Blade Element Momentum Theory (BEMT) is a computationally efficient method of calculating the performance of a tidal stream turbine (TST) generating energy from the ocean. This efficiency is achieved by making several simplifying assumptions; an unintended consequence of these assumptions is the omission of some phenomena that can significantly alter the performance and loads of a TST. We can ameliorate this by incorporating suitable corrections into a BEMT model, which allow us to account for some of the effects of these phenomena. This paper examines the implementation of corrections in an established BEMT solver for two such phenomena: tip/hub losses and high induction conditions.
Tip losses are attributable to the flow of fluid around end of the blade, a flow feature omitted in the classical BEMT treatment of turbines. At high tip speed ratios, above the designed operating range of the device, the theory based on an axial interference factor, a, diverges from experimental results and, indeed, becomes physically untenable. Buhl proposed a high induction correction factor for wind turbines operating in air and a modified version of his correction is implemented here for a TST operating in water. The tip/hub loss and high-induction corrections are well-integrated with one another. The validity of the high-induction correction is checked against experimental results; we find that our model predicts power output well but overpredicts axial thrust compared to laboratory observations.