King's College London
Engineering

Tidal turbines, and the exploitation of tidal energy, show significant promise in developing sustainable energy delivery. The levelised cost of energy (LCoE) is restrained by the maximum tip speed ratio (TSR) of tidal turbines, which is limited by tip vortex cavitation that will occur at increased turbine sizes or rotating speeds. Dr Yabin Liu will explore controlling tip vortex cavitation with micro jets based on the novel principle of jet-induced vortex breakdown. The micro-jets control, originally proposed by Dr Liu and validated to be able to reduce tip leakage vortex and associated cavitation by 50%-60% in turbomachinery under steady inflows, will be transferred to tidal turbines, as it has been found to be more efficient on unshrouded blades. A novel technology of permeable tip treatment will be proposed and developed to achieve the micro-jets control concept during this fellowship. This technology may overcome the TSR limit and significantly improve the mean power output of turbines.

However, as tidal turbines operate in gusty inflows, high-frequency unsteady loadings will occur at higher TSRs, which will decrease the fatigue life and disturb the jets control effect. The existing active control methods are unable to suppress high-frequency unsteady loadings due to the delayed blade response. Therefore, a passive pitch system will be developed to mitigate the high-frequency unsteady loadings that would be experienced at high TSRs, and that would interfere with the jets control. The micro-jets control and passive pitch control are complementary, and thus can simultaneously mitigate cavitation and unsteady loadings. Therefore, this proposal is timely to address these challenges. The potential decrease of LCoE may reach 35% according to one of the key industry partners, and this will be crucial for its commercialisation and competition with other energy sectors.

This research may result in a 5%-10% increase in the harvested tidal power, equivalent to hundreds of GWh/year only in the UK. Furthermore, the outcomes may inspire a novel methodology on controlling vortex- induced vibrations, cavitation and noises in wide applications, such as aerial/underwater vehicles and propellors, which may underpin academic activities over a 10-15 year timescale.