Effect of vacuum scheme on radiative sky cooling performance

Relatively low cooling power density is one of the main barriers to wider promotion of radiative sky cooling (RSC) technology. Vacuum scheme has been proposed to minimize the non-radiative cooling loss and thus improve the cooling capacity. However, systematic research to elucidate the effect of the vacuum mechanism on the RSC performance is still lacking. Therefore, in the present study, an RSC module with four vacuum structures is proposed to evaluate the performance variation resulting from the vacuum scheme. A quasi-steady state mathematical model is developed to characterize the cooling performance of the four RSC modules under different operation conditions. Results suggested that the vacuum strategy can further elevate the cooling ca-pacity if the typical RSC (TRSC) module itself can realize all-day sub-ambient cooling. However, if the TRSC cannot achieve sub-ambient cooling during peak sun hours, the vacuum scheme will deteriorate rather than ameliorate the cooling performance. On a typical summer day in Shanghai, vacuumization in both cavities en-ables a further temperature reduction of 10.21 degrees C during the nighttime, but this value decreases to only 3.39 degrees C during the daytime. The cooling power enhancement resulting from the vacuum scheme is limited in real-world dynamic operation with the thermal carrier. At a reasonable temperature gap of 5 degrees C between the emitter and ambient air, the extra cooling gain is less than 5.10 W/m2. Hence, considering the addition of energy con-sumption and system complexity caused by the vacuum unit, it may not be advisable to pursue better cooling performance of a stand-alone RSC collector/system through introducing a vacuum strategy, unless realizing a deep stagnation emitter temperature is targeted.