## 5 Future Perspectives  
With the accelerating exploitation of marine resources, marine activities continue to extend to the deep sea. An increasing density of marine infrastructure has been deployed, and more and more energy is consumed. At present time, almost all marine activities rely on long-distance electrical power transmission by submarine cables and power supply from self-contained fuels/batteries. Submarine cables transmit electric energy from onshore grids to the seab and provide continuous energy supply to underwater equipment in dendritic and radial structures. The high-voltage direct current transmission is the primary underwater cable power transmission mode. However, expensive submarine cable power transmission is not practical for distributed offshore energy demands and far deep-sea demands [109]. In addition, many studies have found that the electromagnetic fields generated by submarine cables have obvious impacts on marine invertebrates and affect the marine environment temporarily or permanently [110; 111]. An alternative to submarine cables is a traditional self-contained energy supply, such as batteries or fuels. Energy storage with a battery has been widely used by most underwater devices, especially those working in the far deep sea [112]. However, limited by the capacity of a battery, it is more suitable for small devices or devices requiring a short-time power supply. The energy demands of many large underwater infrastructures cannot be met with batteries. Therefore, there is a clear present need for an energy supply mode with acceptable economy and environmental protection. Marine renewable energy with energy storage may be such a solution. The advantages of marine renewable energy power supply are obvious for far deep-sea demands. It follows then that there is a corresponding demand for marine energy storage technology. The harsh marine environment demands more stringent requirements for offshore energy storage than onshore storage. The UWCGES technology discussed in this study is an alternative that can be integrated to serve a variety of marine application scenarios. In the short term, UWCGES technology can serve the application scenarios shown in Figure 6a. Air, natural gas, and hydrogen compressed in gas stations with renewable energy can be stored in underwater gas storage accumulators through underwater gas transportation pipelines. When needed, the compressed gas stored in the underwater accumulators can be fed back to the energy system. With the continuous development and the breakthrough of new technologies in various fields of ocean engineering, UWCGES can serve more application scenarios as shown in Figure 6b in the medium and long term. In the mode of marine renewable energy with energy storage, it can provide energy storage services for marine platforms, large or super large floating bodies, and islands at sea. It can also serve as a marine energy station to provide long-term uninterrupted energy supply for seabed mining, deep-sea fisheries, seabed space stations, subsea observation networks, underwater vehicles, underwater data centers, underwater Internet of Things, etc. [113; 114; 115; 116; 117; 118; 119]. Hydrogen energy is a focal candidate for future low-carbon and renewable energy infrastructure. Offshore renewable energies can be used for producing green hydrogen [120]. Underwater storage underpins this chain and can be treated as a complement to traditional storage and transportation modes. Figure 7 shows an integrated application mode of UWCGES in the future marine clean energy system. Marine renewable energy power can not only be directly supplied to the power grid but also be used to support seawater desalination and green hydrogen production. The freshwater produced in the seawater desalination system can be used for electrolytic hydrogen production. The produced green hydrogen can be compressed by efficient isothermal compression and stored in large underwater gas storage accumulators.