By Kim Robson:
A team of architects and engineers have proposed building vast greenhouses that use seawater combined with solar power plants to provide crops, fresh water and clean energy in desert regions. The would use solar power to evaporate salt water, generating cool air and pure water, allowing food to be grown in otherwise unsustainable deserts.
uses mirrors to focus the sun’s rays on a central tower to generate heat and electricity. The project could transform deserts into viable growing regions without digging freshwater wells. Demonstration plants already are running in Tenerife, Oman and the United Arab Emirates.
Charlie Paton, one of the Sahara Forest team and the inventor of the concept, believes his technology is a proven way to transform arid environments. “Plants need light for growth but they don’t like heat beyond a certain point,” he says. At very high temperatures, too much water is lost via expiration through the leaves, shutting down photosynthesis and growth.
Excess heat from the CSP plant is used to evaporate seawater and then pump the resulting damp, cool air through the greenhouses, thus reducing the interior temperature by about 60 degrees Fahrenheit (15C). “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.”
At the other end of the greenhouse, this water vapor is recaptured via condensation. Most of this fresh water is used to run the turbines and for cleaning the solar mirrors, and the remainder is more than enough to water the crops.
In fact, the greenhouse produces more than five times the fresh water needed to water the plants inside. Some of it can even be released into the desert, creating a local microclimate for hardier plants such as , an energy crop that can be used for biofuel. Similar microclimates already have been created outside the demonstration greenhouses.
CSP provide significant advantages to 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. And CSP needs a supply of clean, demineralized 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.”
In addition, seawater greenhouses could help reverse the environmental damage already caused by greenhouses built in places like in southern Spain. In the past 20 years, more than 40,000 hectares of greenhouses have been built to grow salad greens in this desert region. Paton explains, “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 sterilized large areas of land by using groundwater that’s become increasingly saline,” says 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 , puts the enormous potential of desert technologies into perspective: “Concentrated solar power mirror arrays covering just one percent of the Earth’s deserts could supply a fifth of all current global energy consumption. And one million tons of fresh water could be captured every day from just 20,000 hectares of greenhouses. 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.”
Virtually any vegetable could be grown in the greenhouses, depending on the conditions at which they are maintained. The demonstration plants already produce lettuces, peppers, cucumbers and tomatoes. Fertilizer for the plants could come from local seaweed or be extracted directly from the seawater.
The cost of the Sahara Forest Project should be relatively low since both CSP and seawater greenhouses are proven technologies. Countries across the Middle East, including UAE, Oman, Bahrain, Qatar, and Kuwait have expressed interest in funding demonstration projects.