Talking about wind turbines

Panoramic view of the Whitelee wind farm with Lochgoin Reservoir in the foreground. Date:24 October 2010 Source: Wikipedia Author: Bjmullan

Panoramic view of the Whitelee wind farm with Lochgoin Reservoir in the foreground.
Date: 24 October 2010 – Source: Wikipedia – Author: Bjmullan

NOTE: This is a language guide, not a thesis on wind turbines. My hope is that it will help those of you whose first language is not English to be able to discuss, or write about, wind power.
You can’t have failed to notice strange new trees growing up in windy places. They are usually white and only have three branches, and their trunks are straight and clean. They go by several names, but windmills and wind turbines are two of the most common. Wind power has been utilised for various purposes, like pumping, irrigation and milling, for over two thousand years, but the first electricity-generating wind turbine was a battery charging machine installed in July 1887 by Scottish academic James Blyth to light his holiday home in Marykirk, Scotland (Wikipedia). There have been many designs of electricity generating windmills, but I shall concentrate on the horizontal axis, three-bladed type, pictured above, which have become the most common.

So, everyone knows what they do – they make electricity from the wind,  but exactly how do they do it? First of all, let’s take a look at the component parts of a typical turbine.

Schematic diagram of a modern horizontal-axis, three-bladed wind turbine Date30 November 2006 Author: Office of Energy Efficiency and Renewable Energy - public domain Permission

Schematic diagram of a modern horizontal-axis, three-bladed wind turbine
Date: 30 November 2006 – courtesy of Wikipedia
Author: Office of Energy Efficiency and Renewable Energy – public domain

So, whatever the size of our horizontal-axis wind turbine, it must face into the wind in order for it to work. Very small turbines achieve this by having a big wind vane at the back, just like the one on the anemometer (wind-speed meter) in the drawing above, only much bigger. However, on larger models this is impractical as the vane would have to be absurdly large. However, whatever the design of any major wind turbine, the list of components remains pretty much the same, although their positions may differ. Here follows a description of the main parts and how they work.

Tower: The towers are typically large steel tubes, getting narrower as they get higher, and are delivered in, for example, 20-metre (65 foot) sections, which are then bolted together on site. For generators of less than 1 MW, there are access ladders which lead from a door at the bottom to a fire-proof hatch at the top, giving access to the nacelle. For larger, taller turbines there will also be a lift to carry the maintenance crew to a position close to the top. For the last few metres they will use a ladder. Also inside the tower is the cable which carries the electricity from the generator to the ground. The upper part of the cable hangs free so that it can twist as the nacelle turns. The largest tubular towers are over 125 metres (410 feet) tall, and although there is a theoretical height limit for such towers, greater height could be achieved by using steel lattice structures, similar to power pylons or, indeed, the Eiffel Tower.

The only major component inside this Mervento nacelle is the generator.
Photo: Malcolm Pemberton

Nacelle: The nacelle is the outside casing at the top of the tower. It’s function is to protect the components inside from the weather, provide a safe working environment for the maintenance crew, and also add something aesthetic to what would otherwise be an ugly mess of mechanical parts. The heavy components are not mounted on the nacelle itself, but on a steel chassis inside it. Access to the nacelle is through the yaw ring (it is not a disc as shown on the diagram), and the roof of the nacelle may also open, or have a hatch in it, for servicing the anemometer. Typical nacelles are about the size of a 30-foot
(9-metre) shipping container, but slightly prettier.

Turbine: The turbine is just a propeller in reverse and, like on an aircraft, the blades can pitch (turn) to take best advantage of the wind available. The hub of the turbine is of cast metal and the blades of fibreglass of some kind. On the largest turbines each blade can be over 50 metres (164 ft) long – that’s the length of five buses end to end.

Shaft: The shaft or main axle, which may be hollow or solid depending on the design, is supported on at least two sets of bearings. The shaft must carry the weight of the turbine, and transmit its considerable torque (turning power) to the generator.

Brake: The brake is used to prevent over-speeding, and also to hold the mechanisms in place for maintenance. The actual turning speed of the turbine is controlled by the pitch of the blades, not by applying the brake. The brake can be positioned anywhere along the drive shaft, and is typically of disc type.

Gearbox: Most, but not all, wind turbines have gearboxes, of a single speed variety that increases the shaft speed to something that better suits the generator in use. The gearbox can be the heaviest component, so using a direct-drive version, where the turbine is connected directly to the generator,  can save considerable weight and space. Above left is a photo of a Mervento direct drive design, the only large component in the nacelle being the generator.

Conventional nacelle, about the size of a shipping container - photo: ABB

Conventional nacelle, about the size of a shipping container – photo: ABB

Generator: The generator is the bit that actually makes the electricity, and most of them are of the AC (alternating current) type. In small, vertical-axis windmills, the generator can be housed at the bottom, driven by a shaft inside the tower. However, such a long shaft is not practical in taller towers as shaft weight and vibration become problematic. A generator is just a motor in reverse, with some small differences. If you apply electricity to a generator, it will turn; if you turn an electric motor it will produce electricity.

Yaw drive: The yaw drive turns the whole nacelle on its vertical axis so that the turbine faces into the wind. The yaw ring also gives access to the nacelle from the tower, and has a braking system to control unwanted movement. The yaw drive is controlled by the …

Controller: The controller receives wind-speed and direction information from the anemometer, and then translates this to control the yaw drive and the pitch of the blades. The idea is that the turbine should turn at as constant a speed as possible, supplying a steady amount of electricity. If the wind speed drops too low the blades will be feathered (turned edge on to the wind) so that there is no turning force, the brake will be applied and the turbine will stop. Similarly, if the wind speed is too high, the same thing will happen.

Step-up transformer (not shown on the drawing):  Typically, generators supply about 690 volts, which is too low for long-distance transmission (there are always losses as electricity passes along a wire, and the lower the voltage, the greater the loss), so a transformer increases this voltage to, for example, 21 kV (21,000 volts). This transformer may be housed in the nacelle, inside the base of the tower, or in a small building just outside. At another transformer, close to the grid (network) connection, the voltage will again be stepped up to local transmission level, for example 110 kV or 220 kV (depending on country etc).

Frequency converter: Because the turbine does not turn at a constant speed, despite all attempts to make it do so, the AC current from the generator is first converted to DC (direct current) and then back to AC at a frequency to match the grid. This is done by a frequency converter, or drive. In reverse, the speed of AC electric motors is controlled by drives. Why not use a DC generator in the first place? Because they are actually more complicated and require more maintenance than their AC siblings.

Connecting something like a wind turbine to the grid is challenging. The variable nature of the wind cannot be allowed to generate unreliable electricity, so the idea of Smart Grids has been developed. Smart Grids use computer technology to harmonise wind power with the network.

1 Comment (+add yours?)

  1. Talking about wind power | Malcolm's English Pages
    Apr 20, 2013 @ 16:32:37

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