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Sunday, September 16, 2007

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

I wish I'd thought of that. How dumb could I be. And 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.


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.

These are simple boxes of about a metre by a metre and a meter and a half high, made of fibre glass or galvanized iron. They have louvres 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 louvres 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 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. 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 condenser where the air normally goes through a car radiator. 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. 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.

You can now use the fresh water you produce in a hydroponics system, recirculating the nutrient water either in a batch form or with nutrient make up, or it can be used with a soil growing system with a plastic liner below collecting excess water. It can also 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.

Very long term, such a farm using alkali ground water and returning fresher water to the ground would start to sweeten the ground water in the water table as it removes alkali salts. The salty water coming out of this system can either be piped to the sea or to ponds to evaporate.  In this case, there is the possibility of harvesting various salts as  by-products.

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. 

1 comment:

Anonymous said...

why Oh Why is this idea still dead!!!!!
mikebarnard@lineone.net