Optical materials for spectrally selective heat transfer

Abstract

Techniques for controlling heat transfer or emission using spectrally selective transmissive substrates are provided. Each spectrally selective transmissive substrate may include a plurality of alternating stacked layers, where the spectrally selective transmissive substrate is generally at least 50% transparent on average across a long wavelength infrared wavelength range (8 pm to 13 pm), and greater than 30% reflective on average across infrared wavelengths outside the long wavelength infrared (LWIR) wavelength range. Such substrates may include polymer such as a polyolefin, a nylon, or a polytetrafluoroethylene (PTFE), and may include air voids with a lower refractive index than polyethylene, and / or particles with a higher refractive index than polyethylene, such as ZnO. ZnS. ZnSe, CuO, FeO, Si, etc. Particles sizes are typically selected to behave as an effective medium in LWIR wavelengths, which are 5-20x larger than the particles, while still providing either solar scattering or solar transparency as desired.

Description

[0001]Princeton - 94476 OPTICAL MATERIALS FOR SPECTRALLY SELECTIVE HEAT TRANSFER CROSS-REFERENCE TO RELATED APPLICATIONS The present application claims priority to U.S. Provisional Pat. Appl. No.63 / 523,820, filed June 28, 2023, and U.S. Provisional Pat. Appl. No. 63 / 545,691, filed October 25, 2023, the contents of each incorporated by reference herein in its entirety. TECHNICAL FIELD The present disclosure is drawn to techniques for spectrally selective heat transfer. BACKGROUND Thermal control of human habitats is a growing and urgent challenge one faces as the climate changes around us. In particular, urban environments affected by climate change are threatened by a combination of the urban heat island effect, higher temperatures, and more prevalent heatwaves. The resulting acute thermal control needs in cities has caused air conditioning (AC) usage to soar. However, their costs, energy usage, CO2 emissions, and crucially, net heat generation, make ACs an unsustainable and unequitable heat transfer method. As rising heat threatens lives and livelihoods, sustainably mitigating or adapting to it has become a defining challenge of our times. Passive heat transfer solutions which reflect sunlight and radiate heat to the sky, are a promising solution in this regard. However, advances on those fronts have been centered on a prevalent thermal architecture – an indoor target connected to a radiative cooler (e.g., a rooftop coating) that radiatively loses heat to the sky. Unfortunately, this limits heat transfer. For fastest heat transfer from the indoor target, thermal resistance between the target and the cold sky should be minimal. However, in the traditional architecture, radiative heat transfer units and associated infrastructure act as a thermally massive and opaque intermediate, slowing the heat transfer process. Furthermore, radiative heat transfer units on roofs gain heat by convection to become warmer than the indoors on hot days, which reduces them to minimizing heating of the indoor target, rather than reducing temperature of the indoor target. These longstanding limitations persist despite material advances. BRIEF SUMMARY In various aspects, a heat transfer method may be provided. The method may include allowing heat from at least a portion of a covered or indoor volume of space, in directions Princeton - 94476 within 70° angle from normal to a walking surface of the covered or indoor volume of space, to reach an outdoor atmosphere by only passing through a transmissive portion of a roof structure. The covered or indoor area may be a top floor of a building, an atrium or open area within a commercial building, an outdoor volume of space covered by a canopy, etc. The transmissive portion may be all of the roof structure. The transmissive portion may be less than all of the roof structure. The transmissive portion may include infrared-transparent thermal insulation. A non-transmissive portion of the roof structure may include thermal insulation. Such insulation may be, e.g., transmissive to radiation while insulating from convection and conduction. The roof structure may include one or more rigid portions configured to reinforce each spectrally selective transmissive substrate. The one or more rigid portions may include a metal, a polymer, etc. The transmissive portion of the roof structure may consist of one or more spectrally selective transmissive substrates. The one or more spectrally selective transmissive substrates may be a flexible film. The one or more spectrally selective transmissive substrates may be a rigid material. Each spectrally selective transmissive substrate may be at least 50% transparent on average across a long wavelength infrared wavelength range, and greater than 30% reflective on average across infrared wavelengths outside the long wavelength infrared wavelength range, the long wavelength infrared wavelength range being from 8 µm to 13 µm. Each spectrally selective transmissive substrate may be at least 50% transparent on average across the long wavelength infrared wavelength range, and greater than 50% reflective on average across infrared wavelengths in a second range, the second range including 0.3 µm to 8 µm and 13 µm to 20 µm. Each spectrally selective transmissive substrate may include a plurality of layers disposed atop each other, each layer being different from an adjacent layer. The plurality of layers may include, e.g., 2-50 layers, such as 10-30 layers. Each layer may have a thickness selected based on its contribution to ensuring the one or more spectrally selective transmissive substrates is at least 50% transparent on average across the long wavelength infrared wavelength range, and greater than 30% reflectivity on average across infrared wavelengths outside the long wavelength infrared wavelength range. Each layer may have a thickness of 2.5 µm to 7 µm. For example, the plurality of layers may include a first set of alternating layers, the first set of alternating layers including a first layer and a second layer disposed in alternate layers (e.g., in an A-B alternating structure), where the first layer and the second layer have different compositions. In certain aspects, each first layer is identical, and each second layer is identical. Princeton - 94476 In certain embodiments, at least one first layer has a different thickness from a different first layer, and / or at least one second layer has a different thickness from a different second layer. In certain aspects, the plurality of layers may include at least one additional set of alternating layers, disposed below the first set of alternating layers (e.g., the first set of A-B alternating layers and a second set of C-D alternating layers), such that the at least one additional set of alternating layers is between the first set of alternating layers and the covered or indoor volume of space. In certain aspects, each set of layers may be configured to have at least one reflection band within each of a 0.3 µm to 8 µm range and a 13 µm to 25 µm range, where a reflection band from one set of layers is different from a reflection band of another set of layers. The reflection bands may be complementary, such that together, they cover as much of the 0.3 µm to 8 µm and 13 µm to 25 µm range as possible. In certain aspects, the plurality of layers together have reflection bands covering at least 80% of the 0.3 µm to 8 µm range and the 8 µm to 25 µm range. Each layer may include a polymer. Each polymer may, independently, be a polyolefin, a nylon, or a polytetrafluoroethylene (PTFE). Each layer may independently include, a plurality of air voids with lower refractive index than polyethylene, and / or particles with higher refractive index than polyethylene. The particles may include zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), copper oxide (CuO), Iron (II) Oxide (FeO), silicon, or a combination thereof. A median diameter of the plurality of air voids and / or particles in any single layer may be 500 nm – 1000 nm, 100-500 nm, and / or less than 100 nm. Solar scattering arises because particles have a different refractive index from, e.g., polyethylene, and have a size similar to the solar wavelengths. In some aspects, one or more diameters of the plurality of air voids and / or particles are selected based on effectiveness of transmitting longwave infrared wavelengths, and either solar scattering or solar transparency. In various aspects, the transmissive portion of the roof structure may be an opaque white portion of the roof structure. In various aspects, the transmissive portion of may be a visibly transparent portion of the roof structure (e.g., similar to a skylight). The transmissive portion may include a microtextured anti-reflective layer disposed over the plurality of layers, such that the plurality of layers is between the microtextured anti- reflective layer and the covered or indoor volume of space. The transmissive portion may include a solar reflective, thermal infrared transparent layer, comprising polyethylene and air voids, zinc oxide, and / or zinc selenide. The transmissive portion may include a transparent supporting substrate. Princeton - 94476 In various aspects, a heat emitting method may be provided. The heat emitting method may include emitting heat from a surface through one or more spectrally selective transmissive substrates thermally coupled to the surface, each spectrally selective transmissive substrate being at least 50% transparent on average across a long wavelength infrared wavelength range, and greater than 30% reflective on average across infrared wavelengths outside the long wavelength infrared wavelength range. Each spectrally selective transmissive substrate may be a spectrally selective transmissive substrate as disclosed herein. The surface may be, e.g., an interior or exterior wall. One or more thermally conductive substrates may be disposed between the surface and the one or more spectrally selective transmissive substrates. The plurality of layers may include at least one additional set of alternating layers, disposed below the first set of alternating layers, such that the at least one additional set of alternating layers is between the first set of alternating layers and the surface. A median diameter of the plurality of air voids and / or particles in any given layer may be, e.g., 1000 nm – 2500 nm, 100-1000 nm, or less than 100 nm. The one or more spectrally selective transmissive substrates may include a microtextured anti-reflective layer disposed over the plurality of layers, such that the plurality of layers is between the microtextured anti-reflective layer and the surface. A spectrally selective transmissive substrate may be provided. The substrate may include a plurality of layers disposed atop each other. Each layer may be different from an adjacent layer. The spectrally selective transmissive substrate may be at least 50% transparent on average across a long wavelength infrared wavelength range, and greater than 30% reflective on average across infrared wavelengths outside the long wavelength infrared wavelength range. The spectrally selective transmissive substrate may be at least 50% transparent on average across the long wavelength infrared wavelength range, and greater than 50% reflective on average across infrared wavelengths in a second range, the second range including 0.3 µm to 8 µm and 13 µm to 30 µm. The plurality of layers may include 2-50 layers, such as 10-30 layers. Each layer may have a thickness selected based on its contribution to ensuring the spectrally selective transmissive substrate is at least 50% transparent on average across the long wavelength infrared wavelength range, and greater than 30% reflectivity on average across infrared wavelengths outside the long wavelength infrared wavelength range. Each layer may have a thickness of 2.5 µm to 4.5 µm. Each layer may include a polymer. Each polymer may, independently, be a polyolefin, a nylon, or a polytetrafluoroethylene (PTFE). Layer may independently include, a plurality of air voids with lower refractive index than polyethylene, and / or particles with higher refractive index than polyethylene. The particles may include zinc oxide (ZnO), zinc sulfide (ZnS), zinc Princeton - 94476 selenide (ZnSe), copper oxide (CuO), Iron (II) Oxide (FeO), silicon, or a combination thereof. A median diameter of the plurality of air voids and / or particles may be, e.g., 1000 nm – 2500 