TorqTidal: Mitigating Torque Pulsations in Tidal Current Turbines

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The UK is the global leader in the development of tidal current turbines, which extract energy from the flow of water when the tide moves in and out, and converts the energy into electrical power. This is due to the significant technical tidal current energy resource which has been estimated to potentially supply more than 5% of the UK's electricity demand in a future energy system.At present, the cost of electricity generated from tidal current turbines is many times higher than fossil-fuel generation and even the more established renewables such as wind and solar. This is partly due to the increased capital costs and O&M costs associated with tidal turbines. Turbines experience pulsating torque from the unsteady flow which add stresses to components and lead to either premature component failure or costly over-design of components to cope with these additional stresses. Much of the previous research has been concerned with accurately modelling the loadings on the turbine in order the design components to tolerate these loadings. TorqTidal aims to address this by mitigating torque pulsations in tidal turbines, and the subsequent effects on the turbine, through active control rather than over-design. This will help to toward lowering the cost of energy to a more competitive level.TorqTidal seeks to develop control strategies and implement them in a bi-directional tidal current turbine model simulated under realistic flow conditions. Using the model, the following will be investigated:1. How the control strategies affect power generation and the necessity of reactive power in achieving optimum control2. The performance of control strategies in strong-, weak-, and off-grid conditions and the need for energy storage3. How torque pulsations affect the power quality and the size of energy storage required to smooth the power in weak grid and off-grid conditions for an array of turbinesControl will be achieved using the existing power converter stage which is present in every tidal current conversion system, hence there is no increase in capital cost. The torque applied to the turbine blades is dependent on the flow speed and rotor speed; therefore the optimum rotor speed profile to mitigate torque pulsations will be investigated and a control strategy proposed. This will be verified through experimental work that takes the control algorithm and applies it to a test rig that represents the electrical power take-off system in a tidal turbine and also a scaled tidal turbine deployed in the FloWave basin. The test rig utilises digital signal processors to generate electrical signals from software models, which allows realistic flow to be simulated and control algorithms developed in software to be used directly in hardware. The FloWave basin is able to generate repeatable turbulent flow in order to analyse the behaviour of the turbine under different control strategies.