Disclosed herein is a system for storing and releasing energy, comprising a generator/motor subsystem; a hydraulic pump for pumping hydraulic fluid between first and second pump ports in response to force applied by the generator/motor subsystem and also capable of imparting force to the generator/motor subsystem in response to hydraulic fluid being caused to flow between the first and second pump ports; a first compression/expansion vessel for exchanging hydraulic fluid with the hydraulic pump via the first pump port; a second compression/expansion vessel for exchanging hydraulic fluid with the hydraulic pump via the second pump port; a gas storage subsystem for selectively storing gas from, or releasing gas to, the first and second gas compression/expansion vessels; an internal heat exchanger within each of the first and second compression/expansion vessels for exchanging heat between the hydraulic fluid and the gas therein, each internal heat exchanger having a very large thermal mass relative to the mass of gas within the first and second compression/expansion vessels; a control subsystem for controlling at least a plurality of gas valves associated with the gas storage subsystem to switch between the storing and releasing, wherein, during storing, responsive to hydraulic fluid being pumped by the hydraulic pump between the first and second pump ports, gas is caused to be compressed in the gas storage subsystem thereby to store energy, and wherein, during releasing, responsive to pressure from release of compressed gas from the gas storage subsystem, hydraulic fluid is forced to flow between the first and second pump ports thereby to release energy.