This article shows that several minimum exergy return ratios (ExRRs) can be computed in relation to different aggregate exergy conversion efficiencies. Through the exploitation of available data on exergy conversion efficiency for several countries, our results suggest that as technical change enhances the conversion efficiency of primary-to-final and final-to-useful exergy processes, the minimum ExRR required for society decreases, irrespective of the boundary under consideration. Therefore, the gains in exergy conversion efficiency that mostly occurred between the 1940s and the 1970s have compensated for the concurrent decrease of exergy surpluses of the fossil energy system. However, while the minimum ExRRs required for modern societies have been quite stable, actual ExRRs prevailing for energy systems have continue to decrease. Effectively the increased difficulty in improving exergy conversion efficiency since the mid-1970s has resulted in a tightening exergy constraint on economic growth; this could par- tially explain the global economic slowdown of the last forty years. Further work is needed to estimate actual exergy return ratios that prevailed in the past decades and compare their distance relatively to the minimum levels estimated in the present article, and hence have a more precise idea of the exergy constraint’s magnitude acting on economic growth.
Victor Court is a research associate at the chair
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