Development of a device for characterizing radiative cooling performance

Daytime radiative cooling has recently drawn much attention due to its potential for use in next-generation cooling systems. So far, two indicators have been used to estimate radiative cooling performance: (i) cooling temperature of a radiative cooling surface at equilibrium condition; and (ii) instantaneous cooling power from a radiative cooling surface at ambient temperature. These quantities, however, deal with only a small thermal mass of a sample itself (i.e., they do not consider a real-world system to which a radiative cooling surface is applied) and cannot directly indicate the energy saving caused by the radiative cooling effect. Here, we propose a device that can directly measure daily averaged radiative cooling power as well as the resulting cooling energy reduction. To this end, two enclosures with different top covers (i.e., one with commercially available white paint and the other with radiative cooling paint) are prepared with several thermoelectric coolers attached through the side walls. Two different outdoor experiments are carried out; that is, one maintaining constant enclosure temperature and the other maintaining constant temperature difference between enclosure and ambient. The first test is designed for verifying the cooling energy saving of building, and the second test is for quantifying the cooling density from radiative cooling in an enclosure with a large thermal mass. By measuring the temperature and power consumption in the enclosures, the radiative cooling performance of two different enclosures can be thoroughly and quantitatively analyzed, which potentially can lead to the direct examination of cooling energy saving of buildings exploiting the daytime radiative cooling effects. With the proposed device, we show that the radiative cooling paint can produce the daily averaged radiative cooling of 10.9∼45.2 W/m² and the corresponding cooling load of thermoelectric coolers can be reduced by 15.7∼50.0%. We anticipate this device to be a starting point for more realistic and sophisticated evaluation of radiative cooling performance.