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.

Comments

[azapa]

At what temperature do you propose to "boil" the water? If it is at ambient with suitable vacuum, would the brine, despite its salt content, not tend to freeze as its latent energy is given off to the boil?

[nanoBorg88]

There's also the Knudsen pump, its a pump with no moving parts which works of a thermal gradient. I've only heard of this recently so don't know whether it will be suitable, but I imagine it could easily be powered by the sun also.

[Zues.gods]

The electrical costs for your vacuum pump might be prohibitive. You will also need pumps for thermal fluid circulation, for the salt water in and fresh water out. You have an energy mismatch between your condenser fresh water out and your salt water in. In your sketch you are showing the high-pressure side of your vacuum pump going into your condenser. This will put the Heat of Vaporization when your fresh water condenses into your condenser [965 BTU/lb] as well as cooling it down from 100 C [212 F]. I think you want a condenser coil inside your Brine Tank to condense the vapor and recover its Heat of Vaporization inside your brine. Then pump out the fresh condensed water at 100 C [212 F]. This is the principle of a Vapor Compression Still. You might have 5 degrees of superheat on the pressure side of this vacuum pump that can be recycled into your brine vaporizing water while the fresh water is condensing. You also want to dump the heat from the motor to your vacuum pump here too. You might be able to get a Coefficient Of Performance (COP) of 5 or 10 here, that is, make 5 or 10 units [lbs] of water for what 1 unit of water would cost you otherwise. Good luck.

[67spyder]

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.

[GordieGii]

...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

[67spyder]

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.

[GordieGii]

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.

[azapa]

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!!!

[67spyder]

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.

[azapa]

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

[GordieGii]

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.

[GordieGii]

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.

[azapa]

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).

[GordieGii]

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.

[GordieGii]

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.)

[azapa]

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.

[GordieGii]

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...

[GordieGii]

OK, here's a sketch (please don't laugh)

[azapa]

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

[67spyder]

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.

[tatay66]

Good idea similar to something im working with already. You should try google sketchup so its 3d and more comprehendable to other readers. Also if you have researched solar thermal add in a thermal storage for heating capacity after hours. Thermal storage can be made as simply as using an insulated tank with gravel for the storage. You do have one fatal flaw in this design you havent studied up on reflected angles, the trough wouldnt be able to sit in that position to maintain the heat. Check out the thermo electric towers Solar one, and the australian wind convection tower, the troughs used in the nevada power station are probably a good place to get your idea in a working configuration. Consider using a complex trough called a complex parabolic with an evacuated tube from a solar water heater that would do what you want without moving it around.  PS google sketchup is free.  

[GordieGii]

Actually, the solar trough would be excellent for the vacuum version of this system.

You obviously haven't noticed that the sun crosses the sky in a straight line. If you align your troughs east to west you only need to adjust the plain parabolic reflectors once a week to once a month to keep them in perfect working alignment with seasonal change in elevation. If you were referring the Nevada Power Company's CSP plant 'Nevada Solar One' it seems they are using what I just described. (http://en.wikipedia.org/wiki/Nevada_Solar_One)

I like your idea of the evacuated tubes.

By the way, could you recommend an alternative to Google Sketchup for those of us using Unix or Linux?

[tatay66]

Every intelligent person knows the sun travels from east to west so your comment wasnt special. Anyhow im the one that said to take a look at Nevada solar plant with the troughs so who ever did this project would have an idea what to use. Not so you could act like you know something and demean someones helpful comments. Also the resources for doing the math to know the exact angles for the troughs during the year can be found at (http://eosweb.larc.nasa.gov/sse/) If you had actually paid attention to what i said you would have taken the time to look up a complex parabolic trough. Then your comment to me would be more informed. I believe the idea was to keep a limit on the size of the unit, which is why i suggested the complex trough. Also if you had looked up the complex trough you would know it reaches temperatures over 400 f Using evacuated tubes. Thats more than adequate to do this job. first pic is complex parabolic trough, second pic is point focus dual trough NOTE: the point focus dual trough can melt metals.

[ipeca]

I assume that the production capacity is based on the size, the effectiveness of the parts and the insulation factor. If that is true and hypothetically all is working as it should. How do you calculate the capacity? Thanks,

[GordieGii]

I believe it takes about 40 calories to evaporate 1 gram of water. Of course you get 40 calories back out when it condenses.

Somebody else please do the math.

[GordieGii]

This of course is unnecessary if it works at ambient temperature.

[Rich285]

It would be great to see a sample kit put together along with a parts list and a drawing. We really need something like this.

[Anisa thaslim]

Hey the idea is cool. could u tell me cheap and effective material by which the condensor and the boiler can be built?

