TorqTidal: Mitigating Torque Pulsations in Tidal Current Turbines

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 control
2. The performance of control strategies in strong-, weak-, and off-grid conditions and the need for energy storage
3. 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 turbines

Control 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.

The UK is the global leader in the development of tidal energy. Due to its abundant resource in UK waters, 20% of the electricity demand can potentially be supplied by clean, renewable power generated from tidal energy. Together with other renewable energy technologies, tidal energy can make a significant contribution towards meeting UK emissions and renewable power generation targets, and limit the rise in global temperatures. The industry currently supports 1700 jobs but could rise to 20000 jobs if the UK maintains its lead and manages to gain a significant share of the world market for tidal energy, estimated at £76bn.

For this to happen, greater investor confidence and commercial debt are needed. This is possible if a significant reduction in costs at all levels are achieved, thereby reducing the cost of energy to a point where it competes with more established renewable technologies and eventually fossil-fuel generation. One of the main aims of TorqTidal addresses this by reducing turbine torque pulsations through active control. The risk of component failure is reduced, thus increasing reliability and lowering O&M costs. In addition, active control enables capital costs to be reduced by removing the necessity for over-design, resulting in a reduction in component cost. These help to lower the cost of energy generated from tidal turbines, increase investor confidence, which leads to an increase in investment in the tidal energy sector.

There is an increasing number of community-owned renewable power projects that aim to provide social and economic benefits to the local community. A community-owned tidal turbine located off the coast of Shetland was commissioned in 2014; it managed to generate electricity for the local community and used a local supply chain wherever possible during the manufacture and installation of the turbine. However, to obtain a positive social impact, reliability and availability of supply is essential for these community groups located on islands or remote areas. TorqTidal addresses this by improving turbine reliability; as a result, less failures occur and devices are able to generate electricity more of the time.