6 ms·

Still, pumping water from 230 m is not that expensive energy wise compared to other approaches. Best case scenario, pumping 1 kg of water 230 meters costs 2.3k

Still, pumping water from 230 m is not that expensive energy wise compared to other approaches. Best case scenario, pumping 1 kg of water 230 meters costs 2.3kJ. Elsewhere in this thread it was mentioned that typical desalination approaches need 10-15kJ/kg.

chongli 3y ago
I think you’re still going to get killed on maintenance costs. When the membrane gets clogged, how do you replace it cheaply? How do you repair/replace damaged/worn out pumps? What about sections of pipe and the fittings? All of these sorts of repairs are also required for land-based water treatment facilities, of course, but now you have to perform them all at 230m underwater. That’s gotta be mega-expensive. Furthermore, all of your equipment needs to be able to survive at those depths, with saltwater attacking exposed surfaces and seals, requiring much more expensive materials and tighter tolerances. It really doesn’t seem very economical to me!
- irrational 3y ago
  Is economical important? If we really are desperate for fresh water, does the cost become immaterial?
  - chongli 3y ago
    It’s not just that it’s expensive, it’s also ridiculously complex. About the only thing I can think of that would be more complex is launching rockets into space in order to collect water from comets.
  - function_seven 3y ago
    Yes, economical is always important. In this case, we’re comparing a 231m deep unit with the “boring” existing design, and finding it less economical than the current designs.
  - deleted 3y ago
    [deleted]
  - kelnos 3y ago
    Cost is never immaterial. We look at the various ways we have to provide fresh water, and do whatever is cheapest. If shipping fresh water from places where it's abundant to places where it's scarce is cheapest, we do that. If/when that becomes expensive enough to make desalination attractive from a cost perspective, we start doing that. If the >230m below the surface idea turns out to be cheaper than surface level plants, we do that. But I don't think anyone really knows for sure if the underwater method could be cheaper, so we're all just guessing here. I think a good analogy for this is oil extraction. These days we extract oil from places where it's more expensive and difficult to do so, because we can no longer meet demand with the cheaper options (many of which have simply dried up over time). But we'd never go after the expensive-to-extract oil 40 years ago (or whatever) when cheaper options were available that could meet demand.
- stereo 3y ago
  When your straw gets clogged with ice cream, do you dip your fingers into your milkshake to unclog it? You add your osmosis filter at the end of a long flexible hose. For maintenance, you reel it back in.
  - chongli 3y ago
    Straws don’t work beyond 10 metres. You need all your pumping equipment either at the bottom or distributed along the pipe. Either way, that’s going to put a ton of strain on your “flexible hose” when you try to reel it back in. And what if it breaks? Oops!
    - Enginerrrd 3y ago
      I mean, we do it with oil. ...At least sort of. Usually the wells are pressurized so pumping isn't a concern, but there's a surprising amount of infrastructure involved down at depth and the maintenance of that infrastructure is significant but largely a solved problem. Obviously economics probably favor it when oil is $75/barrel compared to ~$3 maybe for the fresh water, but still.
      - chongli 3y ago
        Your estimate for the water is way too high. I pay $2 USD (as of today's exchange rates) for a cubic metre of water from my local utility. That works out to about 32 cents per "barrel" of water. A $75 USD barrel of oil is 234 times as expensive as the same amount of water. From what I can gather, oil rigs have profit margins below 20% (at best) and margins decline as the pressure in the well head drops, until they become totally unprofitable and shut down to wait for oil prices to go up. Water prices would have to increase by more than 2 orders of magnitude to make this strategy begin to approach profitability!
