The growing contribution of the renewable energy technologies to the world demand is increasing the interest in the marine energy development and particularly in ocean wave energy. Many types of wave energy converters have already reached the precommercial stage and among those heaving point absorbers are one of the most promising concepts. Among the most efficient energy conversion systems for point absorbers are hydraulic power take-off (PTO) systems. Typically hydraulic PTO systems consist in a double-acting cylinder and two accumulators, reserving fluid at different pressures and linked between them by a hydraulic motor connected to an electric generator. This paper presents a simple model (Matlab/Simulink) of a heaving oscillating buoy, represented by a surface-piercing cylinder, extracting power by means of a hydraulic system. Apart from the basic elements listed above, it includes leakages and pressure losses and two small accumulators that take into account the effects of the compressibility of the hydraulic fluid. Also two extra accumulators are considered in order to improve the performance of the hydraulic system by means of properly set control valves. The function of these extra accumulators consists in storing and releasing energy to the system when this is desirable for the improvement of the power output.
The control of the system is managed by means of control valves whose opening will be depending on the sign of the velocity of the buoy. Two control variables are analysed in order to improve the power extraction of the converter: Firstly the torque of the electric generator is considered as a primary way to modify the load of the PTO. Then the two extra accumulators are used as storing devices to perform a kind of phase control on the buoy. The benefit of this effect will be dependent on the instant of activation of the valves that connect them to the circuit and the influence of possible delays or anticipations will be investigated.
