Screening Criteria Of Zeotropic Mixtures For The Trans-critical Organic Rankine Cycle And Experimental Investigation On The Organic Rankine Cycle With Mixture Working Fluid

With the shortage of fossil fuel and the worsening climate,many countries tend to develop renewable energy and more efficient and environmentally friendly energy technology.China is in the crucial period of energy structure adjustment and has set the target of carbon neutrality and peak carbon dioxide emissions.Organic Rankine Cycle(ORC)is an effective medium/low-temperature heat source(industrial waste heat and renewable energy sources,including solar energy,geothermal energy,and biomass energy,etc.)power generation technology that has attracted much attention.Trans-critical ORC using zeotropic mixtures can optimize the temperature match between ORC and heating/cooling source,which is advanced technology research of ORC.At present,the selection of working fluids has blindness and randomness.And there are no zeotropic mixture screening criteria for trans-critical ORC,which is worthy of further study.This article using exergy efficiency as the thermodynamic index,and LEC as the thermo-economic index,under the condition of opening heat source(the heat source outlet temperature is not restricted),established the trans-critical ORC thermodynamic and thermal-economic analysis model.By analyzing the temperature match relations between working fluid and heating/cooling source,established the mixtures screening criteria for trans-critical ORC.The temperature match relations between working fluid and heating/cooling source have been described by two equations.The optimal mixtures can be selected by simple calculation and physical property query.The results show that the optimal working fluid has both good thermodynamic and thermo-economic performance.In this paper,based on the mixtures screening criteria,the thermodynamic and thermo-economic performance of trans/sub-critical ORC using optimal mixtures at the heat source temperature of 200-240℃ have been compared and analyzed.The results show that trans-critical ORC has a better temperature match on the heating side,but the higher operating parameters also increase exergy loss of the expander and pump.Overall,the thermodynamic performance of trans-critical is slightly better than that of subcritical ORC,and the exergy efficiency between the two is about 1%.The higher operating parameters of trans-critical ORC have also lead to higher system cost and LEC than that of subcritical ORC.However,with the increase of heat source temperature,the LEC gap between the two gradually decreases.When the heat source temperature is 240℃,the difference between the two is only 0.0013$/kWh.The 4kW ORC laboratory experimental platform has been improved.A mixture working fluid filling system was built,which could precisely and rapidly adjust the mixture working fluid concentration of the ORC system.R236fa,R123,and their mixtures were used to test the experimental unit’s dynamic and steady performance.The dynamic performance test was carried out using R236fa/R123(0.5/0.5)as working fluid,by adjusting the expander rotating speed while the working fluid flow rate is constant.Results show that the dynamic response of the experimental unit is sensitive,and can quickly turn to stable,and there is no obvious overshoot in the response process of temperature and pressure parameters.The maximum power output is 3.7kW at 1200rpm.In the steady-state test,R236fa/R123(0.5/0.5)was used as working fluid to explore the influence of working fluid flow rate and expander rotation speed on the system output performance.Results show that under different flow rates,the maximum output of the unit was achieved near the speed of 1200rpm.The maximum shaft power is 3782W when the working fluid flow rate is 800kg/h and the rotation speed is 1200rpm.The maximum thermal efficiency is 6.76%when the working fluid flow rate is 600kg/h and the rotation speed is 1200rpm.At 1200rpm,five mixture working fluids R236fa/R123(R236fa mass fraction 0、0.25、0.5、0.75、1)were used for experimental analysis under different working fluid flow rates.Results show that the power output and thermal efficiency of the system vary little under different mixture concentrations,which means that the system performance of mixture working fluids has no obvious advantage.Pure fluid R236fa has achieved the highest shaft power of 3936W.Thermal efficiency is not greatly affected by the working fluid flow rate and has a small fluctuation of 6.5-7%.The maximum 7%occurs when the working flow rate is 600kg/h with pure fluid R123.By analyzing the temperature and pressure of each part of the system,it can be seen that with the pressure difference of the evaporator and condenser itself,using pure fluids can also cause temperature glide.Due to the high evaporator pressure,this effect is not obvious,but this effect is obvious on the condensation side.So that using pure fluids may have a better temperature match than using mixture working fluids.Therefore,the design of the unit should take into account the temperature glide caused by the pressure difference of the equipment itself,and further improve the equipment,to give play the performance advantage of mixture working fluids.