Design and operation of tidal arrays in channels

Tidal currents are a promising source of clean and renewable power in many locations worldwide. In channels with suitable bathymetric conditions for strong currents, arrays of tidal stream turbines have been proposed as a useful method for generating energy. The modelling of tidal arrays is complicated, involving multiple spatial and temporal scales. The large scale deployment of tidal turbines will greatly alter the surrounding flow field, making resource assessment difficult and also complicating the design and operation (‘tuning’) of tidal turbine arrays. This thesis presents a tidal turbine model that is used to explore the interaction between power extraction devices and tidal flows. The resulting knowledge is used to inform different designs and operational strategies for tidal turbine arrays. As an introductory work, a model of the Bohai Sea in China is considered. Flow is simulated through the strait using a depth-averaged numerical model and tidal arrays are introduced using an idealised actuator disc model, while turbine arrangement strategies are considered with the goal of maximising the available power to the array. The results of this study have served to direct the focus of the research to the investigation of a more realistic turbine representation and different tuning strategies for array optimisation. A blockage-corrected blade element momentum theory is implemented to capture more accurately the performance of the tidal turbine. The implementation procedure and key limitations are discussed. A more realistic tidal rotor turbine representation is then used to examine both full-width and partial-width tidal arrays placed within idealised analytical and numerical channel domains. Analysis is undertaken to reexamine and expand on existing theoretical work. Notable results from this research include the reduced significance of tuning for tidal rotors as compared to actuator discs. This is true for both temporally fixed and varying tuning strategies, thus suggesting simpler operational strategies to be sufficient as well as more costefficient. As well, the maximum amount of power produced by tidal rotors, averaged over the tidal cycle, is typically 60–70% of that produced by actuator discs. This range can potentially serve as a useful correction factor generally applicable to present and future resource assessments of candidate tidal sites. Finally, through analysis of the optimal arrangement for partial-width arrays under oscillatory flow and the significance of the local blockage effect, it is revealed that the two-scale theory, though appropriate for a majority of realistic turbine deployments, may underestimate the available power in certain flow conditions where the inertia of the flow becomes important. It is hoped that the present results will assist marine energy developers in selecting suitable models with which to determine optimal operation and arrangements for tidal stream turbines and to estimate the amount of tidal energy resource available.