There are two basic types of wind turbines and several verities within those two categories. These are the Horizontal Axis (HAWT) and the Vertical Axis (VAWT).
Horizontal Axis Wind Turbines
The horizontal axis windmill is much more common. Examples include the Dutch irrigation windmills and the water pumping windmills commonly seen in the plains states in the central and southwest parts of the U.S. The horizontal axis wind turbine (HAWT) properly designed and sited is more efficient in generating power from the total cross section of air that flows through the space encompassed by the blade rotation. The trade off is complexity and expense of the supporting structure. The HAWT needs to be 35 feet above any trees, hills or other structures within a 100 foot radius. This can add quite a bit of cost to the initial installation. Also, the high tower can make maintenance more difficult. These windmills need precision balancing because of the high rotational speed. The blades cut through the air at a faster rate than the wind speed because of the twist in the blades. There is a lifting effect somewhat like an airplane wing provides. In this case the lift pushes the blades around in a circle. Another issue is the wind changes direction and the HAWt windmill has a facing direction. There is usually a tail that drags in the wind and causes the wind turbine to rotate left or right to where it makes best use of the available wind. This means there are actually two axles. One for the spinning blades and one for the orientation. Usually there is a concern that the windmill will turn too fast during high winds and some dampening must be provided to prevent the mill from excessive vibration and torque that could destroy it. This is remedied by a process called furling.
Vertical Axis Wind Turbines
Generally, the vertical axis wind turbine works by creating a drag on the wind flowing across the blades. The radial speed never exceeds the wind speed. This makes the VAWT quieter. Also the balance is a little less critical. This type of generator works by having higher drag in one rotational direction than in the other. It turns out the drag type of windmill peaks in efficiency well below the maximum wind speed. There is little concern of it spinning out of control. Also the VAWT can take advantage of wind from any horizontal direction. This means that even during turbulent winds power is still available. Several of these factors make the Vertical Axis Wind Turbine more practical for the home experimenter. Especially for the first one or two attempts. An easy type to build at home is based on a design by an inventor named Savonius. This type can be build from PVC pipe or discarded buckets or barrels. The circular material is cut in half forming two half circles. The ends of the half circle materials are then mounted between two disks forming the top and bottom. The two halves should be mounted with the open sides facing and offset so there is about 1/3 of the circle overlapping. This allows the air to flow into one curve and pass through the center and out the other curved side. This provides rotational push and increases efficiency compared to having no overlap. This arrangement also helps with the start up of the spin. Even better you can stack another set of pipes or buckets and one more disk to have two blade sets on a common axis. The second set should be mounted 90 degrees around from the first set. This allows the start up to occur in lower wind speeds.
