Research On Working Fluid Selection For Low-grade Heat-driven Supercritical Organic Rankine Cycle

In the low-grade energy utilization technology, organic Rankine cycle (ORC) power generation technology gets great attention of researchers worldwide due to its features of high thermal efficiency, simple equipment and environment-friendly. As working fluids have a significant impact on the cycle performance, selection of working fluids has become one of the most important direction of ORC research. Compared with subcritical cycles, supercritical cycles get better temperatures match between heat source and working fluids in evaporator without isothermal boiling, which effectively improves the thermal efficiency of the cycle. There are three key issues in supercritical cycle working fluid selection research:coupling ORC system with the heat source, the impact of operating parameters on the system performance and the impact of working fluids’physical properties on the performance.This paper was focused on working fluid selection of supercritical ORC. Firstly, a new thermodynamical calculation method was proposed coupling the system with the heat source, which was to analyze the cycle performance with given inlet temperature, outlet temperature and mass flow rate of the heat source and the constraint of pinch point temperature difference. A Fortran code was written to do the computations and the physical properties of the working fluids were computed using the Refprop9.0software.Secondly, for the heat source with an inlet temperature of423.15K and an outlet temperature of343.15K, three working fluids R218, R134a and R236fa were chose. Three types of operating modes were identified:(1) a flexible operating mode for low Tc (critical temperature) fluids having operating states in a rectangular region in a plot of the turbine inlet pressures versus temperatures;(2) a bifurcated operating mode for moderate Tc fluids with one or two pressures corresponding to the turbine inlet temperature;(3) a restricted operating mode for high Tc fluids with only one turbine inlet pressure possible for the turbine inlet temperature. An integrated-average temperature difference was used to quantify the thermal match between the flue gas and the organic fluid in the evaporator. The high Tc organic fluid has a small integrated-average temperature difference that yields large evaporator and system exergy efficiencies. Thus, the useful power is increased. The low Tc organic fluid has a bad thermal match in the evaporator that leads to lower ORC thermal performance.Finally, based on the discussion of impact of the critical temperature on cycle performance, several high Tc working fluids were chose as candidates for working fluids selection at the given heat source. By comparing the thermal efficiency, exergy efficiency of the cycle and net power output per unit mass of the working fluid, R134a is the preferred working fluid for the supercritical cycle heat source423.15K, toxicity, flammability and environmental impact taken into account. R1234ze followed, and R1234ze has excellent environmental characteristics, therefore it is proposed to use in places with high environmental standards.The key innovation point in this paper is that the selection criteria based on the critical temperature of the working fluid at a given heat source temperature was firstly specified internationally, which provides a valuable basis for the design and operation of supercritical ORC cycle and working fluid selection. The major work was published in Energy.