Multipurpose Solar Desalination Plant

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Introduction: Multipurpose Solar Desalination Plant

The most important need of the day in the present circumstances is potable water. Water resources are getting polluted, developing contries are facing growing shortage of fresh water sources, and arid lands are becoming drier. The solution? Water, water everywhere, not a drop to drink. Yes, the sea water. So whats new? Desalination plants have been around for a long time. Mostly in the middle east, where the biproduct of fuel extraction and refining like natural gas is used to do the evaporation of sea water. How about places where you have to exclusively burn fuel for this process? Non green, polluting, expensive fuel. So again look for the most abundant source of energy on earth, the sun. Solar power? Link both resources together and you get a near perennial source of potable water with almost zero energy consumption(Almost? Read on.)

Step 1: Design 1 - Solar Collector

Let us look into the design now. Flat plate collectors are very common and cheap, probably the most widely used for heating or pre-heating water. But not enough to vaporize water as our requirement is. We need a concentrator for that. The parabolic trough reflector is probably the best suited for the task, its geomety ensuring focal concentration of solar radiation. Copper collectors running through the focal point transfer the heat to the brine chamber through a heat transferring medium like Thermic Fluid.

Step 2: Design 2 - Brine Chamber

The brine chamber is a cylindrical, insulated, sealed chamber inside which sea water is stored. As the heat from the thermic fluid gets transfered to the brine inside, the water temperature rises. Parabolic reflectors can transfer enough energy to vaporize a certain volume of water, but if the volume of water in the chamber is high, or the solar input to the collector is low or the ambient temperature is low, the temperature may not rise to 100 degrees. Of course it also has to supply the latent heat of vaporization of water to actually vaporize the water. So now we come to a realistic assumption. It may not be possible to just use solar energy to vaporize water. For heating purposes, it is more than sufficient, but for desalination, we definitely need the water to vaporize. Solution? Flash vaporization. Use a vacuum pump and reduce the pressure inside the chamber. As the chamber pressure reduces, liquid begins to vaporize at a lower temperature. Of course a vacuum pump consumes electric power. That is where thealmost zero energy consumption phrase comes into focus. Think of the pump as suppying the extra energy required to vaporize the liquid. It goes without saying that this power would only be a fraction of what it would cost in a conventional desalination process.

Step 3: Design 3 - Condensor

As the vacuum pump operates, the water vapour generated gets sucked by the pump and is pushed into a condenser. This is where a person can get creative. We have got water vapour and along with it, a lot of thermal energy in the form of heat and latent heat. As the vapour travels through the condensor, it expands and cools eventually coming to liquid form. It also transfers a lot of heat energy to the condenser. Of course, key to effective design is energy efficiency. So this energy ought to be utilized for something. The sea water input to the chamber can be pre-heated by circulating the sea water through the condenser before piping it to the brine chamber. This ensures that

a. The heat energy stored in the vapour as it becomes liquid is not wasted and transfered to the sea water.
b. The amount of energy required to heat the sea water is reduced at the brine chamber as it is already preheated to a certain amount.

Step 4: Design 4 - Float Valve

A float valve controls fresh flow of the sea water into the chamber and has two purposes
1. It regulates inflow of sea water and maintains a constant volume in the chamber.
2. If the sea water input is reduced or shut off for some reason, the float value goes down as the liquid level reduces. This can be sensed and the solar collector covered or an alarm sounded. If the copper collectors are unable to transfer heat to a medium, they would overheat.

Step 5: Useful Byproducts

At the brine chamber as more and more water gets vaporized, the concentration of the brine solution increases ie. the salt content in the chamber increases. As more and more water is desalinated, the liquid in the chamber soon becomes saturated brine. This can be tapped off periodically. What use is saturated brine?

1. It is used to preserve vegetables, fish, meat etc.
2. Brine is also a common fluid used to transfer heat in large refrigerator installations.
3. It can be used to manufacture common salt (who can live without it?). Just wait till the brine becomes completely saturated. Since the solution cannot dissolve more salt, it starts depositing at the bottom as a residue. Just tap it out.
4. Brine solution is used as the electrolyte in manufacture of Chlorine, Sodium Hydroxide, Hydrogen by electrolysis. Potassium and calcium compounds can also be obtained.

Step 6: Enhancements

Since we have so conveniently generated steam(it is low pressure steam containing a lot of water), it can be used for steam cooking as well. A portion of the steam can be tapped and fed to a steamer of some kind, or microwaves(some microwaves allow external steam inside). And how easy is it to adapt the design for this purpose? A single tap from the collector, thats all.

Step 7: Insulate Properly

I save the most important for the last, the one thing that most people misunderstand or deliberately ignore. The NEED for EXCELLENT insulation of the entire apparatus. The insulation MUST be extremely efficient. And must cover not only the solar trough and the brine chamber, but EVERYTHING including the collector tubes, condensers and all piping involved. Even a square inch of uninsulated pipe could waste a considerable amount of energy. The idea is to trap all the solar energy and keep it in the system.

