Patent Application
20180354848
The present disclosure relates to passive cooling. More specifically, this disclosure relates to passive radiative cooling of transparent structures, such as glass windows.
As electronics and communication technologies advance, and research proceeds in flexible and transparent electronics, there is an increasing demand for transparent materials to perform optical, thermal, mechanical, and/or electrical functions. Particularly, functional glass and transparent materials are needed for applications including automotive sunroofs, windshields, windows, as well as other applications.
For example, glazing with transmittance in the visible spectrum (i.e., optically transparent) and structural functionality are desirable for protection against mechanical impact. Similarly, there is a demand for automotive glazing coatings with functionalities such as reflecting sunlight, embedding antennas, self-defrosting by passing current through a coating, and conserving energy (e.g., in electric vehicles). Such applications have driven research in advanced materials, including glasses.
By contrast, although reducing the cost of cooling with air conditioning (e.g., in a vehicle with a sunroof) is commonplace and widespread, additional cooling technologies based on glazing is highly desirable. When considering the heat balance of a window in a hot climate, there are two major contributions that affect the interior temperature (e.g., inside a car or building). The first contribution is the amount of solar radiation transmitted through the window, which contributes to heating objects in the interior. This is quantified by the solar heat gain coefficient (SHGC) of the window or glazing. Window films or coatings on glass that directly modify the SHGC are commercially available. In fact, in existing active glazing systems for hot climates, thermal management is typically approached in this way, i.e. by decreasing the SHGC.
The second temperature contribution involves a balance between thermal radiation emitted by the window to its surroundings, and thermal radiation absorbed by the windowpane from the window's surroundings.
One embodiment described herein provides an apparatus for passive cooling via selective radiative emission. This apparatus includes a transparent substrate and a coating on the transparent substrate for radiating heat from the substrate. 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.
In a variation on this embodiment, the coating has a thermal blackbody emissivity coefficient of at least 0.9 corresponding to an infrared radiation wavelength range.
In a variation on this embodiment, the infrared radiation wavelength range comprises wavelengths in an atmospheric transparency window of 8 micrometers to 13 micrometers.
In a variation on this embodiment, the coating includes a polymer.
In a variation on this embodiment, 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.
In a variation on this embodiment, the transparent substrate comprises a piece of glass, and the chemical coating is coated on the piece of glass.
In a variation on this embodiment, the apparatus further includes a second piece of glass and a second coating on the second piece of glass for radiating additional heat. The second piece of glass is stacked with the first piece of glass.
In a variation on this embodiment, the apparatus further includes one or more layers of reflective coating for reflecting incident sunlight.
In a variation on this embodiment, the transparent substrate is part of a window, an automotive window, or an automotive sunroof.
In a variation on this embodiment, the coating is not coupled to a liquid coolant source or an electrical source.
In the figures, like reference numerals refer to the same figure elements.
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.
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.
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.
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 Tcar. 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 Tcar by carrying heat from the car to the sunroof when Tcar>Tglass, and from the sunroof to the environment when Tglass>Tambient.
Table 1 also lists properties of the sunroof that could be engineered to tune each heating and cooling contribution in order to keep Tcar low. Thus, for the thermal radiation contributions, the thermal emissivity coefficient of the sunroof in the IR wavelength region can affect Tcar. For sunlight 104 and 106, the sunroof's absorbance and transmittance in the solar wavelengths affects Tcar. For both the convection and thermal emission contributions, the sunroof's temperature Tglass would affect Tcar. Although Tglass 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.
In particular, notice that thermal radiation 112 and 114 from the sunroof carries heat away, and therefore tuning the emissivity could help lower Tcar, 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.
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.
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.
In particular, the system has a high emissivity coefficient over the atmospheric transparency window, which corresponds to wavelengths 8 μm<λ<13 μm. 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.
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 μm. This wavelength range is practically outside the 8-13 μm atmospheric transparency window, so glass' passive radiative cooling effect is sub-optimal.
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.
The system can achieve these results by using a chemical coating containing selectively emissive polymers.
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 μm 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 μm window.
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 μm to 2000 μm. (Note that the polymer film thickness is a performance-determining factor, and thicknesses in the range 20 μm to 500 μm 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.
This section describes details of the structure of the present system, including how the selectively emissive coating can be constructed.
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.
In one embodiment, structure 430 can contain an interior selectively emissive chemical coating layer 432 (e.g., 50 μm 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 μm) of polycarbonate 442 as the exterior selectively emissive chemical coating.
Structure 450 can contain an interior selectively emissive chemical coating layer 452 (e.g., 50 μm 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 μm) as the selectively emissive chemical coating.
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
In some embodiments, the layer structures as in
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 μm, through the IR thermal wavelengths, 5 μm to 20 μm). Specifically, the system is transparent to visible light, in order to allow users to see through the window or other glazing.
Together,
As shown in
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.
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.
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.
