 |
BasicsBasics Like all other renewable energy, wind energy comes from the Sun. About 1 to 2 percent of the energy from the Sun is converted into wind energy, about 50 to 100 times as much energy as is converted into biomass by all the plants on earth. Most of this wind energy is at high altitudes where continuous wind speeds of over 100 MPH are common. Sunlight heats different parts of the earth surface differently. Land is heated more quickly during the day (and cools faster during the night) than the sea, and areas near the equator are heated more than areas near the poles. The heated surface heats the air above it, which rises to about 6 miles of altitude (the top of the troposphere) and then spreads out to cooler areas where it falls. This convection system is what drives the earth's winds. The change of seasons, the spin of the earth, the irregular albedo of land and water, and the friction of wind over mountainous areas are some of the many factors which complicate the flow of wind over the surface. The direction, strength, and variability of winds over various parts of the earth have been of great interest for a long time. The first large-scale wind mapping efforts were done to aid sailors who depended on winds to power their ships. In the last four decades a considerable amount of work has been done to map winds which might be used to generate power. The power in the wind can be extracted by acting on a moving wing (or rotor), which converts some of that power into torque on the rotor. The amount of power transferred depends on the wind speed, the swept area, and the density of the air. The mass flow of air that travels through the swept area of a wind turbine varies with the wind speed and air density. As an example, on a cool 15 degrees C day (59 degrees F) at sea level, air density is about 1.22 kilograms per cubic meter (it gets less dense with higher humidity). An 8 m/s breeze blowing through a 100 meter diameter rotor would flow about 76,000 kilograms of air per second through the swept area. The kinetic energy of a given mass varies with the square of it's velocity. Because the mass flow increases with the wind speed, the wind energy available to a wind turbine increases as the cube of the wind speed. The power of the example breeze above through the example rotor would be about 2.5 megawatts. As the wind turbine extracts energy from the wind, the wind slows down, which causes it to spread out, which causes it to divert around the wind turbine to some extent. A German physicist named Albert Betz determined in 1919 that a wind turbine can extract at most 59% of the energy in the wind. This limit applies regardless of the design of the turbine. Most turbines extract much less than this due to aerodynamic and generating inefficiencies. Go to... |