General strategy of passive sub-ambient daytime radiative cooling

Passive sub-ambient daytime radiative cooling has been demonstrated in previous studies by rigorous spectrum tailored coolers with high solar reflection and strong thermal emission. In this work, a general strategy to change the relative position between the radiative coolers and the sun was proposed to decrease solar radiation reaching the coolers. In this way, passive sub-ambient daytime radiative cooling was well demonstrated on the basis of two commonly available coolers, coolers A and B. Cooler A was fabricated by attaching a 20 um-thick polydimethylsiloxane film on a reflective metal substrate, and cooler B was a commercial reflective mirror. A field test was conducted for coolers A and B toward north sky in Hefei (117 degrees E, 32 degrees N), China. Results show that the average temperature drops below ambient temperatures of coolers A and B are 3.2 degrees C and 7.4 degrees C, respectively, which indicate that the proposed strategy can achieve passive sub-ambient daytime radiative cooling. Furthermore, the feasibility of the proposed strategy for daytime radiative cooling was also proved using a universal thermal model. This work provides a new insight into passive sub-ambient daytime radiative cooling and opens up a new avenue for common materials to potentially achieve sub-ambient daytime radiative cooling.