[jandek1]

Desalination: Dutch firms develops high efficiency solar distillation unit: In 2004 Zonnewater BV (The Netherlands) developed a prototype desalination unit based on solar energy (95% thermal and 5% photovoltaic or wind energy), suited for coastal areas with an average temperature of 30 degrees Celsius. However, at lower temperatures, the system will work as well but output will diminish. At higher temperatures the output will proportionally increase. The prototype, installed on the Caribbean island of Bonaire, is a small one cubic meter greenhouse-type construction that produces 40 litres of water per day (lpd).

The extreme high efficiency rate is a result of specialized electronic equipment used to enhance air circulation between the evaporation and the condensation unit. Periodic washing with seawater reduces the negative effect of solar reflection by the salt produced by the unit.

Apart from the small electronic component the whole installation can be made locally with materials available in nearly every part of the world, being glass and PVC piping and appendages. A medium skilled plumber can make the installation, and the electronic device will be shipped in combination with simple drawings. The price per litre of water will therefore be determined by the price of glass, 8 local construction hours of labor and the local price of PVC piping and components, silicon kit, and PVC glue.

The unit has an estimated lifetime of 20 years, thus determinate the number of sunny days the average temperature and your price per litre is known. If buyer need more then 40 L of water per day he should calculate with 30 Litres per day per additional cubic meter. A two cubic meter installation delivers 70 Litres and a three cubic meter installation delivers 100 Litres etc.

For desert areas with higher day temperatures Zonnewater is developing a hybrid solar-still with most likely similar yields as being reached with the Caribbean type. For additional info see our website link

[Permanently_Baffled]

link This looks like good news? It says a cubic metre of fresh water can be produced for 52 cents , could technology like this provide a source for hydrogen at a more acceptable cost?

[dmtu]

I'm not sure why this made the news. The Saudis had a reverse osmosis plant at King Fahd International Airport outside of Darin in the early 90s.
Some of the guys stationed with me actually became licensed operators.

[guest]

According to the article, the breakthrough is the cost ... They even mention Saudi Arabia in the article.

[Grasshopper]

link This looks like good news? It says a cubic metre of fresh water can be produced for 52 cents , could technology like this provide a source for hydrogen at a more acceptable cost?

Good news, yes. Reverse osmosis cost breakthrough to provide better quality water at only 10% more than their water from surface sources or wells. It could be also be used to desalinate groundwater that is too salty to be used at present in desert areas that have salt-water aquifers.
Unfortunately reverse osmosis can't be used to obtain hydrogen from water. You could, however, recover drinkable water from using hydrogen in fuel cells, getting an added benefit that possibly makes the electolysis of unpotable water - fuel cell cycle more viable.

[Permanently_Baffled]

Thanks Grasshopper, what I meant was that I thought clean water was needed for electrolysis to produce hydrogen.(you couldn't use sea water too much crap in there!). But now this can be done cheaply it may make hydrogen a little more viable.

[giengrene]

Yes, you do need cheap clean water, and lots of it to make hydrogen... Maybe one part of the Hydrogen problem has been solved?
Anyhow, we now need a chaep and reliable source of ENERGY to power the Fuel Cells, and A cheap fuel cell (one that does not use platinum).. So only 2 strikes against hydrogen now?

[ernest]

As often, the news article reports the positive news, and ignores the negative. The huge amounts of salt, potassium, heavy metals, bacteria, organic matter and other junk have to go somewhere. if they go back into the ocean at the source then the problem is exacerbated. If they are piled up the problme is not dealt with. The osmosis process does not differentiate the waste product, it just comes out a highly toxic substance that is WATER SOLUBLE and has to be dealt with. TANSTAFFL

[MarkR]

Well, as the residual brine is just seawater with a little bit of pure water removed, it can just go back into the sea. After all, the water extracted with a desalination plant is trifling compared to the amount lost by natural evaporation. Additionally, as the water that is used, will eventually get back into the sea, there is no real problem.

The only issues are to put the return pipe well away from the inlet pipe (so the brine doesn't get sucked up back into the plant) and place the return pipe where there is a vigorous ocean current, so that the brine is rapidly diluted into the sea so doesn't upset the marine life.

[ernest]

I doubt that the residual water is "just seawater". That is a bit like saying the water that is used to process tar sands is "just creek water that has a little petroleum in it." You may not grasp how much mineral content, heavy metal, and just crap is in seawater and has to be removed. Salt is produced in many areas by just evaporating seawater, and in California the detritus in the Cargill Salt Ponds on the SF Bay is laden with toxic materials and is a serious environmental problem, which Cargill cleverly sold to the state to let them deal with.

The idea that it can be dumped into the ocean is probably similar to the idea from fifty years ago the nuclear wastes could be dumped into the ocean. I imagine if you get it out far enough, great. But that is going to be MILES offshore into a deep canyon, not in coastal waters. When you purify seawater you also end up with all the impurities. Do you really want to reintroduce them into the ocean in concentrated state? How large a sterile area do you think that will create at the effluence?

