Keywords: blade, element, theory, tidal, turbine, renewable, energy, yaw, marine.

Accurate modelling of tidal stream turbine systems is vital if reliable, efficient turbine systems are to be produced and installed in the harsh marine environment. The blade element and momentum theory (BEMT) is a computationally efficient approach for modelling the performance of a rotor system in a flowing fluid. Griffiths [1] applied the theory to wind turbine design and Orme [2] continued this research, adapting the approach to take into account tidal and wave effects. The work presented here aims to continue the development of this approach with specific application to tidal stream devices.

Matrix procedures to track blade element positions in a global space are presented, this enables a global velocity vector to be interpolated from a grid of flow vectors for each element in global space. A similar matrix manipulation is then discussed to transform these global flow vectors so that their components are relative to the blade element and can then be used as inputs to the BEMT model.

To test the performance of this system, an investigation was conducted using a one tenth power law velocity profile for a standard turbine rotor in varying water depths. Tests were run over complete rotations of the turbine, over a span of time and over a range of turbine rotational speeds. For each water depth a fluctuation in teeter torque (the torque acting about the hub tending to tilt the rotor plane towards the seabed) was seen as the rotor blades passed through the different flow velocities of the boundary layer profile. The results show that the mapping and tracking procedures are functioning well, capturing the aspects of rotation of the turbine system and the position of the turbine in the flow profile. It was found in this study that the traditional definition of Tip speed ratio, a dimensionless ratio of turbine rotational speed to fluid velocity was inadequate. Different definitions for this ratio were discussed and it was concluded that taking the hub velocity as the reference fluid velocity gave a comprehensible reference for comparison.

1

Griffiths R.T., "Performance of the optimal wind turbine", Applied Energy 4, Applied Science Publishers Ltd., 1978. doi:10.1016/0306-2619(78)90025-9

2

Orme J., "Dynamic Performance Modelling of Tidal Stream Turbines in Ocean Waves", PhD Thesis, Civil and Computational Engineering, Swansea University, 2006.

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