The relation between exergy, sustainability and environmental impact is illustrated in Fig. 1b. There, sustainability is seen to increase and environmental impact to decrease as the exergy efficiency of a process increases. The two limiting efficiency cases appear to be critical as shown in Fig. 1b, such as: (i) As exergy efficiency approaches 100%, the environmental impact associated with process operation approaches zero, since exergy is only converted
from one form to another without loss (either through internal consumption or waste emissions). Also sustainability approaches infinity because the process approaches reversibility. (ii) As exergy efficiency approaches 0%, sustainability approaches zero because exergy-containing resources (fuel ores, steam, etc.) are used but nothing is accomplished. Also, environmental impact approaches infinity because, to provide a fixed service, an ever increasing quantity of resources must be used and a correspondingly increasing amount of exergy-containing wastes are emitted.