3 ms·

I'm curious why pumping the water out, creating a vacuum for water to rush in, is better than pumping air in for water to displace. I had assumed it would be c

I'm curious why pumping the water out, creating a vacuum for water to rush in, is better than pumping air in for water to displace. I had assumed it would be cheaper to have large underwater balloon connected by a hose to a pontoon, and use air. Rather than install and maintain at depth the pumps and a giant concrete sphere able to withstand that sort of pressure. Have I got the economics wrong? Or is there an efficiency gain from dealing with a liquid rather than compressible gas?

potato3732842 1y ago
Tension vs compression loading probably.
- amluto 1y ago
  That sounds like one important consideration — concrete has poor tensile strength. Also, aside from being under water, this is functionally a lot like pumped hydro, which is an established, well developed technology. Compressed air energy storage has been tried, but as far as I know, it has never really been a success.
  - LorenPechtel 1y ago
    The volumes involved are way too low for pumped hydro. This is compressed air--but without the big downsides it usually entails.
    - amluto 1y ago
      Is it compressed air? The article says, and I quote: > To store energy, excess electricity is used to pump water out of the sphere, creating a relative vacuum. To release energy, we open the valve: the water, pushed by the external pressure, rushes into the sphere and turns the turbine, producing electricity. If you take air at ~50 bar and release it into the atmosphere and try to extract energy from the water replacing it spinning a turbine, you are throwing away most of the energy stored in the compressed air. In any case, the math works out for pumped hydro. Ask Google: (4.5m)^3 * 4/3 * pi * 500m * 1 g/mL * 9.8m/s^2 in kWh This gives 520 kWh, which is consistent with the article’s claims. edit: One can ballpark the storage capacity of compressed air storage, too. Assuming isothermal compression (which would be a nice ideal case and is not easy to achieve unless one compresses very slowly), the work is nRT • ln (volume ratio). nRT = PV measured at any point in the process, which is conveniently exactly the calculation above: 520kWh. For the pumped hydro model, I lazily assumed that they pumped all the way to vacuum, which is obviously wrong (some water would boil), but it makes almost no difference. But here we need to compare the actual pressures, and the pressure ratio (equivalently volume ratio) is around 50 between sea level and 500m deep. So multiply by ln 50 to get around 2MWh. But that’s the actual work done in a perfect isothermal process. In the real world, the starting and ending states will be around the same temperature, but the process will be far from isothermal, so a good deal more than 2MWh will be used to compress the gas and a good deal less will come back out.
- LorenPechtel 1y ago
  There isn't much loading at all. Let's examine a vertical core through it: We have a top made of concrete, below that there is air, below that the ocean. This is a pure compression load on the concrete determined only by the amount of air volume, not the depth and thus the pressure the air is under. Make sure the mass of the concrete exceeds the mass of the water the device displaces. In addition you need a skirt around the sides to keep the air from escaping. It experiences an outward force at the top and an inward force at the bottom, but both are once again based on the air column, not the pressure. There's only one part of the system that actually must be beefy--the connection to the surface which will always be pressurized to the depth of the storage. This is compressed air storage, but without the big waste that normally entails as the tank pressure changes. And without the big pressure vessel. I don't know what the round trip efficiency will be, compressed air usually is abysmal because a compressor will be designed for a given pressure and a turbine will be designed for a given pressure. Tank pressure below the compressor pressure is wasted energy, tank pressure above the turbine pressure is wasted energy. But this uses fixed pressure, they won't be mismatched. This is the first mechanical system that I've seen proposed that sounds sensible. (Check the energy density for all the lifting approaches--abysmal.)
  - rcxdude 1y ago
    That doesn't match the described mechanism at all. The load on the concrete is absolutely related to depth, and immense. They describe one option with a connection to the surface, but that's not required. It's not really compressed air, more like pumped hydro with an small lower reservoir but a huge height difference.
voidmain 1y ago
The article says the spheres will be at 500-600m depth, so something like 50-60 atmospheres of pressure. Compressing air enough to displace water at that pressure would heat it to extreme temperatures which must be very inefficient even with some attempt to recover the heat. Water being incompressible can I assume be moved in and out of the sphere without any heating (and, therefore, energy loss). (I'm guessing, of course)
- stubish 1y ago
  I hadn't thought about heat. While the air will be hot, that heat was already there (the heat energy in 50 liters of air compressed into 1 liter, just like the compressor in a heat pump). I think this makes it even more viable, as the air pump/compressor can use that heat to partially power itself. A water pump can't do that.
- amluto 1y ago
  One probably wouldn’t want to store energy by adiabatic compression like that. Instead one might try to approximate isothermal compression, perhaps by using multiple stages with heat exchangers to adjust the temperature between stages. IIRC there was a startup, Lightsail Energy, that had an idea of using a water mist as either a heat reservoir or a phase change medium, and I don’t know whether they ever got it working well.
aaron695 1y ago
[dead]
manarth 1y ago
> "The implementation of the pressure hose led to challenges in the dimensioning even for the small prototype, because conventional pressure hoses are usually designed for high internal pressures, but not for high external pressures. In addition, the installation becomes more demanding as the depth of the water increases. Greater water depths are associated with higher pressures, longer pressure hoses with greater friction losses and higher mechanical stresses. The more stable the hose wall is designed, the larger is its bending radius and weight, which in turn makes the installation more difficult." They tried it with a surface hose and without, and the full charge-dischage cycle were roughly equivalent, so the simpler version without the surface hose is the recommended option. https://www.sciencedirect.com/science/article/am/pii/S2352152X21007891 https://www.sciencedirect.com/science/article/am/pii/S235215... (PDF)

potato3732842 1y ago

Tension vs compression loading probably.

amluto 1y ago
That sounds like one important consideration — concrete has poor tensile strength. Also, aside from being under water, this is functionally a lot like pumped hydro, which is an established, well developed technology. Compressed air energy storage has been tried, but as far as I know, it has never really been a success.
- LorenPechtel 1y ago
  The volumes involved are way too low for pumped hydro. This is compressed air--but without the big downsides it usually entails.
  - amluto 1y ago
    Is it compressed air? The article says, and I quote: > To store energy, excess electricity is used to pump water out of the sphere, creating a relative vacuum. To release energy, we open the valve: the water, pushed by the external pressure, rushes into the sphere and turns the turbine, producing electricity. If you take air at ~50 bar and release it into the atmosphere and try to extract energy from the water replacing it spinning a turbine, you are throwing away most of the energy stored in the compressed air. In any case, the math works out for pumped hydro. Ask Google: (4.5m)^3 * 4/3 * pi * 500m * 1 g/mL * 9.8m/s^2 in kWh This gives 520 kWh, which is consistent with the article’s claims. edit: One can ballpark the storage capacity of compressed air storage, too. Assuming isothermal compression (which would be a nice ideal case and is not easy to achieve unless one compresses very slowly), the work is nRT • ln (volume ratio). nRT = PV measured at any point in the process, which is conveniently exactly the calculation above: 520kWh. For the pumped hydro model, I lazily assumed that they pumped all the way to vacuum, which is obviously wrong (some water would boil), but it makes almost no difference. But here we need to compare the actual pressures, and the pressure ratio (equivalently volume ratio) is around 50 between sea level and 500m deep. So multiply by ln 50 to get around 2MWh. But that’s the actual work done in a perfect isothermal process. In the real world, the starting and ending states will be around the same temperature, but the process will be far from isothermal, so a good deal more than 2MWh will be used to compress the gas and a good deal less will come back out.
LorenPechtel 1y ago
There isn't much loading at all. Let's examine a vertical core through it: We have a top made of concrete, below that there is air, below that the ocean. This is a pure compression load on the concrete determined only by the amount of air volume, not the depth and thus the pressure the air is under. Make sure the mass of the concrete exceeds the mass of the water the device displaces. In addition you need a skirt around the sides to keep the air from escaping. It experiences an outward force at the top and an inward force at the bottom, but both are once again based on the air column, not the pressure. There's only one part of the system that actually must be beefy--the connection to the surface which will always be pressurized to the depth of the storage. This is compressed air storage, but without the big waste that normally entails as the tank pressure changes. And without the big pressure vessel. I don't know what the round trip efficiency will be, compressed air usually is abysmal because a compressor will be designed for a given pressure and a turbine will be designed for a given pressure. Tank pressure below the compressor pressure is wasted energy, tank pressure above the turbine pressure is wasted energy. But this uses fixed pressure, they won't be mismatched. This is the first mechanical system that I've seen proposed that sounds sensible. (Check the energy density for all the lifting approaches--abysmal.)
- rcxdude 1y ago
  That doesn't match the described mechanism at all. The load on the concrete is absolutely related to depth, and immense. They describe one option with a connection to the surface, but that's not required. It's not really compressed air, more like pumped hydro with an small lower reservoir but a huge height difference.

voidmain 1y ago

The article says the spheres will be at 500-600m depth, so something like 50-60 atmospheres of pressure. Compressing air enough to displace water at that pressure would heat it to extreme temperatures which must be very inefficient even with some attempt to recover the heat. Water being incompressible can I assume be moved in and out of the sphere without any heating (and, therefore, energy loss). (I'm guessing, of course)

stubish 1y ago
I hadn't thought about heat. While the air will be hot, that heat was already there (the heat energy in 50 liters of air compressed into 1 liter, just like the compressor in a heat pump). I think this makes it even more viable, as the air pump/compressor can use that heat to partially power itself. A water pump can't do that.
amluto 1y ago
One probably wouldn’t want to store energy by adiabatic compression like that. Instead one might try to approximate isothermal compression, perhaps by using multiple stages with heat exchangers to adjust the temperature between stages. IIRC there was a startup, Lightsail Energy, that had an idea of using a water mist as either a heat reservoir or a phase change medium, and I don’t know whether they ever got it working well.

aaron695 1y ago

[dead]

manarth 1y ago

> "The implementation of the pressure hose led to challenges in the dimensioning even for the small prototype, because conventional pressure hoses are usually designed for high internal pressures, but not for high external pressures. In addition, the installation becomes more demanding as the depth of the water increases. Greater water depths are associated with higher pressures, longer pressure hoses with greater friction losses and higher mechanical stresses. The more stable the hose wall is designed, the larger is its bending radius and weight, which in turn makes the installation more difficult." They tried it with a surface hose and without, and the full charge-dischage cycle were roughly equivalent, so the simpler version without the surface hose is the recommended option. https://www.sciencedirect.com/science/article/am/pii/S2352152X21007891 https://www.sciencedirect.com/science/article/am/pii/S235215... (PDF)