Abstract

To understand interactions of an elastic floating structure with nonlinear mooring dynamics and multiple wind turbines dynamics especially in irregular waves, the elastic mode expansion of Cummins’ equation for an elastic structure in irregular waves is strongly coupled with nonlinear rod element method for the nonlinear mooring lines through Jacobian matrix formed in a time-domain predictor and corrector solver up to the second order. The multiple wind turbines dynamics is resolved by load mapping of stress resultants at wind turbines into the hosting structure. Three numerical studies are presented to investigate linear elastic mode resonance interacting with both of nonlinear mooring excitation and higher elastic modes with the second order irregular waves, nonlinear elastic mode resonance with higher order harmonic excitation from various mooring lines, and three-dimensional elastic deformation interacting with multiple wind turbines dynamics in combination with nonlinear mooring excitation in irregular wave loads. The numerical study further identifies three-dimensional deformation can have substantial interactions with multiple wind turbine dynamics, influenced by outstanding rigid modes, while analogous symmetry in modes imposes strong coupling between elastic and rigid modes as well. Moreover, the relative configuration between mode shapes, mooring, and wind turbines can determine the coupled hydroelastic responses.

Abstract

Considering sequence of energy conversion processes for the wave energy conversion, we establish a sequential optimization for a wave energy converter with adaptive resonance to achieve a cost-competitive Levelized Cost Of Energy. As the ocean waves vary in peak frequency and significant height, the adaptive resonance is devised to locate natural frequency at which Capture Width Ratio is maximized for each sea state using the combination of mass relocation and nonlinear interaction with power-take-off dynamics. The sequential optimization comprises submerged volume optimization to maximize the first conversion from annual irregular waves to excitation relative to radiation damping, operational parameters optimization for adaptive resonance to maximize subsequent conversion to mechanical and electrical energy in terms of Capture Width Ratios at individual sea states occurring annually, system scale determination to get the highest Capture Width Ratio occurring at a target frequent sea state so that Annual Energy Production can be maximized for the given dimensions, and minimization of device costs through structural analysis and generator configuration. We perform the sequential optimization for a Surface Riding Wave Energy Converter in a kilowatt scale, which feasibly change pitch natural frequency by relocating mass units with nonlinear interaction with a linear power-take-off dynamics, featuring cost reduction of Levelized Cost Of Energy by an omni-directional submerged volume, linear power-take-off components sealed inside a tube, and minimum singe mooring line in slack condition. The optimized Surface Riding Wave Energy Converter results in annual average power 17.70 kW, Annual Energy Production 148.8 MWh, and the minimum Levelized Cost Of Energy $0.372/kWh. Contrary to $3.59 to $4.36/kWh of available reference models, the significant improvement of Levelized Cost Of Energy is attributed to the sequential optimization that includes those cost reduction features, extract the maximum energy at individual conversion processes, and results in the adaptive resonance at the optimized scale 3:1, producing the highest Capture Width Ratio 46.44 % among the annually occurring sea states, which is over two times performance of the reference model in equivalent floating condition. The performance of the optimized Surface Riding Wave Energy Converter is validated with fully nonlinear particle-based simulation in time series.

Abstract

To perform an efficient hydroelastic simulation with violent free surface interactions, we extend δ+ SPH to elastic modes of a floating structure through GPU parallelization, which includes the correction of velocity divergence with the deformation and computation of the structure's strain. Free surface interaction is supplemented with a segmented particle shifting and tensile instability correction. We validate the developed hydroelastic simulation for experiments of elastic wedge impacts with aluminum and composite panels. Through comparative analysis with different deadrise angles and impact velocities, we find that the improved free surface interactions reduce early separation from the deforming panels, leading to better prediction of the wedge acceleration and reasonably well-matched profiles of the free surface and panel deformation. The marginal difference is attributable to the water passing through the gaps of the physical test model built in three dimensions, which is absent in the simulation setup. Comparing strain time series, measured at two locations on the elastic panels, through three sets of simulations in different dimensions of the simulation set-up and mode shapes, we see that three-dimensional simulation with correct mode shapes in three dimensions accurately predicts the strain time series at both locations as well as the wedge acceleration. The hydroelastic simulation through the modal expansion in GPU parallelization can be utilized to efficiently predict various hydroelastic phenomena with violent free surface interactions.

Abstract

Since the ocean waves feature irregular waves varying over time, a wave energy converter with adaptive resonance, which feasibly changes the natural frequency for different sea states, is considered. To find its optimum design, multi-dimensional parametric optimization is formed in sequence of setting ranges of submerged geometry for hydrodynamic performance, finding design specifications including the generator for maximum resonance peaks, and locating the natural frequency for individual sea states to achieve the adaptive resonance that converts the maximum annual mechanical power for the given geometry. The multivariable design optimization is performed for a kilowatt-scale Surface-Riding Wave Energy Converter, which controls the pitch natural frequency through relocating a mass vertically. The optimized design presents that the adaptive resonance improves the mechanical power conversion up to 11 times compared to the conventional fixed resonance design. Furthermore, the adaptive resonance for diverse wave spectra indicates that the optimum natural frequencies for individual sea states, maximizing the annual mechanical power conversion, do not coincide with the waves’ peak or energy period. The second-order wave loads entails further change of the optimum natural frequency resulting in subsequent 10 times performance improvement for severe sea states with high peak periods.

Abstract

This paper presents the modeling methodology and performance evaluation of the resonance-enhanced dual-buoy WEC (Wave Energy Converter) by HEM (hydrodynamic & electro-magnetic) fully-coupled-dynamics time-domain-simulation program. The numerical results are systematically compared with the authors’ 1/6-scale experiment. With a direct-drive linear generator, the WEC consists of dual floating cylinders and a moon-pool between the cylinders, which can utilize three resonance phenomena from moon-pool dynamics as well as heave motions of inner and outer buoys. The contact and friction between the two buoys observed in the experiment are also properly modeled in the time-domain simulation by the Coulomb-friction model. Moon-pool resonance peaks significantly exaggerated in linear potential theory are empirically adjusted through comparisons with measured values. A systematic comparative study between the simulations and experiments with and without PTO (power-take-off) is conducted, and the relative heave displacements/velocities and power outputs are well matched. Then, parametric studies are carried out with the simulation program to determine optimum generator parameters. The performance with various wave conditions is also assessed.