Sankey diagrams are a specific type of flow diagram, in which the width of the arrows is shown proportionally to the flow quantity. They are typically used to visualize energy or material or cost transfers between processes.
They are also commonly used to visualize the energy accounts or material flow accounts on a regional or national level. Sankey diagrams put a visual emphasis on the major transfers or flows within a system. They are helpful in locating dominant contributions to an overall flow. Often, Sankey diagrams show conserved quantities within defined system boundaries, typically energy or mass, but they can also be used to show flows of non-conserved quantities such as exergy. Sankey Diagrams drop their arrows when energy is being used.
Sankey diagrams are named after Irish Captain Matthew Henry Phineas Riall Sankey, who used this type of diagram in 1898 in a publication on the energy efficiency of a steam engine (see reproduction in, page 8). While the first charts in black and white were merely used to display one type of flow (e.g. steam), using colors for different types of flows has added more degrees of freedom to Sankey diagrams.
Another example is the United States Energy Use developed by the US Department of Energy's Lawrence Livermore Laboratory.
See also 
- Mario Schmidt (2006). "Der Einsatz von Sankey-Diagrammen im Stoffstrommanagement" (pdf). Beitraege der Hochschule Pforzheim (Nr. 124).
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