FABRICATION METHODS, STRUCTURES, AND USES FOR PASSIVE RADIATIVE COOLING
1. A passive radiative cooling structure comprising:
- a flexible film, transparent to visible light, and incorporating a first material at a volume percentage larger than 25% so as to absorb and emit infrared radiation at wavelengths where Earth'"'"'s atmosphere is transparent;
wherein the flexible film includes a first surface parallel to a second surface, and an inner region between the first surface and the second surface.
Passive radiative cooling structures and apparatus manufactured with such cooling structures conserve energy needs. A flexible film transparent to visible light incorporates particles at a volume percentage larger than 25% so as to absorb and emit infrared radiation at wavelengths where Earth'"'"'s atmosphere is transparent. Another film transparent to visible light is thin and flexible and configured to absorb and emit infrared radiation at wavelengths where Earth'"'"'s atmosphere is transparent, wherein etchings or depositions are present on one or both surfaces. A high efficiency cooling structure has an emissive layer sandwiched between a waveguide layer and a thermal conductive layer. A solar cell panel is covered by a transparent passive radiative cooling film. A container housing an active cooling unit incorporates passive radiative cooling structures on one or more exterior surfaces.
- 1. A passive radiative cooling structure comprising:
a flexible film, transparent to visible light, and incorporating a first material at a volume percentage larger than 25% so as to absorb and emit infrared radiation at wavelengths where Earth'"'"'s atmosphere is transparent; wherein the flexible film includes a first surface parallel to a second surface, and an inner region between the first surface and the second surface.
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
- 10. A passive radiative cooling structure comprising:
a flexible film, transparent to visible light, configured to absorb and emit infrared radiation at wavelengths where Earth'"'"'s atmosphere is transparent; wherein the flexible film includes a first surface parallel to a second surface, and an inner region between the first surface and the second surface; wherein the flexible film comprises a thin sheet of a first material, and wherein etchings are present on one or both of the first surface and the second surface.
- View Dependent Claims (11, 12, 13, 14, 15)
- 16. A passive radiative cooling structure comprising:
one or more cooling stacks; wherein each cooling stack is configured with a first waveguide layer, a first emissive layer and a thermal conductive layer, wherein the first emissive layer is sandwiched between the first waveguide layer and the thermal conductive layer; wherein the first emissive layer is configured to absorb and emit infrared radiation at wavelengths where Earth'"'"'s atmosphere is transparent; wherein the thermally conductive layer has a proximal end and a distal end; and wherein the thermally conductive layer is substantially perpendicular to and in thermal contact at its proximal end with a source of heat to be cooled.
- View Dependent Claims (17, 18, 19)
This patent application claims the benefit of U.S. Provisional Patent Application No. 62/658,146, filed Apr. 16, 2018. The disclosures of this application are incorporated by reference herein in their entirety.
The present invention relates to structures, uses and methods based on passive radiative cooling. As used herein, passive radiative cooling is achieved by the use of devices that emit wavelengths corresponding to the transparency windows in the Earth'"'"'s atmosphere or in other surrounding environments/enclosures. For example, passive radiative cooling can be achieved outdoors using materials that emit at infrared (IR) wavelengths in the range of 7 μm to 13 μm.
In the phenomenon known as the “greenhouse effect”, the atmosphere traps heat in the form of electromagnetic radiation. However, not all radiated wavelengths are equally trapped. For example, there is a so-called “atmospheric window” at IR radiation wavelengths between about 7 μm to 13 μm where radiation emitted from the Earth'"'"'s surface leaves the atmosphere. From a thermodynamic viewpoint, a consequence of these “atmospheric windows” is that thermal radiation emitted in these wavelengths from an object on the surface of the Earth will be transferred to the cold sink of space. In this way, passive emissive cooling can be used to reduce the temperature of an object, even at the surface of the planet, much as evaporative cooling can be used to reduce temperature in a dry climate.
