Thermodynamic Analysis And Multi-objective Optimization Of A Novel Cogeneration System Based On SOFC/GT

Solid Oxide Fuel Cell(SOFC)with its strong adaptability to fuel,high efficiency,easy maintenance,environmental friendly characteristics,has become one of the most potential means of generating in 21st century.It is helpful to ease the world energy crisis and improve the energy structure is of great significance,and is one of current hot research topic in the field of green energy.According to the high operating temperature of SOFC,SOFC/GT combined system is the current mainstream SOFC power generation mode.Due to the high exhaust temperature of SOFC/GT systems,one or more waste heat recovery and utilization systems are usually matched to improve the energy utilization rate of the integrated system.As one of the most common gases,carbon dioxide is not only non-toxic and harmless,but also has good work capacity and wide work range.The transcritical carbon dioxide cycle(TRCC)is one of the more mature thermal cycles in the field of refrigeration,and has a good prospect in nuclear power,heat pump and other fields.In this paper,the matching mechanism of TRCC and SOFC/GT system was studied and analyze the influence pattern of important parameter variation on system performance.The main research contents of this paper are as follows:Firstly,the mathematical and thermodynamic models of SOFC/GT/TRCC system are established.In order to verify the correctness of the system model,the simulation data are compared with the experimental data provided by the references,and the results show that the model is in good agreement with the reference system,which proves that the combined system model established in this paper has good reliability and validity.Through thermodynamic analysis,the important parameters of SOFC/GT system and the influence of cyclic pressure ratio in TRCC cycle on the output power and efficiency of the system are analyzed.The results show that the thermal efficiency and exergy efficiency of the combined system reach 70.47%and 67.99%respectively.The thermodynamic performance of the system decreases with the increase of SOFC current density within the parameter operating range.The optimum value is obtained when the fuel cell input temperature is 490?.Secondly,aiming at the problems of narrow adjustment range of system parameters and poor optimization ability,the system is improved structurally.A high temperature and high pressure exhaust is extracted from the exit of the combustion chamber to preheat the air.After the verification of the new system model,thermodynamic analysis and exergy analysis were carried out.The results show that the thermal efficiency and exergy efficiency of the system are 64.40%and 62.13%respectively under the new design condition.Although the efficiency of the improved system is reduced compared with the original system,it can expand the range of system parameter variation and improve the applicability of the system.Meanwhile,the temperature matching range of the waste heat recovery system is also improved,thus providing feasibility for the subsequent optimization work.Exergy analysis of the system shows that the external losses of the system are mainly concentrated in the condenser and waste heat boiler with large temperature difference,which provides a reference for subsequent improvement and optimization of the system.Finally,the economic analysis and multi-objective optimization of the improved SOFC/GT system are carried out to explore the influence of the variation of each parameter of the system on the cost of each major component.The cost of improved SOFC/GT coupler systems is concentrated in SOFC reactors,DC/AC converters,turbines and heat exchangers.In addition to the main components of the reactor,turbine and heat exchanger are the equipment components that can effectively reduce the system cost.In order to optimize the thermodynamic performance and economic cost of the system simultaneously,this paper introduces genetic algorithm to multi-objective optimization on the improved SOFC/GT system to obtain the optimal state point set(Pareto frontier)satisfying different objectives.TOPSIS decision making method is used to select the optimal result from Pareto frontier.The result shows that TOPSIS decision analysis can get the balance point between two objective functions meeting various operating conditions.At this point,the best thermal efficiency of the system is 63.08%,the best exergy efficiency is 61.1%,and the economic cost is 1.952USD/s and 1.920USD/s respectively.