Minimising turbine thrust variation in multi-rotor tidal fences
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RESEARCH ARTICLE
Minimising turbine thrust variation in multi-rotor tidal fences T. F. L. Stephenson1 · C. R. Vogel2 Received: 18 October 2019 / Accepted: 9 September 2020 / Published online: 20 September 2020 © The Author(s) 2020
Abstract Recent analysis of tidal stream energy devices has focussed on maximising power output. Studies have shown that significant performance enhancement can be achieved through the constructive interference effects that develop between tidal stream turbines by deploying them close together. However, this results in variation in the flow incident on the turbines and hence leads to thrust variation across the turbine fence. This may lead to varying damage rates across the fence with adverse impacts on operation and maintenance costs over the turbine lifetime. This study investigates strategies to reduce thrust variation across fences of tidal turbines using three-dimensional Reynolds-Averaged Navier–Stokes simulations. It is shown that the variation in turbine thrust across a fence of eight turbines can be reduced to within 1% with minimal impact on the fence power. Furthermore, by reducing the rotational speed of inboard turbines, or varying the blade pitch angle of the turbines across the fence, it is possible to reduce overall turbine loads and increase the power to thrust ratio of the turbines. Keywords Tidal stream turbines · Tidal fence · Reynolds-Averaged Navier-Stokes simulation · Blade element momentum theory · Actuator disk
1 Introduction Idealised representations of wind and tidal turbines as actuator disks that reduce the streamwise momentum of a flow have been used to establish theoretical upper bounds on performance. Betz and Prandtl (1919) demonstrated that the maximum power coefficient CP of a turbine in an unbounded flow normalised on the undisturbed kinetic flux through the rotor plane was CP,max = 16/27, achieved when the flow through the actuator disk was reduced to 2/3 of the freestream speed. Garrett and Cummins (2007) showed that the maximum power coefficient increased when flow expansion was constrained by the presence of lateral or vertical boundaries such as the sea bed, sea surface or channel walls. Garrett and Cummins proposed a new limit of CP,max = 16/27 (1 − B)−2 where the blockage ratio B is the ratio of the swept area of the turbine to the cross-sectional area of the flow passage surrounding the turbine. The flow around a turbine can also be affected by adjacent turbines which also act to constrain flow expansion.
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C. R. Vogel [email protected]
1
St Edmund Hall, University of Oxford, Oxford OX1 4AR, UK
2
Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK
The theoretical maxima described by Garrett and Cummins corresponds to a case in which a fence of turbines uniformly spans a channel (Garrett and Cummins 2007). However, turbine deployment constraints due to variations in bathymetry and restrictions due for environmental and other marine users will mean that turbine fences do not uniformly span t
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