nm, 100-1000 nm, or less than 100 nm. Each spectrally selective transmissive substrate may include a plurality of layers disposed atop each other, each layer being different from an adjacent layer. The plurality of layers may include, e.g., 2-50 layers, such as 10-30 layers. Each layer may have a thickness selected based on its contribution to ensuring the one or more spectrally selective transmissive substrates is at least 50% transparent on average across the long wavelength infrared wavelength range, and greater than 30% reflectivity on average across infrared wavelengths outside the long wavelength infrared wavelength range. Each layer may have a thickness of 2.5 µm to 7 µm. For example, the plurality of layers may include a first set of alternating layers, the first set of alternating layers including a first layer and a second layer disposed in alternate layers (e.g., in an A-B alternating structure), where the first layer and the second layer have different compositions. In certain aspects, each first layer is identical, and each second layer is identical. In certain embodiments, at least one first layer has a different thickness from a different first layer, and / or at least one second layer has a different thickness from a different second layer. In certain aspects, the plurality of layers may include at least one additional set of alternating layers, disposed below the first set of alternating layers (e.g., the first set of A-B alternating layers and a second set of C-D alternating layers), such that the at least one additional set of alternating layers is between the first set of alternating layers and the covered or indoor volume of space. In certain aspects, each set of layers may be configured to have at least one reflection band within each of a 0.3 µm to 8 µm range and a 13 µm to 25 µm range, where a reflection band from one set of layers is different from a reflection band of another set of layers. The reflection bands may be complementary, such that together, they cover as much of the 0.3 µm to 8 µm and 13 µm to 25 µm range as possible. In certain aspects, the plurality of layers together have reflection bands covering at least 80% of the 0.3 µm to 8 µm range and the 8 µm to 25 µm range. The spectrally selective transmissive substrate may include a microtextured anti- reflective layer disposed over the plurality of layers. The spectrally selective transmissive substrate may include a transparent supporting substrate. The spectrally selective transmissive substrate may be a flexible film. In various aspects, a system may be provided. The system may include a vertical structure operably coupled to a ground surface. The system may include a roof or covering Princeton - 94476 operably coupled to the vertical structure. The system may include one or more spectrally selective transmissive substrates, as disclosed herein, operably coupled to at least a portion of the vertical structure, or forming at least a portion of the roof or covering. In various aspects, methods of manufacturing a spectrally selective transmissive substrate may be provided. In certain aspects, the method may include providing a plurality of resin streams, each resin stream independently comprising a polymer (such as a polymer as disclosed herein). The method may include forming stacked layers by coextruding the plurality of resin streams as a plurality of alternating layers, and casting the stacked layers onto a casting roll. In certain aspects, the method may include forming a plurality of thin layers by providing a solution comprising a solvent and a polymer as disclosed herein, which may be configured to include air voids and / or particles as disclosed herein, forming a thin layer using the solution, and hot pressing the thin layer at or near a melting temperature of the polymer. Forming the thin layer may include casting the solution as a layer, using a doctor blade, film applicator, and / or an applicator rod. Forming the thin layer may include dip coating the solution onto a substrate, and drying the thin layer. The method may include peeling the thin layer from the substrate after drying. Forming the thin layer may include mechanical rolling or melt rolling of the solution. The method may include combining a plurality of thin layers to form an alternative set of stacked layers. BRIEF DESCRIPTION OF DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention. Figures 1A and 1B are schematic illustrations showing heat transfer via an LW-IR emissive roof (1A) and an LW-IR transparent roof (1B). Figures 2-4 are illustrations showing embodiments of a spectrally selective transmissive substrates. Figure 5 is an example graph showing reflection bands in various wavelength ranges. Figure 6 is an illustration showing an embodiment of a spectrally selective transmissive substrate. Figure 7 is a schematic illustration of heat transfer through a roof or covering. Figure 8 is an illustration of heat emission from a wall surface. Princeton - 94476 Figure 9 is an illustration of heat emission from a wall surface, then through a window. Figure 10 is a graph showing transmittance (T) = 1 – Reflectance (R) for a multilayer optical metamaterial that reflects solar and atmospheric heat in the non LW‐IR wavelengths, and transmits LW‐IR heat from indoors through the atmosphere into space, along with an ideal case is also shown. Figures 11-14 are transmission graphs of substrates comprising multilayer polyethylene and 50% (Figures 11, 13) or 25% (Figures 12, 14) Si-doped polyethylene, for 8 or 16 bilayer substrates (Figures 11-12) or 9 bilayer substrates (Figures 13-14). Figures 15-18 are transmission graphs of substrates comprising multilayer polyethylene and 50% (Figures 15, 17) or 25% (Figures 16, 18) ZnS-doped polyethylene, for 8 or 16 bilayer substrates (Figures 15-16) or 9 bilayer substrates (Figures 17-18). Figure 19 is a graph showing scattering efficiency of ZnS particles in a substrate. Figures 20A-20E are graphs showing complex refractive indices for various substrates: polyethylene (20A), 50% (20B) or 25% (20C) Si-doped polyethylene, and 50% (20D) or 25% (20E) ZnS-doped polyethylene. Figure 21 is a bar chart showing highlighting the NIR-to-SWIR absorptance of twelve commercially available polymers. Figure 22 is a graph showing scattering efficiency of PTFE nanoparticles and fibers of different diameters. Figure 23 is a graph showing an emittance spectrum of PTFE. Figure 24 is a scanning electron micrograph of a bilayer device with both the top surface and cross-section in view; at the right are magnified views of the porous P(VdF-HFP) top surface (top right) and cross section (middle right), and the PTFE cross section (bottom right). Figure 25 is an angular ^^LWIR profile of the radiative cooler from 0°-85°. Figure 26 is a schematic illustration of an experimental setup of a system. It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments have been enlarged or distorted relative to others to facilitate visualization and clear understanding. In particular, thin features may be thickened, for example, for clarity or illustration. Princeton - 94476 DETAILED DESCRIPTION Cooling human habitats is a growing and urgent challenge we face as the world warms around us. In particular, urban environments affected by climate change are threatened by a combination of the urban heat island effect, higher temperatures, and more prevalent heatwaves. The resulting acute cooling needs in cities has caused air conditioning (AC) usage to soar. However, their costs, energy usage, CO2 emissions, and crucially, net heat generation, make ACs an unsustainable and unequitable cooling method. As rising heat threatens lives and livelihoods, sustainably mitigating or adapting to it has become a defining challenge of modern times. A promising solution to this challenge is radiative cooling, which involves the skywards reflection of sunlight (λ~0.3-2.5 μm) and emission of LWIR (λ~8-13 μm) heat, through the transmission windows of the atmosphere, into space. Its zero-energy, zero-carbon functionality, and net cooling effect on earth, has seen it explored as a sustainable and effective way to drastically cut building cooling loads and reduce urban temperatures. This has motivated the development of solar reflective, LW-IR emissive radiative coolers for the built environment, with several technologies commercialized due to their promise. However, these material advances have been centered on a prevalent thermal architecture. Referring to FIG. 1A, showing people (1) in a cooling target (2) (e.g., an indoor space, under a roof (3) with a radiative cooler, such as a coating, on the outer surface of the roof - not shown for simplicity). The cooling target (2) transfers (4) heat to the roof (3) and from the roof to the radiative cooler. The radiative cooler then radiatively (7) loses heat to the sky. Unfortunately, this limits cooling. For fastest cooling, thermal resistance between the target and the sky should be minimal. However, in the traditional architecture, radiative coolers and associated infrastructure act as a thermally massive and opaque intermediate, slowing the cooling process. Furthermore, radiative coolers on roofs gain heat by convection (5) to become warmer than the indoors on hot days, which reduces them to minimizing heating rather than cooling. These longstanding limitations persist despite material advances. To address this, the present disclosure revisits roofs from a thermal and optical perspective: what if one could create a roof that enables the interiors to directly lose heat to space? Referring to FIG.1B, in such a case, the roof would be a broadband reflector (e.g., that prevents (8) solar and (net) atmospheric radiation from heating the interior, but allows the interior to radiate (9) LW-IR heat to space. Such an architecture, where the roof and the interior collectively act as the radiative cooler, but are radiatively more isolated from each other, would Princeton - 94476 overcome the limitations of traditional roofs, and since LW-IR clear sky temperatures are ≲ 0°C, would cool interiors reliably and rapidly. While radically different from the common concept of roofs as thermally insulated, and opaque structures, the disclosed approach is optimal for cooling a wide range of open and closed spaces, such as white / transparent roofs on buildings, cooling shelters, covered public spaces, housing (including low income housing) passenger and freight vehicles, etc. A significant fraction of the built environment consists of open spaces like parking areas, transit hubs, market halls, community centers, and residential courtyards, which are exposed to the elements and not air-conditioned or radiatively cooled. Closed spaces like buildings or shelters may have thin fabric or glass roofs, and for large populations living in low-income housing in the global south, thin metal roofs. In such scenarios, our approach could be deployed as a canopy over open spaces, or a replacement for existing glass, fabric or metal roofs, with significant impact. Heat transfer models (assuming exterior and interior air temperature ~30°C and no solar effects) show that compared to a thermally opaque traditional metal, glass, or fabric roof, which remove around 5-20 Wm-2heat from the interiors to the sky, the LW-IR transparent approachenables underlying objects to radiatively lose as much as 40-90 Wm-2LW-IR heat to space, i.e.typically around five times more. From a radiant temperature viewpoint, the traditional roof stays near the ambient radiant temperature (or slightly sub-ambient if a radiative cooler is used). The LW-IR transmissive roof, by revealing the cold space above, has a radiant temperature that is ≳ 10°C lower, enabling faster cooling. This has multiple potential benefits: 1. Cooling underlying spaces by ~ 2-6°C relative to typical roofs depending on airflow. This may drastically reduce AC usage, or bring relief and lower mortalities in resource-poor settings during extreme heat. 2. Direct enhancement of thermal comfort by allowing human bodies to radiate ~20-50 W of the 80-100 W metabolic heat towards the sky, even if the ambient air is warm, reducing the need for air conditioning. 3. This could transform state-of-the-art heat shelters into completely passive systems. 