Abstract
Variable-pitch (VP) technology is an effective approach to upgrade the aerodynamics of the blade of an H-type vertical-axis wind turbine (VAWT). At present, most of the research efforts are focused on the performance improvement of the azimuth angle owing to the large angle of attack (AoA). On the blade circular path of an H-type VAWT, there are two azimuth positions where torques are negative, and the performance is the poorest. The vicinity zones of the two azimuths also have low performance and greatly weaken the overall productivity of VAWT. In this paper, we propose a new technology that, unlike the traditional VP-technology, focuses mainly on the aerodynamics improvement of the azimuth position with small AoA. The purpose of this novel approach is to widen the band of azimuth positions with high performance and eventually enhance the power efficiency of the overall VAWT. The performance of the new VP-VAWT is predicted using the Double Multiple Streamtubes model and Prandtl's mathematics to evaluate the blade tip loss. Compared with the fixed-pitch (FP) blade, the VP-blade has a wider zone of the max AoA and tangential force in the upwind half-circle and yields the two new larger max values in the downwind half-circle. The new VP-strategy considerably narrows the two low-torque zones near the 0° and 180° azimuths and markedly widens the high-torque azimuth zone; the torque distribution appears in a trapezoidal shape in the upwind region and an M-like shape in the downwind region. The power distribution in the swept area of turbine changes from an arched shape of the FP-VAWT into a rectangular shape of the VP-VAWT. At last, an 18.9% growth in power efficiency is achieved. All of the above results confirm that the new VP-technology can effectively improve VAWT performance and also widens the highest performance tip speed ratio zone which makes the turbines capable of running with high efficiency in wider zones.
Highlights
The effects on the blade revealed by the above studies can be summarized as follows: both 90 and 270 azimuths have the biggest angle of attack (AoA) in FP-vertical-axis wind turbine (VAWT), and the largest growth of performance will be obtained in the same positions of VP-VAWTs
The curve of the pitch angle versus azimuth was designed based on an H-type VAWT rotating in the optimum tip speed ratio (TSR) 1⁄4 5.0
Under FP-VAWT conditions, whether in the upwind or the downwind region, the AoA appears to be in a parabolic shape; the max values obtained are at an azimuth of 90 in the upwind zone and at an azimuth of 270 in the downwind zone
Summary
Its aerodynamics was predicted using the DMST model, and the results showed that only 5 of the pitch angle amplitude increased the power coefficient of the VAWT by 12% Another approach is the cycloidal VP-technology, in which the changes of the pitch angle are much closer to a sinusoidal curve. A substantial amount of research work on cycloidal VP-marine current turbines was conducted to overcome their drawbacks and improve the overall performance of the turbines in China (Wang et al, 2004 and Zhang et al, 2011) Both VP-strategies mentioned above emphasize mainly the performance enhancement of azimuths with a high angle of attack (AoA) in the case of FP.
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