Aerodynamics of Vertical Wind Turbines

For the vertical axis wind turbines, there are more creativity and variety in the design of the blades. Some sub-types turbines use blades that are based on lift forces such as the Darrius. But most of the VAWT rely on drag force to rotate the rotor shafts such as the Savonius and the Zephyr types wind turbine. When the wind hits the blade, the resistance of the blade would create a force called drag. The drag applied on the blades would create a torque and rotate the rotor shaft. The drag force can be obtained from the below equation:

D=1/2ρV^2ACd

Where
ρ=Density of air [kg/m^3]
V=Velocity of the wind [m/s]
A=Surface area [m^2]
Cd=Drag coefficient

The drag coefficient is dependant on the geometry of the blade. The table below gives the coefficient of various shapes. In order to obtain a higher torque output, the drag must be maximized. The design of the blade must therefore be made of geometry of high drag coefficient such as a hollow semi-cylinder or a long flat plate at 90°. The blade must also be as large as possible since the drag is directly proportional to the surface area.

Drag Coefficient