## 5 UK Salt Cavern Exergy Storage Capacity Estimates--Results and Commentary  
We now summarise and present the exergy storage estimates and storage capacities (Figures 3-6) for salt caverns in four of the main UK halite-bearing sedimentary basins: onshore Cheshire, Wessex, eastern England, and the offshore East Irish Sea (Figure 1; S2, Tables S3-S8). Figure 3a,b illustrate the total theoretical UK'static' (one-fill) exergy storage and work from stored exergy for the three models and two depth ranges considered. For the preferred CHT model conditions, the stored exergy to work available in caverns for the 500-1300 m depth range (274 MWh) would almost meet the annual UK electricity requirement of \(\approx\)300 TWh (Figure 3a), whilst for the depth range 500-1500 m, the stored exergy to work available from all three models would prove sufficient to meet UK electricity needs (Figure 3b). Taking just 1% of the potential caverns provides a'static' exergy storage for the CHT scenario of between \(\approx\)3 and 4.7 TWh in the 500-1300 and 500-1500 m cavern depth ranges, respectively (Figure 3a,b, S2, Table S3). Cycled once a month, this could generate between 36 and 56.5 TWh of storage, or up to one-fifth of the UK electricity demand, illustrating the importance of this technology in providing a significant contribution to the UK's energy storage capacity and electricity supply. It should be noted that cavern numbers in eastern England are influenced strongly by depth, with much of the available halite and thus cavern volume being below 1300 m depth. Taking a percentage of the'static' exergy storage estimates derived from the numbers of operating or permitted UGS caverns (148) relative to the number of potential storage caverns (32,185 and 44,849), for the CHT scenario, exergy storage ranges from \(\approx\)1.37 TWh (500-1300 m depth range) to 1.53 TWh (500-1500 m) for the UK as a whole (Figure 3c,d, S2, Table S3). Figure 3e,f shows the influence of the less explored and characterised Wessex Basin halite beds on storage estimates, being significantly greater than other basins, suggesting perhaps 239-367 TWh of storage available and far greater than in the Cheshire Basin (8-9.3 TWh) or eastern England (2.6-39.7 TWh). Figure 3g,h show the effects on'static' storage of applying a filter based upon the number of operational or planned UGS caverns in a basin, with the Wessex Basin storage reducing markedly to 0.24-0.37 TWh, similar to the Cheshire Basin (0.32 TWh) and eastern England (0.1-0.4 TWh). To refine the estimates, the data were filtered to extract those caverns with heights of 100 m and greater and those caverns of 100-150 m in height, as described above. For caverns of 100 m and greater (Figure 4a,b, S2, Table S4), CHT'static' exergy to work storage estimates for the basins range from between 6.6 (500-1300m) and 7.7 (500-1500 m) TWh in Cheshire to \(\approx\)210 to 348 TWh in the Wessex Basin, the latter skewing the estimates. Cycled once a month, this could generate between \(\approx\)79 and 95 TWh of storage in Cheshire and 12-300 TWh in eastern England. For caverns of 100-150 m in height, the results range from 2.6 and 3.8 TWh in the Cheshire Basin, to between 18.3 and 45.5 TWh in the Wessex Basin, the latter again being highest, although estimates appear more realistic than simply taking caverns of 100 m and greater, which takes much of the thick DHM interval as available. For reasons discussed above, cavern construction may not be feasible over much of the upper DHM across the basin. Cycled once a month, this could generate between \(\approx\)79 and 95 TWh of storage in Cheshire and 12-300 TWh in eastern England.