Inertial Sea Wave Energy Converters (ISWEC) convert waves motion into electricity exploiting the conservation of angular momentum principle. The energy transported by the waves depends mainly by the period and the height that change over the time. For this reason, in order to maximize the energy extracted by the WEC, the internal gyroscope rotation speed and the torque applied to the generator shall be continuously modulated so as to follow the sea state.
Proportional-Derivative (PD) control system with specific wave gain tuning has been tested demonstrating the limits on robustness of this simple approach with respect to the sea conditions. Advanced control algorithms, like Linear Quadratic Regulator (LQR) and Model Predictive Control (MPC), provides great alternatives to improve the robustness and the effectiveness of the control system.
ISWEC overall complexity and wear and tear bring uncertainties and approximations on the model parameters used by the control system that can limit the energy extraction performances. The approach described in this paper implements a Tube-based Robust Model Predictive Control, a class of robust controllers which are able to cope with external disturbances and uncertainties which affect the system. The concept of tube has been introduced in classical MPC in order to improve robustness of such controllers. Like the MPC, TRMPC is a linear model-based controller where the optimal control law is obtained by minimization of a cost function.
The robustness of the approach ensures that, unlike the PD, a unique set of control parameters for all the sea conditions considered guarantees the optimal energy extraction performance.
Renewable energy sources play a fundamental role in the decarbonization process of energy production. Among them, energy coming from the sea waves appear to be one of the most promising and with a significant potential.
The total wave power incident on the ocean-facing coastlines around the world is estimated to be 19,000 TWh/year, very close to world's electricity consumption (about 22,000 TWh/year). The amount of energy that can be potentially harnessed from the ocean and sea waves is of 1,500 - 7,500 TWh/year, corresponding to 10% and 25% of the world's yearly electricity demand (IRENA, 2020 Ocean Renewable Energy - Fostering a Blue Economy).
