Energy Resources and Electricity Generation » Electricity from Wind, Water and Hot Rocks
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 4.18P
- The two machines almost every power station needs: a turbine and a generator
- The energy transfers when electricity is generated using wind
- The energy transfers for water: hydroelectric power, tides and waves
- The energy transfers for geothermal resources (hot rocks)
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Unlock This CourseTurbines and generators
This content is examined in Paper 2 only.
Electricity does not come from nowhere. Energy cannot be created, so every power station starts with energy in a store and moves it along a chain until it is transferred electrically to homes and factories. In the exam you must be able to describe that chain for each energy resource. This lesson covers three of them: wind, water and geothermal resources.
All three use the same two machines at the end of the chain.
Key terms:
- Energy resource: something we can take energy from to generate electricity, such as the wind or hot rocks.
- Turbine: a wheel with blades that spins when moving air, water or steam pushes on it.
- Generator: a machine that is turned by a turbine and transfers energy electrically. You will see how it works in the lesson called Generators.
So the last part of every chain in this lesson is the same: something pushes on the turbine and makes it spin, the spinning turbine turns the generator, and the generator transfers energy electrically. Only the start of the chain changes.
Electricity from wind
Wind is moving air, so it has energy in its kinetic store. A wind turbine is a tall tower with large blades at the top. Behind the blades, inside a casing, is a generator.
- The moving air pushes on the blades. Energy is transferred mechanically from the kinetic store of the air to the kinetic store of the turbine blades.
- The spinning blades turn the generator.
- The generator transfers the energy electrically, along cables, to wherever it is needed.
The air leaving the blades moves more slowly than the air arriving. That makes sense: its kinetic store has gone down because energy has been transferred to the turbine.
Electricity from water
The spec says "water", which covers three resources. All of them use moving water to spin a turbine.
💧 Hydroelectric power
A dam is built across a river valley, so a deep lake called a reservoir collects high up behind it. Remember, this water has energy in its gravitational store. When gates open, the water falls down pipes inside the dam. Its gravitational store goes down and its kinetic store goes up. At the bottom, the fast water spins a turbine, which turns a generator.
🌙 Tides
A tidal barrage is a dam built across a river mouth. The rising tide fills the space behind it and the water is held there at a height. When the sea level outside drops, the water is let out through turbines. This is the same chain as hydroelectric power: gravitational store, then kinetic store, then turbine and generator.
🌊 Waves
Waves make the surface of the sea move up and down, so the water has energy in its kinetic store. A wave generator uses this movement to spin a turbine. In one design, the rising water pushes air through a turbine. The turbine turns a generator as before.
Notice the difference at the start. Hydroelectric and tidal power stations begin with water held at a height, so the chain starts with the gravitational store. Wave generators begin with water that is already moving, so the chain starts with the kinetic store.
Electricity from geothermal resources
Deep underground the rocks are hot. They have a large amount of energy in their thermal store. In some places, such as Iceland, hot rocks lie close to the surface, so they can be used as an energy resource.
- Cold water is pumped down a deep hole to the hot rocks.
- Energy is transferred by heating from the thermal store of the rocks to the thermal store of the water. The water becomes steam.
- The steam rises up a second hole and 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.
This is the only resource in this lesson that needs steam. The wind and water chains have no heating step.
The chains side by side
| Resource | Store at the start | What happens next | End of the chain |
|---|---|---|---|
| Wind | kinetic store of the moving air | air pushes on the blades (mechanical transfer) | kinetic store of the turbine, generator, energy transferred electrically |
| Hydroelectric | gravitational store of the water behind a dam | water falls, so energy moves to its kinetic store | kinetic store of the turbine, generator, energy transferred electrically |
| Tides | gravitational store of the water held behind a barrage | water flows out, so energy moves to its kinetic store | kinetic store of the turbine, generator, energy transferred electrically |
| Waves | kinetic store of the moving water | the movement drives a turbine (mechanical transfer) | kinetic store of the turbine, generator, energy transferred electrically |
| Geothermal | thermal store of the hot rocks | water is heated to steam, steam pushes on a turbine | kinetic store of the turbine, generator, energy transferred electrically |
You cannot be shown a diagram here, so learn to draw each chain as a flow chart. Draw a row of boxes joined by arrows. Write one store or one machine in each box, in order, and label each arrow with the type of transfer. For hydroelectric power the boxes read: gravitational store of water, kinetic store of water, kinetic store of turbine, generator. The last arrow, leaving the generator, is labelled "transferred electrically".
Where is energy wasted?
No power station is 100% efficient. In every chain, friction in the moving parts of the turbine and generator transfers some energy by heating to the thermal store of the surroundings. Some is also transferred as sound. So the energy transferred electrically is always less than the energy that left the store at the start. The total still adds up, because energy is conserved.
Worked example
Describe the energy transfers in a geothermal power station.
Step 1, the starting store: energy is in the thermal store of the hot rocks.
Step 2, the first transfer: energy is transferred by heating to water pumped down to the rocks, which turns to steam.
Step 3, the turbine: the steam turns a turbine, so energy is transferred mechanically to the kinetic store of the turbine.
Step 4, the end: the turbine turns a generator, which transfers the energy electrically.
Use the same four steps for any resource: starting store, what pushes on the turbine, turbine, generator.
Common mistakes
- Writing "electrical store" at the end. There is no such store. Energy is transferred electrically by the generator.
- Saying a wind turbine "makes" or "creates" energy. It only transfers energy that was already in the kinetic store of the air.
- Starting the hydroelectric chain with the kinetic store. The water behind the dam is still. The chain starts with its gravitational store.
- Missing out the turbine or the generator. They do different jobs, so name both.
- Putting steam into the wind or water chains. Only geothermal power stations in this lesson use steam.
Exam-style question
A hydroelectric power station has a reservoir behind a dam, high in the mountains.
(a) State the energy store of the water in the reservoir that is used to generate electricity. [1 mark]
(b) Describe the energy transfers that take place when this power station generates electricity. [4 marks]
(c) Explain why the energy transferred electrically is less than the energy lost from the store in (a). [2 marks]
(d) A wind turbine also generates electricity. State the energy store at the start of the chain for a wind turbine. [1 mark]
Model answer
(a) The gravitational store. (1)
(b) As the water falls down the pipes, energy is transferred from its gravitational store to its kinetic store. (1) The moving water pushes on a turbine and makes it spin, (1) so energy is transferred mechanically to the kinetic store of the turbine. (1) The turbine turns a generator, which transfers the energy electrically. (1)
(c) Friction in the turbine and generator (1) transfers some energy by heating to the thermal store of the surroundings, so it is wasted. (1)
(d) The kinetic store of the moving air (the wind). (1)
Exam tip
For a "describe the energy transfers" question, write the chain in order and name a store or a transfer in every sentence. Four marks usually means four links, so for hydroelectric power check you have all of: gravitational store, kinetic store of the water, turbine, and generator transferring energy electrically.