15 ms·

Is it just me, or did this article dance around the question? I am not a physicist but let me give it a stab: except for a few specialized steps like UV or oxi

Is it just me, or did this article dance around the question? I am not a physicist but let me give it a stab: except for a few specialized steps like UV or oxidizing heavy metals, most filtration is mechanical. A series of filters with smaller and smaller pores capture more and more of the mess in the water like bacteria and particulates while UV breaks down viruses, the oxidizer precipitates out metals, and so on. None of those methods work with salt. Salts in general disassociate through ion-dipole interactions - the water dipoles essentially rip the ionic compound apart and surround each ion in what is called a hydration shell. They're bigger than bare water molecules but not much bigger - much too small to target with pore size. This shell also puts them in a thermodynamically stable state and it takes energy to "jostle" the water molecules away from the ions either through evaporation, distillation, or through another chemical reaction that precipitates out the ions. As it turns out, doing that takes a lot of energy, so we use reverse osmosis as a cheaper alternative: we exploit the hydration shell of the ions by putting them behind a semi-permeable membrane with very small pores, "nanopores" if you will. The pores are too small for water to cross normally, but under high pressures bare water molecules can be forced through the pores while the ions trapped in their shells remain and concentrate into a brine. It takes less energy but produces a concentrated liquid waste stream that must be disposed of. Someone please correct any mistakes I've made

