SOMETIMES ideas just pop up out of the blue. Or in Charlie Paton鈥檚 case, out of the rain. 鈥淚 was in a bus in Morocco travelling through the desert,鈥� he remembers. 鈥淚t 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?鈥�
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. 鈥淚 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.鈥�
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 鈥渟eawater greenhouse鈥�. 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 fresh water 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 鈥渁 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鈥�.
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Paton first tried out his seawater greenhouse in 1995 at Grenadilla on the Canary island of Tenerife. This arid island is criss-crossed with abandoned aqueducts that kept it green and productive until the mid-20th century, when the thirst of a burgeoning tourist industry emptied rivers and lowered water tables. Paton鈥檚 project, funded by a European Union grant, might have revived the island鈥檚 agriculture. And tourists would probably have welcomed an alternative to the increasingly saline water trickling out of the island鈥檚 taps. But the EU pulled the plug on the project for political reasons, after what Paton insists were three successful years of crop production (麻豆传媒, 5 December 1998, p 50).
He refused to give up. His friend Davies turned the data from Tenerife into a model of the heat and energy flows in the greenhouse. This led to an improved design, the 45 by 18 metre greenhouse in Abu Dhabi that is now filled with cucumbers, tomatoes, rocket and flowers. The first year of trials ends in February, and Paton believes the technology is about to take off commercially. 鈥淢y patrons in the Gulf are talking about building 400 of these greenhouses. I have a project in the Caribbean ready to go, another in Oman and a third in South Africa,鈥� he says.
Independent observers are very optimistic. The Tenerife greenhouse could theoretically produce a maximum of 40 litres of water a day for every square metre of the greenhouse, and it regularly produced more than 20 litres, says Phil Harris, a plant scientist at Coventry University who studied the project. 鈥淭hink about it. That means that, sitting in the desert, it produces five times as much water as falls on Coventry. That puts it up there with some of the wettest places on Earth, such as the wetter rainforests of Papua New Guinea or central Africa or Colombia.鈥�
Harris, a leading international consultant on organic farming, says a hectare of seawater greenhouses could theoretically produce 350,000 lettuces a year or 50 tonnes of French beans, and still have 80 per cent of its water left over for other uses. He agrees that the technology could transform the production of vegetables and horticultural crops in dozens of arid countries round the world that are hot, sunny and running short of fresh water. If they have a shoreline, of course. Paton is also fielding enquiries from ecotourist companies and others keen on building self-sufficient settlements in arid regions.
The design has three main parts (see Graphic). The greenhouse faces into the prevailing wind so that hot, dry desert air blows in through the front wall of perforated cardboard, kept wet and cool by a constant trickle of seawater pumped up from the nearby shoreline. The evaporating seawater cools and moistens the air. Last June, for example, when the temperature outside the Abu Dhabi greenhouse was 46 掳C, it was in the low 30s inside. While the air outside was dry, the humidity in the greenhouse was 90 per cent. The cool, moist air allows the plants to grow faster, and because much less water evaporates from the leaves their demand for moisture drops dramatically. Paton鈥檚 crops thrived on a single litre of water per square metre per day, compared to 8 litres if they were growing outside.
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.
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 the 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鈥檚 Moroccan bus. Drops of pure distilled water form on the condenser and flow into a tank for irrigating the crops.
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. 鈥淥nce it is tuned to the local environment, you don鈥檛 need anyone there for it to work,鈥� says Paton. 鈥淲e 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.鈥�
The net effect is to evaporate seawater into hot desert air, then recondense 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鈥檚 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 3000 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.
Critics point out that construction costs of 拢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 desalinator. And costs should plummet when mass production begins, he adds.
And the break-even water yield of the Abu Dhabi greenhouse is a worst-case scenario. Seawater from the shallow Persian Gulf is tepid and does a poor job of chilling the condenser plates. Where the water is colder, the greenhouse could churn out vast surpluses. 鈥淭he Tenerife greenhouse was a high-performance water factory,鈥� says Davies. It produced up to 20 times more water than the plants in the greenhouse needed. But that meant extra condensing units and a system to pump cold water up from the ocean depths to chill the condensers, which made it more expensive. In regions such as Morocco and the arid Pacific coast of South America, cold currents or oceanic upwellings bring cold water right to the surface. Still, because of the investment needed, the system seems best suited to growing high-value vegetable and flower crops for sale, not subsistence crops.
