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?
voidmain · 1h 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 · 17m 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.
potato3732842 · 1h ago
Tension vs compression loading probably.
LorenPechtel · 1h 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.)
amluto · 1h 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 · 53m ago
The volumes involved are way too low for pumped hydro. This is compressed air--but without the big downsides it usually entails.
amluto · 36m 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.
mrDmrTmrJ · 2h ago
Is there any reason to need a concrete sphere? Couldn't a robust, durable flexible bag do the trick?
My hydroflask, when compressed, will push water out :)
bell-cot · 3h ago
> In 2026, a sphere nine metres in diameter and weighing 400 tonnes will be submerged off the coast of California at a depth of 500 to 600 metres. It will have a storage capacity of 0.4 megawatt hours (400 kWh), enough energy to power an average household for several weeks.
What is this going to cost? From a quick search, Tesla Megapacks are now about $250/KWh. With battery costs still falling steadily, those might be considerably cheaper by the time the first 9m sphere hits the water.
And with all the recent anchor-dragging incidents, how many countries would be eager to have their energy storage located far off-shore?
engineer_22 · 1h ago
Well concrete and steel are the major material components, fairly cheap, but pumps and turbines require high precision machining, fairly expensive. Lithium will probably be cheaper.
Probably not economical in current conditions, but worth doing to say it was done.
LorenPechtel · 48m ago
But the pumps and turbines are based on how much power it can process, not how much it can store. Most storage systems have a big downside in that the J that you use once a year costs just as much to store as the one you use every day. This perhaps permits putting a lot of air down there as reserve for when the sun doesn't shine for days on end. Lithium is non-viable for dealing with long periods of darkness (big storm on top of your collectors.) To ensure the lights stay on in the worst case you need weeks worth of storage.
cosmicgadget · 4h ago
20-year maintenance intervals for something pumping seawater? Impressive. Way better than a chemical battery.
mrDmrTmrJ · 2h ago
I don't think you're pumping sea water - I think you're pumping air.
engineer_22 · 59m ago
They're pumping water, see technical paper.
engineer_22 · 1h ago
Article claims pumping water out to a "relative vacuum"
How do they do this without causing the pumps to cavitate?
This seems like a workable idea but just electric cranes lifting and lowering weights seems like a simpler approach and I think that has already been proposed.
My guess is there are many ways to balance the grid and the biggest is the utilities not wanting pay.
One thing to consider is the America power grid is in poor shape already and utilities are aiming to avoid modernizing. IE, adding storage to the grid would involve the double of cost of the actual balance equipment and the fixing the old equipment that needs fixing anyway. And utilities are looking avoid both cost.
chrismeller · 1h ago
I was actually wondering why the whole thing needed the complexity of being underwater and felt like I was just an idiot, so thanks.
From the article it sounds like the only reason to deal with the hassle of the water is so you can put these somewhere no one ever sees?
credit_guy · 1h ago
I think when you lift a cubic meter of water by one meter at a depth of 500 meters, it is like lifting an entire column of 500 cubic meters. It's a huge multiplier effect.
LorenPechtel · 1h ago
They specifically talk about putting them deep. There's only one thing to be gained by going deep: pressure.
It sounds like this is simply compressed air storage, except using seawater to contain it so you don't need a pressure vessel.
stubish · 5m ago
There is no air involved. They are pumping water out, creating a vacuum. Water rushing back in generates power. No need to compress air or have a hose to the surface.
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?
(I'm guessing, of course)
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.)
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.
> 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.
My hydroflask, when compressed, will push water out :)
What is this going to cost? From a quick search, Tesla Megapacks are now about $250/KWh. With battery costs still falling steadily, those might be considerably cheaper by the time the first 9m sphere hits the water.
And with all the recent anchor-dragging incidents, how many countries would be eager to have their energy storage located far off-shore?
Probably not economical in current conditions, but worth doing to say it was done.
How do they do this without causing the pumps to cavitate?
Edit: here's the paper https://www.sciencedirect.com/science/article/abs/pii/S23521...
My guess is there are many ways to balance the grid and the biggest is the utilities not wanting pay.
One thing to consider is the America power grid is in poor shape already and utilities are aiming to avoid modernizing. IE, adding storage to the grid would involve the double of cost of the actual balance equipment and the fixing the old equipment that needs fixing anyway. And utilities are looking avoid both cost.
From the article it sounds like the only reason to deal with the hassle of the water is so you can put these somewhere no one ever sees?
It sounds like this is simply compressed air storage, except using seawater to contain it so you don't need a pressure vessel.