Salt caverns are an excellent medium for the purpose of bulk energy storage using compressed gas whereby compressed gas storage can use any gas, such as air, to be compressed. This stored energy is equivalent to the product of pressure and volume. In a cavern with a volume of 500,000 m3 and a pressure of 150 bar or 15 MPa the energy stored is 2 080 MWh,
If a gas is compressed, heat is generated according to the thermodynamic law of conservation of energy.
A compression from 1 bar to 150 bar results in a temperature rise of approximately 500°C. The amount of heat produced would correspond to approximately 40,000 GJ and the efficiency of power-on current basis would be, without the storage of the accumulated heat, about 15%. Hence the storage of the generated heat is required for an efficient storage in this process. But heat can only be added to the process on the surface, so that the cooling in the cavern remains a technical challenge.
Changes in temperature above 100 °C occur at compression. Such temperature changes lead to extreme stresses especially in the cavern, and in addition in the borehole. For the ADELE  CAES project maximum temperatures of 600 °C were specified by RWE Power, at a change in pressure between atmospheric pressure and 150 bar.
Therefore, to overcome these drawbacks it is necessary to store the same amount of energy using a smaller compression ratio. This is possible if gas is compressed from an initial 100 bar to 250 bar instead of from 1 bar to 150 bar. The change in pressure, and as such the stored energy, is the same in both cases, however the temperature change in the latter is now only about 50 °C, as the compression ratio is only 2.5 instead of 150!
The efficiency of the process is about 70 % and higher, i.e. 70 % and more of the energy used for storage will be recovered.
The erneo process requires at least two caverns instead of one, in addition cushion gas would be required.
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The technology is protected by US law. USA patent 14/355,994 in Europe and other countries