Thermo-radiative energy conversion efficiency of a passive radiative fluid cooling system

In the passive radiative cooling process, a sky-facing surface emitting thermal radiation through the bandwidth coincident with the atmospheric window highly transparent to the radiation within 8–13 μm can preserve the temperature below ambient spontaneously. The cold surface can act as a fundamental building block for energy conversion, in which thermo-radiative energy conversion can be the simplest form and realized by a functionalized fluid-wall heat transfer interface. Energy conversion efficiency denotes the ratio of enthalpy converted by the working fluid to the cooling effect harvestable from the sky. In parallel with fluid cooling capacity, they are discussed by thermal and energy responses of a cooling system subjected to a perturbation in fluid flow, and demonstrated by measurement on a wafer-sized system acted by an equivalent heat current. According to interfacial heat transfer characteristics, cooling performance can be classified into inhibition, transition and saturation regimes, where the saturated performance is the most outstanding. However, fluid cooling and energy conversion capacities are always inversely correlated, where the reduction in fluid temperature decreases with increasing flow rate, but efficiency increases with increasing flow rate. Experimental results, in line with the theoretical prediction, show that 12.4 μL/s of water can be chilled by −4.1 °C at an overall efficiency of 14%, but 116 μL/s of water can be weakly chilled by −1.5 °C at an elevated efficiency of 49%. The dilemma in energy efficient collection of cooling fluid is an innate physical mechanism restricted by Newton's law of cooling and the 1st law of thermodynamics.