08 Jun 2016 20:03 IST

To kill heat, bury it

The temperature difference between surface and earth is being tapped as an alternative to ACs and heaters

The outdoor unit of an air-conditioner, as we all know, throws out heat. The AC machine cools your room by sucking out the heat and spewing it outside. If the ambient temperature is not very high, the ‘outside’ absorbs heat easily; conversely, if it is hot outside, the machine has to work harder. And the harder it works the more electricity it consumes.

So somebody thought, why not bury the heat?

It’s not a radically crazy idea. After all, the temperature below the ground, at depths of about 20 feet, is a lot cooler than at the surface in summers, and a lot warmer during winters.

So, the hot air thrown out by the AC’s outdoor unit can be conducted through a pipe, which is inserted 20 feet into the ground. The earth absorbs the heat, which cools the air, and it is then conducted back into your room — closing the loop — thereby reducing the load on the air-conditioner. If there is a small pond or a well nearby, the air can be dumped into it. Pretty cool, (literally) right?

Geo exchange tech

A Mumbai-based start-up called GIBBS has been making systems that bury hot air from air-conditioners into the ground. It works on a principle called geo-exchange.

As a thumb rule, where there is a temperature difference (between surface and underground), there is an opportunity to save energy. And that is exactly what geo-exchange technology makes use of.

Appliances called heat pumps, common in Europe and entering India now, work on this principle. They are so called — heat pumps — because of their primary use, particularly in Europe, which has been not cooling, but heating. In winters, the pumps push cold ambient air into the warmer sub-surface, and warm air is brought back into the rooms to aid heating.

Go deeper into the earth, say several thousands of feet and you can meet pockets that are extremely hot. Ask those who drill for oil. There are, what are known as, ‘high pressure, high temperature’ (HPHT) wells, rich in oil and gas but very difficult to tap into.

Typically, the HPHT wells are located deep inside the earth. But you don’t always have to dig too much to locate a heat source. A farmer in Rajasthan’s Bichardi village (near Beawar) has found, to his dismay, that the water he pumps out to irrigate his field is hot!

But if you harness this underground heat, you make money. Many do. Spread across several countries are around 70,000 MW of electricity generation capacity plants based on this ‘geothermal’ source.

Indian scene

India has no geothermal electricity plant as yet. After the advent of GIBBS, there have been a few geo-exchange units. But it must be remembered that geothermal is a source of clean, continuous energy and geo-exchange is a great way of slashing electricity consumed by air-conditioners, especially in large buildings.

Will India see more of these in the coming years? Looks like it will.

The government is seriously considering geo-exchange technology, and a policy is in the works. A few days back, the Ministry of New and Renewable Energy brought out a draft of the Indian Geothermal Energy Development Framework. It speaks of opportunities, data, incentives, and so on. It is an interesting document, which gives some spicy facts — like 340 hot springs have been identified in India, that contain temperatures ranging between 35° and 98°.

Risky and unexplored

Geothermal is a high-tech affair that calls for big-bucks, and is risky because it is an uncharted territory. However, geo-exchange shows a lot of promise.

Home and office buildings account for 30 per cent of India’s electricity consumption. Almost 55 per cent of the energy consumed in buildings are for heating, ventilation and air-conditioning. And since 70 per cent of the buildings that India will have in 2030 are yet to be built, there is immense potential for energy saving here.

Truly tapping into this energy source is constrained only by our imagination. Where possible, you can bury the heat from the AC outdoor unit. But you can also use the hot air to, say, warm water and milk in your kitchen. Or just pipe the hot air into your overhead tank — your geysers will consume less electricity. If you are a hotel, you could bubble it into the swimming pool and keep the water nice and warm. The uses are many. Question is: will we tap it?

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