**5.5 Ocean energy**

Oceans cover about 70% of earth's surface area, making them the globe's biggest solar batteries. The installed capacity of ocean energy in 2019 was 530 MW [29]. Sunlight warms the surface area of water by a lot compared to the deep ocean water, and also this temperature-level difference develops thermal power. Simply, a tiny low part of the heat trapped within the ocean might power the planet [30]. Underwater compressed air energy storage (UWCAES) and underwater pumped hydro storage (UWPHS) are the second type of ocean energy storage available at the moment [31]. Air is contained in foldable bags on the seafloor in the UWCAES system. This underwater storage system has a great advantage over its ground equivalent. In standard CAES, air is contained in a tank of a set capacity, while the compressed air releases the pressure within the pipe, which reduces the flow to the turbine. UWCAES systems operate well at depths of 400–700 meters below sea level; this water depth provides the pressure required for most turbine compressors where compressed air energy storage is usually used. Seawater is used as a working solvent instead of air in the UWPHS system. This system uses solid steel or concrete spheres. To 'release' as an energy storage unit, the mechanism allows high-pressure seawater to penetrate the sphere through an opening by means of a turbine attached to the generator. Such energy storage systems are best used in peak-saving grid applications. Round-trip efficiencies for UWCAES and UWPHS are in the range of 70–85%. Concerns on how to create the most efficient energy storage of seawater are still on the way and raise the most important question-how to install them and how to do it cost-effectively [31]?
