Fig. 1
From: Passive directional sub-ambient daytime radiative cooling
Passive directional daytime radiative cooling. a Spectral distribution of solar irradiation (AM1.5 G spectrum) and atmospheric transmittance (shown for wavelengths >2.7 µm, Cambridge, MA in October30). b Angular distribution of normalized clear sky radiance in a principal plane that includes the sun (denoted by the circle, shown for a solar zenith angle of 40°) and atmospheric transmittance (shown for 10.5 µm wavelength). c Energy flow diagram showing the possibility of achieving sub-ambient passive cooling during the day by emitting radiation in the mid-infrared wavelength range, while reflecting the angularly confined direct-solar radiation using a broadband reflector and an infrared-transparent cover that reflects diffuse-solar radiation. d Estimated net radiative cooling power Pcooling as a function of emitter temperature (ambient temperature: 25 °C) and constituent contributions for an ideal solar-white emitter (λ < 2.5 µm: ε = 0, λ ≥ 2.5 µm: ε = 1, ∀θ) and ideal solar-black emitter (ε = 1, ∀λ, θ) coupled with a perfect direct-solar reflector (ρrefl = 1, ∀λ, θ) and a representative diffuse-solar cover (λ < 2.5 µm: ρcover = 0.8, λ ≥ 2.5 µm: τcover = 1− ρcover = 0.9, ∀θ)
