Knowledge of the loading patterns associated with dynamic flows is needed to understand Tidal Stream Turbine (TST) array performance, fatigue life and reliability in order to drive the industry towards commercially viability. The introduction of unsteady flow (wave and turbulence) has been shown to alter the average power produced and the loading on the turbine. The systematic fundamental study proposed in this application is required to fully understand the dynamic loading of a single TST before studying array effects. This characterisation of the loading regimes faced by a single turbine is essential in validating early stage simulations and for making comparisons between the loading of singular and array mounted TSTs.
Numerical modelling also shows loadings become increasingly asymmetric within unsteady flows. This dynamic loading will be compounded in an array by the wake and potential focusing effects of upstream and lateral turbines. The interaction between the site conditions and array formation is not fully understood but such knowledge is required to ensure the survivability of the devices in such a harsh environment.
Dynamic Loading of Tidal Stream Turbines (DyLoTTA) is a collaborative British Research Council project between Cardiff University and the University of Strathclyde as well as further international and industrial collaborators. The Dylotta project aims to generate quality data sets relating to turbine operation under realistic flow conditions generated via the combination of wave, turbulence and turbine interaction effects. Fitting into a process of sequential inclusion of flow artefacts, this stage of testing was undertaken to provide high quality data relating to a single turbine under well prescribed wave conditions with minimal turbulence effects.
This report presents an overview of the activities associated with MARINET 2 access to the CNR-INSEAN wave towing tank facility for wave and current testing of a 0.9 m diameter horizontal axis tidal turbine (HATT) model.
Previous evidence [Ordonez-Sanchez et al, 2016] has demonstrated the effects of using control strategies on the loading fluctuations that the turbine is subjected to. Therefore, the testing undertaken during Marinet 2 aims to study the effects of normal, extreme and irregular waves on dynamic turbine loads under the speed and torque control types. This was done by initially studying the turbine characteristics under no wave tow tests for both speed and torque control operation. Secondly, a representative medium monotonic wave was generated of 0.1 m amplitude and 1.44 s period – again under torque and speed control setups. Next the turbine was tested under extreme wave conditions of representative specific of 0.2 m amplitude and 2.0s wave period. Lastly, the turbine was subjected to irregular waves generated in a manner which adhered to the JONSWAP wave spectrum with a significant wave height of 0.1 m and period of 1.44 s.
It was found that generally under the no wave conditions similar thrust loading and power generation were found for both the speed and torque control types. Under the wave cases a larger fluctuation in torque was observed for the speed control case. However, a larger fluctuation in rotor thrust loading was measured for the torque control cases. In terms of the extreme wave case, power fluctuations of up to 52 % of the mean power output were found for the speed control case, this compared with maximum power fluctuations of 33 % measured for the torque control case. Similarly, for the extreme wave case, 18 % and 40% rotor thrust fluctuations were observed, relative to the mean thrust, for the speed and torque control cases respectively.