Study On Characteristics And Optimization Of Combined Cooling,Heating And Power System Based On SOFC/GT And Transcritical CO₂ Cycles

Due to the shortage of energy and environmental pollution,the combined cooling,heating and power(CCHP)system has become one of the future directions of development in energy supply.The solid oxide fuel cell/gas turbine(SOFC/GT)hybrid cycle with high-efficiency and low-emission has a broad prospect in the CCHP field.The transcritical CO2 power cycle could achieve higher power generation than an organic Rankine cycle.In addition,the transcritical CO2 power cycle has advantages of high energy conversion efficiency,compact system,low noise,environment-friendly and so on,and it is suitable to generate power by recovering waste heat from medium and low temperature sources.Therefore,the transcritical CO2 power cycle is used to recover the high-grade waste heat of the SOFC/GT hybrid cycle,and on this basis,CCHP systems are integrated.Simulation analysis is employed as the research method in the thesis.On the basis of domestic and international research results of SOFC/GT hybrid cycle and transcritical CO2 cycle,a series of studies on steady-state modeling,design parameter optimization,dynamic characteristics,and control have been carried out for the CCHP system based on the SOFC/GT hybrid cycle and transcritical CO2 power cycle.The main contents of research include the following three parts.1)The SOFC/GT hybrid cycle,the transcritical CO2 power cycle and the transcritical CO2 refrigeration cycle with two-stage compression and intercooling are integrated in an innovative CCHP system.The first and second laws of thermodynamics and the exergoeconomic analysis method are used to analyze the the CCHP system’s performance under design and off-design conditions,results show that the CCHP system could achieve high power generation and exergy efficienies.The effects of SOFC pressure,air flowrate,CO2 flowrates in transcritical CO2 power and refrigeration cycles,ambient temperature and fuel price on the system’s thermodynamic and exergoeconomic performance(including net power generation,heating output,cooling output,net power generation efficiency,exergy efficiency and unit exergy cost for trigeneration)are studied.The NSGA-II algorithm and the hybrid optimization algorithm based on NSGA-II and the particle swarm algorithm are used for the multiobjective optimization of the CCHP system,and the total energy output,the exergy efficiency and the unit exergy cost for trigeneration are selected as the objective functions.The hybrid optimization algorithm based on NSGA-II and the particle swarm algorithm is found to be more capable of seeking the optimal design parameters and the best design parameters for three objectives with equal weights are obtained.In addition,a set of design parameters whose all three objective values are better than those under design conditions are determined.2)The LNG cold energy utilization cycle,the SOFC/GT hybrid system,the CO2 power cycle and the argon power cycle are intergeted in a new type of CCHP system,which can simultaneously provides cooling at three temperature levels(-25℃,-10℃and 5℃)and domestic hot water with 55℃.The the CCHP system performance under design and off-design conditions are analyzed by using the energy,exergy and exergoeconomic methods.The research results show that this CCHP system could efficiently utilize the LNG cold energy and the waste heat of exhaust from the SOFC/GT hybrid cycle,with high exergy efficiency,power generation and cooling outputs,and annual CO2 reduction of 786.68 tons/year could be reached under the design conditions.The effects of SOFC pressure,LNG pressure,air flowrate,CO2 flowrate,ambient temperature and LNG price on the performance of the CCHP system are studied.The total energy output,the exergy efficiency,the unit exergy cost for trigeneration and the annual CO2 emission reduction are taken as the objective functions for optimization.The hybrid optimization algorithm based on NSGA-II and the particle swarm algorithm is used to optimize the multi-objective problems under two scenarios with different combinations of objectives.The best design parameters for obejectives with four combinations of weighting factors are obtained in two scenarios.This study provides a reference for the multi-objective optimization design and analysis of similar CCHP systems.3)The dynamic model of the CCHP system based on the SOFC/GT hybrid cycle,the transcritical CO2 power cycle and the transcritical CO2 refrigeration cycle is developed in Matlab/Simulink simulation platform by using modular modeling method.The dynamic response of the system is studied when SOFC current,fuel flowrate and air flowrate change in steps.PID controllers are used to control the net power generation of the CCHP system and SOFC’s fuel utilization factor by adjusting SOFC current,fuel flowrate and air flowrate.Then,the controlling dynamic responses of the system are studied.The results show that the net power generation of the system could be stablized near the load demand of power in 20 seconds.The maximum instantaneous error of the system’s net power generation is 6.73%and the average error is 0.14%.The net power generation of the system can follow the power demand well.The maximum instantaneous error of SOFC fuel utilization factor is 0.39%and the average error is 0.01%,SOFC fuel utilization could quickly be stablized at the ideal value of 0.85.The thermodynamic and exergoeconomic performance parameters of the CCHP system based on the SOFC/GT hybrid cycle and transcritical CO2 power cycle under design and off-design conditions are obtained.The Pareto optimal solution set with objective functions including total energy output,exergy efficiency and unit exergy cost for trigeneration is determined.In addition,the control of net power generation and SOFC’ s fuel utilization factor is realized.This thesis provides the necessary theoretical basis for this type of CCHP systems’ characteristic analysis,system integration,multiobjective optimization and operation management.