An apparatus including: - an accumulator (1, 200, 300, 400, 600) including: ∘ an energy storage apparatus with a movable first piston face (9, 309, 409, 609), the movable first piston face (9, 309, 409, 609) configured to reversibly compress an energy storage medium; ∘ a movable second piston face (12, 312, 412, 612) forming at least part of an inner surface of a corresponding second fluid chamber (10, 310, 410, 610) reversibly expandable by said second piston face (12, 312, 412, 612); ∘ a movable third piston face (13, 313, 413, 613) forming at least part of an inner surface of a corresponding third fluid chamber (11, 311, 411, 611) reversibly expandable by said third piston face (13, 313, 413, 613); wherein said first, second and third piston faces (9, 12, 13) (309, 312, 313) (409, 412, 413) (609, 612, 613) are coupled together; - a reciprocating component (28, 553); - a prime mover; - a drive mechanism capable of moving the reciprocating component reciprocally and/or cyclically, said drive mechanism including a hydraulic drive motor (36, 552); - hydraulic fluid interconnections with a combination of hydraulic lines (37, 38, 39, 40, 41, 42) including at least one: ∘ pressure line (37, 38, 39) from the prime mover to: • the drive motor (36, 552); • accumulator second fluid chamber (10, 310, 410, 610); • accumulator third fluid chamber (11, 311, 411, 611); ∘ drain line (40, 41) input to the prime mover from: • the drive motor; • accumulator second fluid chamber (10, 310, 410, 610); ∘ pressure line from the accumulator third fluid chamber (11, 311, 411) to: • the drive motor; wherein the accumulator further includes fluid conduits, valves and/or connections configured to concurrently: input hydraulic fluid to the second fluid chamber (10, 310, 410), and output hydraulic fluid from the third fluid chamber (11, 311, 411) to the drive motor (36, 553). - an accumulator (1, 200, 300, 400, 600) including: ∘ an energy storage apparatus with a movable first piston face (9, 309, 409, 609), the movable first piston face (9, 309, 409, 609) configured to reversibly compress an energy storage medium; ∘ a movable second piston face (12, 312, 412, 612) forming at least part of an inner surface of a corresponding second fluid chamber (10, 310, 410, 610) reversibly expandable by said second piston face (12, 312, 412, 612); ∘ a movable third piston face (13, 313, 413, 613) forming at least part of an inner surface of a corresponding third fluid chamber (11, 311, 411, 611) reversibly expandable by said third piston face (13, 313, 413, 613); wherein said first, second and third piston faces (9, 12, 13) (309, 312, 313) (409, 412, 413) (609, 612, 613) are coupled together; ∘ an energy storage apparatus with a movable first piston face (9, 309, 409, 609), the movable first piston face (9, 309, 409, 609) configured to reversibly compress an energy storage medium; ∘ a movable second piston face (12, 312, 412, 612) forming at least part of an inner surface of a corresponding second fluid chamber (10, 310, 410, 610) reversibly expandable by said second piston face (12, 312, 412, 612); ∘ a movable third piston face (13, 313, 413, 613) forming at least part of an inner surface of a corresponding third fluid chamber (11, 311, 411, 611) reversibly expandable by said third piston face (13, 313, 413, 613); - a reciprocating component (28, 553); - a prime mover; - a drive mechanism capable of moving the reciprocating component reciprocally and/or cyclically, said drive mechanism including a hydraulic drive motor (36, 552); - hydraulic fluid interconnections with a combination of hydraulic lines (37, 38, 39, 40, 41, 42) including at least one: ∘ pressure line (37, 38, 39) from the prime mover to: • the drive motor (36, 552); • accumulator second fluid chamber (10, 310, 410, 610); • accumulator third fluid chamber (11, 311, 411, 611); ∘ drain line (40, 41) input to the prime mover from: • the drive motor; • accumulator second fluid chamber (10, 310, 410, 610); ∘ pressure line from the accumulator third fluid chamber (11, 311, 411) to: • the drive motor; ∘ pressure line (37, 38, 39) from the prime mover to: • the drive motor (36, 552); • accumulator second fluid chamber (10, 310, 410, 610); • accumulator third fluid chamber (11, 311, 411, 611); • the drive motor (36, 552); • accumulator second fluid chamber (10, 310, 410, 610); • accumulator third fluid chamber (11, 311, 411, 611); ∘ drain line (40, 41) input to the prime mover from: • the drive motor; • accumulator second fluid chamber (10, 310, 410, 610); • the drive motor; • accumulator second fluid chamber (10, 310, 410, 610); ∘ pressure line from the accumulator third fluid chamber (11, 311, 411) to: • the drive motor; input hydraulic fluid to the second fluid chamber (10, 310, 410), and output hydraulic fluid from the third fluid chamber (11, 311, 411) to the drive motor (36, 553).