Investigation On The Performance Characteristics And Optimal Theories Of Two Classes Of Typical Energy Conversion Systems-Fuel Cells And Internal Combustion Engine Cycles

Since the 20^th century, the energy conversion technologies have been developed quite quickly. Along with the emergence of the serious energy issue, searching for new clean proper energy sources have became the focus of attention worldwide and enhancing the performance of the energy conversion devices through the improvement of thermodynamic cycles has been the main developing trend of the energy technologies. Under this background, the various thermodynamic analytical methods emerge as the times require to reveal the inherent mechanism of energy conversion and to optimize the performance of the energy systems, which is quite significant for the development of new thermodynamic cycles and the improvement of the system performance.As one of the most important applications of hydrogen energy, fuel cells may be used as distributed electric resources as well as the power plants with large capacities, and consequently, are considered to be one of the best choices for power generation with highly efficient, energy-saving and eco-friendly characteristics in the 21^th century. On the other hand, during the past hundred years, great advancement of the vehicle industry was, first of all, due to the development of the internal combustion engine, and accelerated this technology to a certain extent at the same time. So far the internal combustion engines have been widely used in the areas of industry, agriculture, electric power, national defense, and so on. Even today the internal combustion engine is still the most popular and important power mechanism with the largest range of applications. Therefore, the thermodynamic investigations about these two typical energy conversion systems are of great importance to the practical applications of new energy technologies, the efficient utilization of energy resources, the improvement of environment, and the exploitation of interdisciplinary research.Based on the above reasons, this dissertation is focused on the performance analysis and optimization of the fuel cell systems and several typical internal combustion engine cycles. The performance parameters of the systems under the influence of the various irreversibilities are optimized and the relations between the parameters are searched. The main research contents are organized as follows:In Chapter 1, the brief introduction of the research background and development of the energy conversion systems are given.In Chapter 2, an irreversible model of a class of fuel cells working at steady state is established, in which the irreversibilities resulting from electrochemical reaction, electrical resistance and heat transfer to the environment are taken into account. The physical and chemical performances of the fuel cell are investigated by using the theory of electrochemistry, non-equilibrium thermodynamics, and finite-time thermodynamics from an energetic point of view, and the influence of some design and operating parameters on the performance of the fuel cell is discussed in detail. The results obtained may provide a theoretical basis for both the optimal design and operation of real fuel cells.In Chapter 3, a theoretically modeling approach is presented which describes the behavior of a fuel cell-heat engine hybrid system in steady-state operating condition to provide useful fundamental design characteristics as well as potential critical problems. The different sources of irreversible losses, such as the electrochemical reaction, electric resistances, finite-rate heat transfer between the fuel cell and the heat engine, and heat leak from the fuel cell to the environment are specified and investigated. Energy and entropy analyses are used to indicate the multi-irreversible losses and to assess the work potentials of the hybrid system. The results obtained may give a practical guidance on both the optimal design and operation of real fuel cell-heat engine hybrid systems. This new approach can also provide some theoretical supports for the investigation and development of similar energy conversion settings and electrochemistry systems.In Chapter 4, the irreversible cycle models of five kinds of internal combustion engines such as the Otto, Diesel, Atkinson, Miller, and Dual cycles are established through finite-time thermodynamic analysis. By using these models the influence of multi-irreversibilities coming from the adiabatic compression and expansion processes, finite-rate heat transfer and heat leak loss through the cylinder wall on the performances of the heat engines are investigated. The optimum criteria of some important parameters are obtained, and consequently, the optimally operating regions of the engine cycles are determined.In Chapter 5, the effect of the internal friction dissipation in the adiabatic processes on the performance of an irreversible Otto heat engine is discussed in detail. The cycle model established here discards not only the usual hypotheses of endoreversible cycles but also the over simple description of the piston mean velocity for the irreversible heat engines. The optimum criteria of some main parameters are determined through numerical analyses, and the influence of the major design parameters is investigated. The results obtained may provide a significant guidance for the performance improvement and optimal design of real heat engines.In Chapter 6, the performance optimization and parameter selection issue of the Otto and Diesel engines are investigated with considering the temperature-dependent heat capacities of the working fluid. The adiabatic equation of ideal gases with the temperature-dependent heat capacity is strictly deduced and used to analyze the performances of the Otto and Diesel heat engines. The optimum criteria of some important parameters are given. The results obtained are novel and general, from which some relevant important conclusions in literature may be directly derived.In the last chapter, the conclusions obtained are summed up and the status and prospects of the present research are reviewed briefly.This research may provide a theoretical basis for both the optimal design and operation of real fuel cells and internal combustion engines, and it is also expected that this new method be used in the investigation and optimization of similar energy conversion settings and hybrid systems.