[MarkR]

I doubt that the residual water is "just seawater". That is a bit like saying the water that is used to process tar sands is "just creek water that has a little petroleum in it."

The difference is that all the dissolved solids in the effluent were already in the seawater so start with. Even then, the effluent is not usually much more concentrated than the seawater - typically it's concentrated by about 25-40%. If the water inlet was brackish because it was near a river outflow, the effluent can be less concentrated than the seawater.
When you purify seawater you also end up with all the impurities. Do you really want to reintroduce them into the ocean in concentrated state? How large a sterile area do you think that will create at the effluence?[/quote]
Not very big. In fact studies of commercially operating RO plants (yes, they've been around for many years) suggest no significant effect on the marine life. Studies on the salt concentration near plants demonstrate that the change in concentration of the sea due to natual seasonal variability dwarfs that of the plant.

Remember, the sea is constantly having trillions of gallons per hour of pure water removed by natural processes. It doesn't suddenly start crystalising and the lifeforms within start dieing from build up of toxins. A few million gallons a day for desalination isn't going to make much difference.

[ernest]

a few million gallons a day is not going to change the face of the mideast either. Natural evaporation takes place over the entire surface of the ocean. Desalination takes the brine and concentrates it one place. Not a problem for one plant. As a solution to third world water shortage? I have my doubts.

[Linus]

I think solar energy also can be used to produce water from the sea for some coltivations(sugar cane, sweet sorghum etc...) because of to produce ethanol in the desert.
So what's the energetic efficiency of sea water desalination systems?

[MarkR]

The energy efficiency depends on the type of system used to purify the water and the source of the water. The most efficient desalination plants use a technique called reverse osmosis (RO). Essentially, they pump water at high pressure past special filtration membranes. The salt goes through the filter much more slowly than the water, so the water you get out is purer. Repeat the process about 5 or 6 times, and you get drinkable water.

This type of system needs between 4-6 kWh of electricity for every 1000 litres of drinkable water obtained from sea water. If you use less salty water (from an estuary or tidal part of a river) then you can use less electricity. The problem with this system is that the filter membranes don't last very long - only about 3-4 years before they have to be replaced. This is very expensive as the filters are most of the cost of the plant.

Alternative approaches use several variations on distillation - essentially evaporating water and recondensing it. These need huge amount more energy, although only about 3-6 kWh of that needs to be electricity. The remaining energy (often 20 - 30 kWh per 1000 litres) can be supplied as heat. This can come from cheap low-grade solar panels, or you can use waste heat from a power station.

The advantage of this technique is that it does't matter how salty the water is - a single cycle through the plant will give very pure water - much purer than RO, which means less salt build up in soil if used for irrigation. Even though the nominal energy requirements of this process are much higher than RO, the ability to use low-grade energy means that it is a viable competitor to RO.

[katkinkate]

Wouldn't be viable to use the sun directly? Direct sea water into a shallow black basin, under a glass ceiling. The sun will evaporate the water the warmed wet air could be directed out of the evaporation chamber into a condensing chamber where it's cooler and the water condenses on the walls and runs down into a cistern.

Minimal energy.

Let the tide fill one tank, slow gravity feed into the evaporation chamber and the hot, humid air will flow naturally upwards toward the condensing chamber, gravity feed into the cistern. You might need just a little energy to keep the condensing chamber a little cooler than outside, maybe a heat pump.

[JohnDenver]

Here's a low-tech idea: Float large rafts on the ocean to collect the rain which falls there. This would eliminate the costs involved in occupying land, as well as the energy costs of filtration/distillation.

[katkinkate]

Large open rafts would have a high evaporation rate. Better to have a tank floating in the water with 'sails' surrounding it to catch the water and direct it into the tank. You could have a small alarm beamed to a central point to signal when the water level reaches a certain point and a water tanker could go out and pump it out.

[Doly]

Using the sun for distillation is doable, but the amount of water obtained is fairly low.

[Andy]

There is another potential seawater desalination system available for the tropical maritime areas. It is Ocean Thermal Energy. Basically, the process is flash evaporation of surface seawater with cold deep seawter used for condensation. It can be used in combination with the electricity producing part of the plant whereby a refrigerant type fluid is driven through turbines. This is an extremely appealing technology for small remote tropical islands, especially dry ones. I think the efficiency of the OTEC electricity process is about 3 - 5% but the fuel is warm seawater. The efficiency of a pure desalinating OTEC system (no electricity production) should be similarly in the range of 3 - 5%.

[frankthetank]

in AU...sounds like a good idea

http://www.bbc.co.uk/weather/world/news ... news.shtml

n west Australia drought occurs on a fairly regular basis. Although there is not one at the moment, the local Government have unveiled plans to build the biggest Australian desalination plant, powered by 50 giant wind turbines, in order to be able to provide water to its residents.