A novel model concerning the independence of emissivity and absorptivity for enhancing the sustainability of radiant cooling technology

Radiant cooling technology is a sustainable technology for improving built environment. The past research only studied the thermal performance (e.g., radiant heat flux) based on Kirchhoff's law while the accuracy and its reasons were seldom analyzed. This article points out that it is necessary to analyze the precondition before applying Kirchhoff's law directly, because emissivity may not be equal to absorptivity on radiant surfaces. The independence of the emissivity and absorptivity is considered in the new model based on the inapplicability of Kirchhoff's law. The analysis of sensitivity and relative deviation are performed to investigate the reasons for errors. The sensitivity of emissivity is about 20%-40% more sensitive to radiant heat flux than the absorptivity. Furthermore, the deviation of the heat flux can reach up to 20% when the absorptivity is in the range from 0.4 to 0.9. This deviation is close to the error range of 21.8% estimated in the past. Thence, the discussion based on the theoretical analysis, shows that the errors in past studies were highly caused by the oversimplified preconditions for applying Kirchhoff's law and the impact of surface absorption was ignored. Additionally, the validation in the past experiments was highly coincidence, since the key independent tests of the absorptivity and radiant heat flux were neglected. Comprehensively, the new model is valuable to provide a reliable solution for future design and analysis of radiant heat exchange when a radiant surface is not locally equilibrium.