Passive sub-ambient cooling: radiative cooling versus evaporative cooling

Day-and-night radiative sky cooling has emerged as a potential alternative to conventional cooling technologies such as refrigeration-based air conditioning and evaporative wet cooling. Both radiative cooling and evaporative cooling can passively achieve sub-ambient cooling without consuming electricity. Although both cooling techniques are subject to impacts from various weather conditions, the extents of the impacts under the same conditions are not well understood. In this work, we experimentally and theoretically study the thermal performances of a passive radiative cooler and a passive evaporative cooler when exposed to a clear night sky. We show that evaporative cooling is better suited for high-temperature and low-humidity weather conditions, with the measured sub-ambient temperatures of the radiative and evaporative coolers being -13.5{\deg}C and -15.0{\deg}C, respectively, at a low relative humidity of 13% and a high ambient temperature of 26.0{\deg}C. On the other hand, radiative cooling is relatively more resilient than evaporative cooling under high-humidity and/or low-temperature weather conditions, with the measured sub-ambient temperatures of the coolers being -11.5{\deg}C and -10.5{\deg}C, respectively, at a slightly higher relative humidity of 32.0% and a slightly lower ambient temperature of 17.0{\deg}C. Depending on water availability and weather conditions, both evaporative cooling and radiative cooling can be adopted as mutually supplemental cooling technologies.