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Douglas_Knight comments on Stupid Questions May 2015 - Less Wrong
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Following on from this question, since cheap energy storage is a big obstacle to using wind/wave/solar energy, why is gravity-based energy storage not used more?
Many coasts have some cliffs, where we could build a reservoir on top of the cliff and pump up seawater to store energy. What is the fundamental problem with this? Efficiency of energy conversion when pumping? Cost of building? The space the reservoir would take (or the amount of water it could hold)?
The operating cost for hydro power plants is very low, so the relevant cost is the initial building. If you dam a river, it just takes one wall, while if you want to create a swimming pool, it takes four. Actually, five, and the floor may be the biggest problem. If you dam a river, you already know that it isn't easy for the water to flow through the ground, because it isn't taking that route. Whereas pumping it onto dry ground probably won't work.
Actually, this (water that passes under the dam) is the main problem after water passing directly through the dam. If the bed of the river is ok to hold a height of 10 m of water, it is probably not to hold 20 or 30 or 70 m of water.