Evaluation and optimization of the control system of the Symphony Wave Power Device

Abstract

Raising environmental concerns have stimulated the development of renewable energy, including energy from the oceans, which contain a huge potential. In this thesis, particular emphasis is given to wave energy, which can deliver up to 2 TW on a global scale. The aim of this thesis is to optimize the control system of the Symphony Wave Power Device, which is a point absorber, so that the energy that is being delivered to the electrical grid is maximal and the device functions in a stable way. The device is analytically described in terms of structural parts, operating principle and presentation of all the forces that act on the moving part, which is called the floater. The device is in fact a mass-spring-damper system, for which the spring constant needs to be tuned according to the period of the incoming waves, so as to maximize the energy extraction. For this tuning, not only the actual mass of the floater, but also the added equivalent mass due to the inertia of the inner turbine need to be taken into account.
The whole device is modelled with the help of a Matlab/Simulink programme, in which simulations can be performed, to observe the motion and make certain calculations. The already existing PI controller, which makes use of an energy error, is briefly described and the relevant calculations for the energy extraction are presented. The energy losses in the electrical parts also need to be taken into account.
To evaluate the current controller, it is necessary to calculate the upper boundary of the energy that the Symphony can obtain from a certain wave. This is done with the help of the GAMS software. The code, as well as the necessary assumptions and approximations, are presented in a mathematical way. The results, both in numerical and graphical form, provide a good insight as to how the ideal theoretical control system looks like.
Next, simulations are performed in the Matlab programme and comparisons with the GAMS results are made. The essential parts of the controller are tuned to their optimal values. Only a proportional part for the PI controller is needed and the energy should not flow in two directions.
The results show that, with correct tuning of the proportional part, as well as of the spring constant, the Symphony operates very well in all realistic sea states at the location where it will be placed. A high percentage of the theoretical energy boundary is being extracted from the waves and the motion of the floater is close to the optimal pattern. It is thus concluded that the existing controller has a remarkable performance, if regulated correctly. Finally, recommendations for future research on many levels are given.