An Experimental and Analytical Study of a Radiative Cooling System with Unglazed Flat Plate Collectors

Abstract

On an average about 40% of world energy is used in residential buildings and the largest energy consumption is allocated to the cooling and air-conditioning systems. So every attempt to economize energy consumption is very valuable. In this research a nocturnal radiative cooling system with flat plate solar collectors in a humid area, Babol, Iran, is assessed both experimentally and numerically. Different methods available in the literature are reviewed and by using a widely accepted model, the sky temperature is determined. The mathematical model for a flat plate solar collector is used as a guideline to derive the governing equations of a night sky radiator. Then, a cooling loop, including a storage tank, pump, connecting pipes, and a radiator has been studied experimentally. The water is circulated through the unglazed flat-plate radiator having 4 m² of collector area at night to be cooled by convection and radiation to sky. The experiments were carried out at various mass flow rates and in different weather conditions and the results have been compared to those of the theoretical model. The results indicate that water temperature decreases 7--8°C and the average net cooling will be ranged from 23 to 52 W/m², as the mass flow rate increases from 0.01 to 0.05 kg/s.

Keywords:

• Radiative cooling
• Nocturnal cooling
• Unglazed collector
• Flat-plate collector