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Sankey Diagrams and Efficiency



Know that some of the energy used by a device is wasted.

Be able to draw Sankey diagrams to show the amounts of useful and wasted energy produced by a device.

Be able to calculate the efficiency of a device, given information about the energy input and useful energy output.




Task 1 - Numeracy

Discuss with the person next to you:

What is as a decimal?  What is as a percentage?

What is 75% as fraction?  What is it as a decimal?

What is two thirds as a decimal?  What is it as a percentage?

What is 0.1 as a fraction?  What is it as a percentage?

What is 0.4 as a fraction?  What is it as a percentage?





Task 2

In order to evaluate how efficient a device is, we need to look at the amounts of useful and wasted energy.  Complete the table below with the missing values:



Total energy input


Useful energy output


Wasted energy output


Energy efficient lightbulb 12J electrical 3J light  
Incandescent lightbulb 100J electrical   90J heat
coal power station ________ chemical 500MJ Electrical 1000MJ heat + sound
nuclear power station 2000MJ nuclear 500MJ Electrical _______ heat + sound
Toaster   900J useful heat 100J wasted heat + light
Photovoltaic (solar) cell 20J light   15J reflected light + wasted heat
Kettle   1600J useful heat 400J wasted heat + sound
Laptop computer 60J electrical 20J light + sound  

A copy of the table for students is available here as a word document.



Task 3

Sankey diagrams show energy transfers, using scaled arrows to represent the amounts of useful and wasted energy.  Your teacher will show you how to draw a Sankey diagram for the energy efficient lightbulb on the graph paper below.



Pay attention to the example, and then complete Sankey diagrams for the following devices.  The scale has been given to you in each case.


Energy efficient lightbulb - (3 Joules per square)

Coal power station (500MJ per square)

Kettle (400J per square)

Laptop computer (20J per square)

Incandescent lightbulb (100J per square)


You will need to choose a suitable scale for each one!



Task 4

Your teacher will explain the idea of efficiency to you and explain to you how efficiency is calculated.  Calculate the efficiency of each device in your table.  Show full working for each of your calculations.