A new efficiency parameter for exergy analysis in low temperature processes

With the uncertainty and unreliability of cost estimates in process design, exergy efficiency is an ideal evaluator of design quality since it is firmly based on thermodynamics. In low temperature processes (LTPs), the use of exergy is even more obvious due to the large power (i.e., pure exergy) requirements in refrigeration systems. Due to lack of standardization, however, the use of exergy efficiency as a representative measure of quality is not straightforward. Thus, this paper presents a systematic approach for calculating exergy efficiency based on decomposition of exergy into its components and a careful identification of exergy sources and sinks. An Exergy Transfer Effectiveness (ETE) that works particularly well for LTPs is defined and compared with four classical definitions of exergy efficiency with an LTP as case study. This comparison shows significantly different exergy efficiencies, and a deeper reflection on the reasons for these differences is made in the paper.