4. Potential uses in vehicles, food storage, and depending on the climate, refrigeration. Designing a selectively LW-IR transmissive broadband reflector that can function as this roof is a considerably more difficult challenge than designing radiative coolers, as it entails the use of non-traditional (LW-IR transparent) materials and attaining broadband reflectance without metal. One can achieve this using two common polymers – polyethene (PE) and nylon – which are seldom used for radiative cooling. A spectrally selective transmissive substrate Princeton - 94476 can be created with solid and / or porous layers that simultaneously combines shortwave optical scattering, and longwave effective medium behavior and photonic band‐gaps to achieve a desired optical behavior (see FIG.10). Spectrally selective transmissive substrates and techniques for using such may be provided. Referring to FIG.2, a spectrally selective transmissive substrate (100) can be seen. The spectrally selective transmissive material (100) may be provided in various forms, such as, e.g., a flexible film, a rigid material, etc. As used herein, the term “flexible” generally refers to a property, and materials having such a property, of typically lacking rigidity or stiffness under ambient conditions, such as being deformable by gravity alone, or being capable of being formed into a shape, such as a roll, without cracking, breaking, etc. As used herein, the term “rigid” generally refers to a property, and a material having such a property, of typically tending to maintain a given shape at ambient temperatures and exposure to at least gravity alone without yielding. As used herein, the term “film” generally refers to a relatively thin material, which is typically less than 1 mm thick. The spectrally selective transmissive substrate (100) may include a plurality of layers, including a first layer (110) and a second layer (120). disposed atop each other, each layer being different from an adjacent layer. Each layer of the spectrally selective transmissive substrate (100) may have a top surface (101) and a bottom surface (102), such that a bottom surface (102) of a first layer (110) may be coupled to a top surface (101) of a second layer (120). Each layer may include a polymer (e.g., first polymer (111) and second polymer (121)). Each polymer may, independently, be a polyolefin, a nylon, or a polytetrafluoroethylene (PTFE). As used herein, the term “polymer” generally includes, e.g., homopolymers, copolymers (such as for example, block, graft, random and alternating copolymers), terpolymers, etc., and blends and modifications thereof. This generally includes all possible geometrical configurations of the material. These configurations include, but are not limited to isotactic, syndiotactic, and random symmetries. As used herein, the term “polyolefin” generally refers to any of a series of largely saturated polymeric hydrocarbons composed only of carbon and hydrogen. Typical polyolefins include, but are not limited to, polyethylene, polypropylene, combinations of the monomers ethylene and propylene, etc. As used herein, the term “polyethylene” generally refers not only to homopolymers of ethylene, but also copolymers wherein at least 85% of the recurring units are ethylene units such as copolymers of ethylene and alpha-olefins. Such polyethylenes may include low-density polyethylene, linear low-density polyethylene, and high-density polyethylene. Princeton - 94476 Each layer may independently include air voids (112) and / or particles (122). While FIG.2 shows air voids and particles in separate layers, such is only for ease of understanding; it will be understood that such can be included in a single layer. The plurality of air voids may have a lower refractive index than the polymer forming the layer, and may have a lower refractive index than polyethylene. The particles may have a higher refractive index than the polymer forming the layer, and may have a higher refractive index than polyethylene. Such particles may include, e.g., zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), copper oxide (CuO), Iron (II) Oxide (FeO), silicon, or a combination thereof. The particles, if present in a given layer, may be present in a total amount of 0.1%- 99.9% by volume of that layer. In a preferred embodiment, the particles may be present in a total amount of 10% - 50% by volume. In some embodiments, there may be a plurality of layers with particles, and each layer with particles may have particles at an identical concentration by volume. In some embodiments, there may be a plurality of layers with particles, and at least one layer may have particles with different concentrations by volume than another layer with particles. The general concept with air voids and particles in these substrates is to select a size (e.g., a diameter) range ensuring that: (a) the sizes are not too large, so that the particles behave as a transmissive effective medium (and may have an average transmission of at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, or at least 95%) in the longwave infrared (LWIR) wavelengths (8 µm to 13 µm), which are typically 5-20 times larger than the particle sizes, (b) for particles with a 100-500 nm size, the substrate provides solar scattering, and (c) for particles with < 100 nm size, the substrate exhibits relative transparency. As used herein, “effective medium” refers to a medium that has constituent features that are so small relative to the wavelength of interest, that light does not feel individual features, but rather, feels a homogenous mixture with properties that are some kind of average of the features. For example, consider polyethene with refractive index 1.5, to which silicon nanoparticles with refractive index 3.4 are then added. The particles are nano-size, so much smaller relative to the longwave thermal radiation. So longwave radiation do not feel the individual nanoparticles and do not get scattered by them. However, what the radiation does feel is effectively a continuous medium, with refractive index between 1.5 and 3.4 depending on the polyethene to silicon ratio. So, the particles being added can do two things at once: they can simultaneously make the polyethene-particle mixture a high index effective medium in the long wave, and depending on the particle size, can act as a scattering media in the shortwave. Princeton - 94476 Solar scattering arises because particles have a different refractive index from an underlying polymer (such as polyethylene) and have a size similar to the solar wavelengths. Thus, a median diameter of the plurality of air voids and / or particles in any single layer may be any appropriate size that can satisfy a particular design strategy. Further, multiple layers can be designed to work together to produce a desired effect. In some embodiments, at least one layer has a median diameter of 1000 nm – 2500 nm. In some embodiments, at least one layer has a median diameter of 100 nm – 1000 nm. In some embodiments, at least one layer has a median diameter of 500 nm – 1000 nm. In some embodiments, at least one layer has a median diameter of 100 nm – 500 nm. In some embodiments, at least one layer has a median diameter of less than 100 nm. In some aspects, one or more diameters of the plurality of air voids and / or particles are selected based on effectiveness of transmitting longwave infrared wavelengths, and either solar scattering or solar transparency. In various aspects, the spectrally selective transmissive substrate may form an opaque material. As used herein, the term “opaque” generally refers to a property of having 0% to 20% transmissivity, on average, across a target wavelength range. If not wavelength range is given, this refers to being visually opaque, i.e., opaque across the visual wavelengths, of about 300 nm to about 700 nm. In various aspects, the spectrally selective transmissive substrate may form a transparent material. As used herein, the term “transparent” generally refers to a property of having 80% to 100% transmissivity, on average, across a target wavelength range. If no wavelength range is given, this refers to being visually transparent, i.e., transparent across the visual wavelengths. To control the performance of the substrate (e.g., which wavelengths are reflected, and which are transmitted), the characteristics of the particles in different layers may be controlled. For example, there may be a plurality of layers with particles, and at least one layer may have particles the a different material from another layer. In some embodiments, there may be a plurality of layers with particles, and each layer may have particles of the same material but different median sizes. Referring to FIG.3, the plurality of layers may include any number of layers, including first layer (110), second layer (120), third layer (210), fourth layer (220) and n-th layer (230). In some embodiments, the plurality of layers includes 2-50 layers. In a preferred embodiment, the plurality of layers includes 10-30 layers. For example, the plurality of layers may include a first set of alternating layers, the first set of alternating layers including a first layer and a second layer disposed in alternate layers (e.g., in an A-B alternating structure), where the first layer and the second layer have different Princeton - 94476 compositions. In FIG. 3, such an arrangement would mean first layer (110) and third layer (210) would be “first layers” (type A layers), while second layer (120) and fourth layer (220) would be “second layers” (type B layers). In certain aspects, each first layer is identical, and each second layer is identical. In certain embodiments, at least one first layer has a different thickness from a different first layer, and / or at least one second layer has a different thickness from a different second layer. In FIG. 3, such an arrangement would mean first layer (110) may have a thickness (240) that is different from a thickness (242) of third layer (210), and / or second layer (120) may have a thickness (241) that is different from a thickness (243) of fourth layer (220). In this bilayer approach, in some embodiments, the first material could be a nano / microporous material, and would scatter and reflects ultraviolet (UV)-to-short wavelength infrared (SWIR) wavelengths very well. Its pore sizes would be tailored to ensure its scattering dies down in the medium wavelength infrared (MWIR). It may be intrinsically transparent of emissive in one or more parts of the MWIR-LWIR-far infrared (FIR) range. Similarly, in an some embodiments, the second material could be a microporous material, and could primarily serve as a secondary UV-to-SWIR scatterer and reflector, and a MWIR-LWIR- FIR emitter to compensate for any spectral shortcomings of previous layers. As will be understood, the transmissivity and reflectivity of each layer influence the overall transmissivity and reflectivity of the spectrally selective transmissive substrate, and the thickness of a given layer will influence the transmissivity and reflectivity of that layer. Thus, each layer may have a thickness selected based on its contribution to ensuring the one or more spectrally selective transmissive substrates is at least 50% transparent on average across the long wavelength infrared wavelength range, and greater than 30% reflectivity on average across infrared wavelengths outside the long wavelength infrared wavelength range. In preferred embodiments, each layer may, independently, have a thickness of 2.5 µm to 7 µm. In FIG.3, an embodiment is shown that includes an optional underlying substrate (250). This underlying substrate (250) may be transmissive in one or more infrared ranges (for example, at least 50% transmissive on average across at least the long wavelength infrared wavelength range, at least 50% transmissive on average across the wavelengths from 0.7 µm to around 25 µm, etc.). However, the underlying substrate may inhibit conduction or convection. This underlying substrate may be disposed under all (or only under a portion but not all) of the spectrally selective transmissive substrate. Referring to FIG.4, in certain aspects, the plurality of layers multiple sets of alternating layers. For example, the spectrally selective transmissive substrate (100) may include a first Princeton - 94476 set (310) of alternating layers (e.g., first layer (110) and second layer (120)) and one or more additional sets of alternating layers, including a first additional set (320) of alternating layers (e.g., first layer (321) and second layer (322)). N-th additional set (330) is shown, as there may be multiple sets of such layers. The additional layers are generally disposed below the first set of alternating layers (e.g., the first set of A-B alternating layers is disposed above