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Forum Index : Solar : Solar Evaporation Desalination
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MacGyver Guru  Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Out of curiosity mostly, I was wondering if 4m members thought it worthwhile spending time and cash on constructing an efficient solar desalination system. The on-going problem with this sort of thing is most folks merely cover a black tray of water with plastic film, weighted at its center and as saturated water vapor evaporates from the tray, it condenses on the film and droplets of water roll back into a receiver. Unfortunately, these systems by their design allow the sunlight to pass through the translucent film before landing in the black, water-filled tray, which raises the interior surface temperature of the film and thus, thwarts the process. This tray/film arrangement is okay if you're dying of thirst in the desert, but for on-going production, it just won't cut it. A better way is to collect and concentrate solar energy, then apply it to a closed system where the evaporator (remember that plastic film?) is replaced by a mechanically-chilled region. The solar concentration has to be enough to actually "boil" the salt water. With such a greater temperature difference between the vapor and the condenser surface, water production increases by leaps and bounds. Anyway, I think I have a way to do all this for a song, but was wondering if the crew here thought it worth the time to develop it. Most of the 4m folks are from Australia, a place where I've never been. Pictures of the place look pretty harsh and I wonder if there are sufficient aquifers close to the ground surface in most of the country or if a small, but efficient solar-operated desalination devise might fly there. By the way, I realize most folks are thinking "ocean" when I say salt water, but I was actually thinking about brackish or polluted water as well. Water that is the last to leave an exposed water hole out in the bush will be cram-packed with alkaloids more and more as the concentration of dissolved solids (salts) increases with further evaporation. A desalination device would be just the ticket for extracting useable water from such a place. So, what think you? Should I spend some time on this or just concentrate on the other eleventy-seven-zillion projects I already have going on? Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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wind-pirate Senior Member  Joined: 01/02/2007 Location: CanadaPosts: 101 |
Hello MacGyver. The need for clean drinkable water will always be there. When D.A.R.T. moves in they seem to be able to make water magic. BUT, At a "very high" price. A simple system just might be an answer to the general public. I'm not sure if the goverment's wants us to be able to look after ourselves. they would feel unwanted/unneeded. i'm "Very Very" interested in your flat plate solar/liquid collectors. I'm kind of a homesteader by Heart. I use as much wind and solar (power-heating-water) as I can proudce. I Live in Canada, And in the winter our solar needs all the help it can get  . I have propane, Electric and solar Heat. On a clear sunny day we can just about hold our own with the solar heat,on the shorter days.
Even in our - 30 to -40's.c.or f. So you have realy peaked my intrest with your high out put oil filled collectors. My house is in the hills on the south side of a deep valley. I have the perfect location to collect the sun's Rays. (Summer is another story.) Do you have any other 4m's where you have info posted? Thanks Ron THE Pirate. stealing wind & solar energy is fun |
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| Dinges Senior Member Joined: 04/01/2008 Location: AlbaniaPosts: 510 |
[quote]Unfortunately, these systems by their design allow the sunlight to pass through the translucent film before landing in the black, water-filled tray, which raises the interior surface temperature of the film and thus, thwarts the process.[/quote] Not sure what the absorption coefficent is of water & translucent plastics, but I doubt the sunlight passing through clear plastic and water would heat it up a lot. Of course there will be losses, but whether they justify building a complicated contraption when something as simple as shown below will suffice? I doubt it. [quote]A better way is to collect and concentrate solar energy, then apply it to a closed system where the evaporator (remember that plastic film?) is replaced by a mechanically-chilled region.[/quote] Not sure I agree with the better. But it would sure be a lot more complicated than this: 
http://www.watercone.com/product.html Still haven't figured out a decent way to make my own water cone (and buying is cheating), I very much like the elegance of that design. Simple and functional. Any fool can complicate matters.... 
