An EU project, titled the “Design Study for the European Underground Research Infrastructure related to Advanced Adiabatic Compressed Air Energy Storage” or RICAS2020, which is investigating the possibility of storing very large amounts of energy in underground caverns, has proposed a technique which could almost double the efficiency of such systems.

Currently, there are only two large compressed air energy storage plants in the world – the Huntorf plant in Germany, which has a capacity of 290 megawatts, and the McIntosh plant in Alabama, with 226 megawatts. These store the compressed air in underground chambers in salt formations and release it to power a turbine when electricity is needed.

A major problem with these current facilities is that the process of compressing air produces heat. At both facilities, the heat is simply released. As well as losing a potential source of energy, the air needs to be heated up again, often with natural gas, to run the turbines that generate electricity. This reduces the energy efficiency of the system.

The RICAS2020 participants are proposing a system in which, when the air is first compressed and, when hot, it is passed through a separate cavern filled with crushed rock. The hot air would heat the rock and cool air would continue to the main cavern where it would be stored. When the compressed air is released, it would be passed back through the hot rocks where it would be reheated before reaching the turbine.

It is estimated that this would improve the efficiency of the system to 70-80% compared with efficiencies of no more than 45-55% with current technology.