3 ms·

Desalinization is already with about a factor of 2 or 3 of the thermodynamic optimum, so improvements can't be revolutionary. Desalinization mostly needs cheap

Desalinization is already with about a factor of 2 or 3 of the thermodynamic optimum, so improvements can't be revolutionary. Desalinization mostly needs cheap clean energy.

throwaway91111 10y ago
Also, waste is a major problem, environmentally.
- Baeocystin 10y ago
  What is the best practice for dealing with the brine? Reinjection in to a current? Diffusion over a long pipe? Evaporation in on-shore ponds?
- jessriedel 10y ago
  Are you talking about the brine? This is trivial if you just put a pipe a few hundred yards out to sea. The concentration is diluted extremely quickly. (The only exception is if your desalination plant is below sea level because then you have to pump the brine up hill, which costs some energy.)
  - throwaway91111 10y ago
    I mean, I sure hope so.
  - dredmorbius 10y ago
    I've seen the assumption that the brine dissipates quickly challenged. Fluids can remain subbornly un-mixed, over large distances and for long times. Given that brine is heavier than seawater, it will almost certainly tend to sink and flow along bathymetric contours, perhaps pooling in local low spots. And sealife can be exceedingly sensitive to changes in temperature, ion density, salt content, etc. This reminds me of a book in which an ecologist described a discussion with a chemist over the concentration of some pollutant in seawater. Roughly, "Assuming a well-mixed solution..." starts the chemist. "How are you planning to stir the oceans?" asks the ecologist....
    - jessriedel 10y ago
      I've seen data on this and dilution was extremely fast, as expected.
marmshallow 10y ago
Can you describe this thermodynamic optimum or link me to something? Curious
- jessriedel 10y ago
  > A state-of-the-art facility is now within a factor of two of the theoretical energy minimum, and only 25 percent higher than the realistic minimum for the current reverse osmosis process. In short, it’s going to be tough to squeeze too much more energy out of reverse osmosis, and we’re unlikely to find an alternative method of desalination that will provide a significant boost over that. > But that doesn’t mean that there are no other ways of getting better output for our energy. The total process of desalination turns out to require three to four times the theoretical minimal energy use, since the salt water must be pumped and pretreated, the membranes maintained, and the resulting brine handled afterwards. Some of these things might be amenable to further improvements, and there has been work put into developing membranes that don’t clog up as easily or better pre-filtering of biological materials. http://arstechnica.com/science/2011/08/desalinization-is-this-as-good-as-it-gets/ http://arstechnica.com/science/2011/08/desalinization-is-thi... More: http://science.sciencemag.org/content/333/6043/712 http://science.sciencemag.org/content/333/6043/712 https://www.researchgate.net/profile/Menachem_Elimelech/publication/51547620_The_Future_of_Seawater_Desalination_Energy_Technology_and_the_Environment/links/02e7e51e88c0a7f890000000/The-Future-of-Seawater-Desalination-Energy-Technology-and-the-Environment.pdf https://www.researchgate.net/profile/Menachem_Elimelech/publ...
deleted 10y ago
[deleted]
leot 10y ago
Even with forward osmosis?
- jessriedel 10y ago
  Not really. Basic thermodynamics tells you that if your only input is water with a certain salt concentration and your output is water with a lower salt concentration (plus either brine -- very salty water -- or dry salt), then there is a minimum free energy cost. > Based on thermodynamic principles and practical kinetic requirements, the theoretical minimal energy for desalination with FO is always higher than that without FO. In other words, using FO cannot reduce the minimum energy of separation. https://www.researchgate.net/profile/Shihong_Lin/publication/271706340_Forward_osmosis_Where_are_we_now/links/54d992e10cf25013d042517a.pdf https://www.researchgate.net/profile/Shihong_Lin/publication... The idea behind forward osmosis is to replace the salt in water with some other solute, like sugar. A strict 1-particle-to-1-particle replacement can (I think) have arbitrarily low free energy cost, but then you're just stuck with sugar water instead of salt water. In certain special cases this is fine (like for emergency water generation at sea, when you don't mind drinking sugar water). Insofar as low levels of sugar can't be tasted, and insofar as you have access to free sugar, you can avoid the energetic cost of removing some of the salt by replacing it with sugar. But I don't think this is very much before it becomes noticeable, and sugar isn't free. If you happen to have excess thermal energy from some source, there are also ways to harvest some of this using forward osmosis, but this is really just another way to smuggle in free energy. In principle, you could use the heat gradient to generate electricity and then use that to power normal reverse osmosis, although which is more efficient will depend on the details of your equipment losses.
  - johncolanduoni 10y ago
    > A strict 1-particle-to-1-particle replacement can (I think) have arbitrarily low free energy cost Even better, it can have a (theoretical) energy gain if you use a substance with a lower heat of mixing than (sea) salt and water.
    - jessriedel 10y ago

Desalinization is already with about a factor of 2 or 3 of the thermodynamic optimum, so improvements can't be revolutionary. Desalinization mostly needs cheap clean energy.

throwaway91111 10y ago

Also, waste is a major problem, environmentally.

