Experiments with a ‘psychrometric’ roof pond system for passive cooling in hot-arid regions

Roof pond systems for passive cooling of buildings are often limited in their efficiency because of solar heating loads or insufficient ventilation. Here we test the performance of a roof cooling configuration that resembles a naturally-ventilated psychrometer – designed to maintain a temperature close to that of the ambient wet-bulb by employing an elevated shading structure which allows for a free flow of air and maximizes evaporation. Comparing the performance of this ‘psychrometric’ roof pond (PRP) with a control roof under hot-dry conditions, it was found that the internal surface temperature of the PRP was up to 15 °C lower and that cooling was equally effective whether the roof pond was fully or partially exposed to the night sky. Throughout the hours of a summer day, air temperature in the cell was maintained within accepted comfort limits while the reference cell was continuously overheated. An analysis of energy fluxes above the roof showed that both the modified radiation balance and the conversion of sensible to latent heat through evaporation were significant contributors to the dramatically reduced heat gain and cooling effectiveness. Due to its low water requirement and simplicity, the tested configuration could have a significant potential for resource-efficient cooling.