## 2 UWCAES  
UWCCES derives from UWCAES. Thus, in this section, the working principle and milestones of UWCAES are briefly introduced. In general, there are two technical routes for achieving UWCAES, as shown in Figure 1. One is UWCAES with adiabatic compression and expansion and another is UWCAES with isothermal compression and expansion. This is like onshore CAES. In an adiabatic UWCAES system, no heat is exchanged with the surroundings. The thermal energy of hot compressed air is stored in the thermal energy storage unit. When needed, the storage compressed air is released and the stored thermal energy retrieved. The hot compressed air then expands adiabatically and drives the expansion train to generate electricity. In an isothermal UWCAES system, the compressor is cooled and the compressed air is discharged at a low temperature. Similarly, the expander is heated and the compressed air expands isothermally. It is worth noting that the water body of the ocean/lake is an ideal heat sink/source which could facilitate isothermal compression and expansion. This advantage should be fully exploited in UWCAES systems. This is the reason why many studies on UWCAES are focusing on implementing isothermal compression. Generally, the compressed air can be stored in either human-make accumulators or subseabed caverns/saline aquifers. UWCAES with subseabed caverns/saline aquifers/depleted oil and gas fields are similar to traditional onshore underground CAES. The pressure of compressed air cycles over relatively large pressure ranges in the charging and discharging processes. An important advantage of subseabed storage is a higher storage pressure could be achieved due to the additional hydrostatic pressure of deep water. In addition, the investigation cost could be significantly reduced if depleted offshore gas/oil reservoirs could be reused. The storage volume of artificial accumulators is much less than that of subseabed caverns/saline aquifers/depleted oil and gas fields. Nevertheless, the storage pressure of artificial accumulators can maintain nearly constant levels based on the hydrostatic pressure associated with that depth. Artificial accumulators can be divided into flexible, rigid, and hybrid variants. The flexible accumulator is generally made from polymer composite materials and the shape of the accumulator changes with the changing storage volume of compressed air. The rigid accumulator is generally a steel-reinforced concrete structure. The hybrid accumulator combines the advantages of flexible and rigid ones but is more complex in structure. More details are discussed in the following sections. Figure 2 shows the timeline of the major academic and industrial milestones of UWCAES. As a subbranch of CAES, UWCAES is not a new idea. To our best knowledge, early in 1987, [PERSON] and [PERSON] proposed and improved the UWCAES concept for storing off-peak  
Figure 1: Schematic diagrams of UWCAES: (**a**) UWCAES with adiabatic compression and expansion; (**b**) UWCAES with isothermal compression and expansion. wind electricity [23; 24]. In the first UWCAES concept, human-make accumulators made from flexible material were used for storing compressed air. Many follow-up concepts are very similar to [PERSON] and [PERSON]'s concept. In 1997, [PERSON] at UCSD (University of California, San Diego) proposed the first simple rigid accumulator concept which could be a long pipe or a compact tank with ballast bins [25; 26]. In 2011, a team from the University of Windsor and Hydrostor tested a tiny-scale UWCAES pilot project in Lake Ontario that showed the concept was feasible and promising. The compressed air storage accumulator was a commercial lift bag that is widely used in ocean engineering [27]. In 2012, a team from the University of Nottingham tested their prototype 5 m diameter energy bag in 25 m of seawater at the European Marine Energy Centre off the coast of Orkney [28].