| The issue of energy has always been an important issue related to the national economy and people’s livelihood,and it is also a hot spot of scientific research.For thousands of years,traditional fossil energy has brought wealth to social production,but at the same time,low energy conversion efficiency and limited energy reserves make the energy crisis approaching step by step.Unreasonable development and utilization also leads to frequent natural disasters.In the face of increasingly serious energy and environmental problems,the search for clean and efficient power generation and alternative energy has become one of the main research topics and directions.Solid oxide fuel cell(SOFC)can convert more than 40% of fuel energy into electricity,and has broad research prospects because of its low working noise,long life cycle,high fuel flexibility and less emissions.Combining it with waste heat recovery(WHR)technology can further reduce emissions and improve thermal efficiency,which provides a new idea for the research of distributed power plants and power supply equipment.At present,there are some problems in the research of solid oxide fuel cell cogeneration system.It is embodied in:(1)The design of the system combining fuel cell,gas turbine and waste heat recovery technology is not mature enough.The bottom cycle and temperature do not match well,and the optimal working conditions and working fluid are not discussed in depth.The thermal efficiency of the system can be further improved.(2)Although the new co-supply system of coupled homogeneous compression ignition engine has great potential,there is little related research.The arrangement of preheater and its temperature matching in the system need to be further optimized,and the selection and coupling form of waste heat recovery is single.The prediction of engine operating conditions and the impact of engine parameters on the overall performance of the system need to be studied.(3)Although the concept of coupling between biomass technology and solid oxygen fuel cell has been put forward for a long time,the research focuses on the subsystem formed by gasifier and battery body,and there is little overall design and comprehensive performance evaluation of the combined system.In order to solve the above problems,three new types of solid oxide fuel cell cogeneration systems are designed and analyzed by combining different means of fuel supply,exhaust treatment and waste heat recovery.The thermodynamic and economic performances of the three schemes are compared.Finally,the operating conditions of the system are optimized according to multiperformance indicators.The main work includes:(1)Establish the mathematical model of cell stack and verify it,simulate the influence of cell operating parameters on cell performance and exhaust gas state,so as to select the more influential operating parameters for further research.(2)The mathematical simulation and verification of homogeneous charge compression ignition engine,supercritical carbon dioxide recompression cycle,Kalina cycle,organic Rankine cycle,regenerative supercritical carbon dioxide cycle,absorption refrigeration cycle and biomass gasifier are completed.The optimal operating conditions and working fluids of each subsystem are determined by simulation,and the effects of relevant operating parameters on the overall performance of the system are observed.(3)Establish an evaluation model to evaluate and compare the thermodynamic and economic performance of the three combined systems.Then use the particle swarm optimization algorithm to solve the problem of selecting the optimal working conditions of the system under multiple performance indicators.Based on the above work,the main achievements of this article are as follows:(1)A new combined system is designed and analyzed,which is composed of solid oxide fuel cell-gas turbine subsystem,supercritical carbon dioxide recompression cycle and Kalina cycle.The zeotropic working fluid is used in the supercritical carbon dioxide recompression cycle.The optimum composition and ratio of the working fluid in the bottom cycle are determined by simulation.The overall thermodynamic and economic characteristics of the system are simulated and analyzed.The results show that the thermal efficiency of the combined system is 71.37%,which is 38.65% higher than that of the cell.The longest payback time of the system is 7.492 years,which basically meets the economic requirements.(2)A new system composed of solid oxide fuel cell-preheating subsystem,homogeneous charge compression ignition engine and organic Rankine cycle is designed and analyzed.The effect of engine intake temperature on engine performance is simulated and observed,and the best zeotropic fluid and its ratio of organic Rankine cycle are determined according to its exhaust temperature.Finally,the thermodynamic and economic analysis of the system is carried out.The results show that the performance of the new system is better than that of the traditional gas turbine combined system.The thermal efficiency and thermal efficiency of the system can reach 63.6% and 61.3% respectively,which is at most 18.76% and 17.95% higher than that of the cell stack.The payback period of the system is 2.98 years,which is about half of that of the traditional system,which can fully meet the economic requirements.(3)A new type of solid oxide fuel cell cogeneration system based on agricultural waste is designed and analyzed.The system uses homogeneous charge compression ignition engine to deal with unreacted fuel in exhaust,and uses the combination of regenerative supercritical carbon dioxide cycle and absorption refrigeration cycle to recover waste heat from exhaust gas.Under the selected operating conditions,the thermal efficiency of the system has been increased by 11.10%,and the total energy utilization efficiency can reach59.35%.The payback time of the system is nearly 60% shorter than that of the traditional system,with a minimum of 1.77 years.The whole system realizes the cascade utilization of energy,and is efficient,clean and has good performance.(4)Through performance comparison,it is concluded that the first scheme has the best thermodynamic performance and the lowest economic performance,while the third scheme is on the contrary.The multi-objective optimization of the third scheme is carried out by using particle swarm optimization algorithm.The optimization result sacrifices 3.98% of the power output,but reduces the cost by 18.19% and shortens the payback time by 5.73%.In the case of considering the total system cost,net output power and subsystem payback time at the same time,reducing the number of cell stacks,reducing the inlet temperature of the turbine and reducing the air-fuel ratio will be a better choice. |
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