Energy Balance Analysis and Turbulence Profile of a Darrieus-type Vertical Axis Wind Turbine

Yusri Yusup,Yin Mun H’Ng,Sheikh Ahmad Zaki

doi:10.21315/jps2019.30.1.5

Abstract

Vertical axis wind turbines (VAWTs) extract energy through reduction of the kinetic and turbulent kinetic energies, and the vertical momentum flux. Detailed turbulence profiling and energy balance analysis were attempted through direct turbulence quantification using the Eddy Covariance method. A high-frequency sonic anemometer was employed to measure flow and turbulence upwind and downwind of a Darrieus-type VAWT at different positions that cover the entire windswept area. The data was processed and analysed using a custom Matlab script. Energy balance analysis showed that there was a significant drop in kinetic energy before and after the VAWT (72% to 42%) but a small increase in turbulent kinetic energy (28% to 31%). The results also showed that the rotational direction of VAWT contributed to a higher value of longitudinal turbulence at the windward edge of the VAWT. The VAWT affected the flow field upwind by generating reverse flows. The largest vertical momentum flux transfer occurred downwind of the windward side of the VAWT. Spectral analysis shows that the VAWT produced smaller but faster momentum flux vortices due to the rotating rotors in the 1 Hz to 5 Hz range. This data and results have important applications to accurately model complex flows around VAWTs.

Highlights

Wind energy is a viable renewable energy alternative that has the potential to assist communities and nations to achieve energy sustainability goals
The surface drag increased the amount of momentum transfer to the surface and could be beneficial to the Vertical axis wind turbines (VAWTs) by increasing the overall energy available. These results show that momentum flux energy that could be absorbed by the VAWT would be minimal and insignificant compared to overall kinetic energy (KE) and turbulent kinetic energy (TKE), its effects on larger arrays of VAWTs require further study
The VAWT contributed to high u in the windward edge of the VAWT, which was approximately 1.4 to 2 times greater than other windswept areas

Summary

Introduction

Wind energy is a viable renewable energy alternative that has the potential to assist communities and nations to achieve energy sustainability goals. With more modification and improvement, VAWTs could achieve a maximum power coefficient of 40% and torque coefficient of 43%, which are comparable to the HAWT.[3] A study proposed the use of Maglev to increase the efficiency of the VAWT while another study focused on the design of the generator (e.g., the permanent magnet synchronous generator) to increase power generation at low wind speed conditions.[4,5,6] The use of magnetic gears shows promise as they are found to greatly enhance generator performance even at low wind speed conditions.[7] Aside from generating power, the feasibility to store power using batteries on an off-grid super-capacitator was reported.[8] Some studies have reported that, when installed on building rooftops, small-scale VAWTs (rate 1000 W or less) are practical and sustainable energy providers because it can be positioned close to the end user.[1,9,10,11] A study found that by placing many VAWTs in an array, wind energy extraction per unit area could be significantly increased, making it even more attractive for urban locations with surface area constraints.[12] This finding was found to be correct in a study by Kinzel et al, who showed that the pairing of VAWTs could reduce the size of wind farms. To further improve their performance and longevity in urban settings, it is vital to have an in-depth understanding of the turbulence profile of the VAWT.[14]

Methods

Results

Conclusion