## 5 UK Salt Cavern Exergy Storage Capacity Estimates--Results and Commentary
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 \(\approx\)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 \(\approx\)139-156 TWh (Figures 5a and 6a) and 63-80 TWh (Figures 7a and 8a) stored exergy to work respectively, meeting between \(\approx\)46-52% and 21-27% of the UK electricity demand respectively.