Exergetic optimization of double stage Organic Rankine Cycle (ORC)

The present work focuses on the exergetic optimization of double stage Organic Rankine Cycle (DS ORC) for waste heat recovery. A model of a DS ORC, consisting of a high temperature (HT) stage serially connected to a low temperature (LT) stage is developed, while different combinations of working fluids with variable critical temperatures are considered in each stage. The optimization variables are the evaporation pressures in the HT and LT stages, as well as the evaporator pinch point and condenser temperature in the HT stage. The aim is to explore the exergetic efficiency improvement potential of DS ORCs compared to the single stage cycles and establish optimization guidelines for maximizing their total power output for heat source temperatures ranging from 100 °C to 300 °C. Compared to single stage ORCs, DS ORCs can lead to a relative increase of the exergetic efficiency by up to 25%, depending on the heat source temperature and the working fluids considered. Meanwhile, DS ORCs are especially favorable when the heat source temperature is far lower or between the critical temperatures of the fluids used in their two stages.