4c524e74_y_Storage____Clean_Energy_Wonk__1
[0] Underground Pumped Hydro [2] More sites available, developing application. Might have some water quality issues. [3] Costs Low 75% to 85% expected efficiency. [4] Small turbine/pump makers. [1] 
As above, but water is pumped between an aquifer and an above ground reservoir.
Large Scale Electricity Storage. | Clean Energy Wonk
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