Modelling and geometry optimisation of wave energy converters

Abstract

The ultimate goal of wave energy undertaking is to find a solution that minimises the cost of delivered energy. Not only should a device maximise its energy absorption, but also the costs associated with absorbing and converting that energy into useful forms should be minimised. Towards realising this goal, this thesis contributes in three main areas, namely, numerical modelling, geometry optimisation, and geometry control.

The highlights of numerical modelling include the use of bond graph—a domain-independent, graphical representation of dynamical systems—in developing numerical models of wave energy converters (WECs), and the use of state-space models to represent the wave radiation terms. It is shown that bond graph is well-suited for modelling WECs, which involve interactions between multiple energy domains, and that state-space models of the wave radiation terms are efficient and sufficiently accurate for use in time-domain simulations of WECs. Both bond graph and state-space models are used in the modelling of a floating oscillating water column device, which, from the point of view of hydrodynamics, is a complex device involving various hydrodynamic radiation terms.

The main contribution of geometry optimisation is the incorporation of the cost factor in the design problem through the use of a multi-objective optimisation scheme. Two simplified cost factors are considered, namely, the surface area of the device and the reaction force that the device must withstand. The scheme is applied to find optimum geometries of a class of oscillating-body WECs that oscillate about a fixed horizontal axis. It is shown that when the cost factor is taken into account, a design that maximises the absorbed power is not necessarily the most economical. It is found, for example, that an economical bottom-hinged device has its section spanning only part of the water depth instead of the whole water depth.

Informed by the results of the geometry optimisation study, a design is proposed of a device which allows its geometry to be varied from time to time depending on the prevailing wave condition. An investigation of the device characteristics and potential is reported under geometry control. It is shown that controlling the variation of the geometry has the potential of broadening the power absorption bandwidth and improving its capacity factor in extreme conditions.