tinus_hn 3y ago
To me it doesn’t seem to make sense to first claim that you can’t filter for salt in water, and then talk about semi permeable membranes and osmosis. Those membranes are filters for salt!
scrubs 3y ago
It danced around the question.
eutectic 3y ago
I think it's more a problem of entropy; You're taking a high-entropy mixture and trying to extract a pure substance. Think sorting red and blue lego.
- aidenn0 3y ago
  The comment is still useful; if the red legos and the blue legos are different sizes it's pretty easy to sort them mechanically.
  - eutectic 3y ago
    Yes, but there will always be some necessary energy input.
at_a_remove 3y ago
I only have a BS in Physics but you're basically correct. But to make it even more simple and divorced from method: 1) There's a large difference in energy and entropy between seawater and drinkable "fresh" water. This represents a bare minimum expenditure, below which you can never go, lest you attempt to create a perpetual motion machine. 2) No matter how you do it: Well, now you have a bunch of previously dissolved solids covering everything. How do you get them off of your surfaces and out of your tubes and "away" from everything else? Once you stare at the first factor, then look at the second factor, then go back and forth, you come to your senses and realize that the dream of a jeroboam of colorless, tasteless water next to a little pile of fine powder is just not going to happen, and that the more sensible thing is to release some extra briny water back to your source and hope it doesn't kill too many fish.
- vladraz 3y ago
  A sensible thing to do is to turn the waste brine water into a resource. Since it's already been pumped up, pour it out into an evaporation pond to increase humidity in an area that could benefit from it, and then scoop up the salt to extract valuable minerals.
  - Bost 3y ago
    I guess there aren't that many valuable minerals in seawater. For example, Fritz Haber, a German Nobel Prize winner in chemistry, tried to extract gold from seawater after WWI to pay for the war reparations... long story short, the concentration of gold in seawater is too small. Also, the phase transition for H2O from liquid to gas requires a lot of energy and space (evaporation surface). In other words, it takes ages to evaporate all the water. Also, the larger your pond is, the more expensive it is to scoop up the salt. And then just one rainy afternoon can set you back a lot.
- nephanth 3y ago
  > There's a large difference in energy and entropy between seawater and drinkable "fresh" water I agree with the entropy part, but isn't the energy practically the same for similar quantities/ temperature?
lisper 3y ago
You've pretty much nailed it except for one minor nit: > It takes less energy but produces a concentrated liquid waste stream that must be disposed of. This implies that creating a concentrated waste stream is a problem unique to reverse osmosis. It isn't. No matter what you do you're going to end up with a bunch of salt that you have to get rid of somehow.
- ericlewis 3y ago
  good thing people need salt!
  - stouset 3y ago
    If people needed as much salt as was contained in the water to begin with, we wouldn’t need to remove it in the first place.
    - suddenclarity 3y ago
      No reason to keep the 1:1 ratio. Use the salt to replace our current salt mines/outtakes and then use the water as an addition to our current freshwater usage.
      - deleted 3y ago
        [deleted]
      - Retric 3y ago
        The waste stream doesn’t contain that much more salt than seawater. Extracting salt from mines is much cheaper than extracting it from slightly brackish wastewater from water treatment plants.
      - giantrobot 3y ago
        The brine waste from RO is still mostly water. In order to extract the salt you'd need to evaporate the water which still takes a lot more energy. You could use evaporative ponds to let the Sun do the work but that takes a lot of space. In either case you're spending a lot more money per pound than just digging the salt out of a mine.
dfxm12 3y ago
It takes less energy but produces a concentrated liquid waste stream that must be disposed of. I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere?
- function_seven 3y ago
  It’s toxic only because of concentration. AKAIK, there aren’t any compounds in the brine that weren’t present in the seawater to begin with. The solution is dilution, but I’m sure it’s easier for me to type that than it is to achieve in a desalination plant. But, why not really-long-pipe-with-small-holes-along-the-length? That seems to me like a simple mechanism to send the brine back into the ocean without causing a local disaster on the sea floor. Is there maintenance required that makes it more expensive than I realize?
  - Timshel 3y ago
    I believe I had read just dilute it until concentration is ok then release. But might be more tricky than that ^^ : https://www.sciencedirect.com/science/article/abs/pii/S0048969719334655 https://www.sciencedirect.com/science/article/abs/pii/S00489...
    - dylan604 3y ago
      dilute it with what? the clean water you just removed the salt from?
      - function_seven 3y ago
        No, the salt water you're about to discharge it into. Have a pump that draws in 10L of ocean water for every 1L of brine you need to dispose of, mix 'em up, and discharge the 11L of only-slightly-saltier water back into the sea. Not sure when it makes more sense to do that vs. having a leach-field type of brine discharge. They both ultimately do the same thing, but one requires more mechanicals, the other requires more piping and "passive" infrastructure.
marcosdumay 3y ago
> As it turns out, doing that takes a lot of energy The change in entropy between a batch of saline water and a batch of fresh water and enough saline water that its concentration don't change is about the same as letting that same fresh water fall for 200m and converting the resulting energy into heat (at 300K). What means that desalination will take a lot of energy whatever method you use. There are distillation procedures close to perfect efficiency that wouldn't take much more energy than reverse osmosis; and of course, electrical separation is that one method with lots of promise but that stops due to material related problems every time it's tried. It just so happen that we know how to scale reverse osmosis up cheaply and reliably; but this looks like a feature of our technology and not anything intrinsic.
londons_explore 3y ago
> It takes less energy Distillation and reverse osmosis theoretically use the same amount of energy. Practically, reverse osmosis tech is far closer to that ideal efficiency level, especially if electricity is your starting energy source. But it doesn't seem out of the realm of possibility that someone will figure out efficient distillation in the future. distillation has the big benefit that it can make use of low grade heat which is waste from lots of industrial processes.
gabereiser 3y ago
It’s important to note that the energy needed for RO to work is due to the high pressures needed to ram that little H2O molecule through that virgin nanohole. 700-900wh for a trickle of 14gal/h. At least that what I’m getting on my sailboat.
- theresistor 3y ago
  That's quite inefficient if you're using a Clark pump. I'm currently spending about 1000W to making ~40G/h with a Schenker Zen150.
  - gabereiser 3y ago
    Blame SeaWaterPro for the underpowered HPP.
nine_k 3y ago
