I wish I'd thought of that - growing 'tomatoes' with sea water

I wish I'd thought of that. How dumb could I be. I was working where the desert meets the sea, I had a desert cooler* cooling my apartment and..... I was growing plants in pots in the cool humidified air from the desert cooler. Talk about not connecting the dots.

*An air conditioner that works on the principle of water evaporation instead of using a refrigerator.

It is possible, in fact not very difficult, to grow crops with no source of water except sea water or brackish water or alkali water or for that matter sewage water. You have to be in an area where the air is dry - the dryer the better, and if there is a prevailing wind you can cut your energy costs somewhat. If you want to see a far better explanation than mine, google "sea water green houses". Otherwise read on.


Water has a very high latent heat of evaporation. All this means is that it takes a lot of heat to evaporate water when compared with most other substances. The flip side of this is that if you pass dry air over water and, especially if you contrive to make the water expose lots of surface area, you cool the water and the air. You cool it a lot. I mentioned the desert cooler in my apartment. In Elat where I worked for many years, it worked a treat.

 Desert Coolers are simple boxes of about a meter by a meter and a meter and a half high, made of fiber glass or galvanized iron. They have louvers on all four sides. The bottom of the box is a shallow tray with a simple toilet float valve attached to the municipal water supply. This keeps the water level in the tray constant. A small centrifugal pump pumps the water from the tray and dribbles it over top of excelsior screens which are attached just inside the louvers on the three outside sides of the box. A little water is allowed to go to waste to take care of salt build up but most of the water flows down the screens back into the tray. A drum fan sucks air through the 3 screens and pushes this cooled, humidified air through the fourth side into the house or apartment. The air comes out humidified, much cooled and the water in the tray is too cold to put your hand into for very long. In fact it is a great place to keep the butter hard or the beer cold. These desert cooler boxes are hung outside the apartments in an opening in the wall so that the inner wall of the desert cooler is flush with the inside of the wall of the apartment.

Sea water greenhouses work the same way. One end of a plastic-clad tunnel house is covered in some sort of a screen that allows the outside air to pass through it and the water to dribble down it. If there is a prevailing wind, the tunnel house is aligned with the wind. Sea water or any other waste-type water is pumped to the top of the screen and allowed to dribble down to the bottom where it is collected in a trough. If there is no wind, a fan is inserted into the down-wind end of the tunnel house to suck air through the tunnel house.  

 

A nice touch would be to power the fan with solar panels.  More sun, more heat, more air moved.

Already conditions are far better for the plants. Under the resulting cooler, moister conditions, plants use far less water and are under less stress. However it doesn't stop here.  Here is where it gets really good.  

 

The cold water which is caught in the trough under the screens is then pumped through a lagged (insulated) pipe to the down-wind end of the tunnel house. There it is pumped through a condenser. For the sake of illustration, think of a car radiator with the water being pumped through just where water normally goes through the radiator and the cool moist air from inside the tunnel house passing through the radiator where the air normally goes. The condensers aren't actually built this way but this is just to give you the idea.  Fresh water condenses out and drips down into a second trough and is pumped or 'gravitated' into a tank. This fresh water is used to water the plants in the green house.

Of course, the plants also transpire moisture into the air stream which is available for condensing as well. If you are in the lucky situation that your feed water to the green house is cold, you can also use that for condensing fresh water out of the air flow before it goes to the upwind screens.  

 

Cold sea water exists, for instance, beside deserts which are adjacent to the many upwelling areas of the world. It is also available on volcanic islands in the tropics where the bottom slope of the ocean is so steep that it is practical to put a pipe to below the thermocline.   Cold water is typically found at about 500m depth in tropical oceans but right on the surface in upwhelling areas such as Namibia.

 

If by any chance you are into 'seasteading' you could drop a plastic pipe down to abut 500m and access all this lovely cold (and nutrient rich) water.

You can now use the fresh water you produce in a a number of ways.  You can do hydroponics, recirculating the nutrient water either in a batch form or with nutrient make up.  

 

It can be used with a soil growing system with a plastic liner below collecting excess water for recycling.  

 

And it can  be used in an open system allowing excess fresh water, lightly laced with nutrients, to seep into the ground to replenish the water table. In desert areas, such a flow of fresh water into the water table has been seen to make the area around the green house or areas down slope begin to bloom with whatever seeds are in the soil or with whatever the farmer decides to plant.

Long term, such a farm that uses alkali ground water instead of sea water and returns fresher water to the ground will start to sweeten the ground water in the water table as it removes alkali salts. The salty overflow water coming out of this system can either be piped to the sea or to ponds to evaporate.  If the overflow is evaporated, there is the possibility of harvesting various salts as  by-products.

 

This could be a particularly useful method in areas where conventional agriculture has resulted in the salination of the soil. 

With this open type system, you  will create a lens of fresh water floating on the deeper saltier water just as happens in and tropical atoll island.  

As I said, I wish I had thought of this system. Genius is the recognition of the obvious which no one else sees. I certainly wasn't showing any. I would have thought that Australia with its present crippling drought and ditto, California, would be beating a path to the door of the developers of this system but probably isn't. After all, genius is being able to see the obvious.