Modeling and Concept Design of Wave Energy Device

Sammanfattning: It has been suggested that it is feasible to extract energy from ocean waves which may be used to generate electricity. This thesis is centered on a wave energy converter system, which is known as a point absorber, which in its most simple design may be seen as a buoy moving up and down due to the wave motions. One problem with point absorbers is to achieve a desired motion relative the wave to absorb as much energy as possible. One way to achieve good energy absorption is to control the motion of the buoy by an external force, which for example could be to use a generator partly as a motor. The starting point for this thesis is that a new type of generator is currently being developed at the Royal Institute of Technology. Due to its high efficiency at low velocity, the generator enables a direct driven control setup where the generator partly functions as a motor, which has been deemed unfeasible in the past. The new generator is currently being developed in two variants, linear and rotating, which entail a principal different buoy size and shape due to geometric constraints. To obtain better knowledge about the dynamics of the point absorber and how the generator control forces will affect the dynamics and the energy absorption, a simplified motion response model has been created. In order to study if the generator variants would be more suitable for a particular wave climate, a total of 16 different buoy structures has been presented for 3 different wave climates based on the geometrical differences of the generator variants. To get an estimation of the buoy mass and thereby an estimation of the manufacturing cost, the buoy structures have been dimensioned in accordance with a design standards for loading buoys for the offshore industry. The work has resulted in a simplified hydrodynamic model which can be used to make conceptual analysis for a point absorber with respect to size, mass, control force and energy absorption for a regular sea consisting of one wave component. The simplifications of the model and the control force have been investigated in a sensitivity study. The conceptual design study indicates that the linear generator buoy structures are generally twice the mass of a rotating concept for all investigated wave climates. This is an expected result due to the geometrical constrains the linear generator entail given that it is assumed that the power takeoff unit is integrated in the buoy. The sensitivity study show that the modeling of the added mass needs further refinement and the control force is the parameter affecting the dynamic of the point absorber the most and hence also the energy absorption. Further, are simplifications for the diffraction force and the hydrodynamic damping considered to give a relatively small uncertainty for the total dynamics of the system.