May 7, 2012
Exergy is perhaps an unfamiliar concept. Energy we can fairly easily get to grips with, it’s a measure of the ability of a system to do work. Work is what causes a system to change its energy state…hmm…first law of thermodynamics. But, what is exergy and where does it fit into the energy equation?
By definition: the exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir. If we consider the surroundings of a given system as being that reservoir, then exergy is the potential of the system to cause a change as it comes into equilibrium with its environment.
Now, Enrico Sciubba of “La Sappienza”, the University of Rome, Italy, has used the concept of exergy as an ecological indicator, a way to measure our environmental footprint. He points out that conventional measurements of material and energy balances do not take into account the loss or discharge of exergy. They also usually fail to account for other factors such as the detrimental effects toxicity, pollution and other “externalities” including labour intensity, economic capital, and environmental remediation costs, might have on those energy balances. Extended Exergy Accounting has been mooted as a possible solution to those shortcomings allowing us to consider the complete lifecycle and impact of various processes.
Writing in the appropriately named International Journal of Exergy, Sciubba explains how Extended Exergy Accounting is not merely “another measure of cost”. It is, he claims, the only true measure of cost based, in a lifecycle sense, on the global consumption of primary resources. It’s very much a cradle-to-grave measurement, Sciubba says allowing researchers and decision makers to assess natural and anthropogenic processes with a metric that reflects the “cost” of a resource-to-final-use and disposal. It offers a holistic rather than deconstructionist approach to environmental considerations and policy that avoids simply sweeping the carbon and pollution under the proverbial carpet and reveals how more clearly than other metrics how dwindling resources are, of course, entirely finite.
There are gaps in what EEA can achieve, admits Sciubba. For instance, its foundation in thermodynamics means it cannot be used to pass moral judgements or resolve problems of choice, ethics or politics. However, it can provide a more solid and realistic frame of reference on which to make such decisions.
Enrico Sciubba (2012). An exergy-based Ecological Indicator as a measure of our resource use footprint International Journal of Exergy, 10 (3), 239-266