Passive cooling is a non-mechanical means of cooling homes, using an approach to building design that controls the heat entering the structure and encourages its dissipation. It is a concept that reduces the need for energy-intensive systems like air-conditioning, focusing not only on the architectural design of the building, but also the resources used and its natural surrounds to absorb or dissipate heat.
Passive cooling can be divided into two main techniques – preventative and modulation/heat dissipation – while hybrid cooling systems use simple mechanisms such as pumps and economisers that are integrated into a passive cooling system to enhance its effectiveness.
Preventative techniques shield the building against heat gains and include design principles that take into account the microclimate of the site, with factors such as sun and wind used to determine the application of selected cooling strategies. Shading the structure to minimize the solar radiation that can penetrate it is one means, together with selecting a building orientation and zoning of interior spaces that reduce overheating. Flat, horizontal designs can increase the effectiveness of cross-ventilation, while multi-level buildings can take advantage of natural temperature stratification, with the upper levels generally warmer.
Thermal insulation is a common way of reducing the amount of heat that can penetrate the interior of a building and should be applied not only to roofs and walls, but also windows. Other factors to consider are the use of energy-efficient lighting and electronic equipment to reduce the heat released within internal areas, and the implementation of blinds to reduce solar heat gains through windows.
Modulation and Heat Dissipation Techniques
Modulation and heat dissipation techniques enable a building to store heat in so-called natural ‘heat sinks’ and release it to the surrounding climate. Natural sinks may include the earth’s soil or the night sky, as well as the building’s thermal mass itself.
For example, the thermal mass of a building can absorb and store heat during daytime hours, then release it into the night sky using natural cooling strategies. Natural ventilation such as open windows can release heat, or a cross-ventilation system employed which requires openings on two sides of the space to allow wind to pass through the building. Stack ventilation is another alternative design strategy, taking advantage of the buoyancy of warm air which rises and filters out through ceiling openings, allowing cooler air to enter near the floor.
Night ventilation (or night flushing) is another heat dissipation technique, with the building closed during the day to prevent heat gain (when it acts as a sink), then opened at night when the air is cooler, flushing out heat through natural convection. This is best implemented in areas which have a large difference between maximum and minimum temperatures and most effective when the building has been designed so that its thermal mass absorbs the heat gains of its space. Night cooling can be implemented by opening windows at night to let the natural airflow cool the space, by mechanically forcing air through ventilation ducts at night, or a combination of the two.
Harare’s Eastgate Centre is a good example of night flushing, with the building absorbing a minimum amount of heat during the day and using cool nighttime air temperatures to dissipate heat build up. Its innovative design was inspired by the ventilation passages and chimneys of a termite hill in a design concept known as biomimicry.
Some modern forms of passive cooling design draw on ancient techniques, including the use of passive downdraft evaporative cooling. This technique was inspired by concepts used in traditional architecture from Pakistan, Iran and Egypt, with wind-catching hoods known as malqafs pulling air down and cooling it across a pool or fountain. In modern incarnations, the water source is replaced with wet cellulose pads, as seen in the Torrent Research Centre in Ahemedabad, India.
In Toronto, water has also been used as a natural cooling mechanism, being pumped from 5 kilometres below the surface through metal coils to office buildings where fans blow it into their climate-control systems. While this is a hybrid cooling system (rather than passive), it does significantly reduce the energy demands of cooling the office buildings.
Earth coupling is another means of passive cooling, using the consistent temperate of soil to act as a heat sink and cooling the building through conduction. This is especially efficient in hot climates where the earth temperate is significantly cooler than the air. This can either be employed by direct contact with the earth as a wall buffer, as seen in earth sheltering, or using buried tubes known as earth ducts which transfer heat between the building and soil.
How can Passive Cooling be Used in the Developing World?
The main advantage of implementing passive cooling systems are the energy-saving costs, not only on the environment but also in terms of monetary expense to the occupants. In developing countries where resources are limited and people don’t have the means to invest in mechanical systems such as air-conditioning and fans, it’s a practical way to create more comfortable living conditions.
In some countries, such as India, where temperatures can reach unbearable levels and air-conditioning sales have sky-rocketed in recent years, the demands on the energy supplies can get too much. Major power blackouts in the country during the height of summer have indicated that the demand to cool homes using mechanical means are struggling to be met by the energy resources available.
As home to many of the world’s hottest and most rapidly-growing cities, with expanding middle classes that are purchasing air-conditioning systems, the developing world is experiencing an overwhelming demand on its energy supplies. In some countries it has become a status symbol and, coupled with the rejection of traditional building materials such as mud and bamboo (many of which were sustainable and employed natural passive cooling) in favour of concrete, the requirements on energy resources have escalated.
When you take into account that the total world air-conditioning use annually is more than double the total energy consumption of the African continent, it’s easy to see the problem being faced. Air conditioners are contributing significantly to the effects of global warming, not only by burning coal or gas to fuel them, but also the greenhouse gases emitted from the refrigerants in air-conditioners.
But as passive cooling is being embraced and utilized by architects and engineers, it may be the answer to reducing the energy demands of air-conditioning systems, while ensuring comfortable indoor temperatures across the developing world. The techniques being employed are both old and new, drawing on traditional building methods using modern knowledge and materials.
But its implementation draws largely on governments enforcing stricter building standards to reduce energy-intensive designs and in training architects to utilize passive cooling techniques. Air-conditioning has allowed them to create building designs that don’t take into account their natural surrounds or climatic conditions in the past. But there needs to be an increased understanding and consideration of these processes if we are to address the growing energy demands in the world today.