How does a wind turbine work?

Wind turbines (often colloquially known as wind generators, rather inccurately as they don't generate any wind - quite the reverse) are one of the most cost effective means of collecting free renewable energy. They scale from the very small battery-charging models we sell to enormous farms of hundred-metre turbines, each big enough to power a small village.

In essence wind turbines are very simple. A set of vanes, much like an airplane propellor, are angled so that they face into the wind. The wind blowing past these blades causes them to turn, and inside the hub of the turbine they turn an electrical generator.

On small turbines, the job of angling the vanes into the wind is done by simply having a long tail, like a wind vane, sticking out the back end. On bigger turbines, the head is turned with a motor. One reason for this is to allow the turbine blades to be angled away from the wind if it gets too strong, for turbines can easily spin so fast in high winds that they get destroyed.

Many smaller turbines are also designed to spill wind when it gets too strong. Some achieve this by having slightly flexible blades that bend in strong winds; others have heads that can pivot backwards.

Generating Electricity

The generators found in most small turbines are NOT alternators like those found in cars: they are permanent magnet generators, or PMGs. Alternators only work well at high rotational speed, and this is rarely acheived in small turbines. Permanent magnet generators are good at low speed, are relatively simple to construct, and are simpler in operation: the only part of the generator that moves is the rotor containing the magnets, which doesn't need a supply of electricity, so no commutator and brushes are needed as they would be for an alternator.

The rotors containing the magnets rotate past stators, which have coils of wire embedded in them. As anyone who has done basic physics knows, moving a magnetic field past a coil of wire induces an electrical current in the coil.

The electricity produced in the coil is alternating current (AC) electricity - much like that used in a domestic household, except that it will be at a lower voltage, and the frequency isn't fixed nice and steady at 50Hz, but depends on the rotational speed of the turbine. In practice, it isn't much use in this form, so it is almost invariably rectified to direct current (DC) electricity, which is what is wanted for charging batteries.

Although commutators and brushes aren't needed for the PMG, there is the problem of getting the electricity from the freely-swivelling head of the turbine to the ground, without getting wires twisted up. So a set of slip rings and brushes are usually found in the 'yaw head' (the fitting at the top of the turbine mast that allows the machine to swivel), where the electricity is transferred to wires led down the inside of the mast.

Vertical axis wind turbines

What we have described here is a typical small horizontal axis turbine - but there are a few vertical axis machines available that you might come across. The difference is that instead of having a set of large propellor-like vanes spinning on a horizontal axis, they have a set of vanes standing upright, that spin on a vertical axis. These 'Savonius' vanes are less efficient than horizontal vanes, but the generators inside them are just the same, and they have the advantage that no yaw head is needed. They are also less prone to falling to pieces in high winds.