3 ms·

Even with forward osmosis?

by leot 10y ago

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
    OK, but then, forgetting about desalination, you'd be able to just build a power plant with this substance and sea water, right? The fact that very few such plants exit then tells us that it would probably be cheaper to just do your desalination with reverse osmosis powered by a conventional power plant.
    - johncolanduoni 10y ago
      Baring some yet undiscovered and very bizarre (but thermodynamically viable) chemistry that requires you to involve saltwater in a practical implementation, yes. Otherwise saltwater (which has a positive heat of mixing) is just fighting your actual process.

Even with forward osmosis?

by leot 10y ago

Even with forward osmosis?

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
  OK, but then, forgetting about desalination, you'd be able to just build a power plant with this substance and sea water, right? The fact that very few such plants exit then tells us that it would probably be cheaper to just do your desalination with reverse osmosis powered by a conventional power plant.
  - johncolanduoni 10y ago
    Baring some yet undiscovered and very bizarre (but thermodynamically viable) chemistry that requires you to involve saltwater in a practical implementation, yes. Otherwise saltwater (which has a positive heat of mixing) is just fighting your actual process.