Our civilisation is powered by electricity. Television, radio and the Internet are all, obviously, run directly on electricity. Newspapers are made in electrically-powered printing presses, streets are lit by electric light and even gas-fired central heating systems use electrical pumps and ignition. But where does all this electricity come from? Power stations!
Making Electricity - Generators
The simplest way to make electricity is to move a wire through a magnetic field. By a process known as induction, the combination of magnetism and movement causes electricity to flow in the wire.
To build a generator, take two magnets and arrange them so that there is a gap between the North pole of one and the South pole of the other. There is a magnetic field in this gap. Make a loop of wire in the shape of square. Mount it on an axle so that the axle goes through the midpoints of two opposite sides of the square. Now put the loop in the magnetic field. The axle should lie not along the line joining the N and S poles but at right angles to it. Now spin the loop of wire on the axle. Because the wire moves through the magnetic field, an electrical current will be induced in the loop and will flow around the loop.
We now have a generator, although the electricity produced is circulating only within the generator. The part of the generator which rotates is called the rotor. The bit that remains stationary is called the stator. If we make a small break in the loop and attach a pair of wires to it, we can feed the electricity produced in the rotor along these wires to whatever we like. Of course, the wires will get all twisted as the rotor rotates, so we use clever gizmos called slip rings to take the electricity from the moving rotor and feed it into stationary wires. In a normal power station, these wires then ultimately feed into the power transmission grid. The electricity is sent all over the country.
Real-life generators basically work on that principle, but have a few improvements. Instead of one loop of wire, there are hundreds mounted on the rotor so it can produce hundreds of times as much electricity. Instead of using permanent magnets to provide the magnetic field, an electromagnet is used. This is powered by the generator itself.
A simple generator that many people will have encountered is the dynamo on a bicycle, which is turned directly from the wheel rim. This converts the movement of the wheel into electricity to power the lights. If you've driven a bicycle with a dynamo, you'll know that you have to pedal harder when the dynamo is in operation. You are providing the energy which is being turned into electricity.
Turning the Generator - Turbines
We need to somehow turn the rotor to make electricity. The normal way of doing this is to use a turbine. The turbine is a fan in reverse. A fan takes rotation and turns it into a movement of air. A turbine takes a movement of air, steam or water and turns it into rotation of a shaft. This shaft then feeds directly into the generator.
Wind generators are probably the easiest type of power station to understand. The turbine consists of the rotating blades. The wind turns the blades, these drive the generator. Wind generators are becoming more popular because they are clean and damage the environment very little. Some people consider them unsightly, and they can produce a lot of noise, so they are rarely built in inhabited areas. This means that the power must be transmitted long distances to the cities where most of the power is consumed. Power is lost in transmission1. In recent years experiments have been done with wind generators standing in the sea. Denmark is the world leader in this field.
The next easiest to understand type of power station is the hydroelectric station. Here the turbine is driven by moving water rather than moving air. It would be possible to lower turbines into a giant river such as the Colorado or the Nile and make electricity as the water flowed past. However, this is not very efficient. Much of the water will flow around the turbines without affecting them. The way to get the most power out of the water is to dam the river and collect it in a lake. All the water from the lake is then dropped down through a narrow pipe and through the turbines. This extracts the maximum amount of power from the water. The water turns the turbine, the turbine turns the generator and the generator makes the electricity.
Hydro power stations have a great advantage over other types of power station - they can be turned on very quickly, often within a few seconds, if there is a sudden increase in demand for electricity.
Most of the big rivers of the world have been dammed and used to make hydroelectricity. The Boulder Dam on the Colorado is one of the biggest. Another famous dam is the Aswan Dam on the Nile river, which provides power for Egypt.
Obviously, hydro stations have the problem that their location is determined by the river, which is not necessarily right next to the cities where the power is needed. Again, long transmission distances may be needed. Additionally, large tracts of countryside may need to be flooded to make the lake. If the river is a carrier of silt, the lake behind the dam may silt up, making the station useless. This has happened on a number of the Colorado dams.
