American style wind mills
They typically have many blades and are used by farmers to generate electricity on a small scale. They have low efficiency since the blades operate at a tip speed ratio (TSR) (the ratio between the speed of wind and the speed of the tips of the blades of the turbine) that is equal or less that 1
Conventional wind turbine wind
The common, modern wind turbines produce electricity and are used in many countries for the clean energy they provide. They usually have three blades, because it provides the best balance of high rotation speed and load balancing, but sometimes they only have two blades and sometimes just even one. They have high tip speed ratios, high efficiency and low torque ripple, which subtracts the minimum torque from the maximum during one revolution. As the number of blades decreases, the vibration modes increase in peak intensity, which can cause loud noise and wear and tear of the machine.
Modern three bladed horizontal wind turbine
Unconventional wind turbine
The ducted rotor is a turbine inside a duct. The technology is still under research. It can operate in a wide range of wind, and the generator can operate at high ratio rate. It doesn’t use gearbox, which makes it lighter, but the duct is however heavy, adding more weight to the tower.
Counter rotating wind turbine:-
The counter rotating wind turbine can increase the ratio speed of the electrical generator. The two turbines can either be on the same side of the tower or on the opposite side. When placed on the same side of the tower, the blades on the turbine in the front have to be slightly angled forward in order to not hit the turbine behind. When placed on the opposite side of the tower, the one in the back should be smaller than the one at the front so that they both turn at low wind speeds. At intermediate wind speeds, the front turbine stalls (where the air foil experiences an interruption of airflow resulting in loss of lift and a tendency to drop.) while the rear one keeps on turning. At high wind speeds both turbines stall. The counter rotating wind turbine allows the generator to function at a wider wind speed range than a single turbine generator.
Yet due to gearing losses, additional complexity, waste of some electricity and waste of mechanical power, this type of turbines was not sold at large practically over the last 5 years.
They are four bladed buildings which mainly are used to grind grain or pump water.