Numerical study and experimental validation of a combined diurnal solar heating and nocturnal radiative cooling collector

Neither solar collectors nor nocturnal radiative cooling units could work all day. A novel combined diurnal solar heating and nocturnal radiative cooling (SH-RC) collector was proposed and investigated in this study. The collector can obtain heat at daytime and gather cooling energy at nighttime, therefore offering multi-functionality and enhancing its time utilization ratio. A spatial distributed parameter mathematic model that considers the spectral radiant distribution was established to evaluate the performance of the collector. Besides, the precipitable water vapor amount was introduced to predict the spectral emissivity of atmosphere. Experiments were conducted to validate results obtained from the numerical simulation. The root-mean-square deviations between the experimental and simulation results are only 4.59% for thermal efficiency and 4.90% for cooling power. Based on the validated models, the thermal performance of the collector was investigated under different insulation thicknesses, wind velocities, ambient and inlet temperatures, water flow rates, precipitable water vapor amounts and solar irradiance. Annual performances of four Chinese cities were also conducted. The annual heat gains of the SH-RC collector in Beijing, Hefei, Fuzhou and Urumqi are 3328.76, 2423.53, 2543.31 and 3313.46 MJ/m2, respectively. The annual net radiative cooling gains of the collector in the four cities are 862.44, 831.54, 741.63 and 775.34 MJ/m2, respectively.