Physik | Technik
Leonardo D’Amico, 2003 | Kilchberg, ZH
This paper compares empirically a modified three-bladed small-scale H-Rotor Darrieus type Vertical Axis Wind Turbine (VAWT) to a conventional Savonius rotor, focusing on power coefficient and resulting power over swept area. The modifications include detaching the blades from the shaft and placing them at a distance, supported by arms, to lengthen the lever arm to increase torque. Furthermore, a winglet was added to the blades to add self-starting capabilities. The turbines were tested in an open channel wind tunnel while voltage, RPM and power were measured under wind speeds from 1 m.s-1 to 5 m.s-1. Coefficient of power and tip speed ratio where then calculated. The results obtained showed that the modified design produces similar results at wind speeds under 2.5 m.s-1, while the conventional Savonius rotor produced more power at wind speeds above 2.5 m.s-1. This result can be applied to produce clean and affordable autonomous electricity in rural and urban areas exposed to relatively low and variable winds.
How does the power generate by a H-rotor Darrieus turbine compare with a Savonius turbine?
A conventional Savonius rotor (referred to as prototype 1) and a modified Savonius and Darrieus H-rotor (referred to as prototype 2) were empirically tested in an open wind channel at wind speeds between 1 m.s-1 and 5 m.s-1 in steps of 0.5 m.s-1. The modifications to prototype 2 included detaching the blades from the shaft and supporting them with arms, to lengthen the lever of torque. Measurements were taken at each wind speed. Voltage and current at resistances of Ohms 0, 135, 270, 405 were measured using a digital multimeter. The rotation speed of the rotor was measured in rotations per minute (RPM) with a laser tachometer. Power was then evaluated by plotting the coefficient of power (Cp) against the tip speed ratio (TSR).
Prototype 1 achieved a Cp,max of 0.118 at 4.5 m.s-1. Prototype 2 achieved a Cp,max of 0.0321 at 4.5 m.s-1. Prototype 1 reaches this value at its highest TSR, 0.147. Prototype 2 achieves its maximum Cp (±0.005) at a TSR above ca. 1.5 (1.42, 1.78, and 2.16). Prototype 1 achieved a maximum power density of 6.60 W.m-2 at 4.5 m.s-1. Prototype 2 achieved a maximum power of 2.46 W.m-2 at 5 m.s-1. Cp and power recorded by prototype 2 were never higher than prototype 1’s equivalent.
Both prototypes only start producing power at wind speeds above 2 m.s-1. Prototype 1 has a higher Cp and power at wind speeds above 2.5 m.s-1, making it a more efficient design. Prototype 2’s Cp plateaus at wind speeds above 3.5 m.s-1, indicating its ideal operating speeds in this range. Although Cp for prototype 1 does not plateau, the gradient of the curve decreases at 4 m.s-1, indicating that its ideal operating speeds are around 4.5 m.s-1. Prototype 2 rotates at much higher speeds compared to prototype 1, even though it has a larger diameter. This may be due to the turbine fixation to the shaft of the generator. Since prototype 1 is heavier, the fixation point produces more friction on the shaft. In prototype 2 it is possible that the rotation of the shaft rotates slower than the rotor, because the shaft slips as there is not enough friction. This experiment has multiple limitations. Firstly, these results are only applicable to small scale VAWT at wind speeds lower than 5 m.s-1. The small dataset makes some claims statistically insignificant. The prototypes were tested in an open channel wind tunnel. Therefore, external air flows may have influenced the results. In addition, although the channel utilises two inversely rotating ventilators to minimize swirl, this system is not perfect, resulting in an uneven air distribution on the blades. A cost analysis would have been a useful addition, as these turbines are mostly intended for use in poor rural areas, but this is beyond the scope of this study.
Both prototypes did not produce signifcant power below 2.5 m.s-1. The conventional Savonius rotor was more efficient and generated more power for wind speeds above 3 m.s-1, likely because of its working principle which is more adapted to lower wind speeds. Rotation speeds on prototype 2 were significantly higher than prototype 1, likely due to the fixation to the generator. Overall, although I am fully aware of the limits of my study, I attempted to give a contribution to SDG 7, calling for action “to ensure access to affordable, reliable, sustainable and modern energy for all”.
Würdigung durch den Experten
Prof. Dr. Beat Ribi
Die Arbeit adressiert eine aktuelle und wichtige Fragestellung: Wie kann in ländlichen Gebieten eine nachhaltige Energieversorgung mit vorhandenen Mitteln sichergestellt werden?
Dazu wird ein herkömmlicher Savonius-Läufer verglichen mit einer Windturbine, die eher auf dem Prinzip eines Darrieus-Rotors basiert.
Die Leistung beider Turbinen wurde experimentell für unterschiedliche Strömungsgeschwindigkeiten bestimmt. Das vorgeschlagene Design hat sicherlich noch Optimierungspotential. Mit dem vorhandenen Equipment und den eingesetzten Methoden wurde jedoch eine gesunde Basis dazu gelegt.
SIS Swiss International School Zürich, Wallisellen
Lehrerin: Nina Benisowitsch