Build a Solar Desalinator

Let's Do It
So let's look at building one up using readily available stuff and on a low budget.

It is obvious that solar PV panels could provide the small amount of electric power to operate the auxiliary systems, so we won't worry about that for now and just concentrate on the more interesting problems.

A Trial Solar Collector
The reflector is made up of 70 standard mirror tiles giving an effective area of about 6 square metres. It moves using an azimuth elevation tracker driven by computer controlled electric toy truck motors, and it is mounted on a 5m wooden post.

The collector is a 300mm square water cooled steel target in an insulated single glazed aluminium box. The glass is from a domestic oven door and the insulation is from fibre glass batts as used for insulating the walls of houses.

So the concentration is about 70 suns

and the average power is about 3.5 kilowatts maintained over about 6 hours a day, although it should run for 8 hours.

The designed operating temperature is under 200ºC although the collector would easily go over 500ºC if the control system did not prevent this. Aluminium melts at around 650ºC so obviously it's a good idea to keep the temperature well below this.

It's Like, a Car Radiator
The transfer medium is water mixed with automotive 55% glycol coolant additive. Cooler water (it's not that cool!) comes out the lower struts to the collector and back to the heat transfer tank through the upper strut. The struts are insulated with plumber's foam tube lagging, but even so they are pretty hot. No pump there - works as a thermosyphon.

The heat transfer tank is fitted with a 250kpa (30psi) automotive radiator cap with a coolant recovery bottle, just like in a car. This setup should go to about 150ºC without boiling, so the control system is set to run at 120ºC. If there is any failure the computer will move the reflector off sun.

The Water Separator
The water separator started life as a bulldozer air cleaner - it's not pretty but it's pretty rugged and should stand up to heat and pressure for a while.

Feedwater is pumped up to the separator at a controlled rate. The control system is continually working out how much energy is available and adjusts the flow of water to balance the available heat. The feedwater has some of the fresh water separated from it and then it returns down the wastewater pipe to the waste tank.

The purified fresh water is continually trickling down the delivery pipe to the fresh water tank.

Both waste and fresh water come down through the heat exchanger to transfer heat to the cooler feedwater going up to the separator.

Exactly what sort of makeover happened to the

The grey water goes through an old swimming pool sand filter to get the frogs and other big bits out - actually it looks suprisingly clear after the filter, but of course you wouldn't want to drink it.

The bore water looks quite Ok too, but it tastes salty and you really wouldn't want to drink that either - we have tried to use it in the garden, but there is a lingering suspicion that it has not been kind to some of the plants.

The feedwater is pressurised to 200kpa (30 psi or 2 atmospheres) using the small household pressure pump, and the flow control valve balances the feed water flow against the other system variables.

The garden hose and fittings are Ok but can just stand the pressure and that's all. The white PVC tube going up the pole is the heat exchanger, and it has 2 closewound coils of annealed ½" copper hot water tube the full length of the exchanger - the third line is waste water which is the water jacket.

It's Not Rocket Science
So there it is - just convert sunshine into fresh water. Too easy.

Doesn't need grid power - doesn't contribute to the greenhouse problem - doesn't produce toxic effluent - and anything to dispose of (fats, salts, detergents) all had to be disposed of before so no new problems there.

No real operating costs - can be built from junk - of course to build a commercial installation you can't use scrap parts and so there would need to be a capital investment, but my point is there doesn't have to be some huge cost just because it's solar power.

It is a bit early to be sure about results, but at this stage it looks like production is about 150 litres of fresh water a day. Obviously the real cost is quite low, so using instead the estimate of $400 a square metre this would suggest the trial should cost about $3,000 - and the fresh water should cost about 40¢ per cubic metre (taking into account capital cost - running cost is roughly zero).

Is that something like 50,000% markup to the price of a bottle of water in the shop...?

So the next step is to build a bigger one and make some money from it.

Interested?

Like to email me?