During the charging cycle, the system uses off-peak electricity to work as a heat pump. Argon at ambient temperature and pressure from the top of the cold store is compressed adiabatically to a pressure of 12 bar, heating it to around . The compressed gas is transferred to the top of the hot vessel where it percolates down through the gravel, transferring its heat to the rock and cooling to ambient temperature. The cooled, but still pressurized, gas emerging at the bottom of the vessel is then expanded (again adiabatically) back down to 1 bar, which lowers its temperature to −150 °C. The cold gas is then passed up through the cold vessel where it cools the rock while being warmed back to its initial condition.