Abstract
A mixed time-domain/frequency-domain method is proposed for modelling dense wave energy converter (WEC) arrays with non-linear power take-off (PTO). The model is based on a harmonic balance method which describes the system response in the frequency domain, while evaluating the non-linear PTO force and solving the system equations of motion in the time domain. The non-linear PTO force is computed with Lagrange multipliers. In order to apply the proposed method for WEC array responses in real sea states, the time series is split into time windows and the simulation is carried out individually per window. The method is demonstrated by investigating the dynamics of the Ocean Grazer WEC array (OG-WEC) with an adaptable piston pumping system. The key parameters thought to possibly influence model accuracy, including the number of harmonic components, the length of the time window and overlay, are discussed. It is shown that the proposed model can significantly reduce the computational cost with an acceptable accuracy penalty.
Highlights
Ocean wave energy is a sustainable and abundant energy source but extracting energy from ocean waves has not become a commercially viable technology yet
The hydrodynamic coefficients were obtained from NEMOH [27], the incident waves were generated based on the JONSWAP spectrum with 1001 wave frequencies, and the configuration of the Wave energy converter (WEC) array was chosen to correspond to previous research in Wei et al [22]
In the mixed frequency-domain/time-domain (MFT) model, the input time series of incident waves was replicated from the time domain (TD) model using the same pseudo-random phase, and the implemented Ocean Grazer WEC array (OG-WEC) parameters were chosen to be equivalent to those applied to the TD model, in order to enable the comparison of input and output values of the MFT and TD methods
Summary
Ocean wave energy is a sustainable and abundant energy source but extracting energy from ocean waves has not become a commercially viable technology yet. The paper [22] demonstrated that the HBM can be used to model large dense WEC arrays with non-linear pumping forces They applied a numerical Jacobian in the computation due to occurrence of discontinuity in the pumping force, the vector field is not differentiable everywhere. The present work extends the classical HBM model and propose a mixed frequency-domain/time-domain method with a windowing technique for a large dense WEC array with nonlinear PTO. The appealing feature of the proposed method is that it can describe the non-linear dynamics of the complex WEC array in random sea states while significantly reducing the computational cost In this model, the real sea state is split into time windows with uniform length; the state of each time window can be described with a finite number of harmonic components.
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