Design and Analysis of Hybrid H-Variable Savonius Vertical Axis Wind Turbine
| Autor: | Chen Kuang Yang, 楊宸光 |
|---|---|
| Rok vydání: | 2017 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 105 Since the variation of wind direction does not affect the performance of vertical-axis wind turbines (VAWT), they are suitable for installing in metropolitan area where the wind direction varies constantly due to the vortexes induced by high rise buildings. Among all VAWTs, the power coefficient, CP, of H-rotors can be as high as 0.45, which is even higher than that of some small horizontal axis wind turbines. However, a dead band of negative CP in the lower range of the tip speed ratio (TSR) of the H-rotors prevents them from self starting and consequently reaching the high CP. Therefore, this study is an attempt using a Savonius wind turbine (S-rotor) of variable pitch and smaller radius at the center of an H-rotor that would self start the rotor, carry the H-rotor blades through the dead band, reduce the negative impact in high TSR range, and brake the rotor at high wind. Firstly, an optimization method is used to design the S-rotor blades that have the largest starting torque, which is the smallest torque in all direction for the optimal set of geometric factors. Secondly, the upper limit of the dead band is found as a function of wind speeds with the double-multiple stream tube model. A model of the optimized S-rotor is fabricated and tested in a wind tunnel to find the maximum TSR as a function of wind speed. Then the TSR is adjusted to be the TSR of H-rotor. Comparing the two, one can find the wind speed and the rotational speed of the rotor assembly that passes the dead band. Under this condition, the blades of the S-rotor transform into a cylinder in order to reduce drag as the H-rotor blades pickup the speed. Then, a mechanism is designed to vary the pitch of the S-type blades for performing the above mentioned function as well as braking the rotor with air when rotor speed exceeds a maximum value so as to protect the rotor from breaking by strong winds. Finally, a prototype of the combined rotor is tested on a vehicle mounted test bench. The results show that the experimental data are very different from theoretical prediction. The main reason for this is that the theoretical analysis does not consider the negative torque of the H-rotor and the shielding effect of the H-rotor blades. Therefore, it may be concluded that the H-rotor with fixed pitch blades cannot go across the dead band with the help of S-rotor. Using variable pitch blades for the H-rotor may be the only solution to the problem because the negative torque of variable pitch blades is only one half that of fixed pitch blades. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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