This is correct. But we seem to have colossal amounts of essentially free solar energy, and that energy already evaporates large amounts of sea water. We just don't capture it well. Imagine building a pipe that stands above shallow tropical coastal waters. Make the bottom of it into an almost flat funnel to cover more water surface, using transparent plastic or even glass. Now all the evaporated water and hot air go into the pipe. Build the pipe a kilometer tall. Humans have adequate technologies already, and the pipe does not need to be bearing much internal load, unlike Burj Khalifa or World Trade 1. At 1km, the air is cool enough. The hot air will shoot upwards, cooling on its way up and releasing fresh water. Lightweight collector pipes will bring it down into a reservoir. The remaining dampness of the air will help it produce clouds, and thus shadow, over the land. With a tall enough pipe, we could even generate electricity by putting a turbine inside. Why are we not building it? It's expensive, and most (sub)tropical countries that lack water are poor. They are also politically unstable, and such an installation would be a high-value military and terrorist target. Maybe Singapore or Dubai would some day dare and build it. (California, unlikely; it would never pass an environmental review.)
jkqwzsoo 3y ago
> As it turns out, doing that takes a lot of energy, so we use reverse osmosis as a cheaper alternative: we exploit the hydration shell of the ions by putting them behind a semi-permeable membrane with very small pores, "nanopores" if you will. The pores are too small for water to cross normally, but under high pressures bare water molecules can be forced through the pores while the ions trapped in their shells remain and concentrate into a brine. It takes less energy but produces a concentrated liquid waste stream that must be disposed of. There are no pores, so to speak. Polymer materials form amorphous solids with transient voids which open and close randomly due to thermal motion. They're not "pores" because they aren't permanent over long time scales. Rather, the polymer+water is modeled as a single fluid phase, the same as if you were modeling ethanol+water. The fact that the polymer is a "solid" doesn't affect the fact that it's actually a tangle of vibrating molecules just like any other mixture. Other materials do have well defined pores, like MOFs and zeolites. In this case, the water does sorb as a liquid in the pore space, but is gated by transport between the pores in a similar manner. This is made apparent because water does enter into polymers (even those which desalination) freely, with or without the presence of salt. It is not the case that "the pores are too small for water to cross normally". I can take a polymer that will swell with 50% of its own weight in water, and which has no "free" liquid water (as evidenced by the inability of the water in the polymer to form ice), yet make it reject >90% salt at very high pressures (>3000 psi). If you just let salt water sit on one side without pressure, salt and water will make their way through non-selectively. So it can't be that the water is being physically sieved from the ions to enter into the membrane. Rather, the pressure creates a change in the activity of water (due to the mechanical forces acting on the polymer near the low pressure/support material interface). Since the water is more soluble and more mobile in the polymer, it transports at a more rapid rate than the salt, resulting in desalination.
gameshot911 3y ago
If there's one complaint I have about PE's content, it's that he often give ambiguous answers to the central question. Sometimes it's strung throughout the 10 minute video, but I really wish he'd end all his content with a concise summary.
NullifyNAN 3y ago
One minor thing is that the polyamide membranes that are used are more based on hydrogen bonding capability than pure porosity. Basically the water can hydrogen bond with the polyamide but the salt can not and is therefore left behind.
aerio 3y ago
I don't think the article dodges the question at all. Did you properly read it or just skim it? It goes over multiple ways of desalinating water, distillation and osmosis - as you also cover. The most relevant paragraph to the question and a bit of a conclusion seems to be: >And that’s the problem with desalination. It’s kind of like the nuclear power of water supply. It seems so simple on the surface, but when you add up all the practical costs and complexities, it gets really hard to justify over other alternatives. It’s also harder to compare costs between those alternatives because of desal’s unique problems. It’s just a newer technology, so it’s harder to predict hidden technical, legal, political, and environmental challenges. For example, because of the high energy demands, desalination can strongly couple water costs with electricity costs. During a drought, the cost of hydropower goes up because there’s less water available, increasing overall energy costs and thus making desalination less viable right when you need it most.
- nardi 3y ago
  The article talks about why it's maybe politically or economically difficult, but not why it's physically difficult.
  - 101011 3y ago
    Subtracting economics from the equation, aren't there a very limited number of physically difficult problems?
namibj 3y ago
Using a fractional distillation column you can also separate filtered sea water into water vapor and brine, at equal pressure, with the brine appropriately hotter so the vapor pressure of water over the brine is the same as over the salt-free water droplets that form on the cold end. You'd then have to compress the water vapor until it condenses barely hotter than the brine, and use both distilled water and residual brine at their approximately equal temperature (water at higher pressure than brine, though) in ofc separate counter-flow heat exchangers to pre-heat the filtered source (sea) water to the column's operating temperature (i.e., where the source water just starts boiling at the column's operating pressure (you want a decent vapor pressure to have a reasonable vapor density and thus feasible power density for capex reasons)). Thermodynamically this should match a reverse-osmosis process with equal input/output parameters (I left out that you need pumps/turbines to "losslessly" adapt liquid between ambient pressure and internal operating pressure). One benefit would be that you could directly heat the brine with solar thermal collectors, to get away without having to compress the vapor to condense it, essentially an open-cycle Type-1 absorption heat pump, with solar feed. (Lacking an evaporator, with the condensed output being the desired pure water, and the absorber being fed with source sea water while the return from the generator after the heat exchanger is just warm brine for discharging. If water and brine need to be sub-ambient, you'd evaporate part of the condensed water to chill both the condensate and the brine output streams. That'd be partially-open-cycle.)
markwalllberg 3y ago
So basically, filters. And also. None of this is a challenge at all. R.o. water is simple
TheDudeMan 3y ago
Grady mentioned a maximum theoretical efficiency. How close are we to that?
gcanyon 3y ago
So the obvious question to me is: is there no other physical property that can be used to separate the hydration shell + ion molecule combinations from the just-water molecules? Different magnetic charge? Different mass? Different chemical reactivity? ¯\_(ツ)_/¯ I know the answer must be 'no' since if there were a better answer, none of what I'm thinking of requires more than high-school-level chemistry to discover/exploit.
rvba 3y ago
How are those membranesmade that they have pores + dont break?