Baeocystin 10y ago
What is the best practice for dealing with the brine? Reinjection in to a current? Diffusion over a long pipe? Evaporation in on-shore ponds?
jessriedel 10y ago
Are you talking about the brine? This is trivial if you just put a pipe a few hundred yards out to sea. The concentration is diluted extremely quickly. (The only exception is if your desalination plant is below sea level because then you have to pump the brine up hill, which costs some energy.)
- throwaway91111 10y ago
  I mean, I sure hope so.
- dredmorbius 10y ago
  I've seen the assumption that the brine dissipates quickly challenged. Fluids can remain subbornly un-mixed, over large distances and for long times. Given that brine is heavier than seawater, it will almost certainly tend to sink and flow along bathymetric contours, perhaps pooling in local low spots. And sealife can be exceedingly sensitive to changes in temperature, ion density, salt content, etc. This reminds me of a book in which an ecologist described a discussion with a chemist over the concentration of some pollutant in seawater. Roughly, "Assuming a well-mixed solution..." starts the chemist. "How are you planning to stir the oceans?" asks the ecologist....
  - jessriedel 10y ago
    I've seen data on this and dilution was extremely fast, as expected.

marmshallow 10y ago

Can you describe this thermodynamic optimum or link me to something? Curious

jessriedel 10y ago
> A state-of-the-art facility is now within a factor of two of the theoretical energy minimum, and only 25 percent higher than the realistic minimum for the current reverse osmosis process. In short, it’s going to be tough to squeeze too much more energy out of reverse osmosis, and we’re unlikely to find an alternative method of desalination that will provide a significant boost over that. > But that doesn’t mean that there are no other ways of getting better output for our energy. The total process of desalination turns out to require three to four times the theoretical minimal energy use, since the salt water must be pumped and pretreated, the membranes maintained, and the resulting brine handled afterwards. Some of these things might be amenable to further improvements, and there has been work put into developing membranes that don’t clog up as easily or better pre-filtering of biological materials. http://arstechnica.com/science/2011/08/desalinization-is-this-as-good-as-it-gets/ http://arstechnica.com/science/2011/08/desalinization-is-thi... More: http://science.sciencemag.org/content/333/6043/712 http://science.sciencemag.org/content/333/6043/712 https://www.researchgate.net/profile/Menachem_Elimelech/publication/51547620_The_Future_of_Seawater_Desalination_Energy_Technology_and_the_Environment/links/02e7e51e88c0a7f890000000/The-Future-of-Seawater-Desalination-Energy-Technology-and-the-Environment.pdf https://www.researchgate.net/profile/Menachem_Elimelech/publ...

deleted 10y ago

[deleted]

leot 10y ago

Even with forward osmosis?

jessriedel 10y ago
Not really. Basic thermodynamics tells you that if your only input is water with a certain salt concentration and your output is water with a lower salt concentration (plus either brine -- very salty water -- or dry salt), then there is a minimum free energy cost. > Based on thermodynamic principles and practical kinetic requirements, the theoretical minimal energy for desalination with FO is always higher than that without FO. In other words, using FO cannot reduce the minimum energy of separation. https://www.researchgate.net/profile/Shihong_Lin/publication/271706340_Forward_osmosis_Where_are_we_now/links/54d992e10cf25013d042517a.pdf https://www.researchgate.net/profile/Shihong_Lin/publication... The idea behind forward osmosis is to replace the salt in water with some other solute, like sugar. A strict 1-particle-to-1-particle replacement can (I think) have arbitrarily low free energy cost, but then you're just stuck with sugar water instead of salt water. In certain special cases this is fine (like for emergency water generation at sea, when you don't mind drinking sugar water). Insofar as low levels of sugar can't be tasted, and insofar as you have access to free sugar, you can avoid the energetic cost of removing some of the salt by replacing it with sugar. But I don't think this is very much before it becomes noticeable, and sugar isn't free. If you happen to have excess thermal energy from some source, there are also ways to harvest some of this using forward osmosis, but this is really just another way to smuggle in free energy. In principle, you could use the heat gradient to generate electricity and then use that to power normal reverse osmosis, although which is more efficient will depend on the details of your equipment losses.
- johncolanduoni 10y ago
  > A strict 1-particle-to-1-particle replacement can (I think) have arbitrarily low free energy cost Even better, it can have a (theoretical) energy gain if you use a substance with a lower heat of mixing than (sea) salt and water.
  - jessriedel 10y ago