P3.1 Energy Demands

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: the main energy resources, what we use them for, how power stations generate electricity, and nuclear fuel and biofuels. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Name the main energy resources and sort them into renewable and non-renewable
• Describe the three main uses of energy resources: transport, electricity generation and heating
• Describe how a fuel-burning power station generates electricity
• Compare nuclear fuel with fossil fuels
• Explain what a biofuel is and why it is described as renewable and carbon-neutral

Key words

• Energy resource: Anything that can be used to supply energy in a useful form, such as a fuel, the wind or sunlight.
• Fossil fuel: Coal, oil or natural gas: fuels formed over millions of years from the remains of ancient plants and animals.
• Non- renewable: A resource that is being used up much faster than it can be replaced, so it will eventually run out.
• Renewable: A resource that is replaced by natural processes as fast as we use it, so it will not run out.
• Nuclear fuel: Uranium or plutonium. Energy is released when the nuclei of its atoms split (nuclear fission). It is not burnt.
• Biofuel: A fuel made from living or recently living organisms, such as wood, ethanol from crops or methane from waste.
• Carbon- neutral: Adding no extra carbon dioxide to the atmosphere overall: the CO₂ given out equals the CO₂ taken in.

Energy resources and what we use them for

An energy resource is a source of energy that people can use. The main resources on Earth are: fossil fuels (coal, oil and natural gas), nuclear fuel, biofuel, wind, hydroelectricity, geothermal energy, the tides, the Sun and water waves.

Most of the world’s energy still comes from burning fossil fuels. Energy resources have three main uses:

• Transport; Examples cars, lorries, ships, aircraft, trains; Resources mostly used today oil (petrol, diesel, jet fuel); some biofuel and electricity
• Electricity generation; Examples power stations feeding the National Grid; Resources mostly used today gas, nuclear, wind, solar, hydroelectric, biofuel, some coal
• Heating; Examples heating homes, offices and water; Resources mostly used today natural gas burnt in boilers; electricity; oil; wood

Inside a fuel-burning power station

Coal, oil, gas and biofuel power stations all work in the same way. The fuel is burnt to heat water in a boiler. The water turns into high-pressure steam, which pushes on the blades of a turbine and makes it spin. The turbine turns a generator, which generates electricity for the National Grid. The steam is then cooled and condensed so the water can be used again.

Energy transfers: chemical energy store of the fuel → thermal energy store of the water and steam → kinetic energy store of the turbine and generator → transferred electrically to the grid. Much of the energy is wasted by heating the surroundings, so the power station is far from 100% efficient.

Worked example: power output of a coal-fired power station

A power station burns 80 kg of coal every second. Each kilogram of coal releases 25 MJ of energy. The power station is 36% efficient.
Energy released each second = 80 × 25 = 2000 MJ, so the input power is 2000 MW (2.0 × 10⁹ W).
Useful output power = efficiency × input power = 0.36 × 2000 MW = 720 MW

Answer: input 2000 MW, useful output 720 MW

Nuclear power

A nuclear power station uses uranium or plutonium as its fuel. Nothing is burnt: energy is released from the nuclei of the fuel atoms inside a reactor, which becomes very hot. This energy is used to boil water, and from there it works just like a fossil-fuel power station: steam → turbine → generator. Nuclear fuel is non-renewable, but known supplies of uranium will last a very long time.

• Energy released per kg; Nuclear fuel (uranium) about 3 600 000 MJ; Fossil fuel (e.g. coal) about 24 MJ (coal) to 50 MJ (gas)
• Carbon dioxide given out; Nuclear fuel (uranium) none while running: nothing is burnt; Fossil fuel (e.g. coal) yes: burning releases CO₂
• Waste; Nuclear fuel (uranium) radioactive: must be stored safely for thousands of years; Fossil fuel (e.g. coal) ash and gases, not radioactive
• Other issues; Nuclear fuel (uranium) very expensive to build and to take apart safely; small risk of a serious accident; Fossil fuel (e.g. coal) will run out; causes air pollution

Worked example: how much gas matches 1 kg of nuclear fuel?

mass of gas = energy needed ÷ energy per kg of gas = 3 600 000 MJ ÷ 50 MJ/kg = 72 000 kg

Answer: 7.2 × 10⁴ kg (72 tonnes) of natural gas

Biofuels

A biofuel is any fuel made from living or recently living organisms. Examples: wood pellets and woodchip; ethanol made by fermenting sugar from crops such as sugar beet or maize; biodiesel made from used cooking oil or oilseed crops; methane (biogas) collected from rotting food waste, landfill sites, sewage works or animal manure. Biofuels are burnt in power stations, boilers and (modified) vehicle engines.

• Renewable: new crops and trees can be grown, and waste is produced all the time, so the fuel is replaced as fast as it is used.
• Carbon-neutral (in theory): growing plants take in carbon dioxide by photosynthesis. Burning the fuel releases about the same amount again, so there is no overall increase in CO₂.

Exam tip: In practice biofuels are not completely carbon-neutral: fossil fuels are often burnt to run farm machinery, make fertiliser and transport the fuel. Growing fuel crops can also use land needed for food or cause forests to be cleared.

How much energy do we use?

Energy use is often given in kilowatt-hours (kWh). 1 kWh is the energy transferred by a 1 kW appliance in 1 hour: 1 kWh = 1000 W × 3600 s = 3 600 000 J = 3.6 × 10⁶ J. Average power = energy ÷ time. Use standard form for large numbers and watch the prefixes: k = 10³, M = 10⁶, G = 10⁹.

Worked example: a family’s electricity use

A family uses 3500 kWh of electricity in a year.
In joules: 3500 × 3.6 × 10⁶ = 1.26 × 10¹⁰ J
Hours in a year = 365 × 24 = 8760 h, so average power = 3500 kWh ÷ 8760 h = 0.40 kW (about 400 W)

Answer: 1.26 × 10¹⁰ J; average power about 400 W

Common mistakes

• Saying nuclear fuel is “burnt”. Energy comes from nuclei splitting (fission), so no CO₂ is produced in the reactor.
• Calling biofuels fossil fuels. Biofuels come from organisms alive recently; fossil fuels formed millions of years ago.
• Thinking “renewable” means “causes no environmental problems”. Renewable only means it will not run out.
• Mixing up the turbine and the generator: the steam turns the turbine; the turbine turns the generator, which generates electricity.
• Forgetting to convert MJ to J or kWh to J: 1 MJ = 10⁶ J and 1 kWh = 3.6 × 10⁶ J.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these energy resources is non-renewable? (Choose from: tidal energy, wood pellets, coal, solar energy.)

Answer: coal

Working: Coal is a fossil fuel: it formed over millions of years and is being used far faster than it forms, so it will run out. The others are all renewable.

Question 2. True or false? A biofuel releases no carbon dioxide when it is burnt.

Answer: False

Working: False. Burning a biofuel does release CO₂. It can be called carbon-neutral because the plants took in about the same amount of CO₂ while they grew.

Question 3. The diagram shows a fuel-burning power station. Which label shows the generator? (Choose from: A, B, C, D.)

Answer: C

Working: Steam from the boiler (A) turns the turbine (B), which turns the generator (C). D is the condenser, which turns the steam back into water.

Related lessons

• Previous lesson: P2 RP Required Practical: Specific Heat Capacity
• Next lesson: P3.2 Energy from Wind and Water
• In this chapter: P3.3 Power from the Sun and the Earth
• In this chapter: P3.4 Energy and the Environment
• In this chapter: P3.5 Big Energy Issues

Continue in the full course

This lesson is section P3.1 (AGCSC3201) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.