Radiative cooling has been demonstrated under both nighttime daytime conditions. Raman et al., “Passive radiative cooling below ambient air temperature under direct sunlight,” Nature, vol 515, Nov. 27, 2014, pp 540-544, describe a thermal photonic approach using seven layers of HfO2 and SiO2 that reflect incident light while emitting in the atmospheric window. With this approach, they demonstrated cooling on a rooftop of over 5 degrees Celsius in direct sunlight. Since then, this group has demonstrated a few different versions of materials that cool radiatively, including a visibly transparent etched silica photonic crystal version.
However, the approach of Raman is not practical for commercial applications requiring low-cost materials, flexibility for ease of application, smooth top surface to resist dirt collection and durability for outdoor environments. One aspect of the instant invention is to provide practical and cost-effective radiative cooling structures with high cooling capacity.
In U.S. Application US 2017/0248381, a spectrally transparent but hazy radiative cooling structure is described wherein non-polymer particles are embedded randomly in a polymeric matrix. The preferred range of particle sizes corresponds to average effective diameters of spherical particles between 1 μm and 30 μm. The preferred range of volume percentages of the particles to the polymeric matrix is from 2% to 25%. While particles within the above size range and volume percentages are well suited for capturing and emitting IR radiation in the wavelength range from 7 μm to 13 μm, light scattering is a problem that leads to a significant loss of transparency.
Significant energy expenditures are required to refrigerate products in cargo temperature-controlled truck trailers. Each year the refrigerated trucking industry spends more than $30 B on fuel, making fuel one of their biggest expenses, even more than labor. Moreover, exhaust from burning this much diesel results in 87 MMT of CO2 equivalent emissions (not including emission contributions from refrigerants, as well as the effects of waste heat produced by conventional cooling methods) and makes the refrigerated trucking industry one of the biggest polluters in the U.S. Even a modest reduction in the fuel required for refrigerated truck trailers could significantly enhance profitability for fleet owners, and at the same time provide a major reduction in polluting emissions.
In accordance with aspects of the present invention, passive radiative cooling can be used to cool surfaces outdoors and can increase efficiencies of systems by offsetting heat from sources like engines, electronics, chemical reactions, and the sun (ex. high power electronics including transformers, building envelopes including windows, solar photovoltaics and battery storage, transportation including electric vehicles, water purification/distillation). Another aspect of the instant invention is to provide efficient radiative cooling structures that are transparent to visible light.
In accordance with a preferred embodiment of the invention, a passive radiative cooling structure is provided in the form of a flexible film that is transparent to visible light, and configured to absorb and emit infrared radiation at wavelengths where Earth'"'"'s atmosphere is transparent. The flexible film may be fabricated from a single base material or it may have incorporated greater than 25% by volume of an embedded material.
In some embodiments, the flexible film may incorporate substantially spherical particles having a diameter greater than 30 μm. In other embodiments, particles may be incorporated that have non-spherical shapes, and have an average per particle volume greater than about 14,200 μm3 and being small enough to fit between the top and bottom surfaces of the film.
In some embodiments, the incorporated particles may be arranged in an orderly repetitive manner within the sheet, and in other embodiments the incorporated particles may be distributed in a more random, or disorderly, but deterministicmanner.
In some embodiments, the flexible film is formed by a transparent plastic sheet and the incorporated particles are made of silica glass. In a preferred embodiment, the particles are made of fused silica glass.
In some embodiments, the embedded particles are cylindrical concave or convex sidewall structures, each sidewall structure having a first circular area bounded by a first circular area circumference and a second circular area bounded by a second circular area circumference wherein the first circular area circumference and the second circular area circumference are connected by the sidewall surface, enclosing an inner volume connecting the first circular area and the second circular area, and wherein the first and second circular areas are oriented parallel to the film surface.
In some embodiments, a passive radiative cooling structure made from a transparent flexible film is positioned on top of a panel so as to cool solar cells in the panel sufficiently and increase the efficiency of the conversion of light to electrical energy.
In other embodiments, the transparent flexible film includes a single sheet of one material, wherein etchings are present on one or both of the two parallel surfaces. The etchings may be optionally filled by another material with a refractive index more similar to the surrounding media to improve coupling between the surfaces.
