Passive and low energy cooling
The top four end-uses in the commercial sector are space lighting (20%), space heating (16%), space cooling (14.5%), and ventilation (9%). All together they represent 60% of commercial primary energy consumption (EIA, 2012). In hot climate countries, energy needs for cooling can amount to two or three times those for heating on an annual basis. In very hot climates such as in the UAE, 40% of the total cooling energy can be utilized to offset heat gains from walls and roofs, and it could reach to 75% when combined with the glazing effect (Aboul-Naga et al., 2000). The extensive use of air conditioning is associated with several problems:
• Environmental: extensive use of air conditioning has caused a shift of the energy consumption to the summer season with increased peak electricity demands. This imposes an additional strain on electrical national grids and requires development of extra new power plants. The increased energy production leads to exploiting the finite fossil fuels causing atmospheric pollution and climatological changes. Ozone layer depletion is caused by the refrigerants of the air conditioning units such as CFCs and HFCs. Heat rejection during the air conditioning process increases the phenomenon of the “urban heat island”. The heat island effect has a great impact in exacerbating cooling energy requirements in warm to hot climates in summer. For U.S. cities with populations larger than 100,000, the peak electricity load increases 2.5 to 3.5% for every ◦C increase in temperature (Akbari et al., 1992). It is estimated that 3 to 8% of the urban electricity demand in the U.S. is used to compensate for the heat island effect alone. For the USA, the electricity costs for the summer heat island alone could be over US$ 1 million per hour, or over US$ 1 billion per year (Akbari et al., 1992).