Green the deserts by using seawater to cool greenhouses

Norwegian biologist Joakim Hauge wants to green the deserts using greenhouses that are cooled by seawater. They plan to grow 1200 cucumbers per square metre per year in new greenhouses.

In March, temperatures outside were in the 30s (celsius), but in the low 20s in the greenhouse. In August, it can reach 50 °C outside, but the greenhouse needs to stay below 30 °C for crops to thrive. Hauge is confident that it will. As head of the rather grandly named Sahara Forest Project, the private Norwegian company that built and runs the prototype, he has a lot riding on its success.

Keeping the greenhouse cool and humid reduces the plants’ need for fresh water, but it still has to come from somewhere. The answer again is seawater. A concentrated solar power system provides the heat and electricity needed to desalinate seawater for irrigation, as well as power all the pumps, fans and other machinery. This is the purpose of the 300 square metres of parabolic mirrors, which track the sun and focus its rays onto a pipe containing oil.

But the prototype is more than just a souped-up greenhouse. The aim is to use seawater to maximum effect.

Qatar pilot plant part of the Sahara Forest project

An oasis of green technologies: 1. Concentrated Solar Power; 2. Saltwater greenhouses; 3. Outside vegetation and evaporative hedges; 4. Photovoltaic Solar Power; 5. Salt production; 6. Halophytes; 7. Algae production

Some seawater goes straight into ponds for growing marine algae. Algae farms are in their infancy, says Patrick Brading, a marine biologist fresh from the University of Essex in the UK, who was preparing a batch of algae for release into a pond. “A lot of basic things are still unknown. I am trying to answer those questions, and it is brilliant to work here with a constant supply of brine to do open-air experiments.”

In theory, the algal ponds could provide anything from feedstock for pharmaceuticals and food supplements to food for livestock and farmed fish, Brading says. There are also plans to use seawater to grow a range of native salt-tolerant plants, which could be used for a similar range of purposes.

The leftover brine from all these processes – with a salt content of 10 to 15 per cent – is used to wet the cardboard honeycombs around the small garden plots. “We call them evaporative hedges,” Corless says. Their cooling effect allows desert plants and a few tough herbs and crops to grow where none would grow before. The plots are planted with a variety of grasses, as well as barley, rocket and medicinal plants such as aloe vera. The aim is ecosystem rehabilitation, not just desert farming. The long-term plan, she says, is to grow trees as well, which should eventually take over the cooling role of the cardboard hedges.

Brine from the hedges, now with a salinity of around 30 per cent, is pumped to evaporation ponds to produce salt. This can be sold, and also avoids the need to dump extremely salty waste water back in the sea, which can harm marine life.

None of these technologies is new, says Corless, but they have never been put together before. Well, not quite like this. The seawater greenhouse was actually devised two decades ago by British inventor Charlie Paton, who set aside a successful career devising special lighting effects for films and theatres to pursue a vision of growing crops in deserts.

The Qatari greenhouse cost $6 million to build, with the money coming from QAFCO and the Norwegian agrochemicals company Yara. Given the anticipated annual harvest of 720,000 cucumbers for 10 years, that would work out at a capital cost of almost a dollar per cucumber, even without overheads. That is spectacularly expensive: cucumbers were on sale in the Doha souks for a fifth of the price.

This kind of farming is only economically feasible at a large scale.

There are plans to go large in Jordan, which is one of the most water-stressed countries in the world and increasingly reliant on food imports. Later this year, the Sahara Forest Project hopes to start building a facility near the Red Sea 20 times larger than the Qatar pilot. If all goes well, the idea is to expand to 200 hectares, and eventually to 3000.

1. Algae-facility; 2. Saltwater based Greenhouses; 3. External vegetation and evaporative hedges; 4. Designed stepped protection for flash floods; 5. Facilities for research and accommodation; 6. Concentrated Solar Power facilities; 7. Evaporative ponds

In such a billion-dollar megaproject, the vision is that up to half of the area would be devoted to vegetating the desert with everything from dune grasses to native trees, such as the nitrogen-fixing thorn acacia (Acacia tortilis). Over time, as trees grow and the soil improves, these plots will need less assistance, allowing resources to be switched to other areas. “This is not just about sustainable agriculture. It is about restorative ecology,” says Hauge.

There are cheaper ways to revive desert ecosystems and turn back advancing deserts. In Niger, on the edge of the Sahara, simple measures such as protecting trees and digging ditches to catch rainwater have had a dramatic effect. Then again, these approaches won’t work in drier regions, or deliver cash crops.

In 20 years, Hauge thinks, seawater greenhouses could be sprouting everywhere from the Atacama desert of northern Chile to California and North Africa, where some coastal aquifers have already been pumped dry. In the Egyptian desert, the 19,000-square-kilometre Qattara depression could be exploited. It is around 100 kilometres from the coast, but could receive seawater by gravity as it is below sea level.

Paton has a similar vision. He says the technology could save 20,000 hectares of greenhouses in Almeria in southern Spain, which supply fruit and vegetables to much of Europe but are now running out of underground fresh water. (Another possible solution is setting up closed greenhouses which recycle most of their water).

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