Study on the application of energy storage system in offshore wind turbine with hydraulic transmission

• Hydraulic offshore wind turbine is capable of outputting near constant power.
• Open loop hydraulic transmission uses seawater as the working fluid.
• Linear control strategy distributes total flow according to demand and supply.
• Constant pressure hydraulic accumulator stores/releases the surplus energy.
• Simulations show the dynamic performance of the hybrid system.

A novel offshore wind turbine comprising fluid power transmission and energy storage system is proposed. In this wind turbine, the conventional mechanical transmission is replaced by an open-loop hydraulic system, in which seawater is sucked through a variable displacement pump in nacelle connected directly with the rotor and utilized to drive a Pelton turbine installed on the floating platform. Aiming to smooth and stabilize the output power, an energy storage system with the capability of flexible charging and discharging is applied. The related mathematical model is developed, which contains some sub-models that are categorized as the wind turbine rotor, hydraulic pump, transmission pipeline, proportional valve, accumulator and hydraulic turbine. A linear control strategy is adopted to distribute the flow out of the proportional valve through comparing the demand power with captured wind energy by hydraulic pump. Ultimately, two time domain simulations demonstrate the operation of the hybrid system when the hydraulic accumulator is utilized and show how this system can be used for load leveling and stabilizing the output power.