Interest has increased on the usage of renewable energy in order to reduce the current global dependence on fossil fuels, a finite source of power that is also contributing to climate change. For many coastal countries, this has led to a number of research developments in tidal stream energy extraction, whereby flowing water is harnessed in a manner similar to the way in which wind turbines generate power from moving air. Unlike wind energy, however, the tides are predictable and thus tidal turbines could provide a reliable contribution of renewable energy to meet national energy demands. In 2015, Tidal Energy Ltd. (TEL) installed a grid connected tidal turbine off the Pembrokeshire coast in Wales, U.K., making it one of the first full-scale tidal stream energy demonstration projects underway worldwide. The device has many parallels to a typical wind turbine, featuring three blades on a horizontal-axis rotor that drive an induction generator. However, its power regulation control strategy is unlike the standard methods used in the wind industry. The turbine was designed to limit both power and thrust in high tidal flows through the controlled overspeed of the rotor. This can be achieved with minimal controller complexity, plus there are a number of perceived cost and reliability benefits for the turbine system as a whole by adopting this control philosophy. The control system is assessed in this thesis by firstly developing a numerical model of the turbine. Simulation results from the model were found to agree favourably with those obtained independently from a commercially available tidal turbine design code, predicting that the control strategy yields desirable power and thrust characteristics in all flow conditions. Further verification of the control strategy was subsequently achieved by experimentally testing a 1:30 scale model turbine in a laboratory flume tank.Using only the measurement of rotor speed as an input to the control system, the model turbine successfully tracked the maximum power condition in a time varying flow, before overspeeding to limit power and thrust in the highest flows. Prior to the testing of the full-scale turbine, the flow sensors to be installed alongside the device were tested in both harbour and laboratory environments in order to better understand their capabilities. The results of which provided supplementary evidence to form conclusions on the performance of the full-scale device, including the ability to identify suspected yaw errors and track unsteady upstream flows. Once the turbine was installed at sea, its initial power performance was assessed in accordance to industry guidelines, while a methodology was developed to infer the operational rotor forces from strain gauges placed in the blades. The full-scale turbine demonstrated the expected power and thrust limiting behaviour from overspeeding the rotor, but a control system upgrade is required for the device to fully realise its potential. Thus the research in this thesis contributes to current understanding of numerical modelling, scale model testing, environmental characterisation, and full-scale testing for the purposes of tidal stream energy generation.
|Date of Award||14 May 2017|
- University Of Strathclyde
|Sponsors||University of Edinburgh|
|Supervisor||David Clelland (Supervisor)|