Bingo 2 CTC

The Rainmaker Design

A

Sometimes ideas just pop up out of the blue. Or in Charlie Paton’s case, out of the rain. “I was on a bus in Morocco travelling through the desert,” he remembers. “It had been raining and the bus was full of hot, wet people. The windows steamed up and I went to sleep with a towel against the glass. When I woke, the thing was soaking wet. I had to wring it out. And it set me thinking. Why was it so wet?”

B

The answer, of course, was condensation. Back home in London, a physicist friend, Philip Davies, explained that the glass, chilled by the rain outside, had cooled the hot humid air inside the bus below its dew point, causing droplets of water to form on the inside of the window. Intrigued, Paton – a lighting engineer by profession – started rigging up his own equipment. “I made my own solar stills. It occurred to me that you might be able to produce water in this way in the desert, simply by cooling the air. I wondered whether you could make enough to irrigate fields and grow crops.”

C

Today, a decade on, his dream has taken shape as a giant greenhouse on a desert island off Abu Dhabi in the Persian Gulf – the first commercially viable version of his “seawater greenhouse”. Local scientists, working with Paton under a licence from his company Light Works, are watering the desert and growing vegetables in what is basically a giant dew-making machine that produces freshwater and cool air from sun and seawater. In awarding Paton first prize in a design competition two years ago, Marco Goldschmied, president of the Royal Institute of British Architects, called it “a truly original idea which has the potential to impact on the lives of millions of people living in coastal water-starved areas around the world.”

D

The seawater greenhouse as developed by Paton has three main parts. They both air-condition the greenhouse and provide water for irrigation. The front of the greenhouse faces into the prevailing wind so that hot dry air blows in through a front wall. The wall is made of perforated cardboard kept moist by a constant trickle of seawater pumped up from the ocean. As the hot air blows through the damp cardboard, the water evaporates, and the air temperature drops while its humidity increases. The air then passes through the growing area, where the plants transpire moisture into it.

E

The second feature also cools the air for the plants. Paton has constructed a double-layered roof with an outer layer of clear polythene and an inner, coated layer that reflects infrared light. Visible light can stream through to maximise photosynthesis, while heat from the infrared radiation is trapped in the space between the layers, away from the plants.

F

At the back of the greenhouse sits the third element, the main water-production unit. Just before entering this unit, the humid air of the greenhouse mixes with hot, dry air from between the two layers of the roof. This means the air can absorb more moisture as it passes through a second moist cardboard wall. Finally, the hot saturated air hits a condenser. This is a metal surface kept cool by still more seawater – the equivalent of the window on Paton’s Moroccan bus. Drops of pure distilled water form on the condenser and flow into a tank for irrigating the crops.

G

The greenhouse more or less runs itself. Sensors switch everything on when the sun rises and alter flows of air and seawater through the day in response to changes in temperature, humidity and sunlight. On windless days, fans ensure a constant flow of air through the greenhouse. “Once it is tuned to the local environment, you don’t need anyone there for it to work,” says Paton. “We can run the entire operation off one 13-amp plug, and in future we could make it entirely independent of the grid, powered from a few solar panels.”

H

The net effect is to evaporate seawater into hot desert air, then re-condense the moisture as fresh water. At the same time, cool moist air flows through the greenhouse to provide ideal conditions for the crops. The key to the seawater greenhouse’s potential is its unique combination of desalination and air conditioning. By tapping the power of the sun, it can cool as efficiently as a 500-kilowatt air conditioner while using less than 3 kilowatts of electricity. In practice, it evaporates 3,000 litres of seawater a day and turns it into about 800 litres of fresh water – just enough to irrigate the plants. The rest is lost as water vapour.

I

Critics point out that construction costs of £25 per square metre mean the water is twice as expensive as water from a conventional desalination plant. But the comparison is misleading, says Paton. The natural air conditioning in the greenhouse massively increases the value of that water. Because the plants need only an eighth of the water used by those grown conventionally, the effective cost is only a quarter that of water from a standard desalination plant.

J

The greenhouse Paton built is installed with fans to keep the air flowing if the wind stands still, and it is expected in the future to rely on electricity provided solely by solar panels. Despite the high construction costs compared to a desalination plant, the plants grown in Paton’s greenhouse need much less water, and if produced in large quantities the construction costs could be reduced remarkably. In addition to all these advantages, it is also environmentally friendly because it is clean and pollution-free.