Control of Tethered Airfoils for High-Altitude Wind Energy Generation - Advanced control methods as key technologies for a breakthrough in renewable energy generation [Doctoral dissertation - Ph.D. in Information and System Engineering - Ciclo XXI - Politecnico di Torino]

This thesis is concerned with the development of an innovative technology of high-altitude wind energy generation and with the investigation of the related advanced automatic control techniques. Indeed, the problems posed by the actual energy situation are among the most urgent challenges that have to be faced today, on a global scale. One of the key points to reduce the world dependence on fossil fuels and the emissions of green-house gases is the use of a suitable combination of alternative and green energy sources. Renewable energies like hydropower, biomass, wind, solar and geothermal could meet the whole global energy needs, with minor environmental impact in terms of pollution and global warming. However, they are not economically competitive without incentives, mainly due to the high costs of the related technologies, their discontinuous and nonuniform availability and the low generated power density per unit area. Focusing the attention on wind energy, recent studies showed that there is enough potential in the total world wind power to sustain the global needs. Nevertheless, such energy can not be harvested by the actual technology, based on wind towers, which has nearly reached its economical and technological limits. The first part of this dissertation is aimed at evaluating the potential of an innovative high-altitude wind energy technology to overcome some of these limitations. In particular, a class of generators denoted as HAWE (High Altitude Wind Energy) is considered, which exploits the aerodynamic forces generated by the flight of tethered airfoils to produce electric energy. Numerical simulations, theoretical studies, control optimization, prototype experiments and wind data analyses are employed to show that the HAWE technology, capturing the energy of wind at higher elevation than the actual wind towers, has the potential of generating renewable energy available in large quantities almost everywhere, with a cost even lower than that of fossil energy. Though the idea of exploiting tethered airfoils to generate energy is not new, it is practicable today thanks to recent advancements in several science and engineering fields like materials, aerodynamics, mechatronics and control theory. In particular, the latter is of paramount importance in HAWE technology, since the system to be controlled is nonlinear, open loop unstable, subject to operational constraints and with relatively fast dynamics. Nonlinear Model Predictive Control techniques offer a powerful tool to deal with this problems, since they allow to stabilize and control nonlinear systems while explicitly taking into account state and input constraints. However, an efficient implementation is needed, since the computation of the control input, which requires the real-time solution of a constrained optimization problem, can not be performed at the employed “fast” sampling rate. This issue motivates the research efforts devoted in the last decade to devise more efficient implementations of predictive controllers. Among the possible solutions proposed in the literature, in this thesis Set Membership theory is employed to derive off-line a computationally efficient approximated control law, to be implemented on-line instead of solving the optimization. The second part of this thesis investigates the methodological aspects of such a control strategy. Theoretical results regarding guaranteed approximation accuracy, closed loop stability and performance and constraint satisfaction are obtained. Moreover, optimal and sub-optimal approximation techniques are derived, allowing to achieve a trade-off between computational efficiency, approximation accuracy and memory requirements. The effectiveness of the developed techniques is tested, besides the HAWE application, on several numerical and practical examples.