Theoretical and experimental analysis on the passive cooling effect of transpired solar collectors

The present study provides results about the experimental performance of the transpired solar collector's passive cooling effect for the first time. In order to see the cooling performance of the transpired solar collector, a slope-adjustable experimental setup was built on the campus of Trakya University, Engineering Faculty, Edirne, Turkey. Solar radiation, ambient temperature, absorber temperature, cavity and back plate temperatures were monitored during summer period for different collector tilts. A physical and a mathematical model have been created to describe the heat transfer processes in the collector. The models were used to evaluate the measured data. It has been found that a natural airflow comes to be through the perforated plate, which acts similarly as in open-end double layer roofs. This airflow discharges the heat from the plenum decoupling the back plate from the exposed perforated plate. The temperature of the back plate, which represents the roof under the transpired collector, is significantly lower compared to that of the exposed roof on a typical sunny day. Similarly, the heat gain of the back plate is remarkably lower than the solar radiation received on the exposed roof. It has been found that the passive cooling effect of the transpired solar collector is increasing with rising intensity of radiation, as the heat transfer coefficient between the plenum and the back plate decreases with increasing solar radiation. Due to the natural character of the airflow in the plenum, wind heat losses are strongly dependent on the airflow's characteristics. Convective losses of the perforated plate have been defined by using two methods energy balance equations and a referenced heat loss correlation.