tinus_hn 3y ago

To me it doesn’t seem to make sense to first claim that you can’t filter for salt in water, and then talk about semi permeable membranes and osmosis. Those membranes are filters for salt!

scrubs 3y ago

It danced around the question.

eutectic 3y ago

I think it's more a problem of entropy; You're taking a high-entropy mixture and trying to extract a pure substance. Think sorting red and blue lego.

aidenn0 3y ago
The comment is still useful; if the red legos and the blue legos are different sizes it's pretty easy to sort them mechanically.
- eutectic 3y ago
  Yes, but there will always be some necessary energy input.

at_a_remove 3y ago

I only have a BS in Physics but you're basically correct. But to make it even more simple and divorced from method: 1) There's a large difference in energy and entropy between seawater and drinkable "fresh" water. This represents a bare minimum expenditure, below which you can never go, lest you attempt to create a perpetual motion machine. 2) No matter how you do it: Well, now you have a bunch of previously dissolved solids covering everything. How do you get them off of your surfaces and out of your tubes and "away" from everything else? Once you stare at the first factor, then look at the second factor, then go back and forth, you come to your senses and realize that the dream of a jeroboam of colorless, tasteless water next to a little pile of fine powder is just not going to happen, and that the more sensible thing is to release some extra briny water back to your source and hope it doesn't kill too many fish.

vladraz 3y ago
A sensible thing to do is to turn the waste brine water into a resource. Since it's already been pumped up, pour it out into an evaporation pond to increase humidity in an area that could benefit from it, and then scoop up the salt to extract valuable minerals.
- Bost 3y ago
  I guess there aren't that many valuable minerals in seawater. For example, Fritz Haber, a German Nobel Prize winner in chemistry, tried to extract gold from seawater after WWI to pay for the war reparations... long story short, the concentration of gold in seawater is too small. Also, the phase transition for H2O from liquid to gas requires a lot of energy and space (evaporation surface). In other words, it takes ages to evaporate all the water. Also, the larger your pond is, the more expensive it is to scoop up the salt. And then just one rainy afternoon can set you back a lot.
nephanth 3y ago
> There's a large difference in energy and entropy between seawater and drinkable "fresh" water I agree with the entropy part, but isn't the energy practically the same for similar quantities/ temperature?

lisper 3y ago

You've pretty much nailed it except for one minor nit: > It takes less energy but produces a concentrated liquid waste stream that must be disposed of. This implies that creating a concentrated waste stream is a problem unique to reverse osmosis. It isn't. No matter what you do you're going to end up with a bunch of salt that you have to get rid of somehow.

ericlewis 3y ago
good thing people need salt!
- stouset 3y ago
  If people needed as much salt as was contained in the water to begin with, we wouldn’t need to remove it in the first place.
  - suddenclarity 3y ago
    No reason to keep the 1:1 ratio. Use the salt to replace our current salt mines/outtakes and then use the water as an addition to our current freshwater usage.
    - deleted 3y ago
      [deleted]
    - Retric 3y ago
      The waste stream doesn’t contain that much more salt than seawater. Extracting salt from mines is much cheaper than extracting it from slightly brackish wastewater from water treatment plants.
    - giantrobot 3y ago
      The brine waste from RO is still mostly water. In order to extract the salt you'd need to evaporate the water which still takes a lot more energy. You could use evaporative ponds to let the Sun do the work but that takes a lot of space. In either case you're spending a lot more money per pound than just digging the salt out of a mine.

dfxm12 3y ago

It takes less energy but produces a concentrated liquid waste stream that must be disposed of. I've heard that this brine is toxic. Does this make disposal an issue? Is the toxicity true or hyperbole? I mean, do we know how bad it is, and if we can do anything safely with it? It seems like "salt" is useful in a lot of contexts, including industrial, so can we do something with the brine besides disposing it somewhere?

