**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 ≈188 GW and representing ≈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 During peak demand, air is withdrawn and used in the generation of electricity, and as with PHS, the release of power can be very quick.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),

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 caverns already successfully provide minutes to hours reserve at Huntorf (Germany) and balancing out grid loads over a weekly cycle at McIntosh in the USA [4,9]. However, significant barriers to implementing large-scale CAES plants lie in identifying appropriate geological storage options and thus geographical locations, low round-trip efficiencies of CAES and the low volumetric energy density of compressed air (2.4 kWh/m<sup>3</sup> ) [6,22,23].

Energy in compressed air caverns is stored in the form of physical (mechanical) potential energy, whereas energy in compressed gases is chemical storage (chemical energy bonds). Consequently, the volumetric energy density of air is several orders of magnitude lower than that of gases such as hydrogen (≈170 kWh/m<sup>3</sup> ) or natural gas (≈1100 kWh/m<sup>3</sup> ) [4]. Accordingly, to make CAES economically viable requires very large volumes of air, which can only be achieved through high pressures and large volume storages. Geological storages at depth offer such storage conditions, with typical gas storage salt caverns, in particular, offering rapid cycling and high flow rates to provide ideal storage options. However, the lower volumetric energy density of air means that CAES plants are less suitable for long-term applications and storage because greater storage volume (increased cavern numbers) is required, increasing costs compared to gases with higher value.

Whilst geometrical volumes of compressed air caverns are comparable to those of conventional natural gas storage caverns, CAES operational pressure ranges (and thus storage volumes) will be considerably lower than for gas storage. This is because of the much higher cyclic pressure frequency rate together the with current technological development of compressors, heat storages, and turbines, meaning the operational pressures are also lower, being well below 100 bar [4]. Thus, commercial, central, grid-scale CAES plants will require deep underground (geological) storages such as those already used for natural gas, hydrogen, and the rare examples of already operational CAES plants.

Conventional (diabatic) CAES technology is based upon traditional gas-turbine plants requiring fossil fuel combustion and thus associated emissions during electricity generation, making it less attractive when compared with other EES technologies [24]. Nevertheless, the fitting of recuperators and advances in CAES technologies, particularly if advanced adiabatic or isothermal CAES technologies requiring no external source of energy to heat the withdrawn air eventually prove feasible, together with linking to renewables generation (including offshore wind), all offer the future prospect of improved cycle efficiencies, with the reduction and possibly elimination of emissions.
