Effects of turbulence on tidal turbines: Implications to performance, blade loads, and condition monitoring

Abstract

Laboratory scale testing of tidal turbines has generated valuable datasets to support optimised turbine design and numerical model validation. However, tidal sites are highly turbulent with a broad range of length scales and turbulence intensities that are site specific. In this work we describe an experimental campaign using static grids to generate turbulence and investigate its impact on a model tidal turbine in a circulating water flume. Length scales, energy spectra and turbulence dissipation rates are first considered for centre point measurements before full flow characterisation of the ambient conditions across the turbine rotor area. Six different cases were chosen to observe the performance of a 1/20th scale 0.8m diameter turbine subjected to these flows. The rotor thrust and torque, and flapwise and edgewise blade root bending moments were measured. It was found that the thrust and power coefficients were sensitive to the estimate of ambient velocity. In the most extreme case the Betz limit could be ‘exceeded’ depending on which estimate of ambient velocity was used. Overall variations in the peak power coefficient of over 10% were observed, demonstrating the significance turbulence has on turbine performance. It was also found that there is a strong correlation between fluctuations in blade root bending moments and the rotor loads. As a result we proposed that fatigue loads acting on the blades may be estimated from the fluctuations in power output of the turbine. Therefore maintenance operations maybe optimised from real-time fatigue monitoring of blade loads without the need to install additional instrumentation on the turbine blades. Under this proposed regime the cost of energy will be reduced due to reductions in turbine costs and following optimisation of the maintenance requirements and operational costs. This could also improve turbine reliability which would have significant implications for large multi turbine arrays.