Seeing a solitary spinning wind generator over a farm, or a mass of utility-scale wind turbines on nearby ridges, is a reminder of an unseen resource—moving air. It’s magic to watch these mechanical devices extract renewable energy year after year. But how do they work?
Let’s follow the energy flow, from the wind itself to electricity in your home, your batteries, or the grid. This article will help you understand the basic principles, components, and functions involved in a wind-electric generator.
Wind is created by differences in air pressure—globally, regionally, and locally. Uneven heating of the earth, water, and air create high and low pressure areas, and the air moves to equalize the pressure, moving from high- to low-pressure areas. The earth’s rotation also affects the wind (the “Coriolis effect”), especially away from the equator.
Local geographic features direct, intensify, diffuse, and otherwise influence the wind, with a variety of effects, such as the increase in wind speed over a ridge, or the up-/down-valley phenomenon we see in mountainous areas.
Remembering that wind is a moving mass of air can help you understand the physical demands of capturing it. Imagining this invisible resource as a colored mass can help you understand how hills, valleys, trees, buildings, and wind turbines interact with it.
To capture wind energy, we have to stick something up into the wind that will convert the horizontal flow of moving air into some sort of usable motion. To pump water, we might want a vertical, reciprocating motion, but to make electricity, we need rotary motion.
The simplest conversion might look something like your basic anemometer—a series of half-cups (imagine half of a ping-pong ball) sticking out from a vertical shaft, and being pushed around by the wind. This is an easy way to make a shaft spin, but not the most efficient way to capture the energy in moving air. This strategy uses what we call “drag”—the wind is dragging the collector with it, and the device cannot go faster than the wind itself. The back side of the rotor is moving against the wind, which slows it down—so the efficiency is inevitably very poor compared with other methods.
Most successful wind generator designs rely primarily on another physical phenomenon—“lift.” We see this property in airplane wings, kites, sailboats, and other devices that use moving air to direct mechanical parts in a direction other than the wind direction.
The most common (read “most established, successful, and engineered”) wind generator designs use two or more blades (usually three) on a shaft that turns on a horizontal axis (parallel to the ground). We call these “horizontal-axis wind turbines”—HAWTs. It may seem counterintuitive that these work, because the blades are spinning at roughly right angles to the wind. This works because the design of the blades relies mostly on lift, not drag. Once a wind turbine is spinning, the wind experienced by the front edge of the blade (called “apparent wind”) is a combination of the natural wind direction and the wind created by the motion of the blade itself.