There are two categories of wind turbine, the horizontal axis wind turbine (or HAWT) and the vertical axis wind turbine (or VAWT). HAWT wind turbine is more commonly install and produce by more manufacturers than the VAWT wind turbine. One of the reasons is HAWT wind turbine is more efficient compared to the VAWT wind turbine because of its swept area is larger than VAWT. Another reason is most of the installations are on the area where wind flow is unidirectional.

Before we proceed discussing about vertical and horizontal axis wind turbine, there is one equation that we left from the previous article. This equation is important especially in understanding specifications of wind turbine.

The coefficient of power, C_p of wind energy is given by the following equation:

C_p = Electricity produced by wind turbine/ Total Energy available in the wind

Basically, there is nothing new about the equation. It is just a simplified version from what we have discussed previously. Betz limit is the maximum value for C_p where C_p-max = 0.593.

As what we have discussed previously in real world the value of  C_p is always less than 0.593.

From the previous equation, we simply substitute the C_p value inside the equation, becomes:

Power_turbine = C_p x (1/2 x Swept Area x Air Density x Velocity³)

Thus, from the above equation we could conclude that the power of wind turbine is relatively depends on:

• Efficiency of the wind turbine to convert to electricity (i.e. coefficient of power C_p).
• Swept area, where larger swept area has more energy or power than smaller swept area.
• Velocity, is very important because only small of increase of speed, the power increase by cubic (or power of three).
• Air density, as we have discussed on the first article.

Readers who might be missed those articles could refer at the following links:

• What is Wind Energy? An Introduction
• What is Wind Energy? Limit & Efficiency

Horizontal Axis Wind Turbine

Although horizontal axis is more popular, however to find suitable location for installation of HAWT wind turbine is more difficult than VAWT. For example, HAWT wind turbine need higher wind cut in speed than VAWT.

*Cut-in Speed – Cut-in speed is the minimum wind speed at which the wind turbine will generate usable power.

HAWT wind turbine also could not work in turbulence area, where direction of wind always changing. In urban environment implementation of HAWT wind turbine is almost impossible for our home because of turbulence. Wind follows the path with less resistance by going around obstacles such as hills, mountain passes and buildings.

Vertical Axis Wind Turbines

We have read about the origin of vertical axis wind turbines which were used 3000 years ago in ancient Persia, Green Technology: The Ancient Persian Wind Power Technologies.

VAWT wind turbine is more flexible on which location it could be installed.  VAWT also is less sensitive to turbulence compared to the HAWT. There are two types of vertical axis wind turbine, the lift VAWT and the drag VAWT. From these two types regularly people refer to two classic designs:

• Darrieus wind turbine – lift VAWT
• Savonius wind turbine – drag VAWT

 

From these two classic designs then come a number of wind turbine designs with improvement of efficiency and reliability. We are going to continue this discussion about wind turbines on the next article and below is list of common terminologies which is commonly use in describing wind turbine specifications.  Thanks for reading.

Common Terminology for Wind Turbine

Start up Speed – is the speed at which the rotor and blade assembly begins to rotate.

Cut in Speed – is the minimum wind speed at which the wind turbine will generate usable power.

Rated Speed – is the minimum wind speed at which the wind turbine will generate its designated rated power. For example, a 10 kilowatt wind turbine may not generate 10 kilowatts until wind speeds reach 25 mph (approximately 40km/h).

Cut out Speed – At very high wind speeds, typically between 45 and 80 mph (approximately 72 km/h and 129 km/h), most wind turbines cease power generation and shut down. The wind speed at which shut down occurs is called the cut-out speed, or sometimes the furling speed. Having a cut-out speed is a safety feature which protects the wind turbine from damage.

Survival Wind Speed – is the maximum wind speed that a construction is designed to withstand.

Wind Turbines for Home – reference on some models of wind turbines.