Passive cooling by evapo-reflective roof for hot dry climates

A dynamic mathematical model for an evapo-reflective roof to improve space cooling in buildings for hot arid climates has been developed. The proposed roof design is composed of a concrete ceiling over which lies a bed of rocks in a water pool. Over this bed is an air gap separated from the external environment by an aluminium plate. The upper surface of this plate is painted with a white titanium-based pigment to increase reflection of a radiation to a maximum during the day. At night, the temperature of the aluminium sheet falls below the temperature of the rock bed mixed with water. Water vapour inside the roof condenses and falls by gravity. This heat pipe effect carries heat outwards and cold inwards. Heat exchange is improved by radiation between two humid internal surfaces. The efficiency of this cooling system is studied using finite difference method. Numerical calculations performed for different external temperatures and solar radiation show that the cooling produced by such a system is significant. As a result of this, the mean air temperature in the room may be kept a few degrees above the minimum nocturnal outdoor temperature throughout the day. However, the maximum indoor air temperature was observed at sunset. This could further be lowered by allowing ventilation of the building in the evening. The work is continuing.