*Article* **Analysis of a Wind-Driven Air Compression System Utilising Underwater Compressed Air Energy Storage**

**Lawrie Swinfen-Styles \*, Seamus D. Garvey, Donald Giddings, Bruno Cárdenas and James P. Rouse**

Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, Nottingham NG7 2RD, UK; seamus.garvey@nottingham.ac.uk (S.D.G.); donald.giddings@nottingham.ac.uk (D.G.); bruno.cardenas@nottingham.ac.uk (B.C.); james.rouse@nottingham.ac.uk (J.P.R.)

**\*** Correspondence: lawrie.swinfen-styles@nottingham.ac.uk

**Abstract:** The increasing push for renewable penetration into electricity grids will inevitably lead to an increased requirement for grid-scale energy storage at multiple time scales. It will, necessarily, lead to a higher proportion of the total energy consumed having been passed through storage. Offshore wind is a key technology for renewable penetration, and the co-location of energy storage with this wind power provides significant benefits. A novel generation-integrated energy storage system is described here in the form of a wind-driven air compressor feeding underwater compressed air energy storage. A direct drive compressor would require very high intake swept volumes. To overcome this difficulty, some prior compression is introduced. This paper discusses the constituent technologies for this concept, as well as the various configurations that it might take and the logic behind operating it. Special consideration has been given to the differences resulting from utilising a near-isothermal wind-driven compressor versus a near-adiabatic one. Multiple iterations of the system have been simulated. This has been done using a price-matching algorithm to optimise the system operation and using volumetric air flow rates to calculate exergy flow. Simulated operation has been performed for a year of real wind and electricity price data. This work has been performed in order to clarify the relationships between several key parameters in the system, including pressure and work ratios, volumetric flowrates, storage costs and profit rates. An additional objective of this paper was to determine whether the system has the potential for economic viability in some future energy grid, especially when compared with alternative wind and energy storage solutions. The results of the simulation indicated that, with proper sizing, the system might perform competitively with these alternatives. Maximum one-year return on investment values of 9.8% for the isothermal case and 13% for the adiabatic case were found. These maxima were reached with ~15–20 h of output storage. In all cases, it was found that maximising the power of the wind-driven compressor compared with the initial compressor was favourable.

**Keywords:** generation integrated energy storage; wind-integrated energy storage; compressed air energy storage; underwater compressed air energy storage; wind-driven air compression; intake swept volumes; capture value; alternative wind technology; wind system simulation; optimal operation of energy stores
