A Numerical Simulation of a Variable-Shape Buoy Wave Energy Converter

Wave energy converters (WECs) usually require reactive power for increased levels of energy conversion, resulting in the need for more complex power take-off (PTO) units, compared to WECs that do not require reactive power. A WEC without reactive power produces much less energy, though. The concept of Variable Shape Buoy Wave Energy Converters (VSB WECs) is proposed to allow continuous shape-change aiming at eliminating the need for reactive power, while converting power at a high level. The proposed concept involves complex and nonlinear interactions between the device and the waves. This paper presents a Computational Fluid Dynamics (CFD) tool that is set up to simulate VSB WECs, using the ANSYS 2-way fluid–structure interaction (FSI) tool. The dynamic behavior of a VSB WEC is simulated in this CFD-based Numerical Wave Tank (CNWT), in open sea conditions. The simulation results show that the tested device undergoes a significant deformation in response to the incoming waves, before it reaches a steady-state behavior. This is in agreement with a low-fidelity dynamic model developed in earlier work. The resulting motion is significantly different from the motion of a rigid body WEC. The difference in the motion can be leveraged for better energy capture without the need for reactive power.

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An Efficient Convex Formulation for Model-Predictive Control on Wave-Energy Converters

Volume 10: Ocean Renewable Energy ◽

10.1115/omae2017-62575 ◽

2017 ◽

Cited By ~ 2

Author(s):

Qian Zhong ◽

Ronald W. Yeung

Keyword(s):

Model Predictive Control ◽

Predictive Control ◽

Wave Energy ◽

Optimization Problem ◽

Reactive Power ◽

Tuning Parameter ◽

Primary Concern ◽

Penalty Term ◽

Wave Energy Converters ◽

Energy Converters

Model-Predictive Control (MPC) has shown its strong potential in maximizing energy extraction for Wave-Energy Converters (WECs) while handling hard constraints. As MPC can solve the optimization problem on-line, it can better account for state changes and reject disturbances from the harsh sea environment. Interests have arisen in applying MPC to an array of WECs, since researchers found that multiple small-size WECs are more economically viable than a single large-size WEC. However, the computational demand is known to be a primary concern for applying MPC in real-time, which can determine the feasibility of such a controller, particularly when it comes to controlling an array of absorbers. In this paper, we construct a cost function and cast the problem into a Quadratic Programming (QP) with the machinery force being the “optimizer,” for which the convexity can be guaranteed by introducing a penalty term on the slew rate of the machinery force. The optimization problem can then be solved efficiently, and a feasible solution will be assured as the global optima. Constraints on the motion of the WEC and the machinery force will be taken into account. The current MPC will be compared to others existing in literature, including a nonlinear MPC [1] which has been applied in wave-tank tests. The effects of constraints on the control law and the absorbed power are investigated. Performances of the WEC are shown for both regular and irregular wave conditions. The current MPC is found to have good energy-capture capability and is able to broaden the band-width for capturing wave energy. The reactive power required by the PTO system is presented. The additional penalty term provides a tuning parameter, of which the effects on the MPC performance and the reactive power requirement are discussed.

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Effects of Scale and Configuration of Air Chamber of OWC Type WECs on Air Chamber Characteristics

Volume 5: Ocean Space Utilization ◽

10.1115/omae2020-18762 ◽

2020 ◽

Author(s):

Tomoki Ikoma ◽

Shota Hirai ◽

Yasuhiro Aida ◽

Koichi Masuda ◽

Hiroaki Eto

Keyword(s):

