Wake characterization of tidal turbines in the Pentland Firth using vessel-mounted ADCP measurements

Abstract

Knowledge of the extension and velocity deficit induced by tidal turbine wakes is crucial for the optimization of tidal farm layouts, as the wake induced by an upstream turbine may substantially affect the power and loadings of a device located downstream. The MeyGen project is the largest planned tidal stream energy project in the world: it aims to develop up to 398 MW of installed power in the Pentland Firth, Scotland, a site with current velocities reaching up to 5 m/s. During Phase 1 of the project, four 1.5 MW turbines were installed, providing a valuable opportunity to investigate turbine wake dynamics in full scale. As part of Phase 2, site characterization campaigns were carried out to plan the deployment of additional turbines, representing 28 MW of tidal power capacity. Hence, the influence of the existing devices on the downstream flow was investigated.
 This work introduces the method used for studying wakes downstream tidal turbines using vessel-mounted Acoustic Doppler Current Profiles (ADCP) measurements. During two spring tides, the data was collected using an ADCP Teledyne Workhorse 600 kHz configured for along-beam velocities recording with 1 m resolution, as well as a GNSS Hemisphere Vector V102. The aim of this study was to map the flow downstream the turbines already installed on site, in order to identify and characterize the wake. Cross-flow transect measurements were conducted at various along-flow distances downstream from the devices, and repeated several times for improved accuracy. The cycles of repeated transects were performed both at flood and ebb tides, with the turbine running or switched off to spot potential differences introduced by device operation. Mean flow velocity estimates in the cross-section were obtained from raw data using the location-based velocity solver developed by Vermeulen et al. (2014). The method provides improved flow velocity estimates from vessel-mounted ADCP measurements because it strongly reduces the spatial extent over which flow homogeneity must be assumed, thus decreasing the chances that this assumption will fail. The reduction of the volume across which homogeneity must be achieved may represent a significant advantage for the investigation of tidal turbine wakes. To the authors’ knowledge, this is the first time that this method has been applied to wake characterization in a tidal stream energy site.
 Outputs of the analysis include mean velocity magnitude and direction, as well as velocity deficits associated with the wakes. The mean velocity estimates obtained in each cross-section are compared for cases when the turbine was running or switched off. Findings reveal that vessel-mounted ADCP transects, coupled with the location-based methodology for velocity estimation, provide a powerful tool for tidal turbine wake characterization. The final presentation and paper will provide the results of this study.