Tidal energy has the potential to play a significant part in the renewable electricity generation mix towards meeting green house gas emission reduction targets. To harness tidal energy in a reliable and cost effective way, it is critical to capture and understand the interaction between Tidal Energy Converters (TECs) and the complex tidal stream flow fields, involving large scale turbulence and waves. Hence, accurate flow measurements at TEC hub height, generally midwater column, is essential. Tidal flow measurements are typically performed using off-the shelf oceanographic instruments, thereafter referred as Diverging-beam Acoustic Doppler Profilers (D-ADPs). D-ADPs remotely capture flow speed along each of their diverging acoustic beams. Along-beam velocity is converted to 3D Cartesian components of velocity using the assumption of flow homogeneity between acoustic beams. However, this assumption does not hold at the region of interest for TEC applications, when measuring highly energetic and turbulent flow fields such as tidal currents, resulting in high uncertainty in the measurements.
This thesis investigated a novel sensor concept: the Converging-beam Acoustic Doppler Profiler (C-ADP). The C-ADP is made of multiple spatially separated single-beam Acoustic Doppler Profilers (SB-ADPs), whose acoustic beams are geometrically converging. This enables highresolution measurement at the focal point, where the geometrically converging acoustic beams intersect.
Original tools enabling acquisition of 3D velocity from ADP systems of any geometry are provided. Tank testing of a SB-ADP unit and C-ADP of varying geometry against a reference laboratory instrument achieved a robust assessment of individual unit and full system performance. Testing of the SB-ADP shows that its performance improves with faster flow speeds and bias in mean 1D velocity has a maximum of 2% along the entire profile for flow speeds higher than 0.8 ms-1. Test of the full C-ADP system showed its capability to derive 3D key metrics typically used for tidal flow characterisation along the entire tank profile. Good agreement between the C-ADP and the reference instrument was found at the beam intersection, independently of the C-ADP geometry. Agreement was shown to decline with distance to the beam intersection, demonstrating improved measurement obtained with intersecting beams over spatially separated beams. This highlights limitations of traditional diverging systems, and the improved measurement that can obtained using C-ADPs when measuring non-homogeneous flow fields.
C-ADP tank testing enabled a robust assessment of its performance in a controlled environment, demonstrating its advantage over traditional D-ADPs. It is also a step towards de-risking the C-ADP technique towards field deployments to perform advanced flow measurement at TEC deployment site. Furthermore, a growing use of both SB-ADP and C-ADP as new profiling tools for fast multi-point velocity measurement in the context of tank testing is anticipated. Improved sensor technique for both better understanding of TEC deployment site characteristics and faster TEC tank testing at scale is set to make TEC more reliable and cost effective, ensuring their contribution towards the electricity generation mix
