.

[0042] In one embodiment, the system can contain a polymer film on the outer surface of the glazing, i.e. facing the sky or exterior environment. The thickness of the polymer may range from 5 m to 2000 m. (Note that the polymer film thickness is a performance-determining factor, and thicknesses in the range 20 m to 500 m have been shown to work well). The system may be arranged into a layered structure containing the selectively emissive film, the glazing, and other components, as discussed below.

Layered Structure of the Passive Radiative Cooling System

[0043] This section describes details of the structure of the present system, including how the selectively emissive coating can be constructed.

[0044] FIG. 4A shows an exemplary layered structure 400 of a passive radiative cooling system, according to one embodiment of the present invention. As shown, structure 400 contains a float glass substrate 402, which may be a part of a window, automotive sunroof, etc. Adjacent to substrate 402, structure 400 contains a layer of pyrolytic low-emissivity coating 404. Pyrolytic low-emissivity coating 404 may contain a transparent conducting oxide (TCO) such as fluorine doped tin oxide (FTO), or another low-emissivity coating. Structure 400 then contains an acrylic adhesive 406 to bond the selectively emissive chemical coating 408 to the structure. Finally, the structure contains a layer (e.g., 50 m) of PET 408 as the selectively emissive chemical coating. Note that this coating is typically located closest to the exterior face of the window, i.e. facing the sky.

[0045] FIG. 4B shows an exemplary layered structure 410 of a passive radiative cooling system, according to one embodiment of the present invention. Note that the primary difference between structures 400 and 410 is that structure 410 contains a 500 m layer of polycarbonate 418 as the selectively emissive chemical coating, instead of 50 m of PET. Structure 410 contains float glass substrate 412, which may be a part of a window, etc. Structure 410 contains a layer of pyrolytic low-emissivity coating 414 adjacent to glass 412. Structure 410 further contains an acrylic adhesive 416 and a layer (e.g., 500 m) of polycarbonate 418 as the selectively emissive chemical coating.

[0046] FIG. 4C shows an exemplary layered structure 420 of a passive radiative cooling system, according to one embodiment of the present invention. Instead of a pyrolytic low-emissivity coating such as FTO, structure 420 may contain an infrared reflective thin metal multilayer coating 424. Such a coating can reflect a high proportion of the incident sunlight, thereby producing significant additional cooling. While coating 424 can be strongly reflective at solar infrared wavelengths, the system can nevertheless have strong absorption at thermal IR wavelengths, particularly those in the sky transparency window.

[0047] Structure 420 contains float glass substrate 422, and adjacent to float glass 422 contains the thin metal multilayer 424. Structure 420 also contains acrylic adhesive 426 and polycarbonate layer 428 as the selectively emissive chemical coating.

[0048] FIGS. 4D and 4E show exemplary layered structures 430 and 450 of a passive radiative cooling system, according to embodiments of the present invention. Structures 430 and 450 are similar to structures 410 and 420, respectively. However, in addition to the exterior selectively emissive coating, structures 430 and 450 contain an additional selectively emissive PET coating on the inside (shown as the bottom) of the respective layer stacks. These additional coatings function to absorb additional IR radiation from the inside enclosure (e.g., the interior of a car or room enclosed by the present invention) and can significantly enhance the system's net cooling effect. The layer structure for the inner coating can be similar to the outer one.

[0049] In one embodiment, structure 430 can contain an interior selectively emissive chemical coating layer 432 (e.g., 50 m of PET), and a layer of acrylic adhesive 434 to adhere coating 432 to the structure. Structure 430 also contains float glass 436 and a layer of pyrolytic low-emissivity coating 438. In addition, structure 430 can contain another acrylic adhesive 440 and a layer (e.g., 500 m) of polycarbonate 442 as the exterior selectively emissive chemical coating.

[0050] Structure 450 can contain an interior selectively emissive chemical coating layer 452 (e.g., 50 m of PET), and a layer of acrylic adhesive 454 to adhere coating 452 to the structure. Structure 450 can also contain float glass 456, and adjacent to float glass 456 can contain thin metal multilayer 458. Structure 450 further contains acrylic adhesive 460 and polycarbonate layer 462 (e.g., 500 m) as the selectively emissive chemical coating.

[0051] In some embodiments, the layer structures may also contain a thin, protective outer glass layer, e.g. 0.5-2 mm of alkali-aluminosilicate based toughened glass or float glass, to prevent abrasion. By protecting the polymer from being damaged by scratching, impacts, etc., this additional glass can improve the system's durability. Such protective layers would be located outermost in the layer structure (i.e., at the top of the structures shown in FIGS. 4A-4E). When sufficiently thin, such outer glass layers can be included, while still maintaining the desired cooling properties of the selectively emissive coating. Note that the aforementioned glass types are often found to be non-transparent in the wavelength range greater than 5 m, which might not be ideal for infrared transmission. For more optimized thermal performance, one can use a more infrared-transparent glass that is also transparent in the visible wavelength range, such as calcium fluoride (CaF.sub.2) glass.

