2D design and characteristic analysis of an underwater airbag with mooring for underwater compressed air energy storage

Natural shapes are commonly used for balloons and can also be applied in flexible gas containers for underwater compressed air energy storage (UCAES). However, additional consideration of the mooring is required in this application, and the classic natural-shape design carries the structural risk of a significant mooring force acting at a single bottom point. To address these problems, an underwater airbag with mooring (UAM) is proposed considering the practical fixation and stress distribution. This study employs a 2D axisymmetric design method in a dimensionless form based on the natural-shape method. The weightless optimal UAM shape in the zero-pressure state is obtained when the dimensionless mooring length is 0.62. Compared with the classic design in the zero-pressure state, the optimal design decreases the structural height and significantly decreases the mooring tension. Regarding the deflated shape, the results show that the optimal design has a smoother pressure variation and smaller pressure range, which can reduce energy loss and improve the system efficiency. Moreover, a tank experiment was conducted, and the results show good agreement between the prediction and experimental results. The UAM design is closer to practical applications than the classical natural-shape design and can effectively provide preliminary design references for engineering.