Full-scale experimental studies of a passive cooling roof in hot arid areas

Abstract

A full-scale evapo-reflective roof for 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 aluminum 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 aluminum sheet falls below the temperature of the rock bed mixed with water. Water vapor 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. A full-scale experimental study of passive cooling roof was carried out for a typical summer day of June at Laghouat in Algeria. 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 aluminum plate. The upper surface of this plate is painted with a white titanium-based pigment to increase the radiation reflection process during daytime. Several passive modifications have been introduced to the roof in order to reduce indoor air temperature in hot climates. An experimental investigation, employing a passive procedure, has been carried out to study the possibility of reducing air temperature in buildings. The results show that the air temperature can decrease within a range from 6 to 10 K. This decrease can further be lowered by 2–3 °C if night natural ventilation of buildings is allowed.