A wind turbine isn't a bigger windmill. It's a 150-metre power station that turns slowly, runs itself, and needs the grid to exist.
The size
A recent onshore wind turbine stands 100 to 200 metres tall at blade tip, with a rotor 100 to 170 metres across. Its power: 2 to 6 megawatts. Offshore, the largest exceed 15 MW, with blades more than 110 metres long.
A blade weighs several tens of tonnes, the nacelle several hundred. The foundation of an onshore turbine is a buried block of reinforced concrete of several hundred cubic metres.
How it works
The blades aren't pushed by the wind like a sail. They have the profile of an aircraft wing: air flowing around them creates lift that makes them turn. See Comment vole un avion.
The rotor turns slowly, 5 to 15 revolutions per minute. The blade tips, though, move at more than 250 km/h (155 mph).
The nacelle, at the top of the tower, contains:
- a gearbox that takes it from 10 to 1,500 revolutions per minute for the generator; or, on "direct-drive" models, a giant permanent-magnet generator with no gearbox;
- the generator;
- the braking system;
- power electronics that match the current to the grid;
- an anemometer and a wind vane on the roof.
Two motors steer it constantly:
- yaw turns the whole nacelle into the wind;
- pitch rotates each blade on its own axis to catch more or less wind, as you'd adjust a propeller.
The power curve
- Below about 3 m/s (7 mph), it produces nothing.
- Between 3 and about 12 m/s, power rises very fast: it varies with the cube of wind speed. Twice the wind, eight times the energy.
- Above 12 m/s, it levels off at rated power: the blades feather so as not to exceed it.
- Around 25 m/s (56 mph), it stops and goes into safe mode.
The Betz limit says no turbine can capture more than 59.3% of the energy of the wind passing through it: the air has to keep flowing out behind. The best machines reach about 45 to 50%.
Over a year, an onshore turbine produces on average 20 to 30% of its maximum power (its capacity factor), an offshore one 35 to 50%.
What's rarely said
It needs the grid. Most wind turbines can't start on their own or power an isolated house: their electronics lock onto the frequency of the existing grid, and they use a little power to turn, yaw and heat. During a blackout, a wind turbine stops, even in full wind. Only installations specially designed for it can run as an island.
It's controlled remotely. Each turbine is monitored from afar by its manufacturer, who can stop it, restart it and read its sensors.
It lasts 20 to 30 years. The fibreglass and resin blades recycle badly; the tower, the copper and the steel recycle well.
Tips from people who work on them
- Ice gets thrown. In icy weather, blocks of ice can come off the blades and land tens, even hundreds, of metres away. Signs warn of it at the foot of the machines. Don't stand under an iced-up turbine.
- You climb on foot. Many turbines only have an inside ladder with a fall-arrest rail, or a small lift. A hundred metres of ladder: technicians are trained in rescue at height and always work in pairs.
- Lightning loves wind turbines. The blades have metal receptors and a cable running down to earth. In a storm, move away.
- The noise comes mostly from the blades, a steady whoosh, and the gearbox. At 500 metres, it blends into the wind.
- A nacelle holds hundreds of litres of oil (gearbox, hydraulics), plenty of copper and, on direct-drive machines, rare-earth magnets. It's also an observation platform more than 100 metres up. See Salvaging from any wreck for what you can take from a big immobile machine.
Small wind turbines, for a house or a boat, work on the same principles but without the grid: see Le solaire photovoltaïque, du panneau à la prise for battery storage.