10 ms·

See also: a way to potentially do this with no energy input (exploiting density differences): https://www.science.org/doi/10.1126/science.183.4121.157 https://w

See also: a way to potentially do this with no energy input (exploiting density differences): https://www.science.org/doi/10.1126/science.183.4121.157 https://www.science.org/doi/10.1126/science.183.4121.157 Basically you put a reverse-osmosis membrane at least 231 meters deep in the ocean. If you put it deeper, you could potentially extract both freshwater and energy.

Natsu 3y ago
Just dig a ditch from California to Arizona, then use the sunlight to evaporate the water. Easy, right? :)
- jrockway 3y ago
  That's the most common way of collecting drinking water, right? Sun evaporates water from the ocean, then it condenses in clouds, and the clouds rain into a river which we suck the water out of. Boom! Plus you can get electricity out of it.
MarkMarine 3y ago
That is amazing! I’d never thought of leveraging the pressure of ocean depth for this, it’s a great idea. For anyone reading, the energy that OP is talking about would actually force the fresh water above the surface of the ocean without pumps. I didn’t get to the whole paper, but I imagine the challenges of keeping that membrane clean are remarkable.
- Metacelsus 3y ago
  Yeah, I thought of it a few years ago, then realized I'd been scooped by ~40 years. But given the advances in technology maybe it's possible today?
  - tinus_hn 3y ago
    Technology will never advance enough to enable the perpetuum mobile device, which is what this is, unfortunately.
    - hwillis 3y ago
      No it isn't. This is just like putting a filter at the bottom of a dam, except the filter also blocks salt. The top of the dam is at sea level, so you have to bring the water back up to the surface.
      - postalrat 3y ago
        Water without salt is less dense so that column of water would have less pressure at depth and would be pushed up.
    - AnotherGoodName 3y ago
      Adding salt water is endothermic and separating them is exothermic fwiw. That is separating salt from water releases energy. There's nothing perpetual motion about a process that separates fresh water and salt and also gives energy. I'm not sure if the above works or not but physically there's no issue with this.
    - gadilif 3y ago
      Not exactly, the pressure is an external force working on the system (so, open system). Perpetuum mobiles is a closed system with no energy loss.
      - IshKebab 3y ago
        No he's right, the idea of extracting work from the pressure of the ocean is as impossible as perpetual motion.
        quertered 3y ago
        It is clearly not impossible, the paper clearly states that it's thermodynamically feasible, even if not practical at 1970s
- IshKebab 3y ago
  It's not amazing, it's obviously delusional. Think about it - if it really worked you have invented a perpetual motion machine, since you could just feed the fresh water through a little water wheel back into the ocean. You don't need to do any maths at all to know that his maths has gone drastically wrong somewhere.
  - MostlyStable 3y ago
    In the absence of the sun (which is what ultimately drives ocean mixing), eventually this would lead to a layer of pure fresh water on top of a layer of salt. The energy is being driven by osmotic pressure of salt water. Once you have extracted the water out of the salt, that pressure is exhausted. Thus it is not perpetual.
  - roflc0ptic 3y ago