        jjav 3y ago
        > I pay $2 USD (as of today's exchange rates) for a cubic metre of water from my local utility. A cubic meter is ~264 gallons. Here in my area of California (CA has lots of water agencies, so YMMV) 1000 gallons costs $38.66 (varies based on total consumption). So that's $10.21 for a cubic meter.
jrockway 3y ago
Yeah, 230m sounds like a lot but skyscrapers are taller than that and have running water at the top. So it's definitely possible to do.
GhostVII 3y ago
Wouldn't pumping from 230 meters be exactly equivalent to just pumping through the membrane at sea level? If you have a pump 230 meters below sea level trying to push water up a pipe, to get water to the surface that pump must be exerting the same amount of pressure as you'd be experiencing 230 meters below sea level (since otherwise it wouldn't be strong enough to push up the water column). So you might as well just take the pump up to sea level and have it pump water through the membrane, since it would be exerting the same amount of pressure. Or alternatively, you could just find a hill that is 230 meters high, pump sea water to the top of it, and then have the sea water come down in a pipe and go through a membrane in the bottom. Should be totally equivalent without requiring anything underwater.
- kelnos 3y ago
  I don't think that's correct. Pumping water from below the surface of the ocean is about overcoming gravity. The pipes themselves would need to be constructed to withstand the pressure of the water around it, of course, so the pipes don't collapse. The reason you don't need to "pump" to force the water through the reverse-osmosis filter when you're down that low is because the overall pressure at that depth is sufficient to push the water through as-is. Merely raising water up 230m in the air and "dropping" it through a pipe into a filter sitting on the ground would not give you the same amount of pressure. Put another way, the pressure of the water in a 230m-tall pipe on the filter on the ground is much lower than the pressure of all the water 230m under the surface of the ocean pushing on that submerged filter. While the pipe ensures that the water stays confined, it is not putting pressure on the water in the same way all the water in the ocean is putting pressure on the water that's being pushed into a 230m-submerged filter. I imagine there is some way to do what you describe above-ground, my my intuition is that it would require essentially recreating a large ocean, suspended in the air (not as large as the Atlantic, say, but still fairly large). Much more efficient to just use the ocean we already have. For the submerged filter approach, I think the biggest challenges are probably maintenance and keeping things stable and functioning at the pressures present at >230m below the surface. Those challenges might make it infeasible. I don't think the need to pump the desalinated water back to the surface is all that large a problem in comparison.
the_third_wave 3y ago
Add some wave/tidal/current power generators to the installation and you get those 2.3kJ/kg "for free".
andbberger 3y ago
the single largest consumer of power in california is the DWR's pumping plants. don't underestimate the energy costs of pumping a state's worth of water up several hundred meters. which really puts into perspective how insane desalination, with it's order of magnitude higher energy costs is. why spend all that energy when we could simply build more infrastructure to capture more peak runoff from the sierras
analog31 3y ago
At first glance, this suggests typical desalination has a roughly 15% to 23% efficiency. Also, desalination from some sort of submerged membrane and bringing the freshwater to the surface would itself not be 100% efficient.

