Abstract: The invention provides an underwater energy storage system (UW-ES system) comprising a reservoir structure (5), which is resting at the bottom (4) of a waterbody (1), such as a sea, an ocean, a waterway, etc. The reservoir structure has a pressurizing reservoir (7A, 7B) with a deformable wall structure (17A, 17B) and a depressurizing reservoir (8A, 8B) with a rigid wall structure (6, 14, 18A, 18B), which are holding a working liquid (9) separated from the water (2) of the water body. Energy is stored and retrieved by displacing part of the working liquid from the depressurizing reservoir into the pressurizing reservoir, and vice versa, respectively. Therein use is made of the hydrostatic pressure of the water (2) acting on the deformable wall structure of the pressurizing reservoir. The UW-ES system is compact and economical, and allows for highly synergistical integration with various power take-off systems.

Abstract: The invention provides an underwater energy storage system (UW-ES system) comprising a reservoir structure (5), which is resting at the bottom (4) of a waterbody (1), such as a sea, an ocean, a waterway, etc. The reservoir structure has a pressurizing reservoir (7A, 7B) with a deformable wall structure (17A, 17B) and a depressurizing reservoir (8A, 8B) with a rigid wall structure (6, 14, 18A, 18B), which are holding a working liquid (9) separated from the water (2) of the water body. Energy is stored and retrieved by displacing part of the working liquid from the depressurizing reservoir into the pressurizing reservoir, and vice versa, respectively. Therein use is made of the hydrostatic pressure of the water (2) acting on the deformable wall structure of the pressurizing reservoir. The UW-ES system is compact and economical, and allows for highly synergistical integration with various power take-off systems.