Fig. 1. Temperature-dependent material responses of VO₂. (a) Schematic of temperature-dependent phase. (b) Permittivity at 2 μm when T_c = 298 K. Shaded area represents transition regime. (c) Real and (d) imaginary part of permittivity in transition regime.

Fig. 2. Design of the switchable radiative cooler. Emitter part consists of stacked layers of silver, silicon and VO₂. Solar reflector part consists of three photonic crystals that have 4 pairs of PMMA and silicon. PC_i is designed to suppress absorption at λ_i where λ₁ = 0.52 μm, λ₂ = 0.76 μm and λ₃ = 1.18 μm. Thickness of each layer is λ_i/4n.

Fig. 3. Absorptivity and reflectivity of the switchable radiative cooler. (a, b) Absorptivity and reflectivity of the emitter part when VO₂ is in (a) metallic and (b) insulating state. (c) Absorptivity and reflectivity of the solar reflector part. Three arrows represent the target wavelengths of three photonic crystals. (d, e) Absorptivity and reflectivity of the switchable radiative cooler when VO₂ is in (d) metallic and (e) insulating state. Incident angle is zero. (f) Absorptivity of the switchable radiative cooler when VO₂ is metallic.

Fig. 4. Cooling flux of the radiative cooler under normal incidence of solar energy when T_amb = 303 K. (a, b) Cooling flux when permittivity of VO₂ is assumed to be (a) static and (b) dynamic. Shaded area represents the transition regime.

Fig. 5. Temperature variation in time. (a) Temperature and (b) cooling flux in time for initial temperature of 280 K to 320 K with 5 K step. Temperature indicate the initial temperature of the cooler. T_amb = 303 K. (c, d) A cycle of temperature of a day. (c) T_amb and solar irradiance of July 15, 2018 in Pohang, Korea. (d) Temperature of switchable radiative cooler (blue) and the static radiative cooler when radiative cooling is assumed to be turned on (orange) and off (yellow). T_amb is shown as a reference (black). Initial temperature of the cooler is set equal to the initial T_amb.

Table 1. Specific heat and density of materials.