Storage of electrical energy is a growing problem.

This concept is based on the well-known pumped storage, which is a type of hydroelectric power generation. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric is used to run the system pumps. During periods of high electrical demand, the stored water is released through turbines. The efficiency off a pumped storage system is typically 75 to 80 %.

Countries like Denmark have only few hilly areas, with are suitable or available for pumped storage systems.

This new form of pumped storage system is based on an underground storage reservoir, where the water is enclosed in a bladder placed underground with 25 meters of soil on top. The overlaying soil gives the necessary pressure to store large amount of energy, in corresponding to the pressure created by the level difference in well-known pumped storage system.

The pumped storage systems with a underground storage reservoir is independent of the geography and can be placed close to a water reservoir – sea, lake, and river – with little or no level difference.

The enclosed drawing shows an example of the new underground pumped storage system. The system contains three main elements: (1) The underground storage reservoir, (2) pump/turbine, (3) pipe connections, and (4) the open low level reservoir.