Hydraulically Coupled Compressed-Air Energy Storage Systems: A Review of Configurations and Performance with Emphasis on PHCAES

Growing wind and photovoltaic generation increases the demand for large-scale, long-duration energy storage. Pumped hydro compressed-air energy storage (PHCAES) stores and releases energy through pressure transfer between water and compressed air, using air pressure to provide an equivalent hydraulic head and thereby reducing dependence on natural elevation while retaining hydraulic energy conversion and the potential for near-isothermal operation. This review establishes a taxonomy of hydraulically coupled compressed-air storage comprising PHCAES, liquid-piston systems, hydraulically compensated constant-pressure CAES, and hydraulic–pneumatic cascade or hybrid systems. Variable- and constant-pressure PHCAES are compared with pumped hydro energy storage (PHES) and compressed-air energy storage (CAES) in terms of efficiency, economics and environmental implications; this is followed by a critical analysis of the mechanisms governing PHCAES performance. The results show that improved siting flexibility is the principal conditional advantage of PHCAES, rather than inherently higher efficiency or lower cost. Its net performance depends on pressure–volume matching, gas–liquid heat transfer, hydraulic-machine operation, auxiliary consumption and storage infrastructure. Pressure regulation and thermal enhancement are beneficial only when their gains exceed the associated compression, throttling and auxiliary losses. The principal research gap is the lack of engineering-scale, full-cycle validation using consistent electrical, economic and lifecycle assessment boundaries, which currently prevents robust comparison with PHES and CAES.