image: Schematic of a switchable radiative cooler.
In a new publication from Opto-Electronic Advances; DOI https://doi.org/10.29026/oea.2021.200006, Researchers led by Professor Junsuk Rho from Pohang University of Science and Technology (POSTECH), South Korea consider switchable diurnal radiative cooling by doped VO2.
As the impacts of climate change are increasingly felt, thermoregulation technologies that do not consume external energy have attracted considerable attention in the field of energy-saving applications. Radiative cooling has received much research interest for its ability to cool an object even under direct solar illumination. Nanostructured materials, or multi-stacked layers, can be designed to control reflection and emission spectrum to block solar irradiance and/or to emit heat through the atmospheric window. However, one limitation of radiative cooling for static devices, is that the cooling also occurs in low temperature conditions such as in winter or at night when heating is desired instead. Thus, there has been a strong demand for researchers to develop smart, switchable radiative coolers that cool down objects when the temperature is high but can also heat up in low temperature conditions.
Phase change materials that have distinct phases in response to temperature have been considered good candidates for such requirements. A doped vanadium dioxide (VO2) has a transition temperature at room temperature; the temperature-dependent characteristics of VO2 can be used to design a radiative cooler that cools down at temperatures above the transition temperature while heating up otherwise, without requiring any energy. This passive, but switchable, radiative cooling can directly provide environmentally friendly thermoregulation technology as well as various other applications such as air conditioning and heating.
The authors of this article propose a switchable radiative cooler that satisfies the aforementioned conditions using VO2. They have designed a switchable emitter by stacking VO2 on an insulator-metal layer. The resultant tri-layer structure emits its heat to the atmosphere above the transition temperature. This emitter is combined with a solar reflector, that is composed of several one-dimensional photonic crystals, to form a switchable radiative cooler. Thus, the overall system can block heat from solar energy while emitting thermal energy to the atmosphere.
The authors demonstrated that the switchable radiative cooler maintains a moderate temperature resilient to environmental changes by simulating a cycle of temperature changes under real outdoor conditions. The cooler facilitates self-adaptive thermal control and thus can be implemented in practical applications such as cooling buildings and vehicles.