Airborne Wind Energy Systems (AWES) aim to capture the wind energy potential at high altitudes by using a combination of tethers and bespoke aerofoils. In contrast to conventional horizontal axis wind turbines (HAWT), the tower is replaced with lightweight tethers resulting in a reduction in both the overall mass, and more importantly cost of the system. Currently, there is significant interest from a number of key stakeholders, both academic and industrial, aiming to optimise an airborne wind energy design that captures the wind energy resource found at high altitudes. Two key issues will drive the development of these systems, flight stability and power maximisation. Therefore, the control strategy for these systems will be imperative for reducing costs and optimising system performance. Through collaboration with Altaeros Energies, this thesis addresses the outstanding stability and performance optimisation for a specific AWES known as the Buoyant Airborne Wind Turbine (BAWT). There are three key contributions within this work. Firstly, a comprehensive literature review of different airborne systems is provided with specific consideration given to power optimisation and dynamic stability. This results in a detailed understanding of the BAWT plant model through the introduction of two force ratio’s relating the buoyancy contribution to the aerodynamic contribution on system loads across the operating envelope. The model development is then expanded on to discuss the question of system stability and power optimisation. This is addressed via the development of a hierarchical control structure for the BAWT, which is broken into three distinct regions, low level control, medium level control and high level control. At the lowest level, flight stability, which is vital to providing optimum conditions for energy generation, is guaranteed using a multivariable controller.This is carried out through the development of a PID controller using two methods, a frequency domain method known as MPID and an optimal control scheme, LQR. The results of this chapter inform the interaction of the controller with the underlying plant dynamics. Finally, the broader issue of BAWT optimisation is addressed by implementing a hierarchical control architecture which builds upon the multivariable flight stability controller developed in Chapter 5. Medium level control is implemented using a hierarchical model predictive control scheme (MPC) which provides set-points to the low level controller in roll, pitch and altitude. These set-points are provided such that they are bounded within the defined envelope of operation to ensure that loads on the shroud are not increased beyond acceptable levels i.e. extreme tether loads due to high altitudes. The question of power optimisation is then addressed through the formulation of an Extremum seeking control (ESC) scheme which derives an optimal altitude for the system. This altitude is determined by trading off generated power from the rotor against power losses incurred by reeling the tether in/out at high wind speeds. Implementing a hierarchical control scheme of this type provides an example of how different control techniques can be combined to provide a degree of self-regulation whilst simultaneously providing system stability and power optimisation. Ultimately, this will increase autonomous operation of the BAWT which will help to reduce system costs and make this technology more viable in a competitive marketplace.
|Date of Award||5 Oct 2018|
- University Of Strathclyde
|Sponsors||EPSRC (Engineering and Physical Sciences Research Council)|
|Supervisor||Reza Katebi (Supervisor) & James Biggs (Supervisor)|