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 the directed flow is always drawn between at least two nodes (processes). Thus it shows not only flow values but also information about the structure and distribution of the defined system.
So 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 the energy balance 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 value chains, as this can not be achieved using standard tables (e.g. Excel) or even pie & bar charts.
- Thus they communicate your message more attractively: within your team or to customers and external partners
- Through the visualization data inconsistencies can be detected: such as measurement and transmission error
- Sankey diagrams are suitable for a wide range of applications: energy, material flow & supply chain management or business & marketing analysis
Sankey diagrams are named after the Irish engineer Captain Matthew H.R. Sankey (1853-1925)
A brief history
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 (energy balance) 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 one chart. Thus they fell into oblivion over the years. In the 20th century the Austrian mechanical engineer Alois Riedler (1850-1936) started to use these flow charts 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.
Below you find some examples of several Sankey diagrams. Each diagram was created using our software e!Sankey. Just try out by yourself, how easy it is, to create appealing Sankey diagrams using e!Sankey.
Example 1: Energy use in a passenger car
In this Sankey diagram (in German) you can see the energy balance 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 within the chart can be distinguished by color, and are – by definition – 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.
Example 4: Energy balance of a country
Energy flow charts 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.