Piston control for an Integral Compression Wind Turbine.
PhD thesis, University of Nottingham.
This thesis concerns an analysis of an Integral Compressed Air Wind Turbine (ICWT), in which energy is extracted from a slow-moving renewable source through the use of compressed air. This concept is particularly applicable to large offshore wind turbines, and can be readily integrated with compressed air energy storage methods. The ICWT has a very large rotor with free pistons travelling within the rotor blades, inducting and compressing air to high pressures at each end of the stroke. The compressed air can be stored and expanded when the energy is required, solving the intermittency issue of wind energy. By gathering energy along the rotor blades, rather than at the hub, it also avoids the very high torques associated with extremely large turbines.
This thesis investigates optimal control strategies for ICWTs. Firstly, an initial model of the system using coupled ordinary differential equations (ODEs) is constructed to simulate a single piston pair of an ICWT system. This framework utilises several `modes' which the system passes through in the course of each stroke, with movement between modes controlled by simple algorithms. Calculations of potential and required energy are developed to allow basic control of the valve timings.
The simulation is then extended to include thermal modelling of the walls of the compression tube, using orthonormal polynomials. A long-duration instance of the model is used to identify steady-state values for the orthonormal parameters, which demonstrates that the wall temperatures would remain within 15~K of the ambient temperature.
One possible solution to the high temperatures caused by the near-adiabatic conditions of the compression is added to the model; namely, the injection of water droplets to the cylinder at the start of the compression stage. A method to efficiently simulate a phase transition in MATLAB is developed and implemented, allowing an analysis of the optimum mass balance of water to be injected to reduce the exhausted air temperature. An appendix examines several of the assumptions built into the model, in particular the rigidity of the components and variations in the rotational velocity of the rotor due to Coriolis and gravitational forces.
Two valve control schemes are developed and implemented into the model; firstly, a simple proportional and derivative controller, which acts according to a rule dictating a gradual reduction in the energy surplus. This option proves to be limited in the degree to which it can avoid wasting compressed air. A second scheme, involving a simple version of sliding-mode control with two controllers operating at different timescales, is instead developed and shown to be significantly more effective at improving the system efficiency.
Finally, an optimisation study is carried out on the `kick' stage, in which stored compressed air is used to accelerate the piston before compression. A large dataset of simulations allows for the specification of a set of optimum parameters based on a balance between power extraction from the rotating frame and net power generation.
Thesis (University of Nottingham only)
||Wind turbines, compressed air, storage
||T Technology > TJ Mechanical engineering and machinery
||UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
||07 Sep 2015 13:12
||14 Sep 2016 03:39
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