Radiative cooling during the day: simulations and experiments on pigmented polyethylene cover foils

Nilsson, Torbjörn; Niklasson, Gunnar A.

doi:10.1016/0927-0248(94)00200-2

Cited by 200 publications

(93 citation statements)

References 38 publications

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“…The wind speed reported by local weather stations during the time of the experiment is 2.6 m/s. The nonradiative heat exchange coefficient between the top surface and air corresponding to this wind speed is 10.6 Wm −2 · K −1 (6), which agrees well with the combined nonradiative heat exchange coefficient values we use. From modeling, we observe that the absorber structure with silica photonic crystal is generally predicted to be slightly cooler than the absorber structure with planar silica layer, agreeing with the experimental data.…”

Section: Methodssupporting

confidence: 75%

“…Whereas historically radiative cooling was largely developed for night-time applications (1)(2)(3)(4)(5)(6)(9)(10)(11)(12)(13), recent works have achieved daytime radiative cooling (7,14). In particular, it was shown that the radiative cooling to below ambient air temperature can be achieved (7), with a photonic structure that reflects almost all incident sunlight and simultaneously emits significant thermal radiation in the midinfrared.…”

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confidence: 99%

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Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody

Zhu

Raman

Fan

2015

Proc. Natl. Acad. Sci. U.S.A.

525

351

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A solar absorber, under the sun, is heated up by sunlight. In many applications, including solar cells and outdoor structures, the absorption of sunlight is intrinsic for either operational or aesthetic considerations, but the resulting heating is undesirable. Because a solar absorber by necessity faces the sky, it also naturally has radiative access to the coldness of the universe. Therefore, in these applications it would be very attractive to directly use the sky as a heat sink while preserving solar absorption properties. Here we experimentally demonstrate a visibly transparent thermal blackbody, based on a silica photonic crystal. When placed on a silicon absorber under sunlight, such a blackbody preserves or even slightly enhances sunlight absorption, but reduces the temperature of the underlying silicon absorber by as much as 13°C due to radiative cooling. Our work shows that the concept of radiative cooling can be used in combination with the utilization of sunlight, enabling new technological capabilities.radiative cooling | thermal radiation | photonic crystal | solar absorber T he universe, at a temperature of 3 K, represents a significant renewable thermodynamic resource: it is the ultimate heat sink. Over midinfrared wavelengths, in particular between 8 and 13 μm, Earth's atmosphere is remarkably transparent to electromagnetic radiation. This wavelength range coincides with the peak wavelength of thermal radiation from terrestrial structures at typical ambient temperatures. Thus, a sky-facing terrestrial object can have radiative access to the universe. Exploiting this radiative access has led to the demonstration of radiative cooling (1-7), as well as proposals for direct electric power generation from thermal radiation of terrestrial objects (8).Whereas historically radiative cooling was largely developed for night-time applications (1-6, 9-13), recent works have achieved daytime radiative cooling (7,14). In particular, it was shown that the radiative cooling to below ambient air temperature can be achieved (7), with a photonic structure that reflects almost all incident sunlight and simultaneously emits significant thermal radiation in the midinfrared. Such a structure, being a near-perfect solar reflector, makes no use of incident sunlight. On the other hand, in many applications, including solar cells (15) and outdoor structures (16), the utilization of sunlight through absorption is intrinsic for either operational or aesthetic considerations, but the heating associated with sunlight absorption is undesirable. For these applications, lowering operating temperatures via radiative cooling is only viable if one can simultaneously preserve the absorption of sunlight.Here we experimentally demonstrate a visibly transparent thermal blackbody, based on a silica photonic crystal, using a thermophotonic approach (17-30). When placed on a silicon absorber under sunlight, such a blackbody preserves and even slightly enhances sunlight absorption, but reduces the temperature of the silicon absorber by as m...

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Section: Methodssupporting

confidence: 75%

mentioning

confidence: 99%

Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody

Zhu

Raman

Fan

2015

Proc. Natl. Acad. Sci. U.S.A.

