**3. CAES-Geological Storage Options, Developments, and Restrictions**

Bulk geological storage options and the technologies behind current and future electrical energy storages for compressed air are derived largely from tried and tested storage technologies developed for the underground storage of large volumes of high-pressure natural gas [4]. Most common geological options are porous rock formations (depleted gas fields and aquifers), or man-made (solution-mined) salt caverns. Where such options are not available conventionally mined, non-salt rock caverns and lined rock caverns represent alternatives, but they are significantly more expensive. These same options apply to potential CAES development (S1 and S1, Tables S1–S6).

As alluded to above, CAES has been considered for many decades [7,8] but to date, only three commercially operational CAES plants exist, at Huntorf [9], McIntosh [10], and most recently at Goderich [11]. Between 2012 and 2016, a small 2 MW isothermal CAES demonstration plant using a reconditioned former liquid hydrocarbon storage salt cavern and linked to wind generation, operated at Gaines, Texas, although it is not believed to be currently operating [25,26]. Salt caverns provide important high flexibility with respect to turnover frequency, as the open cavity enables very high flow rates permitting

high 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 [12–14]. 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. Proposed aquifer storage potential for the UK would be offshore [15], thereby increasing costs, which currently thus seems less likely than salt cavern storage.

Depleted field storages appear even more unlikely with a potential hazard posed by residual hydrocarbons in the depleted gas formation. Introducing compressed air presents the risk of ignition and explosion, both underground and during discharge [28].

Additionally, and although more expensive options, gas storages have and still operate in abandoned mines and unlined or lined, conventionally mined rock storages. Similar constructions could host CAES in regions lacking cheaper geological alternatives [7,8] and have been considered (S1, Tables S4–S6). Various CAES test facilities have operated briefly in Japan and Korea, and long-standing plans for CAES in a former limestone mine at Norton, Ohio were finally shelved in 2013 [29]. Small tests for adiabatic CAES are currently ongoing in an unlined Swiss tunnel [30] and a lined old mine working in Austria [31]. Whilst under consideration in, for example, USA, Mongolia, and Australia, such storages may be considered unlikely in the UK.

Non-geological CAES schemes offering storages of small volume. Though not considered here they include aboveground, or shallowly buried steel vessels or pipes [32,33], energy bags secured to the seabed [34], wind turbines linked with energy storage in supporting legs [35], or those in which power is converted directly from the rotor by means of gas/air compression within the rotor blades [36].
