The construction of pumped storage plants is limited due to geographical possibilities. For this reason, the possibility of pumped storage in disused mines or underground storage caverns set up for this purpose is being discussed. Here, at least the lower basin (lower storage volume) is located at a depth of several hundred meters. However, this type of storage is very costly, as a shaft is required and the entire power plant has to be installed underground. 

Schematic diagram of underground pumped storage
according to energy research center Goslar efzn

Sketch of an underground pumped storage project - RAG 

An alternative to this was developed by erneo. The water is pumped between two salt caverns located at different depths. The pumps and turbines can be located above ground. When the system is completed and under sufficiently high pressure, water can easily flow back and forth and electricity can be generated in a turbine.

This method has the advantage that no shaft has to be sunk, as is the case with other underground pumped storage systems, and is therefore much more cost-effective. The principle is illustrated in the following figure, which is taken from our patent application. However, as with other underground pumped storage methods, one disadvantage is that salt can continue to dissolve and one is working with a saturated salt solution, whereby changes in pressure and temperature can cause salt precipitation. This would significantly complicate storage. We have therefore decided to pursue compressed air storage, which is just as efficient.

Underground Pumped Hydro Storage - erneo technology