An experimental study on the effects of winglets on the performance of horizontal axis tidal turbines

Abstract

Horizontal Axis Tidal Turbines (HATT) currently have a lower power coefficient than commercially available wind turbines. This has meant that in recent years less funding goes towards developing such technology. Furthermore, some planned projects have not gone ahead, and there does not seem to be support from the government to promote its development. In the wind industry, turbines can have 5 times the rated power of a tidal turbine because the rotor size is not a problem. In the tidal industry, size is constrained due to technical and environmental factors. An approach to face this issue is to increase the power coefficient for a fixed rotor size. An experimental study is presented to investigate the performance of winglets fitted to a 1:20th scaled 1 m in diameter HATT. Winglets have been extensively employed in the aviation industry to reduce the vortices generated at the end of aircraft wings decreasing drag and hence increasing fuel economy of civilian aircraft. For horizontal axis turbines, winglets facing backwards on the suction side of the blades have been the subject of extensive research almost exclusively based on computer-driven numerical simulations as a means to increase the power capture of the rotor. With the use of oil-based paint flow visualisation, the mechanism behind the phenomenon affecting winglets facing the suction side has been identified as part of this work. Vortices form behind the blade/winglet interface when they are oriented towards the flow direction. These vortices reduce performance due to viscous effects. Power and thrust coefficients were measured from the scale HATT and together with numerical Blade Element Momentum simulations, the bending moments at the root could be calculated. A winglet facing downstream decreases the power coefficient by 12% in average and increases the thrust coefficient by an average 5%. On the other hand, a symmetrically mirrored winglet facing upstream can increase the power coefficient by around 1-2%, at the same time the thrust coefficient increases by 3-4%. Assuming that the increase in thrust is caused by fitting the winglets, their structural cost in the bending moment at the root of the blade is up to 40% more, 4.2-5.6% in this case. Cost-data from industry has suggested that even marginal increases in power coefficients due to winglets can provide a return on investment given the relative costs of blade manufacture and the expected level of subsides awarded to new and developing forms of electricity generation. Therefore, further work to optimise pressure-side winglets should be conducted.

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Identifiers

ORCID for Rodolfo Olvera Trejo: orcid.org/0000-0002-8607-1138

ORCID for Luke Myers: orcid.org/0000-0002-4724-899X

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