    Take a long tube with an osmotic filter on it. Put it in the sea with one end out. Pressure is going to fill the tube up, no, just like a snorkel? Probably back up to sea level, possibly minus whatever force reduction caused by the filter? Boom, water filtration with no expenditure of energy. It may be wrong but it really isn't "obviously delusional."
    - postalrat 3y ago
      You forgot the part where the water in the pipe is less dense and would be pushed up.
  - MarkMarine 3y ago
    This isn’t a closed system. It’s not perpetual motion, there is an EXTRAORDINARY amount of energy added to the system by the sun. Saying “maths” instead of just math sounds quite odd, why are you making it plural? Is solving one equation “a math” and solving multiple equations “maths”?
    - xdex 3y ago
      “Math” is used in USA and Canada. “Maths” is used by the rest of the English speaking world.
bell-cot 3y ago
That article's subtitle: "In principal, but probably not in practice, fresh water can be extracted from our oceans for no expenditure of energy" [Italics mine] Also - 231 meters deep is where, in theory, you'll just start getting fresh water coming through your membrane. Which is 231m deep. Fresh water that you'll spend plenty of $energy to pump up to the ocean surface. (For reference - Hoover Dam, when brim-full, has a slightly lesser drop from the water surface in the reservoir to the output of the hydroelectric turbines. You'll be pumping $energy in, to lift each ounce of your "free" fresh water up to where it is useful.) [Edit - from some quick calculations, and the density of sea water relative to fresh water...it looks like the "don't need to pay to pump the fresh water back up" version of this scheme will need to run its osmotic membranes at the bottom of the Challenger Deep (~11,000m), or something pretty close to that.]
- eslaught 3y ago
  If I built an ocean base 231 meters below the surface, does that mean I'd get free fresh water by sticking a membrane on the outside of my base? (Ignoring the impracticality of building an underwater base, of course.) Sounds like a cool sci-fi premise.
  - brilee 3y ago
    Almost. You would have to maintain your underwater base at 1atm and you would indeed get free freshwater. The catch is that the internal volume of your underwater base would decrease and you would eventually have to eject wastewater by pushing against the deep sea pressure. Nothing's free in physics!
    - nonethewiser 3y ago
      But what would take more energy? Pushing that wastewater out or desalinating?
      - hwillis 3y ago
        Pressure (N/m^2) times volumetric flow (m^3/s) equals power (N-m/s). At 231 meters the pressure is ~340 psi or ~6.3 MPa. 1 liter would take ~6.3 kJ to pump.
cma 3y ago
How much energy does it take to pump it back up from 231 meters, compared to traditional methods? That would be 2.25kJ of energy per liter. Normal desalination I believe is 3kJ per thousand liters.
- greesil 3y ago
  You can power it by pumping down an equal amount of surface water
  - hwillis 3y ago
    If you had a 231 meter dam, with a membrane filter at the bottom, fresh water will come out at atmospheric pressure. It then has to be carried back up to sea level. You could power that pump with another hole in the dam, but then the non-sea side of the dam would constantly be filling up with water. Once it's full, there's no longer any pressure difference to drive a pump.
  - GhostVII 3y ago
    That wouldn't work because the system relies on having a pressure differential across the membrane in order to function.
- paulcole 3y ago
  My idea is to not pump it back up but to let it drain down to Australia.
  - Maxion 3y ago
    This is the most sensible idea in this comment chain

