Performance Research Of CO₂ Mixtures Transcritical Rankine Cycle Using For Engine Waste Heat Recovery

Energy-saving and emission-reduction of engine are greatly significant to deal with the energy crisis and environmental pollution.The waste heat recovery of engine is one of the most effective technologies to realize energy-saving and emission-reduction.In the engine waste heat recovery field,the CO₂ transcritical Rankine cycle has a better adaptability and considerable potential for the waste heat recovery of engine.However,its high operation pressure,strict condensation requirement and lower efficiency are difficult problems limiting the wide application.Therefore,this study extensively explored different CO₂ mixtures as the working fluid.Based on the improvement of cycle performance,condensation conditions and operating pressure,the theoretical exploration and validation experiments were carried out to select the most suitable CO₂ mixtures for transcritical Rankine cycle.In order to study the performance of CO₂ mixtures transcritical Rankine cycle,the first problem to be solved is the calculation of the physical properties of CO₂mixtures.In this paper,the Peng-Robinson equation of state was selected as the basic equation.Through experimental data investigation and software calculation,a high-precision prediction model for the calculation of CO₂ mixtures physical properties was established,and the properties such as enthalpy,entropy,critical temperature and critical pressure were obtained.The error of model calculation results are less than 2%.Taking the preheated and regenerative transcritical Rankine cycle waste heat recovery system as the baseline case,various CO₂ mixtures were selected as the working fluids to implement theoretical exploration and research.Moreover,the influences of different CO₂ mixtures and operating parameters on the cycle performance were studied on the basis of net power,thermal efficiency,exergy efficiency,electricity production cost and total heat transfer area.Based on CO₂mixtures transcritical Rankine cycle with optimal net output power,it was found that the CO₂/R32?0.3/0.7?performed best when the condensation temperature is less than40?,but the CO₂/R161?0.45/0.55?was the most promising when the condensation temperature is higher than 40?.When the condensation temperature was set at 20?,the operating pressure of CO₂/R161?0.45/0.55?and CO₂/R32?0.3/0.7?transcritical Rankine cycle was reduced by 37%and 35%,and the maximum net output power of the system is increased by 4.5%and 12.4%respectively,compared with CO₂transcritical Rankine cycle.For the purpose of transcritical Rankine cycle with optimal total heat transfer area,CO₂/R32?0.7/0.3?was the most promising.For such case,the ambient temperature could meet the cooling requirements of transcritical Rankine cycle.In addition,the net output power of transcritical Rankine cycle worked with CO₂/R32?0.7/0.3?increased by 8.8%and the operating pressure decreased by10.8%compared with pure CO₂,and the total heat transfer area increased by 29.4%compared with CO₂/R32?0.3/0.7?.The validation experimental results showed that CO₂/R134a mixtures could improve the thermodynamic performance of transcritical Rankine cycle,including net output power,thermal efficiency and exergy efficiency compared with pure CO₂.At the same time,the experimental data proved that there was an intermediate mass fraction of CO₂ mixtures making the thermodynamic performance of cycle achieve to best.In addition,the system can output normally and steadily when the condensation source of the CO₂/R134a mixtures transcritical Rankine cycle was only provided by the ambient temperature.