One of the major challenges facing modern industrialized countries is the provision of energy:
traditional sources, mainly based on fossil fuels, are not only growing scarcer and
more expensive, but are also irremediably damaging the environment. Renewable and
sustainable energy sources are attractive alternatives that can substantially diversify the
energy mix, cut down pollution, and reduce the human footprint on the environment.
Ocean energy, including energy generated from the motion of wave, is a tremendous untapped
energy resource that could make a decisive contribution to the future supply of
clean energy. However, numerous obstacles must be overcome for ocean energy to reach
economic viability and compete with other energy sources. Energy can be generated from
ocean waves by wave energy converters (WECs). The amount of energy extracted from
ocean waves, and therefore the profitability of the extraction, can be increased by optimizing
the geometry and the control strategy of the wave energy converter, both of which
require mathematical hydrodynamic models that are able to correctly describe the WEC
uid interaction. On the one hand, the accuracy and representativeness of such models
have a major in
uence on the effectiveness of the WEC design. On the other hand, the
computational time required by a model limits its applicability, since many iterations or
realtime calculations may be required. Critically, computational time and accuracy are
often mutually contrasting features of a mathematical model, so an appropriate compromise
should be defined in accordance with the purpose of the model, the device type, and
the operational conditions. Linear models, often chosen due to their computational convenience,
are likely to be imprecise when a control strategy is implemented in a WEC: under
controlled conditions, the motion of the device is exaggerated in order to maximize power
absorption, which invalidates the assumption of linearity. The inclusion of nonlinearities
in a model is likely to improve the model's accuracy, but increases the computational
burden. Therefore, the objective is to define a parsimonious model, in which only relevant
nonlinearities are modelled in order to obtain an appropriate compromise between accuracy
and computational time. In addition to presenting a wider discussion of nonlinear
hydrodynamic modelling for WECs, this thesis contributes the development of a computationally
efficient nonlinear hydrodynamic model for axisymmetric WEC devices, from
one to six degrees of freedom, based on a novel approach to the nonlinear computation of
static and dynamic FroudeKrylov forces.
