## 4 Materials, Exergy Storage Tool, and Methodology
### Exergy Storage Assessment--Methodology  
Figure 2 summarises the exergy storage assessment process. From borehole log data and map information in the public domain and held by the British Geological Survey, the tops, bases, and thicknesses of the halite deposits and major faults were mapped within each basin. These data were input to ArcGIS, which was used to obtain potential cavern locations, depths, and basic cavern parameters such as heights, diameters, available volumes, spacing and casing shoe depths based upon criteria applied to the design, development, and construction of gas storage caverns in the same strata [42]. The halite beds were evaluated over the depth ranges under consideration for CAES operations, with casing shoe depths (and thus pressures) in general between 500 and 1300 m [23] as well as up to 1500 m depth as at the proposed CAES plant at Larne, Northern Ireland [43]. Then, these basic cavern data were input to a modelling tool and used to estimate the exergy storage potential of prospective UK onshore and offshore East Irish Sea areas, using pressure and temperature ranges derived from gas storage investigations in these areas. Basic theoretical storage estimates are derived (Figure 3) that are only that and which, for various reasons, are clearly unrealistic totals. Most obviously, not all cavern locations will ultimately be available or suitable for cavern creation due to geological constraints, salt quality across the basin, together with economically and operationally viable cavern sizes. Therefore, a series of filters, based upon likely cavern height ranges, sizes, and differing storage operations, have been applied to derive more realistic CHT storage estimates for each basin. These can be compared to the annual UK electricity demand of 300 TWh [44]. In an attempt to obtain realistic assessments of the potential provided by the bedded halite resource, the total exergy storage estimates from each basin were filtered in a variety of ways:  
* Taking 1% of the estimated exergy storage for the 'available' UK caverns
* Calculating the cavern storage estimates based on a percentage reflecting the number of operational or permitted UGS caverns in the UK (148) relative to the total number of possible caverns
* Filtering the caverns to include only those of greater than 100 m height
* Filtering the caverns to include only those of 100-150 m in height
* For the two filtered cavern height datasets, applying filters taking 1% of caverns and a percentage of the storage, using UGS cavern numbers relative to the total number of possible caverns in individual basins. The figure of 1% is not based on industrial experience or previous studies. However, it provides a first pass understanding of the potential cavern numbers and storage capacities over the differing depth ranges, against which estimates set against the constructed or planned UGS cavern numbers in the two most developed basins can be evaluated (S2, Table S1): Cheshire Basin (73 caverns = 3.5-4%, for 500-1300 and 500-1500 m depths, respectively), Eastern England (37 caverns = 2-4%, respectively). Therefore, the figure of 1% is lower than these percentages and thus appears a reasonable gauge against which exergy storage estimates might be assessed initially (Figures 3 and 4). However, Figure 3 reveals very high storage estimates for the Wessex Basin, which is a potentially large region, but one in which the halite beds are less well characterised; halite beds were unknown in the area until oil and gas exploration began in the 1970s [37]. Consequently, further refinement of the estimates was attempted, reflecting more the degree of exploration and the proven potential and capabilities of the halite beds in each of the main halite basins. The greater numbers of storage caverns in the Cheshire Basin (73) and Eastern England (37) mean that these basins represent the most mature areas in terms of exploration and development of the halite beds.