Case Studies in Thermal Engineering (Dec 2021)
Double skin polystyrene- aluminium radiation reflector roofs in arid environments for passive cooling - A case study in Sohar, Sultanate of Oman
Abstract
Global warming increases the indoor temperature and energy consumption of residential buildings in arid regions. No doubt, development of effective passive cooling techniques are crucial to reduce the room air temperature as well as the energy consumption of buildings. The present study analyzes merits of a polystyrene foam - aluminium radiation reflector double skin system to be used as a passive cool roof in an arid region like Oman and assess the effect of air ventilation as a thermal barrier between the double skin layers. The present study incorporates a radiation reflective layer, dynamic air ventilation, and thermal insulation configurations. The study was performed in 3 stages for 0, 20 and 40 cm air ventilation between polystyrene-aluminium radiation reflector layers. It was confirmed by field experiments that a significant decrease in indoor and roof slab temperatures were observed for polystyrene-aluminium radiation reflector passive cool room in comparison to a traditional bare roof. The experimental studies reveal that comparing with a traditional roof, the room air temperature of double skin polystyrene-aluminium radiation reflector was reduced by 2.42 °C (6.7%), 3.69 °C (10.4%) and 4.7 °C (12.8%) for 0, 0.2 m and 0.4 m air ventilation between the polystyrene foam and aluminium reflector, respectively. The corresponding reduction in roof surface temperature was 6.91 °C (16.8%), 8.1 °C (24.5%) and 11.11 °C (31.9%) respectively. It was observed that the indoor air and roof surface temperature of the passive cool roof reduced as the air ventilation increased from 0 to 0.4 m. This proves that dynamic nature of air ventilation has a major role in improving the natural convective heat flux and thereby increase in thermal resistance of double skin roof. Also, Aluminium radiation reflector of the double-skin roof has high solar reflectance (albedo) which results in reduced short wave and long wave radiative heat flux to the ceiling in comparison with the conventional roof. Integration of polystyrene foam thermal insulator, air ventilation and aluminium radiation reflector in the passive cool roof system reduces conduction, convection, and radiation heat transfers, respectively between the roof and ceiling. It is conclusively proven that the double-skin polystyrene-aluminum radiation reflector passive cool roof with air ventilation can appreciably lower the room air and roof slab temperatures during hot sunny days.
