Thermo-economic Optimization Of Power Systems For Waste Heat Recovery

Large quantities of waste heat with exhaust gas as the carrying medium is generated by the industrial processes and working processes of internal combustion engine and gas turbine.The power recovery of the waste heat in an economical and feasible way can reduce the consumptions of the fossil fuels and the pollutions of CO₂and atmospheric pollutants(NO_x,SO_x,and particulate matter),which is consistent with sustainable development strategy.The thermo-economic performance of the waste heat power system is determined by the temperature and capacity of the waste heat source and the technology of the power cycle which includes the cycle configuration,working fluid,cycle parameters and the design of the equipment.In this thesis,the research methods,including the design of heat exchangers,the calculation of turbine efficiency,and the system cost estimation based on Aspen PEA,etc.are employed.The variation of the thermo-economic performance of waste heat power system with the heat source condition and the power cycle technology are studied.Specifically,it includes the preconditions for the economic feasibility of the system,the minimum initial temperature of the flue gas to ensure the economic feasibility of the system,the indicator and optimization objective function of the thermo-economic performance of the system,the optimal power cycle technology under different heat source condition,and the direction of the improve design of the cycle configuration,etc.The main contributions of this work are as follows:The EPC(electricity production cost)of the system reflects the total energy and resource utilization efficiency of the system.Thus,the electricity production cost of the fossil energy power generation system,EPC₀,is introduced as the benchmark value,and we propose that the criterion of the economic feasibility of waste heat power system is EPC?EPC₀.Only if this precondition is satisfied,can the power recovery from the waste heat really bring the benefits of energy saving and the related material resources saving.Therefore,the thermo-economic optimization objective of the waste heat power system is to maximize the net power output with the constraint of EPC?EPC₀,so as to recover the power from the flue gas waste heat as much as possible under the premise of the economic feasibility of the system.For the waste heat source with a mass flow rate of 42.15 kg/s,the minimum initial temperatures of the flue gas to ensure the economic feasibility of the ORC(organic Rankine cycle)and SRC(steam Rankine cycle)are 320?and 350?respectively.When the flue gas initial temperatures are above 410?and 385?,the EPCs corresponding to the maximum net power output condition of ORC and SRC systems are lower than EPC₀,which shows that the power cycle technology cannot fully realize the net power output under the corresponding heat source condition,and suggests to improve the cycle configuration.Compared to ORC and SRC,the OTC(organic trans-critical cycle),SFC(steam flash cycle)and SDC(steam dual-pressure cycle)can give the better temperature match between flue gas and working fluid,which can increase the thermo-economic performance of the system.When the initial temperature of flue gas is higher,the OTC and SDC systems suffer from the poor thermal stability of organic fluid and insufficient temperature matching between flue gas and working fluid respectively,which results in larger irreversible in the heat transfer process between flue gas and working fluid,and affects the thermo-economic performance of the system.The CO₂-TC system can make up the above shortcomings.However,the low critical temperature of CO₂ results in the high reduced condensation temperature and the high average exothermic temperature,which affects the thermal efficiency and net power output of the system.Therefore,four improved cycles including over-expansion CO₂-TC,split heating CO₂-TC,CO₂-TC+ORC,CO₂-TC/ORC are investigated and compared,and the results indicate that the split heating CO₂-TC system shows the optimal thermo-economic performance.The maximum net power output with the constraint of EPC?EPC₀ of nine power systems including ORC,OTC,SRC,SFC,SDC,over-expansion CO₂-TC,split heating CO₂-TC,CO₂-TC+ORC,CO₂-TC/ORC are optimized and compared under various waste heat conditions.For the waste heat source with mass flow rate of m kg/s(20?m?150),when its initial temperature is lower than T₀,the power system is not economic feasibility.When its initial temperature is higher than T₀,and lower than T₁,ORC systems shows the optimal thermo-economic performance.When its initial temperature is higher than T₁,and lower than T₂,OTC systems shows the optimal thermo-economic performance.When its initial temperature is higher than T₂,split heating CO₂-TC systems shows the optimal thermo-economic performance,where,T₀=240.45+389.36e^-m/29.71?,T₁=330.92+449.2e^-m/24.98?,T₂=394.76+563.69e^-m/19.75?.