Wind Turbine

A wind turbine is a set of blades on a tower that spin a generator and make electricity. It is not a windmill, which turns the same wind into mechanical work such as lifting water. The energy carried in moving air rises with the cube of its speed, so a site with a little more wind than another yields a great deal more power.

So the resource is measured, not judged. How windy a place feels is an unreliable guide; the only honest answer comes from leaving a wind meter on the site for a full year. Below an annual average of roughly 4.5 to 5.5 m/s (10 to 12 mph) the machine will not spin often enough to repay its cost. Extension guidance calls for about 0.2 hectares (half an acre) of open ground, and the bottom of the blade circle must clear anything within 90 to 150 m (300 to 500 ft) by at least 9 m (30 ft). Trees and buildings steal wind and stir it into turbulence. Height pays: about half as much energy again is available at a 20 m hub as at 10 m over rough ground. Local height limits can rule out a tower entirely, so ask the planning office first.

Off grid it needs a battery bank sized to carry the calm spells, a large part of the cost. Tied to the grid it can use the grid as its store, but then it shuts down in a power cut unless a battery is fitted too: the cheapest connection is the one that fails when it is wanted most. Size it to the load, not the site: past about 120 percent of yearly consumption the surplus sells back at a poor rate. Most disappointments are underperformance rather than breakage: a guessed resource, a tower too short, or a roof mount, which always runs worse than the same machine on a mast. The quietest month averages only 60 to 70 percent of the yearly mean wind speed, which after cubing can leave a fifth of the energy, so storage or a second source has to carry it. Storms are handled by the machine turning itself out of the wind. Expect about twenty years from a well-maintained machine and roughly 1 percent of installed cost a year in upkeep. Settle on a tilt-down tower at build time: a fixed one turns every service visit into a climb.