Thermodynamic Analysis And Multi-objective Optimization Of CO₂ Power Generation System Driven By Waste Heat

Carbon dioxide is non-toxic,harmless,cheap and easy to obtain,and the thermodynamic cycle system using carbon dioxide as the working fluid has good power generation performance.Supercritical CO2 recompression power cycle(SCRPC)and transcritical CO2 power cycle(TCPC)are two commonly used CO2 cycles with strong heat source adaptability and high energy conversion rate,which can be used to recover exhaust heat from power generation systems such as gas turbine(GT)and high-temperature solid oxide fuel cell(SOFC),and then formed into a combined system to further improve system efficiency.This paper first builds mathematical model of gas turbine based on its characteristic curves and model of SOFC,then conducts thermodynamic study on SCRPC and TCPC cycles,and finally explores the combination of power source and waste heat recovery system.At the same time liquified natural gas(LNG)is introduced,and three combined cycle systems are proposed.The thermodynamic analysis and multi-objective optimization of each combined cycle system are carried out,and the main research contents are as follows:Firstly,the MGT/TCPC combined cycle system is constructed with a micro gas turbine(MGT)as the power source.Under rated conditions,the system thermal efficiency and exergy efficiency reach 51.86%and 32.01%,respectively.The power generation is 212.1 kW,which is 42.81 kW higher than that of the single MGT.The exergy analysis results show that the combustor and condenser are the components with large exergy destruction of the system,which mainly comes from the irreversible loss of the combustion reaction and the large heat exchange temperature difference.Parameter analysis results show that when MGT adopts variable speed control strategy,the shaft speed,air and fuel flow,turbine inlet and regenerative outlet flue gas temperature all decrease with the decrease of MGT load.At the same time,the decrease of MGT load will also cause TCPC net power reduced,which reduces the total power generation of the system.In the multi-objective optimization program,the TCPC cycle high pressure and the cycle low pressure are selected as decision variables,and the particle swarm algorithm and the TOPSIS decision method are used to select the best point in the Pareto frontier.Secondly,the SOFC is coupled into the MGT/TCPC system to construct the SOFC/MGT/TCPC combined cycle system.The thermal efficiency and exergy efficiency of the system are 70.04%and 53.05%respectively,which are greatly improved compared to the uncoupled ones.The exergy analysis results show that SOFC stack becomes the component with the largest exergy destruction due to the overvoltage loss caused by the unbalanced potential difference.Parameter analysis results show that MGT part load rate and changes in parameters such as anode reflux ratio and MGT cycle pressure ratio will have certain impacts on system performance.In the multi-objective optimization study,the system power output and operating cost,thermal efficiency and operating cost,and exergy efficiency and operating cost are selected as three sets of multi-objective optimization functions.Particle swarm optimization and TOPSIS decision theory are used to determine the final optimization result.Finally,a small gas turbine is used as the power source.Aiming at the characteristic of high exhaust temperature,a GT/SCRPC/TCPC combined cycle system is constructed based on the principle of energy cascade utilization.The system can control the flow of flue gas into SCRPC and TCPC according to the gas turbine load to maximize the power generation efficiency by installing a splitter at the tail of the turbine.Under design conditions,the thermal efficiency and exergy efficiency of the system are 52.94%and 30.27%,respectively.GT adopts variable flow control strategy to operate,and its load rise and fall are mainly adjusted by fuel flow.When GT is running at a higher load,the fuel demand will increase substantially,while the air flow and air boost ratio remains basically unchanged.The increase in load is mainly due to the increase in turbine power,and the GT power generation efficiency is also increased.In addition,the influences of parameters including turbine I inlet temperature,TCPC condensation pressure,TCPC heat recovery efficiency,SCRPC and TCPC high pressure,HTR and LTR heat exchange efficiency on system performance are analyzed,and these seven parameters are used as decision variables for five sets of multi-objective optimization functions.The genetic algorithm and entropy-based TOPSIS decision-making method are adopted to select the optimal design parameters,and the corresponding system performance indexes are compared comprehensively.