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Solar Powered Seawater Greenhouses for Deserts

Challenge

Use seawater and solar power to create green spaces in deserts.

Problem

Plants cannot grow in deserts because of the extreme temperatures and lack of nutrients and water. 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.

Solution

The Sahara Forest project aims to exploit seawater greenhouses that work by using concentrated solar power (CSP) to drive seawater evaporators and then pump the damp, cool air through the greenhouse. This reduces the temperature by about 15˚C 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. The greenhouses are therefore humid and at a lower temperature, ideal for growing crops. Demonstration greenhouses running in Tenerife, Oman and the United Arab Emirates already produce lettuces, peppers, cucumbers and tomatoes. The nutrients needed to grow the plants could come from local seaweed or even be extracted from the seawater itself.

Seawater greenhouses and CSP work in synergy as both function extremely well in hot, dry desert locations. CSP produces a lot of waste heat that can evaporate more seawater from the greenhouse and CSP needs the supply of clean, demineralised water from the greenhouse in order for the electricity generating turbines to function, and to keep the mirrors at peak output. The seawater feed is key, as many regions are destroying water tables to feed large greenhouses in desert regions to grow food. The seawater greenhouses are largely passively (wind) driven and so use only a small amount of energy for pumps and fans – leaving most of the energy generated available for use/export.

In fact the seawater greenhouse produces more fresh water than 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.

Benefits

The fresh water produced is pure and distilled from seawater, with no need for chemical treatment.
Unlike traditional greenhouses, which often rely on gas or other fossil fuels for temperature control and CO₂ enrichment, Seawater Greenhouse systems use only seawater and sunlight to control the growing environments, with equal effectiveness.
The seawater evaporators have a biocidal and scrubbing effect on the ventilation airflow. This greatly reduces or eliminates the need for pesticides.
Enables the development of land normally considered unsuitable for agriculture.
Commercial grade crops yields, coupled with significantly lower capital and operating costs result in enhanced operator economics.
Salt gained in the process can be sold and other minerals used as crop nutrients.
Creation of new jobs.

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