Sankey Diagrams

This lesson covers:

  1. What a Sankey diagram is 
  2. How to do calculations with Sankey diagrams 

Sankey diagrams are a visual representation of all the energy transfers taking place in a process.

The thicker the line or arrow - the greater the amount of energy involved.

Sankey diagram for an electric lamp showing energy transfers with 100 Joules of electrical energy, 80 Joules of heat energy, and 20 Joules of light energy.
Sankey diagram showing energy transfer in an electric lamp with 100 Joules of electrical energy, 80 Joules of heat energy, and 20 Joules of light energy.

A Sankey diagram for an electric lamp is shown above. It shows that most of the electrical energy (100 Joules) is transferred as heat (80 Joules) rather than light (20 Joules).

Sankey diagram for an electric lamp showing 100 Joules of input energy, 80 Joules of waste energy as heat, and 20 Joules of useful energy as light.

In the case of this lamp, the energy that is transferred as light is considered 'useful energy output' - but the energy transferred as heat is not useful for providing light, so is considered 'waste energy'.

Sankey diagrams are normally drawn to scale:

Sankey diagram showing energy flow with input energy of 15 Joules, useful output of 10 Joules, and wasted energy of 5 Joules.

For most Sankey diagrams, the width of each arrow is directly proportional to the energy value it represents.


In this diagram, each square of an arrow's width is equal to 1 Joule of energy.

What type of diagram is used to show energy flows?

Sarcher diagram

Sankey diagram

Sotchi diagram

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Sankey diagram showing energy transfer for an electrical motor lifting a load with input energy of 50 J, mechanical work of 28 J, and thermal energy.

The diagram above shows the energy transfer for an electrical motor lifting a load.


How much thermal energy does the motor produce?

22

J

J

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Sankey diagram showing a bulb with 100 J input energy and 70 J wasted as heat energy.

This Sankey diagram shows a bulb with an input energy of 100 J. 70 J is wasted as heat energy.


Calculate the useful energy output of the bulb.

30

J

J

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As well as the flow of energy (joules), Sankey diagrams can also be used to show the power (watts) of a system. 

Sankey diagram showing the input power for a kettle with 1.6 kW for heating the water and 0.2 kW wasted energy.

Above is a Sankey diagram for a kettle. 


What is the input power of the kettle? Give your answer in kW.

1.8

kW

kW

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