Hybrid Wind-Tidal System Holds Potential to Guarantee Continuous Availability of Grid Power

Offshore wind power is subject to short-term fluctuations, limiting the potential for this technology to serve as a source of continuously available grid power. Scientists at the Graduate School of Energy Science in Kyoto have suggested a hybrid system that combines an offshore wind turbine with a corresponding tidal turbine to make offshore power available to the grid at a constant level (Rahman et al.2011). In the proposed system, tidal power is used to balance the variations in the load of offshore wind power by operating a flywheel motor/generator system. When wind power exceeds a specified level, the tidal system functions as a motor to store surplus power as rotational energy. When wind power falls below a certain level, the tidal system works as a generator to complement the wind power and counter large fluctuations in wind power that can affect the frequency and voltage of output. The hybrid system could enable the development and utilization of offshore renewable energy sources by proposing new load fluctuation control strategies. A laboratory performance analysis favorably evaluated the feasibility of this system. —Meredith Reisfield
Rahman, M., Shunsuke, O., Shirai, Y., 2011. Hybrid offshore wind and tidal turbine power system to compensate for fluctuation (HOTCF). Green Energy and Technology. doi: 10.1007/978-4-431-53910-0_24.

The proposed system utilizes two types of power generation, the tidal motor/generator and the offshore wind turbine generator. While the tidal generator creates smooth output power, the output power of a wind turbine is directly dependent on wind velocity. Rahman and his colleagues built a laboratory scale prototype model of the hybrid system.
The offshore wind turbine generator component of this hybrid system consists of a coreless synchronous generator and a servo-motor. Servo-motors can be combined with encoders to provide an information feedback about position and speed and continuously correct performance. The servo-motor, controlled by a computer, simulated the rotation of an offshore wind turbine. The rpm (rotations per minute) of the motor determined the generation of electrical energy. A 6-pulse diode rectifier converts the AC power generated by the wind turbine to DC power. The tidal turbine component of the systems seeks to apply and control a two way energy flow scheme, so that energy can either be injected into the offshore wind turbine or stored as kinetic energy as wind power fluctuations demand. The tidal component combines a servo-motor with a generator/motor. The servo-motor serves as an input of tidal energy to the generator, which converts the mechanical energy from tides into electrical energy. The tidal turbine induction output is connected to a DC capacitator through a dual way converter. The researchers also placed several small controllers at both ends of the system to monitor operating conditions.
Tests of the system found that the tidal system turned to generator mode was successfully able to compensate for variations in wind generator output. Conversely, the tidal system could store rotational energy as a flywheel with small losses. The main challenge facing this model is to reduce the delay in recovering grid power to initial value after a drop in wind power generation, which suggests the control flexibility and overall stability of the system can be improved. This framework can produce a relatively stable power output when connected to a commercial grid, avoiding the inherent power fluctuations of traditional offshore wind technologies. The hybrid design makes the system stable, adaptable and easily scalable. Wind and tidal resources can complement each other to general large amounts of power in an economically feasible manner. Successful evaluation of load demands and resource forecasting could make the hybrid system method a successful technique for converting tidal energy and wind energy into electricity. 

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