Comparative evaluation of the performance and costs of hydrogen and compressed air energy storage in salt caverns

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Due to their excellent potential for holding compressed gases, salt caverns are widely considered to be a key resource in the UK for long duration energy storage. Candidate gases primarily include hydrogen and compressed air for energy storage. The properties (i.e. operation, energy storage, efficiency, etc.) of these systems differ significantly, but so far, comparative analysis of these two key options has been limited. Therefore, in this work we perform a comparative analysis of these two long-duration energy storage technologies at two caverns depths, a shallow cavern (circa 500m depth) and a deep cavern (circa 1500m depth). At each depth, we undertake a high-level design of the system based on the operational pressure, as determined by the lithostatic pressure and cavern structural requirements. We find that the energy density of hydrogen systems is considerably higher, however the ACAES systems have higher efficiency at both depths (~64%). Conversely, the hydrogen storage efficiency is lower in the deeper system due to the extra compression work required (~40% compared with ~38% for the shallow and deep systems respectively). For both systems, the capital expense per unit of stored energy is considerably lower for the deeper systems. Our analysis highlights that each of these systems offers considerably different storage capabilities, and therefore the best option is likely to be highly dependent on the energy system that sits above the surface.