Passive Cooling for Buildings

Passive Cooling at BCA Academy ( Ref: https://architizer.com/idea/1054673/)

Building energy consumption is a vital component of the global energy mandate. A major part of the building energy is consumed in cooling the buildings and providing thermal comfort to occupants. There has been a drastic increase in the use of building energy for cooling the buildings through mechanical systems, which has led to environmental pollution resulting in global warming and ozone layer depletion.

In India, the building sector represents about 40% of total energy consumption and in recent years, the usage of air conditioning systems has been rapidly increase for the purpose of cooling the building which results in high energy consumption. Also, it is expected to further increase because of improving standards of living and increasing population. Moreover, air conditioning use has increasingly penetrated the market during the last few years and greatly contributes to the upsurge of absolute energy consumption.

Reduction of energy consumption in buildings can be achieved by simple methods and techniques using an appropriate building design, energy-efficient system, and low energy cooling techniques. Passive cooling and Active cooling are the two main types of cooling systems. In the Active cooling technique, mechanical energy in one or another form is used to cool the interior of the building (eg: Air- Conditioning) which requires a power source to provide the desired effect, whereas, Passive cooling technique is a natural method of cooling buildings is least expensive, and it mainly depends on the interaction of building and its surrounding. Passive cooling techniques can be a promising alternative to satisfy the cooling requirements of the building as well as to reduce the building energy consumption.

Passive Cooling strategies

Passive cooling is a building design approach that is used to control heat gain and promote heat dissipation in a building in order to improve thermal comfort with low or zero energy consumption. Passive cooling works either by preventing heat from entering the interior (heat gain prevention) or by removing heat from the building (natural cooling).  

Four major common strategies are discussed below.

1)    Natural Ventilation

There are two major techniques in natural ventilation systems: cross ventilation and single-sided ventilation. Cross ventilation is attained when rooms with a double orientation with at least two walls face externally in opposite directions, and single ventilation is achieved when there is only one external façade.

In the cross-ventilation system, the action of any wind will then generate pressure differences between those openings and so will promote a robust airflow through an internal space. But in the single ventilation system, wind-driven ventilation flow is dominated by the turbulence of the wind, as caused by temporal changes in wind speed and direction. Hence, cross ventilation is the design type of choice.

2)    Stack Ventilation

Stack ventilation (also known as stack effect or chimney effect) creates airflow using the natural force that emerges from changes in air pressure, temperature, and density levels between corresponding internal and external environments.

The effectiveness of stack ventilation is influenced by:

·         The effective area of openings.

·         The height of the stack.

·         The temperature difference between the bottom and the top of the stack.

·         Pressure differences outside the building.

Stack ventilation can be effective in tall buildings that include vertical spaces which rise throughout their height, for example, buildings with central atriums. Also, this can be useful in deep buildings, where cross ventilation may not be sufficient to penetrate to spaces in the heart of the building.

 3)    Evaporative Cooling

Evaporative cooling is a technique based on the effect of evaporation as a heat sink. The cooling of air is obtained as an amount of sensible heat is absorbed by the water and used as a latent source for evaporation.

The four major factors that affect the rate of evaporation are relative humidity, air temperatures, air movement, and surface area. Evaporative cooling can be direct or indirect. In the former, the water content of the cooling air increases, being the air in contact with the evaporated water. In the latter, the evaporation takes place inside a heat exchanger, without a change in the water content of the air.

 4)    Earth Coupling

Earth coupling is a technique that is used for passive cooling as well as heating of buildings, which is made possible by the earth acting as a massive heat sink. At depths beyond 4 to 5m, both daily and seasonal fluctuations die out and the soil temperature remains almost constant throughout the year. Thus, the underground or partially sunk buildings will provide both cooling (in summer) and heating (in winter) to the living space. A building may be coupled with the earth by burying it underground or berming.

In summary, implementing passive cooling systems in building design has many advantages over using the active cooling systems, as they produce no environmental impacts and Green House Gas (GHG) emissions. However, the incorporation of passive cooling techniques needs to be taken from the earliest design stages in order to reach its fullest possible potentials. Additionally, these systems need to be integrated within the design itself, rather than being a standalone or additional solution that is forced into a building.

Written by Inderjit Ahuja