While wind power only makes up about 1% of current world power generation, it is one of the fastest-growing methods to create power. Wind power requires very little maintenance after installation, can be scaled to meet individual or community needs, and creates no emissions or by-products. Windmills have been in use for over two thousand years, but only in the last 150 years has man harnessed the wind to create electricity. Small wind turbines brought electrification to a large section of rural America, and today, countries such as Germany and Denmark rely on wind power for a significant fraction of their energy needs.
Creating Electricity from Rotational Input
The rotational motion of the wind turbine blades is converted to electricity using an electrical generator. On horizontal-axis wind turbines, the generator is located at the top of the tower in line with the rotating horizontal shaft. On vertical-axis wind turbines, the generator is located at the base of the structure in line with the rotating vertical shaft. The generator uses electromagnetic induction to generate electricity, which can either be used immediately or stored for later use. A source of energy loss in this configuration is friction between the drive system, which may contain a gearbox, and the generator itself. In order to reduce friction and increase efficiency, turbine manufacturer Enercon has developed a gear-free annular generator. This design reduces the stress and wear on the turbine's moving parts.
Power Generation and Efficiency
Wind turbines are available in small sizes on the order of hundreds of watts for remote site electricity needs, up to giant 6MW generators with a blade diameter of 126 m (413 ft). The amount of power that can be generated by a wind turbine depends on a variety of factors including wind speed, rotor size and air density (which varies with altitude). Because wind is not a constant resource, wind power generation often contains a capacity factor, usually in the range of 20-40% of the theoretical maximum. The German physicist Albert Betz calculated "that a wind turbine can extract at most 59% of the energy that would otherwise flow through the turbine's cross section", assuming a perfect and frictionless system.
Storing Electricity from Wind Turbines
Wind, despite its benefits, does not provide a constant an continuous input for electricity generation. Because of this, power generated from wind turbines must either be used immediately either locally or within a regional grid, or stored for later use using batteries, capacitors, or grid storage techniques. Grid storage methods capture the energy from the wind turbines and can release that energy during off-peak periods. These methods include pumped water, compressed air and flywheels. Newer techniques still under development include superconducting magnetic energy storage and a hydrogen liquification method.
Wind is an attractive power generation medium because it does not create emissions and is renewable. The intermittent nature of wind restricts wind power from being a primary power source for large areas, but as energy storage methods improve, wind power will be poised to take over a larger fraction of world power generation.
Sources
Betz, A., Introduction to the Theory of Flow Machines. (D. G. Randall, Trans.) Oxford: Pergamon Press, 1966.
Manwell, McGowan, & Rogers, Wind Energy Explained: Theory, Design and Application, Wiley, 2002
