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'No-water' hydropower turns England's hills into green and pleasant batteries
(www.rechargenews.com)
This is a most excellent place for technology news and articles.
I agree you need much less capacity because you'd usually just want to even out fluctuations, but I think the general gist of the comment is still true: you need just 2,5x the amount of water to produce the same amount of energy. The article says very little about the liquid, and very little about why this would enable them to build this capacity much quicker. A little more data would be nice.
More information is alway s useful. But it's pretty obviously quicker to build because it only needs to handle 40% of the liquid and it's not on a mountain.