3 ms·

You will get the latent heat back when the water condenses again.

by Hermel 7y ago

bluGill 7y ago
This is only true in theory. Thermodynamics says you get it all back, but in the real world it is hard to get it back in a useful form.
dredmorbius 7y ago
That's a principle already present in extant designs from at least the 1960s, see MSF processes (https://blog-en.condorchem.com/evaporation-systems-water-desalination/ https://blog-en.condorchem.com/evaporation-systems-water-des...). Reverse-osmosis is still far more energy efficient. The major use of evaporative distillation (based on a few minutes of research, though some general familiarity) is in chemical processing, including but not limited to petroleum distillation. There's well over a century of history and R&D here (the first commercial petroleum distillation occurred in the 1830s, though larger-scale plants date from the 1870s and onward, as kerosene was used for lighting). Optimising for energy use and production volumes, controlability, predictability, safety/risk, etc., has been going on for a while. Keep in mind that in evaporative desalination you need to both heat the input supply and cool the output, and that processing is rate-limited at both ends. So yes, you could pre-heat the inflow via a condensing jacket, but heat in must equal heat out, and if there's an insufficiency in cooling from native inflow, you'll need an additional cooling working fluid. Other options include partial vacuum on the salty side, overpressure on the pure side (reducing and increasing boiling and condensation points, respectively), though PV = nRT (Boyle's Law) says that that isn't free either. And better minds than mine have looked at this for a while. The issues of input purity and filtration (a major problem with RO desal) is another major factor -- you don't want to just dump raw seawater over your osmosis membrane, but need to pre-filter that through other materials (I'm largely assuming gravel, sand, and possibly clay filters), which will foul with time and require replacement, as well as increase the water-handling energy costs (filtration isn't free). Reverse osmosis has also seen energy recovery research, e.g., https://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=4330 https://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=4... Possibly interesting reading: https://duckduckgo.com/?q=energy+recovery+evaporative+desalination&t=ffab&ia=web https://duckduckgo.com/?q=energy+recovery+evaporative+desali... https://scholar.google.com/scholar?q=energy%20recovery%20evaporative%20desalination&btnG=Search&as_sdt=800000000001&as_sdtp=on https://scholar.google.com/scholar?q=energy%20recovery%20eva...
dotancohen 7y ago
Right, but you "get it back" on the condenser surface, not in the fluid that needs to be boiled. At it is exactly the temperature gradient that you need to maintain, so cooling the condenser surface with the to-be-boiled fluid will work only to a point. There are systems that cool the condenser with the water before sending the water to the heater, but the rate at which heat flows is much slower than the rate at which the water must flow. That's the problem with heat. It is not a source of energy. It is a store of energy, often one that simply needs to be disposed of.
- deleted 7y ago
  [deleted]

You will get the latent heat back when the water condenses again.

by Hermel 7y ago

You will get the latent heat back when the water condenses again.

This is only true in theory. Thermodynamics says you get it all back, but in the real world it is hard to get it back in a useful form.

dredmorbius 7y ago

That's a principle already present in extant designs from at least the 1960s, see MSF processes (https://blog-en.condorchem.com/evaporation-systems-water-desalination/ https://blog-en.condorchem.com/evaporation-systems-water-des...). Reverse-osmosis is still far more energy efficient. The major use of evaporative distillation (based on a few minutes of research, though some general familiarity) is in chemical processing, including but not limited to petroleum distillation. There's well over a century of history and R&D here (the first commercial petroleum distillation occurred in the 1830s, though larger-scale plants date from the 1870s and onward, as kerosene was used for lighting). Optimising for energy use and production volumes, controlability, predictability, safety/risk, etc., has been going on for a while. Keep in mind that in evaporative desalination you need to both heat the input supply and cool the output, and that processing is rate-limited at both ends. So yes, you could pre-heat the inflow via a condensing jacket, but heat in must equal heat out, and if there's an insufficiency in cooling from native inflow, you'll need an additional cooling working fluid. Other options include partial vacuum on the salty side, overpressure on the pure side (reducing and increasing boiling and condensation points, respectively), though PV = nRT (Boyle's Law) says that that isn't free either. And better minds than mine have looked at this for a while. The issues of input purity and filtration (a major problem with RO desal) is another major factor -- you don't want to just dump raw seawater over your osmosis membrane, but need to pre-filter that through other materials (I'm largely assuming gravel, sand, and possibly clay filters), which will foul with time and require replacement, as well as increase the water-handling energy costs (filtration isn't free). Reverse osmosis has also seen energy recovery research, e.g., https://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=4330 https://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=4... Possibly interesting reading: https://duckduckgo.com/?q=energy+recovery+evaporative+desalination&t=ffab&ia=web https://duckduckgo.com/?q=energy+recovery+evaporative+desali... https://scholar.google.com/scholar?q=energy%20recovery%20evaporative%20desalination&btnG=Search&as_sdt=800000000001&as_sdtp=on https://scholar.google.com/scholar?q=energy%20recovery%20eva...

dotancohen 7y ago

Right, but you "get it back" on the condenser surface, not in the fluid that needs to be boiled. At it is exactly the temperature gradient that you need to maintain, so cooling the condenser surface with the to-be-boiled fluid will work only to a point. There are systems that cool the condenser with the water before sending the water to the heater, but the rate at which heat flows is much slower than the rate at which the water must flow. That's the problem with heat. It is not a source of energy. It is a store of energy, often one that simply needs to be disposed of.

deleted 7y ago
[deleted]