Of the many different methods to reduce the cooling load, passive cooling of buildings is the most suitable and sustainable method. Passive systems use non mechanical methods to maintain a comfortable indoor temperature and are a key factor in mitigating the impact of buildings on the environment.
According to the International Institute of Refrigeration (IIR), there are more than 240 million air-conditioning units installed worldwide. IIR’s study shows that the refrigeration and air conditioning sectors consume about 15% of all electricity consumed worldwide (IIR, 2002). It is evident that the total energy consumption of buildings for cooling purposes varies as a function of the quality of design and climatic conditions. Therefore, sustainable architecture can have a large impact on environmental sustainability. It is an approach where building technology is integrated with the concept design to reduce the need for high-tech systems and the energy consumption of buildings.
Traditional Cooling Systems
Passive Downdraft Evaporative Cooling (PDEC) technique was extensively used in the vernacular architecture of Pakistan, Iran, Turkey and Egypt. The tradition of ‘cooling without air-conditioning’, has its origins in ancient Egypt; it subsequently spread eastwards through the Middle East and Iran to north India with the Mughal empire and westwards across North Africa to southern Spain. In the Middle East there is a long tradition of using various techniques to encourage evaporative cooling both within and between buildings.Wind catchers called ‘malqafs’ captured wind and directed it over porous water pots, causing evaporation and bringing a drop in temperature as a result of latent heat of vaporization (Ford, 2001). This system maintained a balance between two important parameters of passive cooling – thermal performance and ventilation effectiveness. In this tradition, wind-catchers guide outside air over water-filled porous pots, inducing evaporation and bringing about a significant drop in temperature before the air enters the interior.
In north India, the Mughal palaces and gardens exploited evaporative cooling to delight the eye and other senses as well as providing thermal relief. Thin water chutes (salsabil) and other evaporative cooling techniques were features of Mughal architecture from the thirteenth to the seventeenth centuries. The intense dry heat and dust of the summer in north India calls for the creation of an internal refuge or haven from the extremes of the external world. The diurnal swing in temperature is dampened by the mass of stone and earth, and the air is further cooled by the evaporation of water in the ventilation air flow path. This is exemplified perfectly in the beautifully atmospheric Rai Pravina Mahal in Orcha ((Ford and Hewitt, 1996).
Passive Evaporative Cooling
Passive-cooling techniques concentrate mainly on reducing unwanted heat gains into the building. In the twentieth century, evaporative
cooling was applied in buildings throughout the world in conjunction with a mechanically driven air supply (known widely as desert coolers). Recently, attention has returned to the potential of exploiting the benefits of direct evaporative cooling while avoiding mechanical assistance by using buoyancy or wind forces to drive the air flow. In the late 1980s, a number of successful experiments were undertaken which tested the evaporation of water within a downdraught tower, hence the term Passive Downdraught Evaporative Cooling (PDEC).
The device consists of single or multiple towers equipped with a water vapour supply placed on the top. This innovation consists of replacing the wetted pads with rows of atomisers (nozzles, which produce an artificial fog by injecting water at high-pressure trough minute orifices). During the constant injection of water, droplets descend through the tower and conditions close to saturation along its length. Cool air descends the tower and exits at its base where it is delivered to the adjacent spaces. The situation of the micronisers in a tower gives rise to a naturally downdraught effect.
In hot climates, commercial buildings with appropriate heat and solar protection and careful management of internal loads may reduce their cooling load down to 5 kWh/m2/year, while buildings of low quality environmental design may present loads up to 450 kWh/ m2/year (Santamouris and Daskalaki, 1998).