a second set of C-D alternating layers). Said differently, the first set of alternating layers may be the most outward or external of layers, while the additional sets of alternating layers may be disposed between the first set of alternating layers and whatever the spectrally selective transmissive substrate is operably coupled to (e.g., a roof or a wall surface), or disposed over (e.g., over a floor, ground, etc.). By selecting appropriate polymer-particle combinations, including appropriate particle materials and sizes, one can control the bands of reflectivity within the various layers or sets of layers. That is, there may “reflection bands”, or a range of wavelengths across which the average reflectivity is at least 10%, at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, or at least 50%. Referring to FIG.5, a simplified example of reflectivity of two sets of alternative layers is shown. For a first set (410) of alternating layers, there is a first reflection band (411) in the 0.3 µm to 8 µm range, and a second reflection band (411) in the 13 µm to 25 µm range. Similarly, for a second set (420) of alternating layers, there is a first reflection band (421) in the 0.3 µm to 8 µm range, and a second reflection band (421) in the 13 µm to 25 µm range. Thus, in certain aspects, each set of layers may be configured to have at least one reflection band within each of a 0.3 µm to 8 µm range and a 13 µm to 25 µm range. A reflection band from one set of layers may be different from a reflection band of another set of layers – as shown in FIG.5, this may be both a degree of reflectivity, as well as which wavelengths exhibit the desired degree of reflectivity. In some embodiments, the wavelengths defining the reflection bands may overlap (see, e.g., the overlap of the first reflection bands (411, 421) of the first and second sets of alternating layers in FIG.5). In some embodiments, there may be at least one gap (423) between two reflection bands where the reflectivity of each layer or set of layers in that gap of wavelengths may be insufficient to be considered part of a reflection band. The reflection bands are preferably created such that the majority of the LWIR wavelength range (of 8 µm to 13 µm), i.e., the desired transmission window, is less than 20% reflective, less than 15% reflective, less than 10% reflective, less than 5% reflective, or less than 1% reflective. Princeton - 94476 In one preferred embodiment, the spectrally selective transmissive substrate may include a solar reflective, thermal infrared transparent layer, comprising polyethylene and air voids, zinc oxide, and / or zinc selenide. As will be understood, the reflection bands may be configured to be complementary, such that together, they cover as much of the 0.3 µm to 8 µm and 13 µm to 25 µm range as possible. In certain aspects, the plurality of layers together have reflection bands covering at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, or at least 90% of wavelengths in the 0.3 µm to 8 µm range and the 8 µm to 25 µm range. Referring to FIG.6, the spectrally selective transmissive substrate (100) may include a microtextured anti-reflective layer (510) disposed over the plurality of layers (e.g., first layer (110) and second layer (120), such that the plurality of layers may be disposed between the microtextured anti-reflective layer and whatever the spectrally selective transmissive substrate is operably coupled to (e.g., a roof or a wall surface), or disposed over (e.g., over a floor, ground, etc.). That is, in some embodiments, the microtextured anti-reflective layer (410) may be coupled to a top surface (101) of a first layer (110)). The microtextured pattern may be, e.g., a symmetric pattern (such as a triangular wave pattern, a square wave pattern, rods, etc. In certain embodiements, this layer, due to its textured nature and low refractive index, would enhance medium wavelength infrared (MWIR), LWIR, and far infrared (FIR) emittance, and its intrinsic ultraviolet (UV) – to – short wavelength infrared (SWIR) transparency would minimize solar absorptance. The patterned / textured nature of the layer also makes it highly insensitive to angle. This would result in essentially a solar transparent, and MWIR-LWIR-FIR antireflective layer. This may be intrinsically emissive in one or more parts of the MWIR- LWIR-FIR wavelengths. The spectrally selective transmissive substrate (100) may include a LWIR (or visually and LWIR) transparent support substrate (520) The bottom surface (521) of the support substrate may form a bottom surface of the spectrally selective transmissive substrate (100) (for example, the support substrate may be coupled to a bottom surface (102) of a second layer (120)). Each spectrally selective transmissive substrate may be at least 50% transparent on average across a long wavelength infrared wavelength range, and greater than 30% reflective on average across infrared wavelengths (as used herein, this range is considered to be from 0.7 µm to 25 µm) outside the long wavelength infrared wavelength range (8 µm to 13 µm). In some embodiments, each spectrally selective transmissive substrate may be at least 50% Princeton - 94476 transparent on average across the long wavelength infrared wavelength range, and greater than 50% reflective on average across wavelengths in a second range, the second range including 0.3 µm to 8 µm and 13 µm to 20 µm. In various aspects, a heat transfer method utilizing such spectrally selective transmissive substrates may be provided. Referring to FIG. 7, the method may include allowing heat from at least a portion of a covered or indoor volume of space (e.g., heat from ground area (631) on ground surface (630), in one or more directions (621) within 70° angles (622) from normal (620) to a walking surface of the covered or indoor volume of space, to reach an outdoor atmosphere (640) by only passing through a transmissive portion of a roof structure (610). The transmissive portion may consist of one or more spectrally selective transmissive substrates. The covered or indoor volume of space may be a top floor of a building, an atrium or open area within a commercial building, an outdoor area covered by a canopy, etc. In some embodiments, the transmissive portion may be all of the roof structure. In some embodiments, the transmissive portion may be less than all of the roof structure. For example, the roof structure may include both a portion that is only a spectrally selective transmissive substrate (100), and a thermal insulation portion (611). In certain preferred embodiments, the thermal insulation portion may be, e.g., transmissive to radiation (and specifically to infrared radiation), while insulating from convection and conduction. For example, the thermal insulation may be at least 50% transmissive, on average, across the LWIR wavelengths. In certain other embodiments, the thermal insulation portion may be non-transmissive to infrared radiation (e.g., the thermal insulation may have an average transmission across the LWIR wavelengths of less than 20%, such as less than 10%). Such thermal insulation that is IR transparent may be composed of various materials, such as insulation formed of multiple layers of thin polyethene films with a few millimeters (e.g., 1-5 mm or 1-10 mm) of air between each layer, or one or more layers of polyethylene having many large (e.g., at least 1 mm, preferably at least 5 mm) size bubbles. The roof structure may include one or more rigid portions (650) configured to reinforce each spectrally selective transmissive substrate. The one or more rigid portions may include a metal, a polymer, etc. In various aspects, a system may be provided. The system may include a vertical structure (612) operably coupled to a ground surface (630). The system may include a roof or covering (610) operably coupled to the vertical structure (612). The system may include one or more spectrally selective transmissive substrates (100), as disclosed herein, operably Princeton - 94476 coupled to at least a portion of the vertical structure, or forming at least a portion of the roof or covering. In various aspects, a heat emitting method may be provided. Referring to FIG. 8, the heat emitting method may include emitting heat (720) from a surface (711) through one or more spectrally selective transmissive substrates (100) as disclosed herein, which is / are thermally coupled to the surface. A bottom surface (102) of the spectrally selective transmissive substrate may be coupled to the surface (711). The surface may be any appropriate surface, such as, e.g., an interior or exterior wall (710). One or more thermally conductive substrates (730) may be disposed between the surface (711) and the one or more spectrally selective transmissive substrates (100). Referring to FIG.9, if the surface is an indoor surface, the heat may be transmitted through a first spectrally selective transmissive substrate (100) directly coupled to the surface, and may then transmit through a second spectrally selective transmissive substrate (810) forming some or all of a roof, window, etc. In various aspects, methods of manufacturing a spectrally selective transmissive substrate may be provided. In some embodiments, the multilayer optical films can be produced by coextrusion of alternating polymer layers. See, e.g., U.S. Pat. Nos.3,610,729 (Rogers), 4,446,305 (Rogers et al.), and 9,833,956 (Neavin et al.). In certain aspects, the method may include providing a plurality of resin streams, each resin stream independently comprising a polymer (such as a polymer as disclosed herein). The resin streams may be from a plurality of extruders, and may be provided through a manifold. The method may include forming stacked layers by coextruding the plurality of resin streams as a plurality of alternating layers, and casting the stacked layers onto a casting roll. In certain aspects, the method may include forming a plurality of thin layers by providing a solution comprising a solvent and a polymer as disclosed herein, which may be configured to include air voids and / or particles as disclosed herein, forming a thin layer using the solution, and hot pressing the thin layer at or near a melting temperature of the polymer. Forming the thin layer may include casting the solution as a layer, using a doctor blade, film applicator, and / or an applicator rod. For example, the solution can be applied to a substrate using an Wire-wound Mayer film applicator rod (sold by Gardco), the solution can hten be drawn down, and allowed (or caused) to dry. Forming the thin layer may include dip coating the solution onto a substrate, and drying the thin layer. The method may include peeling the thin layer from the substrate after drying. Forming the thin layer may include mechanical rolling (e.g., reducing thickness of a layer by applying pressure via rollers) or melt rolling (e.g., Princeton - 94476 using a heated roll coater) of the solution. The method may include combining a plurality of thin layers to form an alternative set of stacked layers. FIG.10 shows transmittance for an example spectrally selective transmissive substrate (multiple polyethylene / nylon layers), along with that of an ideal substrate. FIG. 11 illustrates how, for example, different particle sizes can impact scattering efficiency of particles in a polymer matrix (here, ZnS in polyethylene). In FIG.11, scattering efficiency for three different size ZnS particles are shown – 750 nm, 500 nm, and 50 nm. Higher scattering efficiency (1000) generally provides better, e.g., scattering and reflection of a given wavelength, while lower scattering efficiency (1010) is indicative of a generally transparent effective medium for those wavelengths. As can be seen, the ZnS particles embedded in polyethylene scatter and reflect sunlight in sufficiently large sizes, but lead to a high index effective medium behavior in longwave infrared wavelengths, which are much larger than the particles. If the particles are small enough (diameter < 100 nm), they act as effective media in the visible wavelengths as well, leading to a high index transparent effective medium behavior in the visible – something that is suitable for visibly transparent variants. Further, it can be seen that by adjusting particle size, for the same material (here, ZnS), the shape of the curves shifts, in addition to shifting the wavelengths at which effective scattering / reflection or transparency is seen. For example, moving from 750 nm to 500 nm ZnS particles, the point at which the scattering