A windmill does work by harnessing the energy of moving air. Its name is derived from its initial use -- the grinding or milling of grain -- though windmills have also been used to pump water uphill, behind dikes, and into rivers flowing to the sea. You may be familiar with the classical windmill design, which features a rotor composed of four wooden panels with cloth stretched over them. Today, much larger wind generators called wind turbines are widely being used to produce electricity to power homes, businesses, and city buildings and services.

Though windmills and wind turbines vary in design, they operate in the same basic way. Kinetic energy carried in the wind is translated into rotational motion of the blades, which are attached to a rotor hub assembly atop a tall structure. The turning motion then powers a generator that feeds electricity into a transmission grid or else is applied to some other mechanical function.

Some rotor blades are designed like airplane wings so that wind passing over and under the blades creates a pressure differential that generates a lifting force. Other, flatter blade designs simply allow the wind to push the blades. These are known as drag devices. Whichever design is used, it's essential that the blades rotate at a rate of speed that optimizes energy transfer -- neither too quickly nor too slowly.

Because no combustion is involved in wind energy generation, harmful gases, pollution, and other damaging emissions that result from burning fossil fuels like coal and oil are not produced. In many countries, wind farms with hundreds of wind turbines generate as much power as stations fueled by fossil fuels. Though the world's winds have the potential to supply as much as ten times the current total world energy demand, not all parts of the world have equally good wind resources. This means that renewable wind energy -- like many other renewable energy sources -- is more likely to supplement, rather than replace, fossil fuel resources for many years to come.