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Its a neat idea, but the devil is always in the detail.The idea of pumped hydro as n energy storage for excess solar/wind is important. One option that won't be environmentally ugly or prohibitively costly is the system developed by Rheenergise. There are plenty of small hills that would suit this option.
Engineering news
High-density pumped hydro ‘could be installed on thousands of small hills’
08 Feb 2021
Professional Engineering
Unlike conventional hydro power, the system from RheEnergise uses dense liquid instead of water. The fluid is two-and-a-half-times denser than water, and could therefore potentially provide two-and-a-half-times the power of equivalent conventional systems.
The High-Density Hydro systems would be built underground. Its developers said it could offer long-term energy storage at relatively low costs, with high energy efficiency
Like conventional pumped hydro, it would use excess energy – such as that generated by wind turbines on a windy day with low demand – to pump the liquid uphill from underground storage tanks. After travelling uphill through underground pipes, the liquid would then be released to power downhill turbines when electricity demand is higher.
The part that is not mentioned is that the 2.5 times as dense fluid will require somewhat more than 2.5 times the energy to raise it up to the required height. Not being a fluid dynamics guy, I don't know what sort of efficiency losses there are with higher density fluids, but I am going to take an educated guess that the boundary layer losses are going to be significant.
Mick