In another preferred embodiment, a passive radiative cooling structure includes one or more cooling stacks. Each cooling stack of the cooling structure is configured with a waveguide layer, an emissive layer configured to absorb and emit infrared radiation at wavelengths where Earth'"'"'s atmosphere is transparent, and a thermal conductive layer. In this embodiment, the emissive layer is sandwiched between the waveguide layer and the thermal conductive layer. The thermally conductive layer is in thermal contact with a source of heat to be cooled. This may be direct or through a thermally conductive interface. The thermally conductive layer can be configured substantially perpendicular to the source of heat to be cooled. These structures can be utilized on horizontal or vertical surfaces to be cooled.
In another embodiment, each cooling stack of the passive radiative cooling includes two waveguide layers, and two emissive layers, each emissive layer being configured to absorb and emit infrared radiation at wavelengths where the Earth'"'"'s atmosphere is transparent, wherein the waveguide layers form the outer layers of the stack, the two emissive layers form the next outermost layers of the stack, and wherein the inner core of the stack is a thermally conductive layer, sandwiched between the two emissive layers. These structures can be utilized on horizontal or vertical surfaces to be cooled.
In accordance with a further embodiment of the invention, a refrigerated container includes a passive radiative cooling structure configured to absorb and emit infrared radiation at wavelengths where the atmosphere is transparent on the outside of the refrigerated container, in order to reduce the amount of energy required to refrigerate the container. For the passive radiative cooling structure made of a flexible film, a reflective layer may be inserted beneath the film. In other embodiments, the passive radiative cooling structure is situated on top of a thermal switching layer, which is in thermal contact with the roof of the container, wherein the thermal switching layer includes channels that can be filled, alternatively, with a heat-conducting fluid and with a heat-insulating fluid. In another preferred embodiment, a transparent passive radiative cooling structure is used to cover one or more of the container sidewalls, thereby allowing both enhanced passive cooling and the ability to allow ads or other information on the sidewalls of the container to be seen through the transparent structure.
In further embodiments, passive radiative cooling structures are configured to cool a variety of structures and devices, including but not limited to transformers, walls, rooftops, chillers in manufacturing, cooling towers, articles of apparel, vehicles including electric vehicles, shade structures, water fountains and batteries.
The foregoing features of embodiments will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which:
Definitions. As used in this description and the accompanying claims, the following terms shall have the meanings indicated, unless the context otherwise requires:
“Substantially transparent” means allows wavelengths through sufficiently for the objects below to operate as designed within acceptable parameters, or to otherwise minimize absorption of unwanted wavelengths.
“Flexible” means that the material can flex enough to not be damaged while rolling onto a spool as is typical for roll-to-roll fabrication techniques.
The size, chemical composition and distribution of the embedded particles 35 can be controlled in order to control IR absorbance and emission intensities and bandwidths. As a consequence of using larger particles, volume fractions of particles greater than 25% may be used, and low-scattering transparency is attainable. Fortuitously, even when larger particles result in a reduced absorbance/emission maximum intensity in the desired wavelength region, and will shift the wavelength of the absorbance maximum, larger particles will also provide a broadband absorption response, thereby partially compensating for the loss of intensity at any single wavelength. The broader band distribution of wavelengths associated with larger particles can a) partially compensate for losses of intensity at a particular wavelength while still providing comparable total intensity across the atmospheric window, and b) provide a more robust ability to adapt to changes in the IR window due to changing atmospheric conditions.
Based on the above considerations of the interrelated effects of particle size and volume percentage of particles on scattering in the visible, overall absorption in the desired atmospheric windows, and the ability to perform despite changing atmospheric conditions, preferred embodiments of the disclosed invention utilize particles having an average per particle volume size of greater than 14,200 μm3, and volume percentages above 25%. More particularly, the volume percentage in preferred embodiments is between 25% and 73%.
