Energy Resources and Electricity Generation » Electricity from Sun, Fossil Fuels and Nuclear

Making steam to spin a turbine

This content is examined in Paper 2 only.

Remember, a turbine spins when something pushes on its blades, and it turns a generator that transfers energy electrically. In the lesson called Electricity from Wind, Water and Hot Rocks, moving air or water did the pushing. Most of the world's big power stations use steam instead. They heat water until it boils, and the steam rushes through the turbine at high pressure.

So fossil fuel power stations, nuclear power stations and solar thermal power stations all end in the same way: thermal store of steam, kinetic store of the turbine, generator. They only differ in how the water is heated. Solar cells are the odd one out, as you will see.

Key terms:

• Fossil fuel: coal, oil or natural gas. These fuels formed over millions of years from the remains of dead plants and animals.
• Boiler: the part of a power station where water is heated until it turns to steam.
• Nuclear fuel: a material such as uranium that has energy in its nuclear store.
• Solar heating system: equipment that uses radiation from the Sun to heat water.
• Solar cell: a device that transfers energy from light directly to an electrical output.

Electricity from fossil fuels

Coal, oil and gas all have energy in their chemical store. A fossil fuel power station releases it by burning the fuel.

• The fuel is burned in a boiler. Energy is transferred by heating from the chemical store of the fuel to the thermal store of the water in the boiler. The water turns to steam.
• The steam, at high pressure, pushes on the blades of a turbine. Energy is transferred mechanically from the thermal store of the steam to the kinetic store of the turbine.
• The spinning turbine turns a generator.
• The generator transfers the energy electrically, along cables, to homes and factories.

In short: chemical store of the fuel, thermal store of the steam, kinetic store of the turbine, generator, energy transferred electrically.

Electricity from nuclear power

A nuclear power station has no fire at all. Its fuel is uranium, which has energy in its nuclear store. Inside a part of the power station called the reactor, uranium nuclei split and energy is released. You will learn how in the lesson called Nuclear Fission. For this lesson you only need the chain.

• Energy is transferred from the nuclear store of the uranium fuel to the thermal store of the reactor, which gets very hot.
• This energy is used to heat water. Energy is transferred by heating to the thermal store of the water, which turns to steam.
• The steam pushes on a turbine. Energy is transferred mechanically to the kinetic store of the turbine.
• The turbine turns a generator, which transfers the energy electrically.

From the steam onwards, a nuclear power station works just like a coal power station. The only real difference is the start of the chain: a nuclear store instead of a chemical store, and no burning.

Electricity from the Sun

Energy leaves the Sun and crosses space as light and infrared waves. This is energy transferred by radiation. There are two very different ways to use it.

The chains side by side

Resource	Start of the chain	How the turbine is driven	End of the chain
Fossil fuels	chemical store of the coal, oil or gas	fuel is burned to heat water; steam pushes on the turbine	kinetic store of the turbine, generator, energy transferred electrically
Nuclear power	nuclear store of the uranium fuel	hot reactor heats water; steam pushes on the turbine	kinetic store of the turbine, generator, energy transferred electrically
Solar thermal power station	energy transferred by radiation from the Sun	mirrors focus sunlight to heat water; steam pushes on the turbine	kinetic store of the turbine, generator, energy transferred electrically
Solar cells	energy transferred by radiation (light) from the Sun	no turbine	energy transferred electrically, straight from the cell

To draw the fossil fuel chain as a flow chart, draw four boxes in a row joined by arrows. Write in the boxes, in order: chemical store of fuel, thermal store of steam, kinetic store of turbine, generator. Label the first arrow "by heating", the second "mechanically", and the arrow leaving the generator "transferred electrically". For nuclear power, change only the first box to "nuclear store of uranium fuel".

Where is energy wasted?

Energy is wasted at every stage of a steam power station, so the energy transferred electrically is much less than the energy taken from the fuel.

• At the boiler: in a fossil fuel power station, hot gases from the burning fuel escape up the chimney and heat the surroundings.
• At the turbine: the steam is still warm when it leaves the turbine. It is cooled back to water, and the energy taken out of it is passed to the thermal store of the surroundings, often through large cooling towers. This is the biggest waste of all.
• At the turbine and generator: friction in the moving parts heats the surroundings, and some energy is transferred as sound.

In a typical coal power station, more than half of the energy from the coal is wasted in these ways. Solar cells waste energy too. Only part of the light that hits a cell is transferred electrically. The rest is reflected or heats the cell.

Exam-style question

A country generates electricity using a gas power station and a large field of solar cells.

(a) State the energy store of the gas that is used to generate electricity. [1 mark]

(b) Describe the energy transfers that take place when the gas power station generates electricity. [4 marks]

(c) Describe one way in which the energy transfers in a solar cell are different from those in the gas power station. [2 marks]

(d) Explain why the energy transferred electrically by the gas power station is less than the energy transferred from the gas. [2 marks]