Thermodynamic Analysis of a Two-stage Organic Rankine Cycle with Ejector

orc2019 Tracking Number 18

It is well known that lowering the expander backpressure is beneficial for the performance of steam and organic Rankine cycles (ORC). Typically, the temperature level of the coolant medium governs the achievable backpressure level, but in several turbine applications diffusers are also applied for lowering the exhaust pressure level. In the case of small and medium organic Rankine cycles it is in principle possible to achieve significant backpressure level reductions by means of ejectors (or jet pump devices). This approach was recently proposed by Xinguo Li and co-workers. In their so-called EORC concept two independent evaporators were applied, and the vapour from the secondary evaporator worked as primary fluid for the ejector to suck the exhaust from the expander so as to decrease the backpressure. Some minor technical advantages were identified for that EORC, but a thermodynamic analysis indicated a substantial exergy penalty for driving the ejector by means of an independently generated vapour. With a look to a second-law thermodynamic analysis, it is obvious to drive the ejector by second-stage vapour extracted after a first expansion. Then, a two-stage organic Rankine cycle with ejector results without the need to use a secondary evaporator. Furthermore, the vapour extraction lowers the low-pressure volume flow rate, and a rather compact turbine design including the ejector can be achieved. In this contribution results of a detailed thermodynamic analysis of the two-stage organic Rankine cycle with ejector are presented. The different cycle concepts were analyzed by means of the commercial process simulation tool EBSILON Professional developed by STEAG. This tool permits the simulation of cycle performance under the assumption of realistic component performance behaviour and fluid properties based on REFPROP data. The ejector performance was modelled in detail, and implemented as sub-routine within the simulation process. The thermodynamic analysis showed promising results for this novel cycle concept in terms of both output capacity and efficiency. This work was financially supported by the academic exchange program “NRW-Nahost (Israel, Palästina, Jordanien)”. The fruitful discussion about ejector modelling with the development team of EBSILON Professional is also acknowledged.