## 3 Current Status
### Adiabatic UWCAES
Furthermore, they investigated the feasibility of balancing the entire seasonal demand of UK domestic heating with subseabed gas field hydrogen storage. It was found that only a few offshore gas fields were required and hydrogen storage would not compete for the subsurface space required for carbon storage or CAES [56]. [PERSON] et al. determined that a value of 6900 TWh of available hydrogen storage capacity was present in gas fields and 2200 TWh in saline aquifers on the UK continental shelf [57]. [PERSON] et al. analyzed the prospects and scientific challenges in subseabed hydrogen geological storage and concluded that there was great potential to achieve net-zero by 2050 with subseabed hydrogen geological storage [58]. [PERSON] et al. from University College Cork also integrated subseabed hydrogen geological storage with offshore wind farms [59]. [PERSON] et al. from Kiel University and Flensburg University of Applied Sciences assessed subsurface renewable energy storage capacity for hydrogen, methane, and compressed air in the North German Basin [20]. [PERSON] et al. from the University of Virginia investigated the techno-economic performance of UWCAES in subseabed saline aquifers for balancing offshore wind power [11]. The result showed that the levelized cost of electricity of a 350 MW UWCAES system with 168 h of storage could be 81% less than that with 10 h lithium-ion battery energy storage [11]. Industrial progress in this space is trailing far behind academic studies. Too many enterprises begin well but fall off towards the close. Hydrostor's world's first UWCAES demonstration is the only existing commercial UWCAES facility. The critical issue is that the onshore section works well but the underwater section remains problematic. The marine components of UWCAES are still the greatest challenge to large margin returns on investment. Thus planned UWCAES projects in Lake Huron and Aruba were terminated. Instead, several onshore CAES projects based on adiabatic compression/expansion and thermal energy storage have been built and contracted in recent years [60]. Another UWCAES project in Hawaii with 12 MW (56 MWh) capacity was announced by Brayton Energy several years ago [61]. However, no detailed engineering progress has been revealed in recent years In addition, SEGULA Technologies is developing a UWCAES concept named \"REMORA\" [62]. They are now focusing on isothermal compression/expansion while not on underwater systems. TechnipFMC is leading an underwater hydrogen energy storage project named \"Deep Purple\" [14]. Green hydrogen is produced with offshore wind power and subsequently stored in artificial pressure vessels on the seabed. In 2021, Tractebel and partner companies developed an offshore infrastructure and processing facilities concept for storing hydrogen at large scale in the subseabed caverns [13]. It is understandable that the UWCAES is trailing behind the onshore CAES, not to mention underwater natural gas, hydrogen, and CO\({}_{2}\) energy storage. That said, flourishing offshore renewable energies are pushing UWCGES forward and there is a trend of resurgence. Before large-scale applications of UWCGES will proliferate, many challenges must be addressed.