Thermal-Mechanical Coupled Assembly Mechanics In Proton Exchange Membrane Fuel Cells

Fuel cells are electrochemical devices which directly convert the chemical energy of hydrogen and oxygen into the electrical energy, and the final reaction product is water. Since the energy crisis and the pollution situation are deteriorated at present, fuel cells have received considerable attentions as a kind of effective, clean and environmentally friendly energy equipment. As one of the most interested and useful fuel cells, proton exchange membrane fuel cell (PEMFC) has a good potential in the applications of aviation, space, navigation, mobile power sources and distributed power plants owing to the advantages of quick-start at room temperature, no loss of electrolyte, long operating lifespan and so on. Especially in the automobile industry, PEMFC has been regarded as the optimal power source in a modern car for environmental protection.Most PEMFC products in practice are assembled as large cell stacks, therefore it is necessary to study and solve the mechanics problems existing in the assembled PEMFC stack, and this has significant importance to the development of PEMFC technology and the improvement of PEMFC's performance, reliability and lifespan. Among those problems, the thermal influence, the assembly load determination and the optimal end plate design are especially critical. Taking PEMFC stack as the study object and numerical simulation as the study method, a series of researches on the above structure dependent mechanics problems of the PEMFC stack assembly are carried out in this dissertation.There are five chapters in this dissertation, and the research work mainly covers the following aspects:In chapter 1, types and current research status of the fuel cells are introduced, especially the operating principle, structure features and component functions of the PEMFC. Moreover, some key mechanics problems during the assembly mechanics research, such as the thermal management, the assembly load determination and the optimal end plate design, are separately discussed in detail.In chapter 2, combining the computational fluid dynamics (CFD) and the finite element method (FEM), three-dimensional thermal-mechanical coupled numerical models for a PEMFC single cell and a multi-cell stack are respectively established on the basis of typical fuel cell structures. The corresponding distribution and variation regularity of temperature, stress, strain and the membrane electrolyte assembly (MEA) surface contact pressure under different working conditions are analyzed and compared in turn. Also the deformation roles of the whole multi-cell stack and the bipolar plates are studied. Finally, effects of several heat dissipation factors, such as heat dissipation of the working environment, velocity and selection of the coolant fluid, on the temperature and stress distributions are discussed.In chapter 3, a mechanical equivalent stiffness model is proposed for large PEMFC stack assembly according to the basic idea of equivalent stiffness, for the purpose of dealing with the difficulty of analyzing mechanical performance for the whole large PEMFC stacks. Using this model, mechanical responses of the stack system under the assembly load can be quickly obtained, and then influences on the main structural bearing and deformation regularity of the PEMFC stack, which are caused by the assembly load, external environment and operating temperature can be studied. Verified with FEM, it shows that the equivalent stiffness model not only saves at least one order of time but also gives good calculation accuracy when calculating the whole structual stiffness of a typical PEMFC single cell, which may provide a new methodology for designing the assembly load.In chapter 4, the complete assembly load design for a typical PEMFC stack using the equivalent stiffness model is introduced considering factors as structural strength, product performance and temperature effect. In addition, application field of the equivalent stiffness model is expanded into the fuel cell structural reliability design, and the structural reliability of a typical PEMFC stack is studied based on the equivalent stiffness model. It also proceeds to show that the equivalent stiffness model offers a new design solution and an efficient and convenient tool for the large PEMFC stack assembly research.In chapter 5, based on a typical PEMFC stack, a multi-objective topology optimization model for designing the optimal end plate is established combining with FEM. The proposed optimization model is converted into a single-objective optimization problem by means of the weighting coefficient method. Also the equivalent stiffness model is used for proper simplifications to reduce the unnecessary calculation. After topology optimization of the end plate, the corresponding design requirements such as light weight and high stiffness can be well satisfied. Therefore, not only material cost can be reduced, but also the product performance can be promoted simultaneously. In the end of this chapter, influences of several design parameters, such as the number of clamping bolts, the number of design cells, and the different weighting coefficients, on the stack optimization design are discussed.