## 2 Mechanical, Bulk Electrical Energy Storage (EES), and the Potential of CAES  
PHS is the most mature, proven bulk energy storage technology, whereby energy is stored in the form of the gravitational potential energy of water pumped from a lower to a higher elevation reservoir. Pumps are typically run by low-cost surplus, off-peak electrical power, and during periods of high/peak electrical demand, release of the stored water through turbines generates electric power. Used by electric power systems for load balancing, it reliably provides a large-scale and fast-responding storage option, with a current worldwide grid-connected capacity of \(\approx\)188 GW and representing \(\approx\)96% of the total global energy storage capability [18]. Significant potential for hydro-storage capacity may still exist in many other areas around the world [19]; however, ultimate development and capacity for PHS in most developed countries, including the UK, is considered limited and constrained by social, environmental, availability, and geographical considerations [5; 20; 21]. CAES, with a modest surface footprint and greater siting flexibility relative to PHS, represents a low-cost technology that is capable of a power output of over 100 MW. CAES is based on large quantities of electrical energy stored as high-pressure, compressed air in an underground storage'reservoir' (currently salt caverns). During peak demand, air is withdrawn and used in the generation of electricity, and as with PHS, the release of power can be very quick. Worldwide, CAES capacity is currently around 431 MW [18], and CAES is viewed increasingly as offering bulk storage potential and a solution to levelling intermittent renewables generation (wind-power and solar photovoltaic [PV] plants),  
Figure 1: General outcrop map of the main halite basins studied onshore England and offshore East Irish Sea. Note area indicated in the East Irish Sea is that of the Triassic Preesall Halite at depths investigated (500–1500 m). Refer also S2, Table S1 for details on UGS facilities. and capable of maintaining system balance (S1, Tables S1-S6) [3; 5]. CAES technology has advantages over PHS, including a lower visible impact on the landscape and a greater scope for building CAES facilities nearer the centres of wind-power production, especially in parts of Europe and regions of the USA. CAES facilities in salt caerhigh injection/withdrawal rates required for rapid cycle storages. They also offer ideal conditions for compressed air storages because unlike porous reservoirs, the rock salt is inert to oxygen [4]. Thus solution-mined salt caverns are a likely first choice for CAES in the UK, and for CAES proposals linked with renewables, they are the overwhelming majority (S1, Table S1). Many regions of the world lack suitable salt deposits, and so, the suitability of porous rock storage has long been and remains under investigation [12; 13; 14]. However, serious doubts exist over the likely development of porous rock storage (principally aquifers), with no CAES plants having operated commercially and only a few small test facilities having been constructed, with variable results (S1, Tables S2 and S3). The King Island project in California demonstrated the technical feasibility of using an abandoned natural gas reservoir for a 300 MW, 10 h CAES facility, with the reservoir capable of accommodating the flow rates and pressures necessary for the operation of the facility. Originally planned for opening around 2020, its progress appears stalled due to the high cost of a CAES facility relative to alternative energy storage technologies [27]. All test facilities encountered problems with one of more of the following: wells and economics, pressure anomalies, variations in reservoir quality and performance, formation of the 'air bubble' in the storage reservoir, and reaction between the oxygen of the injected air and minerals in the reservoir rock leading to oxygen depletion and/or potential for bacterial/micro-organism growth and porosity reduction.