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Comment by sliken | Hacker News Reader

Seems a bit limited and expensive. Pumping water uphill when there's a surplus seems like a no brainer, doubly so when you are already using hydro and hydro production is already tied into the grid. Seems like pretty much the only way to cost effectively bank substantial amounts of energy for 6 months.

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It's good but not perfect. Dams require a reservoir, which takes land out of use and changes the flow dynamics of a river. It can interrupt wildlife patterns both up and downstream. It introduces some risks of dam failure, and decommissioning a dam requires careful management of built up sediment. Flooded organic matter anaerobically produces methane.

It also requires significant space and particular configurations (must be on a river, must be in a hilly or mountainous area, etc.) It also isn't necessarily scalable -- how many more dams can we build?

You can build a crane and blocks or drill a big hole in lots and lots of places.

So yes, pumped hydro is better than fossil fuels. But also it's not something we can consider to be the ultimate solution to the issue of storage. By all means, let's use it (and we do use it today), but not stop there.

Dams are expensive and have significant side effects. However the duty cycle on a dam is pretty low (they rarely run the turbines at 100%). So there's substantial additional capacity that could be had by pumping water upstream whenever there is a surplus of power.

I am not sure that this form of storage is comparable to a traditional dam. The fact that blocks of matter need to be physically moved by a crane severely limits the total capacity of the system. The most dense metals are only about 20x as dense as water, so we are not talking about a huge volume of water. The equivalent mass of water can be stored in a tank - no dam required and moved through a pipe - no river required.

Paying for 20 denser material is usually more expensive than just "buying" 20 times as much material. Gravity storage will always be a scaling problem because you would rather have 1000 times the material than 20 time the density. Doing something as nonsensical as using a mine shaft whose size is fixed is not going to work out.

The crane system could work out in theory if you make the crane arms 200m or longer. The covered surface area grows much faster than the length of the crane arm. In practice it's an engineering nightmare.

The piston idea has the benefit that you just use a wire saw and cut out a circle. The primary challenge is building what is basically the largest seal on the planet. The pressure itself is not a problem but the sheer size is.

Compared to hydrogen, reflow batteries, compressed air, heat storage, etc gravity energy storage is extremely questionable unless it's pumped hydro.

The piston idea seems like a appealing design requiring minimal new technology. The plant is the same as existing pumped hydro plant, the weight can come from the native rock.

Using a rolling membrane for the seal is discussed at

https://heindl-energy.com/technical-concept/engineering-chal...

Naively that does seem plausible but it's a very large sheet of material to manufacture. What are your thoughts on that approach?

It seems like using hydro as a demand-smoother changes the aspect of how much damming you need to do. Instead of having to construct a massive reservoir to provide constant power if the hydro is only intended to provide peaking or off-demand power then it seems like the reservoir could be much smaller - and conversely less environmentally impactful.

Once you've committed to a dam, much of the environmental damage is baked in, whether small or large. And dams have huge environmental impacts across the entire river system.

They still have their place. There is a pumped hydro system here in the Bay Area (San Luis Reservoir) that is coupled to a hydro station, and produces power when it drains back in to the aqueducts. It's primary purpose is water capture and distribution, not power storage, but it does both.

The other way is to electrolyze water and store hydrogen in underground salt caverns.

There are a lot of places water/ hydro isn't an option.

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