Date Published: February 04, 2016
Publisher: John Wiley and Sons Inc.
Author(s): Md. Muntasir Hossain, Min Gu.
The recent progress on radiative cooling reveals its potential for applications in highly efficient passive cooling. This approach utilizes the maximized emission of infrared thermal radiation through the atmospheric window for releasing heat and minimized absorption of incoming atmospheric radiation. These simultaneous processes can lead to a device temperature substantially below the ambient temperature. Although the application of radiative cooling for nighttime cooling was demonstrated a few decades ago, significant cooling under direct sunlight has been achieved only recently, indicating its potential as a practical passive cooler during the day. In this article, the basic principles of radiative cooling and its performance characteristics for nonradiative contributions, solar radiation, and atmospheric conditions are discussed. The recent advancements over the traditional approaches and their material and structural characteristics are outlined. The key characteristics of the thermal radiators and solar reflectors of the current state‐of‐the‐art radiative coolers are evaluated and their benchmarks are remarked for the peak cooling ability. The scopes for further improvements on radiative cooling efficiency for optimized device characteristics are also theoretically estimated.
Passive cooling systems offer substantial impact as energy saving devices due to their ability to operate without external energy. Radiative cooling is such a promising cooling method with their ability to offer a cooling power of more than 100 W m−2 with optimized device designs and under suitable atmospheric conditions.1, 2 The potential of this passive cooling method for energy saving applications has also been realized.3, 4, 5, 6 The key behind this unmatched ability stems from releasing energy via the radiative heat exchange where the heat is dumped directly to the outer space. The significance of radiative cooling and its potential application was emphasized a few decades ago and practical cooling during the nighttime operation was also demonstrated.7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 The use of bulk materials comprising intrinsic infrared (IR) emissions for considerable radiative cooling were discussed earlier.10, 18, 21, 23, 24 However, radiative cooling was mostly limited during the nighttime as suitable materials with high IR emission within the atmospheric window and yet delivering strong solar reflection during the day was not achieved. Although some solar reflecting materials were reported, daytime cooling below the ambient temperature was not achieved as the absorbed solar energy exceeded the emitted energy by thermal radiation.25, 26 It was only the recent demonstrations where the use of advanced nanophotonics led to daytime radiative cooling well below the ambient temperature.3, 27 The radiative coolers with photonic designs can simultaneously possess a high solar reflection up to 97% and strong IR emission within the atmospheric window.1, 3, 27 On the other hand, microstructure based thermal emitters can also offer highly efficient cooling power with their strictly selective and strong IR emission within the atmospheric window.2 These emerging photonic devices, offering substantial passive cooling during the day and night, have triggered significant research interest.
The Earth’s atmosphere has a highly transparent window in the infrared (IR) wavelength range between 8 and13 μm, i.e., the atmosphere’s radiative emission is very weak in that window. Outside the atmospheric window, the Earth’s atmosphere is highly emissive. Coincidentally, the atmospheric window falls within the peak thermal radiation of a black body defined by Plank’s law at the ambient temperature (at around 300 K). This feature permits for a passive cooling mechanism for a terrestrial body at the ambient temperature by eliminating heat via radiative emission through the atmospheric window. The emitted radiation escapes through the Earth’s atmosphere to the outer space which acts as a gigantic thermal reservoir. The atmospheric window allows the terrestrial body’s outgoing radiative emission to exceed its absorbed incoming atmospheric radiation and thus to passively cool below the ambient temperature. Figure1a shows the modeled atmospheric transmission28 in the zenith direction and the downward atmospheric radiation in a cloud‐free day for a typical mid‐latitude summer atmospheric conditions (Melbourne, Australia).29 Figure 1b shows the reference global AM1.5 solar irradiance spectra.30 The atmospheric window from 8 to 13 μm is clearly visible. A secondary atmospheric window also exists between 16 and 23 μm; however, it is fairly weak and its effect on radiative cooling can be neglected in general.
