A note on the tuning of tidal turbines in channels

Abstract

Maximising the performance of a tidal turbine array requires the turbine resistance to be 'tuned' for a given channel and turbine arrangement. In many cases, tuning the turbines to produce the maximum power coefficient presents too great a resistance to the incoming flow and results in a lower power output. Recent studies have shown that, as compared to using a fixed tuning, considerably more power can be produced by varying the turbine resistance over the tidal cycle. To our knowledge, however, this higher power output has only been demonstrated for highly idealised models, which use an additional drag force or a uniformly porous disc to represent the turbines. In this study, we re-examine these tuning strategies using a more realistic turbine model, which is based on the blockage-corrected blade element momentum theory. We find that, as compared to the idealised actuator disc, the importance of tuning is significantly reduced for the more realistic tidal rotor, and that this is particularly true for rotors with fixed blade pitch. We also find that the maximum amount of power produced by the blade element momentum rotor, averaged over the tidal cycle, is typically 60-70% of that produced by the actuator disc.

A Note on the Effects of Local Blockage and Dynamic Tuning on Tidal Turbine Performance

“…Figures 7 and 8 also show that the amount by which dynamic tuning can enhance P av(max) increases both with B L for a given N, and with N for a given B L . These results are consistent with previous works on turbine tuning, which have shown that the importance of tuning increases with the potential for turbines to exert thrust on the incoming flow and hence affect their own performance [15,18]. The results are also consistent with present understanding of array performance: as B L increases (and B A reduces) for a given N (or as N increases and B A reduces for a given B L ), an array-scale bypass flow is introduced and a reversal boost dynamic is established [8].…”

“…As before, different turbine arrangements are considered and the maximum time-averaged available power is compared between the different turbine models. This part extends not only the work of Bonar et al [8] to include dynamic tuning, but also the work of Chen et al [15] to include partial-width turbine arrays. Although the computational demands of the numerical scheme used to calculate the optimal tuning curves limit the study to a single channel and small number of turbine arrangements, the present paper is, to our knowledge, the first to explore the combined effects of local blockage and dynamic tuning on tidal turbine performance.…”

“…Although the rotor provides a more accurate description of turbine performance than the disc, it retains some limitations that restrict its ability to model large blockage ratios and extreme off-design conditions. For large T SR, for instance, the rotor attains a high throughflow induction factor and the model begins to produce non-physical solutions as the wake flow becomes turbulent [15]. In the present study, such solutions are precluded by limiting B to values ≤ 0.3125 and T SR to a realistic range of operation: 3 ≤ T SR ≤ 8.5.…”

“…Although the variations in C P and C T are broadly similar, Fig. 2 shows that the magnitudes of C P and C T are lower for the more realistic rotor, which experiences both lift and drag forces, than for the idealised disc [15]. Figure 3 shows that the variations in η are also quite different.…”

“…Turbine thus the amount of power dissipated in wake mixing, so η decreases. For the fixed-pitch rotor, however, the relationship is more complex: as k increases from zero, η initially increases as the T SR increases and flow begins to attach to the surfaces of the blades, peaks at the design T SR, and then decreases as the T SR continues to increase and drag forces begin to predominate [15].…”