function_seven 3y ago
It’s toxic only because of concentration. AKAIK, there aren’t any compounds in the brine that weren’t present in the seawater to begin with. The solution is dilution, but I’m sure it’s easier for me to type that than it is to achieve in a desalination plant. But, why not really-long-pipe-with-small-holes-along-the-length? That seems to me like a simple mechanism to send the brine back into the ocean without causing a local disaster on the sea floor. Is there maintenance required that makes it more expensive than I realize?
- Timshel 3y ago
  I believe I had read just dilute it until concentration is ok then release. But might be more tricky than that ^^ : https://www.sciencedirect.com/science/article/abs/pii/S0048969719334655 https://www.sciencedirect.com/science/article/abs/pii/S00489...
  - dylan604 3y ago
    dilute it with what? the clean water you just removed the salt from?
    - function_seven 3y ago
      No, the salt water you're about to discharge it into. Have a pump that draws in 10L of ocean water for every 1L of brine you need to dispose of, mix 'em up, and discharge the 11L of only-slightly-saltier water back into the sea. Not sure when it makes more sense to do that vs. having a leach-field type of brine discharge. They both ultimately do the same thing, but one requires more mechanicals, the other requires more piping and "passive" infrastructure.

marcosdumay 3y ago

> As it turns out, doing that takes a lot of energy The change in entropy between a batch of saline water and a batch of fresh water and enough saline water that its concentration don't change is about the same as letting that same fresh water fall for 200m and converting the resulting energy into heat (at 300K). What means that desalination will take a lot of energy whatever method you use. There are distillation procedures close to perfect efficiency that wouldn't take much more energy than reverse osmosis; and of course, electrical separation is that one method with lots of promise but that stops due to material related problems every time it's tried. It just so happen that we know how to scale reverse osmosis up cheaply and reliably; but this looks like a feature of our technology and not anything intrinsic.

londons_explore 3y ago

> It takes less energy Distillation and reverse osmosis theoretically use the same amount of energy. Practically, reverse osmosis tech is far closer to that ideal efficiency level, especially if electricity is your starting energy source. But it doesn't seem out of the realm of possibility that someone will figure out efficient distillation in the future. distillation has the big benefit that it can make use of low grade heat which is waste from lots of industrial processes.

gabereiser 3y ago

It’s important to note that the energy needed for RO to work is due to the high pressures needed to ram that little H2O molecule through that virgin nanohole. 700-900wh for a trickle of 14gal/h. At least that what I’m getting on my sailboat.

theresistor 3y ago
That's quite inefficient if you're using a Clark pump. I'm currently spending about 1000W to making ~40G/h with a Schenker Zen150.
- gabereiser 3y ago
  Blame SeaWaterPro for the underpowered HPP.

nine_k 3y ago

This is correct. But we seem to have colossal amounts of essentially free solar energy, and that energy already evaporates large amounts of sea water. We just don't capture it well. Imagine building a pipe that stands above shallow tropical coastal waters. Make the bottom of it into an almost flat funnel to cover more water surface, using transparent plastic or even glass. Now all the evaporated water and hot air go into the pipe. Build the pipe a kilometer tall. Humans have adequate technologies already, and the pipe does not need to be bearing much internal load, unlike Burj Khalifa or World Trade 1. At 1km, the air is cool enough. The hot air will shoot upwards, cooling on its way up and releasing fresh water. Lightweight collector pipes will bring it down into a reservoir. The remaining dampness of the air will help it produce clouds, and thus shadow, over the land. With a tall enough pipe, we could even generate electricity by putting a turbine inside. Why are we not building it? It's expensive, and most (sub)tropical countries that lack water are poor. They are also politically unstable, and such an installation would be a high-value military and terrorist target. Maybe Singapore or Dubai would some day dare and build it. (California, unlikely; it would never pass an environmental review.)

