Electricity generation

Electricity is generated in a power station. In previous grades, we have looked at how electricity is generated within coal-powered power stations and distributed to the country in the national electricity grid. We are going to revise some of these concepts.

The general method for producing electricity is to turn a turbine which will turn a generator. In South Africa most of the power stations use coal for fuel. The coal is mined out of the earth. The coal is transported to the power station in large trucks or trains.

Let's take a closer look at what happens inside a coal-powered power station. Have a look at the following diagram.

An overview of the steps in a coal-powered station:

1. The large chunks of coal are first crushed into a fine powder. This is called pulverisation.

2. The coal is then transported to a furnace where it is burnt.

3. The thermal energy from the burning coal is used to boil water and generate steam.

4. The steam pushes the blades of the turbine and so the turbine spins.

5. The turbine is connected to the shaft of the generator which then rotates large magnets within wire coils, which generates electricity.

6. The electric current is sent through the power lines to businesses and homes.

Is coal a renewable or non-renewable energy source? Explain your answer.

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What are the disadvantages of South Africa's reliance on coal as the main source of energy in power stations?

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There are many different ways to get enough energy to turn the turbine. What are some of the alternative energy sources which can be used instead of coal? List them below.

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The Gariep Dam on the border of the Free-State and Eastern Cape provinces is a hydroelectric power station which uses the water falling out of the dam to turn the turbine. How does the falling water make the turbine turn? Let's investigate that a little more.

A large dam wall is often built to collect the water. The water then flows through the hydroelectric power plant.

A turbine can be used to transfer kinetic energy from the falling water to the generator. A generator is a device which converts mechanical energy to electrical energy. A generator consists of large metal coils which move within a magnetic field. In some generators the coils are stationary and the magnet is rotated and in other generators the magnets are stationary and the coil is rotated. Turning a set of conducting metal coils inside a magnetic field generates an electric current.

The modern-day generator works on the principle of electromagnetic induction discovered by Michael Faraday in 1831-32. Faraday discovered that you could cause an electric current to flow by moving an electrical conductor, such as a wire that contains electric charges, in a magnetic field. The movement creates a potential difference between the two ends of the wire or electrical conductor. This then causes the electric charges to flow through the conductor as current.

There are many different types of generators used in different situations, not only in power stations. Some yachts and boats use water or wind-powered generators to charge their batteries through the use of small propellers in the water or a wind turbine. Portable generators are often used in homes and businesses when there are power outages to keep certain appliances running, such as the lights and refrigerator. Portable generators run on fuel, such as petrol, diesel or gasoline to turn the shaft to generate electricity.

Small generators, called dynamos, can be rotated by a person rotating a crank and are used in devices such as portable radios and torches, especially on mining helmets. Dynamos are also used in bicycle lights. The dynamo in a bicycle consists of a permanent magnet and some surrounding coils of wire, and is attached to the wheel, which rotates as the bicycle moves. As the dynamo rotates it generates a changing magnetic field that generates electricity in the surrounding coils of wire.

As we have seen, most of the electricity generated in South Africa uses the burning of coal to provide steam to turn the turbines. Some of the electricity is generated using alternative energy sources. Why are they called alternative energies? This is because they are not the main source of energy. Most alternative energy sources are renewable forms of energy.

Nuclear power in South Africa

South Africa only has one commercial nuclear power station, the Koeberg Power Station in Cape Town. We are going to take a closer look at nuclear power.

Before we look at nuclear fission, let's revise the model of an atom which we have already learned about. Label the following diagram of the model of an atom.

Most atomic nuclei are stable. But, there are some elements which are not stable. The nuclei in these unstable elements spontaneously emit particles, which is referred to as radiation. A nucleus that emits radiation is said to be radioactive.Radioactive decay is the process when an unstable nucleus of an atom emits particles. It then 'decays' into another type of atom with a different mass.

Nuclear power was first pursued for electricity generation in the beginning of the 20th century when researchers discovered that radioactive materials, such as radium and uranium, release large amounts of energy when they decay. For a long time, however, the use of nuclear power was not seen as practical or possible to generate electricity.

