Seawater Greenhouses – A New Approach to Restorative Agriculture

In arid and semi-arid areas such as around the Persian Gulf, the Red Sea, and the Mediterranean Sea, the scarcity of freshwater resources has led to increasing use of desalination plants to produce water. However, all conventional desalination techniques eject concentrated brine back into the sea at roughly double the salinity of the intake. As a consequence, the salinity in these semi-enclosed seas rises.

Increased salinity has an adverse effect on all marine life and there are very few plants or fish that can survive a doubling of salinity from 3.4% to 6%. In 2008 about 18.4 million m3/day was discharged into the Persian Gulf, 9.8 million m3/day into the Mediterranean Sea, and 6.8 million m3/day into the Red Sea. That is a total of 35 million tons/day and the volume is expected to grow. This effect is made worse by reduced inflow from rivers such as the Euphrates and Tigris, and high rates of evaporation.

Salinity in Persian Gulf. Source: Allsop & Yao (2010)
Salinity in Persian Gulf. Source: Allsop & Yao (2010)

For a number of technical reasons, conventional desalination techniques have to discharge concentrated brine as their processes cannot function with high salinity. However, at Seawater Greenhouse, they have developed a technology that can. The idea of the Seawater Greenhouse is to convert these two seemingly intractable problems – a shortage of fresh water and brine discharge from desalination – into an elegant solution for crop cultivation, reforestation and realizing the value chain of salt, minerals and nutrients from seawater.

Seawater Greenhouses:

Just as with desalination, the last few decades have seen tremendous growth in conventional greenhouses around the world. There are now some 200,000 hectares of greenhouses around the Mediterranean, and over 1 million in China, where 30 years ago, there were almost none. This is because yields that are achieved in greenhouses can be 10 to 100 times greater than yields achieved outside. They also enable high value crops to be grown ‘out of season’.

The Seawater Greenhouse enables year-round crop production in some of the world’s hottest and driest regions. It does this using seawater and sunlight. The technology imitates natural processes, helping to restore the environment while significantly reducing the operating costs of greenhouse horticulture. In addition to not having to discharge concentrated brine, it also benefits from the fact that high salinity water has a powerful biocidal or sterilising effect on the air that passes through it. This reduces or eliminates airborne pests.

The most important benefit of the Seawater Greenhouse is that it cools and humidifies large volumes of air at very low cost, and to do this, it must evaporate large volumes of seawater, thereby dealing with the discharge from desalination. One hectare of Seawater Greenhouse near the coast will typically evaporate 50 tons of water/day, but this will increase 2-3 fold in regions of low humidity. The effect is illustrated here.

Evaporative cooling properties of air at 30ºC
Evaporative cooling properties of air at 30ºC

The most significant benefit of the process is that the combination of lower temperature and higher humidity reduces plant transpiration up to 10-fold and enables delicate crops such as lettuce and French beans to grow in a hot, arid location. Further, the beneficial effect of the humid exhaust air creates a zone of locally higher humidity which encourages vegetation. The photographs below were taken two years apart in Oman.

Seawater Greenhouse, Oman
Seawater Greenhouse, Oman

Evaporating large volumes of water in the GCC region could have many environmental benefits and the Seawater Greenhouse has a similar effect on the local environment to an area of forest in terms of the amount of water vapour it produces and the consequent cooling achieved. For example, one hectare of greenhouse will evaporate ~ 100 tons of water/day, consuming 60MWh of heat in the process. Effectively, it reduces the temperature of air from the dry bulb to the wet bulb temperature.

If implemented on a large scale, it makes sense to evaporate the water some distance from the coast. It may also be beneficial to evaporate it at the base of a mountain, as air cools with increasing height, typically by 1ºC for every 100m of elevation, so there is a greater chance of contributing to rain or dew by increasing the humidity of air that blows up a mountain.

Just add water:

Drought, desertification, food shortages, famine, energy security, land use conflict, mass migration and economic collapse, climate change and CO2 sequestration are all issues that can be overcome by increasing the supply of water. Present methods of supply in arid regions include: over-abstraction from groundwater reserves, diverting water from other regions, and energy-intensive desalination. None of these are sustainable in the long term and inequitable distribution can lead to conflict.

The growth in demand for water and increasing shortages are two of the most predictable scenarios of the 21st century. Agriculture is the primary pressure point (see: The state of the world’s land and water resources). A shortage of water will also affect the carbon cycle as shrinking forests reduce the rate of carbon capture, and will disrupt the regulating influence that trees and vegetation have on our climate. Fortunately, the world is not short of water, it is just in the wrong place and too salty. Converting seawater to fresh water and water vapour in the right places offers the potential to help solve all these problems.

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