**7. Conclusions**

A study of the main halite-bearing strata of the UK onshore and East Irish Sea areas in which UGS caverns have been constructed or planned has been undertaken to assess their potential for the construction of salt caverns for CAES purposes and their exergy storage potential. Storage depths investigated are between 500 and 1500 m. Revisions to an earlier exergy modelling tool, equations for which were validated by operational data from the Huntorf CAES plant, have led to a series of exergy storage capacity estimates for three differing heat models. Both the 'static' one-fill exergy storage capacity and a series of 'dynamic' exergy storage capacities based on various fill and empty rates are derived. From a theoretical storage of over 300 TWh, more realistic storage estimates of many tens of TWh are achieved by way of filtering the estimates based on cavern dimensions and different storage cycles considering UGS projects and operational modes. Significant exergy storage capacity exists for CAES in salt caverns, which could provide important support to the UK electricity grid, requiring 300 TWh per year. As the contribution of intermittent renewables generation to the grid rises, it is suggested that salt cavern storages constructed onshore, rather than porous rock storages located offshore, are likely to be the main CAES storage technology available in the UK at least in the short term.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/app11114728/s1, S1, Tables S1–S6, S2, Tables S1–S8 and S3.

**Author Contributions:** Conceptualisation, D.E., J.B., J.W. (Jihong Wang) and S.G.; Formal analysis, D.E., D.P., M.D., P.W. and J.W. (John Williams); Funding acquisition, D.E., J.B., J.W. (Jihong Wang) and S.G.; Investigation, D.E., D.P., M.D., P.W. and J.W. (John Williams); Methodology, D.E., D.P., M.D., P.W., J.W. (John Williams) and W.H.; Project administration, D.E., J.W. (Jihong Wang) and S.G.; Software, D.P., M.D., P.W. and W.H.; Writing—original draft, D.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded from the UK Government's Engineering and Physical Science Research Council (EPSRC), UK grants EP/L014211/1 and EP/K002228/1.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The authors thank three anonymous referees for comments on the manuscript all of which improved the final version. Financial support from the Engineering and Physical Science Research Council (EPSRC) funded Grand Challenge [Energy Storage] IMAGES project (EP/K002228) is gratefully acknowledged. This paper is published with permission of the Executive Director of the British Geological Survey (NERC).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
