Energy versus exergy: An empirical analysis of thermodynamic metrics at the macro scale

While energy analysis considers the quantity of energy and its conservation, exergy analysis also accounts for the quality of this energy and its degradation. Exergy analysis therefore offers the possibility of an enhanced... [ view full abstract ]

While energy analysis considers the quantity of energy and its conservation, exergy analysis also accounts for the quality of this energy and its degradation. Exergy analysis therefore offers the possibility of an enhanced assessment of the improvement potential of thermodynamic systems and has gained growing popularity as a tool in this regard.

This study investigates the application of exergy analysis in assessing the improvement potential of macro scale systems (e.g. industrial sectors and economies). These systems are complex and there are many factors that may confound the interpretation of their exergetic improvement potentials, relative to simpler thermodynamic systems that can be primarily described by their energetic output. An assessment of how the theoretical advantages of exergy analysis translate to empirically verifiable benefits at the macro scale is the basis of this study. Specifically, we use historic data to examine three considerations relating to the use of the improvement potential:

• Predicting efficiency improvements: Although the “exergetic improvement potential” has a clear theoretical grounding, many other factors are relevant in realising this potential. We analyse the historic correlation between exergetic improvement potential and the actual improvement that has occurred. This is then compared to the correlation obtained with energy analysis. This comparison is conducted over a range of sectors and regions, highlighting practical considerations regarding the relationship between the energy and exergy metrics, at the points for which data is available.
• Significance of embodied energy: While energy efficiency improvements have the scope to reduce the direct energy requirements of sectors, interventions that target embodied energy inputs (e.g. material efficiency) also have significant potential. This potential is not directly related to the exergetic improvement potential of the sector. We consider the relative importance of these metrics for different sectors in determining priorities for interventions that might reduce energy related impacts.
• Consistent definition of system output: As the scope of analysis is expanded to entire industrial sectors, the energetic and exergetic efficiencies are defined relative to selected energy uses, rather than the actual output of the sector. This involves some subjectivity. By comparing trends in exergy efficiency to those in exergy productivity across sectors, we consider the extent (and consistency) to which energy saving opportunities have been exploited outside the scope of the exergetic improvement potential.

We conclude that exergetic improvement potential is of great value when assessing thermodynamic systems but care should be taken when applied to systems that are not primarily defined by an energy supply or transformation process. Energy and exergy analyses find complementary roles in assessing the scope for improvement; using either to the exclusion of the other is likely to result in missed insights. Developments within industrial sectors are complex with a range of drivers; some of these will relate to energy and exergy use but others are relatively independent of them.