Performance Enhancement of Submerged Ocean Wave Energy Converter using Nonlinear Stiffness
Date
2021
Authors
Schubert, Benjamin W.
Editors
Advisors
Robertson, William
Cazzolato, Benjamin
Sergiienko, Nataliia
Ghayesh, Mergen
Cazzolato, Benjamin
Sergiienko, Nataliia
Ghayesh, Mergen
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Thesis
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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.
School/Discipline
School of Mechanical Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2021
Provenance
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