Research On Low-Temperature Startup Characteristics And Control Strategies For Automotive Fuel Cell Systems

As global energy demand escalates and environmental challenges intensify,the exploration and promotion of sustainable green energy solutions have become increasingly urgent.Proton exchange membrane fuel cells(PEMFC),as a novel and efficient clean energy technology,demonstrate significant innovative potential in promoting green development and low-carbon transformation in emerging fields such as new energy vehicles and energy storage.However,fuel cell vehicles encounter significant challenges in low-temperature environments,including forced shutdowns and notable efficiency reduction,which limit their application and commercial expansion in cold regions.Currently,research on improving the cold start characteristics of fuel cell vehicles and their key control technologies for environmental adaptability has become one of the hot topics in the field.The systematic analysis and understanding of the internal mechanisms and external characteristics of low-temperature fuel cell startup,along with the development and proposal of rational and efficient control strategies,are crucial technological paths for advancing the industrialization of fuel cell vehicles.This paper focuses on researching the low-temperature startup characteristics and control strategies for automotive fuel cell systems.Analyzing the mass and heat transfer mechanisms inside fuel cells and their system operation characteristics,a model for low-temperature startup of fuel cell systems was established.Aiming to improve the low-temperature cold startup performance,a load current density control method based on thermal equilibrium was proposed without auxiliary conditions,and corresponding control strategies were developed to shorten the cold startup time of fuel cells.Furthermore,addressing the low-temperature auxiliary cold start requirements of fuel cell vehicles in extremely cold environments,a multi-stage heating-load coordinated control strategy for fuel cell systems was proposed,and genetic algorithms were used for multi-objective optimization to achieve comprehensive regulation and improvement of low-temperature startup performance and energy-saving effects.Addressing the issue of reduced system efficiency due to auxiliary heating energy consumption in fuel cell systems,an optimized thermal management system circuit with the introduction of a thermal storage unit was developed.The phase-change material combined with cooling fluid and a control method for intermittent operation of the water pump are designed,significantly extending the time the fuel cell remains at the lowest self-starting temperature.The specific research content of this paper is as follows:(1)Analyzing the phase division of the cold start process,this study delves into the competition between heat production and ice accumulation,exploring their coupled effects under non-linear conditions to clarify the necessary conditions for successful startup.Furthermore,a cold start model for individual fuel cells was constructed using the lumped parameter method,providing tools to solve the output characteristics and temperature distribution states of fuel cells.(2)Building on the analysis of the operational characteristics of fuel cell systems and their key components,the stack,gas supply system,and thermal management system were modeled through simulation.The model’s accuracy was validated against low-temperature start-up experimental data,enabling accurate estimation of the fuel cell system’s state in low-temperature environments and comparison of energy consumption by auxiliary systems during the cold start process.The results indicated that the constructed system model effectively reflected the operational states of key components under low-temperature start conditions.The energy consumed by the air compressor,hydrogen circulation pump,and coolant pump during cold start accounted for 3.49%,0.63%,and 2.08% of the total energy consumption,respectively,while the PTC heater’s energy consumption constituted 33.4% of the total energy consumption of the fuel cell system.(3)In addressing the issue of mutual contradiction and coupling between ice formation and heat production during passive cold starts of fuel cells,an analysis of critical conditions for cold start was conducted.Subsequently,a current density control scheme based on waterthermal balance was proposed,and a control strategy was developed using fuzzy control theory.This proposed strategy,in controlling the load current,took into account both the fraction of ice volume and the heat absorption of the membrane.It utilized inertia elements to "smooth out" the load current,meeting start-up requirements while reducing damage to the fuel cell.The feasibility and effectiveness of this strategy were validated through simulation.The results showed that compared to other load modes,the start-up mode proposed in this paper reduced the cold start time by 25.6s to 41.6s at an initial environmental temperature of-10°C,and decreased the ice volume fraction by 29.4% to 31.8%.(4)Addressing the technical challenge of coordinating energy consumption and start-up time in the thermal management system caused by external heating during active cold start conditions,an in-depth study was conducted on system heating and load operating states under assisted start conditions.A multi-stage heating-load collaborative control strategy for the fuel cell system was proposed to achieve synergistic control of heating power and electric current.Based on this,with start-up time and heating energy consumption as optimization objectives,a multi-objective optimization of the control strategy was conducted using genetic algorithms,leading to the development of an optimal control strategy for the fuel cell system that balances start-up time and system energy efficiency.Compared to strategies focusing solely on start-up time,the proposed strategy resulted in a 5.20% increase in start-up time and a 17.54% reduction in energy consumption of the PTC heater.(5)Considering the insulation needs of fuel cells in shutdown state and aiming to further enhance cold start energy efficiency,a low-temperature thermal management system for fuel cells,integrating phase change materials(PCM)with coolant,was designed.A scheme for intermittent operation of the water pump for stack insulation was proposed and its feasibility was established and verified through simulation.Different thermal management schemes were compared in terms of insulation and startup performance.The results showed that the proposed insulation scheme could transfer the heat released during the PCM phase change process to the stack via the coolant pump,thereby maintaining the minimum temperature of the fuel cell stack above the lowest self-start temperature for about 72.58 h in a-20°C low-temperature environment.This thesis,based on an in-depth analysis and study of the low-temperature start-up characteristics of automotive fuel cell systems,proposes and develops control strategies targeted at key application scenarios of fuel cell vehicles in cold conditions.The research significantly enhances the environmental adaptability of fuel cell vehicles under low-temperature operating conditions.The findings provide theoretical guidance for the study of the mechanisms and key control technologies of fuel cell low-temperature start-up performance and offer important technical support for accelerating the demonstration operation and industrial application of fuel cell vehicles.