Passive sub-ambient daytime radiative cooling

Passive daytime radiative cooling approaches primarily rely on spectrally selective surfaces whose performance is limited by heat gain due to solar absorption and parasitic conduction and convection. We propose and experimentally demonstrate a directional approach to perfectly reject direct solar radiation and an optically-selective thermally-insulating emitter cover that minimizes parasitic losses that can lead to low-cost, high-performance daytime radiative coolers.