ABSTRACT
Passive radiative cooling, making use of a sky-facing surface simultaneously reflecting solar radiation and emitting mid-infrared radiation to preserve a sub-ambient surface temperature, inspired many novel engineering applications. This article reviews recent progresses on radiative cooling applications for thermal and electrical energy harvesting. Chilled water collection is a form of thermophotonic energy conversion enabled by a fluid-wall heat transfer interface. Discussed by thermal and energy responses of a chilled water collection system subjected to a perturbation in fluid flow and verified by a proof-of-concept experiment, it is shown that water temperature reduction and energy conversion efficiency are always inversely correlated, in which temperature reduction decreases with increasing flow rate, but efficiency increases with increasing flow rate. Moreover, thermoelectricity generation is a form of thermoelectrical energy conversion facilitated by radiative cooling. Demonstrated by a field investigation, it enables direct electricity generation from a renewable resource at nighttime, realizing 24-hour unstopped electricity generation with a deliverable electrical voltage up to 20 mV. However, radiative cooling and derived energy conversion performances can be heavily hindered by local weather conditions. Field investigative results on the surface temperature reduction, as a cardinal indicator on radiative cooling performance, can be correlated with the sky temperature, which is a lumped parameter of ambient temperature, relative humidity, and cloudiness. This reveals the difficulty in tropical and subtropical radiative cooling, where the cooling demand is the highest. Possible solutions for overcoming these issues are highlighted for further research and development.
