ABSTRACT

This paper examines the energy capture of a vertical floating truncated cylinder used as a Wave Energy Converter (WEC). An eigenfunction matching methodology is employed to validate the Boundary Element Method (BEM) solutions from the NEMOH solver (the first open-source BEM code developed and released by Ecole Centrale de Nantes) for heaving motions. Comparison of capture widths for individual and combined degrees of freedom (DOF) reveals maximum efficiency in uncoupled motions. The theory of optimal wave power absorption explains this behaviour. A parametric study shows that under specific wave angles, power absorption in coupled modes can be fully recovered in uncoupled modes.

INTRODUCTION

Climate change stands out as the foremost challenge confronting the 21st century, and its resolution is imperative to address within the next 20 to 30 years to avert impending irreversible alterations in our ecosystem. This impending crisis threatens existential consequences and may instigate the 6th mass extinction event. Urgency and significance underscore the need to harness alternative energy sources, displacing reliance on fossil fuels while ensuring energy security. Despite being an underutilized asset, renewable energy holds immense promise, potentially satisfying a notable percentage of global energy demands by 2050. However, realizing this potential commercially necessitates the unleashing of technological advancements.

A floating truncated cylinder is a classical model for WEC and has been a matter of study in several papers e.g., Yeung (1981); Garrett (1971), etc. focusing on the underlying hydrodynamics. Calculation of power absorption by a cylindrical WEC fully utilizes the prediction of wave forcing using linear wave structure interaction theory (i.e., only the first order term is retained in the free surface boundary conditions, referring to small amplitude waves) and has been investigated by many e.g., Sergiienko et al. (2017). In many of these studies, the chosen degree of freedom of the WEC is often restricted to heave only and it is not fully understood how the simultaneous motions (which often led to coupling between modes) of the cylinder in all degrees of freedoms may affect the power absorption characteristics. Thus, in this paper, we attempt to find the power absorption of a floating truncated cylinder using the standard Boundary Element Method for solving hydrodynamic coefficients (namely added mass, damping and excitation forces) from the linear wave body boundary value problem. The theory of optimum wave power absorption by Evans (1980) readily provides the formula for power using these coefficients and we adopt this formulation to investigate power absorptions. In more recent literature, similar to this paper, Heikkinen, Lampinen & Böling (2013) also did investigations on the horizontal cylinder. Some research has also been conducted on how cylindrical wave converters behave when placed in an array. E.g. a stochastic approach was developed by Bailey, Robertson, and Buckham (2018) to predict the array power time series solely using the knowledge of individual WECs. A similar numerical and experimental study has been conducted for pitch mode only by placing the center of rotation off-centered is comparable to that of an asymmetric WEC in Sik Ko et al (2019). The study was also tested for different axes of rotation Sik Ko et al (2018). Most of these studied focused on one or two degrees of freedom. Apart from the study by Wu (2020), there are not many investigations on how the mutual coupling between different degrees of freedom might affect the performance of a wave energy device. This is the topic; thus, we pursue in this paper for a WEC of cylindrical shape.

This content is only available via PDF.
You can access this article if you purchase or spend a download.