Power Generation from Large-Scale Tidal Stream Turbine Arrays

  • Christoph Hachmann

Student thesis: Phd

Abstract

The investigation of power generation from large-scale tidal stream turbine arrays has become of increased interest following the progressive developments in the tidal stream energy sector. This research develops a multi-scale modelling approach, spanning from device scale through array scale up to channel scale, to assess the impact of alternative turbine operating points on the power output, energy yield and the resource. Initiating the multi-scale modelling approach, Reynolds-averaged Navier-Stokes (RANS) based device-scale computational methods are suitable to model operating points in the form of the local thrust coefficient and tip speed ratio of a turbine. Rotor and blade loading shows a good agreement within 13% against Linear Momentum Actuator Disc Theory, numerical and scaled experimental rotor studies. Both methods were found to give accurate wake predictions within 2% to 7% in the intermediate and far-wake of the rotor. The Constant Actuator Disc (AD) method is chosen to be extended to an array model because of the similarly accurate single turbine wake prediction, the capacity to apply pre-defined operating points, and the direct analogy to porous disc type models which are commonly used in array experiments at small geometric scale. The extended RANS-AD array model shows that the net array power coefficient does not vary by more than 15% between 5.26 and 6.15 as it is largely insensitive to a change of operating point or flow direction. The net thrust coefficient varies significantly with flow direction and operating point, ranging from 6.77 to 12.83. In-array turbine loading ranges between +30% and -70% for constant array operating points, indicating unfavourable operation of turbines far from design conditions which must be accounted for in order to avoid a detrimental impact on device performance and life expectancy. Mitigating measures which halve the rotor load variation can be achieved by employing row-specific operating points. This is confirmed by an experimental porous disc study, showing a discrepancy of in-array and net thrust predictions within 10%. The range of expected basin efficiency values as ratio of net array power to thrust of arrays with three rows is between 0.79 and 0.45, reaching favourable values above 0.7 for low thrust coefficients. The time variation of the array performance is assessed in a channel-scale model. The definition of the channel velocity change as a function of the removed power, incorporating the pre-determined basin efficiency, captures the interaction between array and resource more realistically. The impact of row-specific operating strategies on energy yield as well as power and drag force profile depends on the local flow regime, favouring peak flows above rated speed. Simplistic operating strategies over-predict the annual energy yield by almost 5% and are unable to capture the power output profile and extent of hourly interval changes.
Date of Award1 Aug 2019
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorPeter Stansby (Supervisor) & Timothy Stallard (Supervisor)

Keywords

  • Operating strategies
  • Experiments
  • Computational Fluid Dynamics
  • Tidal arrays
  • Tidal stream energy

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