## 4 Materials, Exergy Storage Tool, and Methodology
### Exergy Storage Tool  
The exergy storage system is represented by a thermal modelling tool developed during the EPSRC-funded IMAGES project [38] and augmented during this study (S3) to calculate stored exergy for individual caverns of known depths and size/volume, in two operational modes: constant volume, variable pressure (isochoric), and constant pressure, variable volume (isobaric) modes. The tool, equations for which were validated using operational data from the Huntorf CAES facility [38], considers three wall conditions to approximate and model the unsteady heat transfer (flux) between the injected air and cavern walls and models. Two cavern wall conditions represent idealistic and somewhat unrealistic, end-member models:  
* Adiabatic boundary conditions in which heat flux into the surrounding rock mass is zero
* Isothermal boundary conditions in which heat flux is infinite with perfect conduction into and through the surrounding rock massFigure 3: Plots of theoretical ‘static’ (one-fill) exergy storage estimates for the three thermal models for all potentially available caverns over the two depth ranges for all caverns with the basins studied. Parts (**a,b**) show graphs for combined totals from each basin for the two depth ranges, together with the estimated stored exergy to work for each thermal model. Parts (**c,d**) show graphs for the estimated stored exergy to work for each thermal model based upon percentages related to UGS numbers of the combined totals from each basin for the two depth ranges. Parts (**e,f**) show graphs breaking storage down by basin for the three thermal models, including stored exergy to work estimate for the CHT model also shown, with outlines data ranges being those pertinent to CHT model storage data presented in Figure 4. Parts (**g,h**) show graphs for estimates based upon a percentage related to the number of operation and/or planned UGS caverns in the basins. Figure 4: Plots of ‘static’ (one-fill) exergy storage estimates for the preferred CHT model, over the two depth ranges and cavern sizes (100 m+ and 100–150 m height) considered for CAES. Graphs for all potentially available caverns, 1% of available caverns and estimates based upon a percentage related to the number of UGS caverns in the basin. Parts (**a,c**) show data for the 500–1300 m depth range and parts (**b,d**) those data for the 500–1500 m depth range. Key common to all: blue = stored exergy, brown = stored exergy to work. Figure 5: Plots of dynamic energy storage and energy to work estimates for the preferred CHT model, over the depth range 500–1300 m and cavern heights 100 m + considered for CAES. Parts (**a**–c**) show graphs for differing injection/withdrawal rates (108/108 kg/s and 108/417 kg/s) or fill and pressure reduction rates (108 kg/s/1.5 MPa/h) for all potentially available caverns, 1% of available caverns and estimates based upon the number of UCS caverns in the basins. Additionally shown, by basin, the percentage of UK electricity demand for 92% of stored energy to work. Key common to all, see Figure 3.  
Figure 6: Plots of dynamic energy storage and energy to work estimates for the preferred CHT model over the depth range 500–1500 m and cavern heights 100 m+ considered for CAES. Parts (**a**-c) show graphs for differing injection/withdrawal rates (108/108 kg/s and 108/417 kg/s) or fill and pressure reduction rates (108 kg/\(\sim\)1.5 MPa/h) for all potentially available caverns, 1% of available caverns, and estimates based upon the number of UGS caverns in the basins. Additionally shown, by basin, the percentage of UK electricity demand for 92% of stored energy to work. Key common to all, see Figure 3.  
Figure 7. Plots of dynamic energy storage and energy to work estimates for the preferred CHT model, over the depth range 500-1300 m and cavern heights 100-150 m considered for CAES.