Numerical Calculation ◽

Water Column ◽

Wave Energy ◽

Experimental Models ◽

Experimental Results ◽

Oscillating Water Column ◽

Wave Energy Converters ◽

Air Chamber ◽

The Difference ◽

Energy Converters

Abstract This paper describes scale effects and influence of configurations of oscillating water column type wave energy converters from model tests and theoretical calculations. Many researches regarding wave energy converters (WECs) have been conducted. The behavior of an oscillating water column of an OWC type WEC is complicated because of including wave-air-turbine interaction, and thus several issues remain. One of the issues is that influence of difference in scale between small scale experimental models and full scale models is unclear. It is important to understand its characteristics accurately to improve design technologies for such as complicated systems. In this study, we carried out forced oscillation tests using multiple scales and shapes of OWC models in still water, and measured the pressure inside the air chamber and the internal mean water level with a multi-line wave probe. The experimental models used have a box like air chamber or manifold type air chamber, and which scales were 1/1, 1/2 and 1/4.The difference of the two air chambers is an orifice or a duct to be inlet-outlet of air. As a result, the difference in scale and configuration of the air chamber affected the characteristics of the air chamber. In addition, as a result of numerical calculation using the linear potential theory and comparison with experimental results, the experimental results could be reproduced by numerical calculation. Besides, we could discuss the effects and the influences of the air chamber basically.

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A Comparison of Selected Strategies for Adaptive Control of Wave Energy Converters

Journal of Offshore Mechanics and Arctic Engineering ◽

10.1115/1.4002735 ◽

2011 ◽

Vol 133 (3) ◽

Cited By ~ 104

Author(s):

Jørgen Hals ◽

Johannes Falnes ◽

Torgeir Moan

Keyword(s):

Wave Energy ◽

Reactive Power ◽

Average Power ◽

Volume Ratio ◽

External Input ◽

Irregular Waves ◽

Optimal Velocity ◽

Wave Energy Converters ◽

Absorbed Power ◽

Energy Converters

Wave-energy converters of the point-absorbing type (i.e., having small extension compared with the wavelength) are promising for achieving cost reductions and design improvements because of a high power-to-volume ratio and better possibilities for mass production of components and devices as compared with larger converter units. However, their frequency response tends to be narrow banded, which means that the performance in real seas (irregular waves) will be poor unless their motion is actively controlled. Only then the invested equipment can be fully exploited, bringing down the overall energy cost. In this work various control methods for point-absorbing devices are reviewed, and a representative selection of methods is investigated by numerical simulation in irregular waves, based on an idealized example of a heaving semisubmerged sphere. Methods include velocity-proportional control, approximate complex conjugated control, approximate optimal velocity tracking, phase control by latching and clutching, and model-predictive control, all assuming a wave pressure measurement as the only external input to the controller. The methods are applied for a single-degree-of-freedom heaving buoy. Suggestions are given on how to implement the controllers, including how to tune control parameters and handle amplitude constraints. Based on simulation results, comparisons are made on absorbed power, reactive power flow, peak-to-average power ratios, and implementation complexity. Identified strengths and weaknesses of each method are highlighted and explored. It is found that overall improvements in average absorbed power of about 100–330% are achieved for the investigated controllers as compared with a control strategy with velocity-proportional machinery force. One interesting finding is the low peak-to-average ratios resulting from clutching control for wave periods about 1.5 times the resonance period and above.

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Extending Complex Conjugate Control to Nonlinear Wave Energy Converters

Journal of Marine Science and Engineering ◽

10.3390/jmse8020084 ◽

2020 ◽

Vol 8 (2) ◽

pp. 84

Author(s):

David G. Wilson ◽

Rush D. Robinett ◽

Giorgio Bacelli ◽

Ossama Abdelkhalik ◽

Ryan G. Coe

Keyword(s):

Energy Harvesting ◽

Limit Cycle ◽

Wave Energy ◽

Limit Cycles ◽

Reactive Power ◽

Complex Conjugate ◽

Linear Wave ◽

Wave Energy Converters ◽

Linear Limit ◽

Energy Converters

This paper extends the concept of Complex Conjugate Control (CCC) of linear wave energy converters (WECs) to nonlinear WECs by designing optimal limit cycles with Hamiltonian Surface Shaping and Power Flow Control (HSSPFC). It will be shown that CCC for a regular wave is equivalent to a power factor of one in electrical power networks, equivalent to mechanical resonance in a mass-spring-damper (MSD) system, and equivalent to a linear limit cycle constrained to a Hamiltonian surface defined in HSSPFC. Specifically, the optimal linear limit cycle is defined as a second-order center in the phase plane projection of the constant energy orbit across the Hamiltonian surface. This concept of CCC described by a linear limit cycle constrained to a Hamiltonian surface will be extended to nonlinear limit cycles constrained to a Hamiltonian surface for maximum energy harvesting by the nonlinear WEC. The case studies presented confirm increased energy harvesting which utilizes nonlinear geometry realization for reactive power generation.