chongli 3y ago

I think you’re still going to get killed on maintenance costs. When the membrane gets clogged, how do you replace it cheaply? How do you repair/replace damaged/worn out pumps? What about sections of pipe and the fittings? All of these sorts of repairs are also required for land-based water treatment facilities, of course, but now you have to perform them all at 230m underwater. That’s gotta be mega-expensive. Furthermore, all of your equipment needs to be able to survive at those depths, with saltwater attacking exposed surfaces and seals, requiring much more expensive materials and tighter tolerances. It really doesn’t seem very economical to me!

irrational 3y ago
Is economical important? If we really are desperate for fresh water, does the cost become immaterial?
- chongli 3y ago
  It’s not just that it’s expensive, it’s also ridiculously complex. About the only thing I can think of that would be more complex is launching rockets into space in order to collect water from comets.
- function_seven 3y ago
  Yes, economical is always important. In this case, we’re comparing a 231m deep unit with the “boring” existing design, and finding it less economical than the current designs.
- deleted 3y ago
  [deleted]
- kelnos 3y ago
  Cost is never immaterial. We look at the various ways we have to provide fresh water, and do whatever is cheapest. If shipping fresh water from places where it's abundant to places where it's scarce is cheapest, we do that. If/when that becomes expensive enough to make desalination attractive from a cost perspective, we start doing that. If the >230m below the surface idea turns out to be cheaper than surface level plants, we do that. But I don't think anyone really knows for sure if the underwater method could be cheaper, so we're all just guessing here. I think a good analogy for this is oil extraction. These days we extract oil from places where it's more expensive and difficult to do so, because we can no longer meet demand with the cheaper options (many of which have simply dried up over time). But we'd never go after the expensive-to-extract oil 40 years ago (or whatever) when cheaper options were available that could meet demand.
stereo 3y ago
When your straw gets clogged with ice cream, do you dip your fingers into your milkshake to unclog it? You add your osmosis filter at the end of a long flexible hose. For maintenance, you reel it back in.
- chongli 3y ago
  Straws don’t work beyond 10 metres. You need all your pumping equipment either at the bottom or distributed along the pipe. Either way, that’s going to put a ton of strain on your “flexible hose” when you try to reel it back in. And what if it breaks? Oops!
  - Enginerrrd 3y ago
    I mean, we do it with oil. ...At least sort of. Usually the wells are pressurized so pumping isn't a concern, but there's a surprising amount of infrastructure involved down at depth and the maintenance of that infrastructure is significant but largely a solved problem. Obviously economics probably favor it when oil is $75/barrel compared to ~$3 maybe for the fresh water, but still.
    - chongli 3y ago
      Your estimate for the water is way too high. I pay $2 USD (as of today's exchange rates) for a cubic metre of water from my local utility. That works out to about 32 cents per "barrel" of water. A $75 USD barrel of oil is 234 times as expensive as the same amount of water. From what I can gather, oil rigs have profit margins below 20% (at best) and margins decline as the pressure in the well head drops, until they become totally unprofitable and shut down to wait for oil prices to go up. Water prices would have to increase by more than 2 orders of magnitude to make this strategy begin to approach profitability!
      - jjav 3y ago
        > I pay $2 USD (as of today's exchange rates) for a cubic metre of water from my local utility. A cubic meter is ~264 gallons. Here in my area of California (CA has lots of water agencies, so YMMV) 1000 gallons costs $38.66 (varies based on total consumption). So that's $10.21 for a cubic meter.

jrockway 3y ago

Yeah, 230m sounds like a lot but skyscrapers are taller than that and have running water at the top. So it's definitely possible to do.

GhostVII 3y ago

Wouldn't pumping from 230 meters be exactly equivalent to just pumping through the membrane at sea level? If you have a pump 230 meters below sea level trying to push water up a pipe, to get water to the surface that pump must be exerting the same amount of pressure as you'd be experiencing 230 meters below sea level (since otherwise it wouldn't be strong enough to push up the water column). So you might as well just take the pump up to sea level and have it pump water through the membrane, since it would be exerting the same amount of pressure. Or alternatively, you could just find a hill that is 230 meters high, pump sea water to the top of it, and then have the sea water come down in a pipe and go through a membrane in the bottom. Should be totally equivalent without requiring anything underwater.

kelnos 3y ago
I don't think that's correct. Pumping water from below the surface of the ocean is about overcoming gravity. The pipes themselves would need to be constructed to withstand the pressure of the water around it, of course, so the pipes don't collapse. The reason you don't need to "pump" to force the water through the reverse-osmosis filter when you're down that low is because the overall pressure at that depth is sufficient to push the water through as-is. Merely raising water up 230m in the air and "dropping" it through a pipe into a filter sitting on the ground would not give you the same amount of pressure. Put another way, the pressure of the water in a 230m-tall pipe on the filter on the ground is much lower than the pressure of all the water 230m under the surface of the ocean pushing on that submerged filter. While the pipe ensures that the water stays confined, it is not putting pressure on the water in the same way all the water in the ocean is putting pressure on the water that's being pushed into a 230m-submerged filter. I imagine there is some way to do what you describe above-ground, my my intuition is that it would require essentially recreating a large ocean, suspended in the air (not as large as the Atlantic, say, but still fairly large). Much more efficient to just use the ocean we already have. For the submerged filter approach, I think the biggest challenges are probably maintenance and keeping things stable and functioning at the pressures present at >230m below the surface. Those challenges might make it infeasible. I don't think the need to pump the desalinated water back to the surface is all that large a problem in comparison.

the_third_wave 3y ago

Add some wave/tidal/current power generators to the installation and you get those 2.3kJ/kg "for free".

andbberger 3y ago

the single largest consumer of power in california is the DWR's pumping plants. don't underestimate the energy costs of pumping a state's worth of water up several hundred meters. which really puts into perspective how insane desalination, with it's order of magnitude higher energy costs is. why spend all that energy when we could simply build more infrastructure to capture more peak runoff from the sierras

analog31 3y ago

At first glance, this suggests typical desalination has a roughly 15% to 23% efficiency. Also, desalination from some sort of submerged membrane and bringing the freshwater to the surface would itself not be 100% efficient.