Spectral modeling of an oscillating surge wave energy converter with control surfaces

• Published in: Applied ocean research Vol. 56; pp. 143 - 156
• Main Authors: Tom, Nathan, Lawson, Michael, Yu, Yi-Hsiang, Wright, Alan
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 01.03.2016
• Subjects: Fatigue damage; Irregular wave; Marine; Nonlinear optimization; NREL; Oscillating surge wave energy converter; TIDAL AND WAVE POWER; Variable structures; Viscous linearization; Irregular wave; Oscillating surge wave energy converter; Viscous linearization; Variable structures; Fatigue damage; Nonlinear optimization
• ISSN: 0141-1187; 1879-1549
• DOI: 10.1016/j.apor.2016.01.006

•Spectral techniques were applied to a novel WEC concept to circumvent time-domain simulations.•WEC variable geometry provides an additional degree of freedom from which to control device performance.•Performance was evaluated not only in terms of power production but also foundation loads and fatigue damage accumulation.•Nonlinear optimization was used to maximize power under motion constraints and under the influence of nonlinear dynamics.•A nonlinear quadratic viscous drag torque flattened the capture width and foundation load curves. The aim of this research is to use spectral techniques in evaluating the irregular wave performance of a novel wave energy converter concept that combines an oscillating surge wave energy converter with control surfaces. The control surfaces allow the wave energy converter to have a time-varying geometry that enables the hydrodynamic exciting and radiation coefficients to be altered. In the current state of development the device geometry is controlled on a sea state-to-sea state time scale and combined with control of the power take-off (PTO) on a wave-to-wave time scale to maximize power capture, increase capacity factor, and reduce design loads. Analysis begins with the application of linear hydrodynamic theory to evaluate the device performance in terms of absorbed power, foundation loads, and accumulation of fatigue damage on the PTO. To determine the linear PTO damping coefficient that maximizes the time-averaged absorbed power for a given sea state, an optimization problem was constructed while incorporating a motion constraint on the maximum pitch amplitude of motion. The inclusion of the motion constraint prevents linear scaling of the performance results with the significant wave height. Previous studies on the modeling of oscillating surge wave energy converter designs have included nonlinear hydrodynamics. Therefore, a quadratic viscous drag moment was added to the system dynamics through use of a quasi-linear viscous damping coefficient. The same performance quantities were calculated for both the linear and nonlinear models while assuming an irregular wave surface elevation described by a Bretscheider spectrum. One major effect of including the viscous drag moment was flattening of the capture width and structural load curves with respect to the wave spectrum peak frequency while reducing the sensitivity with respect to the significant wave height compared to the linear analysis.