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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.
It's just good ol' fashioned mercury.
The article claims it's 2.5x as dense as water, which according to this density chart is probably bromine.
I actually bet it's just very very fine stonedust and water in suspension.
I'm not sure which would be worse for the machinery
Supposedly it's exactly that, but stabilized by a polymer to keep it in suspension for up to 60 days without mixing. https://online.flippingbook.com/view/1025707592/10/#zoom=true
Edit: I've concluded it's likely calcium carbonate.
rotor blades are going to love this