Exergy analysis of a solid oxide fuel cell system

The analyzed 5kW SOFC system has the theoretical maximal electric working power of 12.3kW. Systems maximum working power is reached without intermediate heat tappings to external heat exchangers. The exergy losses in the SOFC system are at maximum when all the heat is taken out from the final heat exchanger. However this leads to a decrease in the effective temperature. With intermediate heat tapping the exergy losses in the system can he reduced and the effective temperature increased. Therefore the theoretical working power does not differ significantly from the cases where one or more extemal heat recovery units are placed in the system. The results show that the effect of the anode exhaust gas flow is the most critical factor for exergy losses. In order to minimize the entropy generation in the burner, stoichiometric quantities of the air should be fed into the burning process. If the losses in the burner are at minimum the power loss for the whole SOFC system is at minimum and the effective temperature is the highest. The temperature levels in all the components of the system are at highest when 75% of the air is fed into the external heat exchanger. The analysis of the pre-reforming of methane in the SOFC system was made in order to find the optimal method to produce a synthesis gas for the system. Different reforming methods; steam reforming, partial oxidation and auto-thermal reforming; were compared. Steam reforming is the most effective method in terms of hydrogen production but it consumes a lot of heat energy. With auto-thermal reforming and anode gas circulation the heat demand can be decreased and the electrical efficiency of the SOFC system increased