Principles of exergy analysis in HVAC and evaluation of evaporative cooling schemes

Principles of exergy analysis in heating ventilating and air conditioning (HVAC) are discussed through an analytical review of some pertinent works. An unusual selection of dead-state novel to HVAC applications is suggested, which simplifies the exergy analysis by eliminating the necessity of calculating the exergy of water at ambient temperature (T0). Such a selection will also make it easier to evaluate the theoretical limit of evaporative cooling capacity. In this paper, the authors also suggest a functional classification to break down HVAC project in a hierarchy structure for exergy analysis. Energy flows are also classified according to their functions as source energy flow and service energy flow to assist the evaluation of the performance of the systems or components. If free energy is involved in the processes, then exergy efficiency ratio is defined to evaluate the effective use of the purchased available energy. The rationality is demonstrated with the analysis of application examples. For the evaluation of evaporative cooling, the exergy of moist air are analyzed and broken down into three components: thermal, mechanical and chemical. Each of them is represented with a special devised diagram applicable to various conditions and ambient states among HVAC applications. Evaporative cooling processes are analyzed with the aid of these diagrams. Five cases of four existing schemes of evaporative cooling are properly evaluated. Results show that regenerative evaporative cooling has the best performance and the effectiveness for indirect evaporative cooling heat exchange has great importance in improving the exergy efficiency ratio of regenerative scheme.