Strategy for the zeotropic organic rankine cycle operation to match the heat sink variation

The zeotropic organic Ranking cycle (ORC) is considered a high-efficiency utilization of low-grade energy, caused by its nature of temperature slide, which could reduce the irreversibility loss of evaporation and condensation, and make the ORC more adaptable to the heat source and heat sink. Regarding this advantage, the ‘fH’ and ‘fL’ are defined in this work to describe the system matching degree with the heat source and heat sink, respectively. By considering ORCs employed by different types of mixtures, it is found that when the fL is equal to 1, the relationship between ORC and heat sink is considered the highest matching degree, and the system would exert the best performance. Thus, a system operation strategy is presented for system optimization with the guidance of fL by adjusting the condensation pressure under varied heat sinks. Considering a 0.5R1233zd(E)/0.5R1234yf driven ORC system, the monthly condensation pressures are evaluated according to the supply cooling water. Compared to the referred system, the annual net output and thermal efficiency of the optimized system under the heat source of 120 °C could improve from 785.55 MWh to 905.66 MWh and 8.27 %–9.13 %, respectively. The presented operation strategy contributes to the zeotropic ORC optimization with varied heat sinks. •The matching mechanism of the ORC operation pressure and heat source/heat sink was stated.•An ORC operation strategy was presented to match the heat sink variation.•The universal applicability of the operation strategy was verified by different working fluids.•The annual ORC system was optimized by considering the environmental condition of Beijing.