Response of Tidal Energy Converters to Combined Tidal Flow, Waves, and Turbulence

Tidal currents are known to have complex turbulent structures. Whilst the magnitude and directional variation of a tidal flow is deterministic, the characteristics of turbulent flow within a wave-current environment are stochastic in nature, and not well understood. Ambient upstream turbulent intensity affects the performance of a tidal turbine, while influencing downstream wake formation; the latter of which is crucial when arrays of tidal turbines are planned. When waves are added to the turbulent tidal current, the resulting wave-current induced turbulence and its impact on a tidal turbine make the design problem truly challenging. Although some very interesting and useful field measurements of tidal turbulence have been obtained at several sites around the world, only limited measurements have been made where waves and tidal currents co-exist, such as in the PFOW. Also, as these measurements are made at those sites licensed to particular marine energy device developers, the data are not accessible to academic researchers or other device developers.

Given the ongoing development of tidal stream power in the Pentland Firth, there is a pressing need for advanced in situ field measurements at locations in the vicinity of planned device deployments. Equally, controlled generation of waves, currents and turbulence in the laboratory, and measurement of the performance characteristics of a model-scale tidal turbine will aid in further understanding of wave-current interactions. Such measurements would provide a proper understanding of the combined effects of waves and misaligned tidal stream flows on tidal turbine performance, and the resulting cyclic loadings on individual devices and complete arrays. The availability of such measurements will reduce uncertainty in analysis (and hence risk) leading to increased reliability (and hence cost reductions) through the informed design of more optimised tidal turbine blades and rotor structures. An understanding of wave-current-structure interaction and how this affects the dynamic loading on the rotor, support structure, foundation, and other structural components is essential not only for the evaluation of power or performance, but also for the estimation of normal operational and extreme wave and current scenarios used to assess the survivability and economic viability of the technology, and to predict associated risks. The proposal aims to address these issues through laboratory and field measurements.

This research will investigate the combined effect of tidal currents, gravity waves, and ambient flow turbulence on the dynamic response of tidal energy converters. A high quality database will be established comprising field-scale measurements from the Pentland Firth, Orkney waters, and Shetland region, supplemented by laboratory-scale measurements from Edinburgh University's FloWave wave-current facility. Controlled experiments will be carried out at Edinburgh University's FloWave facility to determine hydrodynamic loads on a tidal current device and hence parameterise wave-current-turbulence-induced fatigue loading on the turbine's rotor and foundation.