Nocturnal Passive Cooling By Transpired Solar Collectors

A novel use of the commercially available transpired solar collector is presented in the current paper. The reliable solar air heating system loses heat to the night sky if mounted on a building roof so it can be used as a passive cooling system based on thermal radiation. The collector plate cools down below ambient temperature and has the potential to cool the air as it is drawn through the perforations by a fan. A model has been elaborated for the cooling process based on heat transfer between the system components and energy balance equations. A method has been developed in order to choose the most suitable equivalent sky temperature model, as the radiative heat flow to the sky is the driving force of the cooling process and thus its accuracy is of utmost importance. The model has been validated by a series of field measurements carried out using a 5 m(2) setup in Edirne, Turkey. It has been found that the collector plate cools down up to 4.3 K below ambient temperature and it has the potential to cool air by up to 4.0 K. The system reached a maximum cooling performance of 66.5 W/m(2), while the average cooling performance was 34.6 W/m(2). It has been found that the collector plate cools down below ambient temperature an hour before sunset and does not reach ambient until one hour after sunrise under clear sky. A new Nusselt number correlation has been developed for the convection heat transfer between a perforated plate and the transpiring air flow.