**6. General Discussion**

This study has attempted an estimate of the exergy storage (and stored exergy to work) potential of major bedded halite deposits of the UK onshore and offshore East Irish Sea areas. Storage would be using salt caverns constructed in the massively bedded halites and storage estimates are based on three thermodynamic models for the temperature and pressure variations within CAES caverns developed by ref [38]. Clearly, a number of significant assumptions and generalisations have been necessary when assessing entire sedimentary basins. However, current salt cavern hosted gas storage facilities prove that the UK halite beds studied are capable of hosting large, stable caverns for high-pressure gas storage. 'Static' theoretical storage volume is enough to meet the UK electricity demand of 300 TWh, although this is unrealistic. Various filters applied to the cavern storage data together with cycle numbers based upon gas storage operational parameters provide more realistic dynamic exergy storage and stored exergy to work estimates of at least 36 MWh, illustrating that salt caverns onshore and in the EIS could deliver significant EES and grid-scale support.

Estimates for future UK electrical energy storage capacity needs for a net-zero system in 2050 range from about 1 TWh in total [50] to the latest National Grid Future Energy Scenario (NGFES) estimates of about 200 GWh [51]. In both cases, the majority of capacity requirement will be for large-scale, long-duration energy storage, with CAES in all three NGFES net-zero scenarios contributing about 20–40 GWh. Currently, PHS accounts for the majority of the UK energy storage capacity, which has 2.8 GW power capacity and 27.6 GWh storage capacity. In 2019, the total energy discharged by PHS in the UK was 1.7 TWh, which is only about 1/70 of the total gas power generation. Due to the potential site-specific negative environmental and ecological impacts of PHS and the limited availability of favourable sites, further expansion of PHS capacity in the UK will be difficult. Lithium-ion battery storage and hydrogen are two promising technologies that may fulfil this required capacity. Lithium-ion batteries have attracted attention and undergone significant development in the last 5 years. However, the cost structure (high CAPEX of energy in \$/kWh) renders it suitable only for mainly daily cycling applications, instead of energy storage operations at timescales greater than 10 h, even with a significantly reduced cost in future (e.g., \$150/kWh) [52]. 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 highpenetration (>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 decarbonisation

progress 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 largescale 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 Pittsfield, Illinois, USA and ran from 1982 through to mid-1984 [14,62]. Following eight years of investigations and research (2003–2011) funded by the US DOE at the Dallas Centre, Iowa USA, the Iowa Energy Storage Project, which aimed to develop a utility-scale, bulk energy storage facility linked to renewable wind energy, was shelved [12]. All of the above porous rock projects encountered problems with one of more of the following: pressure anomalies, variations in reservoir quality and performance, 'air bubble' formation in the reservoir, reaction between the oxygen of the injected air and minerals in the reservoir rock leading to oxygen depletion and/or potential for bacterial/micro-organism growth and porosity reduction. Aquifer storage for the UK, which would be remote offshore, thereby increasing costs, thus seems less likely than salt cavern storage, at least in the short term.

By contrast, our results illustrate the main halite-bearing strata of the UK onshore and East Irish Sea areas offer very significant CAES exergy storage possibilities and capacity, which being closer to demand could play a major role in grid support, load-levelling, and helping to meet the UK's annual electricity demand, which is currently at a level of ≈300 TWh [44]. Such resources in combination with renewable energy generation, particularly solar and wind, could replace the current flexible power generation at a national scale. Open-source data [63] illustrate that although the UK has achieved substantial carbon emission reductions in its power sector in the last decades by reducing the coal-based generation by almost 95%, from about 100 TWh in 2009 to 6 TWh in 2019, gas power is still an essential source in offering flexibility to maintain the second-by-second balance between the power supply (including intermittent renewable power) and varying demand. In 2019, gas power provided 114 TWh electricity that is 42% of all the electricity generated. To decarbonise the gas power and provide the flexibility sacrificed, energy storage will play a significant role and the use of salt cavern-hosted CAES could underpin decarbonisation of the UK power system by offering large-scale flexibility over multiple timescales.
