Thermodynamic analysis of serial dual-pressure organic Rankine cycle under off-design conditions

As a novel and promising cycle architecture, the serial dual-pressure organic Rankine cycle (STORC) is proved with higher thermodynamic performance than the basic organic Rankine cycle (BORC) in design condition. Nevertheless, system thermodynamic performance may severely deteriorate when the heat source and ambient temperature fluctuate. Thus the performance of STORC under off-design conditions is still unclear and needs further discussion. Besides, the serial layout of evaporators in STORC makes the mass and energy flow between different pressure levels coupled together, which presents new challenges for off-design analysis. This study proposes a design and off-design model of STORC using R1234ze(E) as the working fluid. Fluctuations of the heat source and ambient temperature are analyzed and their effects on the off-design performance and stability of STORC are explored. The net power output is maximized in each operation condition and the optimized results are compared with BORC. Results show that, compared with BORC, STORC exhibits better thermodynamic performance in both design and off-design conditions. On the one hand, under all calculated operation conditions, STORC achieves higher net power output and exergy efficiency, which are up to 24.3% and 21.4% higher than BORC, respectively. On the other hand, STORC better adapts to external fluctuations and runs closer to the design point in underload state with less thermodynamic performance degradation. Besides, it's revealed that the variation of heat source characteristic mainly affects the evaporation process and working fluid mass flow rates, while the ambient temperature mainly affects the condensation process and has little effects on the evaporation process. Moreover, the low-pressure turbine accounts for 70% of the overall net power output, but its performance degradation is more serious than that of the high-pressure turbine. Thus the enhancement of the low-pressure turbine is expected to further improve the off-design performance of STORC in practical operation conditions.