Experimental and theoretical study on cooling performance of membrane roof with circulated water film

Abstract

Membrane roofs are widely used in buildings for excellent daylight and visual effects. Nevertheless, excessive solar penetration results in an overheated indoor thermal environment. The experimental and theoretical study on the cooling performance of circulated water film technology on membrane roofs was conducted. The scale experiment results indicated that with circulated water film the membrane surface temperatures decreased rapidly to a level slightly above the water temperature, where the polyvinylidene fluoride (PVDF) and ethylene tetrafluoroethylene (ETFE) membranes were 0.4 and 2.4 °C higher than water temperature. While the indoor air temperature gradually decreased in instances where PVDF roofs were utilized, the application of ETFE roofs did not yield a significant decrease in temperature. Subsequently, we derived an analytical solution for the dynamic two-dimensional heat and mass transfer process of the water film flowing on the membrane with incident solar radiation, which was validated by experiment. The analysis of influential factors indicated that the temperature drop increased with absorbed solar radiation and outdoor dry-bulb temperature, and decreased with the ambient wet-bulb temperature. Conversely, the cooling performance exhibited minimal sensitivity to variations in water film thickness. The indoor operative temperature dropped by 0.5–0.8 °C in the membrane roofed atrium with water film. The water film effectively cools the indoor thermal environment with membrane roofs.