Abstract
Harnessing the energy of tidal currents has huge potential as a source of clean renewable energy. To do so in a reliable and cost effective way, it is critical to understand the interaction between tidal turbines, waves, and turbulent currents in the ocean. Scaled testing in a tank test provides a controlled, realistic, and highly reproducible down-scaled open ocean environment, and it is a key step in gaining this understanding. Knowledge of the hydrodynamic conditions during tests is critical and measurements at multiple locations are required to accurately characterise spatially varying flow in test tank facilities. The paper presents a laboratory technique using an acoustic velocimetry instrument, the range over-which measurements are acquired being more akin to open water applications. This enables almost simultaneous multi-point measurements of uni-directional velocity along a horizontal profile. Velocity measurements have been obtained from a horizontally mounted Single Beam Acoustic Doppler (SB-ADP) profiler deployed in the FloWave Ocean Energy Research Facility at the University of Edinburgh. These measurements have been statistically compared with point measurements obtained while using a co-located Acoustic Doppler Velocimeter (ADV). Measurements were made with both instruments under flow velocities varying from 0.6 ms−1 to 1.2 ms−1, showing that flow higher than 1 ms−1 was more suitable. Using a SB-ADP has shown the advantage of gaining 54 simultaneous measurement points of uni-directional velocity, covering a significant area with a total distance of 10 m of the test-tank, at a measurement frequency of 16 Hz. Of those measurement points, 41 were compared with co-located ADV measurements covering 8 m of the profile for a tank nominal flow velocity of 0.8 ms−1, and four distributed locations were chosen to to carry out the study at 0.6 ms−1, 1.0 ms−1, and 1.2 ms−1. The comparison with the ADV measurement showed a 2% relative bias on average.
Highlights
The exploitation of marine energy to support the transition to a low carbon future requires an extensive understanding of the harsh and complex ocean environment
The Acoustic Doppler Velocimeter (ADV) data used from the comparison at 0.8 m/s were collected during the SuperGen Marine tidal array project [8,29], the remaining ADV data and all the Single Beam Acoustic Doppler (SB-Acoustic Doppler Profilers (ADPs))
The results have shown that the SB-ADP estimates the velocity with a difference in mean velocity varying from close to 0% to up to 2% of the velocity measured by the ADV for flow velocities of 0.8 ms−1 to 1.2 ms−1, while, at 0.6 ms−1, the difference varies between 0.1% and 5%
Summary
The exploitation of marine energy to support the transition to a low carbon future requires an extensive understanding of the harsh and complex ocean environment Such an understanding facilitates the efficient design, cost-effective install, and reliable operation of tidal turbines. Test tanks that can recreate waves and currents at scale (representing the open ocean environment in a controlled, realistic, and highly reproducible manner) enables turbine testing to be performed at small scales (∼1/30–1/15). This supports companies and their investors without the need to make the significant financial commitments needed to construct and deploy full-scale devices [1,2]. Further experimental studies have been carried out in order to assess the flow, loads, and performance of turbines in second generation arrays [7,8]
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