Advancements in wave energy conversion: a numerical analysis of the C4 CorPower WEC’s dynamic response in various wave scenarios

This study examines the dynamic response of a selected wave energy converter (WEC) in long-crested waves ranging from linear to steep scenarios. First, an overview on the state-of-the-art for WECs is documented, also highlighting the fundamental role of renewable energy and carbon emission reduction in addressing environmental challenges. Various WECs technologies are discussed comparatively and, the innovative device, C4 CorPower WEC, proposed by CorPower is selected for an in-depth analysis. This is based on a point absorber solution, already at a high Technology Readiness Level (TRL). The theoretical framework for the dynamic analysis is based primarily on linear potential-flow theory, but weakly-nonlinear hydrodynamic effects are also examined; moreover, viscous-flow effects are included through empirical formulas. The linear numerical approach includes: 1) a frequency-domain solution based on the zero-order boundary element method FFLOB, developed at CNR-INM, using the Green’s function, and 2) a time-domain solution following the Cummin’s approach, with a Runge-Kutta time scheme, including nonlinear body dynamics and using a body-fixed reference frame. Weakly-nonlinear effects in the incident waves and in their induced loads can be included in the time domain formulation, stretching the applicability of the Cummin’s approach. Preliminary numerical convergence studies are performed for a surface-piercing spherical buoy and validation analysis against available free-decay tests in heave at three different initial positions of the body, highlighting the importance of nonlinear effects. The sphere represents a relevant geometry as it shares similarities with the C4 CorPower WEC, which is an axisymmetric body with a substantial variation in the horizontal cross section. The selected device is described in detail, together with the simplified model of the different components and examined within a stepwise strategy. Within the frequency-domain analysis, it is first studied at the expected mean draft and the response amplitude operator (RAO) is evaluated for the relevant rigid degrees of freedom, then different values of the mean draft are assumed to quantify the effects on hydrostatic restoring, added mass and natural period in heave. Within the time domain analysis, free-decay tests in calm water and analyses in regular and irregular waves representing operational and severe sea environments, are examined. Both fully linear and weakly-nonlinear models are used. The selected wave conditions are taken from Agucadoura, in Portugal, a site of particular interest to CorPower, and for which, reference experimental and numerical data were made available by the company for comparison. Present study focuses on a detuned case where the device is not generating energy but is instead in a condition of storm protection. In this scenario, the device employs a high PTO (Power Take-Off) stiffness to ensure minimal heave movements, demonstrating the possibility to adapt in harsh sea environments. This approach explores paths for possible improvements of the system design, particularly in enhancing the operational flexibility of the WEC in facing extreme weather conditions.