A system for compressing gas includes a source of gas, a gas output location, first and second pressure vessels, first and second gas input lines for directing gas from the source of gas respectively to the first and second pressure vessels, first and second gas output lines for directing gas respectively from the first and second pressure vessels to the gas output location, and a hydraulic system for moving hydraulic fluid back and forth between the first and second pressure vessels to compress gas in the first and second pressure vessels in an alternating manner. Gas is pressurized in the first pressure vessel and the second pressure vessel. A heat absorbing media is positioned within the first and second pressure vessels to control an amount the gas increases in temperature during compression.