Thermal performance of transparent radiative cooling: selective vs blackbody emitter in combination with low-emissivity coating

• Thermal performance of radiative cooling in terms of appropriate metrics, e.g. net reduction of heat flux has been estimated for the first time. • Fundamental limit of radiative cooling estimated by solving the heat equation analytically. • Radiative cooling that selectively emits between 8 and 13 μ m can reduce indirect heat flow by 15–22 %. This estimate is based on year-round thermal simulation (numerical) of RC coated windows. • A multi-coated glass with a selective emitter on the outside and Low-E on the inside can save about 50 % energy. • The blackbody radiator performs just as well as the selective emitter, eliminating the need for complicated structures to fabricate a selective emitter. Radiative cooling has been studied for many years for the purpose of cooling surfaces such as solar panels and building roofs. The idea utilizes the atmospheric transparency window that exists between 8 and 13 μ m where electromagnetic waves are mostly transmitted with little absorption. If a surface’s emittance is aligned with this window, the incoming radiation from the sky is greatly reduced. However, such selective emitter, as we show in this paper, also has a compromised ability to radiate heat back to the atmosphere due to the reduced emissivity. In this paper, we analyze the net heat flux through a transparent surface - relevant to a glass-based building - for the two types of radiative cooling - selective emitter and blackbody emitter with unity emissivity. The fundamental limit of cooling power is found by solving the heat equation analytically. We find that the selective emitter works best when convection near the radiative cooling surface is low. Numerical solution in the presence of convection shows that the two types of radiative cooling perform similarly. We propose a device structure where radiative cooling on transparent glass can be combined with low emissivity (Low-E) coating on the interior side. We show that the two types of coatings work in a complementary way and minimize the overall inward heat flux. We calculate the net energy footprint for different configurations of radiative cooling and Low-E coatings and discuss the optimum structure for various applications.