Exergy analysis of novel dual-pressure evaporation organic Rankine cycle using zeotropic mixtures

Organic Rankine cycle is a power system for the widely use of medium- to low-grade (<200 °C) thermal energy. Adopting the dual-pressure evaporation cycle and zeotropic mixtures are important approaches to increase the efficiency of organic Rankine cycle system. The combination of two approaches presents considerable potential to integrate their advantages and further increase the system efficiency significantly. However, for the dual-pressure evaporation organic Rankine cycle using zeotropic mixtures, the studies on the exergy performance are blank up to now. The obtained efficiency increments by combination, reached exergy efficiency, and characteristics of irreversible loss distribution remain unclear. This study carried out a comprehensive exergy analysis on the dual-pressure evaporation organic Rankine cycle using isobutane/isopentane mixtures. The exergy efficiency and irreversible loss distribution were analyzed. The superiority of combining dual-pressure evaporation cycle and zeotropic mixtures was evaluated. Results show that the largest exergy efficiency of dual-pressure evaporation cycle using isobutane/isopentane mixtures is 43.1%–61.7% for heat sources of 100–200 °C, and the largest exergy efficiency initially increases and then decreases with increasing heat source temperature. The largest irreversible loss occurs in the heat absorption, expansion, and heat release processes for heat sources of 100–160 °C, 170–180 °C and 190–200 °C, respectively. The exergy efficiency of dual-pressure evaporation cycle using isobutane/isopentane mixtures is the largest for heat sources below approximately 180 °C, and the maximum relative increments are 11.6% and 25.6% compared with the dual-pressure evaporation cycle using isobutane and single-pressure evaporation cycle using isobutane/isopentane mixtures, respectively.