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 1/20 th 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.

Characterisation of Tidal Flows at the European Marine Energy Centre in the Absence of Ocean Waves

“…These include information varying across annual and seasonal time scales to fluctuations in velocity at time scales of seconds and below, with different scales of motion understood to have varying effects on energy extraction devices [10]. Similarly, knowledge of the spatial variation of flow parameters is required across a wide range, from orders of tens of blade diameters for, e.g., power curve production [11] to variations of metres and below for investigations into device loading and blade fatigue [12,13] and quality of power [14]. No single instrument system at present can economically provide the measurements required to meet the multi-scale nature of tidal flows.…”

Tidal energy “Round Robin” tests comparisons between towing tank and circulating tank results

“…As shown in section 2.5, the acquisition time may also be responsible for some few percent of variation: 0.3 to close to 4% depending on the tanks, from peak-to-peak results. In addition, flow velocities (or accuracy of the carriage speeds) were not measured during these tests; as 300 shown by [20], power and thrust measurements are strongly sensitive to the estimation of the ambient velocity for flume tanks.…”

“…It is likely that a better 355 characterization of the turbulence inflow, also in term of frequency and scale distribution is necessary to fully and accurately investigate that point [20].…”

“…The turbulence intensity is 3.0% at IFREMER and 2.5% at CNR-INSEAN, yet the standard deviations of power and thrust are in some cases twice as large. In order to investigate this discrepancy further, the flow turbulence should be charactered more fully, with reference to parameters such as turbulence length scales or dissipation rate (see [20]). …”

Re-Creating Waves in Large Currents for Tidal Energy Applications

“…Matching turbulence statistics or depth profiles to site equivalents was not possible at FloWave and is typically not achievable in any of the larger test facilities. The nature of the turbulence will have significant implications of TST loads and performance [28] and as such remains a major challenge if true similitude to site wave-current conditions is to be attained. It is, in certain facilities, possible to deploy turbulence-modifying grids as detailed and discussed in [29][30][31], and this approach has been used to vary turbulence in test conditions for TSTs [28,32].…”