SOFCs are promising candidates for advanced distributed power generators. However, if we consider the good use of exergy, SOFC systems are insufficient. One reason is that the fuel utilization cannot be so high because of the durability of cells. As a result, around 20 % of the input fuel is unused and exhausted from the cell as an off-gas. The other reason is that abundant air is introduced to the cell to suppress the increase of the cell temperature. These waste the high exergy and decrease the exergy rate of the fuel. Although the exhaust gas has much heat, the exergy rate of the heat is low and the heat energy is not used well.
One effective solution for the subject is producing hydrogen using the waste heat of the exhaust gas. The amount of the heat energy of the exhaust gas is around 50% of the input fuel. Thus through the steam reforming reaction, much hydrogen can be produced using the waste heat. By producing hydrogen, the low exergy rate of the exhaust gas is increased again.
In this paper, several hydrogen producing systems with SOFC are presented. The produced hydrogen can be used as fuel of PEFCs. The SOFC and PEFC combined system is proposed and the overall conversion efficiency is evaluated. From the evaluation it is made clear that the overall conversion efficiency is higher by 20% than that of the stand alone SOFC system.