[0052] In some embodiments, the layer structures as in FIGS. 4A-4E can be assembled by applying the polymer film directly onto the glass (by spin coating, doctor blading, slotted die coating, etc.). Optionally, the structures can be assembled by attaching the film to the glass with acrylic adhesive (e.g., optical-grade clear acrylic adhesives), as shown.

Characteristics and Performance of the Passive Radiative Cooling System

[0053] FIG. 5A shows an exemplary emissivity measurement of the passive radiative cooling system, according to one embodiment of the present invention. As shown in FIG. 5, the system has a high thermal blackbody emissivity coefficient >0.8 over a range of wavelengths 3 m<<15 m. In particular, is high over the atmospheric transparency window 510, which has wavelengths 8 m<<13 m. Due to these desirable properties, the system can emit thermal radiation strongly in the window 510, a range of wavelengths that is unlikely to be absorbed by the atmosphere or other surroundings. As a result, the system can maintain a non-equilibrium state, emitting more thermal radiation to its environment than it receives in the atmospheric transparency window.

[0054] In some embodiments, the system does not need to maintain a uniformly high emissivity throughout the entire spectrum of relevant wavelengths. (This relevant region may be roughly identified as the solar spectrum, 250 nm to 2.5 m, through the IR thermal wavelengths, 5 m to 20 m). Specifically, the system is transparent to visible light, in order to allow users to see through the window or other glazing. FIG. 5B illustrates an exemplary transmittance measurement of the passive radiative cooling system, according to embodiments of the present invention. Also shown is the range of visible wavelengths 550, running from 400 nm to 700 nm. As shown, the system has a high transmittance coefficient of greater than 80% in the visible spectrum.

[0055] Together, FIGS. 5A and 5B demonstrate that the system can have high transmittance in the visible spectrum 550, while simultaneously having high emissivity in the atmospheric transparency window 510. This unique combination of properties allows the system to perform better than existing systems at passive cooling, while simultaneously being optically transparent.

[0056] FIG. 6 shows exemplary cooling effects, based on experimental results using the passive radiative cooling system. As shown, several schemes are possible for the structure of the passive radiative cooling system. Scheme 1 makes use of a thin layer (e.g., 50 m) of PET as the selectively emissive chemical coating and contains a layer of pyrolytic low-emissivity coating, as in example structure 400 shown in FIG. 4A. Scheme 2 instead uses a thicker layer (e.g., 500 m) of polycarbonate as the chemical coating, and also contains a layer of pyrolytic low-emissivity coating, as in structure 410 shown in FIG. 4B. Scheme 3 contains the thicker layer of polycarbonate as the chemical coating, but contains a reflective thin metal multilayer coating instead of the pyrolytic low-emissivity coating, as in exemplary structure 420 shown in FIG. 4C. Finally, scheme 4 contains both an exterior and an interior layer of selectively emissive coating. Scheme 4 can further contain either pyrolytic low-emissivity coating, as in structure 430 shown in FIG. 4D, or reflective thin metal multilayer coating, as in structure 450 shown in FIG. 4E.

[0057] As shown in FIG. 6, schemes 1 through 3 of the present invention have been measured to achieve from 3 to 11 C. (20 F.) cooling in sunlight, compared to a control system. Scheme 4 has been measured to achieve 2 C. additional cooling (not shown) relative to scheme 2 in the embodiment of FIG. 3D with top and bottom layers of high-emissivity chemical coating and a layer of pyrolytic low-emissivity coating/FTO. Scheme 4 is likewise projected to achieve a total of 13 C. (23 F.) cooling vs. the control, in the exemplary embodiment of FIG. 4E with top and bottom layers of high-emissivity coating and an optically reflective thin metal multilayer coating. Thus, the present invention achieves extraordinary cooling of a transparent substrate in ambient sunlight, without input of electricity or liquid coolants required.

Exemplary Applications

[0058] While this disclosure has focused on an example of an automotive sunroof, the passive cooling system and methods are not limited by the present disclosure, and may be used for any application. For example, the disclosed passive cooling system may be used as part of an automotive windshield or rear or side window, a commercial or residential window, a transparent or translucent wall or structure, transparent electronics, glasses, sunglasses or other wearable items, a container for food or biological matter, or any application known or later developed.

[0059] An advantage of the disclosed passive cooling system is that it can be manufactured with inexpensive materials, without complex techniques such as lithography. The passive cooling system can be manufactured either by applying the polymer film directly onto glass (e.g., by spin coating), or attaching to the glass with acrylic adhesive (e.g., optical-grade clear acrylic adhesives from 3M). The manufacturing methods for the optional inner surface are similar.

[0060] The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention.