CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The application claims priority to U.S. Provisional Patent Application No. 62/941,593 filed on November 27, 2019, the disclosure of which are included herein by reference in their entirety.

FIELD OF THE INVENTION

[0002] The present invention generally relates to methods for tuning radiative heat flows between interior surfaces and human occupants; and more particularly to improved heating and cooling efficiency by incorporating tunable emissivity materials.

BACKGROUND OF THE INVENTION

[0003] Energy consumption in residential and commercial buildings contributes to 30% of total greenhouse gas emissions worldwide. In the United States, the buildings sector accounts for 41% of primary energy consumption, of which heating and cooling alone is responsible for over 35%. Heating can pose a profound challenge for broader decarbonisation goals in temperate and cool climates. With energy consumption for heating and cooling expected to grow dramatically worldwide, improving the efficiency of these systems may be an important part of mitigating climate change.

BRIEF SUMMARY OF THE INVENTION

[0004] Methods and systems for tuning radiative heat flows between interior surfaces and human occupants are illustrated.

[0005] One embodiment of the invention includes a surface for controlling heat flow between an interior space and at least one human occupant comprising at least one tunable emissivity material, where the tunable emissivity material has a low emissivity state wherein the emissivity is smaller than about 0.5 over wavelength range from about 7 microns to about 14 microns, and the material minimally emits heat and reflects heat from the interior surface and the human occupant; where the tunable emissivity material has a high emissivity state wherein the emissivity is larger than about 0.8 over wavelength range from about 7 microns to about 14 microns, and the material absorbs heat from the interior surface and the human occupant; and where the tunable emissivity ranges from 0 to 1 in long-wave infrared wavelength.

[0006] In a further embodiment, the surface is a wall, a floor, or a ceiling.

[0007] In another embodiment, the tunable emissivity material is a conducting polymer, an inorganic oxide, or a thermochromic material.

[0008] In a still further embodiment, the conducting polymer is poly(3,4- ethylenedioxythiophene), poly(3,4-ethylenedioxythiophene) tosylate, poly(3,4- ethylenedioxythiophene) polystyrene sulfonate, or polyaniline.

[0009] In still another embodiment, the surface further comprising a solid electrolyte layer between the conducting polymer and an electrode.

[0010] In yet another embodiment, the inorganic oxide is tungsten trioxide, hydrogen tungsten bronzes or vanadium dioxide.

[0011] In a yet further embodiment, the inorganic oxide is ion-irradiated vanadium dioxide.

[0012] In another embodiment, the tunable emissivity material comprises lithium titanate.

[0013] In a further embodiment, tuning the surface to the low emissivity state decreases a set point temperature of the interior space in cold weather.

[0014] In yet another embodiment, the decrease in the set point temperature of the interior space in cold weather results in energy saving.

[0015] In another additional embodiment, tuning the surface to the high emissivity state increases a set point temperature of the interior space in warm weather.

[0016] In a yet further embodiment, the increase in the set point temperature of the interior space in warm weather results in energy saving.

[0017] In yet another embodiment, the tunable emissivity material has at least about 70% transmittance over wavelength range from about 400 nm to about 900 nm. [0018] In a further embodiment, the surface is capable of exhibiting at least about 70% transmittance over wavelength range from about 400 nm to about 900 nm in the low emissivity state, and less than about 50% transmittance over wavelength range from about 400 nm to about 900 nm in the high emissivity state.

[0019] In yet another embodiment, the surface is capable of reflecting near-infrared wavelengths of the solar spectrum between about 900 nm to about 3000 nm.

[0020] In a still further embodiment, the surface is a window.

[0021] In another embodiment, the surface further comprising at least one power source; and at least two electrodes, wherein the tunable emissivity material is placed in between the at least two electrodes.

[0022] Still another additional embodiment includes a surface for controlling heat flow between an interior space and at least one human occupant comprising at least two sides, where at least a first side has a low emissivity in the long-wave infrared wavelength, and at least a second side has a high emissivity in the long-wave infrared wavelength, and the at least two sides are rotatable mechanically.

[0023] In another embodiment, rotating to the low emissivity side decreases a set point temperature of the interior space in cold weather.

[0024] In still another embodiment, the decrease in the set point temperature of the interior space in cold weather results in energy saving.

[0025] In a yet further embodiment, rotating to the high emissivity side increases a set point temperature of the interior space in warm weather.

[0026] In a still further embodiment, the increase in the set point temperature of the interior space in warm weather results in energy saving.

[0027] In yet another embodiment, the at least two sides are transparent over visible wavelength.

[0028] In a further embodiment, the surface is capable of exhibiting at least about 70% transmittance over wavelength range from about 400 nm to about 900 nm in the low emissivity, and less than about 50% transmittance over wavelength range from about 400 nm to about 900 nm in the high emissivity. [0029] In yet another embodiment, the surface is capable of reflecting near-infrared wavelengths of the solar spectrum between about 900 nm to about 3000 nm.

[0030] In another additional embodiment, the surface is a window.

[0031] Another further embodiment again includes a method for tuning radiative heat flow between at least one interior surface and at least one human occupant comprising providing at least one interior surface comprising at least one tunable emissivity material; and tuning the emissivity of the interior surface to regulate the radiative heat flow depending on surrounding temperature; where the tunable emissivity material has a low emissivity state wherein the material reflects heat from the interior surface and the human occupant, and the tunable emissivity material has a high emissivity state where the material absorbs heat from the interior surface and the human occupant; and the tunable emissivity of the material ranges from 0 to 1 in long-wave infrared wavelength.

[0032] In an additional embodiment, the at least one interior surface is a wall, a floor, or a ceiling.

[0033] In a still further embodiment, the tunable emissivity material is a conducting polymer, an inorganic oxide, or a thermochromic material.

[0034] In yet another embodiment, the conducting polymer is poly(3,4- ethylenedioxythiophene), poly(3,4-ethylenedioxythiophene) tosylate, poly(3,4- ethylenedioxythiophene) polystyrene sulfonate, or polyaniline.

[0035] In another embodiment, the inorganic oxide is tungsten trioxide, hydrogen tungsten bronzes or vanadium dioxide.

[0036] In still yet another embodiment, the inorganic oxide is ion-irradiated vanadium dioxide.

[0037] In a further embodiment again, the tunable emissivity material comprises lithium titanate.

[0038] In still another embodiment, tuning the surface to the low emissivity state decreases a set point temperature of the interior space in cold weather.

[0039] In a further additional embodiment, the decrease in the set point temperature of the interior space in cold weather results in energy saving. [0040] In a still further embodiment, tuning the surface to the high emissivity state increases a set point temperature of the interior space in warm weather.

[0041] In yet another embodiment, the increase in the set point temperature of the interior space in warm weather results in energy saving.

[0042] Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosure. A further understanding of the nature and advantages of the present disclosure may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The description will be more fully understood with reference to the following figures, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:

[0044] Figs. 1a and 1b conceptually illustrate the tunable emissivity surfaces and human occupants, where Fig. 1a conceptually illustrates low-emissivity, high-reflectivity interiors reflect radiative heat back to the body in cold weather conditions, and Fig. 1b conceptually illustrates high-emissivity interior surfaces absorb the radiative heat released from the occupant inside during warm-weather conditions in accordance with embodiments.

[0045] Fig. 2 illustrates a 3D computational model simulating a single occupant standing in a conditioned space, to assess the impact of tuning the thermal emissivity of the spaces that surround the human occupant in accordance with embodiments.

[0046] Fig. 3a illustrates under the cold weather and single-occupant condition, radiative heat loss of the occupant decreases with the emissivity of their surroundings in accordance with embodiments.

[0047] Fig. 3b illustrates under the cold weather and single-occupant condition, the set point temperature for heating decreases by 7°C as the emissivity of the interior surfaces surrounding the human occupant go from 0.9 to 0.1 in accordance with embodiments.

[0048] Fig. 4a illustrates under warm weather and single-occupant condition, radiative heat loss of the occupant increases with the emissivity of their surroundings in accordance with embodiments.

[0049] Fig. 4b illustrates under the warm weather and single-occupant condition, the set point temperature for cooling increases about 4°C as the emissivity of the interior surfaces surrounding the human occupant increases from 0.1 to 0.9. in accordance with embodiments.

[0050] Fig. 5 illustrates a 3D computational model simulating multiple occupants standing in a conditioned space, to assess the impact of tuning the thermal emissivity of the spaces that surround the human occupant in accordance with embodiments.

[0051] Fig. 6a illustrates the radiative heat loss as a function of emissivity in both cold weather and warm weather conditions in accordance with embodiments.

[0052] Figs. 6b illustrate the heating set point decreases from about 22°C for an emissivity of about 0.9 to about 12°C for an emissivity of about 0.1 , while the cooling set point increases from about 16.5°C to about 20.5°C as the emissivity increases from about 0.1 to about 0.9 in accordance with embodiments.

[0053] Fig. 7 illustrates the air temperatures in August and December in Ancona, Italy over a 24-hour period in accordance with the prior art.

[0054] Fig. 8 illustrates energy savings as a function of interior surface emissivity for energy use variation for heating and cooling in a building in winter and summer in accordance with embodiments.

[0055] Fig. 9 illustrates tunable emissivity devices incorporating conducting polymers in accordance with embodiments.

[0056] Fig. 10a and 10b illustrate emissivity of PEDOT and PANI layers in doped and undoped states in accordance with embodiments.

[0057] Fig. 11 illustrates transfer matrix electromagnetic simulation of 1 D photonic design of alternating thin layers of ZnS and LTO to enhance emissivity contrast in accordance with embodiments. [0058] Fig. 12 illustrates switchable emissivity blinds by coating the two sides with different emissivity paintings in accordance with embodiments.

DETAILED DESCRIPTION OF THE INVENTION

[0059] Turning now to the drawings, methods and systems for tuning radiative heat flows between interior surfaces and human occupants are described. Many embodiments implement tunability of interior surfaces for maximal year-round energy efficiency. In many embodiments, the tuning can be accomplished by dynamically tuning the thermal emissivity of interior building surfaces at long-wave infrared (LWIR) wavelengths to maintain thermal comfort. A number of embodiments implement materials that have tunable emissivity in the LWIR spectrum. Several embodiments provide systems to mechanically tune their LWIR emissivity. Some embodiments implement materials with electrochromic properties to tune emissivity over LWIR wavelengths. Examples of tunable emissivity materials include (but are not limited to): conducting polymers, inorganic oxides, and lithium titanate. Examples of conducting polymers with tunable emissivity in the LWIR spectrum include (but are not limited to): poly (3,4-ethylenedioxythiophene) tosylate (PEDOT), poly (3,4-ethylenedioxythiophene) tosylate (PEDOT:Tos), poly (3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) and polyaniline. In some embodiments, the doping concentration of conducting polymers can affect emissivity in LWIR wavelengths. Several embodiments show that doped conducting polymer can have a relatively low emissivity in LWIR ranges while undoped conducting polymer may have a relatively high emissivity. Examples of inorganic oxides with tunable emissivity in the LWIR spectrum include (but are not limited to): tungsten trioxide (WO3), hydrogen tungsten bronzes (HxWCte), and vanadium dioxide. Some embodiments implement ion- irradiated vanadium dioxide for tuning emissivity in interior spaces. In many embodiments, emissivity of the tunable materials in LWIR wavelengths can be tuned electrically including (but not limited to) by applying a voltage. Applied voltage may change the doping concentrations of conducting polymer and hence tune the emissivity in accordance with some embodiments. In several embodiments, tunable emissivity materials can be incorporated in mechanical devices. Emissivity in LWIR ranges can be tuned mechanically in accordance with certain embodiments. In a number of embodiments, low emissivity and high emissivity materials can be applied to different sides of mechanical blinds, and different sides can be switched in response to the weather.

[0060] Several embodiments implement that in cold weather conditions, tuning the emissivity of interior surfaces including (but not limited to) walls, floors, windows, and ceilings to a low value can decrease set point temperatures of heating systems. In many embodiments, tunable emissivity materials in LWIR wavelength are transparent over visible wavelengths such that they can be applied to transparent substrates including (but not limited to) windows. In some embodiments, tuning the emissivity of interior surfaces to a relatively low emissivity at about 0.1 can decrease the set point temperature up to about 7°C. The decrease of the set point temperature can correspond to energy saving in accordance to various embodiments. In certain embodiments, low emissivity interior surfaces can achieve an energy saving of approximately 67.7% relative to high emissivity materials, which may have an emissivity of about 0.9. Many embodiments exhibit that in warm weather, tuning interior surfaces including (but not limited to) walls, floors, and ceilings to a high emissivity can result in energy savings relative to low emissivity surfaces of cooling systems. In several embodiments, high emissivity interior surfaces can have achieve about 38.5% energy savings relative to low emissivity surfaces.

[0061] Many embodiments provide tunable emissivity materials can maintain high visible and solar transmittance to enable their use on the interior and/or exterior surfaces of transparent substrate including (but not limited to) windows. In several embodiments, tunable emissivity materials can maintain high visible and solar transmittance between about 400 nm and about 900 nm wavelengths. In such embodiments, tunable emissivity materials would allow windows to function as they normally do, but reflect and/or absorb heat from the occupants and interior objects emitted as thermal radiation at long-wave infrared wavelengths. In certain embodiments, tunable emissivity materials may allow for simultaneously tunability of both visible transparency as well as infrared emittance. For instance, in cold weather settings tunable emissivity materials would have low emissivity at infrared wavelengths to reflect interior heat back to the occupant and high transmittance over visible wavelengths in accordance to some embodiments. In hot weather settings tunable emissivity materials would have lower transmittance over visible wavelengths but high emissivity at infrared wavelengths to absorb heat from occupants. [0062] Many embodiments reveal energy savings potential by better controlling the flows of heat that surround human occupants in the form of thermal radiation. Several embodiments demonstrate incorporating materials choices and/or structural parameters that enable emissivity tuning in the LWIR part of the electromagnetic spectrum.

[0063] With energy consumption for heating and cooling expected to grow dramatically worldwide, improving the efficiency of these systems may be an important part of mitigating climate change. One goal of heating and cooling in buildings with human occupants is to maintain their thermal comfort (See, e.g. Fanger, P.O., Copenhagen: Danish Technical Press., 1970, the disclosure of which is incorporated herein by reference). Thermal comfort is both a quantitative and qualitative judgment that connects an individual’s physiological and emotional perceptions of being in a thermally comfortable state (See, e.g. Fanger, P.O., Copenhagen: Danish Technical Press., 1970; and Flansen, J., Build Env., 1990, 24, 309-316, the disclosures of which are incorporated herein by reference).

[0064] While thermal comfort can be linked to the air temperature set point in a conditioned space, a human occupant’s perception of comfort is subject to a range of other factors. These can include light intensity, material properties, metabolic heat production, heat transfer coefficients and radiative heat losses to external surfaces (See, e.g. Moon, J. H., et al., Int. J. Therm. Sci., 2016, 107, 77-88; and Cheng, Y., et al., Energy Build, 2013, 64, 154-161 , the disclosures of which are incorporated herein by reference). Fluman skin temperature is about 33°C in comfortable conditions, while the average heat generation rate of a standing adult is about 70 W/m ² (See, e.g. American Society of Heating, 2013 Ashrae Handbook: Fundamentals, 2013; and Okamoto, T., et al., Sci. Rep., 2017, 7, 11519, the disclosure of which are incorporated herein by reference). Previous work has examined how temperature, air velocity and humidity may affect thermal comfort (See, e.g. Coutts, A. M., et al., Theor. Appl. Climatol., 2016, 124, 55-68; Yang, L, et al., Appl. Energy, 2014, 115, 164-173; and Rupp, R.F., Energy Build., 2015, 105, 178-205, the disclosures of which are incorporated herein by reference). Given the complex array of factors that influence the perception of comfort, and the need for reducing energy use for heating and cooling, it is to be noted that an increase in the set point temperature for cooling, or a decrease in the set point for heating, by about 4°C can reduce energy use by up to 45% and 35% respectively (See, e.g. Hoyt, T, et al. , In International Conference on Environmental Ergonomics, 2009, 608-612, the disclosure of which is incorporated herein by reference).

[0065] In an indoor environment, where most people stay in a sedentary state, more than 50% of the heat generated by the human body is released through thermal radiation in the LWIR part of the spectrum (See, e.g. Cai, L, et al., Nat. Comm., 2017, 8, 496; and Winslow, C.-E., et al., Am. J. Physiol., 1939, 127, 505-518, the disclosures of which are incorporated herein by reference). The effect of radiative heat transfer on thermal comfort has been explored (See, e.g. Marino, C., et al., Sol. Energy, 2017, 144, 295-309; and Arslanoglu, N., et al., Energy Build., 2016, 113, 23-29, the disclosures of which are incorporated herein by reference) but remains a comparatively untapped mechanism for efficiency gains. One approach can be tuning the radiative properties of clothing through optical approaches, making the clothing fabric more or less transparent to thermal radiation emitted from the human wearer, depending on weather conditions (See, e.g. Qiu, Q., et al., Nano Energy, 2019, 58, 750-758; Cai, L., et al., Joule, 2019; Zhou, H., et al., Ind. Eng. Chem. Res., 2019; Yue, X., et al., J. Colloid Interface Sci., 2019, 535, 363- 370; Hsu, P.C., et al., Science, 2016, 353, 1019-1023; Guo, Y., et al., ACS Appl. Mater. Interfaces, 2016, 8, 4676-4683; Hsu, P.-C., et al., Nano Lett., 2015, 15, 365-371 ; and Tong, J. K., et al., ACS Photonics, 2015, 2, 769-778, the disclosures of which are incorporated herein by reference). While conceptually attractive, this approach poses practical challenges, as it requires the human occupants of a conditioned space to wear specialized clothing depending on weather conditions. On the other hand, materials whose emissivity can be tuned in the LWIR part of the spectrum relevant to room temperature blackbody radiation, including by electrochromic control have been reported (See, e.g. Mulford, R. B., et al., J. Heat Transfer, 2019, 141 , 32702; and Zhang, X., et al., Sol. Energy Mater. Sol. Cells, 2019, 200, 109916, the disclosures of which are incorporated herein by reference).

[0066] Many embodiments implement methods to make the environment surrounding the human occupants responsive to their radiative heat flows, and enable improved heating and cooling efficiency. In cold weather conditions, lower radiative heat loss from human occupants may be desirable, as the air temperature could then be maintained at a lower temperature for the same level of thermal comfort. In these conditions it would be preferable to have low emissivity (high reflectivity) materials in the interior surfaces including (but not limited to) floor, ceiling, windows and walls, surrounding an occupant. Low emissivity materials may have emissivity between about 0.1 to about 0.5 in LWIR wavelength in accordance with some embodiments. By contrast, in summer or warm weather conditions, the heat generated by a human should be dissipated to interior surfaces, as these surfaces are typically colder than skin temperature. Thus, high emissivity (and low reflectivity) materials of the surroundings would be desirable. High emissivity materials may have emissivity between about 0.8 to about 1 in LWIR wavelengths in accordance with some embodiments. In winter, having low emissivity interior surfaces could reduce the heat loss of a radiator in a room with no human occupant (See, e.g. Robinson, A. J., et al. , Energy Build, 2016, 127, 370-381 , the disclosure of which is incorporated herein by reference). However, one main purpose of space heating and cooling is to ensure the thermal comfort of the human occupants inside. Previous studies lack the understanding of how much energy use can be reduced by controlling the thermal emissivity of interior surfaces, while maintaining the same thermal comfort level for human occupants of a conditioned space. Many embodiments show that the desired radiative properties can change from low emissivity to high emissivity depending on weather conditions and the heat load, enabling maximal heating and cooling efficiency year-round.

[0067] Many embodiments implement tunable emissivity surfaces for interior spaces in built environment. To evaluate the possible set point change and thus energy savings, several embodiments implement computational fluid dynamics (CFD) simulations of an office environment with a human occupant. In some embodiments, the emissivity of the inner walls can be tuned from that of a near blackbody to a very low value. Several embodiments exhibit that in cold weather conditions a decrease in the set point of up to about 7°C relative to conventional materials if a low emissivity surface is used. Certain embodiments show that a decrease of up to about 10°C in the set point when multiple occupants are in the conditioned space. Conversely, in warm weather conditions, a number of embodiments show that an increase in the set point of up to about 4°C can be achieved when the emissivity of the interior surfaces increases to a high value.

[0068] Many embodiments analyze the building scale to evaluate the impact of heating and cooling energy use on a typical summer and winter day in a temperate climate. Building scale analysis can be done using EnergyPlus™ (See, e.g. Kant, K., et al. , Sustain. Through Energy-Efficient Build., 2018, 209-223; Yu, Y., et al., Building Simulation, 2019, 12, 347-363; and Shen, P., et al., Energy, 2019, 173, 75-91 , the disclosures of which are incorporated herein by reference). In some embodiments, a low- emissivity interior surface can result in up to about 67.7% energy savings relative to conventional materials, when heating is needed. Several embodiments show that in warm weather conditions however low emissivity interior surfaces are no longer appropriate and would result in about 38.5% energy penalty relative to high emissivity interior surfaces. Thus, tuning the interior surface’s thermal emissivity can enable maximal energy efficiency throughout the year, and in response to varying heat loads and conditions in accordance with a number of embodiments.

Tunable Emissivity Interior Surfaces

[0069] Many embodiments implement reducing radiative heat loss in interior spaces to keep occupants comfortable at lower air temperatures. In cold weather conditions, the human body can lose a significant amount of heat through both convective and radiative heat transfer to its surroundings. Since more than half this heat loss is from thermal radiation (See, e.g. Marino, C., et al., Sol. Energy, 2017, 144, 295-309, the disclosure of which is incorporated herein by reference), embodiments according to the instant disclosure show reducing radiative heat loss in interior spaces can be an effective way to keep occupants comfortable at lower air temperatures. Most building materials including (but not limited to) paints have high emissivity (and absorptivity) in the LWIR wavelength. Thus interior surfaces can absorb the thermal radiation from the human occupant and emit back a smaller amount of thermal radiation corresponding to the lower temperature of the walls, ceilings and floors. An example of having low emissivity, high reflectivity, interior surfaces in cold-weather conditions in accordance with an embodiment of the invention is schematically illustrated in FIG. 1A. The interior surfaces can send back a large fraction of the radiative heat lost by the human occupant back to them, allowing lower air temperature setting, thereby reducing the need for heating energy while maintaining the same level of thermal comfort. Many embodiments assess the level of thermal comfort by the maintenance of constant skin temperature and fixed amount of body heat released by the human occupant.

[0070] By contrast, in summer and warm-weather conditions more generally, it is desirable to maximize the net heat rejected by the human occupant to their environment. An example of having high emissivity, high absorptivity, interior surfaces in warm-weather conditions in accordance with an embodiment of the invention is schematically illustrated in FIG. 1 B. High emissivity (high absorptivity) materials in the interior of the building can be used to absorb the heat radiated by the occupant. By doing so one can maintain the same level of thermal comfort while using less cooling energy than it would be needed if the walls were low emissivity or high reflectivity. In many embodiments, tunability in the emissivity of the surfaces surrounding human occupants can optimize heat losses and/or heat savings depending on weather conditions and overall heat loads.

Computer Fluid Dynamics Simulations

[0071] Many embodiments analyze the impact of emissivity on the interior air temperature set point. An example of a 3D computational model to simulate a standing person in a proto-typical conditioned space in accordance with an embodiment of the invention is illustrated in FIG. 2. The room, with dimensions of about 3 m x 3 m x 3 m, has an air inlet (201 ) where heating or cooling air is supplied, and multiple outlets (202) to ensure adequate distribution and flow of air in the space. In winter, the heating air comes through the inlet, heats the room, and then comes out the room through the outlets. In summer, the cooling air comes from the inlet, cools the room, and then exits through outlets. The human occupant (203) can be modeled as a volume heat source of about 103 W (See, e.g. Marino, C., et al. , Sol. Energy, 2017, 144, 295-309, the disclosure of which is incorporated herein by reference). The occupant is set to have 1.0 clo and 0.6 clo clothing insulation in the cold weather and warm weather case respectively (See, e.g. Takada, S., et al., Build. Environ., 2016, 99, 210-220; and Ogulata, R. T., et al., Fibres Text. East. Eur, 2007, 15, 61 , the disclosures of which are incorporated herein by reference). The wall temperature is set as a constant value during the simulation to capture its large thermal mass and typical observed behavior in commercial buildings. [0072] To assess the change in set point temperature as a function of emissivity, the air temperature can be adjusted to maintain a skin temperature of about 33°C in response to the change in the emissivity of the interior surfaces. The possible change in air temperature set point can be determined while maintaining the human occupant’s thermal comfort. For a closed system with two gray and diffuse surfaces, the radiative heat flux of the two surfaces can be analytically described as:

In the equation (1 ), Q ₁₂ is the heat flux, A is the area of the hot surface and A ₂ is the sum of the three cold surfaces area, E _bl and E _b2 are the black body emission at the temperature of hot surface and cold surface respectively, e _c and e ₂ are the emissivity of the hot surface and the cold surface, X is the view factor. When surface 1 is a plane or convex equation (1 ) can be simplified as: equation (2) can be used as a simplified way to calculate the radiative heat transfer between the occupant and their surroundings. Equation (2) shows that the decrease of the emissivity of the cold surface can result in the decreased heat flux as well. Low Emissivitv Surfaces Under Cold Weather Conditions

[0073] In the winter, or cold-weather conditions more generally, the temperature of the walls in the interior space can be set to 13°C (See, e.g. ANSI/ASHRAE, Ashrae, 2013, 58, the disclosure of which is incorporated herein by reference). The heater is turned on and heating air is delivered through the inlet to the room. At the same time, the occupant inside is exchanging heat both through convection to the air and radiation to the surrounding interior surfaces. While the temperature of the occupant and the surrounding interior surfaces are difficult to change, the emissivity of the walls can be adjusted to reduce the radiative loss in accordance to several embodiments.

[0074] In many embodiments, the CFD models can determine the radiative heat exchange between the occupant and the surrounding surfaces as a function of emissivity. An example of radiative heat loss in response to interior surfaces of different emissivity in accordance with an embodiment of the invention is illustrated in FIG. 3A. The radiative heat loss from the occupant can be reduced from about 99.1 W to about 72.3 W when the emissivity is decreased from about 0.9 to about 0.1 in accordance with some embodiments. The heat loss response to the emissivity change matches well with the analytical model of equation (2). When the interior surfaces have a lower e ₂ , the radiative heat loss can be decreased.

[0075] In several embodiments, since the occupant’s heat loss is reduced, the set point temperature can be decreased to save heating energy. An example of set point temperature in response to interior surfaces of different emissivity in accordance with an embodiment of the invention is illustrated in FIG. 3B. The set point temperature decreases from about 25°C to about 18°C, when the emissivity is tuned from 0.9 to 0.1 in accordance with certain embodiments. Several embodiments exhibit that a common trend shown in FIG. 3A and FIG. 3B is that the rate of change of both the radiative heat loss (FIG. 3A) and set point temperature reduction (FIG. 3B) can be accelerated when the emissivity approaches 0. Equation (2) shows that as e ₂ approaches zero, the impact of the denominator increases sharply. Hiah Emissivitv Surfaces Under Warm Weather Conditions

[0076] In warm weather conditions, the temperature of the walls can be set to 20°C (See, e.g. Joudi, A., Appl. Energy, 2011 , 88, 4655-4666, the disclosure of which is incorporated herein by reference), which is lower than human skin temperature. The air conditioner is turned on and cooling air is delivered through inlet to the room. An example of radiative heat flux between the occupant and the surrounding walls as a function of emissivity in accordance with an embodiment of the invention is illustrated in FIG. 4A. The heat released radiatively from the occupant increases significantly when the emissivity of the walls goes to near that of a blackbody (emissivity about 0.9). The radiative heat loss increases from about 57.7 W for an emissivity of about 0.1 to about 77.2 W when the emissivity is about 0.9 in accordance with some embodiments. This trend corresponds well with equation (2).

[0077] In several embodiments, since the occupant can release more heat through radiation at a higher interior surface emissivity, the cooling temperature set point can thus be increased to save energy. An example of set point temperature and the surrounding walls as a function of emissivity in accordance with an embodiment of the invention is illustrated in FIG. 4B. The set point temperature can rise up to about 4°C, from 18°C to 22°C when the emissivity increases from about 0.1 to about 0.9.

Interior Space with Multiple Occupants

[0078] Many embodiments investigate the presence of more than one occupant in an interior space. An example of more than one standing person in a proto-typical conditioned space in accordance with an embodiment of the invention is illustrated in FIG. 5. The room has an air inlet (501 ) where heating or cooling air is supplied, and multiple outlets (502) to ensure adequate distribution and flow of air in the space. In winter, the heating air comes through the inlet, heats the room, and then comes out the room through the outlets. In summer, the cooling air comes from the inlet, cools the room, and then exits through outlets. Multiple human occupants (503) are placed inside the room. The multiple occupants scenario is implemented numerically in both cold weather and warm weather conditions. When there is more than one occupant in the same room, the heat source can be effectively multiplied and there is not only heat transfer between occupant and surroundings, but also between occupants in accordance with many embodiments. In typical cases, such as a classroom or movie theater, the space between different occupants is quite narrow. If the multiple occupants are treated as one and larger heat source, the emissivity effect will be enhanced according to equation (2) because of the area of the hot object increases. Extending this analysis when multiple occupants are in a same room, the low emissivity walls will not only reflect the radiative heat back but also the occupants’ heat to them, which makes the occupants feel warmer than single occupant case. On the other hand, in summer, if the walls reflect most of the radiative heat back, the multiple occupants can result in a significant cooling load for the air conditioner. However, if the walls can absorb most of the heat, the energy used in cooling can be substantially decreased.

[0079] An example of the radiative heat flux resulting from varying emissivity from 0.1 to 0.9 in both winter and summer in accordance with an embodiment of the invention is shown in FIG. 6A. In the cold weather case shown in FIG. 6A, 602 shows the average radiative heat loss of each occupant decreases from about 96.2 W to about 55.5 W when the emissivity decreases from 0.9 to 0.1. There is also an acceleration of change after emissivity goes below than 0.3. In the warm weather case, 601 shows the radiative heat loss of each occupant is increased from about 39.9 W to about 72.1 Wwhen the emissivity increases from 0.1 to 0.9. There is also an acceleration in the heat transfer reduction as emissivity goes below than 0.3.

[0080] An example of the set point temperature change resulting from varying emissivity from 0.1 to 0.9 in both winter and summer in accordance with an embodiment of the invention is shown in FIG. 6B. In the cold weather conditions shown in FIG. 6B, 604 shows a set point temperature decrease of about 10°C when the emissivity decreases from 0.9 to 0.1. In the warm weather conditions, 603 shows a set point temperature decrease of up to about 3.8°C when the emissivity decreases from 0.9 to 0.1 , necessitating substantially more cooling. Several embodiments show that in dense spaces like classrooms, theaters and indoor stadiums, a significant amount of energy can be saved by implementing a tunable emissivity surface on the walls, ceilings and floors. Enerav Savinas with Tunable Emissivitv Interior Surfaces

