Technical Information

Horizontal vs. Vertical Axis Wind Turbines

Horizontal Axis Wind Turbine                                                   Vertical Axis Wind Turbine


The vertical axis marketing mantra is “they work in turbulent air", "no tower is needed" and “you can mount them on your rooftop”. However, the same siting rules apply to Vertical Axis Wind Turbines (VAWTs) as Horizontal Axis Wind Turbines ( HAWTs). They need the same fuel, i.e. a clean, laminar airflow across the rotors. Because VAWT’s are omni-directional they are a little less sensitive to rapidly changing wind directions, but if the wind at the site changes direction frequently it normally means there is a lot of turbulence as well. No wind turbine, VAWT’s included, will produce well at such a site. VAWT’s also have the inherent disadvantage that half of their swept area rotates "against" the wind and does not contribute to energy production.

There are many vertical axis designs now being touted as the latest invention in wind turbines. Not quite so. Vertical axis wind energy devices have been around for a long time, approximately 5,000 years. They are the original “wind extracting” equipment, developed in Persia and China. Both vertical-axis and horizontal-axis wind turbine designs (wind electric generators) were invented in the 1920s, following the successful development of water-pumping windmills in the late 1800’s.

There are two basic vertical axis designs. One is called a Savonius rotor, which looks like a 55 gallon drum cut it in half with the two halves offset 180° and mounted opposite each other on a vertical shaft that rotates. It is similar to a cupped wind anemometer. The other design is called a Darrieus rotor, which resembles an egg beater. It is two vertically oriented blades revolving around a vertical shaft. The Darrieus models use an airfoil design.

A wind turbine airfoil works in the same way as an airplane wing. It has a flat side and a curved side. As the air passes over the two sides it creates a force known as “lift”. For the airplane, that lift is created from the engine forcing the wing through the air. For the wind turbine, the air forces the rotor to turn the generator, thereby creating power.

When you are assessing or purchasing a wind turbine, the most important information you need to know is how many kilowatt-hours (kWh’s) the turbine will generate at a certain wind speed, preferably the wind speed at your site and not somewhere else.