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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.
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