A key aspect of the SustainX technology is the use of isothermal (constant temperature) compression and expansion of the gas, as opposed to the adiabatic (no heat transfer) compression and expansion employed by other CAES techniques. Thermal efficiencies in excess of 90% have been achieved experimentally for the compression and expansion cycles, as compared to ~50% thermodynamic efficiencies for adiabatic techniques over the same pressure range.