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Kristian 24-02-2010 09:08

Water Desalination by Using the Sun - Will it Work ?
I might be too stupid to understand why the described thing below not should work.
Im sure there is someone that can give it thumbs up or down.

Before reading the thing below it might be a good thing reading the thread started by Anjou : Water water everywhere around , the thread inspired me into a lot of thinking in if we can get our freshwatersupply in a new way.

Lets start with the general idea. Water heated by the sun boiling of stem that is condensed back to pure water.

I started in looking at what lowpressure is possible to get using a very low cost pump such as a reversed tire pump for your bike or a tirepump 12V for your car or why not a danfoss compressor from the refrigerator not working any more..
I got the result that the lowpressure achivable will make water boil in the 50-60 degrees celcius range! A lot better than I first thought .
I belive this temperature is very easy to achive with sun energy!

Getting the lowpressure ( Vacuum) is easy but you shuld not really have to be pumping all day to keep the low pressure as the water boils away so this little problem needed solving.

I belive this is fairly easy on a Sailing boat with a 12 Meter + high mast.
What Im thinking of is using a thin tube high up connected to a coarse tube ( some 15 cm diameter) down below with a watertight piston within that is free to move upwards but can not fall out in the bottom.
You would start by filling this tower with potable water some 9 meters high. Next you pump out untill reaching your low pressure of aprox - 26 inches mercury and this way you will be lifting the watercolumn that will keep the lowpressure in the system with its own weight.

How will it work ones the water starts to boil?
The vapour will go in a tube from the "sun boiler" to the top of the fine tube in the mast. As this happens the watercollumn will start to sink keeping the low pressure. Vapour is around 16000 times bigger when it comes to volume than water. This fact works both ways and it will contract just as much ones it is cooled back to water in the column.

The thing will have to be restarted ones you have produced enough fresh water and the movable piston hits the lower restriction.

At this point you tap of the produced water and start all over again.

To Easy to work??




anjou 24-02-2010 09:13

Just a quick observation Kristian

A 6'' dia pipe up the mast when filled will weigh rather a lot and may affect the boat ballast.

Jetexas 24-02-2010 09:29

In other words, you want to distill salt water like this:
But with two long pipes instead of beakers and using low pressure and sunlight to generate the heat needed on the saltwater pipe.

Even if you use a thin tube, so that it's easier to maintain temperature and pressure along the length of the tube, I think as water starts to boil off, because of the length of the tube, it's going to be hard to maintain temps, and it would just evaporate and condense within the tube without ever making it to the pure water collector.

You also have to remember that you have salt as a precipitant filling the raw water tube, which will need to be cleaned out and could gum up your vacuum pump, etc.

Kristian 24-02-2010 09:46

Anjou, agree!
I was thinking about a thin tube high up and the thick one really low perhaps all the way down in your bilge.

I belive you might need cooling fins on the tube high up so the condensation is effective. From a theoretical point of view I think it really only affects the speed of production.

Jetexas , agree but do you not think it is possible using a really thin tube with good isolation to minimise this problem?

The other possible solution is using double collection reservoirs for the freshwater. One as described and one just before the climb of the thin tube up the mast!?

As to gumming and saltwater yes that has to be adressed but I do not really think it will kill the consept.

Thanks for your inputs!



rustypirate 24-02-2010 09:47

There have been solar stills around for making distilled water from salt water without the need for pressure lowering pumps for a long time. The problem with them has allways been the volume of water generated was extremely small. I assume that you would want to increase the volumen by reducing pressure to force generating more steam....

I would agree with what has already been posted about maintaining temps, etc. You may be better off with a smaller presure vessel painted matte black to absorb more heat from the sun. Thsi would make it easier to control temp, pressure, etc.

ViribusUnitis 24-02-2010 09:47

That is what is called vacuum distilation, and is one of the most energy efficent forms of distilation when done properly.

The usual solution to the high energy draw of the vacuum pump is to utalize a condenser to cool the vapor, and then run the liquid water through the vacuum pump. This greatly reduces the energy required by the vacuum pump, as it only has to move a small volume of liquid, instead of a large volume of vapor. At a maximum it's about a 14 psi pressure diffrental, so even a small cheap pump can hanndle it.

The trick to making it work is you can't allow any air into the system. The air will not condense, and will thus remain traped in the system. As the air is traped, the pressure will rise. Eventualy the pressure will get too high for the system to work.

There are two usual solutions to that, a vacuum pump to remove the air when it becomes a problem, or to flush the entire system with steam at atmospheric pressure. The steam removes the air, then the system is closed in, and then condenses to reform the vacuum.

Both solutions will require some complexity, and arn't exactly energy cheap. It works well for large scale equipment, as the process controls become a rather small part of the over all expence. Not so well for small scale boat sized equipment.

Pete7 24-02-2010 09:48

Jetexas, thats quite good, can I suggest a couple of large bin lid sized solar reflectors aim with the focal length at the first beaker to boil the water. Seem to remember a TV program years ago were the Aussies had set up a reflector to cook sausages on a BBQ. Not sure how they got the BBQ flavour but it certainly cooked a sausage from a bin lid sized refector.


Pete7 24-02-2010 09:54

1 Attachment(s)

Originally Posted by Jetexas (Post 409267)
You also have to remember that you have salt as a precipitant filling the raw water tube, which will need to be cleaned out and could gum up your vacuum pump, etc.

BTW do you know how much people pay in Paris or London for French Sea Salt collected in vast shallow marsh ponds in Brittany :thumb: forget the distilled water, the sea salt is financially far more attractive. Mixed with Lanolin (sheep coat wax) it sells 25 for a jam jar sized container for cleaning your hands without water and very effective it is.


Pete7 24-02-2010 10:05

Anjou, don't you have a secret recipe from an old Grand mother on how to make a superb hand cleaner and face defoliater :thumb: The lads in the Carib produce the sea salt which they ship to you for making a concoction and then pass to Harods to sell to the Arabs :D

What are the forum rules about running a business on here? should we start buying shares and futures in the Cruiser Forum incase this takes off?


anjou 24-02-2010 10:14

It must be 5 years since I was accosted into buying a jar of Dead Sea salt crystals. Back then it was 20 a go. I find sharp beach sand is just as good and rinsing off in the water after leaves skin feeling baby soft.

Go on, be an Anjou baby today to look and feel years younger.

(appologies to Msrs johnson and johnson)

Randy 24-02-2010 10:33

And everyone wants to be an Anjou baby!

mintyspilot 24-02-2010 10:43

OK - these are impractical but they illustrate the point.


(about 40 seconds in)

Tellie 24-02-2010 11:01

Keep trying boys it's an interesting read. Might look at smaller easier to handle and store Fresnel lens instead of a parabolic dish.

anjou 24-02-2010 11:06

Ken Hom has reported to police that several woks have been stolen from his garage.

mintyspilot 24-02-2010 12:21


Originally Posted by Tellie (Post 409353)
Keep trying boys it's an interesting read. Might look at smaller easier to handle and store Fresnel lens instead of a parabolic dish.

Indeed, but it does rather neatly illustrate the amount of energy that can be gathered and how intense it can be.

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