## 3 Current Status
### Adiabatic UWCAES  
The majority of studies have gone in different directions based on more mature adiabatic CAES (A-CAES) with thermal energy storage. Since 2019, several onshore commercial A-CAES systems have been successfully operated worldwide, such as Goderich A-CAES facility (2.2 MW, 10 MWh) [39], Jintan A-CAES facility (60 MW, 300 MWh) [40], Zhangjiakou A-CAES facility (100 MW, 400 MWh) [41], etc. Thus, for now, the pathway of A-CAES is more feasible than the isothermal CAES pathway. Based on the world's first grid-connected UWCAES facility, [PERSON] et al. from the University of Windsor and Hydorstor revealed that the real round-trip exergy efficiency could reach about 53%. About 75-82% of the exergy destruction was avoidable, thereby showing significant potential for improvement [42; 43]. [PERSON] et al. from Dalian Maritime University designed a hybrid energy system for the island that integrates marine renewable energy with UWCAES, BES and diesel generation. It was found that an efficiency of 59% was achievable in terms of UWCAES subsystem [44]. [PERSON] and [PERSON] from the University of Salerno investigated the feasibility of carbon-free renewable energy feeding in Sicily by introducing UWCAES [45]. [PERSON] et al. from Polytechnic University of Milan conducted a preliminary design and performance assessment of UWCAES considering the off-design properties of the overall system and realistic power input. It was found that a round-trip efficiency in the range of 75-85% could be achieved [46; 47]. [PERSON] et al. from Xi'an Jiaotong University designed an autonomous renewable seawater reverse osmosis system by introducing underwater compressed air energy storage and investigated the feasibility from perspectives of technology and economy [48; 49]. They also proposed underwater compressed CO\({}_{2}\) energy storage by replacing air with CO\({}_{2}\)[22]. [PERSON] et al. from Qingdao University of Science and Technology and Xi'an Jiaotong University proposed a trigeneration system with UWCAES. It was found that an overall exergy efficiency of about 56% could be obtained [50]. Cheaterfrom GustoMSC proposed the ECO concept with UWCAES [51]. The results showed that it was economically competitive with PHS when the compressed air was stored in ultra-deep water [51]. The underwater system is the distinction between UWCAES and onshore CAES. The underwater system can be divided into the gas storage unit and the gas transportation unit. Researchers from the University of Windsor and Dalian Maritime University are still collaborating on evolving UWCAES. [PERSON] et al. investigated the numerical and experimental properties of flow around a balloon-shaped flexible accumulator [52]. Moreover, they proposed a general accumulator concept that could be used for storing fluids less dense than seawater. The general accumulator combined the advantages of traditional flexible and rigid ones. A large-eddy simulation and modal analysis of a 1000 m\({}^{3}\) model revealed that the risk of vortex-induced vibration fatigue damage was very low [21]. [PERSON] et al. designed flexible risers for gas transportation in UWCAES systems. The catenary riser and lazy wave riser were compared under different environments and internal pressure levels [53]. [PERSON] et al. established a theoretical model for describing slugging flow in a hilly-terrain tube. This was a step toward accurately predicting the status of liquid accumulation in gas transportation pipelines of UWCAES [54]. After the investigation on flexible energy bags in 2012 [28], [PERSON] et al. from the University of Nottingham stopped updating their progress along this line. Subsea geological storage of compressed air and hydrogen has emerged as an advanced variant of CAES in just the last 4 years. Researchers from the University of Edinburgh conducted several pioneering studies in this field. [PERSON] et al. revealed that UWCAES in porous rocks of sedimentary basins could completely satisfy the seasonal energy storage demand of the United Kingdom with acceptable economy [55].