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Open AccessArticle

Performance Analysis of Hydrofoil Shaped and Bi-Directional Diffusers for Cross Flow Tidal Turbines in Single and Double-Rotor Configurations

Department of Energy, Systems, Territory and Constructions Engineering, University of Pisa, l.go Lucio Lazzarino, 56122 Pisa, Italy
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Energies 2019, 12(2), 272; https://doi.org/10.3390/en12020272
Received: 6 December 2018 / Revised: 8 January 2019 / Accepted: 11 January 2019 / Published: 16 January 2019
(This article belongs to the Section Sustainable Energy)
With the aim of finding efficient solutions for cross flow turbine (CFT) bi-directional diffusers able to harvest non perfectly rectilinear tidal currents, a 2D CFD analysis of ducted CFTs was carried out with focus on the effects of diffuser shape and yaw angle. The HARVEST hydrofoil shaped diffuser, equipped with a pair of counter-rotating turbines, and a bi-directional symmetrical diffuser were compared in terms of coefficient of power (CP), torque ripple, overall thrust on diffuser and wake characteristics. Slightly better CP were predicted for the symmetrical diffuser, due to the convergent walls that address the flow towards the blade with a greater attack angle during early and late upwind and to the viscous interactions between the turbine wakes and strong vortices shed by the diffuser. A CP’s extraordinary improving resulted when yaw increased up to 22.5° for the hydrofoil shaped and up to 30° for the symmetrical diffuser. Similar behaviour in yawed flows also occurred in case of a ducted single rotor, demonstrating that it is a characteristic of CFTs. The insertion of a straight throat in the diffuser design proved to be an effective way to mitigate torque ripple, but a CP loss is expected. View Full-Text
Keywords: tidal currents; cross flow turbine; diffuser; vortex shedding; yawed flows tidal currents; cross flow turbine; diffuser; vortex shedding; yawed flows
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Zanforlin, S.; Buzzi, F.; Francesconi, M. Performance Analysis of Hydrofoil Shaped and Bi-Directional Diffusers for Cross Flow Tidal Turbines in Single and Double-Rotor Configurations. Energies 2019, 12, 272.

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