Abstract
Constructive interference between tidal stream turbines in multi-rotor fence configurations arrayed normally to the flow has been shown analytically, computationally, and experimentally to enhance turbine performance. The increased resistance to bypass flow due to the presence of neighbouring turbines allows a static pressure difference to develop in the channel and entrains a greater flow rate through the rotor swept area. Exploiting the potential improvement in turbine performance requires that turbines either be operated at higher tip speed ratios or that turbines are redesigned in order to increase thrust. Recent studies have demonstrated that multi-scale flow dynamics, in which a distinction is made between device-scale and fence-scale flow events, have an important role in the physics of flow past tidal turbine fences partially spanning larger channels. Although the reduction in flow rate through the fence as the turbine thrust level increases has been previously demonstrated, the within-fence variation in turbine performance, and the consequences for overall farm performance, is less well understood. The impact of turbine design and operating conditions, on the performance of a multi-rotor tidal fence is investigated using Reynolds-Averaged Navier-Stokes embedded blade element actuator disk simulations. Fences consisting of four, six, and eight turbines are simulated, and it is demonstrated that the combination of device- and fence-scale flow effects gives rise to cross-fence thrust and power variation. These cross-fence variations are also a function of turbine thrust, and hence design conditions, although it is shown simple turbine control strategies can be adopted in order to reduce the cross-fence variations and improve overall fence performance. As the number of turbines in the fence, and hence fence length, increases, it is shown that the turbines may be designed or operated to achieve higher thrust levels than if the turbines were not deployed in a fence configuration.
Highlights
Article Highlights Designing tidal turbines for closely spaced operation can improve overall fence performance. Cross-fence variations in blade loads and power arise in short fence configurations. The magnitude of the cross-fence variations depends on fence length and turbine operation. Differential blade pitch control can be used to mitigate the effect of the variations and further improve array performance.A key difference between wind turbines and tidal stream turbines is that tidal stream turbines generally lie in close proximity to the flow passage boundaries, such as the seabed, sea surface, as well as the presence of neighbouring turbines
Garrett and Cummins (2007) demonstrated that the maximum power coefficient can increase by a factor of (1-BL)−2 above the Betz limit if a sufficient level of resistance is applied to the flow by a uniform fence of tidal turbines
Schluntz and Willden (2015) used actuator line and blade element actuator disk simulations to investigate the effect of rotor design on turbine power, and showed that turbine performance is improved in high blockage configurations if the turbines are designed or operated to harness the increased static pressure difference
Summary
Experimental work on fences of actuator disks partially spanning a tidal channel by Cooke et al (2015) demonstrated that cross-fence variations in thrust and power develop as a result of the changing incident flow conditions across the fence. This cross-fence variation, in which turbine power reduces towards the ends of the array, can be significantly detrimental to the overall farm power for short fences of tidal turbines.
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