The Generation Potential of Diffuser Augmented Tidal Stream Turbines

Abstract

This thesis investigates the power generation potential of diffuser augmented horizontal axis tidal stream turbines. The use of diffuser augmentation for such devices is not a new concept, though many questions remained unanswered regarding the device performance and economics. This work highlighted a number of limitations within the literature on such devices and aimed to clarify the main factors which affect their performance and economics in tidal stream flows.

Extant numerical modelling methods for diffuser augmented turbines were shown to produce inaccurate power prediction results due to modelling simplifications. The accurate numerical modelling of diffuser augmented turbines was made possible by the derivation of an extended blade element momentum turbine model within computational fluid dynamics. This model was verified experimentally and found to reproduce the experimental data accurately, matching both power outputs and diffuser surface pressures.

The investigation was undertaken using a combination of numerical modelling and wind tunnel experimentation. It was found that effective diffuser augmentation requires a rotor with a thrust of approximately 1/2 that of a bare rotor. The rotor geometry was found to have a significant impact upon the boundary layer flows and therefore the diffusion and power output. The impact of the hub and the component geometry interactions were also highlighted. It was also found that diffuser augmentation allows sustained performance under yawed flows and potential for improvement in highly turbulent flow. It was shown that diffuser augmented devices with lower area ratios are more efficient at power capture.

An economic analysis was performed, combining a derived cost model and an annual energy production analysis using real tidal stream data, incorporating flow yaw and turbulence effects. It was shown that using current technology, bare rotor type devices are the more economically viable means of power generation.