For more water, you can't beat the simplicity of something like this: 
http://www.txses.org/epsea/watersolar/purification [quote]This tray/film arrangement is okay if you're dying of thirst in the desert, but for on-going production, it just won't cut it. [/quote] Refrigeration cycles are fun and everything.... but sometimes the simple solutions may be better - just scale it up a factor n if you want n times more output.... Peter. |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
[Quote=Wind-pirate]Do you have any other 4m's where you have info posted? Nope, this be it! [Quote=Dinges]Refrigeration cycles are fun and everything.... but sometimes the simple solutions may be better - just scale it up a factor n if you want n times more output.... Well, here's the thing: If we "scale up" either of your two scenarios, can you imagine the size of either if we want to say take a 20-minute shower? I don't know about your lifestyle, but here in the US we waste EVERYTHING including water. Like I said, if you were dying of thirst, a small contraption like either of those you mention will suffice, but I'm talking about being able to supply gobs of water for an entire family or maybe even a small community. When I started working on this original post, I had in mind more along the lines of a settlement 100 miles from nowhere, like in the Australian outback, where people live and want more than to merely survive. Thanks for the posts. Edit: Oops! Just thought of something more I wanted to add to Peter's reply, which will help wind-pirate as well: I had in mind a jacketed condenser. By that, I was intending to circulate some of the product water through a jacket surrounding the main exhaust from the desalination boiler after first passing that water through a small (decorative perhaps) spraying fountain. Doing this (spraying fountain), some of the water in the water feature would evaporate and as it did, it would lose a portion of its "latent heat of vaporization" causing a drop in the cooling water's temperature. The increased efficiency of my system will count on two things: First, I'll be actively heating (boiling) yucky water and secondly, it'll be condensing inside a cold-water-jacketed "still" arrangement. Doing each together like this will boost water production several fold. Hope that puts us all on the same page. I'm going to wait to see how much other interest there is before tooling up, but don't get discouraged wind-pirate; if I don't build it soon, I'll share the plans and you can have a go at it on your own.  Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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| Dinges Senior Member Joined: 04/01/2008 Location: AlbaniaPosts: 510 |
[quote]Well, here's the thing: If we "scale up" either of your two scenarios, can you imagine the size of either if we want to say take a 20-minute shower?[/quote] Surely you jest - showering with potable water in such a situation? It's insane we shower and flush toilets with potable water in the West as it is, but where I live at least water is plentiful and cheap. Where it isn't, showering or flushing with potable water should be a crime. Peter. (<-- hands MacGyver a bar of saltwater soap....) PS: using 35 l/day over here. The average in the USA is about 600 l/day, I understand. |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Yup. Sounds about right. Like I said, here in the USA, we waste EVERYTHING! It's the American way. Apparently, many people in Australia capture the rain water off their roofs and use it for household use. When it rains in California, the run-off from all the roofs causes the salinity of the Pacific Ocean to change drastically for several miles out to sea. Still (no bun intended) the thing I'm proposing could be used to turn a muddy water hole into many gallons of pure water without any energy input except sunshine. Of course, that's assuming solar cells could power the little water pump for the fountain, the water transfer from the fountain through the evaporator jacket and so on. I was thinking the water from the mud hole could merely be scooped in by hand. No sense ruining a perfectly good transfer pump with dirt, eh? Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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| GOFJ Newbie  Joined: 22/12/2009 Location: AustraliaPosts: 27 |
A couple of years ago there was an article on the ABC radio which said the CSIRO had inproved the efficiency of solar distalation to (I think) 6 litres per square metre per day (or it might have been 9 litres). They did not say how but a little research in that direction may turn up a few gems. The rate of evaporation is directly proportional to the temperature and inversly proportional to the vapor pressure so you can speed it up by heating the dirty water and applying a partial vaccume to it and cooling and pressurising the resulting vapor, perhaps a combination of solar heating and wind power pumping. Jack |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
