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Question: why do desalination plants use a filtering system, instead of evaporation? (like that desert survival trick of spreading plastic over a hole) (guessi

by 6ren 14y ago

Question: why do desalination plants use a filtering system, instead of evaporation? (like that desert survival trick of spreading plastic over a hole) (guessing) Is it because (1) evaporation is actually very energy inefficient compared to filtration, especially at scale; or (2) evaporation doesn't separate out impurities that also evaporate.

kijin 14y ago
(1) is definitely a factor. It takes a very large amount of energy to boil water. https://en.wikipedia.org/wiki/Desalination#Methods https://en.wikipedia.org/wiki/Desalination#Methods > Reverse osmosis plant membrane systems typically use less energy than thermal distillation, which has led to a reduction in overall desalination costs over the past decade. But the wikipedia article says that 85% of desalination worldwide still uses distillation.
- ams6110 14y ago
  Seems like a good area to use solar energy though. Water is easy to store and if you can make enough on sunny days to carry you through nights and overcast times, efficiency doesn't really matter as much since your energy source is "free."
  - excuse-me 14y ago
    But then you need a somewhere sunny, with low land prices on a coast with a large demand for fresh water. Otherwise you have to pipe the fresh water uphill to the inland cities - much easier to pipe it downhill from the mountains. Apart from Australia, agricultural bits of california and some parts of the middle east there aren't many places that fit.
  - planetguy 14y ago
    I'm sure it's possible, and that someone has sat down and done the mathematics, and figured out that it's just not worthwhile. The most efficient way to do it is to use solar thermal, not solar electric, power -- build a huge array of mirrors to focus on heaters which boil your water and distill it out. But it's not really "free" -- you need a lot of land, a lot of mirrors, and a staff of people who keep the mirrors clean and replace the broken ones. Oh, what the hell, I'll do the maths myself. To heat up and boil one litre of water from 20 degrees C takes 2.6 million joules. Sunlight, on a sunny day, is one kilowatt per square metre; let's generously assume we can get 50% efficiency over the eight hours of the day when the sun is high in the sky [don't forget, this is solar thermal power, it's more efficient than photovoltaics]. So each square metre of collecting area in our plant can boil 5.5 litres of water per day. The desal plant in this article produces 27.5 million gallons per day so to replace it with solar we'd need nineteen million square metres, or nineteen square km, of mirrors. That's a lot of mirrors, and a lot of squeegee men. If you wanted a solar-powered desal plant I'm sure you could do it cheaper using photovoltaics plus reverse osmosis.
    - phreeza 14y ago
      Wouldn't it be possible to somehow recover the latent heat of the boiling water when it passes back into the liquid phase, minus the salt? I'm pretty sure I learned the theoretical maximum efficiency for this in my undergrad physics class but I can't quite remember.
      - planetguy 14y ago
        Good point, you could get some energy back that way.
      - kijin 14y ago
        Maybe you could use the steam to pre-heat the water that enters the system? If the water is at 60C instead of 20C to begin with, that will reduce the amount of energy needed to boil it. Besides, all that steam needs to be condensed back to water anyway if people are going to drink it. (I think that's what they already do in advanced "multi-stage" distillation plants.) Theoretically, you could also add a steam turbine to make some extra electricity on the side, but I don't know how efficient such an add-on might be.
excuse-me 14y ago
4200 J/Kg/Kelvin and 2260 J/kg Pretty much anything you can do is cheaper than boiling water
- uvdiv 14y ago
  (Your second number is off -- it's 2,260 kJ/kg or 2,260,000 J/kg) There's no therodynamic objection that you can't recover (virtually) all the heat you spent creating steam, when the steam is recondensed back to water. Which is what they do. Distillation methods (multi-stage flash (MSF), multiple effect (MED)) "only" use around 200-400 kJ/kg of heat per water output. http://www.desware.net/desa4.aspx http://www.desware.net/desa4.aspx Some perspective: at $4/thousand cubic feet natural gas (US of course), this is a heating cost of $0.75-$1.50/cubic meter of desalinated water. High, but not impossible. http://www.eia.gov/dnav/ng/ng_pri_sum_dcu_nus_m.htm http://www.eia.gov/dnav/ng/ng_pri_sum_dcu_nus_m.htm
simcop2387 14y ago
My understanding is that it's all about efficiency. The impurities I don't think is too much of a problem (think about distilled water, they make that with evaporation), but filtering is much easier to scale. You can increase pressure, increase the number of filters, etc. to get it all to happen at a faster rate. combined with the fact that you don't lose any water from it and you can probably do very very well there.
- excuse-me 14y ago
  Depends what your impurities are. If you have a lot of other liquids mixed in (oil , organic solvents etc) then distillation can be easier - it also lets you separate them for disposal. Small scale pure 'distilled' water eg. for battery top-up, is typically made by ion-exchange resin - it's more effective than osmosis for removing very small amounts of inorganics and cheaper than actual distilling if you are starting with fresh water.
stcredzero 14y ago
that desert survival trick of spreading plastic over a hole Solar stills are cost effective at very small scales, even for devices that are built in a more permanent fashion than plastic sheeting over a hole. There was a program where the US government gave rural families solar stills in the North American southwest a number of years back.

