Advanced Turbine Concepts

Tidal stream turbines are deployed environments subject to a range of challenging conditions including significant levels of turbulence, strong shear profiles and waves. The flow approaching a short fence of turbines will vary across the fence due to the array-scale bypass flows, resulting in cross-fence performance variation. Turbines deployed on floating platforms will operate in the wave zone and likely experience platform-induced flow variations which may contribute significantly to unsteady loading. We employ numerical and experimental approaches to investigate these phenomena and explore techniques to mitigate their consequences for turbine performance.

Cross-fence performance variation

fence_varCross-fence variation in turbine performance arises as a result of the interaction between the resistance presented by individual turbines and the overall array-scale flow dynamics. Towards the end of a partial fence of turbines, the reduced resistance to flow expansion in the array bypass (see image on right) means that the flow incident on the outer turbines see a laterally sheared flow, affecting their performance. The extent of this lateral shear depends on the operating condition of the turbines, but is a factor that limits overall fence performance. We are exploring cross-fence control strategies that seek to reduce the variation in turbine performance across the fence which may help to reduce turbine loading whilst minimising the reduction in power capture by the fence of turbines.

velocityAn alternative approach to minimising the cross-fence variation and performance loss associated with the array-bypass is to constrain flow expansion. The concept of turbine ducting has been proposed by a number of groups and companies as a means by which to align and accelerate the flow through the rotor, which has also been studied by our group. A similar concept can be used to improve flow alignment through a short fence of turbines, which we have investigated numerically and also tested experimentally during the ATHENA project at SSPA. Hydrodynamically designing the buoyancy of a floating system may present an opportunity to provide array-scale flow control to improve overall fence power. We are also interested in investigating alternative flow confinement approaches that will help to boost fence performance.

Turbine blade hydrodynamics

deformationJust as with wind turbine blades, tidal turbine blades deform when operating, which changes the blade hydrodynamics. Most reduced-order turbine models, such as blade element momentum (BEM) theory utilise the assumption that the rotor blades are perpendicular to the direction of the oncoming flow and consequently that the spanwise flows along the blade are negligible. Whilst blades tend to be stiff in the edgewise direction, hydrodynamically significant deformations can take place in the twist and flapwise directions.

We employ high-fidelity blade resolved simulations to investigate the effects of blade deformation on turbine performance. The spanwise flows that develop along blades that have deformed hydroelastically can result in significant changes to the spanwise forces and consequently overall turbine thrust and power. The changes in loading are important to understand in improving overall prediction of turbine performance. The detailed physics that can be extracted from blade resolve simulation is contributing to the development of improved understanding of how to capture the leading-order effects of hydroelastic deformation in reduced order models. Understanding blade hydroelastics is also important for better predicting turbine response to flow unsteadiness, such as that due to turbulence and waves.

Another area of blade hydrodynamics that we are interested in is individual blade pitch control, in which turbine blades are individually pitched in order to mitigate some of the unsteady loading and fatigue damage that can arise in sheared or turbulent flow conditions. High-fidelity simulation allows the interconnected investigation of turbine and blade rotation to be modelled, and results to date indicate that a significant reduction in thrust load fluctuation can be achieved, leading to reduced blade fatigue.

Floating platforms

platformExploiting the benefits available from constructive interference effects requires turbines to be deployed in closely spaced fence arrangements. This, in conjunction with the need for access for deployment and maintenance, leads to structurally interconnected devices in order to achieve reduced cost of infrastructure, installation and maintenance per turbine. Floating tidal turbine systems are being pursued by a number of companies to exploit these benefits. However, system complexity is increased, relative to a more traditional seabed mounted system, due to the six degree-of-freedom response of the platform and increased loads due to operation in the wave zone. We are investigating the complex coupled turbine-platform dynamics of floating turbine systems in order to help improve understanding and modelling of these systems.