efficiency drops below 1 moves from around 1.9 µm wavelengths to around 1.25 µm wavelengths. Thus, it will be understood that by controlling particle sizes within one or more layers, one can control the performance of the substrates. FIGS. 12-15 show transmittance graphs of various polyethylene / Si-doped polyethylene bilayer substrates. Specifically, FIG. 12 shows a transmission spectrum of a multilayer film comprising 8 or 16 bilayers (16 or 32 total layers), with each bilayer comprising a first layer of polyethene and a second layer of polyethene doped with 50% Silicon nanoparticles by volume. FIG. 13 shows a transmission spectrum of a multilayer film comprising 8 or 16 bilayers (16 or 32 total layers), with each bilayer comprising a first layer of polyethene and a second layer of polyethene doped with 25% Silicon nanoparticles by volume. FIG. 14 shows a transmission spectrum of a multilayer film comprising 9 bilayers (18 total layers), with each bilayer comprising a first layer of polyethene and a second layer of polyethene doped with 50% Silicon nanoparticles by volume. The bilayers are optimized to reflect different non-longwave infrared bands of light (e.g., layer thicknesses were selected to reflect specific thermal bands). FIG. 15 shows a transmission spectrum of a multilayer film Princeton - 94476 comprising 9 bilayers (18 total layers), with each bilayer comprising a first layer of polyethene and a second layer of polyethene doped with 25% Silicon nanoparticles by volume. The bilayers are optimized to reflect different non-longwave infrared bands of light. FIGS.16-19 similarly show transmittance graphs of various polyethylene / ZnS-doped polyethylene bilayer substrates. Specifically, FIG. 16 shows a transmission spectrum of a multilayer film comprising 8 or 16 bilayers (16 or 32 total layers), with each bilayer comprising a first layer of polyethene and a second layer of polyethene doped with 50% ZnS nanoparticles by volume. FIG. 17 shows a transmission spectrum of a multilayer film comprising 8 or 16 bilayers (16 or 32 total layers), with each bilayer comprising a first layer of polyethene and a second layer of polyethene doped with 25% ZnS nanoparticles by volume. FIG. 18 shows a transmission spectrum of a multilayer film comprising 9 bilayers (18 total layers), with each bilayer comprising a first layer of polyethene and a second layer of polyethene doped with 50% ZnS nanoparticles by volume. The bilayers are optimized to reflect different non-longwave infrared bands of light. Here, the bilayers were computationally optimized to maximize LWIR transmission, and maximize non-LWIR reflection. A center of the reflection band and a number of bilayer pairs were chosen, and then modified to get the optimal combination of LWIR transmission and non-LWIR reflection. FIG.19 shows a transmission spectrum of a multilayer film comprising 9 bilayers (18 total layers), with each bilayer comprising a first layer of polyethene and a second layer of polyethene doped with 25% ZnS nanoparticles by volume. The bilayers are optimized to reflect different non-longwave infrared bands of light. Referring to FIGS. 20A-20E, as scattering by the pores or particles in polyethene diminishes, effective medium behavior kicks in. The inclusion of high index particles ZnS and Si (n > 2) turns the composite into a higher refractive index effective medium. The inclusion of low index (n~1) air voids has the opposite effect. The complex refractive index (n + iK) of polyethene, and polyethene doped with various volume fractions of ZnS, Si and air voids are shown below. Example In this example, a substrate comprising a bilayer architecture, consisting of porous poly(vinylidene fluoride-co-hexafluoropropene) (P(VdF-HFP)) topcoat which acts as a low- index, rough LWIR effective medium and a modest solar scatterer, and a nanofibrous polytetrafluoroethene (PTFE) underlayer which is a highly efficient solar scattering medium. The differing behavior of the example design’s microstructure in the solar and thermal infrared Princeton - 94476 (TIR, λ~4-20 μm) yields an ^^solarof ~0.991, a near- normal ^^^^ூோ,^of ~0.98, and hemispherical ^^^^ூோof ~0.96. The solar reflectance, ^^solar of a material is defined as: ಮ ^^^ ூೞ^^ ^ఒ^∙ோ^ఒ^ௗఒ^^^^^ൌ బೌ^^ಮ(1) బ ூೞ^^ೌ^^ఒ^ௗఒ where ^^ is the wavelength, ^^^^^^^^ ^^^ is the AM1 or ASTM G173 Global solar intensity spectrum and ^^^ ^^^ is the material’s spectral reflectance. The directional LWIR emittance ^^^^ூோ^ ^^^ is defined as: భయ µ^^்,ఒ^∙ఌ^ఏ,ఒ^ௗఒ ^^ ^ ^^^ ൌ^^ µ^ூಳಳ^^ூோ ^భయ µ^^ூಳಳ^்,ఒ^ௗఒ(2) a to be 25 ̊C) and ^^^^^, ^^^is the material’s spectral directional thermal emittance. In this example, the near-normal emittance ^^^^ூோ,^corresponds to the case where ^^ is 8°, as it is difficult to practically make accurate measurements at near-normal incidence. The hemispherical emittance ^^^^ூோis given by: ഏ / మ ^^^బఌ^ೈ^ೃ^ఏ^∙ூಳಳ ୡ୭^^ఏ^ ^୧୬^ఏ^ௗఏ^^ூோൌഏ / మ(3) ୡ୭^^ఏ^ ^୧୬^ఏ^ ^^^^^ ^^^^^^ െ ^^^^௬^ ^^^^^, ^^ ^^ ^^^ (4) where ^^^^^is ambient temperature, TPW is total precipitable water, and ^^^^௬is thedownwelling atmospheric irradiance (see, e.g., ref 900 in FIG.10). The near-ideal values of the example enable the design to attain sub-ambient cooling of 2.3°C even under humid, partially cloudy skies. The design can be conveniently made using established materials and fabrication techniques. An ideal sky-facing radiative cooler, whether it is a broadband or selective LWIR emitter, has an ^^^^^^^ൌ 1 and ^^^^ூோൌ 1. While there is an abundance of radiative coolerswith ^^^^^^^and ^^^^ூோ,^> 0.90, designs with ^^^^^^^and ^^^^ூோ,^> 0.95 are few, and thoseapproaching ^^^^^^^and ^^^^ூோ,^=1 are rarer still. Near those limits, fundamental material and structural properties limit what designs limitations can achieve. For instance, radiative cooler architectures that have a solar- transparent LWIR emitter on a solar reflective metal like silver, rarely have ^^^^^^^> 0.96 because of the intrinsic ^^^^^^^of silver (~0.97) and the fact that a less than perfectly smooth silver surface and the intrinsic absorption of the emitter above lower the Princeton - 94476 reflectance from the intrinsic value. Consequently, most radiative cooling designs with ^^^^^^^> 0.96 are thick, optically inhomogeneous porous polymers and composites that scatter and reflect light, although intrinsic absorption of materials limit their ^^^^^^^as well. Similar material limitations also occur for ^^^^ூோ. A survey of the literature indicates that emitters with ^^^^ூோ,^> 0.90 are quite common, but emitters with ^^^^ூோ,^> 0.95 are few. If one considers the true, hemispherical emittance ^^^^ூோ, which is typically lower than ^^^^ூோ,^but is seldom reported, designs with ^^^^ூோ> 0.95 are exceptionally rare. One reason behind this is the often-overlooked LWIR surface reflectance of radiative coolers. Although most non- metallic materials used in radiative cooling designs are good intrinsic emitters, their surface reflectance, which is complementary to their emittance, limits ^^^^ூோ. Many radiative cooling designs have planar or microscale-smooth surfaces, which have a significant surface reflectance, particularly at high angles, because of the refractive index contrast at the air-emitter interface. By contrast, designs with microstructured surfaces have lower surface reflectance and higher ^^^^ூோ. Another factor that applies to both smooth and rough surfaces, is the emitter’s refractive index. The use of strong LWIR emissive materials like SiO2in radiative cooling designs also means that their refractive index is higher than that of air (n~1), as dictated by the Kramers-Kronig relations. This can lead to a high backscattering of light from heterogeneous media or high reflections off smooth surfaces, again limiting ^^^^ூோ. Since the use of metal mirrors limit ^^^^^^^, this example initially opts to use an optically heterogeneous medium with ~0.5-1 µm features to scatter sunlight. Given that features in that size range are Mie scatterers in the solar wavelengths, in the absence of absorption, they couldtheoretically yield ^^^^^^^ =1 if the scattering medium is sufficiently thick.Promisingly, since the ~0.5-1 µm heterogeneities are roughly an order of magnitude or smaller than the LWIR wavelengths, the heterogeneous medium could effectively act as a homogenous medium in the LWIR wavelengths. Thus, as long as the scattering and non- absorption in the solar wavelengths were kept intact, one could choose materials and morphologies to create an effective medium with a high ^^^^ூோ. Crucially, this would enable going beyond the limitations of single materials that are intrinsically solar transparent but LWIR emissive. If one constituent of the heterogenous medium was a low-index material, this would not only help scatter sunlight, but also lower the effective LWIR refractive index of the medium towards that of air, taking ^^^^ூோclose to 1. In fact, if the other constituent(s) of the medium were higher index, solar-transparent, LWIR-emitters, this constituent could simply be Princeton - 94476 air. In other words, a nano-to-microporous media with ~0.5-1 μm features, and containing air in the pores, could hypothetically achieve near-ideal ^^^^^^^, and ^^^^ூோ. With the general design in mind, materials that have a near-zero solar absorptivity, and high LWIR emissivity were considered. Polymers, which are generally transparent to sunlight, radiate TIR light due to their molecular vibrations, and are highly processable into porous forms, are promising in this regard, and have long been used as thermal emitters. For this example, a wide range of commercially available polymers were considered, and systematically screened by their near-to-shortwave infrared (NIR-to-SWIR, λ~1-2.5 μm) solar absorptance, and then by their ^^^^ூோ. The decision to first screen by NIR-to-SWIR absorptance was because of the large magnitude of peak solar intensity (> 1000 Wm-2) relative to the LWIR cooling potential (10-150 Wm-2), and that polymers primarily absorb in the NIR-to-SWIR. FIG. 21 shows the twelve least solar absorptive polymers among the polymers considered in this example. As evident, polytetrafluoroethene (PTFE, (C2F4)n), whose C-C and C-F bonds have minimal absorption in the solar wavelengths, has the lowest solar absorptance among all polymers. Ethylene-chlorotrifluoroethylene (ECTFE), silicone and P(VdF-HFP) have the second, third and fourth lowest absorptances respectively, with the rest of the polymers having increasingly higher absorptances. An examination of the chemical structures of these polymers reveals that near-IR absorptances increases with the relative abundance of CH, CH2and CH3groups in the molecular structure, particularly evident in the case of polyethene and polypropene, and additionally, C=O and C-OH groups, in the case of acetal and PMMA. Other bonds, in particular, C-C and C-F bonds, have little NIR-to-SWIR absorption, which is why polymers like PTFE and P(VdF-HFP) absorb little sunlight. PTFE’s exceptionally low absorptance could make it a near-ideal solar reflector when made porous, as optical scattering in the absence of absorption can lead to a high reflectance (see FIG.22). In FIG.22, the impact of size and shape of the dopant can be seen, with scattering efficiency ( ^^^^^) of spherical particles and fibers of the same diameters being compared at 1.2 µm, 0.8 µm, and 0.2 µm diameters. However, its molecular composition lacks strong vibrational modes precisely in the LWIR wavelengths where they are needed (see FIG. 23). Thus, for weakly absorptive porous PTFE, scattering-induced reflectance would lead to a low ^^^^ூோ. ECTFE, Silicone and P(VdF-HFP), which are highly emissive (average emissivity > 0.9 in the LWIR window of 8 µm – 13 µm), could be potential alternatives. However, the intrinsic NIR-to-SWIR absorptance of these materials, even when lower than most polymers, Princeton - 94476 could limit ^^^^^^^of porous films made from them. This has been observed for porous P(VdF-HFP) and silicone, whose ^^^^^^^plateau at ~ 0.98 for thick films.These considerations led to the disclosed bilayer architecture, which may include, e.g., a LWIR emissive porous polymer first layer and a solar reflective porous PTFE second layer, which overcomes the above limitations. Central to the design is the fact that absorptances of polymers are typically much stronger in the LWIR than NIR-to-SWIR. This would enable a thin topcoat of a porous LWIR-emissive polymer to enhance ^^^^ூோfar beyond that of PTFE, while minimizing solar absorptance. The solar reflective PTFE underlayer would, at