[xarlock667]

Grandpa taught me how to build a still when I was about 12. (Thanks Grandpa!) and most of it can be easily constructed. For a tiny one use a pressure cooker, drill a hole in the lid and use some hi-temp water tubing run through some cool water to condense the steam back into water. Simple enough.

If you do not own a pressure cooker, then any pan that has a lid that can be clamped on will work fine. Just make sure that your clamps are SECURE before you start. Steam burns hurt bad.

[johnny3h]

I once made a condenser for another purpose [Ethanol Still] and it should work great for this applicaiton also.  However, the sizing will depend on the flow rate of the "boiler," and thus I cannot give dimensions for this Solar Still application.

For my condenser outer shell, I used a 24 inch section of 4 inch diameter PVC plastic pipe.  For the actual condenser tube I used 1/4 inch soft Copper tubing.  I "wound" the Copper tubing on a piece of 2 inch pipe used as a mandrill.  This resulted in a coil of Copper tubing that looked like a big machine screw.  I then gently pulled the ends of the Copper coil to slightly "spread" the coils apart to allow liquid flow around the outside to contact all of the tubing surface.

I then drilled 1/4 inch holes in two PVC pipe caps for the Copper tube ends to pass through.  Since the operating pressure was going to be so little, and the maximum operating temperature something like 190 F, I used an ordinary Hot Glue Gun [with 160 degree F glue sticks] to seal the Copper tubing where it passed through the PVC end caps.

I assembled the condenser unit by forcing the end caps on the 4 inch pipe, AND to allow for disassembly if needed, I sealed the end caps to the pipe with the hot glue.  I do not recommend using PVC pipe cement as once assembled, there is no way to disassemble if necessary.

I had earlier drilled an addition hole in each of the end caps to insert barbed hose bibs for cooling water inlet and outlet, also which was sealed with hot glue.

I used plastic tubing to connect to the condensate inlet and outlets, and for connection of the cooling water inlet and outlet supply.

That worked great for my application, and should be fine for this application also.

[Anisa thaslim]

Thank u for the information!

[kurtiskrump]

Great concept!!!!

In the  first stage, using copper as a heat exchanger in a saltwater enviroment  may work for awhile, but very soon it will corrode and you will have a green mess...
  I am a Scuba Diver, and  I have seen how fast this happens...I think, though expensive, you would want to use pure titanium as the heating coils.  I have a friend in the saltwater fish tank industry, and he made a cooling coil for the tank and he had to go with this....

Also, I like the idea of a vacuum pump, but it seems like a weak element in the system, i.e., any hole of any size will compromise the vacuum...

[pubdc]

Great artcile. How could this be cobstructed cheaply ? Can a boiler function as condenser ? When they use molten salt as the transfer medium in solar towers, can we use the heavy-concentrated-brine as medium to pass through the concentrated solar throughs ? Would be cool if some further practical help could be offered ... anyone ?

[very_best13]

nice project, i think its useful on a limited output, but i've got a question -about the vaporized water, is it a 100% H2O or some of the water ingredients can go with the steam? i mean the sodium is bad for human body and we don't want it with the output. if we need more heat we can put a parabolic heater before the tank so the water enters hot and another parabolic heater "which u've already put on the tank" is heating more and more.

[PKM]

Some of the things that can be dissolved in water will evaporate so would come out with the vapor, some don't. Sodium (as part of sodium chloride) is one of the things that doesn't- it stays in the brine solution. The resulting water may not be 100% pure, but it will be free of the vast majority of salts (hence "desalinated"), and of course the plankton, seaweed, bacteria, plastic bottles etc. you find in seawater :) If you are thinking of building one of these, I'd recommend treating the water as if it's non-drinkable fresh water and sterilising by boiling or using Steritabs if you want to drink it. In large-scale use the water output would presumably be treated with chlorine like regular tap water.

[very_best13]

Aha, i have thought of this later, you are right, and its also very useful thing to do. thanks for the information and I hope to make this project :D

[Johenix]

The waste brine would also be a good source of Bromine, Iodine, and Magnesium as well as Sodium, Chlorine and Potassium. Distilled water is absolutely pure H2O (with only a trace of Deuterium Oxide- Heavy Water).

[catprog]

Can you run a turbine off the steam for energy?

(and then use it to make provide energy for http://en.wikipedia.org/wiki/Magnesium#Magnesium_from_sea_water )

[tech-king]

dont think so; not enough pressure, too much water. you need to raise the pressure and superheat the steam. plus, turbines are designed specifically for a certain input pressure.

[problah]

I'm sure you can run a solar panel/chargecontroller/battery/inverter configuration off of the side of the plant to provide power for the vaccum pump, considering this is most efficiently a daylight operation.