Embodiments relate generally to energy storage systems, and in particular to energy storage systems using compressed gas as an energy storage medium. In various embodiments, a compressed gas storage system may include a plurality of stages to convert energy into compressed gas for storage, and then to recover that stored energy by gas expansion. In certain embodiments, a stage may comprise a reversible compressor/expander having a reciprocating piston. Pump designs for introducing liquid for heat exchange with the gas, are described. Gas flow valves featuring shroud and/or curtain portions, are also described.