Page 98 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Thermodynamics 75

the waste heat of HE2 (for T < TerTaP). An auxiliary burner must be used for the
reforming process if the waste heat of the SOFC cannot cover the heat
requirement of the reformer and the evaporator. This auxiliary burner is not
shown in Figure 3.11. It is possible to use efficiencies q

to describe any real heating process (heat exchanger, burner). The internal
reforming in the SOFC is included in this modelling of the system (TsoFc = Tref).
The external reforming is included in this model as well if the heat engine HE1 is
replaced by a burner. The parameters listed in Table 3.2 have been used for
analysis of both the systems. The water surplus was ked at 2 to avoid coke
formation [9].

Table 3.2 Standard parameters for analysis of SOFGheat engine hybrid cycles
SOFC temperature Tsopc 900°C
Reformer temperature T,d 750°C
Evaporator temperature Tevnp 200°C
Ambient temperature To 2 5°C
Excess air h 2
Water surplus nw 2
Exergetic efficiency SOFC &oFc 0.60
Exergetic efficiency heat engine SHE 0.70
Efficiency of air heater AH 0.90
Efficiency of heat exchangers qHpx 0.98

The system efficiencies qsyst of combined SOFC cycles with integrated and
external reforming have been calculated and compared [7,8]. The possible
system efficiencies qsyst of systems with external reforming are about 57%
lower than of a system with integrated reforming. The differences between the
processes with external or integrated reforming are caused by the utilisation of
the waste heat of the SOFC within the system. External reforming systems use
an external burner with an additional entropy production. This increases the
usable heat of the heat engine HE3 operating with the heat source SOFC and
the heat sink environment and thus the waste heat of the system increases. An
external reformer cannot be used as a heat sink of the waste heat of the SOFC
and the entropy cannot be recycled as within an integrated reforming. An
internal reforming in the SOFC has no temperature difference available for a
power generation during the heat transport. This leads to a slight decrease of
the system efficiency qsllst of the internal reforming compared with the
integrated reforming.
An important influence on the performance of combined SOFC cycles with an
integrated reforming is caused by the operation of the air heater under real
conditions. The excess air h and the efficiency qAH of the air heater (Eq. (83))