Wind Power
From Thermal-FluidsPedia
Not all the kinetic energy present in winds can be extracted by a wind turbine. In addition to various losses, such as slippage and drag, some of this energy must also be carried downstream of the turbine in order to maintain the air flow.
The power generated by a wind turbine is proportional to the kinetic energy of the mass of air swept through its rotor. The mass itself increases with the air density, the size of the rotor, and the speed of the wind. As a result, total power generated by a wind turbine increases with the air density, the cube of the wind velocity, and rotor area, that is, the square of the rotor diameter (See Box “Power and Torque”).
Wind speed increases with height above the ground, so the height of the tower on which the rotor is mounted indirectly affects power production. Figure 1 shows the increase in power with the height of the tower. For example, a wind turbine mounted on a tower 120’ (36 m) tall will be exposed to wind speeds twice that of a tower 30’ (9 m) tall. This alone results in the production of eight times more power.
Air density (and power output) decreases with altitude at the rate of roughly 2.5% for every 1000’ above sea level. Temperature affects the power derived from wind energy in two ways. First, they affect wind patterns and intensity. Second, they affect the air density and thus the mass of air drawn through the turbine. In desert and open areas, where most wind turbines are installed, temperature differences as high as 60°C are possible between summer and winter seasons. As a result, as much as 20% more power can be extracted in winter when the air is denser.
References
(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Further Reading
Gipe P., Wind Energy Basics, —A comprehensive guide to modern small wind technology. AWEA (http://www.awea.org).
Elliott, D. et al., Wind Energy Resource Atlas of the United States, by American Wind Energy Association (http://rredc.nrel.gov/wind/pubs/atlas).
Khennas, S., Small wind systems for rural energy services, London: ITDG Pub., 2003.
Solar Energy, Direct Science Elsevier Publishing Company, the official journal of the International Solar Energy Society ®, is devoted to the science and technology of solar energy applications, and includes the indirect uses such as wind energy and biomass.
Home Power Magazine—bimonthly magazine for farm and home wind turbines (http://www.homepower.com).
External Links
Energy Efficiency and Renewable Energy Clearinghouse (http://www.eren.doe.gov).
National Wind Technology Center, National Renewable Energy Laboratory (http://www.nrel.gov/wind).
Energy Efficiency and Renewable Energy: Wind Energy Technologies, US DOE (http://www.eere.energy.gov/windandhydro/wind_technologies.html).
American Wind Energy Association (http://www.awea.org).