the same time, maximize ^^^^^^^within the bounds of the solar absorption of the topcoat. Since ECTFE, Silicone and P(VdF-HFP) all have high emittances and similarly NIR-to- SWIR absorptance, they can be considered for emissive layer. A comparison of the emissivities of the polymers shows that P(VdF-HFP) has the highest emissivity in the LWIR. Thus, for this example, a thin, porous P(VdF-HFP) film as the first layer was chosen. Nano-to-microporous polymers usually have fibrous or bicontinuous / closed air void morphologies. Scattering efficiencies of PTFE filaments and spherical voids in PTFE matrix (n~1.38) show that an array of fibers or voids with nano-to-microscale (~0.2-1.2 μm) sizes scatter all solar wavelengths (see FIG.22.). In the absence of intrinsic absorption (see FIG.23), this would yield a high ^^^^^^^. The same nano-to-microscale features can lead to microscale surface roughness, and provided that they are ≲ 10x smaller than the LWIR wavelengths, cause the porous polymer to behave as a low- effective medium. Simulation of bulk P(VdF- HFP) with surface roughness of ~5 μm shows that surface roughness has an antireflective effect, leading to a higher emittance than the emissivity of smooth bulk P(VdF-HFP). To simulate the effective medium behavior, one can calculated the effective complex refractive index of 50% porous P(VdF-HFP) using Maxwell-Garnett effective medium theory, and calculated its emissivity. This also leads to a high emittance, as would be expected of a low-index material. To fabricate the bilayer porous polymer, different fabrication pathways can be considered. Although PTFE’s optical properties make it highly appealing, its chemical inertness, resistance to solvents, and incompatibility with melt processing make it particularly difficult to process. One pathway to make porous PTFE is to sinter PTFE particles under heat and pressure. While that can yield high reflectances, sintering often leads to fusion of particles and porosities of ~40%, leaving too few air voids for efficient scattering. Consequently, sintered PTFEs reported in the literature need to be several mm thick to achieve near-unity Princeton - 94476 reflectance. An alternative is to use PTFE in its expanded form (ePTFE). When heated to high temperatures and rapidly stretched, PTFE expands into a highly porous (~65%), nanofibrillar form, with fibril sizes comparable to those in the above-described simulation (see also FIG. 22). This indicates the potential for a high solar reflectance. Indeed, ePTFE is known for its brilliant white color, and is commercially available, at very large scales. Given its promise, ePTFE was explored as the solar reflective underlayer for an example design.0.8-1 mm thick ePTFE sheets were purchased from EqualSeal, and examined under a QUANTA® 200 field emission gun (FEG) Scanning Electron Microscope. As shown in FIG.24, , the ePTFE comprises of numerous 0.2-0.5μm thick fibrils branching from nodes. As expected from the optical properties and scattering efficiencies, the microstructure imparts an exceptional ^^^^^^^of 0.992 for ~1 mm thick films. However, as expected, ^^^^ூோ,^^ ^^^ is low, and is calculated to be 0.86. To augment ^^^^ூோ, a porous P(VdF-HFP) layer was created on the ePTFE using a highly scalable phase inversion method, as understood in the art. A precursor solution of acetone, P(VdF-HFP) and water in 8:1:1 mass ratio was coated on the ePTFE using an applicator. The rapid evaporation of acetone, followed by phase separation of P(VdF-HFP) and water, and eventual evaporation of water, left nano porous P(VdF-HFP) films with microscale surface roughness on the ePTFE (see FIG. 24). As expected from the theoretical results, the surface roughness and the effective medium behavior due to the nanopores lead to a considerably higher emittance than a solid P(VdF-HFP) film. To find the best combination of ^^^^^^^and ^^^^ூோ, different film thicknesses of the precursor solution were coated on the ePTFE, and spectral reflectance measurements were taken. For a nominal film thickness (corresponding to applicator setting rather than physical value) of ~75-100 μm, the porous film reaches a plateau at ^^^^ூோ~0.98, compared to ePTFE’s 0.86, for only a 0.001 drop in ^^^^^^^. Higher thicknesses reduce ^^^^^^^with little gain in ^^^^ூோ, so a nominal film thickness in the ~75-100 μm range for the example bilayer design was targeted. A superwhite bilayer porous polymer resulted from the design process. As shown in FIG.24, the porous P(VdF-HFP) topcoat has 5μm surface features at the top, and ~0.2-0.8 μm pores within, while the underlayer comprises of 0.2-0.5 PTFE nanofibrils. Notably, the 75-100 μm nominal film thickness corresponds to a ~30 μm thin P(VdF-HFP) topcoat above the 0.8-1 mm thick ePTFE. The structural and material properties of the two components described earlier complement each other to yield an exceptional optical performance (for an example visualization of this, see FIG.10). The bilayer porous polymer has a near-ideal ^^^^^^^of 0.991 Princeton - 94476 at near normal incidence, and likely greater at high angles due to a higher effective thickness for incident light. The near-normal LWIR emittance ^^^^ூோ,^is calculated at 0.981. Angular radiometric measurements (see FIG. 25) revealed the emittance stays nearly constant up to 60° angle from the surface normal, and only drops below 0.90 after 75°, which is desirable as the atmospheric irradiance is high near the horizon. The hemispherical emittance^^^^ூோis calculated at 0.96. Collectively, the measured values of ^^^^^^^and ^^^^ூோrepresents anear-ideal combination, and validates the design approach towards ideal optical performance. The steady state radiative cooling performance of the example bilayer design was tested under an open sky on a late summer day Princeton, USA. The experimental setup is shown in FIG.26. A 15 cm x 15 cm ePTFE-P(VDF-HFP) film, serving as a radiative cooler (2503), was adhered to a 1 mm thick 6061-aluminum sheet (2508) and mounted on roof (2501) formed from a R13 insulation foam board. This board was covered with Tyvek and aluminum foil featuring 93% solar reflectance to minimize solar heating or radiative cooling of the setup. The roof was attached to supports (2502) that placed the roof approximately 2 meters above a light- colored roof to avert local hotspots. Two k-type thermocouples were employed, a first thermocouple (2509) for measuring temperature of the radiative cooler (TRC) and a second thermocouple (2507) that was shaded and below the setup to measure Tamb. Importantly, the measured Tamb matched readings from the local weather station. An APGOGEE® SP510 pyranometer (2505) and an APOGEE® SL510 pyrgeometer (2506) were installed on the installation to record solar and atmospheric TIR irradiances respectively. Conductive losses were deemed negligible due to the high thermal resistance of the foam board. All sensors were linked to an OCTPRO® datalogger from Omega Engineering, Inc. that collected data throughout the experiment, after allowing the setup to reach steady state temperature. Steady state temperature measurements and solar / thermal irradiance were documented for roughly three hours. The subsequent period experienced intermittent cloud cover obstructing direct sunlight. There appears to be no relationship between the variations in Isolar and Δ ^^= Tamb - TRC, indicating that the exceptionally high Rsolar of the radiative cooler makes solar heating negligible. Indeed, the radiative cooler achieves a sub-ambient temperature throughout the period, with an average sub-ambient cooling of 2.31°C observed for the first 45 mins, and 2.26°C observed over the entire experiment. While the observed sub-ambient cooling is lower than those of previously reported radiative coolers, this is attributable to meteorological conditions. During the first part of the Princeton - 94476 experiment, a moderately high solar intensity of ~750 Wm-2, a high total precipitable water (TPW) ~38.1 mm, a partial cloud cover of ~20-25%, and fluctuating windspeeds of 0.5 - 2 ms-1, made sub-ambient radiative cooling quite difficult. The mean sky temperature calculated from pyrgeometric measurements was ~25°C, only 8°C cooler than the mean Tamb. This indicated a radiative cooling potential of 50 Wm-2or less, since the pyrgeometer does not register sky radiation at grazing incidences. Calculations performed with MODTRAN® (MODerate resolution atmospheric TRANsmission) computer code from Spectral Science, Inc., using Tamb and TPW as inputs yielded cooling potential of 50-60 Wm-2for cloud covers of 25-15% . The observed sub-ambient cooling Tamb - TRC were compared with theoretical calculations involving solar absorption by the radiative cooler, MODTRAN® computer code sky irradiances based on TPW, Tamb and cloud cover, and different windspeeds. Importantly, using MODTRAN® computer code enabled spectral calculations using previously determined optical properties. The results show that the experimental results lie within the bounds of the different cloud cover and windspeed scenarios. Indeed, much of the variation in Tamb- TRCappear to be due to windspeed, with differences between average observed and theoretical Tamb- TRC being within 0.17-0.4°C for windspeeds of 1-2 ms-1and cloud covers of 20-25%. The factthat radiative cooling is observed in far-from-ideal conditions and cooling potentials is due tothe high ^^^^^^^and ^^^^ூோof the bilayer radiative cooler. Furthermore, the design’sexceptionally high ^^^^^^^is corroborated by the remainder of the experiment, which showsthat even large fluctuations in solar intensity (~500-700 Wm-2) due to shading by clouds has no effect on the sub-ambient cooling. In other words, sunlight does not discernibly impact cooling performance. The optical performance of the porous P(VdF-HFP)-ePTFE bilayer made is complemented by its scalability. ePTFE can be made at large scales through mechanical extrusion, followed by uniaxial or biaxial stretching at high temperatures. The P(VdF-HFP) toplayer can likewise be made by a scalable phase inversion technique used to make polymer filtration membranes. These processes are both established, and suitable for industrial production. The fact that the bilayer design uses commercially available polymers also adds to its potential for scalable manufacturing and use. The high optical performance and scalability of the porous P(VdF-HFP)-ePTFE bilayer makes it attractive for use in a range of applications, such as cooling HVAC systems, passive Princeton - 94476 ventilation, direct cooling of vehicles, infrastructure, water harvesting from air, and freezing desalination, and potentially, cooling rooftops of buildings. Although the porous P(VdF-HFP)-ePTFE bilayer attains a near-ideal optical parameters for radiative cooling, it is understood that such an approach may still be further augmented / optimized. One way to minimize thickness could be to replace the lower part of the ePTFE underlayer with a thinner, porous polymer, where the polymer has a higher refractive index for more efficient scattering. Potential materials include cellulose acetate (n~1.47), which can be easily phase inverted and mylar (n~1.6). With regard to morphology optimization, we note that ePTFE fabrication parameters can be tuned to change both the nanofibrous structure and porosity, which in turn would impact backscattering of sunlight. One particularly intriguing possibility is to tailor the morphology to maximize scattering at λ~2 μm, where ePTFE shows some absorption. It may also be possible to morphologically alter the porous P(VdF-HFP) top coat to further enhance ^^^^ூோ, particularly at high angles. The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” The phrase “and / or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and / or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and / or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and / or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A Princeton - 94476 and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and / or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including”, “comprising”, or “having”, “containing”, “involving”, and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof, either by themselves or in addition to one or more additional items. In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including”, “carrying”, “having”, “containing”, “involving”, “holding”, “composed of”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively. Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.

