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.
Highlights
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 inner two turbines are controlled together, and it is sought to minimise the difference in turbine thrust coefficient whilst maintaining the reduction in fence power to within 1% of the maximum when all turbines operate at λ = 5.6
Summary
Nishino and Willden (2012) and Vogel et al (2016) developed analytical models of fences of tidal turbines partially spanning a channel with non-deforming and deforming free-surfaces respectively, and showed that reducing inter-turbine spacing resulted in increased the fence power coefficient. These two scale models distinguish between the global blockage BG, the ratio of the swept area of all turbines to the channel cross-sectional area, and the local blockage BL, the ratio of the swept area of a single turbine to the surrounding flow passage bounded by neighbouring turbines. Reductions in fence thrust have the additional benefit of reducing turbines loads and may increase the turbine power to thrust ratio
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