Performance analysis of a novel organic Rankine cycle with a vapor-liquid ejector

Organic Rankine cycle (ORC) is a promising technology of converting the low-grade thermal energy to electricity. But the pump work accounts for a large percentage of the expander output power in the small-scale system, which greatly worsens the system efficiency. In this paper, a novel organic Rankine cycle with a vapor-liquid ejector (EORC) is proposed to enhance the system performance. It is compared to the conventional ORC and a regenerative organic Rankine cycle (RORC), and the results show that it has higher system efficiency when the pump has a low efficiency and the evaporating temperature is high. A parametrical study on this novel system is further carried out with three working fluids, namely, R123, R1233zd(E) and R1336mzz(Z). The ejector behavior and system performance are strongly interacted, which are elaborated and discussed. There exists an optimal ejector entrainment ratio that minimizes the pump work and maximizes the system efficiency. The ejector area ratio and the subcooling at the condenser outlet have great influence on the ejector pressure lift, leading to the significant variations of temperature in the evaporator I, however, their influence on the expander output power is moderate. As for the condensing temperature, it has remarkable effect on system performance except the ejector pressure lift. The three candidates have similar features of variations for the considered variables. R1233zd(E) is recommended as the good working fluid since it has higher system efficiency than R1336mzz(Z) and is more favored by the environment than R123.