A less obvious disadvantage of damming rivers in hot countries is that evaporation from the surface of the lake may reduce the amount of water in the river significantly. Where water is scarce, this can trigger an environmental disaster, as well as reducing the output from the power station.
The third, and by far the most important way of driving the turbines is steam. Lots of steam is produced and driven through the turbines, which turn the generator, producing electricity. This leads to two questions:
- How do you make the steam?
- What do you do with the steam afterwards?
Making the Steam
Steam is made by heating up water. This is done in a boiler and can be done by burning just about anything. Coal, oil and gas are the main fuels for power stations, but anything will do. If you have plenty of forests you can burn trees, providing you plant enough trees to replace them. This is known as biomass generation. In Ireland they burn peat, which is a type of brown substance made from thousands of years of decaying sphagnum moss. Russia also burns a lot of peat. However, coal, oil and gas make the steam for most power stations in the world.
Unfortunately, burning fuel produces large quantities of carbon dioxide. This seemingly harmless gas has been blamed for the greenhouse effect which appears to be heating up the world.
Another way to make steam is to heat water using a lump of active uranium. A nuclear reaction in the uranium generates a lot of heat which boils the water. This is described in more detail in the entry How a Nuclear Plant Works. Nuclear stations produce no carbon dioxide, but they do produce small amounts of extremely hazardous waste instead.
Getting Rid of Used Steam
You could just release the steam into the atmosphere, but there are plenty of reasons not to do this. The water used to make the steam has to be very pure so that it does not corrode the turbines. Throwing away this water as steam would be a waste. Releasing high-pressure steam would be both noisy and dangerous. So it makes sense to send the steam back around to be used again. The steam circulates in closed loop from the boiler through the turbine and back around to be boiled again.
For the steam to push through the turbine, the pressure on the other side must be less. To achieve this, the steam is cooled after it leaves the turbine so that it condenses back into water. This water is then fed into the boiler to make more steam. So the water/steam in the closed loop follows a continual cycle of boiling, driving the turbine and condensing.
Cooling the Steam
If the power station is built beside a large river, the easiest way to cool the steam is to take a large amount of cold water from the river, run it over pipes containing the steam and discharge it back into the river. The water taken in from the river will rise in temperature slightly and the steam will drop in temperature a lot, causing it to condense into water within the closed water/steam system. The device in which the steam is cooled is known as a heat exchanger. Care must be taken not to heat the river too much as it might have an adverse effect on the wildlife in the river. Animals and plants have been used to the temperature in the river for millennia and changing the temperature may kill some, and encourage others to thrive, killing off others and altering the food cycle. So for the least effect on the river, your power station should take in a large volume of water and heat it only very slightly.
If there are no rivers nearby, cooling towers are the preferred method for cooling the steam. The steam passes through a heat exchanger, heating up large quantities of water by a small temperature. This water must itself be cooled before being collected and recycled to cool some more steam. This cooling process is done by spraying the water over a huge grid of wooden laths (thin strips of wood). The water drips down and the wind blowing through it cools the water.
There isn't much wind at ground level, but there are always movements of air higher up in the atmosphere. By erecting a giant chimney on top of the wooden laths, the movement of air higher up can be brought down to ground level so that there is always wind to cool the water. Many people, seeing the giant cooling towers of a land-locked power station, think that the towers are packed with machinery or pipework. In fact they are vast empty chimneys, with all the workings at the base, within two metres of the ground. Standing inside a cooling tower is a moving experience, as the clouds flit by overhead, the constant wind blows past with a roaring sound and the water drips and drips.
There you have it. All power stations have generators. The only difference is how you make them turn. It may be directly from the wind, using the power of a dammed river, or using steam heated up by any method you care to imagine. Still, the generators keep turning and the electricity keeps flowing, to keep us all in the 21st Century lifestyle to which we have become accustomed.