Wave energy converter: hydrodynamics and control

Waves, just like wind and solar radiation, is a widely available renewable energy source. Waves are created when winds move across the sea surface. The estimated potential of wave energy is abundant, but the commercial harvesting technologies are still in their infancy. There are different wave energy converter designs, one of which is developed at Uppsala University and is based on a permanent magnet linear generator. A semi-submerged buoy on the water surface absorbs the energy of the wave and converts the mechanical energy into electricity with a direct drive linear generator. One of the main goals in wave energy research is to enhance the absorbed power for a single device and multiple devices in wave energy parks. The energy harvest can be increased in different ways: by optimizing the buoy, the generator, and/or by implementing control strategies. To assess the best optimization strategies numerical modeling is an inexpensive tool, aimed to predict the complex behavior of the system. This licentiate thesis focuses on the study of wave energy converters in irregular waves for testing sites, such as in Lysekil (Sweden) and Wave Hub (UK). The numerical model is used to analyze the wave energy conversion power performance. The hydrodynamic model involves radiation force approximation for a state-space model. It has been shown that a higher order of approximation can be achieved by vector fitting than by the transfer function fitting in the frequency domain, especially for the interaction of several bodies with the incident wave. Wave energy converter concepts are evaluated in terms of absorbed power for the resistive load connection, representing the passive control of the currents in the generator windings. Additionally, RC-load intends to model a grid-connected generator with active rectification, such that phase angle is compensated. Finally, a power-hardware-in-the-loop study of a grid-connected wave energy converter is presented. The current and voltage profiles of a grid-connected wave energy converter are shown with a suggestion on the implementation of RLC filter for power smoothing