Thermodynamic analysis of double-stage biomass fired Organic Rankine Cycle for micro-cogeneration

摘要

A biomass fired double-stage Organic Rankine Cycle (ORC) for micro-cogeneration is studied. Focus is laid on optimizing thermal efficiency in summer mode by appropriate working fluid and pressure level selection. Simulation and thermodynamic analysis show that in double-stage ORC, the working fluid in the low-temperature circuit (LTC) effects total efficiency more than the working fluid in the high-temperature circuit (HTC). Within the chosen boundary conditions, isopentane gives best thermal efficiency, whereas R227ea is the least efficient in the LTC. Among the working fluids for the HTC, maximum total efficiency is similar for several working fluids. Simulations demonstrate that a prediction of thermal efficiencies with respect to physico-chemical characteristics of different working fluids is only feasible within certain chemical classes. In the HTC, low critical temperature, low molar mass, and high critical pressure increase the efficiency, whereas in the LTC, condensation pressure is most crucial for high efficiency. Constructional analysis indicate that in the majority of cases, an increase in thermal efficiency is connected with high-volume flow rates at the outlet of the turbine, which leads to voluminous expansion units and high investment costs, respectively. Appropriate working fluid combinations within a double-stage ORC reach total efficiencies of up to 35 at flue gas temperatures from 950 to 150 °C.