Model Predictive Control of a Wave Energy Converter with Discrete Fluid Power Power Take-Off System

Wave power extraction algorithms for wave energy converters are normally designed without taking system losses into account leading to suboptimal power extraction. In the current work, a model predictive power extraction algorithm is designed for a discretized power take of system. It is shown how the quantized nature of a discrete fluid power system may be included in a new model predictive control algorithm leading to a significant increase in the harvested power. A detailed investigation of the influence of the prediction horizon and the time step is reported. Furthermore, it is shown how the inclusion of a loss model may increase the energy output. Based on the presented results it is concluded that power extraction algorithms based on model predictive control principles are both feasible and favorable for use in a discrete fluid power power take-off system for point absorber wave energy converters

Model Predictive Control of Low-Speed Partial Stroke Operated Digital Displacement Pump Unit

To enhance the use of the Digital Displacement Machine (DDM) technology as the future solution for low speed fluid power pump and motor units, a Model Predictive Control (MPC) strategy is presented. For a low speed DDM, the conventional full stroke operation strategy is unsuitable, since the control update rate is proportional to the machine speed. This creates an incentive to utilize sequential partial stroke operation where a fraction of the full stroke is used, which thereby increases the control update rate and control signal resolution. By doing this, the energy loss is increased and may become undesirable large if the control objective is purely set-point tracking, why a trade-off is considered advantageous. Discretizing the full stroke based on a chosen update rate results in a Discrete Linear Time Invariant (DLTI) model of the system with discrete input levels. In this paper, the Differential Evolution Algorithm (DEA) is used to determine the optimal control input based on the trade-off between set-point tracking and energy cost in the prediction horizon. The paper presents a flow and a pressure control strategy for a fixed speed digital displacement pump unit and shows the trade-off influence on the optimal solution through simulation. Results show the applicability of the control strategy and indicate that a much higher energy efficiency may be obtained with only a minor decrease in tracking performance for pressure control

Control of an Energy Efficient Hydraulic Cylinder Drive with Multiple Pressure Lines

This paper concerns a novel energy efficient hydraulic cylinder drive concept, consisting of three pressure lines used to supply a hydraulic cylinder(s). The proposed concept allows for reduced throttling losses compared to conventional solutions, while maintaining accurately cylinder control. One application where the concept shows great potential is as the actuation system for life tests of large mechanical structures, where structure deflections is made through cylinder force control. This work contributes to the development and control of this hydraulic concept, where the purpose is to obtain a high energy efficiency and an accurate cylinder force control. The final concept design is implemented in simulation models, where the performance of the developed control system is investigated to verify that the control performance is satisfactory. Furthermore, the energy efficiency is compared to a conventional hydraulic concept to verify the feasibility. Through simulation models, control performance similar to a conventional hydraulic concept has been obtained with the proposed control structure, while reducing the hydraulic energy consumption by up to 50%. Based on the findings in this paper it is assessed that the concept is ready for experimental validation

Optimisation of Control Algorithm for Hydraulic Power Take-Off System in Wave Energy Converter

Wave energy converters are still a maturing technology and, as such, still face a series of challenges before they can compete with already-established technologies. One of these challenges is optimising the amount of energy extracted from the waves and delivered to the power grid. This study investigates the possibility of increasing the energy output of the existing hydraulic power take-off system of a wave energy converter made by Floating Power Plant during small-scale testing of their hybrid wind and wave energy platform. This system consists of a floater arm that rotates an axle when displaced by the waves. When the axle rotates, two hydraulic cylinders are actuated, displacing oil to run through a hydraulic motor driving an electric generator. The energy extraction is controlled by implementing a control algorithm on a series of on/off valves, which decouples the two hydraulic cylinders driving the hydraulic motor, and by varying the applied torque from the generator to match the wave conditions. Finally, it is investigated whether adding high-pressure pathways to the cylinder pressure chambers is beneficial for maximum power point tracking with reactive control. The analysis is conducted through a numerical model developed in Simulink and verified by comparison to the experimental setup supplied by Floating Power Plant. The study finds that a continuous valve switching strategy is optimal compared to end-point switching and reactive control with high-pressure pathways