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I am always mystified by assertions that underground CAES systems suffer inefficiency. Earth is a superb insulator, so any heat pumped into an underground reservoir along with the compressed air would be still there when you draw it down later.

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If you use water to compress the air as it goes down, you get isothermal compression. The trompe, which originated in the ancient Catalan forges, has been widely used over the centuries. Anecdotally, a trompe in a mine in Switzerland powered a whole bunch of ~1000 horsepower rock drills, provided fresh air to the underground miners, and powered turbines that generated enough electricity to power a town of about 1500 people.

Here's the vid on trompes I always see come up: https://youtu.be/50fJ8Av_g7Q

I know "free energy" devices don't really exist, but a trompe seems pretty darn close, IMO.

https://news.ycombinator.com/item?id=27066295

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