Abstract: 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