*4.1. Massively Bedded UK Halite Deposits Available*

Important massively bedded halite deposits are developed in the UK and have been associated with, or identified as potential hosts for, large solution-mined natural gas storage caverns (Figure 1; S2, Table S1). The halite deposits considered extensive and thick enough for cavern construction occur in four main basins (with ages) [37]:


• The Fordon Evaporite Formation, on- and extending offshore Eastern England (Upper Permian, Zechstein [Z2]).

**Figure 2.** Workflow for the estimation of exergy storage provided by solution-mined salt caverns in the main halite-bearing basins of the UK.

These deposits offer important alternative energy storage capacity, and this study has assessed their potential for large-scale exergy storage through CAES. Differing from energy that is always conserved, exergy which takes its basis from the second law of thermodynamics, measures the loss of energy quality in every energy transformation process. Exergy tends to be destroyed during any conversion or storage processes, and therefore, exergy storage capacity quantifies the maximum useful work of the stored air that could be used in subsequent power generation. Exergy analysis is employed in applications with electricity output and power generation processes, and an exergy analysis tool was developed to estimate the exergy losses in energy conversions associated with a salt cavern-based CAES system. This permitted an estimate of the exergy storage capacity of the compressed air stored in a salt cavern for generating electricity during the discharging period [38]. Compared to conventional static thermodynamic exergy analysis, our developed tool also considers time-dependent factors that affect the overall electrical efficiency of a CAES system, such as dynamic internal air responses in the cavern and the coupled thermal effects of surrounding rocks [S3].

The Triassic and Permian bedded halite deposits in Northern Ireland have not been included here, as they are poorly defined and largely identified for UGS purposes [37]. Equally, the available Preesall Halite in NW England has also been identified for UGS and is not included here [37,39]. The Zechstein halite beds extend offshore from eastern England into the Southern North Sea, where due to halokinesis, they may attain great thicknesses. For various reasons, they have not been included in this study: they occur often far offshore and show significant changes in thickness over short distances, with some salt structures rising to shallow depths, even approaching close to sea bed, and are often in association with existing producing gasfields [40]. However, they should not be ruled out as CAES hosts, perhaps linked to the growing number of offshore windfarms. If existing hydrocarbon infrastructure (platforms, pipeline and cable routes, etc.) could be re-purposed, development costs, which are high for proposed gas storage caverns (A. Stacey, pers comm.), might be reduced significantly.
