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Loss Mechanisms in Tidal Stream Turbines
This paper explores the loss mechanisms critical to the operation of a tidal stream turbine. There is an upper limit to the amount of power that may be removed from a tidal stream. The designer may therefore aim to minimise the ratio of loss to useful power. Computational predictions were undertaken on a horizontal axis turbine. At design point, the total loss for this case was 69.2% of the useful power extracted. This may be broken down to different sources: rotor profile loss, structural loss, and wake mixing loss. Wake mixing loss is shown to dominate. It is shown that in addition to the ‘idealised’ radial variation of velocity through the wake there was also significant circumferential variation. This circumferential variation is responsible for ~one third of total wake mixing loss, while the remaining two thirds is due to radial variations. This result implies that wake mixing loss could be reduced by designing turbines which produce wakes with lower circumferential non-uniformity