Abstract
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.
Highlights
A solar absorber, under the sun, is heated up by sunlight
The fabricated silica photonic crystal has a square lattice with a periodicity of 6 μm, and with an etching depth of 10 μm, as shown in the scanning electron microscope (SEM) images in Fig. 1 C and D for normal and side views, respectively
We have experimentally demonstrated a thermophotonic approach to substantial, passive, radiative cooling of a solar absorber, by radiating heat to the cold of outer space through the atmosphere’s infrared transparency window
Summary
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. 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. 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.
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