Temperature non-uniformity in the radiative cooler and its effect on performance under various humidity conditions

Sub-ambient diurnal radiative cooling is an effective, environment-friendly and energy-saving passive cooling approach as it cools a surface without anthropogenic energy input. The cooling power of a radiative cooler is strongly influenced by the meteorological conditions, in particular humidity, on which quite a few investigations have been conducted so far. However, the cooling power evaluation of a radiative cooler usually considers the radiative cooler to be isothermal, which ignores the interior temperature difference resulted from the low thermal conductivity of the prevalent radiative materials, leading to an overestimation of the cooling power. Therefore, this work focuses on the theoretical evaluation of the radiative cooling power along with the interior temperature distribution under varying humidity conditions through incorporating coupled heat and radiation transfer approach. The results show that higher relative humidity leads to less cooling power and minor interior temperature difference in the radiative cooler. In addition, the position of the lowest temperature inside the radiative cooler, which is usually taken at the bottom of the radiative cooler, gradually approaches the surface facing outer space with the increase in humidity during the diurnal time. Our work provides a unique viewpoint for predicting the cooling power by taking interior temperature non-uniformity resulted from the low thermal conductivity into account, which elaborates a vivid picture of the evolution of radiative cooling power as well as the internal temperature distribution with the variation of humidity. Such an approach is beneficial for the further design, efficiency enhancement and on-site deployment of radiative coolers.