Control strategies for a wave energy converter connected to a hydraulic power take-off

Among the various types of wave energy converters currently being developed, heaving point absorbers are one of the simplest and most promising concepts. A typical efficient energy conversion system for point absorbers is based on hydraulic power take-off (PTO) systems, consisting in a double-acting cylinder, a hydraulic motor and two or more accumulators. This paper presents a simple model of a heaving oscillating buoy extracting power by means of a hydraulic system. The hydrodynamic behaviour of the absorber is modelled through application of the linear water wave theory. Apart from the basic elements listed above, the model of the hydraulic system includes leakages and pressure losses and takes into account the compressibility of the fluid. Also, possible control accumulators are considered in order to improve the performance of the hydraulic system by means of properly controlled valves. Different control variables are analysed depending on the wave inputs considered in order to improve the power extraction of the converter. The results prove that it is possible to achieve a great enhancement of the power extraction with the implementation of these control strategies and that a possible combination of some of them might be beneficial for improved efficiency of the components.