Design, Operation and Diagnostics of a Vertical Axis Wind Turbine

The need for sustainable energy sources becomes greater each year due to the continued depletion of fossil fuels and the resulting energy crisis. Solutions to this problem are potentially in the form of wind turbines which have been receiving increased support at a micro level. At present a number of wind turbines are being developed that are of cross-flow vertical axis operation which have shown significant increases in performance compared to existing technologies. From an extensive literature review a number of key issues have been highlighted which are concerned with design, operation and diagnostics of this new wind power technology which have been used to formulate the scope of this research.

A design procedure for a cross-flow machine that features both a multi-blade rotor and fixed outer stator guide vanes has been derived in which both rotor and stator blade profiles have been generated for a low wind speed urban application. Using these blade profiles a prototype wind turbine has been fabricated and used for full scale development testing.

In the presented work both experimental and numerical investigations have been carried out to determine the operational characteristics of this new technology. The experimental data obtained under controlled laboratory conditions has been used to validate a Computational Fluid Dynamic (CFD) model which has been used throughout. A flow field analysis of the machine has highlighted large asymmetries in both pressure and velocity about the central axis of the machine in both stationary and rotating frames of reference. This has identified primary inefficiencies within the design which limit the torque generating capability of the rotor due to blockage effects and downstream blade interactions. This asymmetry has been quantified in the form asymmetry ratio and used to determine downstream rotor effects and the optimum location of multiple wind turbines which is seen to be x/D >10 in order to minimize performance reductions.

The torque and power generation capabilities of the machine have been characterised at both 'design' and ‘offdesign' conditions in which individual blade torque contributions have been quantified. This has highlighted specific energy transfer zones within the turbine namely at a few key blades on the windward side of the rotor. It has also shown counter-rotating torques generated on the leeward side of the machine at specific blade positions during the cycle. Overall performance has been quantified in which a maximum CT = 1.7 and CP = 0.24 has been observed which has some similarities to the Savonius rotor. Geometric effects on torque and power response have been quantified in which a strong dependence on stator blade number is noticed. Further, maximum performance output of the machine is generated at the baseline design condition. Using torque response data a multiple regression model has been developed in which a design equation for crossflow rotor torque has been derived which can be used during the conceptual design phase.

Finally, the effectiveness of a two-dimensional transient CFD model to predict cross-flow wind turbine rotor blade loss has been evaluated against full scale experimental data. It has shown that from analysis in the frequency domain specific blade faults can be recognised which agrees well with experimental data obtained. The use of this model for wind turbine performance emulation has been described.