Claims

Princeton - 94476 What is claimed:

1. A heat transfer method, comprising: allowing heat from at least a portion of a covered or indoor volume of space, in directions within 70° angle from normal to a walking surface of the covered or indoor volume of space, to reach an outdoor atmosphere by only passing through a transmissive portion of a roof structure, wherein the transmissive portion of the roof structure consists of one or more spectrally selective transmissive substrates, each spectrally selective transmissive substrate being at least 50% transparent on average across a long wavelength infrared wavelength range, and greater than 30% reflective on average across infrared wavelengths outside the long wavelength infrared wavelength range, the long wavelength infrared wavelength range being from 8 µm to 13 µm.

2. The heat transfer method of claim 1, wherein each spectrally selective transmissive substrate being at least 50% transparent on average across the long wavelength infrared wavelength range, and greater than 50% reflective on average across wavelengths in a second range, the second range including 0.3 µm to 8 µm and 13 µm to 20 µm.

3. The heat transfer method of claim 1, wherein the transmissive portion is all of the roof structure.

4. The heat transfer method of claim 1, wherein the transmissive portion is less than all of the roof structure.

5. The heat transfer method of claim 3, wherein the transmissive portion includes infrared-transparent thermal insulation.

6. The heat transfer method of claim 1, wherein the covered or indoor volume of space is a top floor of a building.

7. The heat transfer method of claim 1, wherein the covered or indoor volume of space is an atrium or open area within a commercial building.Princeton - 94476 8. The heat transfer method of claim 1, wherein the covered or indoor volume of space is an outdoor volume of space covered by a canopy.

9. The heat transfer method of claim 1, wherein each spectrally selective transmissive substrate comprises a plurality of layers disposed atop each other, each layer being different from an adjacent layer.

10. The heat transfer method of claim 9, wherein the roof structure includes one or more rigid portions configured to reinforce each spectrally selective transmissive substrate.

11. The heat transfer method of claim 10, wherein the one or more rigid portions comprise a metal.

12. The heat transfer method of claim 10, wherein the one or more rigid portions comprise a polymer.

13. The heat transfer method of claim 9, wherein the plurality of layers includes 2-50 layers.

14. The heat transfer method of claim 13, wherein the plurality of layers includes 10-30 layers.

15. The heat transfer method of claim 9, wherein each layer has a thickness selected based on its contribution to ensuring the one or more spectrally selective transmissive substrates is at least 50% transparent on average across the long wavelength infrared wavelength range, and greater than 30% reflectivity on average across infrared wavelengths outside the long wavelength infrared wavelength range.

16. The heat transfer method of claim 9, wherein each layer has a thickness of 2.5 µm to 7 µm.

17. The heat transfer method of claim 9, wherein the plurality of layers includes a first set of alternating layers, the first set of alternating layers including a first layer and a secondPrinceton - 94476 layer disposed in alternate layers, where the first layer and the second layer have different compositions.

18. The heat transfer method of claim 17, wherein each first layer is identical, and each second layer is identical.

19. The heat transfer method of claim 17, wherein at least one first layer has a different thickness from a different first layer, and / or at least one second layer has a different thickness from a different second layer.

20. The heat transfer method of claim 17, wherein the plurality of layers includes at least one additional set of alternating layers, disposed below the first set of alternating layers, such that the at least one additional set of alternating layers is between the first set of alternating layers and the covered or indoor volume of space.

21. The heat transfer method of claim 20, wherein each set of layers is configured to have at least one reflection band within each of a 0.3 µm to 8 µm range and a 13 µm to 25 µm range, where a reflection band from one set of layers is different from a reflection band of another set of layers.

22. The heat transfer method of claim 21, wherein the plurality of layers together have reflection bands covering at least 80% of the 0.3 µm to 8 µm range and the 8 µm to 25 µm range.

23. The heat transfer method of claim 9, where each layer comprises a polymer.

24. The heat transfer method of claim 23, wherein the polymer is a polyolefin, a nylon, or a polytetrafluoroethylene (PTFE).

25. The heat transfer method of claim 23, wherein each layer includes a plurality of air voids with lower refractive index than polyethylene, and / or particles with higher refractive index than polyethylene, the particles include zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), copper oxide (CuO), Iron (II) Oxide (FeO), silicon, or a combination thereof.Princeton - 94476 26. The heat transfer method of claim 25, wherein a median diameter of the plurality of air voids and / or particles is 500 nm – 1000 nm.

27. The heat transfer method of claim 25, wherein a median diameter of the plurality of air voids and / or particles is 100 nm – 500 nm.

28. The heat transfer method of claim 25, wherein a median diameter of the plurality of air voids and / or particles is less than 100 nm.

29. The heat transfer method of claim 25, wherein one or more diameters of the plurality of air voids and / or particles are selected based on effectiveness of transmitting longwave infrared wavelengths, and either solar scattering or solar transparency.