[0081] Many embodiments assess the building-level energy savings with tunable emissivity interior surfaces. Energy savings assessment can be analyzed using EnergyPlus™, a building energy analysis tool (See, e.g. Fumo, N., Energy and Buildings, 2010, 42, 2331-2337, the disclosure of which is incorporated herein by reference). The CFD simulation results are applied to EnergyPlus™. Several embodiments estimate the energy savings in a hotel-type commercial building if it uses tunable emissivity interior surfaces. A small hotel reference building located in Ancona, Italy is used as the simulated environment. This hotel has a low window/wall ratio (184.2 m ² /1 , 695 m ² ) making it similar overall to the structure simulated in CFD analysis (See, e.g. Commercial Reference Buildings, Office of Energy Efficiency & Renewable Energy, the disclosure of which is incorporated herein by reference). Furthermore, Ancona has both a cold winter and a hot summer, allowing to assess both heating and cooling energy savings. The analysis period is a typical day in August for warm weather conditions, and December for cold weather conditions (See, e.g. Commercial Reference Buildings, Office of Energy Efficiency & Renewable Energy, the disclosure of which is incorporated herein by reference). FIG. 7 shows the average air temperature in the August (701 ) and December (702), corresponding to weather patterns common in a wide range of temperate climates throughout the world.

[0082] Many embodiments use the change in set point temperature associated with a change in the interior surfaces emissivity from 0.9 to 0.1 in winter, to model the change in heating energy use as a function of emissivity. Several embodiments use the change in set point temperature associated with a change in the interior surfaces emissivity from 0.9 to 0.1 in summer, to model the change in cooling energy use as a function of emissivity. An example of energy used for heating in winter and for cooling in summer as a function of emissivity in accordance with an embodiment of the invention is illustrated in FIG. 8. 801 shows the energy used for heating decreases from about 910 kWh to about 300 kWh, when the emissivity decreases from 0.9 to 0.1 . The decrease in energy used in cold weather conditions corresponds to about 67.7% savings in heating energy. Conversely, during the summer day modeled, 802 shows the cooling energy use increases from about 740 kWh to about 1050 kWh when the emissivity changes from 0.9 to 0.1. The energy increase leads to about 38.5% energy penalty if the low emissivity interior surfaces are not tuned back to high emissivity ones. Some embodiments implement emissivity tunability may be necessary within a single day depending on variable weather conditions between day and night, and variable occupancy and heat loads.

EXEMPLARY EMBODIMENTS

The following embodiments provide specific combinations of materials and structures that enable tuning emissivity in the long-wave infrared part of the electromagnetic spectrum. Many embodiments implement materials with electrochromic properties to tune emissivity over LWIR wavelengths. It will be understood that the specific embodiments are provided for exemplary purposes and are not limiting to the overall scope of the disclosure, which must be considered in light of the entire specification, figures and claims.

Example 1 : Conducting Polymers

[0083] Many embodiments implement conducting polymers applied to interior surfaces for tunable emissivity. Examples of conducting polymers with tunable emissivity in the LWIR spectrum include (but are not limited to): poly (3,4-ethylenedioxythiophene) tosylate (PEDOT), poly (3,4-ethylenedioxythiophene) tosylate (PEDOT:Tos), poly (3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) and polyaniline. In several embodiments conducting polymer can be applied to the interior surfaces. In many embodiments, emissivity of the tunable materials in LWIR wavelengths can be tuned electrically including (but not limited to) by applying a voltage. A voltage can be applied to the film and thereby changing its state from a high emittance to low emittance one. Applied voltage may change the doping concentrations of conducting polymer and hence tune the emissivity in accordance with some embodiments. In some embodiments, flexible electrodes made of conducting polymer on flexible substrates could enable roll to roll applications. Certain embodiments implement flexible electrodes comprising (PEDOT: PSS) deposited on a PET and/or plastic substrate. Some embodiments implement polyaniline on the interior surfaces. Polyaniline can exhibit a contrast in emissivity in the long-wave infrared when a voltage is applied. IR electrochromic devices assembled using those polymers could have applications in thermal camouflage and control.

[0084] An example of tunable emissivity devices incorporating conducting polymers in accordance with an embodiment of the invention is illustrated in FIG. 9. 901 in FIG. 9 illustrates a device incorporating poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer. The top layer 902 is a gold film acting as one electrode. The gold film can be about 10 nm thick. The thin gold layers can enable infrared wave transmits through without significant loss in accordance with some embodiments. The second layer 903 is PEDOT, and third layer 904 is a solid electrolyte comprising UCIO4. The fourth layer 905 is ITO coated glass acting as counter electrodes. A voltage source 906 can be used to apply voltage to tune emissivity of PEDOT.

[0085] 911 in FIG. 9 illustrates a device incorporating FICI04-doped polyaniline (PANI) conducting polymer. The top layer 912 is a gold film acting as one electrode. The gold film can be about 10 nm thick. The thin gold layers can enable infrared wave transmits through without significant loss in accordance with some embodiments. The second layer 913 is FICI04-doped polyaniline, and third layer 914 is a solid electrolyte comprising HCIO4 The fourth layer 915 is ITO coated glass acting as counter electrodes. A voltage source 916 can be used to apply voltage to tune emissivity of PANI.

[0086] In some embodiments, the doping concentration of conducting polymers can affect emissivity in LWIR wavelengths. Several embodiments show that doped conducting polymer can have a relatively low emissivity in LWIR ranges while undoped conducting polymer may have a relatively high emissivity. Many embodiments demonstrate that doped and undoped states of electrochromic polymers can yield high and low emissivity in the LWIR wavelengths for controlling radiative heat flows between humans and their surroundings. By fitting the Drude model: the Drude plasma frequency with m * being the effective mass of the free carriers, n is the free carriers’ concentration and e is the carrier charge, emissivity of conducting polymers by transfer matrix method can be calculated.