GOFJ Pulling a vacuum on the boiler is a bit too high-tech for what I'm talking about, but you're absolutely right about the physics. I was an engineer on a Coast Guard ship and we made our own fresh water exactly that way while under way (at sea). We injected high-pressure steam through a thing called an "eductor" that passed a high-pressure stream of steam through a void and caused a tremendous vacuum. This is the kind of vacuum that would instantly kill you (you'd explode) if you were in the space it was drawing air from. Anyway, the vacuum pulled off the sea water boiler and all the water "flashed" into steam, then the steam was cooled through a sea-water-jacketed condenser and we made hundreds of gallons of fresh water in just a few minutes. After each use, the flash tank had to be flushed, because it would be full of salts. Worked like a champ, but a little too high-tech for the bush! Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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KarlJ Guru Joined: 19/05/2008 Location: AustraliaPosts: 1178 |
I dont quite understand why when perfectly good water falls from the sky. Although I live in the city I use only 30L of mains water per day for a family of 4 a have a 5000L tank (not very big) and we get about 60,000L of rainwater collected each year, I run a simple rope type 6 micron filter (which is just fine) on the outlet and the rubbish bins I use to collect from the downpipes feed into a buried rubbish bin via 1" low pressure hose and then pumped to the tank via a submersible pump and mesh filter (this catches the big stuff to prevent too much buildup in the tank) Water here is too cheap at present at about $2/1000L but with the introduction of a desalination plant soon using Reverse osmosis I envisage that cost will rise significantly. Here in Australia we invented the dual flush toilet which uses ~3L for yellow and 6L for brown but I dont think I've ever needed the bigger flush, water saving shower heads that only flow 8L/min are also underkill as I usually run mine at least half that after all how wet do you need to get? as for drinking water, i'll happily drink the rainwater and ontop of that I find it more pleasant than the chlorinated water from the tap, as for fluoride, there is plenty of that in toothpaste anyway. Luck favours the well prepared |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi All To add my little bit, we designed and built a desal plant for remote use a few years ago it was powered by 6 solar panels about 25 amps average power, to explane the principal there was a solar collector like a HWS only bigger about 10 sq meters and a black painted dome made out of 3mm gal iron that sat on a concrete base six mtr in diameter this was heated by the sun inside temp over 120 deg c on a sunny day the base acted as a shallow tray in the moddle the hot water from the solar collector was injected against a SS flat disk to break up the water into vapour the pressure was 20 psi to prevent boiling in the collector The whole dome was under a low pressure caused by a vacume cleaner type turbine made of SS running on the 24 volt solar panels as was the injector pump. The turbine sucked out the vapor and it was condensed by a buried pipe 4" diameter 20 meter long leading to a storage tank. This thing supplied a huge amount of fresh water just on sunlight, I think it was about 20 ltr min. Note that the 24 volt panels where load panels designed to run without battery connections Solar preheat panels and solar panels where tracked by water pressure bleed and the power was turned on and off by light intensity. Very simple and easy to make. We designed a huge one but it never went ahead due to crashed contract. All the best Bob Foolin Around |
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| Farmer Newbie Joined: 23/02/2010 Location: AustraliaPosts: 2 |
If your close enough to get a good supply of salt water your prolly close enough to make a fog wall and harvest to water instead of desal. A fog wall is basically a car radiator that either has coolant pumped through of cooled via something like a Hilsch vortex tube. The water in the air condenses against the radiator like water on a beer glass, and runs down into a collector. Most coastal areas have enough water in the air to make this work. Cheers Rob |
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| Redman Regular Member  Joined: 12/06/2010 Location: AustraliaPosts: 41 |
To heat in Australia is not an issue unles you live in Tasmania. Solar output in summer is 9000 w x 10m^2 That's enough. To cool and condense just use Geo Thermal. Run a big pipe into the ground about a meter deep. Your local service station has the ground temps at 2 meters written on the pump which is a constant. In Tasmania you go the other way. The ground is warmer... |
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Poski Newbie Joined: 26/06/2011 Location: New ZealandPosts: 2 |