Step 8: Conclusion

Conclusion. A very simple idea but one that is cheap, simple, effective, green, energy efficient and with 100% byproduct utilisation. Try it and feel the difference you make to the world. Unlimited potable water as a main product and brine, salt and steam as byproducts each of which has many uses. It cannot get better than this.

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    46 Discussions

    The heat exchanger should be constructed with two hot sources so that the sea water coming in cools both the fresh water coming out of the system and the brine from the evap chamber.  If all (or most) of the heat is recovered from the brine then you can continuously be purging the brine so that the increase in salinity is not very significant and it can just be dumped back into the ocean.  Eg: 10L sea water in, 1L fresh water out and 9L slightly saltier sea water back into the ocean.

    Since most coastlines are heavily populated the desalination plant could be located inland with 3 pipes running to it, bunch the pipes together and wrap them with insulation.  This would be your heat exchanger with 2 of the pipes going into the ocean (sea water in and slightly saltier sea water out) and the 3rd pipe branching off to be distributed into the public water distribution.

    In a place like California where inland also means higher elevation then the entire system would run under a natural vacuum due to the weight of the water.  The entire desalination plant would have to run as a closed system, with the only openings being at the lower end of each of the 3 pipes for this to work.

    18 replies

    ...and of course if the plant were about 30 to 33 feet above sea level the water would boil at ambient temperature so you wouldn't have to worry about reclaiming the heat from the brine, although I suppose it could increase efficiency. The only problem I can see is the dissolved oxygen coming out of solution and contaminating the vacuum

    That is a problem. It is not only O2 , actully dissolved air

    It's mostly oxygen, other gasses dissolved in water wouldn't be any worse problem. It would just be nice if there weren't any dissolved gasses at all because then it could be totally maintenance free. If somebody came up with a vacuum pump with no moving parts that would be great.

    I sure did not know that it would only take 33' of water vacuum to drop the boiling point to ambient. But when I say it out loud it makes sense, 100 inches of vacuum is a lot and 33' is nearly 400" of vacuum. This is a difficult amount of vacuum to manage and it would most likely be easier to rely on heat recovery to boost the efficiency. I can't help thinking that if the system were efficient enough it could be solar powered.

    It should work beautifully!
    The only problem I am having with it in my head is how to reliably clear the brine.
    I want to have an inverted Y with the fork right at the edge of the vacuum so the salt water comes up one side and the brine goes down the other with the vapor going up and over to the fresh side. But if the temperature rises then the pressure of the vacuum goes up (from say 0.1 atmospheres to 0.11 atm) the the level will drop below the fork and the brine will be trapped. (and as I said before, dissolved gases coming out of solution and contaminating the vacuum which would have the same effect)
    If someone could solve those two problems you could have a completely passive desalination plant. Anyone could walk down to the beach next to the 40 ft tall tower and pour themselves a glass of free, pure water.

    I love this post and thread! I am desperate to make obtain solar powered fresh water and have the conditions: right by the ocean, about 12 Celsius all year round, rocky (not silty) and high ambient temps, about 22C with 300 clear days per year. also, we have about 100ft elevation.
    Could you please explain how:
    - you assume a 33ft head over the sea level will produce allow boiling at ambient temp. on what principal is this based?
    - you propose to produce a vacuum with this height difference. Is it more a "suction" imparted by the flow of stored sea water from 33ft down to sea level again? I know that 10m of head (about 33ft) = 1 bar or 14PSI in pressure when measured at the base, but are you saying that the opposite, draw, would produce a -1Bar vacuum?

    Thanks!!!

    The principle is simple.
    It has to do with vapor pressure. As you heat a liquid its vapor pressure rises and the rate at which it evaporates goes up. When the vapor pressure equals the pressure pressing down on it can evaporate from the middle. We call this boiling.
    Now, if the pressure pressing down on the liquid is lower then it doesn't need to be as hot to overcome the atmospheric pressure. People who live on mountains know this because they have to boil their eggs longer or add stuff to the water to raise its boiling point back up to 100C.
    If you reduce the atmospheric pressure enough then the water will boil at room temperature. You can see demonstrations of this on YouTube.
    Now, boiling takes energy, no matter how low the pressure is, so you still have to heat it or it will get colder and stop boiling. But you only need to keep it at room temperature.

    re. your second question, take a glass and stick it in a sink full of water. Turn it upside-down and slowly lift it out of the sink. Why does the water stay in the glass? Why does it get heavier the higher you lift it? It has nothing to do with flow.

    Sorry I am getting too tired to think. If the rest doesn't come to you then I will explain another day.

    Good night,
    Gordie.

    Thanks very much for your speedy reply Geordie.
    I get it now, and some youtube videos really helped :) indeed by flow and vacuum we were talking of the same thing: with a pump fill a tank 50feet up, shut off the flow, and the water will drop to 33ft, the space above will be a vacuum.