30. The heat transfer method of claim 25, wherein the transmissive portion of the roof structure is an opaque white portion of the roof structure.

31. The heat transfer method of claim 25, wherein the transmissive portion of the roof structure is a visibly transparent portion of the roof structure.

32. The heat transfer method of claim 9, wherein the transmissive portion further comprises a microtextured anti-reflective layer disposed over the plurality of layers, such that the plurality of layers is between the microtextured anti-reflective layer and the covered or indoor volume of space.

33. The heat transfer method of claim 9, wherein the transmissive portion further comprises a solar reflective, thermal infrared transparent layer, comprising polyethylene and air voids, zinc oxide, and / or zinc selenide.

34. The heat transfer method of claim 9, wherein the one or more spectrally selective transmissive substrates is a flexible film.

35. The heat transfer method of claim 9, wherein the transmissive portion further comprises a transparent supporting substrate.Princeton - 94476 36. A heat emitting method, comprising: emitting heat from a surface through one or more spectrally selective transmissive substrates thermally coupled to the surface, each spectrally selective transmissive substrate being at least 50% transparent on average across a long wavelength infrared wavelength range, and greater than 30% reflective on average across infrared wavelengths outside the long wavelength infrared wavelength range.

37. The heat emitting method of claim 36, wherein each spectrally selective transmissive substrate being at least 50% transparent on average across the long wavelength infrared wavelength range, and greater than 50% reflective on average across infrared wavelengths in a second range, the second range including 0.3 µm to 8 µm and 13 µm to 30 µm.

38. The heat emitting method of claim 36, wherein the surface is an interior or exterior wall.

39. The heat emitting method of claim 36, wherein one or more thermally conductive substrates are disposed between the surface and the one or more spectrally selective transmissive substrates.

40. The heat emitting method of claim 36, wherein each spectrally selective transmissive substrate comprises a plurality of layers disposed atop each other, each layer being different from an adjacent layer.

41. The heat emitting method of claim 40, wherein the plurality of layers includes 2-50 layers.

42. The heat emitting method of claim 41, wherein the plurality of layers includes 10-30 layers.

43. The heat emitting method of claim 41, wherein each layer has a thickness selected based on its contribution to ensuring the one or more spectrally selective transmissive substrates is at least 50% transparent on average across the long wavelength infraredPrinceton - 94476 wavelength range, and greater than 30% reflectivity on average across infrared wavelengths outside the long wavelength infrared wavelength range.

44. The heat emitting method of claim 41, wherein each layer has a thickness of 2.5 µm to 4.5 µm.

45. The heat emitting method of claim 41, wherein the plurality of layers includes a first set of alternating layers, the first set of alternating layers including a first layer and a second layer disposed in alternate layers, where the first layer and the second layer have different compositions.

46. The heat emitting method of claim 45, wherein each first layer is identical, and each second layer is identical.

47. The heat emitting method of claim 46, wherein at least one first layer has a different thickness from a different first layer, and / or at least one second layer has a different thickness from a different second layer.

48. The heat emitting method of claim 46, wherein the plurality of layers includes at least one additional set of alternating layers, disposed below the first set of alternating layers, such that the at least one additional set of alternating layers is between the first set of alternating layers and the surface.

49. The heat emitting method of claim 41, where each layer comprises a polymer.

50. The heat emitting method of claim 49, wherein the polymer is a polyolefin, a nylon, or a polytetrafluoroethylene (PTFE).

51. The heat emitting method of claim 49, wherein each layer includes a plurality of air voids and / or particles, the particles include zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), copper oxide (CuO), Iron (II) Oxide (FeO), silicon, or a combination thereof.Princeton - 94476 52. The heat emitting method of claim 51, wherein a median diameter of the plurality of air voids and / or particles is 1000 nm – 2500 nm.

53. The heat emitting method of claim 52, wherein a median diameter of the plurality of air voids and / or particles is 100 nm – 1000 nm.

54. The heat emitting method of claim 52, wherein a median diameter of the plurality of air voids and / or particles is less than 100 nm.

55. The heat emitting method of claim 41, wherein the one or more spectrally selective transmissive substrates further comprises a microtextured anti-reflective layer disposed over the plurality of layers, such that the plurality of layers is between the microtextured anti- reflective layer and the surface.

56. A spectrally selective transmissive substrate, comprising: a plurality of layers disposed atop each other, each layer being different from an adjacent layer, where the spectrally selective transmissive substrate is at least 50% transparent on average across a long wavelength infrared wavelength range, and greater than 30% reflective on average across infrared wavelengths outside the long wavelength infrared wavelength range.

57. The spectrally selective transmissive substrate of claim 56, wherein the spectrally selective transmissive substrate is at least 50% transparent on average across the long wavelength infrared wavelength range, and greater than 50% reflective on average across infrared wavelengths in a second range, the second range including 0.3 µm to 8 µm and 13 µm to 30 µm.

58. The spectrally selective transmissive substrate of claim 56, wherein the plurality of layers includes 2-50 layers.

59. The spectrally selective transmissive substrate of claim 58, wherein the plurality of layers includes 10-30 layers.Princeton - 94476 60. The spectrally selective transmissive substrate of claim 56, wherein each layer has a thickness selected based on its contribution to ensuring the spectrally selective transmissive substrate is at least 50% transparent on average across the long wavelength infrared wavelength range, and greater than 30% reflectivity on average across infrared wavelengths outside the long wavelength infrared wavelength range.

61. The spectrally selective transmissive substrate of claim 56, wherein each layer has a thickness of 2.5 µm to 4.5 µm.

62. The spectrally selective transmissive substrate of claim 56, wherein the plurality of layers includes a first set of alternating layers, the first set of alternating layers including a first layer and a second layer disposed in alternate layers, where the first layer and the second layer have different compositions.

63. The spectrally selective transmissive substrate of claim 62, wherein each first layer is identical, and each second layer is identical.

64. The spectrally selective transmissive substrate of claim 63, wherein at least one first layer has a different thickness from a different first layer, and / or at least one second layer has a different thickness from a different second layer.

65. The spectrally selective transmissive substrate of claim 63, wherein the plurality of layers includes at least one additional set of alternating layers, disposed below the first set of alternating layers.

66. The spectrally selective transmissive substrate of claim 56, where each layer comprises a polymer.

67. The spectrally selective transmissive substrate of claim 66, wherein the polymer is a polyolefin, a nylon, or a polytetrafluoroethylene (PTFE).

68. The spectrally selective transmissive substrate of claim 66, wherein each layer includes a plurality of air voids and / or particles, the particles include zinc oxide (ZnO), zincPrinceton - 94476 sulfide (ZnS), zinc selenide (ZnSe), copper oxide (CuO), Iron (II) Oxide (FeO), silicon, or a combination thereof.

69. The spectrally selective transmissive substrate of claim 68, wherein a median diameter of the plurality of air voids and / or particles is 1000 nm – 2500 nm.

70. The spectrally selective transmissive substrate of claim 68, wherein a median diameter of the plurality of air voids and / or particles is 100 nm – 1000 nm.

71. The spectrally selective transmissive substrate of claim 68, wherein a median diameter of the plurality of air voids and / or particles is less than 100 nm.

72. The spectrally selective transmissive substrate of claim 56, further comprising a microtextured anti-reflective layer disposed over the plurality of layers.

73. The spectrally selective transmissive substrate of claim 56, wherein the spectrally selective transmissive substrate is a flexible film.

74. The spectrally selective transmissive substrate of claim 56, further comprising a transparent supporting substrate.

75. A system, comprising: a vertical structure operably coupled to a ground surface; a roof or covering operably coupled to the vertical structure; and one or more spectrally selective transmissive substrates operably coupled to at least a portion of the vertical structure, or forming at least a portion of the roof or covering, each spectrally selective transmissive substrate comprising: a plurality of layers disposed atop each other, each layer being different from an adjacent layer, where each spectrally selective transmissive substrate is at least 50% transparent on average across a long wavelength infrared wavelength range, and greater than 30% reflective on average across infrared wavelengths outside the long wavelength infrared wavelength range.

76. A method of manufacturing a spectrally selective transmissive substrate, comprising:Princeton - 94476 providing a plurality of resin streams, each resin stream independently comprising a polymer, wherein the polymer is a polyolefin, a nylon, or a polytetrafluoroethylene (PTFE), and wherein each polymer is optionally configured to include a plurality of air voids with lower refractive index than polyethylene, and / or particles with higher refractive index than polyethylene, the particles include zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), copper oxide (CuO), Iron (II) Oxide (FeO), silicon, or a combination thereof; forming stacked layers by. coextruding the plurality of resin streams as a plurality of alternating layers; and casting the stacked layers onto a casting roll.

77. A method of manufacturing a spectrally selective transmissive substrate, comprising: forming a plurality of thin layers by: providing a solution comprising a solvent and a polymer, wherein the polymer is a polyolefin, a nylon, or a polytetrafluoroethylene (PTFE), and wherein each polymeris optionally configured to include a plurality of air voids with lower refractive index than polyethylene, and / or particles with higher refractive index than polyethylene, the particles include zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), copper oxide (CuO), Iron (II) Oxide (FeO), silicon, or a combination thereof; forming a thin layer using the solution; and hot pressing the thin layer at or near a melting temperature of the polymer.

78. The method of claim 77, wherein forming the thin layer comprises casting the solution as a layer, using a doctor blade, film applicator, and / or an applicator rod.

79. The method of claim 77, wherein forming the thin layer comprises dip coating the solution onto a substrate, and drying the thin layer.

80. The method of claim 79, further comprising peeling the thin layer from the substrate after drying.

81. The method of claim 77, wherein forming the thin layer comprises mechanical rolling or melt rolling of the solution.Princeton - 94476 82. The method of claim 77, further comprising combining a plurality of thin layers to form an alternative set of stacked layers.

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