Award-winning environmentalist and sustainable living expert, ANIL K RAJVANSHI shows you how to set up a low cost, non-electrified airconditioning system for your house
During these times of COVID-19 pandemic and when air pollution and temperatures in cities is increasing and more and more people and families are indoors during lockdown, it is necessary for them to be comfortable. I would like to share our small experiment of developing a very economical and efficacious air conditioning system for households which does not require any electricity.
I stay in the rural town of Phaltan in western Maharashtra. Phaltan is about 100 km southeast of Pune. Phaltan weather is mild but in April and May, the air is dry and temperatures can reach up to 45 degree Centigrade.
For the last 37 years, I have been living in a house which is passively air-conditioned without using electricity. I designed and built the house and its cooling system in 1984.
The house has 16 inches thick stone walls and on the roof, we have put a green shade net on a simple angle frame to stop the sun heating up the roof’s surface. The surface is covered with discarded jute gunny sacks which are sprinkled with water once a day at around 2 p.m. in the afternoon. The shade net also stops the sun from heating the gunny sacks thus allowing the soaked sacks to remain cool for 24 hours.
This simple system cools the house very well. The evaporating water from the soaked gunny sacks cools the roof and the ceiling fan inside the room brings down the cool air. Around 80 per cent of the heat into the house comes from the roof. So a cool roof can easily bring down temperatures inside the room and make the living space very comfortable.
We also close the windows and draw the curtains in the morning so that the radiative heat and glare does not come inside the rooms. The trees surrounding the house also reduce the heat. Thus when the outside daytime temperatures are between 40 to 45 degree Centigrade, our house remains cool with inside temperatures of 28 to 30 degree Centigrade and under the ceiling fan it can sometimes even feel chilly.
The 16” thick walls provide thermal lag of about 10 to 12 hours and thus allow the heat to come inside the house through the walls only late at night. At that time with windows open, the outside cool night air can dissipate this heat rapidly. We also put a table fan in the window of our bedroom to pull in the cool night air. This cools the room really fast and most of the times we have to shut off this fan early morning around 4 am since it becomes very chilly.
This simple roof top cooling system is a highly effective air-conditioning system with little energy and monetary costs involved. The gunny sacks that cost Rs 10 per square metre last for about two years after which they need to be replaced. This is because the evaporating water leaves salts behind which rots the jute sacks. The water requirement is only 1.5 litres per square metre of roof. We feel that this system will also reduce our earth warming footprint.
The costing that I have done has shown that this system is one-tenth the cost of a regular electric air-conditioning (AC) system. Besides, when the electricity supply in most Indian cities is erratic, especially during the long hot summer months, the roof top evaporative system provides a very cost-effective and passive method of cooling the house. In addition, the capital cost of this system including the shade net and gunny sacks is only Rs 250 per square metre.
This system can also work well in existing houses which have thinner brick walls since 80 per cent of the total heat input comes from the roof only.
Generally in dry hot regions, ubiquitous desert evaporative coolers are used. These coolers work on the principle of water evaporation for cooling air. However, they increase the humidity inside the house which sometimes is not comfortable. With this roof cooling system, there is no effect on the humidity inside the house; in other words, the humidity does not increase.
Let me, however point out that this system works best in areas which have dry summers. With high humidity conditions, the roof evaporation will be less and hence there will be less cooling.
We also use this system in our offices and lots of people have copied this simple technique for cooling their homes and offices.
(Featured Image shows gunny sacks covering the terrace soaked with water and covered with green shade net)
Anil Rajvanshi is an IIT Kanpur graduate. After getting his Doctorate in Mechanical Engineering from USA he came back to rural India to run a rural NGO, Nimbkar Agricultural Research Institute, in Western Maharashtra. Besides his technology work he is also interested in spirituality and likes to be called a spiritual engineer.
Readers can contact the author at email@example.com
This article was first published in the South Asia Monitor