Performance Enhancement of Submerged Ocean Wave Energy Converter using Nonlinear Stiffness

Abstract

Ocean waves are a source of renewable energy with an enormous potential to augment current renewable energy markets. Historically, the levelised cost of wave energy has been higher than conventional renewable energy sources such as wind or solar. While significant progress has been made in improving the economic viability of wave energy, a robust control system for wave energy converters is an important step to progress their technology readiness level. Utility scale wave energy systems typically require large capital investment. Therefore, tools are required to accurately and reliably model systems to predict the dynamic response and performance of potential control systems. This thesis presents a passive control system in the form of a nonlinear stiffness to improve the robustness of wave energy systems in situ as the ocean wave conditions change over time. In the preceding work in the literature, two common shortcomings, which may undermine the investigations, are: (i) the lack of comparisons against optimal conditions; and, (ii) the simplistic representation of hydrodynamic forces in fluid-structure interactions. These two gaps underpin the purpose of each chapter of this thesis and are systematically addressed in the context of a submerged point absorbing wave energy converter. Many differing designs of wave energy converters have been proposed in literature, with fundamentally different modes of operation. This thesis initially compares the application of a passive control system to point absorbing wave energy devices in both floating and submerged contexts. It was found that the application of nonlinear stiffness did not improve upon a system controlled by an optimised linear stiffness in both floating and submerged scenarios for regular wave excitation. Since many floating point absorbers experience a large hydrostatic stiffness, mechanisms to provide large negative stiffness are required for tuning purposes. The nonlinear stiffness — which can provide negative stiffness—offers a notable improvement in power production capacity compared to the scenario with no control stiffness in floating systems. For a submerged system, a position-dependent force is inherently required to counteract the constant buoyancy force, so the system may be optimally tuned by a linear stiffness. For irregular waves, which are more representative of ocean conditions, a floating system without an optimised linear stiffness experiences a significant benefit, while systems with optimal linear parameters do not benefit in terms of the power converted. However, as ocean conditions change in terms of significant wave height, energy period, and wave phase relationships, the addition of a nonlinear stiffness mechanism provides an improvement by enhancing the robustness to changing ocean conditions and by desensitising the system to wave phasing. The fidelity of simulations involving nonlinear stiffness may be improved by extending the model to three degrees of freedom to capture geometric nonlinearities and dynamic coupling between different degrees of freedom. In this work, the nonlinear stiffness was parametrised and varied to demonstrate how and why the system responds either positively or negatively depending on particular wave conditions. It was shown that when the system is optimally tuned for a regular wave, the nonlinear stiffness is not able to improve the amount of power generated. For irregular waves, the optimal performance is observed when the system is tuned with a linear stiffness to give a particular natural frequency—depending on wave condition. However, the same performance is also achieved with a nonlinear stiffness augmentation when the system is oscillating about any equilibrium point if the position dependent natural frequency is close to the optimal natural frequency. A consistent beneficial trend is seen under different irregular wave excitations. The nonlinear stiffness exposes the system to a changing effective resonance frequency varying with position. As a result, performance improvements over the linear system are observed when the system is tuned for one irregular wave and excited by a different irregular wave. Therefore, the primary benefit of a nonlinear augmentation is the improvement to robustness of such systems for varying sea conditions. The hydrodynamic modelling of the fluid-structure interaction of a submerged wave energy device is often achieved using linear potential flow theory. This limitation is explored by comparing both linear and nonlinear hydrodynamic models (using a validated computational fluid dynamics simulation) with a novel pseudo-nonlinear model, which extends the linear model to incorporate pose-dependent hydrodynamic parameters during simulation through pre-calculated values. The results showed that linear hydrodynamics do not adequately represent all the important nonlinear effects. The trends in motion also indicates the presence of frequency dependent fluid-structure interactions associated with the resonance of body of water above the buoy. It is not possible to represent such phenomena using standard linear potential flow methods. Therefore, higher fidelity models should be employed to obtain more reliable indications of performance. The three degrees of freedom model was further extended by including nonlinear stiffness into the validated computational fluid dynamics model. It was shown that inclusion of nonlinear hydrodynamics shifts the optimal natural frequency of the system. For regular waves, the nonlinear stiffness did not provide a consistent improvement. Under irregular conditions, a small amount of nonlinear stiffness was shown to provide a 5.5% improvement. The nonlinear stiffness was parametrised relative to the potential energy of the incident wave, leading to the observation that the peak in time-averaged power generation occurred when the nonlinear stiffness potential at the nominal equilibrium position was around 25% of the potential energy of the incident wave. While the trend in power results between the models using linear and nonlinear hydrodynamics with the nonlinear stiffness were reasonably similar, in the nonlinear hydrodynamics model, the nonlinear stiffness more rapidly detunes the system than in the linear model. This finding indicates that a nonlinear stiffness mechanism may be an effective method to detune the device to protect components from extreme operating conditions.