Building energy savings using high-albedo-high-emittance (cool) roof materials
Zingre, Kishor Tarachand
Date of Issue2014
School of Mechanical and Aerospace Engineering
In the tropics, the earth surface receives abundant solar radiation annually contributing significantly to building heat gain and, thus, cooling demand. An effective method that can curb the heat gains through opaque surfaces could provide significant energy savings. This study investigates the effectiveness of “high-albedo-high-emittance (or cool) roof” on reducing heat gain through opaque surfaces into buildings. It is hypothesised that building heat gain can be reduced by increasing the albedo of opaque surfaces to reflect off the incident solar radiation. At the same time, by increasing the thermal emittance to emit off the absorbed heat (as infrared radiation) to outdoor. Computational simulations (calibrated using experiments) are conducted to compare the thermal performance of a cool concrete flat roof to that of a green roof and a thermal insulation roof under the tropical climate of Singapore. The cool roof provides higher annual net heat gain reduction of about 81-83% as compared to the green roof (about 60-75%) and the thermal insulation roof (about 68-80%). A novel and general cool roof heat transfer (CRHT) model that is based on the spectral approximation method is proposed. The CRHT model provides very concise and easy-to-use expressions to evaluate the impact of cool coating on roof temperature, ceiling temperature, indoor air temperature and heat flux on single-skin roof (SSR, i.e., solid primary roof with no air-gap between roof layers) and double-skin roof (DSR, i.e., has an air-gap in between two solid roof layers). The model can handle transient outdoor and indoor boundary conditions as experienced by naturally ventilated buildings. The CRHT models are verified against analytical (the conduction transfer function) method and validated against experiments performed in real-scale apartments in Singapore. On a sunny day, cool coating reduces the peak temperatures and daily (integrated) heat gain of a concrete flat SSR by up to 14.1oC and 0.74 kWh/m2 (or 58%), respectively. The same cool coating reduces the peak temperatures and daily heat gain of a concrete flat DSR by up to 14.7oC and 0.21 kWh/m2 (or 47%), respectively. A new roof thermal transfer value (RTTV) model is proposed based on the CRHT model to accurately assimilate the effect of solar reflectance changes (due to application of cool roof) into RTTV or equivalent models for air-conditioned buildings. The proposed RTTV model incorporates new formulations for modelling the equivalent-thermal resistance increment due to the solar reflectance effect on opaque roofs. The new model shows significant improvement in accurately capturing the cool roof effect in RTTV calculation compared to existing model. This study sheds new insights on cool roof performance in tropical climate. The proposed CRHT and RTTV models provide the foundation for accurate modelling of cool roof heat transfer characteristics. These proposed models are also applicable to other climates.