There are two main types of wind turbine; vertical axis and horizontal axis. Although early versions of vertical axis turbines date back to around 500AD in Persia the horizontal axis turbines are now more prevalent and produce more power. However, the type of turbine may depend upon its suitability to the site.
A vertical axis wind turbine (VAWT) has the main rotor shaft arranged vertically. This means that it does not have to be pointed into the wind to be effective, which is an advantage on a site where the wind direction is highly variable. An additional benefit is the integration of the turbine into the whole structure, making it significantly less steerable. Also, the generator and gearbox can be placed near the ground, improving accessibility for maintenance.
Vertical axis turbines may be mounted on a roof, which may help to increase the wind speed as the wind is redirected over the building. To gain the optimum for maximum wind energy and minimum wind turbulence the height of the rooftop mounted turbine should be approximately 50% of the building height. Nevertheless, the ability of the roof to resist the additional stress has to be taken into account.
These advantages may be far outweighed by the superior energy generation performance of the horizontal axis wind turbine (HAWT). This is inherent in the design as when the wind blows on a vertical axis turbine only a fraction of the blades generate torque, compared with the horizontal type, in which all the blades contribute to energy production. However, horizontal axis turbines have the main rotor shaft and electrical generator at the top of the tower and must be pointed into the wind, which may be difficult to achieve on some sites.
There are advantages and disadvantages of both types of wind turbine. A vertical axis turbine does not need to be facing into the wind and may be more suitable for small sites. Horizontal axis turbines can be scaled-up, making them ideal for commercial energy generation; however they may not be as easy to place in an urban setting. Despite preferences, the site may determine the turbine type.