What is a Sankey diagram?

Sankey diagrams visualize material, energy and cost flows shown proportionally to the flow quantity. They are  often used in energy management, manufacturing or in fields of science.

Sankey diagrams - a definition:

Sankey diagrams feature directed arrows that have a width proportional to the flow quantity visualized: if a flow is twice as wide it represents double the quantity. Flows in the diagram can show e.g. energy, materials, water or costs. 

Within a Sankey diagram a flow is always drawn between at least two nodes (processes). 

They are a great alternative to common flow or bar & pie charts.

They are gaining popularity in energy management, facility management, process engineering and process control and many other kinds of data visualization.

Thus Sankey diagrams draw the attention to the largest entries in the system.

Why should I use them?

In comparison to conventional bar or pie charts and even flow charts, they are more suitable for visualizing energy or material flows:

  • The width of the arrows is proportional to the flow quantity: the larger the width of an arrow, the larger the material or energy flow. The viewer's focus is drawn to the most significant flows.
  • The arrows show flows from one node to another node: this fact makes them ideal for production systems or for value chains, as this can not achieved using standard tables (e.g. Excel) or pie or bar charts.
  • Thus they communicate your message more attractively: within your team or to customers and external partners.

Sankey diagrams are named after the Irish engineer Captain Matthew H.R. Sankey (1853-1925)

A brief history on Sankey diagrams

The first illustration of energy with arrows proportional to the amount were done by the Irish engineer Captain Matthew Henry Phineas Riall Sankey in 1898. He compared the energy efficiency of steam engines. Before that this kind of diagram had been used by the French engineer Charles Joseph Minard to visualize Napoleon's Russian Campaign of 1812.

Captain Sankey only drew that chart. Thus Sankey diagrams fell into oblivion over the years. In the 20th century the Austrian mechanical engineer Alois Riedler (1850-1936) started to use Sankey diagrams to analyze the power and the energy losses of passenger cars.

In the course of this Sankey diagrams gained popularity, especially in Germany, where the economy focused on material and energy efficiency due to the reparation payments after World War I.

Today this kind of chart is used worldwide for data visualization, e.g. in material flow analyzes and energy management systems.

Examples:

Below you find some examples of several Sankey diagrams. All of them were created using our software e!Sankey. Just try out by yourself, how easy it is, to draw appealing Sankey diagrams using e!Sankey.

Example 1: Energy use in a passenger car

In this Sankey diagram (in German) you can see energy use for a passenger car. Besides the energy actually used for motion at the wheel ("mechanische Energie") a large part of the energy is lost, especially as heat losses. Additionally we can identify the additional energy consumers (water pump, steering support, etc.) in the car.

The figures are given as percentage values, as well as in absolute numbers (based on a fuel consumption of 6,57 Liter/100 km). Flows on the Sankey arrows can be distinguished by color, and are - as is the rule in Sankey diagrams - proportional in their width to the flow quantities they represent.

(Source: Prof. Mario Schmidt, INEC, Pforzheim University)

Example 2: Food Supply Chain

In this rather simple chart we can see losses along a process chain for food. In every step the losses are shown as an arrow branching out to the bottom, labeled with percentages (mass-%).

So here we have no absolute quantitites (although the diagram is based on real data), but only proportions.  One could call this the "efficiency" of food production, processing and consumption.

Source: David Lisle, 'Know The Flow' blog, based on data from a study by FAO.

Example 3: Battery of an ELV

Very often we find diagrams which have a left-to-right orientation (just as the reading direction for many, but not all scripts). However, in e!Sankey you do not have any limitations as to the flow directions. This is helpful when depicting material flows in a production system, or, as is the case in this diagram, when visualizing loops.

This Sankey diagram displays a battery cycle for an electric vehicle (ELV) with losses branching out at every node of the cycle.

Source: sankey-diagrams.com

Example 4: Energy balance of a country

Energy flow charts and Sankey diagrams are used very often for energy balances in a region or in a country. Thus the different use of energy and energy sources can be seen.

This example is an energy balance from Malaysia in 2011. The unit of the flows is 'Million tonnes of oil equivalent' ('Mtoe').

Source: Chong, C.; Ni, W.; Ma, L.; Liu, P.; Li, Z. The Use of Energy in Malaysia: Tracing Energy Flows from Primary Source to End Use. Energies 2015, 8, 2828-2866.

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Summary:

  • Sankey diagrams are used for visualization of material, cost or energy flows.
  • They show energy or mass flows with arrows proportional to the flow quantity.
  • They have directed arrows (between at least two nodes), they feature flows in a process, production system or supply chain.
  • They draw the attention of the reader to the largest flows, the largest consumer, the main losses. Flow quantities that have different dimensions are understood intuitively.
  • Using Sankey diagrams you communicate effectively and get your message across: Whether it is to external stakeholders or within your project team.
  • There is no standardized definition of how a Sankey diagram should look or must be set up. There are numerous design and layout options.

The software e!Sankey is a dedicated tool for drawing Sankey diagrams. The program is suited for the visualization of energy, material or money flows. With e!Sankey you can easily draw flows and nodes to visualize your most important data.