Paul Brockway, University of Leeds
Timothy Foxon, University of Leeds
Julia Steinberger, University of Leeds
John Barrett, University of Leeds
This paper explores how useful work and exergy analysis at a national scale can give new global insights to global energy systems and energy efficiency policies.
Exergy, a term introduced by Rant (1956) is ‘available work’, and at a national scale – primary exergy input to an economy is very close (in energy units) to Total Primary Energy Supply (TPES). Useful work is the work done by end uses such as mechanical drive (e.g. transport), heat (e.g. in steel making) and electricity (e.g. cooking and lighting).
First, to understand how exergy and useful work can be of use in understanding OECD energy consumption, we present our work on an analysis of national exergy efficiencies for the US and UK for the period 1960-2010, which builds on the work of Ayres and Warr (Ayres et al, 2003). National level exergy efficiencies measure the efficiency with which exergy inputs into an economy are converted into useful work outputs, by appropriately combining estimated exergy efficiencies of current energy conversion processes for different end uses.
The results show that end useful work has doubled for both US and UK in this period. However, the contribution of TPES for each country is quite different, and exergy efficiency offers an explanation. For the US, we see that exergy efficiency has remained at around 11% since 1960, which means that TPES has also had to rise by 220% to meet the 220% growth in end useful work. However for the UK, 180% growth in exergy efficiency (from 9% to 15%) means that TPES has only had to grow by 130% to deliver the 220% growth in end useful work.
Next, to show how exergy and useful work may differ for non-OECD countries, our analysis for China in the period 1971-2010 is presented. The results show that useful work has grown by a factor of ten in this period, which has been met by a factor five increase from TPES combined with a factor gain in exergy efficiency of two (5% to 11%).
There are several key implications of this research for the BIEE conference theme Global Energy Trends. First, for OECD countries, the US and UK analysis results indicate that we are nearing peaking of exergy efficiency due to approaching asymptotic technical efficiency limits (e.g. car engines, steel making) being coupled to an ‘efficiency dilution’ (Williams et al., 2008) simultaneously occurring – i.e. more use of less efficient processes, e.g. low temperature heat and air-conditioning.Second, for non-OECD countries, TPES in non-OECD countries may grow even faster than thought as current projections of energy demand include an implicit continuous gain in exergy efficiency, and does not account for a slowing of exergy efficiency growth – as for example it is hard to see China doubling again its efficiency (i.e. from 11% to 22%) by 2050 when maturing economies such as US and UK are only at 11-15% overall efficiency. Third, if energy efficiency is stagnating at a national scale, then energy efficiency policies will need a rethink, in order to account for efficiency dilution – perhaps by capturing savings before rebound occurs to less efficient processes.
Ayres, R. U., Ayres, L. W., & Warr, B. (2003). Exergy, power and work in the US economy, 1900-1998. Energy, 28(3), 219-273. doi:10.1016/S0360-5442(02)00089-0
Rant, Z. (1956). Exergy, a new word for “technical available work.” Forsch. Ingenieurweser, 22(1), 36-37.
Williams, E., Warr, B., & Ayres, R. U. (2008). Efficiency Dilution : Long-Term Exergy Conversion Trends in Japan. Environ. Sci. Technol., 42(13), 4964-4970.
energy demand, energy efficiency, exergy, exergy efficiency, useful work
Categories: Academic PapersBrockway-Peaking-of-Exergy-Efficiency.pdf 1.52 MB