Dr. Mohammad Arif Kamal, Department of Architecture, Aligarh Muslim University evaluates building integrated passive cooling techniques and their significance in climatic control and conserving energy in buildings.
Ancient architecture has used various passive techniques to restrict the flow of heat to and from a building. But recently the emphasis on these passive cooling techniques has been neglected due to the availability of air-conditioning systems. 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. The problems associated with the use of air-conditioning vary from the serious increase in the peak electricity load due to the absolute energy consumption of buildings and poor indoor air quality to emissions from refrigerants used in air-conditioning adversely impacting ozone levels and global climate.
Evaporative cool i ng is a passive cooling technique in which outdoor air is cooled by evaporating water before it is introduced in the building whose physical principle lies in the fact that the heat of air is used to evaporate water, thus cooling the air, which in turn cools the living space in the building. It is a low energy passive system. There are basically two methods of evaporative cooling: direct and indirect evaporative cooling.
In direct evaporative systems (e.g. desert coolers used in North India), the main disadvantage is the increased moisture content of the ventilation air supplied to the indoor spaces. High evaporation may result in discomfort due to high humidity.
Passive Downdraught Evaporative Cooling
The system consists of single or multiple towers equipped with a water vapour supply placed on the top. Functioning like wetted pads of desert cooler, are the 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 concept is based on the relatively large amount of energy required to convert water from its liquid to gaseous form within a local thermal imbalance with subsequence differences in air density. This leads to the movement of air from a zone of high pressure, where air is hot and less dense (top of the tower) to a zone of lower pressure, where air is colder and denser (bottom of the tower).
These towers are often described as reverse chimneys. While the column of warm air rises in a chimney, in this case the column of cool air falls. The air flow rate depends on the efficiency of the evaporative cooling device, tower height and cross section, and resistance to air flow in the cooling device, tower into which it discharges.
Limitations: The hardness of water is a significant factor, therefore water quality has to be good otherwise nozzles will block. High pressures (>40 Bar) are required to minimize water droplet size and maximize evaporation, which implies more expensive pumps and plumbing. The risk of microbiological contamination of the water supply to the misting nozzles must also be minimized. This can be addressed by a combination of design measures (including the use of UV filters in the supply line to the micronizers), regular maintenance, and testing, but it would clearly be better if this was not an issue. In many parts of the world, the potential disadvantages of using micronizers (risks of microbiological contamination, blockage of micronizers, high-pressure stainless-steel plumbing fittings etc.), are a powerful disincentive.
The indirect system includes cooling the roof with a pond, wetted pads or spray, transforming the ceiling into a cooling element that cools the space below by convection and radiation without raising the indoor humidity.
Sustainable buildings are related to the notion of climate-responsive design, which aim to achieve building comfort through the interaction with the dynamic conditions of the building environment. In hot climates, commercial buildings with appropriate heat and solar protection and careful management of internal loads may reduce their cooling load down to 5kWh/m2/year.
Roof Surface Evaporative Cooling
In a tropical country like India, the solar radiation incident on roofs is very high in summer, leading to overheating of rooms below them. Roof surfaces can be effectively cooled by spraying water over suitable water-retentive materials spread over the roof surface.
As the water evaporates, it draws most of the required latent heat from the surface, which acts as a radiative cooling panel for the space, thus lowering its temperature and reducing heat gain. The indoor temperature gets lowered without elevating the humidity level. The solar radiation falling on the water film is utilized in water evaporation and thus being prevented from entering the room below. Besides, evaporation also cools the air above the roof. The cool air slides down and enters the living space through infiltration and ventilation, providing additional cooling. The effectiveness of the technique depends on:
- Ambient air temperature and humidity
- Intensity of solar radiation
- Wetness of the roof surface
- Roof type
Limitations: The roof has to be kept wet throughout the day using a water sprayer (manual or automatic) and a suitable waterproofing treatment of the roof is essential. The roof must be covered with water absorptive and retentive materials such as brick ballast, sintered flyash, coconut husk or coir matting which on account of their porosity, when wet, behave like a free water surface for evaporation, but they have to be treated for fire safety. During the peak period of summer, the quantity of water needed can be approximately 10 kg/ day/ m2 of roof area.
The PDEC and RSEC can be considered the most sustainably viable methods, especially for hotdry and composite climatic regions in India where the cooling requirement is around 6-7 months in a year. Lowtech PDEC solutions may be more appropriate in locations where water quality is poor or where high-pressure plumbing is unfamiliar. The practical integration of such systems within the building envelope is fundamental to the feasibility of this approach. If simpler techniques currently under investigation do prove technically and financially viable, the market potential could be significant.