525

351

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“…The earliest adoption of radiative cooling has been traced back to the courtyard architectures of ancient Iran [4]. In the modern era, the first scientific studies found that certain materials have potential for limited selectivity, notably polymeric materials [5,6], titanium dioxide [7][8][9], silicon nitrides [10], and silicon monoxide (SiO) [11]. While cooling to 40°C below ambient with SiO is theoretically possible [11], the temperature difference is much smaller in experiments [2].…”

Section: Introductionmentioning

confidence: 99%

Radiative sky cooling: fundamental physics, materials, structures, and applications

et al. 2017

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Radiative sky cooling reduces the temperature of a system by promoting heat exchange with the sky; its key advantage is that no input energy is required. We will review the origins of radiative sky cooling from ancient times to the modern day, and illustrate how the fundamental physics of radiative cooling calls for a combination of properties that may not occur in bulk materials. A detailed comparison with recent modeling and experiments on nanophotonic structures will then illustrate the advantages of this recently emerging approach. Potential applications of these radiative cooling materials to a variety of temperature-sensitive optoelectronic devices, such as photovoltaics, thermophotovoltaics, rectennas, and infrared detectors, will then be discussed. This review will conclude by forecasting the prospects for the field as a whole in both terrestrial and space-based systems.

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“…The same conceptual approach can help to improve thermal management of tents, buildings and vehicles (3,7,8), and will undoubtedly lead to many exciting advances of passive cooling technologies resulting in significant energy savings. …”

Section: Panels D and E)mentioning

confidence: 99%

Nanoporous fabrics could keep you cool

Boriskina

2016

Science

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Convection, conduction and thermal radiation all play important role in the personal thermal management. From the early days when animal skins were the season's fashion till modern times, clothes have been typically engineered for comfort in cold environment by tailoring their thermal conduction. Air pockets in feathers, furs and woolen fabrics help to reduce thermal conduction and to keep warmth inside. Cooling effect is however much harder to achieve without the use of external active devices such as fans, air conditioners or wearable thermoelectric coolers. The wicking technology used in the modern athletic apparel to enhance convective cooling is not ideal for everyday clothes, as it only works once perspiration occurs. On page XX of this issue, Hsu et al.(1) demonstrate a passive way to cool an object by a few degrees by simply allowing thermal radiation to pass efficiently through a nanoporous fabric. This demonstration opens the possibility to develop new wearable technologies for personalized cooling and paves the road to significant energy savings through reduced use of air conditioning.Human body is an almost perfect emitter of thermal radiation in the mid-infrared spectral range. However, conventional fabrics block infrared waves by partially reflecting them and partially absorbing the thermal energy. The stark variations in the intensity of the thermal emission from the bare skin and through clothes can be easily observed in photos taken with an infrared camera (see the Figure, panel A). Therefore, clothes made from fabrics that are transparent to the infrared waves emitted by the skin offer an opportunity to shed energy via radiation (2-4). Transparent clothes might sound like an odd idea though, and thus the challenge is to keep the fabric opaque for the visible light that human eyes are sensitive to while making it transparent for infrared radiation.A simple solution to this dilemma is offered by the Mie theory of resonant scattering from objects with sizes either comparable to or much smaller than the wavelength of the propagating electromagnetic field. This is the same physical effect that is behind the blue color of the sky, which is caused by the scattering of the short wavelength part of the solar spectrum by the small molecules of the atmosphere. Likewise, fabrics with pore sizes that are comparable on average to the wavelength of visible light (400-700 nm) scatter visible light strongly and make the fabric opaque to human eyes. However, if the pore sizes are much smaller than the body infrared radiation wavelength (7-14 micron), such fabrics are still highly transparent to the thermal emission (see the Figure, panel B) (2).Hsu et al.(1) used a commercially available polyethylene material NanoPE, which has interconnected pores 50-1000 nm in size (see the Figure, panel C), to experimentally demonstrate the radiative cooling effect. Their spectral transmission measurements revealed that the nanoPE exhibits over 90% total infrared transmittance for wavelengths longer than 2 μm and at the same ...

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Radiative cooling during the day: simulations and experiments on pigmented polyethylene cover foils

Cited by 200 publications

References 38 publications

Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody

Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody

Radiative sky cooling: fundamental physics, materials, structures, and applications

Nanoporous fabrics could keep you cool

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