Carwyn Frost

Effects of extreme wave-current interactions on the performance of tidal stream turbines

The main objective of this paper is to analyse extreme cases of wave-current interactions on tidal stream energy converters. Experiments were undertaken in the INSEAN tow tank facility where carriage speeds of 0.5 and 1m/s were used with and without waves. The waves studied in this testing campaign had wave heights of 0.2 to 0.4m with a 2s wave period in a stationary reference frame. These wave conditions were considered extreme cases considering the use of a turbine with a rotor diameter of 0.5m. The turbine was equipped with a torque transducer, an encoder and a strain gauge to measure both the rotor torque and the forces on a single blade root. The results of the experiments showed that extreme wave-current cases can result in significant variations in power. Investigating the time histories of the blade root loading in wave-current conditions illuminated the importance of the relationships between the wave phase and blade angular position, and the number of blade rotational periods in a wave period. These affected the loading patterns and also the loading range seen by the blade, both of which have important implications for the fatigue life of the blade.

Modelling tidal stream turbines

The transient behaviour of the sea and the rotation of a turbine rotor can result in high asymmetric loadings, which are transmitted to the drive shaft. A turbine mounted on a circular stanchion has been used to highlight the effects of introducing more realistic boundary conditions, over a rotational cycle of the turbine. The consequences on the turbine’s performance characteristics and crucial structural loading are shown. The position of the turbine relative to the support structure and its alignment to the flow direction can have significant temporal hydrodynamic and structural effects. Depending on their wavelength, waves can also have a significant effect on the overall design decisions and placement of devices. Thrust loading and bending moments applied to the drive shaft can be of the order of hundreds of kN and kNm, respectively. This leads to the need to not only size the drive shaft and bearings to account for axisymmetric thrust but also consider large asymmetric loads.

The Effect of Control Strategy on Tidal Stream Turbine Performance in Laboratory and Field Experiments

The first aim of the research presented here is to examine the effect of turbine control by comparing a passive open-loop control strategy with a constant rotational speed proportional–integral–derivative (PID) feedback loop control applied to the same experimental turbine. The second aim is to evaluate the effect of unsteady inflow on turbine performance by comparing results from a towing-tank, in the absence of turbulence, with results from the identical machine in a tidal test site. The results will also inform the reader of: (i) the challenges of testing tidal turbines in unsteady tidal flow conditions in comparison to the controlled laboratory environment; (ii) calibration of acoustic Doppler flow measurement instruments; (iii) characterising the inflow to a turbine and identifying the uncertainties from unsteady inflow conditions by adaptation of the International Electrotechnical Commission technical specification (IEC TS): 62600-200. The research shows that maintaining a constant rotational speed with a control strategy yields a 13.7% higher peak power performance curve in the unsteady flow environment, in comparison to an open-loop control strategy. The research also shows an 8.0% higher peak power performance in the lab compared to the field, demonstrating the effect of unsteady flow conditions on power performance. The research highlights the importance of a tidal turbines control strategy when designing experiments.