Thermodynamic Cooling Potential of Passive Radiative Cooling Based on Entropy Flux Analysis and Ideal Boundary Conditions

Passive radiative cooling (RC) through the atmospheric window has great potential in reducing fossil energy dependence and greenhouse gas emissions. This technology has made rapid progress recently, with new cooling records and emerging application fields. Taking the ideal emissivity model and actual multilayer structures as examples, this study explores the minimum cooling temperature of RC from a thermodynamic perspective via entropy flux analysis. It finds that the minimum temperature is lower than what is previously thought to be achievable at heat flux equilibrium. In analogy with the Carnot cycle and the Landsberg limit in solar energy harvest, the maximum Carnot output work and Landsberg limit output work corresponding to the thermoelectric generator operating between the ambient and the radiative cooler are derived, based on the ideal emissivity model and actual multilayer film structure, which are much higher than the thermoelectric generation reported in experiment. This shows that the utilization of RC in thermoelectric generation has huge room for improvement.