Energy storage system

A method of operating an energy storage system configured for installation at an underwater location, the method comprising:

• providing an energy storage system comprising a vessel to store water at a low pressure, an energy conversion device to selectively convert between potential energy and electric energy, and a connection line connecting the vessel with the energy conversion device, the energy conversion device comprising a housing, a pump turbine unit arranged in the housing, and a motor generator unit, the housing comprising a low pressure opening to receive water at the low pressure, and a high pressure opening to discharge water at a high pressure, the pump turbine unit comprising a first shaft to rotate about an axial direction, the motor generator unit comprising a second shaft to rotate about the axial direction, and the second shaft coupled to the first shaft to transmit a torque between the first shaft and the second shaft, and
• selectively operating the energy storage system in a pump mode or in a turbine mode, in the pump mode the pump turbine unit operated to discharge the water from the vessel through the high pressure opening, and in the turbine mode the water enters the housing through the high pressure opening, drives the rotation of the first shaft and is discharged through the low pressure opening to the vessel,
• the pump mode started by operating the pump turbine unit with a zero flow until the rotational speed of the first shaft is sufficient to generate a positive flow from the low pressure opening to the high pressure opening.