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Hydrodynamic Analysis of Variable-Shape Wave Energy Converters

10.1115/1.0000839v ◽

2021 ◽

Author(s):

Shangyan Zou ◽

Ossama Abdelkhalik

Keyword(s):

Wave Energy ◽

Hydrodynamic Analysis ◽

Wave Energy Converters ◽

Variable Shape ◽

Energy Converters

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Numerical Wave Tank Simulation of a Variable Geometry Wave Energy Converter

Volume 9: Ocean Renewable Energy ◽

10.1115/omae2020-18802 ◽

2020 ◽

Author(s):

Shangyan Zou ◽

Ossama Abdelkhalik

Keyword(s):

Wave Energy ◽

Reactive Power ◽

Irregular Waves ◽

Variable Geometry ◽

Numerical Wave Tank ◽

Wave Tank ◽

Wave Energy Converters ◽

Highly Nonlinear ◽

Numerical Wave ◽

Energy Converters

Abstract This paper presents a high-fidelity numerical wave tank simulation for Variable Geometry Wave Energy Converters (VG-WECs). Typically, wave energy converters require reactive power to optimize the energy conversion, which significantly jeopardizes the economic index of the system. The proposed VGWECs allows comprehensive shape-changing not only in response to ocean climate but also to reduce the reactive power requirements on the power take-off (PTO) unit. This design aims at eliminating reactive power with minimal impact on optimality in terms of energy production. To investigate the dynamic behavior of the VGWEC, this model is simulated in a Computational Fluid Dynamics (CFD) based Numerical Wave Tank (CNWT) using ANSYS 2-way Fluid Structure Interaction (FSI) tool. The interaction between irregular waves and the VGWEC is simulated. The numerical results show that the proposed VGWEC has large deformation and motion in response to the incoming wave. This highly nonlinear interaction between waves and VGWEC can be leveraged to eliminate reactive power.

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Wave-to-Wire Power Maximization Control for All-Electric Wave Energy Converters with Non-Ideal Power Take-Off

Energies ◽

10.3390/en12152948 ◽

2019 ◽

Vol 12 (15) ◽

pp. 2948

Author(s):

Sousounis ◽

Shek

Keyword(s):

Wave Energy ◽

Reactive Power ◽

Energy Resource ◽

Wave Energy Converter ◽

Floating Body ◽

Maximum Efficiency ◽

Energy Converter ◽

Electric Wave ◽

Wave Energy Converters ◽

Energy Converters

The research presented in this paper investigates novel ways of optimizing all-electric wave energy converters for maximum wave-to-wire efficiency. In addition, a novel velocity-based controller is presented which was designed specifically for wave-to-wire efficiency maximization. In an ideal wave energy converter system, maximum efficiency in power conversion is achieved by maximizing the hydrodynamic efficiency of the floating body. However, in a real system, that involves losses at different stages from wave to grid, and the global wave-to-wire optimum differs from the hydrodynamic one. For that purpose, a full wave-to-wire wave energy converter that uses a direct-drive permanent magnet linear generator was modelled in detail. The modelling aspect included complex hydrodynamic simulations using Edinburgh Wave Systems Simulation Toolbox and the electrical modelling of the generator, controllers, power converters and the power transmission side with grid connection in MATLAB/Simulink. Three reference controllers were developed based on the previous literature: a real damping, a reactive spring damping and a velocity-based controller. All three literature-based controllers were optimized for maximum wave-to-wire efficiency for a specific wave energy resource profile. The results showed the advantage of using reactive power to bring the velocity of the point absorber and the wave excitation force in phase, which was done directly using the velocity-based controller, achieving higher efficiencies. Furthermore, it was demonstrated that maximizing hydrodynamic energy capture may not lead to maximum wave-to-wire efficiency. Finally, the controllers were also tested in random sea states, and their performance was evaluated.

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