NOVELTY - The system (100) has first and second vertical low pressure vessel units coupled to a low pressure hydraulic vessel on opposite sides of a low pressure piston. Off-the-shelf heat transfer devices are placed within upper regions of the first and second low pressure vessel units, where the low pressure piston is moveable along first direction to displace heat transfer liquid from the low pressure hydraulic vessel to the first vessel unit and the low pressure piston is moveable along second direction to displace the liquid from the low pressure hydraulic vessel to the second vessel unit.    USE - Horizontal-actuation modular compressed air energy storage system for use at electric power plants, intermittent electric power production sites e.g. wind farms, electric power grid or solar power sources, and electric power consumption sites e.g. buildings, towns, cities, commercial facilities and military facilities.    ADVANTAGE - The system improves process flow dynamics for reductions in non-pass-through volume to improve thermodynamics and for elimination of choke points in a flow stream to ensure higher efficiency. The system allows half-unit sub-system to be coupled to additional half-units or full units to build a system of capacity so as to satisfy desired project rating. The system uses common interstage pipes for each energy subassembly so as to reduce length and complexity of a pipe-system to simplify water exchange piping function, thus improving water management capability with conveniently located liquid exchange connections. The system eliminates point loads on the working vessels so as to eliminate a need for thermal expansion bellows to deal with thermal loads. The system ensures elimination of heat transfer device shrouds and interstage fins to produce a less restrictive air flow path in a cost-effective manner with reduced risks, so that surface tension of the heat transfer liquid can be broken without potential for shock associated with fins contacting the liquid when gas is in contact with heat transfer fins during compression and expansion. The system allows internals of the working vessels to be simplified, thus reducing a need for frequent service of the vessels and limiting horizontal configuration of the pistons to be limited in height without being suspended to wind/seismic loading, while reducing the required wall thickness of the vessels and reducing coefficient of variation for the vessel performance with no unique pressure chamber configurations. The system allows rising and falling liquid pistons to make contact with the heat transfer devices located in the upper regions of the pressure vessel units to transfer thermal energy efficiently between compressing or expanding gas and the liquid in the vessel units. The system reduces low pressure lines and high pressure lines with additional valves to accommodate for the reduced number of lines, where half-units or full-units can be operated separately from or in parallel with the other units such that modular nature of the system allows various sub-assemblies of the system can be isolated for preventative maintenance or repair of the system and for quick service and replacement of the system in an easy and safe manner with no internal work on-site and without splitting vessel body flanges, thus providing exceptional system availability and overall system reliability with reduced assembly time. The system allows a controller to operate various components of the system to set piston velocity profiles, liquid flow rates, heat transfer rates, valve timings and other system properties to attain a desired gas compression and expansion rate and efficiency.    DETAILED DESCRIPTION - The off-the-shelf heat transfer devices are heat wheels. The heat transfer liquid is water. An INDEPENDENT CLAIM is also included for a method for operating a modular compressed air energy storage system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of an energy storage and delivery system.    Horizontal actuation modular compressed air energy storage system (100)    Power source (102)    Power routing subsystem (110)    Conversion sub-system (112)    Storage sub-system (122)    Power grid (124)