Back in the 1970s, a German utility wanted to build a flexible storage plant that could respond to sudden peaks in electricity demand, since its conventional plants — mainly coal — weren’t designed to dial up or down quickly.
It didn’t have the hilly terrain needed for a hydroelectric plant, which can start operating much more quickly when demand surges. But here’s what it did have: ancient, underground salt deposits.
Borrowing a technique commonly used to store natural gas and oil deep underground, it piped water into the salt beds to dissolve the salt and create two caverns roughly a half-mile below the grassy fields in Huntorf. The plant, which opened in 1978, uses electricity from the grid, when it’s cheap because demand is low, to compress and store air in the salt caves.
Then, when electricity demand surges, a motor pushes the air to the surface and into a combustion system, where it burns natural gas that spins a turbine to produce electricity. Compressing the air allows it to deliver more oxygen to the turbines, making them more efficient.
A similar plant opened in 1991 in McIntosh, Ala. Several energy companies, mainly in the United States and Europe, are exploring mining their salt deposits for storage as well.The article has a bunch of different examples, including the classic "fill a reservoir with water, let 'er rip when you need it" one. (I really like the flywheel one - that's pretty cool.)
I don't think anyone thinks these could be practical solutions across the board, but they are pretty fun.