Sankey Visualization: Managing energy and manufacturing

Visualize industrial information flows with Sankey diagrams

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

A brief start about Sankey

When working with data, flows or any set of processes related to industrial systems, diagrams come in to play and provide global view on the information we focus. Without them, identifying problems may become pretty hard.

Talking about diagrams, Sankey diagrams in industrial ecology are helpful in identifying inefficiencies, visualizing energy balance, material flows and drawing attention to the largest flows of the diagram. So today, we are going to explore this helpful diagram by rolling briefly back in its history. Then we shall see where, when and how to apply it to various situations.

A rollback in history

As most of the diagrams or mathematical formulas change names; graphical heat balances (Minister of Fuel and Power 1944) , energy flow charts (Christensen 1990) [1] were what is called Sankey diagrams today.

The diagram was published in 1898 [2] by the Irish naval engineer Riall Sankey and named after him.

Interestingly, 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 [3] to visualize troop losses throughout the Campaign.


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

Today, it is used worldwide for data visualization such as in material and energy management systems that we shall drill down in some of its use cases.

Applying Sankey diagrams

Just before seeing our use cases, it is important to know how these diagrams can be prepared. For instance, one of the crucial steps, is to define and understand perfectly our industrial context in data visualization. When a diagram is well designed, it clearly stands out the data it illustrates, guiding us in taking the best decisions. “Visualization gives you answers to questions you didn’t know you had.” — Ben Schneiderman


Identifying the main flow

Let’s go step by step in understanding how Sankey diagrams work.


sankey diagram
Figure 3. Distribution of heat and power in 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 flow between two nodes is called a process. And a node is actually an type of context. In our example, we can check out that there is a directed flow from Natural gas ( element of a context most probably named “ sources” ) to CHP unit 1 ( an element of a context surely named “ converters” ). Moreover, our flow represents a quantity or KPI ( Key performance indicator ), thus in our case, it is the value of energy produced.


I can interpret what is going on in the red rectangle by saying “ 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 are proportional to the quantity of the value it transfers. And it is this feature that marks out usually the most important flows.


Secondly, when using dashboards, the set of bars ( for instance, Power supply and Natural gas) form up a Node sets called “ source” ( mentioned previously).


Coming back to our example, we can display our value in a process using dashboards just as shown in the red rectangle. We can also display the value that contains a Node element before flow arrival or departure. Therefore, without reading any values, we can see that our main energy generator is Natural Gas. And we can understand that our converters produce mostly heat. Even if it not significant in this example, we recognize that our system looses energy when we convert our energy into heat or power supply.


We should note our interpretation will depend on the widths of our flows between Node sets.


energy balance dashboard
Figure 4. Energy balance in a compressed air system.

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

Without getting lost

sankey diagram example
Figure 5. Line production of cosmetic products

As we might guess, every diagram has its own downside and we should be aware of the fact that when those downsides occur, they cause loss in productivity.

Indeed, we could notice that Sankey diagrams might become complex by the number of nodes and the number of processes . In other words, the longer our process chain is, the more possibilities of loosing sight of information such as the most productive line in an factory. Moreover, they can make it difficult to differentiate and compare flows with similar values (widths). While developing a Sankey diagram, it is important to not represent a process chain in which we loose sight of the main flow. In Figure 5. “Line production of cosmetic products”, I can hardly understand which Line produces the most cosmetic products if this is what I am trying to figure out in my scenario.


If we are primarily concerned about comparing our production lines , we might consider using a (stacked) bar graph.




Coming to an end

As we have seen above, we can say that Sankey diagrams are very efficient diagrams used in visualizing data. They draw attention to the largest flows of the system. And they also provide us information about the structure of our system.

References

[1] 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.

[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.

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