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September 07, 2008

Seawater greenhouses to bring life to the Sahara desert

Photograph: Exploration Architecture

The Sahara Forest project will use seawater and solar power to grow food in greenhouses across the desert. The planned project would use solar power to evaporate salt water, generating cool air and pure water thereby allowing food to be grown

Vast greenhouses that use seawater to grow crops could be combined with solar power plants to provide food, fresh water and clean energy in deserts, under an ambitious proposal from a team of architects and engineers.

The Sahara Forest project would marry huge greenhouses with concentrated solar power (CSP), which uses mirrors to focus the sun's rays and generate heat and electricity. The installations would turn deserts into lush patches of vegetation, according to its designers, and without the need to dig wells for fresh water, which has depleted acquifers in many parts of the world.

Demonstration plants are already running in Tenerife, Oman and the United Arab Emirates.

Plants cannot grow in deserts because of the extreme temperatures and lack of nutrients and water. Charlie Paton, one of the Sahara Forest team and the inventor of the seawater greenhouse concept, said his technology was a proven way to transform arid environments.

"Plants need light for growth but they don't like heat beyond a certain point," said Paton. Above a particular temperature, the amount of water lost through the holes in its leaves, called stomata, gets so large that a plant will shut down photosynthesis and cannot grow.

The greenhouses work by using the solar farm to power seawater evaporators and then pump the damp, cool air through the greenhouse. This reduces the temperature by about 15C compared to that outside. At the other end of the greenhouse from the evaporators, the water vapour is condensed. Some of this fresh water is used to water the crops, while the rest can be used for the essential task of cleaning the solar mirrors.

"So we've got conditions in the greenhouse of high humidity and lower temperature," said Paton. "The crops sitting in this slightly steamy, humid condition can grow fantastically well."

The designers said that virtually any vegetables could be grown in the greenhouses, depending on the conditions at which it is maintained. The demonstration plants already produce lettuces, peppers, cucumbers and tomatoes. The nutrients to grow the plants could come from local seaweed or even be extracted from the seawater itself.

Michael Pawlyn of Exploration Architecture, who worked on the Eden Project for seven years and is now part of the Sahara Forest team, said the seawater greenhouse and CSP provided substantial synergies for each other. "Both technologies work extremely well in hot, dry desert locations – CSP produces a lot of waste heat and we'd be able to use that to evaporate more seawater from the greenhouse," he said. "And CSP needs a supply of clean, demineralised water in order for the [electricity generating] turbines to function and to keep the mirrors at peak output. It just so happens the seawater greenhouse produces large quantities of this."

Paton said that the greenhouse produces more than five times the fresh water needed to water the plants inside so, in addition to producing water to clean the CSP mirrors, some of it can be released into the local environment. This can create a local microclimate just outside the greenhouses for hardier plants such as jatropha, an energy crop that can be turned into biofuel. The ability to create similar microclimates has already been proven in the demonstration greenhouses Paton has built.

The cost of the Sahara Forest project could be relatively low since both CSP and seawater greenhouses are proven technologies – the designers estimate that building 20 hectares of greenhouses combined with a 10MW CSP scheme would cost around €80m (£65m). Paton said groups in countries across the Middle East, including UAE, Oman, Bahrain, Qatar and Kuwait, have expressed interest in the possibility of funding demonstration projects.

He added that using seawater greenhouses could reverse the environmental damage done by the greenhouses already built in places such as Almeria in southern Spain. More than 40,000 hectares of greenhouses have been built in this desert region during the past 20 years to grow salad vegetables. "They take water out of the ground something like five times faster than it comes in, so the water table drops and becomes more saline. The whole of Spain is being sucked dry. If one were to convert them all to the seawater greenhouse concept, it would turn an unsustainable solution into a more sustainable one."

"In places like Oman, they've effectively sterilised large areas of land by using groundwater that's become increasingly saline," said Pawlyn. "The beauty of the Sahara Forest scheme is that you can reverse that process and turn barren land into biologically-productive land."

Neil Crumpton, an energy specialist at Friends of the Earth, said the potential of desert technologies was huge. "Concentrated solar power mirror arrays covering just one per cent of the Earth's deserts could supply a fifth of all current global energy consumption. And one million tonnes of sea water could be evaporated every day from just 20,000 hectares of greenhouses."

He added: "Governments around the world should invest serious money in these solar energy and water technologies and not be distracted by lobbyists promoting dangerous nuclear power or nuclear-powered desalination schemes."

The Guardian

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