In this thesis the performance of high temperature fuel cell systems is studied using a new method of exergy analysis. The thesis consists of three parts: ⢠In the first part a new analysis method is developed, which not only considers the total exergy losses in a unit operation, but which distinguishes between different types of exergy losses; ⢠The second part describes the development of a fuel cell model. A detailed model is used to determine the relevant aspects of the performance of the fuel cell. Using this knowledge, a simpler model for the off-design performance of a fuel cell is designed for the system calculations. ⢠In the last part the advanced exergy analysis method is used to compare different high temperature fuel cell system configurations. The focus is on understanding the effect of changes in the conceptual design on the performance of the system. The value of the exergy analysis method is discussed and the main factors influencing the system efficiency are identified. The fuel cell is a single component in a complex system. As this study, and earlier studies on fuel cell systems show, th design of the fuel cell system largely determines the efficiency of the system. To achieve a high efficiency a high degree of integration is necessary. But a high degree of integration generally tends to make optimisation of systems difficult. The focus of this thesis is therefore on: ⢠Identifying how the interaction between subsystems in the fuel cell system affects straight-forward optimisation; ⢠Identifying how exergy analysis can be used in the optimisation process.