Experimental Investigation into the Influence of Turbulence on the Performance of a Small-scale Vertical Axis Wind Turbine in a Wind Tunnel Environment

Abstract

Wind power is ranked among the fastest growing renewable energy technologies. Globally, there is renewed interest in research and utilization of wind energy both onshore and offshore. Horizontal axis wind turbines (HAWT) have been preferred because of existing knowledge on their performance and productivity. However, with an increasing global population and urbanization, there is higher demand of power in urban environments. The behavior of wind in these built-up areas is affected by extremely complex interaction amongst incident wind, vertical velocity gradient, shapes, sizes and layouts of buildings and resultant turbulence. In such conditions, small-scale wind turbines have been utilized as sources of power. Vertical axis wind turbines (VAWT) have proven particularly suitable for power production in these environments. The aerodynamic performance of VAWT operating in non-uniform turbulent flow regimes has not been comprehensively researched leaving a knowledge gap on their optimization and utilization for these environments. Different studies conducted on this field have not specifically agreed on the effect of turbulent non-uniform flow conditions on the performance of VAWTs. In this study an experimental method was used to investigate the effect of free-stream turbulence intensity on the aerodynamic performance of a VAWT under turbulent flow operating conditions in an open wind tunnel. To carry out the investigation, a mechanism to generate a turbulent flow was created to analyze the effect of induced turbulence intensity on the aerodynamic performance of a small-scale VAWT. The turbulence levels were chosen to match prevailing wind characteristics of the unsteady urban environment target site in Kenya. In addition, a systematic analysis of torque, power and energy coefficients, including their variations at uniform flow have been presented to predict the aerodynamic performance of a small-scale VAWT model. The VAWT was subjected to free-stream wind speeds ranging between 5 m/s and 10 m/s at different tip speed ratios. For uniform flow, the power coefficient, CP distribution power curve continued to increase with increasing free stream velocity throughout the range of TSR under study. The power increased up to an optimum CP of 0.2822 at uniform wind speeds of 10 m/s. Under the non-uniform conditions fluctuations in power production were recorded with the optimum power produced at CP of 0.36 at wind speed 5 m/s. The study findings revealed that non-uniform turbulent wind impacted the aerodynamic performance of the turbine with higher power coefficients being observed at lower wind speeds and improved self-starting ability in the presence of turbulence.