Table of Contents
8.1.3 – Sankey Diagrams
  • Definition & Description
  • Sankey Diagrams for various systems
Bibliography

8.1.3 - Sankey Diagrams

Definition & Description
A Sankey Diagram is a flow diagram that illustrates the energy transfers that take place in a process that produces energy. No machine can produce 100 % useful work, so the Sankey Diagram illustrates where the energy is degraded. It is usually represented as a big arrow pointing in one direction with several other arrows pointing the perpendicular direction. The width of the arrows is proportional to the amount (percentage) of the total energy used up. The big arrow pointing in one direction represents the energy input and the eventual energy output, while the other arrows pointing in perpendicular directions signify the degraded energy. As more energy is degraded (because of more perpendicular arrows), the size of the main arrows get smaller and smaller. This shows how the energy input converted into useful work gets smaller and smaller as a fair amount of the energy is degraded.

Sankey Diagrams for various systems
Steam engine used to light a lamp
Figure 8.1 represents the Sankey diagram for a steam engine used to light a lamp. The initial energy input from the chemical energy eventually dissipates due to exhaust from flames, friction in mechanism, and the other processes shown on the diagram below, leaving only a tiny portion of it used as light energy from the lamp.


Sankey_Diagram.JPG
Figure 8.1: Sankey diagram for a steam engine used to light a lamp

Heat engine used to power a car
Figure 8.2 shows the Sankey diagram for a heat engine used to power a car travelling at constant speed. In this case, the power is shown, not the energy. The initial 72 kW of power input eventually dissipates into 9 kW of power output, an overall efficiency of 12.5 %. The power input of 72 kW is shown to dissipate into thermal energy through the lights, fan, alternator, transmission and during driving the car itself.

Sankey.gif
Figure 8.2: Sankey Diagram for a heat engine used to power a car.



Bibliography
· "» Sankey Definitions." Sankey Diagrams. 12 Mar. 2009 <http://www.sankey-diagrams.com/sankey-definitions>.
· "Topic 5 Second Law of Thermodynamics and Engines." Welcome to the Antonine Education Website. 12 Mar. 2009 <http://www.antonine-education.co.uk/Physics_A2/Options/Module_7/Topic_5/topic_5_second_law_of_thermodyna.htm>.