Page 100 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Thermodynamics 77

heat sinks : I
I variation of I

- 1

0 0,2 0,4 0,6 0,8 1
exergetic efficiency <,,E [-I

so, = 0.6, SHE = 0.7 (Variation HE1/2/3) integrated reforming
excess air h = 2, excess water n, = 2
t&oFc= 900 "C, = 750 "C, 6, = 200 "C
6,
Figure 3.7 3 The influence of the heat engine design on the system eficiency vsyrt of SOFC-heat engine
hybridcycles.

temperature differences between the SOFC and the heat sinks. This result
confirms that entropy recycIing by integrated reforming is more important to
achieve high efficiencies than the power generation in the heat engine HEl. But
the heat engine between the SOFC and the evaporator is important especially
with a good exergetic efficiency CHE2. The shape of qsgst of the variation of CHg2
seems to be unexpected compared with the variations of CHEl and cHE3.
But it can be explained easily if we look at the conditions that lead to
the required amount of the SOFC's waste heat to supply the evaporator or the
reformer. We obtain for the necessary waste heat QsoFc to operate the heat
engines (HE1, HE2 = HEprocess) and to supply the processes (reformer or
evaporator) with the necessary heat Q,,.ocBss

This result is governed by the exergetic efficiency of the heat engine LHEproccss
and the relation of the temperature of the heat sink TprOCeSS and the temperature of
the heat source TsoFc (T in K). Table 3.2 shows that the relation Tprocess/T~~~~
is about 0.4 in the case of the evaporator and about 0.9 in the case of the
reformer. Recycling of the anode outlet flow is another option to supply
the reformer with steam.

3.6 Design Principles of SOFC Hybrid Systems
The process models as given in Figures 3.9 and 3.11 can be realised by different
heat engines; Figure 3.14 gives an example. The heat source can be the flue gas