In the last two decades, the Indian metros have witnessed a severe energy crisis especially during the peak summer season. This is primarily due to cooling load requirements of buildings. There has been a drastic increase in the use of air-conditioning systems for cooling buildings everywhere and needless to say, the ever increasing consumption of energy has also had its impact on the environment.
In this background, passive cooling systems, which are non-mechanical methods to maintain a comfortable indoor temperature, could become a key factor in not only mitigating the energy crisis, but also reduce the impact on the environment. Passive cooling uses free, renewable sources of energy such as the sun and wind to provide cooling, ventilation and lighting needs for a household. This additionally removes the need to use mechanical cooling. There are many types of passive cooling strategies that can be recommended for use in a hot climate such as Hyderabad. Design strategies that minimise the need for mechanical cooling systems include proper window placement and daylight design, the selection of suitable glazing for windows or skylights, proper sized shading of glass when heat gains are being avoided, the use of light or reflective-coloured materials for the building envelope and roof, careful sitting and wise orientation decisions alongside appropriate landscaping design.
The passive design approach can include the structure of the building itself, including building orientation, window placement, skylight installation, insulation and building materials, or specific elements of a building, such as windows and window shades. Building envelopes also play a major role in improving the overall energy efficiency as well as in providing a healthy indoor air quality for occupants in a building.
Thermal insulationAs most of the buildings today have glass façade, a high performance glazing can be used which can perform better than normal glazing in terms of thermal insulation and solar heat control.
The high performance glass wall system offers good thermal and optical performance to lower cooling load, reduces the reliance on artificial lighting and hence reduces energy consumption. Along with high-performance glazing, an envelope provided by external shading devices contributes to the ultra low overall thermal transfer value of the building.
External shading devices also play a major role in effectively reducing solar heat gain and glare of the building envelop. Deep overhang projection on the south-east facade blocks high-angled sun. Trellises providing additional shade on the south-east façade can rule heat island effect. Vertical shading fins on the north-west facade block the low angle sun in the late afternoon. The lower the heat gain of a building, the higher would be the building energy efficiency.
Optimisation of Window to Wall Ratio (WWR) can contribute a lot in natural thermal comfort scale. The WWR adopted effectively reduces the solar heat gain and the cooling load for air-conditioning while maximising natural ventilation and good views. High WWR for the north-west façade, coupled with fritted glass and external shade, with high transparency to enhance view and north light. High WWR for the south-east façade, coupled with the deep overhang, for good views and large open able windows for cross ventilation provides a comfortable environment.
Low WWR for south-west facade (about 10 per cent) and north-east facade (<25 per cent) can minimise heat gain of the building. Light reflection by light shelf onto light-coloured and angled ceiling soffit improves daylight distribution.
The use of natural light reduces the electricity demand and the cooling load from artificial lighting. Use of cool paint can add to the energy efficiency, it reflects and emits the sun’s heat back to the sky, reduces surface temperature by up to 5° C.
Passive solar energy-efficient building design should be the first aim of any building designer, because, in most cases, it is a relatively low-cost exercise that will lead to savings in the capital and operating costs of the air-conditioning plant. Incorporation of these passive cooling techniques would certainly reduce our dependency on artificial means for thermal comfort and minimise the environmental problems due to excessive consumption of energy and other natural resources and hence will evolve a built form, which will be more climates responsive, more sustainable and more environmental friendly for tomorrow.
(The writer is an architect and environmentalist. She can be reached at tapaswini.03@gmail.com)
