Model Predictive Control of Hydraulic PTO for the ISWEC

In 2015, world leaders agreed to 17 Global Goals. The objective of these goals is to create a better world by 2030. Among the 17 goals, the seventh entails obtaining cheap, reliable, and most importantly, sustainable energy. The reason sustainable energy is most important is because our dependence on fossil fuels is unsustainable and harmful to the planet. This is why, new renewable energy sources must be discovered and developed. Among the solutions already in place and developed, wave energy is an excellent alternative as about 70% of the earth’s surface is covered by oceans. ENEA and RSE show that the amount of electricity that could be produced from wave motion is about 80 trillion (KWh), which would correspond to five times the annual energy needs of the entire planet. There are various types and forms of devices that convert the energy content of waves into electrical energy known as Wave Energy Converters. In this thesis the WEC analysed is the ISWEC; it consists of a floating and sealed hull inside which gyroscopic systems intercept the pitch motion caused by the impact of the waves on the hull and transfer the torque produced to the generator unit by a PTO. Different PTO technologies exist in literature; in this evaluation, a hydraulic PTO was considered. The choice was guided by the advantages a hydraulic PTO offers, such as increased performance, reduced size and weight for sites handling high power production. Its controllability allows for greater variability in the sea site where WEC may be used. The control systems used for the PTO analysed in this work are two and they work in parallel. A proportional control that uses torque applied to the generator and is proportional to the speed, and a Clutch-declutch control system that uses a valve to inhibit the energy conversion performed by the PTO under certain conditions. The aim of this thesis is to investigate the optimisation of clutch-declutch valve control using Model Predictive Control, improving the productivity of the device. The thesis is divided into three main sections. In the first one, an introduction is given about the different technologies used to obtain energy from wave motion, the different components that make up the ISWEC are explained and the equations needed to model it are given. The second part of the thesis shows an analysis of the PTO by evaluating the electrical power that can be extracted for different combinations of control parameters. In the third part of this thesis, the MPC logic is introduced and explained, along with two different formulations. This last part is most significant in terms of results, two different formulations. The first one is simplified and the second takes into account the valves’ pressure drops inside the PTO. A detailed analysis of influence of the prediction horizon and time-step is also reported. Several tests have been carried out considering irregular waves, the characteristics of which are based on the Alghero sea site. In addition, values are collected in terms of electrical power, total device efficiency and annual extractable electrical energy. These studies consider the different occurrences of waves throughout the year. The latter simulations have been performed using the classical clutch-declutch valve control with threshold value and employing the MPC algorithm, all done for the formulation with and without pressure drop. Finally for some sea conditions, the stress evaluation of the valve is reported.