Aerodynamic study of single stage multi-blade drag-based vertical axis wind turbines

Abstract

Harnessing wind energy in built-up areas to generate power by a wind turbine for domestic application still remains a challenge despite considerable research has been undertaken in this area. The complex wind conditions in built-up areas, manufacturing complexity and the cost make most wind turbines unattractive for domestic application. For the use in built-up areas, vertical axis wind turbines are preferable due to their omni-directional characteristics, aesthetics, low noise and safety. Despite having these advantages, most vertical axis wind turbines available in the market are not producing appreciable power. An innovative and low cost vertical axis wind turbine with appreciable power generation capacity can provide a competitive edge to its peer. The primary objective of this study is to develop a micro vertical axis wind turbine suitable for built-up areas. To achieve this objective, a series of vertical axis wind turbines with multiple blades have been designed, manufactured and studied experimentally. A total of thirteen single stage multi-blade drag-based vertical axis wind turbines (6 with 300 mm diameter, four with 200 diameter and three with 600 mm diameter) with various blade configurations were investigated using RMIT Industrial Wind Tunnel. The blade shape of each turbine is semi-circular with zero twisting. Each turbine’s rotational speed (RPM), torque (T) and power (P) were determined under the wind speed ranging from 4.5 m/s (16 km/h) to 8.5 m/s (31 km/h). Smoke and wool tuff flow visualisations techniques were used to understand airflow characteristics in and around each prototype turbine. For each prototype wind turbine, a graphical relation between power and rotational speed, power coefficient and tip speed ratio, and maximum torque and number of blades were constructed. The effect of wind turbulence on power output has also been studied. At all speed range studied, the 300 mm diameter turbine with 32 blades produced highest torque and power amongst all 300 mm diameter configurations. Similarly, the 200 mm diameter turbine with 19 blades generated highest torque and power amongst 200 mm diameter turbines. An empirical relationship has been established to determine the optimal blade number for a constant diameter multi-blade single stage drag-base vertical axis wind turbine. The experimental data obtained for two different diameter prototype turbines have confirmed the validity of the developed empirical relationship. Power is highly dependent on blade number and blade spacing. With the increase of blade number, the power increases till it reaches an optimal blade number thereafter it starts to decrease. The optimal blade spacing was determined to be in between 5.5 mm and 6.5 mm for all turbines studied. A flow enhancing device (cowling) was designed and employed to explore its effectiveness on turbine rotational speed (RPM) and torque. The flow enhancing device had shown a positive impact on the turbine RPM however, the overall torque and power was found to be lower compared to turbines without its employment. Wind turbulence intensity has significant impact on power output. The power decreases with an increase of turbulence. However, the airflow rate or volume flow rate has much higher impact on the power output. The power can be increased by approximately two times when the blade diameter is scaled up. Doubling the blade diameter increases the amount of wind energy the turbine blades are able to extract. However, it is found that the power output does not increase proportionally as the optimal power output depends on a) blade number, b) blade spacing, c) blade angle, d) turbulence intensity, and e) airflow rate. In an open environment, small scale vertical axis wind turbine blades can experience wind with varied gustiness due to variety of structures. This can have effect on aerodynamic parameters optimised in controlled wind conditions in the wind tunnel. Therefore, it would be useful to undertake a field study to determine the wind gustiness effect on power output