Role of preload in determining the performance enhancement of tidal turbines by passive pitch

In this study, we present results from an experimental and analytical study on a turbine equipped with three independent passively pitching blades and show the effect of different levels of spring preload. Results demonstrate that passive pitch blades can produce the same power and thrust as a fixed pitch turbine at any design condition. If the preload is set below the optimum value, the power captured is reduced, but the thrust load is reduced by a larger amount, leading to an increase in hydrodynamic efficiency. Setting the preload too high results in increased thrust and reduced power at the turbine design point. At off-design conditions, the passive pitch turbine with optimal preload shows reduced thrust compared with the fixed pitch turbine while matching the power output, giving an efficiency increase of up to 20%. Therefore, passive pitch can be used to improve efficiency when working at off-design conditions. The passive pitch system is also shown to mitigate load fluctuations at frequencies below the turbine rotational frequency. Results from the analytical study show that blade element momentum theory can be used to model the dynamics of a passive pitch turbine, and it is therefore an appropriate design tool for such systems. • Passive pitch is tested on a model turbine with different levels of spring preload. • With optimum preload, passive pitch system matches design point power of fixed pitch. • At off-design conditions, the thrust is reduced while the power output is matched. • Load fluctuations at frequencies below the blade rotational frequency are mitigated. • BEMT captures the system dynamics and gives insights into blade behaviour.