A system for storing and releasing energy, the system having a storing state and a releasing state and comprising: a generator/motor subsystem; a hydraulic pump configured to pump hydraulic fluid between first and second pump ports (14, 16) in response to force applied to the hydraulic pump by the generator/motor subsystem during the storing state and also configured to impart force to the generator/motor subsystem in response to hydraulic fluid being forced to flow through the hydraulic pump during the releasing state between the first and second pump ports (14, 16); 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 comprising at least a plurality of gas valves (112, 114, 122, 124) and configured to receive and store gas forced via the hydraulic fluid from the first and second gas compression/expansion vessels (20,30) during the storing state, the gas storage subsystem configured to release gas during the releasing state thereby to impart force to the generator/motor subsystem via the hydraulic fluid; an internal heat exchanger (22, 32) within each of the first and second compression/expansion vessels (20, 30) for exchanging heat between the hydraulic fluid and the gas therein, each internal heat exchanger (22, 32) having a very large thermal mass relative to the mass of gas within the first and second compression/expansion vessels (20, 30); and a control subsystem configured to control at least the hydraulic pump and the gas valves (112, 114, 122, 124) thereby to switch between the storing state and the releasing state. a generator/motor subsystem; a hydraulic pump configured to pump hydraulic fluid between first and second pump ports (14, 16) in response to force applied to the hydraulic pump by the generator/motor subsystem during the storing state and also configured to impart force to the generator/motor subsystem in response to hydraulic fluid being forced to flow through the hydraulic pump during the releasing state between the first and second pump ports (14, 16); 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 comprising at least a plurality of gas valves (112, 114, 122, 124) and configured to receive and store gas forced via the hydraulic fluid from the first and second gas compression/expansion vessels (20,30) during the storing state, the gas storage subsystem configured to release gas during the releasing state thereby to impart force to the generator/motor subsystem via the hydraulic fluid; an internal heat exchanger (22, 32) within each of the first and second compression/expansion vessels (20, 30) for exchanging heat between the hydraulic fluid and the gas therein, each internal heat exchanger (22, 32) having a very large thermal mass relative to the mass of gas within the first and second compression/expansion vessels (20, 30); and a control subsystem configured to control at least the hydraulic pump and the gas valves (112, 114, 122, 124) thereby to switch between the storing state and the releasing state.