Exergy Analysis .Applications

Following on from [BE 8 015] , this second article on exergy analysis is devoted to applying the concept of exergy analysis to heat transfer and dithermic machines. While determining the entropy production and irreversibilities associated with heat transfer is a straightforward matter, estimating the exergy yield during heat transfer can become complex when temperature levels overlap with the reference ambient temperature, leading some to apply absolute values to avoid negative values and others to set the ambient temperature in a very arbitrary way, thus totally erasing the usefulness of exergy analysis. Generalized expressions for irreversibilities and exergy efficiency are developed, enabling them to be integrated into calculation codes for optimization purposes. The same difficulty is encountered in the exergy analysis of driving and driven machines. Energy efficiency, rather than efficiency or COP, has long been overlooked, but is now back in the limelight, as it has been shown to correspond to exergy efficiency under conditions specific to the type of machine. This article uses simple, practical illustrations to help you understand and master the calculation of irreversibilities and exergy efficiency. The role of the reference ambient temperature is highlighted, as it is the key factor in exergy analysis.