Hello All I am a newbie to this and came across the forum while researching solar still options, some great info on here. I have friends who have run an aid program in Vanuatu for several years now on an outer island that has no fresh water source & relies on bore water (brackish). In drought, drinking water must be collected from the main island by boat and there is a lot of sickness from the poor water quality. The plan was to install a commercial boat desalination system to provide water for both drinking and washing but these are all too complex/unsustainable to be maintained &/ or repaired in this situation. So, the plan is now to look into a solar desal option that is as simple & efficient as possible so any help/ ideas from the forum would be much appreciated. VK4AYQ - I was very impressed with the sound of your system and if it is not breaching any trade secrets, would love to get some more details on how this worked. Was the solar collector evaporated tubes? Or could a more homespun system of pipes be used (and this is used as a preheater before the water was sent to the dome/ solar oven?) Is the hot water then hosed at pressure onto a flat SS (stainless steel?) plate and this spray vaporises due to the heat? We would probably use corrugated iron painted black as the dome/oven portion on a concrete base/ local stone base. I am also a bit hazy on the pumps/vacuum, one pumps the hot saltwater into the oven and the other is an exhaust/vacuum for the vapor? Could you recommend a model/ type that would be minimal maintenance. Also with the condensing, does the vapour condense naturally due to the long length of pipe? We would run all pumps directly via solar panels with no battery set up, I have looked into more simple passive glasshouse type options but none seem to provide enough water output. Anyway, sorry for all the questions and cheers in advance for any suggestions/ advice on this. Many thanks Paul Posk |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Paul For starters, we need to know how much water you need to manufacture. We also need to know how much you're starting with and whether it's salty, brackish or just dirty water. If it's dirty, the product water has to be run through again to be sure to kill all the wee beasties. We'll also need to know your solar situation, meaning how much sun you have and whether skies are normally clear or overcast. As long as you have water to start with and clear skies, extracting the pure water is merely an engineering project. Using oil as a working fluid lets you use a smaller collector, raises the temperatures you can attain, but also adds a degree of danger, since the hot oil can burn and even catch fire if it gets oxidized. Fill in the blanks and maybe we can get you on your way making your own water. Try to keep it as simple as possible. . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Paul The heart of this unit is a dome formed of heavy gauge galvanized steel, it is formed as low pressure vessel to lower the boiling point of water and act as a solar collector, the idea being that the water vaporised as soon as it was blown up into the flat plate at pressure by doing this the amount of water could be vaporised nearly instantly. The water vapor was sucked out by the impeller type of the vacuum pump and blown into a cyclone condenser where it condensed into water due to lower temperature and higher pressure. From memory the air pressure differential was about - 4psi and incoming water was around a 80C. a smallish pressure pump at 50 PSI is all thats required as it is only dependent on the amount of water to be distilled. I think we used a 15 ltr min pump on each heat exchange. You could use black poly pipe laid on wind proof racks above ground to get the water hot enough. possibly a wood base with clear lexan roofing over the top. A low voltage high speed motor drove a stainless steel impeller about 12 inches in diameter, this is the main power consumer in the device as to lower the pressure in the dome was a big job, -.2 ats required six or eight solar load panels, these where designed to operate without batteries to simplify the operation for non skilled people. A possible substitute for this would be a high volume airator turbine used in aquaculture, from memory about a 1000 watt unit would work, this would require an inverter and battery setup and have the risk of electrocution to unskilled workers. Hope thats some help All the best Bob It was switched on and off by voltage from the sun and water temp sensors in the solar heaters. Instead of batteries there was a large bank of low voltage capacitors to act as a starter reserve for the motors. The concrete base tub was fitted with a float switch so when the water level reached capacity the process was switched off and the drain solinoid was activated to drain the base of concentrated salts, this was run into a pond lined with plastic and as the heat of the sun evaporated the water out of this the salts where collected and sold to pay for the on going run costs of the machine. If you use underground water this may not be possible as the chemical cocktail from underground could contain toxic components. Foolin Around |
