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.
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.
My hydroflask, when compressed, will push water out :)
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?
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.
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.)
(I'm guessing, of course)
> "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."
https://www.sciencedirect.com/science/article/am/pii/S235215... (PDF)
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...
"The most important finding was that an air-connection to the surface is not needed, reducing the technical effort significantly." [1]
"Charging requires more time and energy with closed ventilation. In return the higher pressure difference results in a higher turbine power and energy during the discharging phase…ventilation is not beneficial in this regard." [2]
[2] https://www.sciencedirect.com/science/article/am/pii/S235215... (PDF)
That implosion would have a localised short-term effect on water currents as water moves to fill the void, which may affect wildlife in the very immediate area, at the point of implosion (e.g. fish swimming directly next to the sphere may be sucked in and knocked into or trapped by the collapsing concrete).
I can't envisage a failure mode which would cause an explosion or cascade to other spheres in the locality.
In this example, the energy source is the pressure of the water column, which is equipotent with the depth, and applying equally to all external faces of the sphere. It'll be dramatic for the sphere, but fairly inconsequential for anything not immediately adjacent.
The target depth is 750m (compared to 100m for the pilot in Lake Constance).
cosmicgadget•9mo ago
mrDmrTmrJ•9mo ago
engineer_22•9mo ago
stubish•9mo ago