Measurable metrics for characterisation of large-scale turbulent structures in tidal races for the marine tidal energy industry

The marine renewable energy industry is vital to the future energy security of the UK as climate change necessitates the shift towards low carbon energy sources. Ocean energy represents a vast and largely untapped resource and the shallow seas around the UK represent one the best tidal energy resources globally, accounting for some 10% of the total resource. In consequence, the tidal energy industry is an emerging and steadily growing sector of the UK economy. However, the potentially highest yield tidal environments (i.e. tidal races) are particularly challenging environments for development due to the strong currents and in particular very turbulent flows. Optimum and efficient design of tidal energy convertors (TECs) therefore requires the characterisation of both the flow and turbulence at potential sites for development.

The aim of this project is to develop techniques, which are directly applied to enhance the technological capability of oceanographic measurement equipment, for the characterisation of key aspects of turbulence, and hence identification of appropriate turbulence parameters, to aid the design and operation of TECs.

The main technical challenge in the development of the marine tidal energy sector is the design, deployment and operation, over the long term, of cost effective in-stream TEC devices which are able to survive the extreme conditions associated with potentially high yield regions (ie. rapid tidal currents). A key requirement in the development of the industry is therefore methodology for site survey which provides characterisation of appropriate turbulent parameters. Measurements demonstrate that turbulent kinetic energy dissipation is strongly linked to tidal current speed, in a confined channel it is highly variable, fluctuating by over an order of magnitude for a given flow speed. However, commonly used acoustic techniques fail to resolve this variability in dissipation, which results from the formation of coherent structures. Furthermore, these measurements do not provide accessible information on the larger scale structure of turbulence, ie. the scales, structure/coherency and stress, which are most likely to compromise the structural integrity of the tidal energy infrastructure and effect power yield.

Here we propose to develop and test new methodologies, using measurements from the latest generation of off-the-shelf 5-beam acoustic Doppler current profilers (ADCPs), to provide a more appropriate and comprehensive characterisation of turbulence at length scales directly relevant to the design of tidal stream energy generation infrastructure. The immediate relevance of these developments will be insured through collaboration with our partners, ADCP manufacturer Nortek and marine energy site survey company Partrac. In particular, we will exploit our previous observations that structure function techniques applied to multi-beamed acoustic current profilers can be used to provide a measure of anisotropy of turbulence.

Expected Impacts - The expected impacts of this proposal are twofold: (1) an increased scientific understanding of turbulence in the marine environment; and (2) the direct application of the technology into the marine measurement and marine renewable energy industries.

The research has the potential to advance the measurement of turbulence by providing an appropriate parameterisation of turbulence scale, structure and stress that may be incorporated into the system software of ADCP instruments. Subsequently, these provide a standardised method of quantifying these turbulence properties, which is readily deployable into the resource assessment and operational phases of the development of marine renewable energy infrastructure.

Who will benefit - The principal benefits of improved metrics for the quantification of coherent turbulent structures in energetic tidal flows will be observed by the scientists, engineers and developers of Tidal Energy Convertors (TECs) working within the marine renewable industry, and the manufactures of instrumentation used to measure these parameters; our project partners (Nortek and Partrac) are key organisations within these industries and will assist in the dissemination of impact from this proposal.

How will they benefit - In addition to the conventional science dissemination pathways through journal papers and conference presentations, we will disseminate the knowledge gained from this project to end users via direct engagement with our project partners at all stages of the project from inception to delivery.

Nortek plan to integrate the newly developed turbulence measurement methods into the deployment planning software of their acoustic Doppler instruments. This will help to transfer the knowledge and expertise from the realm of specialised scientific researchers to the many engineering teams responsible for the advancement of marine tidal power development. Furthermore, this knowledge will impact the marine tidal power research projects, which Nortek supports (EMEC, FORCE, Islay Strait, etc) and several companies (Open Hydro, Andrtiz Hydro) who are actively integrating Nortek instruments directly onto TECs for improved efficiency and feed-forward control development. As evidenced by the failure of full-scale TECs at FORCE and EMEC, significant uncertainty exists around the appropriate parameterisation of turbulence characteristics and researchers and engineers do not have a deep understanding of how to measure and interpret turbulence as it relates to developing engineering and operating principals. The collaboration between Bangor University and Nortek will ensure that the impact of this proposal provides the critical information required by the marine renewable energy industry.

Partrac will help to transfer the research effectively from the academic area into sector areas where it will be of genuine benefit to the TEC community. However, as one of the principal survey companies working in this area alongside the project, we envisage Partrac would itself be a benefactor of the research outputs, and hopefully this would create a commercial lead in projects which they bid for. A specific objective of Partrac is to: "develop and implement a safe, fit-for-purpose and quality assured marine survey methodology for the tidal industry, allowing measurement of tidal flows including turbulence" (Clark et al., 2016). It is essential to capture coherency in a characterisation of tidal flows and the new set of turbulence metrics generated through this proposal is directly relevant to Partrac and the marine renewable industry as a whole, allowing a reduction of both capital and operational expenditure, and risk.