**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 ≈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 ≈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 ≈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 ≈210 to 348 TWh in the Wessex Basin, the latter skewing the estimates. Cycled once a month, this could generate between ≈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 ≈79 and 95 TWh of storage in Cheshire and 12–300 TWh in eastern England.

When the 'static' stored exergy to work estimates for each basin are assessed in relation to the numbers of operational or permitted gas storage cavern numbers in the basins over the two depth ranges (Figure 4c,d, S2, Table S4), then the potential exergy storage offered is highest in the Cheshire Basin (up to 0.30 TWh) and eastern England (0.25 to 0.32 TWh) areas, with much less estimated for the EIS (<0.1) and Wessex Basin (0.1 to 0.12 TWh). Cycled once a month this could generate between ≈3.6 TWh of storage in Cheshire and 3–3.84 TWh in eastern England.

Gas storage caverns are cycled more than once a year, and CAES caverns more than gas storage caverns, effectively increasing the 'static' working gas storage capacity and giving rise to a larger 'dynamic' working gas volume [39], or 'dynamic exergy storage', as considered here. At this stage, it is impossible to determine precise cavern depths, sizes, and temperatures and thus undertake detailed geomechanical and thermodynamic modelling for all potential cavern locations, volumes, storage pressures, operating scenarios, and cycle times. Therefore, we outline the processes behind an attempt to calculate the general 'dynamic exergy storage' potential for both the 100 m and greater, and the 100–150 m cavern sets. This was undertaken by estimating the number of cycles per year, which is based upon flow rates calculated from cavern fill and withdrawal rates in UGS and CAES operations and taking the average values for the outputs of maximum cavern pressure, exergy stored and air mass in caverns for CHT conditions from the exergy modelling tool (see S3 and S2, Table S5).

Thus, cavern emptying or withdrawal times were calculated for three scenarios based upon an injection phase (cavern charging), involving a conservative mass injection rate of 108 kg/s, as reported from the Huntorf CAES facility [9] and three differing withdrawal rates (generation or cavern discharge phase): 108 kg/s (equivalent to injection rate), 417 kg/s (from Huntorf [9]), and a general maximum pressure rate reduction of 15 bar/h (1.5 MPa/h) for gas storage operations [9,49]. For the latter, an approximate equivalent air mass withdrawal rate in kg/s was calculated to assess how realistic the rate might be for any particular scenario. For the higher flow rates, it may be that caverns would require more than one withdrawal well to achieve the air mass withdrawal rates. Then, the calculated injection and withdrawal rates were used to derive an estimate of the number of cycles per year and thus determine the 'dynamic' exergy capacity available for the different model categories (Figures 5–8, S2, Tables S6–S8).

Figures 5–8 illustrate the 'dynamic' exergy storage (and stored exergy to work) increases of the 100 m plus and 100–150 m cavern height subsets over the 'static' storage estimates presented in Figures 3 and 4. Dynamic exergy storage estimates based upon the Huntorf operational parameters (Figure 5a, Figure 6a, Figure 7a and Figure 8a, S2, Table S6) are lower than those using faster withdrawal rates, which increase the number of storage cycles possible (Figure 5b,c, Figure 6b,c, Figure 7b,c and Figure 8b,c, S2, Tables S7 and S8). The 'dynamic' exergy storage results illustrate more markedly the potentially significant contribution of exergy storage through CAES in salt caverns to the UK's energy storage capacity and electricity supply. This is highlighted by taking the Cheshire Basin as an example. Here, exergy storage estimates from the operational cycle based on 108 kg/s fill rates and 108 kg/s withdrawal rates suggest that caverns of 100 m and greater or 100–150 m in height over the two depth ranges have the potential to provide between ≈139–156 TWh (Figures 5a and 6a) and 63–80 TWh (Figures 7a and 8a) stored exergy to work respectively, meeting between ≈46–52% and 21–27% of the UK electricity demand respectively. For the operational cycle based on 108 kg/s fill rates and maximum withdrawal rates of 1.5 MPa/hr, stored exergy to work estimates range between ≈264–296 TWh (Figures 5c and 6c) and 119–151 TWh (Figures 7c and 8c) respectively, meeting between ≈88–98% and 40–50% of the UK electricity demand, respectively. Taking the estimates based on UGS cavern numbers for the two depth ranges and cavern sizes, stored exergy to work estimates could, using the most cycled operational mode (Figures 5c and 8c), provide between 13.3 and 13.7 TWh, respectively, meeting ≈4.5% of the UK electricity demand.

The other halite basins provide similarly important additional exergy storage and exergy to work support, with for example, just 1% of all caverns of 100 m and greater in the 500–1500 m depth range in the basins providing a further ≈34–65 TWh of work and meeting ≈12–22% of the UK electricity requirements, depending on the mode of operation and cycle numbers (Figure 6a–c, S2, Tables S6–S8). Whilst 1% of caverns 100–150 m

height in the same depth range might provide a further ≈9.53–18.1 TWh of work, meeting ≈3.2–6.0% of the UK electricity requirements (Figure 8a–c, S2, Tables S6–S8). Relative to UGS cavern numbers in the basins, the figures for 100 m plus caverns in the depth range 500–1500 m might provide an additional ≈12.1–23.2 TWh of work, meeting ≈4–7.7% of the UK electricity requirements (Figure 6a–c, S2, Tables S6–S8), whilst 100–150 m height caverns might provide ≈7.5–14.2 TWh of work, meeting ≈1.5–4.7% of UK electricity requirements (Figure 8a–c, S2, Tables S6–S8). These data illustrate the potential importance of CAES and salt cavern storage to UK electricity demand and supply.
