Parallel and in-series arrangements of zeotropic dual-pressure Organic Rankine Cycle (ORC) for low-grade waste heat recovery

Due to the recent interest in renewable energy and waste heat for power generation, organic Rankine cycle (ORC) has drawn considerable attention. By applying a dual-pressure evaporation strategy, the exergy loss in the evaporators can be reduced significantly, resulting in an enhanced system performance. In this study, detailed modelling analyses have been conducted to evaluate the effect of zeotropic working fluids on the performance of two dual-pressure ORC (DPORC) arrangements, namely, DPORC-P (parallel) and DPORC-S (in-series), where the working fluid pair R245fa and R152a has been selected to form a representative zeotropic mixture. It has been found that as a zeotropic mixture is used, both systems show improved cycle net power outputs. Interestingly, the highest power outputs occur at mass fractions that result in matching temperature profiles between the hot and cold fluids in the condenser. At a heat source temperature of 120 °C, a DPORC-S always has a greater net power output regardless of the mass fraction of the zeotropic mixture; as heat source temperature reduces, the difference of power output between the two systems tends to drop: for a heat source temperature of 90 °C, a DPORC-P has a better performance as the mixture mass fraction is between 0.4 and 0.9. The influence of zeotropic fluids to the heat transfer of the condenser has also been investigated and it indicates that a significantly larger heat transfer area is required for a zeotropic fluid; based on a fixed heat transfer area, the performance enhancement of a zeotropic system is only seen as the condenser heat transfer area is considerably large.