| The development in China is in a critical period of transition to a green economy.In order to improve the energy consumption structure,the proportion of coal in the energy consumption for power generation is gradually reduced,and the proportion of cleaner energy such as solar energy and natural gas should be increased.With actively dealing with the shortage of coal resource,efforts should be also made to alleviate the environmental pollution caused by the process of burning coal for power generation.At present,the technology of power generation by using natural gas in China is mainly based on gas turbine(GT)or internal combustion engine.However,the power generation efficiency is seriously restricted by the combustion technology and the great energy loss in the combustion process.Additionally,the efficiency can not exceed 50%due to the limitation of carnot engine efficiency.Solid oxide fuel cell(SOFC)is the most efficient fuel cell for power generation.The chemical energy in natural gas can be directly converted into electricity by electrochemical reaction in SOFC,which can effectively decrease the energy loss caused by the energy conversion from chemical energy to electricity.The operating temperature of SOFC is high,and the recovery of waste heat in the exhaust is of great value.SOFC can be coupled with micro gas turbine(MGT)into more efficient hybrid power system with a good prospect in distributed poly-generation system under the background "environmental friendly and green development" advocated by the Chinese government.Energy can be efficiently utilized according to the principle of "temperature matching,cascade utilization" for cooling,heating and power in the poly-generation system for multiple energy products.The poly-generation system is a complex system owning many energy conversion and utilization devices.It is necessary to achieve the optimal comprehensive system performance though the component design and matching are diffciult to confirm.The performances of system components are affected by the design,technical and economic factors.It is hard to determine the specific influence of these factors on the component performance by the traditional performance evaluation methods based on the first or second law of thermodynamics.It is also impossible to determine the potential of system component performance and put forward feasible performance improvement measures.To sum up,this paper presents a research on the integration characteristics and performance optimization of poly-generation system based on SOFC/MGT.A new exergoeconomic analysis method is developed to essentially analyze the potential of performance improvement in the system components,which is of positive significance to investigate and improve the performance of poly-generation system.On the basis of the above research backgrounds and major scientific issues,the research work carried out in this paper mainly includes the following aspects:Firstly,the integration law and thermal performance of the SOFC/MGT hybrid power system and the combined cooling,heating and power(CCHP)system based on the SOFC/MGT are analyzed.On the basis of establishing the mathematical models of SOFC and MGT,the energy conversion law of the hybrid power generation system is studied.The influences of the key design parameters,such as fuel flow rate,fuel utilization factor,steam to carbon ratio,air to fuel ratio and compressor compression ratio on the system power output and energy efficiency are analyzed.Then,the energy conversion and thermal matching characteristics of the CCHP system based on SOFC/MGT are studied.The thermodynamic performance of the system is investigated from the aspects of the first law of thermodynamics(energy efficiency)and the second law of thermodynamics(exergy efficiency).The influences of air to fuel ratio,SOFC working pressure/inlet temperature and ORC working medium flow rate on the thermal performance are analyzed in detail.The results show that the hybrid power system coupled with SOFC and MGT has obvious advantage in energy conversion efficiency compared to the single SOFC or MGT.The thermal efficiency can be increased to 90%by utilizing the waste heat of the hybrid power system for further providing cooling and heating.Secondly,a poly-generation system for providing cooling,heating,power and freshwater is proposed based on GT or SOFC/MGT and multi-effect desalination(MED)of seawater.The matching characteristics and system thermal performance are investigated.On the basis of constructing system mathematical models,the effects of key design parameters such as compressor pressure ratio,air temperature at the inlet of the compressor,pinch point temperature difference in the waste heat boiler,motive steam pressure of MED and TVC compression ratio on the power generation,exergy and overall energy efficiencies and the freshwater production are revealed.Then,a novel poly-generation system based on SOFC/MGT and MED is developed,and the system performance is comprehensively evaluated from the aspects of energy,exergy,economy,environment and freshwater production.The effects of the key design parameters such as fuel utilization factor,air to fuel ratio,feed seawater temperature,evaporator pressure of generator and entrainment ratio in organic steam ejector refrigerator(OSER)on the system performance are analyzed.The results of parametric study reveal that the system electrical,exergy and overall energy efficiencies all increase with the fuel utilization factor in SOFC.It is also illustrated that compared to the basal SOFC/MGT,the poly-generation system can extra provide cooling,heating and freshwater at loads of 14.33 kW,8.096 kW and 0.1303 kg/s with the system overall energy efficiency being increased by 4.65%under the design condition.Further,a multi-objective optimization of the the poly-generation system based on SOFC/MGT by using NSGA-Ⅱ method is carried out.The objectives in the optimization are system power output,overall energy efficiency,exergy efficiency,total cost and freshwater production,and the two dimensional(2D)and three dimensional(3D)optimizations are conducted individually.The 2D and 3D Pareto frontiers in optimizations are depicted and analyzed.The equilibrium point and TOPSIS(Technique for Order Preference by Similarity to an Ideal Solution)methods are applied for selecting the optimal system design parameters in differnent restrictions.At last,the optimal system performances on energy,exergy,economy and environment are obtained.Finally,a new method for evaluating the thermal-economic performance of poly-generation system based on SOFC/MGT by the advanced exergy and exergoeconomic analyses is developed.On the basis of constructing the energy and exergoeconomic models of the poly-generation system,the results of the traditional and advanced exergy and exergoeconomic analyses are shown and comparied.Compared to the traditional analysis method based on the first and second laws of thermodynamics,the advanced exergy and exergoeconomic analysis methods could reveal the potential of thermodynamic and economic performance of system components from both qualitative and quantitative perspectives.The potentials for improving the exergoeconomic performance of key system components are clearly revealed,and three possible strategies for improving the performances are developed according to the analysis results. |
PDF Full Text Request