Characterization of the wake generated downstream of a MW-scale tidal turbine in Naru Strait, Japan, based on vessel-mounted ADCP data

Abstract

With tidal energy demonstration projects with one or a small number of turbines having provided very positive results, the technology is moving to the commissioning and operation of tidal energy farms. For this next step, the understanding of the wakes generated downstream of the turbines is crucial to optimize the array performance. To date, the analysis of wakes of MW-scale tidal turbines has been made by numerical methods or experimentally with downscaled rotors. However, no consensus on a methodology to characterize wakes generated by full-scale turbines based on data measured on-site has yet been reached. The present paper introduces a new method to compare current velocity data measured before and during turbine operation that minimizes the impact of the spatial and temporal variability of tidal currents, thus enabling the estimation of the velocity deficit caused downstream of the turbine. Through this method, a characterization of the near wake was possible, with velocity deficits of 0.537, 0.463, 0.469 and 0.431 at 2D, 3D, 4D and 5D from the turbine. Results from this paper present a very valuable tool for the validation of numerical models aiming to estimate the wake losses in tidal energy farms.