In the embodiment shown in
As further detailed in
In another embodiment, illustrated in
Flexible thin-film absorbers can be fabricated using the device layer of standard Silicon on Insulator (SOI) wafers and/or equivalent panel or roll-to-roll versions in materials to achieve desirable optical and mechanical properties. This would work like this: 1) optional preprocessing of the device layer to enhance atmospheric window absorption as with the films described in
Embodiments with a flexible, transparent passive radiative cooling structure 10, including those shown in
In one embodiment the thin film comprising the substantially transparent passive radiative cooling structure is held in uniform contact with a surface of the solar panel, or an alternative surface to be cooled, by means of capillary forces. In a preferred method of holding the film on the solar panel or alternative surface by means of capillary forces, the film is affixed by a method involving wetting the back of the structure, disposing the structure onto the surface and squeegeeing (applying uniform linear pressure) across the surface of the structure, thereby eliminating the water and air bubbles trapped between the film and the surface, and facilitating capillary adhesion. This can be followed by adhesive application at the edges of the film to restrict the edges from peeling up and disengaging with the surface. This method of installation allows for easy de-installation as well, by removing the edge bead of adhesive and peeling up the flexible film from one edge or corner.
A transparent passive radiative cooling structure 10 may be advantageously used as shown in
Thin film based radiative cooling structures such as those described in the preceding embodiments typically give a cooling power of roughly 100 watts per square meter. Daytime cooling is further reduced by the added heat load of incoming visible and near-infrared radiation from sunlight and the surrounding environment. To achieve the cooling goal of 1 Ton of refrigeration (1 TR), 35 square meters of such radiative cooling panels are needed. Consequently, available surface area remains a limiting factor in the implementation of radiative cooling. As an added complication, the large area footprint means the heat load from the sun and the ambient environment will be partially nullifying the benefits of increased surface area.
In other embodiments of the invention, the efficiency of passive radiative cooling structures can be enhanced by right angle deflection and layer stacking. While such embodiments typically include opaque thermal conductors, preventing them from being see-through, they can provide a dramatic increase in the effective cooling panel surface area, while minimizing the surface area exposed to sunlight. Such embodiments use well-understood waveguide principles widely used in photonics and in consumer applications involving flat-panel displays.
Such embodiments make use of thin optical waveguides, which allow the IR radiation emitted over a large area to be concentrated and deflected 90 degrees to allow the radiative areas to be stacked so that a very large total surface area of radiative cooling can be housed in a much smaller area footprint. Such embodiments can thereby allow 100 square meters or more of cooling panel area to occupy 1 cubic meter, on a 1 square meter footprint. There are efficiency losses from this arrangement per each square meter of cooling panel area, but the multiplication effect on total cooling capacity, due to larger effective surface area, is greater than the losses introduced by the coupling and absorption losses in the waveguides. In addition, since the IR radiation is emitted over a much smaller area, the area that is subject to daytime heating is greatly reduced, without the addition of any special filtering.
In such embodiments, IR radiation is emitted by a radiant cooling absorber/emitter over a large area and is focused and/or coupled into a waveguide typically at an angle of 90 degrees from the cooling surface. The advantage of this arrangement is that emissive layers where cooling takes place can now be stacked in a very dense arrangement.
One embodiment using optical waveguides is shown in
In cooling operation, heat from the source of heat to be cooled is conducted through the thermally conductive layer 16. Heat from the thermally conductive layer 16 in the form of IR radiation is absorbed and emitted from the emissive layer 14 and coupled into the waveguide layer 22, where it is redirected upwards towards the sky to radiate through the atmospheric window into the depths of outer space.
Note that these stacks could be configured to remove heat from surfaces in any orientation, including normal to the sky, perpendicular to the sky, and anywhere in between, for example, cooling surfaces in any orientation by changing the angle of thermal wavelength redirection and propagation in the waveguide layer to head skyward.
A further goal of an embodiment of the invention is to reduce fuel usage by refrigerated cargo containers on truck trailers through the use of passive radiative cooling structures on the roofs and sides of such containers. These containers house active cooling units, such as refrigeration compressors, for maintaining a low temperature inside the container.
In other embodiments as shown in
The upper layer 56 could be a transparent passive cooling structure as set forth in
For applications of passive radiative cooling structures of
In other embodiments, the passive radiative cooling structure 54 is fitted with a conductively and convectively insulating window that is transparent to wavelengths where the atmosphere is transparent. This would enhance cooling to lower temperatures, because it would inhibit warming from the surrounding media but still allow for the emissive cooling. The window could be foam 65, for example polyethylene foam, placed directly on the emissive cooling material as in
The embodiments of the invention described above are intended to be merely exemplary; numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.