SustainX compressed air energy storage (CAES) technology combines the low cost of compressed-air energy storage with the long lifetime and high efficiency of hydraulic systems. In contrast to classical underground CAES, the SustainX technology need not be paired with a secondary heat source such as a gas combustion turbine; rather, it generates electricity directly from the expansion of the compressed air itself. No fuel input is required. In addition, SustainX utilizes above-ground storage in the form of industrial-grade, off-the-shelf gas cylinders, eliminating the siting constraints and permitting concerns associated with classical underground CAES.