Theoretical And Experimental Research Of CO₂-based Transcritical Power Cycle For Waste Heat Recovery Of Internal Combustion Engine

As internal combustion engines?ICEs?becomes a main source of petroleum consumption and CO₂ emission,energy-saving of them shows great significance to energy security of a country and reduction of CO₂ emission.Among energy-saving technologies of ICEs,waste heat recovery?WHR?possesses the most potential of efficiency improvement and energy saving.Current project proposes that CO₂-based Transcritical Power Cycle?CTPC?as a WHR bottoming cycle can fit well to engine waste heat sources with various grade,large gradient temperature and highly transient characteristics,and also satisfies the demand of miniaturization and lightweight.Aiming at fully understanding of the matching law between CTPC and heat sources,theoretical and experimental researches of CTPC for ICE-WHR are conducted in this project.According to the advantage of supercritical CO₂ in simultaneously recovering both high-and low-grade waste heat sources,a modified CTPC with preheater and regenerator is constructed to improve poor performance of basic CTPC.Models based on the first law of thermodynamics,second law of thermodynamics,component evaluation and economic performance examination are built to comprehensively investigate the performance of the modified CTPC.In comparison with the basic CTPC and R123-based organic Rankine cycle,the modified CTPC achieves great improvement in net power output,thermal efficiency and exergy efficiency,reducing electricity production cost and showing miniaturization potential in turbine and heat exchangers.Based on the modified CTPC,two novel design methods of cycle optimization are proposed from the aspect of configurations and parameters.Considering different layouts including basic CTPC,preheated CTPC,regenerative CTPC and both preheated and regenerative CTPC,selection MAPs are proposed to satisfy different demands with different applications,which realizes configuration optimization of CTPC.In order to realize the best recovery of both high-and low-grade waste heat with or without regeneration,the‘optimum design point'method is proposed to complete parameter optimization.Results find the matching law between critical temperature and the optimal design parameters.The thermophysical property of low critical temperature is the prerequisite for CO₂ to realize the optimum parameter design.After analysis of optimization methods,a 4.5 kW CTPC prototype is designed and developed for combined recovery of high-and low-grade waste heat of ICE,with the design pressure and temperature of 10 MPa and 231°C,respectively.The test bench can be switched easily to form four CTPC configurations.Gas heater,one of the key components,is designed and manufactured independently,which show stable operating performance at high pressure and high temperature.Finally,operating strategies and control methods of the prototype are proposed according to the characteristics of CTPC and ICE.Based on the CTPC prototype,tests are conducted with different waste heat sources,cycle configurations and operating conditions.Under a specific ICE condition and CTPC condition,there is an optimal frequency for the preheating pump to balance the adequate heat dissipation requirement of ICE and maximum heat recovery demand of CTPC,avoiding a lower coolant temperature due to excess heat recovery or CTPC performance deduction because of insufficient heat recovery.In the test bench,the optimal frequency of preheating pump is 8.75 Hz.Besides,from experimental comparison between four CTPC configurations,it can be found that preheater and regenerator bring two main benefits to CTPC,i.e.improving net power output and reducing total cooling load.Compared with the basic CTPC,the estimated net power output,thermal efficiency and exergy efficiency of the modified CTPC with both preheater and regenerator is increased by 110.6%,69.9%,and 79.5%,respectively.At the same time,total cooling load of the CTPC-ICE combined system is reduced by18.1%.The maximum thermal efficiency improvement of ICE is from 39.4%to 41.4%.Moreover,to obtain the best net power output,pump speed and pressure ratio are chosen as variables for parameter optimization research.Through the experiments,pressure ratio shows the greatest impact on net power output while the pump speed has unapparent relation to net power output.After regression analysis of available experimental data by SPSS software,a predictive model is obtained to find the optimal condition.Considering real operating conditions of ICEs,various responses of systems are tested under different types of engine conditions.Results show that the preheating process can improve dynamic performance of the CTPC by setting ICE to start and warm condition,stop and idle condition,stop and restart condition,and random changing condition.The reasons are as follows.Firstly,the preheating process can raise expansion inlet pressure and temperature which can improve thermodynamic performance of CTPC and stabilize liquid height of tank,which could keep stable operation of CTPC.Secondly,it can maintain continuous power output capacity for more than twice extended time when ICE is going to stop or idle and also avoid pressure uprush that appears under the stop condition of ICE.Thirdly,owing to large heat capacity of engine coolant,it can play an important role in the restart and random condition of ICE since the preheater acts as a buffer.