Importance of drivetrain optimisation to maximise electrical power from wave energy converters

Abstract

This article demonstrates the benefits of optimising the drivetrain to improve the level and quality of electrical power output from a wave energy converter. The study considers a spherical buoy connected to a permanent magnet synchronous generator through a mechanical drive. The wave energy converter is equipped with a model predictive control system that maximises electrical power from the generator. Three different scenarios are compared: (i) when the drivetrain is not optimised, (ii) when only the gear ratio is optimised, (iii) and when both gear ratio and flywheel inertia are optimised. The performance of all three configurations is compared in terms of their effect on the generator operating range, the natural frequency of the system, the amount of generated electrical power, and control forces. The results demonstrate that the drivetrain optimisation leads to a significant increase in the electrical power output while shifting the generator's operating range to areas with the highest efficiency. Moreover, drivetrain designs that utilise a flywheel reduce the power take-off loads and facilitate smoother power production.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Modelling and Optimisation of Wave Energy Converters

“…In order to extract more energy from the wave environment, it is also important to optimise both the device geometry and the power take-off (PTO) system. At present, there is a significant amount on research on the optimisation of PTO systems using control theory to enhance the performance of WECs [10,461,274,624,706,308]. Some of the key considerations include balancing the peak-to-average power ratio to ensure smoother operation of the device and maximising the average power extracted by the PTO system.…”

“…where M, B, and K are the matrices of inertia, damping and stiffness, respectively; the hydrodynamic coefficients of added mass, A m (ω), radiation damping, B rad (ω), and excitation force, F e (ω), are usually obtained through BEM codes or analytically as per Section 2.2.2. In this formulation, additional forces from the PTO, mooring and control system are assumed to be functions of the displacements and velocities; hence, are described as a combination of linear stiffness and dampers that compose matrices B and K. Note that some PTO systems present relevant inertial effects, e.g., translators, and flywheels [706], hence, inertia terms from the PTO appear in matrix M. It is also important to highlight that some sources of nonlinearities vanish during the linearisation, such as the viscous drag loads under zero current speed.…”

“…Meanwhile, a practical PTO unit is non-ideal and has its own limits. Therefore, a co-design framework is required for WEC modelling, performance assessment, optimisation, control, and so forth [319,706].…”