Wind turbines and windmills uses wind energy to produce electricity. They are mounted on a high tower--100 feet (30 meters) or more, to to have a good exposure to the wind and to harvest the bulk of energy. At height they can benefit from the faster and less turbulent wind.
The heart of a wind turbine is the rotor. The rotor is aligned into the wind by the tail manually or via a computer-controlled tail-fan tail.
Source U.S. Department of Energy at http://www1.eere.energy.gov/windandhydro/wind_how.html
The wind's energy is captured with the blades, that look like propellers, but narrow. Typically there are 2 or 3 blades mounted on a shaft to form a rotor. A blade acts almost like an airplane wing. Wind is caused by differences in pressure. When the wind blows on a blade a low-pressure air forms a pocket on the downwind side of the blade. Since the wind always blow from high to low pressure, air from downwind side of the blade rush in to equal the air pressure. The blade is pulled toward the low pressure starting the rotor to turn. The process is called a lift. The lift force is much stronger than the wind force acting on the front side of the blade, called the drag that exerts a force which increases as the square of the wind velocity.
The combination of lift and drag causes the rotor to spin like a propeller and then the spinning of the turbine’s shaft. The rotary force produced by the turning shaft subsequently drives an electrical generator that converts the force into electricity.
Blades, like wings of an airplane, are called airfoils, because with their curved surfaces their shapes--designed to give the most favorable ratio of lift to drag, are very efficient in converting the wind energy into rotational energy. In wind turbine design, the objective is to have a high lift-to-drag ratio. This is accomplished by twisting the blades. The blades are twisted so that the wind hits them at the correct angle of attack. This twist is known a pitch.
The blades rotate at 10-22 revolutions per minute. At 22 rotations per minute the tip speed exceeds 300 ft per second. A gear box is commonly used to step up the speed of the generator, although designs may also use direct drive of an annular generator. Some models operate at constant speed, but more energy can be collected by variable-speed turbines which use a solid-state power converter to interface to the transmission system. All turbines are equipped with shut-down features to avoid damage at high wind speeds. Tip speeds of over 200 miles per hour (320 km/h), high efficiency, and low torque ripple contribute to good reliability. The ratio between the speed of the wind and the speed of the blade tips is called tip speed ratio. High efficiency 3-blade-turbines have tip speed/wind speed ratios of 6 to 7.
According to the Albert Betz' law and the hypothetical ideal wind-energy extraction machine, no more than 16/27 (59.3%) of the kinetic energy of the wind can be captured. Modern turbines may reach 70 to 80% of this theoretical limit.
Wind turbines come in various sizes. Normally for the commercial production the rotor diameters range from 130 to 300 ft (40 to 90 meters). Offshore turbine have larger rotors up to 370 ft (110 meter) diameter. Small wind turbines intended for residential or small business have rotor diameters of 27 ft (8 meters) or less and would be mounted on towers of 130 ft (40 meters) in height or less. The blade length range from 65 to 130 ft (20 to 40 meters) or more. Tower heights for horizontal-axis wind turbines (HAWT), as the balance of costs versus efficiency, are approximately 2 to 3 times the blade length and range from 200 to 300 ft (60 to 90 meters).
The towers are mostly tubular and made of steel. The blades are made of fiberglass-reinforced polyester or wood-epoxy. The blades are usually colored light gray to blend in with the clouds.
There are 2 types of wind towers: floating towers and land-based towers. Floating towers are mounted on a floating structure that allows the turbine to generate electricity on the water. The electricity generated is sent to shore through undersea cables. The initial capital cost of floating turbines is competitive with bottom-mounted, near-shore wind turbines while the rate of energy generation is higher out in the sea as the wind flow is often stronger, steadier and unobstructed by topographic features.
Source http://www.windaction.org/pictures/2942
The output of a wind turbine depends on the turbine's size and the wind's speed through the rotor. Wind turbines now have power ratings ranging from 250 watts to 5 megawatts (MW). Utility-scale turbines come typically in the range of 700 kW to 2.5 MW. As of 2010 the most powerful turbine is rated at 7 MW.
Wind turbines can be used as stand-alone, or they can be connected to a utility power grid or even combined with a photovoltaic (solar cell) system.
Utility-scale (megawatt-sized) turbines are usually installed in groups called windfarms. Several electricity providers today use wind plants to supply power to their customers. Stand-alone wind turbines are typically used for water pumping or communications. However, homeowners, farmers, and ranchers in windy areas can also use wind turbines to cut their electric bills. Small wind systems have potential as distributed energy resources. It refers to variety of small, modular power-generating technologies that can be combined to improve the operation of the electricity delivery system.
An average U.S. household uses about 10,655 kilowatt-hours (kWh) of electricity each year. 1MW wind energy can generate from 2.4 to more than 3 million kWh annually. Therefore, a megawatt of wind generates about as much electricity as 225 to 300 households use. Wind energy cannot be, however, the only power source, since the wind does not blow all the time. The other energy sources are required. No power plant is 100% reliable.
Wind turbines produce some noise. The turbine blades produce a whooshing sound as they hit turbulence in the air, but most of this noise tends to be masked by the background noise of the blowing wind. An operating modern wind farm at a distance of 750 feet - 1000 feet is no more noisy than a kitchen refrigerator.
At the end of 2009, worldwide nominal capacity of wind-powered generators was 159.2 GW. Energy production was 340 TWh, which is about 2% of worldwide electricity usage; and is growing rapidly, having doubled in the past few years.
Inside a Wind Turbine
