Evaluation and optimization of a hybrid wave energy converter using excited motion response in two degrees of freedom

As waves in China seas are not high, a wave energy converter consisting of a coaxial annular buoy and a cylindrical buoy that extracts wave energy using two generators through the relative heave motion between the buoys and the pitch motion of the cylinder could be a more efficient choice. A dynamic model considering constraints and assuming linear power take-off is established to evaluate the power performance of the device. The influences of two key factors, the diameter of the annular buoy and the power take-off stiffness of the pitching generator, and their couplings on the power performance are analyzed. The power of the pitching generator accounts for a major proportion of the total power. An increase in the annular buoy diameter increases the power of the heaving generator while greatly decreases the power of the pitching generator. An increase in the power take-off stiffness of the pitching generator greatly decreases its power while has little influence on the power of the heaving generator. These two factors also influence the peak period of the total power. Based on the findings and practical limitations, an optimization strategy is proposed. Further, the device is optimized based on a real wave environment in Shandong Province, China.