Apparatus (100) comprising a power plant or air motor utilizing compressed air or liquid air for energy storage. The apparatus includes an electrical plant (200), a mechanical plant (300), and a pneumatic plant (400). When operating as a compressor, the plant receives electrical and/or direct mechanical power as an input to drive the plant, compress air, and store its output in the form of compressed or liquefied air. When operating as an engine, the plant consumes the compressed or liquid air to drive a mechanism of the engine and deliver mechanical power and/or electrical power as an output.