A Sankey diagram shows energy flowing through a system as bands whose widths are proportional to the amounts. The input splits into the useful output (straight on) and the wasted outputs (bending away). You can read the efficiency off it and see at a glance where energy is wasted, which is how engineers compare real systems.
About two thirds of the energy in the coal burned at a power station never becomes electricity. You can see exactly where it goes in one picture.
You can calculate efficiency. This lesson draws it, and uses the drawings to compare real systems.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: reading the bands
ExploreTry it on the board and compare what happens with your prediction.
Explore: build two power stations
Drag the sliders, or use + and −.
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A coal power station: 1000 J of chemical energy gives 350 J of electricity, 550 J of heat in the cooling water and 100 J of heat up the chimney. Build its Sankey diagram.
Use the board. This ticks when you get there.
A hydro-electric station: 1000 J from falling water gives 900 J of electricity, 60 J of heat and 40 J of sound. Build it.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: reading and using Sankey diagrams
In your book: sketch the coal and hydro Sankey diagrams side by side. Under them, write which is more efficient and one reason a country might still use the less efficient one.
A solar panel's Sankey diagram shows 1000 J of light in and 200 J of electricity out. What is its efficiency (%)?
Elaborate TypicalUse the same idea in a new context.
Elaborate: a kettle
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A kettle takes in 2000 J: 1800 J heats the water, 180 J heats the kettle and the air, and 20 J becomes sound. Build its diagram.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: choosing an energy source
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
A Sankey diagram shows energy flows as bands whose widths are proportional to the amounts.
The input band splits into the useful output (straight on) and wasted outputs (bending away); outputs always add up to the input.
Read the efficiency as useful ÷ input, and find the biggest waste by the widest wasted band.
Sankey diagrams show energy only, not cost or pollution.
For teachers
Students construct and interpret Sankey diagrams, read efficiency and the largest waste from them, and apply efficiency and conservation to compare real energy systems (AC9S9U05, nodes 3 and 4). About 26 minutes.
I can read the efficiency and the biggest waste from a Sankey diagram.
I can say what a Sankey diagram doesn't show.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider. It closes Year 9 energy, after How much is useful?
Neither is a hinge node. The common slips are reading band widths as unrelated numbers and judging efficiency by the size of the useful band rather than its share of the input. The prediction targets the second.
Board notes: the board draws one split (input to outputs), not multi-stage chains. Power station figures are rounded and typical: coal about 35%, hydro about 90%, solar panels about 20%.
Running the lesson
Engage: ask students who chose 'can't tell' what the widths are drawn to show.
Explore: ask which station's useful band is wider compared with its input.
Explain: the biggest-waste question and the solar efficiency are the success criteria; 'what it hides' is the critique.
In your own words: the two sketches with a comparison.
The hard part (on purpose)
Reading widths as proportions. When a student compares two diagrams by the width of the useful band alone, ask what share of each input band it is.
When students get stuck
A student's outputs don't add up to the input. Ask: What does conservation of energy say about the total out? Fix: It must equal the total in. The board shows the missing amount.
A student says the widest band is the useful one. Ask: Which band carries energy the device was made to produce? Fix: The useful band goes straight on; in a coal station the widest band is waste heat.
Stretch and support
Stretch: A power station's waste heat warms a nearby town's homes. Redraw its Sankey diagram. What happens to its overall efficiency? (Some waste becomes useful, so the efficiency of the whole system rises.)
Support: Give a Sankey template with the input band drawn and gaps for each output's label and amount.
Where it leads
Next: Year 10 energy (climate and the energy balance) uses the same accounting.
In class: draw Sankey diagrams on grid paper (1 square = 10 J) for three appliances from their labels or data sheets.
What each step is for
Engage: reading the bands: Students who read Sankey bands as separate quantities, or think wider means more efficient, need the 'width = amount, all bands add up' rule.
Explore: build two power stations: Building the diagrams for two power stations from data, and seeing the widths, makes the comparison visible before it is calculated.
Explain: reading and using Sankey diagrams: Reading values, spotting the biggest waste, judging what the diagram hides, and comparing systems covers construction, interpretation and application.
Elaborate: a kettle: A device whose useful output is heat checks that 'useful' depends on the job, not on the type of energy.
Elaborate: choosing an energy source: Efficiency is one factor among several in real energy decisions; Sankey diagrams inform but don't decide.
Now you can
Build a Sankey diagram from energy data.
Read the efficiency and the biggest waste from a Sankey diagram.