Sankey diagram - a definition:
Sankey diagrams are a great alternative to a common flow chart, especially when visualizing energy, cost or material flows.
They are gaining popularity in energy management, facility management, process engineering and process control.
This kind of flow chart feature colored, directed arrows, whose width (magnitude) is proportional to quantity of flow along the arrow: if an arrow is twice as wide it represents double the quantity, be it energy, water, or cash flows.
They draw the attention to the largest flows in the system.
Why should I use Sankey flow charts?
In comparison to conventional flow charts, they are more suitable for visualizing energy or material flows:
- The width of the arrows is proportional to the material and energy flow quantity displayed. 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 (e.g. stock, process, machines) to another node. This fact makes them ideal for production systems or for value chains. This can not achieved with Excel tables.
- 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)
Example 1: Food Supply Chain
In this rather simple diagram 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 2: 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 systems, or, as in the case in this diagram, when visualizing loops.
In this example the arrows actually run counter clockwise, down on the left side, and up again on the right side: The diagram shows a battery cycle for an electric vehicle (ELV) with losses braching out.
Example 3: 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').
A picture is worth a thousand words. A Sankey diagram is worth thousand pie charts.
Sankey diagrams are used for visualization of material and energy flows.
- They show energy or mass flows with arrows proportional to the flow quantity
- They have directed arrows, they feature flows 'from' > 'to' 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 flow chart 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 to visualize energy flows (energy flow diagrams, energy balance) and material flows (mass/goods, value streams).