Effects Of The Non-condensable Gas On The Thermodynamic Performance Of Organic Rankine Cycle

Nowadays, the problem of energy shortage which is caused by excessive consumption of primary energy becomes more and more obvious. There exists fairly large number of low-grade energies, such as solar, geothermal, industrial waste heat and automobile exhaust heat in China. Making full use of the energies can alleviate environmental pollution and increase energy efficiency. Over the years, organic Rankine Cycle(ORC) has a broad application prospects in thermoelectric conversion technology and has become an effective way to utilize low-grade energy. And so far, there have been many researches on performance and parameters optimization of ORC, such as selection of working fluids, and designs of new ORC applications. However, the effects of non-condensable gas on thermodynamic performances of ORC still don’t get enough notice. Therefore, the physical model of the mixture of non-condensable gas and organic working fluid is built in this paper. Also, to analysis the thermophysic property of the mixture, study the influence of non-condensable gas on organic working fluid’s phase transition, and discuss the thermodynamic performance of organic Rankine cycle under the given operating conditions, the Peng-Robinson EOS and the van der Waals mixing rules are applied to the model. The main research contents and conclusions in the paper are as follows:(1) The phase diagram of binary zeotropic mixing working fluids(R125/R134 a, R245fa/R601 a, R1234fy/R1234 ze, R245fa/R601) are given in this paper with the data calculated by the PR equation of state and the van der Waals mixing rules used to get the pressure, volume and the temperature(PVT). Under the given conditions the system is supposed to experience a constant pressure process and a theoretical model is established to build a gas-liquid equilibrium equation, which is used to calculate and analyze the influence of non-condensable gas on phase transition of organic working fluid. The results shows that the pressures of liquid phase and gas phase of working fluid are different because non-condensable gas results in a non-isothermal process during phase transition of organic working fluid.(2). The Peng-Robinson EOS and the van der Waals mixing rules are used to calculate thermodynamic property of the mixture of non-condensable gas and organic working fluid and analyze the influence of the concentration of non-condensable gas on performance of ORC. The results show that the mixture of organic substance and non-condensable gas has larger entropy and lesser enthalpy than the pure working fluid on the same condition. In the process of evaporation and condensation, non-condensable gas can cause the temperature glide. Under the given conditions of the outlet temperature of the evaporator and the condenser, the outlet temperature of turbine will decreases and outlet temperature of pump will increases in case of the existence of non-condensable gas, and both the pump power and power output of the turbine rise. The net power output will gain the maximum value at the point that the nitrogen mole fraction is about 0.2%, and the effect of non-condensable gas is positive under 0.3%. However, hydrogen changes the net power output work in a way of only negative effect. With the increase of non-condensable gas concentration, mass flow rate increases, while cycle thermal efficiency declines, and the exergy loss in evaporator, pump and of expander increases, while the exergy loss of the condenser decreases. Nevertheless, the total exergy loss will increases, of which the exergy loss in pump affected by nitrogen is the largest. Hydrogen is in line with nitrogen on the influence of each part but stronger.