On exergy
[Aug 2024]
TLDR: Exergy is an under used metric.
This quick post covers the merits of exergy as a metric, how it simplifies efficiency analysis and goes beyond energy efficiency. This is not a deep technical post. I will keep it very high-level. Those already familiar with the relevant thermodynamics may find my view on the uses of exergy analysis interesting. If you disagree with anything, please reach out at dm@duncanmcmillan.xyz.
So, what is exergy?
Exergy is the ability to do useful work. More accurately it is the maximum theoretical work that can be done as a system comes into equilibrium with its environment. Alternatively – and conversely - it is the minimum theoretical work to bring the ‘dead state’ to the system state, where the dead state is the system matching environmental conditions. So, unlike energy, exergy is a reference between two states. Also, unlike energy, exergy is not conserved.
In a sense this makes exergy a measure of the ‘quality’ of energy. Knowing the exergy of a black-boxed system tells you more about what you can with that system than plain-old energy. More can be done with 100J of electricity than 100J of natural gas. Why? Because 100J of electricity can do 100J of work, i.e. it has 100J of exergy. This is not the case for natural gas. This is why to create 100J of electricity in a power plant requires >100J of fossil fuel.
Extending this example out leads to the primary energy fallacyr1 in general. We often see the build out of renewables compared to our primary fossil fuel use, with hot takes claiming renewables will never fill the gap, and that fundamentally society requires perpetual primary energy growth to thrive1. This is false, people don’t want cheap and plentiful crude oil, they want cold homes in summer and warm homes in winter. They want cheap useful energy.
The energy transition is not ‘just’ a switch to low carbon fuels. It’s a transition from heat to workr2. It’s a transition away from low exergy fuels.
Taking the lens of exergy makes it clear why electricity is superior to thermal fuels, why directly electrifying steam production is a waste, why fossil fuel boilers are actually very efficient (and superior to 100% efficient electric boilers), and how heat pumps reach their >100% energy efficiencies.
Beyond clarifying energy systems exergy can also be applied to resource efficiency more generallyr3. An exergy optimisation company could do far more than improve energy efficiency! It could analyse material value add and waste2, and even spot where products can be improved or outcompeted on a material sourcing and use basis!
Footnotes:
1 I don’t deny the statistical link between primary energy growth and GDP growth, but blindly following data incorrectly is a sin. Data exists to test explanations, not create them. Primary energy is a means to an end, the end being useful energy.
2 How do you trade off saving X% on energy by producing Y% more waste? Exergy is the metric.