Comment by chongli | Hacker News Reader

Comment by chongli | Hacker News Reader

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gravity (notably pumped hydro) is cheap for even more capacity but has limited opportunities, and it’s not nearly as fast to trigger as the other, so it’s better used to store capacity over one week. Evaporation is a problem, but cheaply fixable.

I wonder whatever happened to the earth piston idea I saw floated a bunch of years ago. It involved cutting a cylindrical piston in the earth and drilling beneath it at an angle. You then pump water down into the space beneath the piston, causing it to lift. To extract the stored energy you simply let the pressure provided by the piston force the water back out through a turbine.

The main trick to getting it all to work is with the surfaces. You want the piston to be able to slide up and down smoothly without water infiltrating the earth or going around the side of the bore, so the whole thing needs to be sealed and smooth like an engine cylinder. The other trick is for your pumps, valves, and turbines to be able to operate reliably with huge water pressures.

I haven’t heard about the idea in years so I have no idea how it went, though I would guess it didn’t pan out for some reason.

It sounds like quite an engineering challenge and a lot of ground to excavate to store a significant amount of water.

Geothermic energy and CO2 storage sound a lot more useful if you’re going to dig that much

I think the idea was to avoid excavating all the ground in the middle. You just cut around the circumference of the piston and along the bottom. The thing I never could understand was how they’d seal the walls of the cylinder against the bore so that water doesn’t fill that space and gush right out the top!

That sounds massively expensive. Being very generous and assuming that the piston is the length of the hole and can rise out completely, then the energy storage is still only a few times that required to pump all the water out of the hole. It doesn't start to compare to the volume behind a hydro dam (which may then have a large vertical drop to the generator further down-river), but is still much more complex.

Another proposed idea for places that have deep seas or lakes is pumping air down into a storage at the bottom of the water. This storage can even being a flexible plastic - there isn't any high loading on it because the pressure balances out. The issue with that tech is that it's not that efficient, as compressing the air going down generates a lot of heat - some of that could potentially be recovered, but there's a trade-off with simplicity. Also if the containment fails then a lot of air bubbles to the surface, potentially sinking any ship on the surface at the time.

As ben_w points out elsewhere in the thread, renewables are so cheap that losses matter less than you think.

Put another way: Today in Denmark, electricity is free. Literally 0 cents (øre) before taxes and transport fees (of about 5 cents, 36 øre). Just before Christmas it was about $1 (700 øre) per kWh. There's no inefficiency where it would be bad to store energy with swings like that.

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