Johnson, P; (2012) Hydrodynamics of tidal stream energy devices with two rows of blades. Doctoral thesis , UCL (University College London).

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Abstract

Tidal stream energy is an emerging low-carbon technology which could meet 5% of UK electricity demand. Current developments use ‘axial-flow’ rotors, which are efficient but limited in size, to generate electricity from ocean currents. This thesis investigates the hydrodynamics of a previously undeveloped rotor concept which has two rows of blades and also has no inherent size limit, hence it might achieve greater economies of scale. The rotor concept, called the ‘Moonraker’, is a cross-flow device with an oval blade path in the horizontal plane. This thesis presents research into the hydrodynamic performance of the Moonraker, focussing on the forces exerted on the blades by water currents and thereby deriving the thrust on and power generated by a Moonraker. The point vortex method was used to model the Moonraker and predicted high power coefficients when compared to a conventional cross-flow turbine with a circular blade path. A lab-scale Moonraker device was built and tested in the towing tanks at UCL and QinetiQ. The device was 2 m wide, 0.5 m high, with up to six blades and was towed at up to 0.7 m/s (blade Reynolds numbers were in the range 65,000--112,000). One of the blades was instrumented with strain gauges so that two components of blade loading could be recorded. Comparisons of predictions and measurements of blade loading showed some encouraging agreement, but also some disagreement, leading to suggested improvements in the modeling of the blade forces. The vortex model was subjected to further verification and validation tests in order to explore the issue of double actuator surfaces in close proximity. The extension of this work could help optimise the spacing between the two rows of blades on a Moonraker.

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