Eh. (imho) Hydro pumping is way more simple, and is just limited by the fact that you can't have it wherever you want.
Also concrete is not exactly environmentally friendly to make.
Also concrete is not exactly environmentally friendly to make.
Anything heavy will work. I wonder about bags of stones & rubble, or earth itself. The challenge would be making such bags not break apart due to fall.
If they're falling and there's an impact at the end, something has already gone wrong.
When operating correctly, the descent is a smooth glide.
Think that through.
A dam probably holds 100 or 1000 tons of water for every ton of dam material.
A concrete weight holds exactly on ton of concrete for one ton of concrete weight material.
And "amount of weight helt", be it water or concrete, is Exactly what matters here.
You don't necessarily need a river to be involved at all for pumped storage. Just build two reservoirs at different heights.