In 1892 the wonderfully named Captain Matthew Henry Phineas Riall Sankey became lead engineer and steam engine designer for Willans and Robinson. The efficiency of steam engines was a hot topic, and he needed to understand and communicate where the steam energy went and where he could make improvements.

The birth of the Sankey diagram

In 1898 he published An Introductory Note on the Thermal Efficiency of Steam Engines in the proceedings of the Institution of Civil Engineers that included this chart.

As he explained:

No portion of a steam plant is perfect, and each is the seat of losses more or less serious. If therefore it is desired to improve the steam plant as a whole, it is first of all necessary to ascertain separately the nature of the losses due to its various portions; and in this connection the diagrams in Plate 5 have been prepared, which it is hoped may assist to a clearer understanding of the nature and extent of the various losses.”

This became known as a Sankey diagram. A chart that records flows using arrows with a width proportional to the amount of the flow. It reduces a complex system to a single graphic that quickly allows the reader to see the most important flows and their interactions. Sankey diagrams are one of my favourite tools for looking at systems. Easy to understand, great at triggering debate, and encouraging focus on the most important intervention points.

In the early days, Sankey diagrams were mostly used to look at energy flows, but over the years they have been applied to any situation where the creation or use and fate of a resource needs tracking. The resource can be energy, raw materials, money, carbon emissions, waste; anything where you need accurate accounting in a dynamic system.

Following different stocks and flows

Sankey diagrams are excellent tools where you want to visualise how a system works. These two versions show the material flows in the UK economy in 2000 and 2010. You can quickly see how much raw material goes into the UK economy, how much ends up as waste and how much is recycled. The comparison shows that despite GDP increasing by 20% and the population by 6%, fewer raw materials were going into the economy in 2010, less waste was produced and more recycled. The economy became more materials efficient over the ten years.

Sankey diagrams are very effective in industrial ecology; understanding the stocks and flows of industrial processes and looking for ways to improve resource efficiency and create a more circular economy. This example shows global material flows in the paper industry.

Finance and economics is another area where stocks and flows are very important, but often hard to understand. This diagram shows the landscape for climate finance in 2015/16. It shows where the money comes from, what form the investment takes, who are the recipients and how they use the money.

Sankey diagrams can become pretty complicated. For example, this is the flow of food in the EU. It is fascinating to see how much of the EU’s primary cereal production goes into animal feed, and how much of that is for dairy products. Also interesting that the waste in production, processing and distribution is about the same as end-user waste.

The next example shows a model of the UK energy system. What are the primary sources of energy, how is it delivered,  and what is it used for?

And finally, global greenhouse gas emissions for 2017 compiled by Navigant.

You could argue that a few bar or pie charts would easily capture the conclusions from any of these diagrams. What is the balance of inputs, outputs, and some intermediate steps? The genius of the Sankey diagram is in visualising the flows between the various buckets of energy, materials, money etc. Knowing how much of your crude oil is used in industry rather than in transport, or which are the key sources of NOx, helps understanding. Focusing on flows emphasises the dynamics of a system rather than the static view of pie charts and tables.

Building Sankey diagrams

So, if Sankey diagrams are so great, how do I go about producing them? I can’t find that option in PowerPoint.

The first problem is getting the data. National or market statistics often have major gaps. When I have tried to build Sankey diagrams myself, it has been very difficult (and sometimes impractical) to get all the evidence I need. What you tend to find are the fixed points, the stocks data and the pie charts. Finding out about the flows is harder. So there is one immediate benefit from trying to focus on stocks AND flows, you see the weakness in the available information.

Computing has made creating Sankey diagrams much easier. There are programmes that will take a table of volumes and links and turn it into a nice graphic. There are libraries for statistical programmes and online tools that will create the diagram for you for free. These allow you to include mouse-over access to the underlying numbers and highlighting of specific paths, improving communication. However, most of these on-line tools use the D3.js JavaScript library, and D3 cannot handle feedback loops. You can have a diagram that smoothly moves from inputs to outputs, but you can’t deal with recycling. And for many questions about sustainability and resource efficiency, that is exactly what you want to do. That limitation makes it harder to use a Sankey diagram like any another visualisation tool. I haven’t found a free solution out there yet, but perhaps one is under construction right now.

Even a crude Sankey diagram has value. I once worked with a real estate company that was trying to work out a future strategy. Should they optimise their existing business, or should they expand into adjacent areas? An end to end process flow map of the real estate sector turned into a value chain map that turned into a rough Sankey diagram. That enabled a debate about which areas of the overall process they would need to include to increase their value add and how workable would it be.

So use Sankey diagrams where you can. Remember, to get under the skin of a dynamic system you need to understand both stocks and flows.

A Powerful Tool to Focus on Stocks and Flows
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