Abstract
A radiative cooling device, based on a metamaterial able to mirror solar radiation and emit heat toward the universe by the transparency window of the atmosphere (8–13 µm), reaching and maintaining temperatures below ambient air, without any electricity input (passive), could have a significant impact on energy consumption of buildings and positive effects on the global warming prevention. A similar device is expected to properly work if exposed to the nocturnal sky, but during the daytime, its efficacy could be affected by its own heating under direct sunlight. In scientific literature, there are only few evidences of lab scale devices, acting as passive radiative cooling at daytime, and remaining few degrees below ambient air. This work describes the proof of concept of a daytime passive radiative cooler, entirely developed in ENEA labs, capable to reach well 12 °C under ambient temperature. In particular, the prototypal device is an acrylic box case, filled with noble gas, whose top face is a metamaterial deposited on a metal substrate covered with a transparent polymeric film. The metamaterial here tested, obtained by means of a semi-empirical approach, is a spectrally selective coating based on low cost materials, deposited as thin films by sputtering on the metallic substrate, that emits selectively in the 8–13 µm region, reflecting elsewhere UV_VIS_NIR_IR electromagnetic radiation. The prototype during the daytime sky could reach temperatures well beyond ambient temperature. However, the proof of concept experiment performed in a bright clear June day has evidenced some limitations. A critical analysis of the obtained experimental results has done, in order to individuate design revisions for the device and to identify future metamaterial improvements.
Highlights
The average ambient air temperature has raised by about 0.12 ◦ C, per decade, since 1951 because of climate change [1]
The second step is the development of an engineered cool metamaterial, coolcool metamaterial (CM), able to satisfy some stringent optical requirements as reported in Figure 3, and the deposition of it on an aluminum plate acting asmetamaterial heat exchanger, by means of reproducible, scalable and low
This electricity saving can be achieved if the cooler located on the roof is able to grant, under direct sunlight, a temperature well below ambient temperature
Summary
The average ambient air temperature has raised by about 0.12 ◦ C, per decade, since 1951 because of climate change [1]. This trend has been emphasized at a local scale by the Urban Heat Island (UHI). Aligned with the ambient temperature increase, a relevant rise of the electricity consumption can be expected in the future [5]. Energies 2020, 13, 4192 countries exceeded by 7% that of Northern European ones, due to the occurrence of higher temperature levels often concentrated in short periods, attributable to building space cooling [7]. The so-called cool materials, which are characterized by high solar reflectance and high thermal emissivity, can limit the temperature rise of urban fabrics
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