The compressed-gas electricity storage sector has recently advanced beyond laboratory prototypes, with significant projects now operational or under construction globally. China commissioned a 600 MW/2.4 GWh compressed-air facility at Huai’an, Hydrostor achieved California certification for its 500 MW/4 GWh Willow Rock project, Highview Power began construction on a 50 MW/300 MWh liquid-air plant, and Energy Dome deployed a first-of-a-kind 20 MW/200 MWh CO₂ battery system in Sardinia. These represent substantial engineering achievements beyond initial research phases. However, the existence of these demonstration projects does not establish a viable commercial storage technology, as building individual large facilities provides limited evidence about whether similar projects should be replicated at scale.
The fundamental challenge lies in the thermodynamic requirements of these systems. Compressed-air storage must manage heat generated during compression through complex thermal storage and heat recovery systems. Liquid-air technology addresses air’s large gaseous volume by cryogenic refrigeration, requiring extensive cooling and heating infrastructure. Energy Dome’s approach using CO₂ offers genuine thermodynamic improvement over liquid-air methods, as carbon dioxide can be condensed under more manageable industrial conditions. Despite these refinements, all compressed-gas systems require substantial process plant infrastructure to compress, cool, condense, store, reheat, and expand their working fluids.
When measured against existing alternatives, compressed-gas storage underperforms economically. Mature pumped-hydroelectric systems achieve approximately 80 percent round-trip efficiency while relying on proven civil engineering. Battery storage benefits from advanced manufacturing systems that achieved one-third price reductions between 2020 and 2025. Compressed-gas storage operates between these reference points, carrying significant process and civil-engineering burdens without matching either pumped hydro’s mature efficiency or batteries’ manufacturing advantages.
Hydrostor’s Willow Rock project exemplifies the limitations of engineering innovation without compelling economics. By excavating purpose-built hard-rock caverns instead of relying on conventional salt caverns, the design broadens geographic feasibility but sacrifices economic efficiency. The California Energy Commission assessed the facility at approximately 60 percent round-trip efficiency—requiring substantial infrastructure to return roughly three-fifths of input electricity. Liquid-air storage faces similar challenges, with 2025 technical reviews identifying round-trip efficiencies between 50 and 60 percent in standalone systems and noting limited economic benefits alongside technical complexity.
Energy Dome’s CO₂ system claims exceed demonstrated operating history. While advertised efficiency levels of more than 70 percent may prove testable, the company’s thirty-year plant-level lifecycle claims rest on installations with insufficient operating duration. Equipment including compressors, turbines, pumps, valves, bearings, seals, and heat exchangers typically requires maintenance, overhauls, and eventual replacement—factors that should feature in comprehensive economic models rather than assumptions of indefinite durability.
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