Traditional wind turbines are mounted on horizontal shafts and rotate in a vertical plane. These are referred to as horizontal axis wind turbines (HAWTs). Alternative designs consisting of rotating blades mounted on vertical shafts, known as vertical axis wind turbines (VAWTs), have existed for hundreds of years. There are two major types of vertical axis wind turbines:
Savonius-style turbines - these are very simple turbines that consist of several scoops mounted on a vertical shaft. The open face of the scoop experiences greater drag forces from the wind than the curved back face of the scoop. This differential drag causes the Savonius turbine to spin. A wind anemometer is perfect example of a Savonius turbine. The history of this type of turbine can be traced back to an Italian engineer in 1622; however, the Finnish engineer Sigurd Johannes Savonius perfected the design and invented the Savonius turbine in 1922. Savonius turbines are less efficient than vertical axis turbines that utilize lift as opposed to drag, but simpler in design, and thus are used when cost or reliability is much more important than efficiency.
Darrieus-style turbines - these turbines consists of a number of curved aerofoil blades mounted on a vertical shaft. This design of wind turbine was patented by Georges Jean Marie Darrieus, a French aeronautical engineer in 1931, although examples of similar devices can be found much earlier in history. Darrieus turbines tend to be more efficient than Savonius turbines but with some disadvantages. As examples, Darrieus turbines are not self-starting and require a mechanism to begin spinning, and early designs with radially symmetric airfoils create a pulsing load that can cause structural problems in the turbine as well as undesirable electrical pulses. New designs using helical airfoils have solved some of these problems.
VAWTs have an advantage over HAWT in that they are equally effective no matter which direction the wind is blowing and do not need to be rotated to face into the wind. This also gives them an advantage in converting a more turbulant wind flow into electricity (see OWOE Amazing Energy: Capture Mobility Wind Turbine). However, simple physics indicate that HAWTs are more efficient than VAWTs given the same wind velocity. HAWT blades can be turned into the wind such that they are always sweeping the most undisturbed air possible. Wake eddies do not interfere with the motion of the blades as they dissipate downwind. VAWT blades, by comparison produce maximum torque on the rotor at only a few points as they travel around the shaft. The remainder of the time the blades must travel through the turbulance that they have created.
A good indicator of which turbine configuration is the important in the quest for renewable energy comes from a simple look at installed electrical generation capacity. There are currently about 300,000 utility scale HAWTs in operation with a capacity of over 300 GW in over 100 countries around the world. There are a number of demonstration VAWTs installed, but no utility scale turbines operating today.