Abstract
The results from an experimental study of a model diffuser augmented tidal stream turbine are presented with a particular focus on the impact of the diffuser upon the turbine׳s performance in yawed flows. This study is the first to examine yaw effects with quantification of blockage corrections and is the first study of the wake recovery characteristics of such devices. The device was designed using an innovative optimisation procedure resulting in a diffuser that was able to maintain the turbine׳s performance to yaw angles of up to ±30°. It is shown that the diffuser׳s performance is strongly influenced by its length to diameter ratio and by the jet flow that develops through the turbine׳s tip gap. Although the performance characteristics of an individual turbine can be significantly improved by diffuser augmentation under yawed flow, a wake recovery rate that is less than half that of a bare rotor raises doubts about their suitability for array deployment.
Highlights
It is estimated that the UK could produce 15–20% of its electricity demand from tidal resources (Callaghan, 2006)
Research into the yaw performance of diffuser augmented wind turbines has been presented by a number of authors (Kogan and Seginer, 1963; Foreman and Gilbert, 1979; Igra, 1981; Phillips, 2003)
A generic diffuser geometry has been derived using an optimisation methodology based around Kriging surrogate modelling and a genetic algorithm
Summary
It is estimated that the UK could produce 15–20% of its electricity demand from tidal resources (Callaghan, 2006). Secondary characteristics which affect the power generation capabilities of diffuser augmented tidal stream devices, such as performance in yawed flows and wake recovery have not been investigated . Research into the yaw performance of diffuser augmented wind turbines has been presented by a number of authors (Kogan and Seginer, 1963; Foreman and Gilbert, 1979; Igra, 1981; Phillips, 2003). The wake of a diffuser augmented turbine has received little attention, with the experimental and numerical work of Abe et al (2005) and Ohya et al (2012) respectively, on the flow within the cavity of a flanged diffuser structure and that of Grumman Aerospace (Oman et al, 1977) on the exit plane flow of a multi-slotted diffuser being the only studies to the author's knowledge. The implications of these properties of diffuser augmented devices on power generation are discussed
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