Full daytime sub-ambient radiative cooling film with high efficiency and low cost

There is an urgent need to develop sustainable alternative strategy for the cooling of buildings other than the current air conditioning technique. Passive daytime radiative cooling (PDRC) dissipates heat directly into deep space without extra energy demand and therefore is very promising. Here we reported a low-cost, scalable composite film with novel dendritic cell like structures and superior radiative cooling properties. The novel composite film was prepared by simple phase inversion process using cheap cellulose acetate and calcium silicate as raw materials. The composite film reflects 97.3% of solar irradiation owning to the scattering effect of calcium silicate particles and broad size-distributed pores formed by phase inversion. The high sky-window emittance of 97.2% is attributed to the abundant vibrating bonds of Si–O, C–H, C–O and C–O–C in the composite film. The composite film delivers a maximum temperature drop up to 7.3 °C and an average net cooling power of 90.7 W m−2 under clear sky around noon (solar intensity 803–885 W m−2) in early September in Qingdao. The figure of merit RC of our composite film is 0.78 and is on par with or exceeds those reported results for PDRC materials or structures.