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vawtwindy Newbie Joined: 23/10/2010 Location: IndiaPosts: 31 |
Interesting VK4AYQ, do you have any pictures of this system.? |
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| Poski Newbie Joined: 26/06/2011 Location: New ZealandPosts: 2 |
Mac and Bob Thank you both very much for the replies. Mac the ideal would be at least 6000 litres per day as that would be the size limit of the current tanks. Even if it efficiently got less than that, then at least it would supply drinking water, washing is a bonus. The water source would be seawater (tidal lagoon rather than open sea)but the actual salt water source is unlimited. In the season when the water is most needed, pretty much every day is clear skys and around 28-30c air temp. I also had an idea borrowing from the solar tower/chimney design, where a corrugated iron solar oven would be linked to a tall updraft chimney. Seawater would be pumped into a base tray in oven and the thermal updraft would hopefully increase the rate of evaporation. The length of chimney would also hopefully be enough to condense the steam enough to remove usable water. would probably try some sort of heat exchange so that incoming cold water helps condense chimney steam and may use some type of local screen or fabric to increase surface area exposed to draft. I completely agree that it needs to be simple and I am a bit lacking in the engineering skills for anything too complex, but any suggested welcomed. Bob, thank you for the detailed set up, this sounds like an amazing system and I was really interested in the sheer volume of water you were able to get. Ideally we would have a more passive system that could be maintained by locals if needed, but I will discuss the set up with my friends and see what budgets are in place. We could probably work out a cut down /localised version with a bit of brainstorming. Thanks again for your help. Posk |
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Rastus Guru Joined: 29/10/2010 Location: AustraliaPosts: 301 |
Hi Mac, I'm surprised to see the underwelming support from Aussies so far to your proposal!We have a very arid country and the recent floads brought a 10yr to 12yr drought to an end.Thats why we can have a shower by running around really fast and catch every drop of water that comes out of the rose.One of the few people not collecting his shower water in a bucket was the prime minister.We also have the "She'll be right" when offered a hand,we're a proud bunch you know.During the last drought a large town where asked to drink their own recycled urine. things were desperate.I don't want to get offside with my freinds here so I'd like to suggest that stock would significantly benefit from improved water quality.Large amounts of artesian water used for them is brackish and its tuff to say the least on young livestock.Older animals can tolerate it better to a point.Having cheap effecient desal units available would be a great asset.Australians are poised to pay ever increasing sums for water.These comments have been made without market reseach into commercially available units and the 1 zillion other projects you have.Cheers Rastus see Rastus graduate advise generously |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Posk It can be scaled up and down as required and with a bit of scrounging can be got running at a reasonable cost, one we did used a old fuel tank 12 ft in diameter as the evaporator and a 200 liter metal drum and some sheet metal as the cyclone. The main costs are the solar panels and pumps. It may sound complicated but can be run by unskilled people with a little training. Using sea water the salt recovered can pay for the unit in time if a market is sourced. All the best Bob Foolin Around |
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Everard Newbie Joined: 30/08/2011 Location: TanzaniaPosts: 3 |
I came across your discussion while looking for ideas on how to produce irrigation water for my fruit trees from brackish well water. Very interesting indeed. 2 questions that I would like to pose: 1. In East Africa we pretty much have to use readily available materials and adapt them. We have large 5000 litre black plastic tanks that could act as heat absorbers and might serve as the 'dome' for a system such as you describe but without the pressures described. 2. Another resource we have are the large clear plastic 9 litre water containers - seeing the watercone idea, I wondered whether it might not be possible to adapt these 9 litre containers - particularly in multiple banks of them - to act as a kind of watercone factory to provide drinking water for our coastal dwellers. At present the locals have to travel by foot or bike very long distances to collect potable water in gallon containers and anything like a watercone, even if not so efficient, might do the trick. Any ideas? East African coastal farmer |
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