This changed in the 1930's with the discovery of nuclear fission. During nuclear fission, scientists split the nucleus of an atom into two smaller atoms. This releases a huge amount of energy. There is also another way in which the energy in an atom can be released during nuclear fusion. Nuclear fusion is when two atoms are brought together to make a new, bigger atom. During both of these nuclear reactions, huge amounts of heat and radiation are released.

Nuclear power uses nuclear fission, nuclear fusion and nuclear (radioactive) decay. Uranium is an element which is unstable and undergoes radioactive decay at a very slow rate. This makes uranium a good choice to use as a fuel in nuclear power stations. Nuclear power stations therefore use uranium and induce nuclear fission to release the heat and radiation. Let's take a look inside a nuclear power plant, such as Koeberg Power Station.

The main difference between a nuclear power station and other power stations, such as coal power stations, is the way in which the water is heated to produce the steam.

A nuclear power station has a nuclear reactor vessel. The nuclear power station needs to control the huge amount of energy given off during nuclear fission of uranium in order to generate electrical energy. The nuclear reactor is the device in which the nuclear reactions take place and are controlled. The uranium is formed into pellets which are arranged into long rods, called the reactor core. The rods together make a bundle which is inserted into water to prevent overheating and melting. The bundle of uranium rods also contains control rods which help to control the process.

The large amounts of energy produced by the nuclear fission reactions in the uranium fuel rods heat the water to produce steam. The steam is used to turn large turbines which transfer the kinetic energy to generators, which produce electricity in the same way as in other power stations.

Most of South Africa's power stations use the burning of coal to produce enough heat to boil the water. The only difference in a nuclear power station is how the energy is produced to heat the water and produce steam.

As was mentioned before, the nuclear fuel is radioactive. The radiation that the fuel emits is dangerous and can be very harmful as it can enter our bodies and damage our cells. The workers in nuclear power plants therefore need to take extra precautions. The nuclear reactor is also contained within a special container that acts as a barrier to radiation.

You might have heard some of the debate surrounding the use of nuclear fuel in power stations. There are many supporters but also many critics. Let's look at some of the advantages and disadvantages of using nuclear fuel.

We are now going to look at what happens to the electricity that is generated in a power station, whether it is coal, nuclear or a hydroelectric power plant.

National electricity grid

The national energy grid is a network of interacting parts which form one big system to provide electricity to all sectors of the economy. It starts at the power stations where the electricity is generated. The power stations then feed the electric current into large power lines called transmission lines. There is a massive network of transmission lines that extends throughout the entire country. The transmission lines are supported by pylons.

The following diagram illustrates a coal-powered power station connected to the national electricity grid.

Very high currents are generated at the power stations. The transmission lines have some resistance. If the power stations transferred electricity at high currents, what do you think would happen in the transmission lines? Hint: Remember what we learned about the effects of resistance.

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To prevent the waste of energy, the electric current is rather sent through the transmission lines at very high voltages and low current.

However, these voltages are too high for use in both private homes and commercial buildings. In our homes and buildings we need a low voltage and high current again.

Transformers are used to change the voltages at different points in the grid. As you can see in the previous diagram, the electricity first goes through a transformer before entering the national transmission lines. This is a step-up transformer as it increases the voltage and lowers the current. When the electricity reaches the substation to be distributed locally, there is a step-down transformer which decreases the voltage again and increases the current.

All the systems in the national electricity grid are connected and this means that a power surge or a grid overload can cause blackouts and disruptions throughout the network. What is a power surge?

A power surge is a sudden increase in the voltage somewhere in an electric circuit. The power surge causes an increase in the current strength. This sudden increase in the current strength can damage sensitive circuits.

Lightning strikes near a transmission line can cause a power surge. Other causes of power surges or a grid overload include faulty wiring, or appliances or equipment that require a lot of energy when they are switched on and off. These sudden demands or excesses of energy cause brief changes in voltage that can cause a surge. There are several points in the national grid which can detect a power surge or grid overload. If these are detected then the power supply to that area is cut off.

Concept Map

Complete the concept map on the following page to summarise this chapter on electricity production and delivery.