[0087] An example of emissivity of PEDOT in doped and undoped state by applying positive and negative voltage in accordance with an embodiment of the invention is illustrated in FIG. 10A. 1001 shows undoped PEDOT has relatively high emissivity at about 1.0 in the LWIR wavelength ranges between 8 pm and 12 pm. 1002 shows doped PEDOT has relatively low emissivity at about 0.2 in the LWIR wavelength ranges between 8 pm and 12 pm. FIG. 10A shows a significant contrast between doped and undoped state of PEDOT by applying positive or negative voltage.

[0088] An example of emissivity of PANI in doped and undoped state by applying positive and negative voltage in accordance with an embodiment of the invention is illustrated in FIG. 10B. 1003 shows undoped PANI has relatively high emissivity at about 0.8 in the LWIR wavelength ranges between 8 pm and 12 pm. 1004 shows doped PANI has relatively low emissivity from about 0.1 to about 0.3 in the LWIR wavelength ranges between 8 pm and 12 pm. FIG. 10B shows a significant contrast between doped and undoped state of PANI by applying positive or negative voltage.

Example 2: Inorganic Oxides

[0089] Some embodiments implement inorganic oxides on the interior surfaces to tune emissivity. In many embodiments, inorganic oxide materials include tungsten trioxide (WO3). Certain embodiments implement hydrogen tungsten bronzes (HxWOs). The materials can experience a contrast in emissivity in the long-wave infrared when a voltage is applied in accordance to a number of embodiments.

[0090] Many embodiments utilize thermochromic materials to interior surfaces for tunable emissivity. Several embodiments implement vanadium dioxide for tuning emissivity in interior spaces. In some embodiments, ion-irradiated vanadium dioxide can experience a phase-change temperature near room temperature, thus enabling interior surfaces including (but not limited to) walls to natural turn from high to low emissivity as it cools down from a warm temperature.

Example 3: Lithium Titanate

[0091] Many embodiments implement Lithium Titanate (LTO) as tunable emissivity materials. In several embodiments, LTO can have a state change in an electrochemical cell, going for being lithiated to de-lithiated, which can induce a change in its emissivity. [0092] In several embodiments, a 1 D photonic design can be used to enhance emissivity contrast. An example of transfer matrix electromagnetic simulation of 1 D photonic design in accordance with an embodiment is shown in FIG. 11. The structure of 1 D photonic design including alternating thin layers of ZnS and LTO (1101) can enable two capabilities. In the lithiated state, thermal emittance from the LTO is high. The ZnS structured with the LTO in varying thicknesses, a chirped and/or an aperiodic photonic crystal architecture, enables high reflectance over the solar spectrum, even though LTO presents high intrinsic solar absorption. In the de-lithiated state, LTO is effectively a wide- bandgap semiconductor. The ZnS does not alter its low infrared emissivity, but it can augment its solar absorption by virtue of the index contrast between LTO and ZnS. This then allows imparting visual color to an occupant of an interior space while altering the infrared emissivity for heat transfer purposes.

Example 4: Mechanical Devices

[0093] In several embodiments, tunable emissivity materials can be incorporated in mechanical devices. Emissivity in LWIR ranges can be tuned mechanically in accordance with certain embodiments. In a number of embodiments, low emissivity and high emissivity materials can be applied to different sides of mechanical devices, and different sides can be rotated between low emissivity and high emissivity states in response to the weather.

[0094] A mechanical blinds device with two sides coated with low and high emissivity paint respectively in accordance with an embodiment of the invention is illustrated in FIG. 12. One side of the mechanical blind 1201 is coated with low emissivity materials. A second side of the mechanical blind 1202 is coated with high emissivity materials. The low and high emissivity sides can be easily switched between the emissive and reflective mode according to the weather.

[0095] In a number of embodiments, tunable emissivity materials can be materials that experience a change in emissivity due to mechanical deformation. Some embodiments provide poly-dimethyl-siloxane (PDMS) embedded with nanoparticles and/or microparticles. In certain embodiments, nanoparticles and/or microparticles embedded PDMS can be in forms of films. Some embodiments provide that stretching the films can tune the emissivity from below about 0.3 to above about 0.8 depending on the mechanical state.

[0096] Many embodiments include mechanically tuning emissivity of porous low- emissivity polymers. Some embodiments implement a porous low-emissivity polymer polyethylene, which has inherently low emissivity of below about 0.5. By filling the polymer with a fluid including (but not limited to) water, the emissivity can increase to above about 0.9 in accordance with some embodiments.

[0097] Several embodiments implement mechanical tuning emissivity systems for application on transparent surfaces including (but not limited to) windows as well as opaque surfaces including (but not limited to) walls, ceilings and floors. In some embodiments, For instance, mechanical blind devices can be semi-transparent with one way transmission of visible light that depends on which side faces the window in accordance with certain embodiments.

DOCTRINE OF EQUIVALENTS

[0098] As can be inferred from the above discussion, the above-mentioned concepts can be implemented in a variety of arrangements in accordance with embodiments of the invention. Accordingly, although the present invention has been described in certain specific aspects, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that the present invention may be practiced otherwise than specifically described. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive.