Abstract
An embedded Reynolds-Averaged Navier-Stokes blade element actuator disk model is used to investigate the hydrodynamic design of tidal turbines and their performance in a closely spaced cross-stream fence. Turbines designed for confined flows are found to require a larger blade solidity ratio than current turbine design practices imply in order to maximise power. Generally, maximum power can be increased by operating turbines in more confined flows than they were designed for, although this also requires the turbines to operate at a higher rotational speed, which may increase the likelihood of cavitation inception. In-array turbine performance differs from that predicted from single turbine analyses, with cross-fence variation in power and thrust developing between the inboard and outboard turbines. As turbine thrust increases the cross-fence variation increases, as the interference effects between adjacent turbines strengthen as turbine thrust increases, but it is observed that cross-stream variation can be mitigated through strategies such as pitch-to-feather power control. It was found that overall fence performance was maximised by using turbines designed for moderately constrained (blocked) flows, with greater blockage than that based solely on fence geometry, but lower blockage than that based solely on the turbine and local flow passage geometry to balance the multi-scale flow phenomena around tidal fences.
Highlights
The importance of the blockage ratio, the ratio of turbine swept area to the cross-sectional area of the flow passage surrounding the turbine, BL, was established in the context of tidal stream turbine performance by Garrett and Cummins [1]
The importance of the blockage ratio in determining the theoretical limit of power of a tidal turbine in a constrained flow passage was demonstrated by Garrett and Cummins [1], showing that the power coefficient increases by a factor of (1 − BL)−2 above the Lanchester-Betz limit if a sufficient level of resistance is applied to the flow
For a single device in a blocked flow passage, the implication is that a higher power coefficient than the Lanchester-Betz limit can be achieved if the device is designed and operated to utilise the additional streamwise static pressure gradient that develops in the flow passage in order to increase rotor torque, and power
Summary
The importance of the blockage ratio, the ratio of turbine swept area to the cross-sectional area of the flow passage surrounding the turbine, BL, was established in the context of tidal stream turbine performance by Garrett and Cummins [1]. This work investigates the role of flow phenomena that scale on the turbine diameter and array width, described by the local and global blockage ratios respectively, on overall multi-rotor fence power and thrust characteristics. Single turbines were hydrodynamically designed by varying blade twist and solidity ratios for operation under five different blockage conditions with a fixed tip speed ratio. Performance of the turbines was analysed for a range of different tip speed ratios and off-design blockage conditions These turbines were arranged in a cross-stream tidal fence arrayed normal to the flow direction in order to study the relative importance of global and local blockage ratios. The effect of pitch-to-feather power control on array performance was investigated
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
