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Monday, May 8, 2017

Greening the desert

If you could set up a whole bunch of desalination plants, you could pump the water on to a desert somewhere and over time it would green and you would establish a fresh water aquifer.  It wouldn't work.  No one is going to go to that expense for some future nice-to-have ecological result.  No, you need a way of doing it, that in the mean time generates revenue.  Fortunately such a way exists already.

Some time ago I wrote a blog on Sea Water Greenhouses.  You can see it here.

Image result for image sea water greenhouse
Sea water greenhouse can be at any scale.  Note the solar panels powering the pumps and fan.

To recap briefly, you set up a tunnel house with it's long axis parallel to the prevailing wind.  Note that sea water greenhouses will only work if the humidity is low.  You close the upwind end of the tunnel house with a screen made of some wettable material such as excelsior or some types of cardboard such that the wind can blow through this end of the greenhouse.  The downwind end of the Greenhouse has a Solar powered DC fan sucking the air out of the greenhouse and a condenser to condense the moisture out of the air stream.  You dribble your sea water down the screen at the upwind end and collect it in a trough.  You will find it is amazingly cold.  You pipe this water to the condenser at the downwind end in a lagged (insulated) pipe and it will condense out fresh water from the air passing the condenser.  You collect and use this fresh water to irrigate your plants.  You pipe the sea water back to the upwind area and dribble it down the excelsior screen again. Somewhere in this return system is a bleed off that you can adjust so that the water never gets too salty.  This brine is piped back to the sea or into an evaporation pond.

There are many ways to run the agricultural side of such a green house but here I am talking about an open system in which the plants grow in soil on to which you drip the fresh water.  You don't use a closed hydroponic system. The reason is simple.  You want the excess water to flow into the soil and over time create a fresh water, water-table. All by themselves, plants will start to grow around the farm where none grew before.  As more and more of these salt water greenhouses are set up, more and more fresh water will flow into the ground.  You can plant trees and you are on your way to transforming the desert.

The key is in having a farming enterprise that is profitable.

Of course, you may have some other source of water.  There may be, for instance, a salt water aquifer you can tap far from the sea.  No problem and perhaps an extra opportunity.  Not all such aquifers are simply condensed sea water.  They may have valuable minerals in them.  For instance some brines are rich in Lithium, some in Borax.  Whatever the composition of your sub surface salt water, you can let the overflow brine go into lined ponds to evaporate and precipitate out whatever salts are in the water. In the mean time the lighter fresh water going into the soil from your tunnel houses will float over top of the salty aquifer just as occurs in coral atoll islands. The salty aquifer will be sucked down over time as you utilize it and be replaced with fresh water.

In some locations in Australia trees have been cut down to farm the land.  The trees were keeping the salty water table down and without the trees, the area has become a salty swamp.  This would be a perfect place to use such greenhouses and over time replace the salt water with fresh water.  In the mean time, you could have solar ponds to generate electricity.

Incidentally, this will work as a remediation system in areas where salination has ruined the soil.  Often there is salty sub surface water which can be used and replaced by the fresh water your tunnel houses produce.