Ever since windmills were first used to grind grain, or pump water, wind has been a useful source of renewable energy. Today’s turbines look a little different to their cousins – as tall as the Statue of Liberty, in some instances, and with a rotor diameter larger than the wingspan of an Airbus A380 aircraft. However, the process by which they generate electricity has hardly changed in centuries.
The first purpose-built wind turbines to generate electricity were built in the late 19th century in Scotland, Denmark and the US. Wind turbines were widely used to make electricity in rural areas where access to established power systems was not possible. But it has only been in the past two decades that wind turbines have been widely employed to generate electricity to send into existing regional power grids.
BP has the largest operated renewable energy business of any major international oil and gas company, with a stake in 14 onshore wind farms across the US. Together, they gross a generating capacity of 2,259 megawatts – enough electricity to power all the homes in a city the size of Philadelphia.
But, as Stanton Peterson, a mechanical engineer for BP Wind Energy, explains, a lot of work and research is done prior to breaking any ground on a wind farm. “Before building anything, we spend a lot of time studying optimal locations for a wind farm. We will put out sensors in an area to measure wind speeds anywhere from three to nine years before a farm is designed and built. All that is done upfront to make sure we get as much wind to the machines as possible.”
The huge rotor blades on the front of a wind turbine have a curved shape similar to the wing of an aircraft. When air moves across an aircraft wing, it moves faster across the top than the bottom. This results in lower pressure on top of the wing than on bottom, which creates a lifting force on the wing. Similarly, when wind passes a turbine's rotor blade, the pressure difference on either side of the blades causes them to turn. This motion turns a shaft that sits inside the nacelle - the structure that sits on top of the wind turbine tower and looks a bit like an airline cockpit.
The nacelle also contains a gear box, a second ‘output’ shaft and a generator. The gear box increases the speed of the second shaft, which turns the generator, converting mechanical energy into electrical current. This process is based on the law of induction, discovered by English scientist Michael Faraday in 1831, and occurs by moving an electrical conductor, such as a coil of wire, within a magnetic field.
“Electromagnetic induction is the interaction between magnetism and electricity,” says BP Wind electrical engineer Negel Ernesto Martin. “If you move a permanent magnet relative to a conductor—or vice versa—you create what is called an electromotive force. This causes a current to flow producing electrical energy."
The electricity created in the nacelle flows through cables from the generator to the bottom of the tower and then via a transformer and into a nearby electricity substation. All the wind turbine towers in a wind farm send their power to the substation, where it is then passed along into the area’s electrical power grid.
Martin says that to attain the most efficient use of wind and turbines, a number of other technologies are employed in wind farms. “All of our turbines are between 80 and 100 metres tall, and the rotor diameters are larger than the wingspan of an Airbus A380 (80 metres), so these are really large machines,” he says.
The towers are built this tall because there is more wind to be found higher off the ground, and the longer the rotor blade, the more wind it can capture and the more energy it can create.
The blades work best when they are facing the wind and, in order to keep them turned into the wind, each nacelle is equipped with at least one anemometer – a device that senses wind direction and speed and then applies a transfer function, a mathematical representation that relates the response of a system to its input. This process activates a motor that turns the nacelle towards the wind.
“It also regulates the pitch of the blades to capture the most wind possible, and it stays that way until the turbine reaches its rated power [the maximum capacity of a turbine to produce electrical power],” Martin says, “Once it reaches rated speed, the unit stays close to that speed.”
When winds reach speeds between 56-67 miles per hour (90-107 kilometres per hour) - depending on the type of turbine - the nacelle controllers will change the pitch of the blades so that the straight edge faces the wind in a process called feathering. This prevents the blades from catching the wind and rotating, and is the same process used on aeroplane propellers when an engine is shut down.
And, while strong winds usually are desirable, too much wind can also present problems. “We don’t want the rotation speeds to get too high,” says Martin. “This is because too much wind can actually damage the turbine. So, when it reaches maximum speed, the turbine will shut down.”