## 6 General Discussion
The design space for large-scale, long-duration electrical energy storage is plausibly set to be up to $20-40/kWh for balancing a grid with high-penetration (>90%) variable renewable energy generation [53,54]. Alternatively, hydrogen energy storage is at the other end of the storage spectrum, being particularly suitable for long-duration energy storage. Compared to technologies such as PHS, batteries, and CAES, hydrogen is still in the development phase of prototype or demonstration in order to validate its technical performance. Its cost reduction may require massive infrastructure construction (e.g., centralised electrolysis) that enables convenient transmission and distribution of hydrogen [54,55] and further research on currently less mature technologies such as high-temperature solid oxide or molten carbonate fuel cells that may enable low-cost scalable hydrogen production [56]. Either of these will add to the uncertainty in timescale and system-scale of the technology in decarbonising the power system. Although a diverse range of large-scale long-duration energy storage technologies are needed to deeply decarbonise electrical systems, technologies with relative high technology maturity and resource availability will help mitigate the risk and ensure an early and steady decarbonisationprogress in the next decade, which may also help reduce the cost required for meeting the net-zero goal [57].  
Amongst all the EES technologies, CAES is a relatively mature technology with large-scale conventional (diabatic) CAES having been commercially operational since 1978 at Huntorf, in systems of over 100 MW capacity and employing salt cavern storages. In this time, pilot ACAES plants have been considered, with a small (2 MW) pilot plant having operated between 2012 and 2016 in Texas [25,26], and the commissioning in 2019 of the world's first ACAES plant in Canada [11], also using salt caverns. Demonstration plants on the scale of 1-10 MW have been under appraisal (S1, Tables S1-S6) in Europe [30] and China, where there has also been a successful integration test of the world's first 100 MW CAES expander [58]. In comparison with other EES technologies, CAES has very low energy-storage costs ($3-6/kWh) [59], which makes it a cost-effective solution for long-duration grid-scale energy storage. The cost of CAES is described as low compared to all other energy storage technologies, which is evidenced by [59,60]. This includes the cost of hydrogen energy storage, amongst other energy storage technologies. The works are considered by the authors to be suitable resources for comparing the costs and other important performance methods of energy storage technologies as opposed to providing too much detail in this manuscript. Therefore, there exists the real possibility for the deployment of CAES to offer flexibility at a scale currently provided by fossil fuels in the system balance on various timescales from short duration (minute to hourly) to long duration (days/weeks). In contrast to the alternative large-scale storage technology, PHS, recent studies, and our analysis indicate that substantial exergy storage potential exists for CAES in the UK area. It is suggested that saline porous rocks (aquifers) in sedimentary basins of the UKCS area could provide inter-seasonal electricity storage amounting to approximately 160% of the UK's electricity consumption for January and February of 2017 [15]. However, this storage is offshore and distant to demand centres onshore. Additionally, whilst there has long been interest in the potential for CAES in porous rock formations [13,14], serious doubts exist over the likely development of porous rock CAES, with no plants having operated commercially and only a few, mostly small, test facilities having been constructed: a small 25 MW R&D CAES demonstration facility operated between 1987 and 1991 at Sesta, Italy [61], while an aquifer field test facility was built at Pittsburgh, Illinois, USA and ran from 1982 through to mid-1984 [14,62].