by 6ren 14y ago

kijin 14y ago

(1) is definitely a factor. It takes a very large amount of energy to boil water. https://en.wikipedia.org/wiki/Desalination#Methods https://en.wikipedia.org/wiki/Desalination#Methods > Reverse osmosis plant membrane systems typically use less energy than thermal distillation, which has led to a reduction in overall desalination costs over the past decade. But the wikipedia article says that 85% of desalination worldwide still uses distillation.

ams6110 14y ago
Seems like a good area to use solar energy though. Water is easy to store and if you can make enough on sunny days to carry you through nights and overcast times, efficiency doesn't really matter as much since your energy source is "free."
- excuse-me 14y ago
  But then you need a somewhere sunny, with low land prices on a coast with a large demand for fresh water. Otherwise you have to pipe the fresh water uphill to the inland cities - much easier to pipe it downhill from the mountains. Apart from Australia, agricultural bits of california and some parts of the middle east there aren't many places that fit.
- planetguy 14y ago
  I'm sure it's possible, and that someone has sat down and done the mathematics, and figured out that it's just not worthwhile. The most efficient way to do it is to use solar thermal, not solar electric, power -- build a huge array of mirrors to focus on heaters which boil your water and distill it out. But it's not really "free" -- you need a lot of land, a lot of mirrors, and a staff of people who keep the mirrors clean and replace the broken ones. Oh, what the hell, I'll do the maths myself. To heat up and boil one litre of water from 20 degrees C takes 2.6 million joules. Sunlight, on a sunny day, is one kilowatt per square metre; let's generously assume we can get 50% efficiency over the eight hours of the day when the sun is high in the sky [don't forget, this is solar thermal power, it's more efficient than photovoltaics]. So each square metre of collecting area in our plant can boil 5.5 litres of water per day. The desal plant in this article produces 27.5 million gallons per day so to replace it with solar we'd need nineteen million square metres, or nineteen square km, of mirrors. That's a lot of mirrors, and a lot of squeegee men. If you wanted a solar-powered desal plant I'm sure you could do it cheaper using photovoltaics plus reverse osmosis.
  - phreeza 14y ago
    Wouldn't it be possible to somehow recover the latent heat of the boiling water when it passes back into the liquid phase, minus the salt? I'm pretty sure I learned the theoretical maximum efficiency for this in my undergrad physics class but I can't quite remember.
    - planetguy 14y ago
      Good point, you could get some energy back that way.
    - kijin 14y ago
      Maybe you could use the steam to pre-heat the water that enters the system? If the water is at 60C instead of 20C to begin with, that will reduce the amount of energy needed to boil it. Besides, all that steam needs to be condensed back to water anyway if people are going to drink it. (I think that's what they already do in advanced "multi-stage" distillation plants.) Theoretically, you could also add a steam turbine to make some extra electricity on the side, but I don't know how efficient such an add-on might be.

excuse-me 14y ago

4200 J/Kg/Kelvin and 2260 J/kg Pretty much anything you can do is cheaper than boiling water

uvdiv 14y ago
(Your second number is off -- it's 2,260 kJ/kg or 2,260,000 J/kg) There's no therodynamic objection that you can't recover (virtually) all the heat you spent creating steam, when the steam is recondensed back to water. Which is what they do. Distillation methods (multi-stage flash (MSF), multiple effect (MED)) "only" use around 200-400 kJ/kg of heat per water output. http://www.desware.net/desa4.aspx http://www.desware.net/desa4.aspx Some perspective: at $4/thousand cubic feet natural gas (US of course), this is a heating cost of $0.75-$1.50/cubic meter of desalinated water. High, but not impossible. http://www.eia.gov/dnav/ng/ng_pri_sum_dcu_nus_m.htm http://www.eia.gov/dnav/ng/ng_pri_sum_dcu_nus_m.htm

simcop2387 14y ago

My understanding is that it's all about efficiency. The impurities I don't think is too much of a problem (think about distilled water, they make that with evaporation), but filtering is much easier to scale. You can increase pressure, increase the number of filters, etc. to get it all to happen at a faster rate. combined with the fact that you don't lose any water from it and you can probably do very very well there.

excuse-me 14y ago
Depends what your impurities are. If you have a lot of other liquids mixed in (oil , organic solvents etc) then distillation can be easier - it also lets you separate them for disposal. Small scale pure 'distilled' water eg. for battery top-up, is typically made by ion-exchange resin - it's more effective than osmosis for removing very small amounts of inorganics and cheaper than actual distilling if you are starting with fresh water.

stcredzero 14y ago

that desert survival trick of spreading plastic over a hole Solar stills are cost effective at very small scales, even for devices that are built in a more permanent fashion than plastic sheeting over a hole. There was a program where the US government gave rural families solar stills in the North American southwest a number of years back.