Wave energy, as a renewable resource, has the potential of contributing two TW power to the present energy mix. The feasibility of marine energy extraction through wave energy converters (WECs) has been numerically and physically verified over the past decades. However, only a few devices such as the Oscillating Water Columns&nbsp; (OWCs) and Wave Activated Bodies (WABs) have been qualified to be economically competitive in the global energy market. The Point Absorber Wave Energy Converters (PAWECs) belong to the WAB type and extract energy through the relative oscillation between a buoyant floater excited by wave forces and a rigid structure. The narrow-banded frequency response of PAWECS, while being subjected to the stochastic nature of impinging waves, results in the application of robust control to optimize the power output capacity of the device. In addition to the linear radiation damping, the nonlinear viscous forces excited at the controlled condition as a resultant of the amplified buoy excursion results in attenuation of output power. Hence, in the present work, we propose to optimize a scaled-down PAWEC by employing a Superposed Hydrodynamic Model (SPHM) formed by integrating both the linear and nonlinear wave forces. A mechanical-based direct-drive Power Take-Off (PTO) mechanism is used to convert wave energy into electrical power. Two variants of SPHM are considered: 1. SPHM1 (Nonlinear viscous model) and 2. SPHM2 – (Linearized viscous model). Linearization is carried out in SPHM2 to establish a direct relationship between the system's output response and the excitation force via a transfer function. The optimization is guided by employing the 'latch control' strategy based on non-predictive methods. Different targets such as the phase optimum and the maximum velocity criterion are pursued to evaluate the latching and de-latching intervals for both variants. The hydrodynamic coefficients such as the added mass and radiation damping along with the excitation forces are supplemented by Ansys AQWA using a Boundary Element Method (BEM) &nbsp;method to model the linear part of SPHM. The superposition of hydrodynamic forces is carried out to approximate the fully nonlinear model. The deviations are then corrected by tuning the viscous damping coefficient ‘Cd’ evaluated from the experimental free-decay test. The resultant SPHM is numerically solved using the fourth-order Runge-Kutta method, and the output power from both the optimization techniques are compared. Further, the effectiveness of latching is investigated by evaluating the absorption efficiencies at different wave heights for monochromatic wave conditions. Results show that a latch-controlled device can produce approximately seven times higher power compared to an unoptimized PAWEC. It is seen that the system performs efficiently for long period waves implying that the role of damping is to increase the response bandwidth of the device. Also, SPHM2 predicts a significantly higher energy conversion than SPHM1, at this condition.

Optimizing Power Output of a Wave Energy Converter by Employing Superposed Hydrodynamic Model

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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