Enhancing cold energy harvest with daytime passive radiative cooling and phase change materials integration: A numerical exploration

Abstract

Daytime passive radiative cooling (DPRC) is an emerging technology to dissipate heat to the ultimate cold sink, the universe, with zero energy input. However, the achieved cold surface temperature of an object by DPRC can readily increase close to the ambient temperature. Sensible heat readily induces changes in the temperature of objects, but it generates a heat leakage path to surroundings with the largely varying temperature gradients. To address the challenge, we designed a DPRC coating integrated with phase change materials (PCM) so as to harvest and store the cold energy, respectively from the universe. To evaluate the feasibility of this concept, a numerical model, which integrates a self-programmed MATLAB code to calculate the net cooling power and a FLUENT-assisted enthalpy-porosity method to simulate the phase change process, was developed and validated. Also, the cold energy harvesting performance of the proposed device was explored under different climate types and conditions. We found that the surface temperature of the PCM with a DPRC coating can be reduced to −15.9 °C and −4.5 °C in a day in the temperate continental climate and the subtropical desert climate, respectively, when the non-radiative heat transfer is completely avoided. The temperature difference in the PCM is less than 1.0 °C when aluminum fins are added to enhance the heat transfer performance of the PCM. This study provides new insights into the continuous steady cooling of DPRC using PCMs, and the harvested cold energy can be used to cool water for the condenser of an air conditioner to save energy consumption