Ever since the potentials of radiative cooling were realized, continuous efforts have been made to increase the cooling efficiency of the radiative emitters and practical cooling at night was also demonstrated.7, 8, 9, 10, 11, 15, 16, 17, 18, 19 However, the use of naturally available materials and synthetic polymers has always been the limiting factor. The possibilities of daytime radiative cooling were also discussed in different studies.4, 25, 26 The lack of suitable materials with high solar reflection but possessing strong emission within the IR window restricted any effective radiative cooling under the direct sunlight. For the realization of efficient nocturnal radiative cooling, selective IR emitters of various bulk materials have been studied. Among those, polymer films,10, 12, 13 white pigmented paints,18, 32 silicon monoxide (SiO) films, 20, 21, 22, 33 and other solid materials43, 44 have been widely investigated. Although these radiators provide somewhat selective radiation within the atmospheric transmission window, many of them suffer from weak emissivity, limiting the cooling performance. Furthermore, the lack of strict selectivity of the IR emission/absorption, results in significant absorption of the atmospheric radiation outside the transparency window, not allowing the cooling device to achieve steady state temperature significantly below the ambient temperature.10, 12, 32, 33
The recent progress on developing radiative coolers based on photonic devices has opened a new window to achieve highly efficient cooling and the ability to operate directly under the sun, reaching temperature below the ambient temperature.1, 3, 27, 62 Unlike radiative coolers based on the intrinsic optical properties of bulk materials, these approaches utilize engineered photonic properties combined with the intrinsic properties to realize improved cooling abilities. The unmatched potential of photonic structures is that they can offer strictly selective but strong thermal emission2 and very high solar reflection.3, 27 Different types of photonic devices have been investigated for efficient and daytime radiative cooling. In the following subsections, design principles and photonic characteristics of these devices will be analyzed.
From the analysis in the previous sections, it is clear that the radiative cooling performance depends on the maximized thermal radiation within the atmospheric window, the radiator emissivity profile, the nonradiative heat gains, solar power absorption, and atmospheric conditions. To further estimate the variation of cooling performance on the emissivity profile of the radiator, we consider a selective radiator with a varying emission bandwidth where the center emission band is at 10.5 μm fitting within the mid of 8–13 μm window range. The atmospheric model for Perth with transmittance within atmospheric window (Figure 4a) is adopted for optimal performance. Figure7a shows the achievable Ta−Tr for the variable emission bandwidth including a fixed 3% solar absorption3, 27 and two high and low values of nonradiative heat gain coefficients. The right side of the vertical dashed line represents the radiators with bandwidth >|8–13| μm of the ideal selective radiator and left side represents the case for bandwidth <|8–13| μm. It is evident that if the cooling device is exposed to a small but practical nonradiative heat gain (2 W m−2 °C−1),33 the ideal selective radiator can provide with a lower cooling temperature (Ta−Tr > 15 °C) than a broadband radiator. For increasing bandwidth of the radiator, the cooling performance decreases and becomes almost stable for bandwidth >11 μm, where it starts to behave like a blackbody radiator. There is a sudden small increase of the cooling performance for a radiator emission bandwidth >12 μm where the radiator starts to interact with the weak secondary atmospheric window from 16 to 23 μm wavelengths range. For decreasing emission bandwidths, i.e., for bandwidths <|8–13| μm, Ta−Tr rapidly decreases and does not provide any effective cooling for a bandwidth <1.25 μm. As discussed in Section 2.2, for a radiator with a very narrow emission bandwidth, the small amount of radiated power is unable to offset the solar and nonradiative heat gain. Radiative cooling promises a vital impact with its highly efficient passive cooling potential. In particular, the recent demonstrations for daytime radiative cooling below the ambient temperature3, 27 already indicate their ability for practical energy saving applications.4, 5 A strictly selective but highly emissive radiator with high solar reflectance (≈97%) can deliver a substantial passive cooling during the daytime, leading to temperatures as low as 15 °C below the ambient temperature, provided that insulations for minimized but realistic nonradiative heat gain are employed. Further minimization of nonradiative heat gain and solar power absorption can substantially improve the cooling performance. However, the realization of strictly selective radiators with substantial solar reflection is still a great challenge. Besides, the atmospheric conditions in a particular geographical location will also play a vital role for meaningful radiative cooling efficiency. For applications where integration of nonradiative insulations may not be suitable, broadband radiators can easily replace selective radiators. Furthermore, the cooling efficiency can also be enhanced when radiative cooling can be combined with other passive cooling devices, such as, phase change materials.86 Source: http://doi.org/10.1002/advs.201500360