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Comment by eliaspro | Hacker News Reader

A local company [1] is taking a similar approach to the extreme and is developing a concept which involves cutting a 100-250m (328-820ft) diameter piston out of solid rock and then pumping water underneath to store up to 8GWh of energy.

Sounds quite crazy at first but actually looks quite reasonable after looking at it a little closer [2].

[1] https://heindl-energy.com/ [2] https://heindl-energy.com/technical-concept/engineering-chal...

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That's amazing!

Any idea how they will separate the bottom of the piston?

I wonder how they will balance it too, so that it doesn't seize in the cylinder. It seems like (re)moving material would be straightforward enough, but figuring out what the current balance is, maybe not so much.

How do they make the seal at the bottom of the piston?

“The Gravity Storage is cut out of surrounding rocks. Because rocks nearly always have crevasses and fine cracks through which water can flow, it is necessary to completely seal the piston and the surrounding piston cylinder against the surrounding area.

For this purpose, all freely exposed surfaces are sealed with a geomembrane and concrete.”

There still needs to be the equivalent of a series of O-rings between the piston and the cylinder bore to prevent the piston from sinking. I'd guess that it is the second-hardest technical problem, after the considerable challenge of isolating the piston.

The 'rolling membrane' that they are proposing is described here: https://heindl-energy.com/technical-concept/engineering-chal...

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