The principles of electricity generation were discovered by Michael Faraday in 1831. He found that moving a bar magnet through a wire coil generated electricity. Modern generators are more complex, but the difference is mainly one of scale.
Power stations range in size from single wind driven devices to major industrial sites, employing many hundreds of staff, but what they are all doing is converting one kind of energy into another. Different stations use a variety of energy sources but they all generate electricity in the same way.
Simplified to its essentials, a power station consists of just two major items. First, there is a machine that generates electricity when its shaft is turned - the generator. Secondly, there is some kind of engine to turn the shaft. The generated voltage can be up to 25,000 volts, which is transformed to a higher voltage for transmission on the grid.
Generators need to turn fast and continuously, and the most efficient type of engine for this is the turbine. In the United Kingdom, most power stations use steam-driven turbines.
In a power station generator, the equivalent of Faraday’s bar magnet is a powerfulelectromagnet - a coil energised by direct current to produce a magnetic field. This is mounted on the central rotating shaft, and is called the rotor. Around the rotor is a series of coils called the stator, in which the electrical voltage is generated by the rotating magnetic field. Both rotor and stator may weigh several hundred tonnes.
The rotor turns at 3000 revolutions per minute - 50 per second - to produce alternating current with a frequency of 50 hertz (cycles per second). Modern generators typically produce 500 megawatts of power, the largest generating up to 700 megawatts - enough to light seven million 100 watt bulbs!
The Boiler
The diagram below shows how coal is used to drive a turbine. Firstly the coal is pulverised into a fine powder. Mixed with preheated air, the coal powder burns fiercely to heat water in the boiler tubes. The steam emerging at the top of the boiler is returned to the furnace to be superheated. This increases its energy before it is piped to the high-pressure cylinder of the steam turbine. Superheated steam may be hot enough to make the steam pipe glow a dull red – over 560°C.
The hot gases leaving the boiler on their way to the chimney are used to preheat both the air needed for combustion and the condensed water returning to the boiler (in the economiser).
- Coal is pulverised into dust
- Hot air blows coal dust into the furnace
- The dust burns like a gas and boils the water
- Superheated steam drives the turbines
- The generator produces electricity
- Steam is cooled and converted into water by the condenser
- The warm water is cooled by air blowing through the tower
- Water is recirculated to maximise use
A modern boiler can burn over 260 tonnes an hour of pulverised coal. Transporting such quantities is expensive; so many coal-fired power stations are built close to coalfields. Some coastal stations have coal brought in by sea, but inland power stations generally have to be supplied by train. These stations have their own loop line (‘merry-go-round’), where special hopper wagons discharge their coal load on the move, into bunkers beneath the rails.
For the same reasons, most oil-fired power stations are near oil refineries, or are located on the coast or large estuaries. A typical 500 megawatt boiler can burn up to 2,750 tonnes of oil per day – over 115 tonnes per hour.
Power stations waste a lot of the energy in the fossil fuels they burn. The best convert only about 38% of it into electricity. Most of the wasted energy is heat – in the flue gases, and in the water used to condense the steam as it leaves the turbine cylinders. Combined heat and power (CHP) units make use of this ‘waste’ heat to provide hot water for room heating. The electricity generated can be used locally or supplied to the National Grid. Although many CHP schemes are currently being set up in Britain, the economics are not always favourable, mainly because we have plenty of cheap natural gas for home and industrial heating. It may be worthwhile for certain users, such as small factories and schools, leisure centres, hospitals and office blocks. CHP could become more important in the future, because it can help reduce the emission of carbon dioxide – a ‘greenhouse gas’ – into the atmosphere.
Distribution
Electricity arrives in your area from the national supply network (the National grid) at 275,000 or 400,000 volts. It is reduced to 132,000 volts at a substation for distribution within each area of the country, travelling to further substations known as grid supply points. From these it is distributed on overhead lines or underground cables at 33,000 volts - the primary distribution networks - to the intermediate substations.
At the intermediate substations, electricity at 33,000 volts is reduced to 11,000 volts for secondary distribution. The secondary distribution networks then carry it at 11,000 volts to individual towns, industrial areas and groups of villages.
Particularly heavy users such as manufacturing industries are supplied at 33,000 volts. Electrified railways have their own substations which draw electricity direct from the grid supply point - the latest overhead-line systems run at 25,000 volts.
At the final substations, transformers reduce the 11,000 volt supply to 230 volts for small scale customers such as homes and shops. A typical substation serves 200 to 300 houses. Larger users such as farms take electricity at 415 volts.
