Visualize industrial information flows with Sankey diagrams

Figure 1. Distribution of heat and power to a factory site.

A brief start about Sankey

A rollback in history

As stated in “The Sankey Diagram in Energy and Material Flow Management.”, the diagram was published in 1898 [2] by the Irish naval engineer Riall Sankey and named after him.

One of the most famous Sankey diagrams existed before Riall Sankey, created in 1869 by Charles Joseph Minard, a French civil engineer who covered the map of Napoleon’s Russian Campaign of 1812 cited in “Mapping Time : Illustrated by Minard’s Map of Napoleon’s Russian Campaign of 1812.” [3] to visualize troop losses throughout the Campaign.

Figure 2. Minard’s classic diagram of Napoleon’s invasion of Russia.

Today, it is used in material and energy management systems that we will drill down in some of its use cases.

Applying Sankey diagrams

“Visualization gives you answers to questions you didn’t know you had.” — Ben Schneiderman

Identifying the main flow

Figure 3. Distribution of heat and power to a factory site.

Let us take our example illustrated in Figure 3. “Distribution of heat and power in a factory site”. First, as we have discussed earlier, this diagram represents flow “circulation”. A flow is actually pointing out a value from a node directing it to another one. A process is a flow between two nodes. A node is actually a type of context. In our example, we can check out that there is a directed flow from Natural gas to CHP unit 1. Moreover, our flow represents a quantity or KPI ( Key performance indicator ). In our case, it is the value of energy produced.

Looking at the red rectangle, we can deduce that “ 4.3 kWh of energy was transferred from Natural gas to a CHP unit 1 converter”.

A must-know in Sankey diagrams is that the width of our flows is proportional to the quantity of the value it transfers. This feature marks out the most important flows.

Secondly, the set of bars ( for instance, Power supply and Natural gas) form up a Node sets called “ sources”. This is equivalent to column names “sources” in a spreadsheet or database table.

Coming back to our example, we can display our value in a process using dashboards as shown in the red rectangle. We can also display the value that contains a Node element before flow arrival or departure.

In this example, we immediately notice that our main energy generator is Natural Gas. We can also observe that our converters produce mostly heat. Lastly, we realize that our system loses energy when converting energy into heat or power supply.

The key to interpret Sankey diagrams is to observe the widths of our flows between Node sets.

Figure 4. Energy balance in a compressed air system.

In our second example above, our system generates 5280kW of compressed air per day. Then the energy is distributed throughout our compressed air system. At the first glance at it, I notice that most of the produced energy is lost to heat waste and other losses. This can be relevant to analyze the efficiency of my compressed air System.

Without getting lost

Figure 5. Line production of cosmetic products

As we might guess, every diagram has its own downside. When those downsides occur, our diagrams lose efficiency and clarity.

In Sankey Diagrams, the number of nodes decreases our readability. In other words, the longer our process chain is, the more we lose sight of the information we wish to communicate. Moreover, they can make it becomes difficult to compare flows with similar values (widths). We should visualize a main flow distinguished from the others when we use Sankey diagrams. In Figure 5. “Line production of cosmetic products”, notice how hard it is for us to identify the line which produces the most cosmetic products.

Besides, we should consider using a (stacked) bar graph to compare our production lines.

Coming to an end

References

[2] Schmidt, Mario. “The Sankey Diagram in Energy and Material Flow Management.” Journal of Industrial Ecology, vol. 12, no. 1, Feb. 2008, pp. 82–94, 10.1111/j.1530–9290.2008.00004.x.

[3] Menno-Jan Kraak. “Mapping Time : Illustrated by Minard’s Map of Napoleon’s Russian Campaign of 1812.” CiNii Books, ESRI Press, 2014, ci.nii.ac.jp/ncid/BB19280683.

Knowledge enthusiast. Mostly about science, education and rhetoric. Writing about data viz in dashboards.