Performance optimization of a U-Oscillating-Water-Column wave energy harvester

This paper deals with the problem of optimizing the performance of a U-Oscillating Water Column (U-OWC) wave energy converter equipped with a Wells turbine by controlling the turbine dynamics in a variety of incident wave conditions. The controller methodology is based on the identification of the reference turbine rotational speed providing the maximum average converted power. Two different approaches to the problem are proposed. The first relies on a sea-state based controller providing the optimum value of a reference rotational speed assumed constant during a sea state via a Maximum Power Point Tracking (MPPT) algorithm. In this context, it is shown that the significant wave height of the incident sea state is eligible as reference parameter for performing the tracking of the optimum conditions. The second solution is a wave-to-wave control algorithm relating the reference turbine rotational speed to the actual dynamic conditions of the plant. Numerical comparisons show that the assumption of a constant reference rotational speed within the sea state duration works better than a fast acting control. Finally, the consequences of the controller on the phase distribution of the dynamic system response is analysed. In this context, numerical results highlight the fact that the U-OWC responds differently in wind-dominated and swell-dominated sea states. Specifically, in wind-dominated sea states the maximum converted average wave energy occurs during the resonance condition; while, in swell-dominated sea states, the maximum converted average wave energy is obtained by adopting an adequately large turbine rotational speed, irrespective of the achievement of the resonance condition.