Natsu 3y ago

Just dig a ditch from California to Arizona, then use the sunlight to evaporate the water. Easy, right? :)

jrockway 3y ago
That's the most common way of collecting drinking water, right? Sun evaporates water from the ocean, then it condenses in clouds, and the clouds rain into a river which we suck the water out of. Boom! Plus you can get electricity out of it.

MarkMarine 3y ago

That is amazing! I’d never thought of leveraging the pressure of ocean depth for this, it’s a great idea. For anyone reading, the energy that OP is talking about would actually force the fresh water above the surface of the ocean without pumps. I didn’t get to the whole paper, but I imagine the challenges of keeping that membrane clean are remarkable.

Metacelsus 3y ago
Yeah, I thought of it a few years ago, then realized I'd been scooped by ~40 years. But given the advances in technology maybe it's possible today?
- tinus_hn 3y ago
  Technology will never advance enough to enable the perpetuum mobile device, which is what this is, unfortunately.
  - hwillis 3y ago
    No it isn't. This is just like putting a filter at the bottom of a dam, except the filter also blocks salt. The top of the dam is at sea level, so you have to bring the water back up to the surface.
    - postalrat 3y ago
      Water without salt is less dense so that column of water would have less pressure at depth and would be pushed up.
  - AnotherGoodName 3y ago
    Adding salt water is endothermic and separating them is exothermic fwiw. That is separating salt from water releases energy. There's nothing perpetual motion about a process that separates fresh water and salt and also gives energy. I'm not sure if the above works or not but physically there's no issue with this.
  - gadilif 3y ago
    Not exactly, the pressure is an external force working on the system (so, open system). Perpetuum mobiles is a closed system with no energy loss.
    - IshKebab 3y ago
      No he's right, the idea of extracting work from the pressure of the ocean is as impossible as perpetual motion.
      - quertered 3y ago
        It is clearly not impossible, the paper clearly states that it's thermodynamically feasible, even if not practical at 1970s
IshKebab 3y ago
It's not amazing, it's obviously delusional. Think about it - if it really worked you have invented a perpetual motion machine, since you could just feed the fresh water through a little water wheel back into the ocean. You don't need to do any maths at all to know that his maths has gone drastically wrong somewhere.
- MostlyStable 3y ago
  In the absence of the sun (which is what ultimately drives ocean mixing), eventually this would lead to a layer of pure fresh water on top of a layer of salt. The energy is being driven by osmotic pressure of salt water. Once you have extracted the water out of the salt, that pressure is exhausted. Thus it is not perpetual.
- roflc0ptic 3y ago
  Take a long tube with an osmotic filter on it. Put it in the sea with one end out. Pressure is going to fill the tube up, no, just like a snorkel? Probably back up to sea level, possibly minus whatever force reduction caused by the filter? Boom, water filtration with no expenditure of energy. It may be wrong but it really isn't "obviously delusional."
  - postalrat 3y ago
    You forgot the part where the water in the pipe is less dense and would be pushed up.
- MarkMarine 3y ago
  This isn’t a closed system. It’s not perpetual motion, there is an EXTRAORDINARY amount of energy added to the system by the sun. Saying “maths” instead of just math sounds quite odd, why are you making it plural? Is solving one equation “a math” and solving multiple equations “maths”?
  - xdex 3y ago
    “Math” is used in USA and Canada. “Maths” is used by the rest of the English speaking world.

bell-cot 3y ago

That article's subtitle: "In principal, but probably not in practice, fresh water can be extracted from our oceans for no expenditure of energy" [Italics mine] Also - 231 meters deep is where, in theory, you'll just start getting fresh water coming through your membrane. Which is 231m deep. Fresh water that you'll spend plenty of $energy to pump up to the ocean surface. (For reference - Hoover Dam, when brim-full, has a slightly lesser drop from the water surface in the reservoir to the output of the hydroelectric turbines. You'll be pumping $energy in, to lift each ounce of your "free" fresh water up to where it is useful.) [Edit - from some quick calculations, and the density of sea water relative to fresh water...it looks like the "don't need to pay to pump the fresh water back up" version of this scheme will need to run its osmotic membranes at the bottom of the Challenger Deep (~11,000m), or something pretty close to that.]

eslaught 3y ago
If I built an ocean base 231 meters below the surface, does that mean I'd get free fresh water by sticking a membrane on the outside of my base? (Ignoring the impracticality of building an underwater base, of course.) Sounds like a cool sci-fi premise.
- brilee 3y ago
  Almost. You would have to maintain your underwater base at 1atm and you would indeed get free freshwater. The catch is that the internal volume of your underwater base would decrease and you would eventually have to eject wastewater by pushing against the deep sea pressure. Nothing's free in physics!
  - nonethewiser 3y ago
    But what would take more energy? Pushing that wastewater out or desalinating?
    - hwillis 3y ago
      Pressure (N/m^2) times volumetric flow (m^3/s) equals power (N-m/s). At 231 meters the pressure is ~340 psi or ~6.3 MPa. 1 liter would take ~6.3 kJ to pump.

cma 3y ago

How much energy does it take to pump it back up from 231 meters, compared to traditional methods? That would be 2.25kJ of energy per liter. Normal desalination I believe is 3kJ per thousand liters.

greesil 3y ago
You can power it by pumping down an equal amount of surface water
- hwillis 3y ago
  If you had a 231 meter dam, with a membrane filter at the bottom, fresh water will come out at atmospheric pressure. It then has to be carried back up to sea level. You could power that pump with another hole in the dam, but then the non-sea side of the dam would constantly be filling up with water. Once it's full, there's no longer any pressure difference to drive a pump.
- GhostVII 3y ago
  That wouldn't work because the system relies on having a pressure differential across the membrane in order to function.
paulcole 3y ago
My idea is to not pump it back up but to let it drain down to Australia.
- Maxion 3y ago
  This is the most sensible idea in this comment chain