One beauty of the system is that you can run the thermodynamic model to predict the greenhouse鈥檚 output in different places. 鈥淲e have only built two full-size greenhouses, but we can produce virtual greenhouses endlessly,鈥� says Davies. This means you can 鈥渢une鈥� the design to balance water output, air conditioning and cost for different sites.
Dozens of hot, dry coastal countries could benefit hugely from the seawater greenhouse. Many struggle to feed themselves despite using all their available water for irrigation. Across North Africa and the Middle East, underground water reserves are being pumped dry. On coasts, this often means that seawater seeps into the empty aquifers, permanently polluting them. Such countries badly need new sustainable sources of fresh water.
Abu Dhabi, one of the seven oil emirates that formed the United Arab Emirates when the British left the region in 1971, may not be short of cash to buy food. But it鈥檚 still uncertain about the security of its food supply because of tensions with Iran, across the Gulf. So it is keen to increase production on farms in the desert. The government recently levelled 30 square kilometres to transform it into 3000 irrigated farms. But watering these farms now involves expensive and energy-intensive desalination of seawater.
Paton鈥檚 seawater greenhouses might replace much of this. 鈥淢y hosts are planning to build another greenhouse, probably on the mainland, next year,鈥� says Paton. 鈥淎nd after that, who knows? There are big plans and there is plenty of land. Provided the seawater can be piped in, then you can build them even some kilometres away from the shore.鈥�
Best of all, the greenhouses should be environmentally friendly. 鈥淚 suppose there might be aesthetic objections to large structures on coastal sites,鈥� says Harris, 鈥渂ut it is a clean technology and doesn鈥檛 produce pollution or even large quantities of hot water.鈥�
Paton sells his technology under a licence agreement with his government clients. Next stop could be the Batinah coast on the north-east shores of neighbouring Oman. Once famous for its fruit trees, the coastal plain has a long history of irrigated farming. But it鈥檚 all gone horribly wrong, says Paton, because seawater has got into the aquifer. 鈥淓ven the palm trees are dying. They call it Hiroshima,鈥� he says. Paton hopes his partnership with the Agricultural Experimental Station at Sultan Qaboos University to build seawater greenhouses there can turn the tide.
The Omanis are hopeful. 鈥淭he concept is a very promising alternative way of generating fresh water in Oman, both for coastal regions and inland, where we have brackish underground water. Farms are literally dying and it will allow us to reclaim the abandoned land,鈥� says Johan Perret of the Sultan Qaboos University鈥檚 soil and water engineering plant.
Paton鈥檚 latest plan is to start building among the flamingos on the flat coral-fringed island of Grand Turk in the Caribbean. The island couldn鈥檛 have been better designed for a seawater greenhouse. It is perched right on the edge of a deep ocean trench, known to divers as 鈥渢he wall鈥�. Within a mile of the shore the sea plunges down 700 metres and the water at that depth is around 10 掳C. By exploiting the 20 掳C difference between the water and air temperature, you could generate very large amounts of water very cheaply, says Paton. The Canadian government is considering funding the project as part of its foreign aid programme.
Paton also wants to position a greenhouse next to one of the ocean鈥檚 upwelling zones, where ocean circulation pumps cold water from the depths to the surface. One example is the west coast of Morocco, where the surface of the ocean is around 20 掳C. 鈥淓verybody agrees it is a great idea, but nobody wants to put in the cash,鈥� says Paton. Britain鈥檚 Foreign Office is typical. It lists Paton鈥檚 idea on its website as an example of 鈥渙ne of the things Britons are good at鈥�. But when its embassy in Morocco suggested putting up some cash to start a greenhouse there, the Whitehall mandarins vetoed it as 鈥渢oo risky鈥�, because the technology is still experimental.
But the deserts seem poised to grow hotter and drier in the coming decades, and a glasshouse that turns seawater into fresh water-and produces more of it the hotter it gets-looks like a technology for the future. It could transform the economy of islands such as Tenerife, Grand Turk and maybe hundreds of others in the Indian and Pacific Oceans. Certainly Paton thinks he is onto something big. 鈥淭he seawater greenhouse is in the position today of wind turbines a generation ago, when they were seen as the stuff of hippies and the Whole Earth Catalogue,鈥� he says. 鈥淏ut the pioneers [from] then are running big engineering companies today and the technology is taking off. We have that potential.鈥