jkqwzsoo 3y ago

> As it turns out, doing that takes a lot of energy, so we use reverse osmosis as a cheaper alternative: we exploit the hydration shell of the ions by putting them behind a semi-permeable membrane with very small pores, "nanopores" if you will. The pores are too small for water to cross normally, but under high pressures bare water molecules can be forced through the pores while the ions trapped in their shells remain and concentrate into a brine. It takes less energy but produces a concentrated liquid waste stream that must be disposed of. There are no pores, so to speak. Polymer materials form amorphous solids with transient voids which open and close randomly due to thermal motion. They're not "pores" because they aren't permanent over long time scales. Rather, the polymer+water is modeled as a single fluid phase, the same as if you were modeling ethanol+water. The fact that the polymer is a "solid" doesn't affect the fact that it's actually a tangle of vibrating molecules just like any other mixture. Other materials do have well defined pores, like MOFs and zeolites. In this case, the water does sorb as a liquid in the pore space, but is gated by transport between the pores in a similar manner. This is made apparent because water does enter into polymers (even those which desalination) freely, with or without the presence of salt. It is not the case that "the pores are too small for water to cross normally". I can take a polymer that will swell with 50% of its own weight in water, and which has no "free" liquid water (as evidenced by the inability of the water in the polymer to form ice), yet make it reject >90% salt at very high pressures (>3000 psi). If you just let salt water sit on one side without pressure, salt and water will make their way through non-selectively. So it can't be that the water is being physically sieved from the ions to enter into the membrane. Rather, the pressure creates a change in the activity of water (due to the mechanical forces acting on the polymer near the low pressure/support material interface). Since the water is more soluble and more mobile in the polymer, it transports at a more rapid rate than the salt, resulting in desalination.

gameshot911 3y ago

If there's one complaint I have about PE's content, it's that he often give ambiguous answers to the central question. Sometimes it's strung throughout the 10 minute video, but I really wish he'd end all his content with a concise summary.

NullifyNAN 3y ago

One minor thing is that the polyamide membranes that are used are more based on hydrogen bonding capability than pure porosity. Basically the water can hydrogen bond with the polyamide but the salt can not and is therefore left behind.

aerio 3y ago

I don't think the article dodges the question at all. Did you properly read it or just skim it? It goes over multiple ways of desalinating water, distillation and osmosis - as you also cover. The most relevant paragraph to the question and a bit of a conclusion seems to be: >And that’s the problem with desalination. It’s kind of like the nuclear power of water supply. It seems so simple on the surface, but when you add up all the practical costs and complexities, it gets really hard to justify over other alternatives. It’s also harder to compare costs between those alternatives because of desal’s unique problems. It’s just a newer technology, so it’s harder to predict hidden technical, legal, political, and environmental challenges. For example, because of the high energy demands, desalination can strongly couple water costs with electricity costs. During a drought, the cost of hydropower goes up because there’s less water available, increasing overall energy costs and thus making desalination less viable right when you need it most.

nardi 3y ago
The article talks about why it's maybe politically or economically difficult, but not why it's physically difficult.
- 101011 3y ago
  Subtracting economics from the equation, aren't there a very limited number of physically difficult problems?

namibj 3y ago

Using a fractional distillation column you can also separate filtered sea water into water vapor and brine, at equal pressure, with the brine appropriately hotter so the vapor pressure of water over the brine is the same as over the salt-free water droplets that form on the cold end. You'd then have to compress the water vapor until it condenses barely hotter than the brine, and use both distilled water and residual brine at their approximately equal temperature (water at higher pressure than brine, though) in ofc separate counter-flow heat exchangers to pre-heat the filtered source (sea) water to the column's operating temperature (i.e., where the source water just starts boiling at the column's operating pressure (you want a decent vapor pressure to have a reasonable vapor density and thus feasible power density for capex reasons)). Thermodynamically this should match a reverse-osmosis process with equal input/output parameters (I left out that you need pumps/turbines to "losslessly" adapt liquid between ambient pressure and internal operating pressure). One benefit would be that you could directly heat the brine with solar thermal collectors, to get away without having to compress the vapor to condense it, essentially an open-cycle Type-1 absorption heat pump, with solar feed. (Lacking an evaporator, with the condensed output being the desired pure water, and the absorber being fed with source sea water while the return from the generator after the heat exchanger is just warm brine for discharging. If water and brine need to be sub-ambient, you'd evaporate part of the condensed water to chill both the condensate and the brine output streams. That'd be partially-open-cycle.)

markwalllberg 3y ago

So basically, filters. And also. None of this is a challenge at all. R.o. water is simple

TheDudeMan 3y ago

Grady mentioned a maximum theoretical efficiency. How close are we to that?

gcanyon 3y ago

So the obvious question to me is: is there no other physical property that can be used to separate the hydration shell + ion molecule combinations from the just-water molecules? Different magnetic charge? Different mass? Different chemical reactivity? ¯\_(ツ)_/¯ I know the answer must be 'no' since if there were a better answer, none of what I'm thinking of requires more than high-school-level chemistry to discover/exploit.

rvba 3y ago

How are those membranesmade that they have pores + dont break?