    The problem I now see is how to extract the water vapor to be condense else where, or the water itself if condensed in the upper vacuum chamber space, without braking the vacuum!! One solution mounts the fresh water collection at sea level (0 feet), which would work when the condenser was installed in the upper vacuum space (www.vacuumdesalination.com), but requires a second pump, and more energy, to return the water where it is to be used (a small tank at about 33ft).

    Actually the system I had in mind was more like this http://www.youtube.com/watch?v=ZZsvGGBVsCc&NR=1
    but with a few improvements.
    I like your idea of the pump. You would only need about 25 PSI pump which should be cheaper than a vacuum pump.
    Then you use a solar collector to heat the salt water and put the fresh water tube in the shade with fins on it. The solar collector doesn't have to be very big, just enough to raise the temperature a few degrees above the air temperature at the condenser.

    Now that I think about it, a vacuum pump would be better for two reasons. First, using a water pump would contaminate the fresh side. Second, the vacuum pump could be used to 'refresh' the vacuum as needed. (ie. remove any dissolved gases and/or tiny bubbles that may come out of the sea water.)

    How about a aspirator vacuum pump circuit powered on the fresh water side? You vacuum cylinder pre-charged with sea water, and closed, with suitable "topup" float valves activating the sea water filling pump. Then a fresh water loop producing additional vacuum via the venturi effect http://en.wikipedia.org/wiki/Eductor-jet_pump that would draw off the vapor from the now boiling sea water and place this fresh water in the loop. Fresh drinking water is tapped off from this loop. http://www.youtube.com/watch?v=ZRZhzX67xHY&feature=related in this video 30in Hg produces a nice boil.

    As you mention, a small capilary solar water heater (pool type) would produce a few more degrees for a stronger boil.

    I like the Bernoulli pump because there are no moving parts but I'm not sure how efficient it is. Also, you would need a source of high pressure water, that's definitely not passive.

    There's only one vacuum chamber. It separates the salt water from the fresh water. You only need one vacuum pump to keep it 'clean' of air.

    I guess I should make a sketch to be clear...

    the sketch is perfectly clear, thanks. What sizing would you give to the main tube (boiling cylinder)?? I have the perfect conditions to try this: sun, ocean, height and power. I would like to start building ;)

    Check out this:

    http://www.eng.usf.edu/~abutayeh/Files/Solar%20Flash%20Desalination%20under%20Hydrostatically%20Sustained%20Vacuum.pdf

    (for some reasion the link meses up, but google "Solar Flash Desalination under Hydrostatically Sustained Vacuum.pdf" and you'll find it

    What Gordie said. Vacuum is of course just a lack of pressure. I watched a show about people who treked to the top of Everest and near the top it showed them drinking tea while it was boiling and they said it was barely warm enough to drink. After thinking about this more I am thinking that maintaining a vacuum while in a boiling condition may be too difficult. especially since you can just run the input and output lines bunched together and wrapped with insulation. If the run is sufficiently long and well enough insulated then by the time the sea water gets to the bioling vessel it should be almost at boiling. At the other end when the waste and drinking water exit they should be almost at sea water temp. How large of a system are you building? I had always thought of this as a municipal sized system.

    Hello Spyder. I would see these devices as single household units, for ocean front second residences in remoter parts of the world. This would mean:
    - water production could be slow, just a couple of gallons a day would be fine, most second residences are not used that often
    - the unit would be small enough to be almost portable, at least would fit in a car. The feeds and returns would be the standard black plastic water pipe, very cheap.
    - ideally it would be 100% solar powerable with less than a 100W panel, this enabling small DC pumps (or vacuum pumps) to run in daylight hours = no accumulation battery

    If you want something that small then why not just use a greenhouse style desalinater? You could use a small solar panel powered DC pump to supply it with salt water.

    Forgot to mention the other advantage to contiuously purging the brine is that there won't be any solids persipitating out and pluggin up the system.  My ex-wife is a chemist and her lab has a glass still for producing distilled water that operates like this.  This system can also be run continuously without being shutdown except for periodic maintenance.

    Used to run 6 stage flash type evaporator plants shipboard in the Navy. 6k gal an hr per plant. We had two plants. Heated with 150psi saturated (wet) steam. Vacuum was drawn on each stage using air ejectors. The water (distillate) would flash over into the next stage where the process was started over again. The brine was drawn off the bottom and pumped overboard. All of the stages had windows so the process could be monitored. Target was 1/2 full max. As for vacuum leaks we used lacquer or varnish on them while it was running. The pressure diff would suck it in and it dried almost on contact.
    There were many times the fuel tanks were stripped (pumped to remove condensation and sediment ) and the evap watch was not told. This meant we drank and cooked with and bathed with water that smelled and tasted like fuel oil. Makes me shudder to this day.
    I brought that up to remind all that if water will flash so will other things so know what is in your source.
    We also had a rupture disc to prevent accidental over pressurization of the final stage.