Fuel cells and electrolysers in future energy systems - VBN

they supply. In transport applications, fuel cells are combined with batteries in order to
ensure good start‐up capabilities. [25;26;38]
Fig. 9, Principle diagram of grid‐connected fuel cells.
For low temperature fuel cells, such as PEMFCs and AFCs, start‐up time is only a few min‐
utes or instant, depending on the system design [26‐28;32]. Efforts are also being made to
design these systems in such way that they enable a fast cold start‐up from below‐freezing
temperatures [45;54]. The low operating temperatures enable a fast fuel supply and a rapid
heat up of the cell to the operation temperature without material problems; however, due
to pressure control and liquid water, the system designs meet challenges when operating
at low temperatures. The system design of the intermediate temperature HT‐PEMFC is
promising in this respect, as the systems are less sensitive to pressure changes because the
water is gaseous. The question is whether the water in the membranes can flush the acid in
the electrolyte; however, no evidence of this effect has been documented yet.
For high temperature fuel cells, such as MCFCs and SOFCs, the situation is somewhat dif‐
ferent. Here, the temperature poses a challenge because of problems with temperature
gradients within the cells. Because of their high operating temperature and the tempera‐
ture gradients, these cells have a start‐up of several hours. These problems, however, can
be reduced by making intelligent stack designs, such as placing the manifolds horizontally
along the stacks or making hexagonal designs; fitting the cells with start‐up burners; intro‐
ducing buffers enabling a smooth start‐up load; or, as a potentially more promising alterna‐
tive, keeping the cells at a high temperature by operating them periodically and with insula‐
tion. Insulation has been investigated for SOFCs [55]. It was found that SOFCs can be oper‐
ated on low amounts of fuel; producing very little electricity, but keeping the temperature
at the right operation level. This enables a very fast start‐up. However, when temperatures
are high, the regulation ability of the fuel cell is system‐dependent, i.e. depending on the
supply and support system and gas turbine in hybrid systems. Hence, it is possible to elimi‐
nate start‐up time, start‐up and idle fuel consumption in SOFCs by operating at least once a
day or by cyclic reheating. This, however, requires the development of an integrated fuel
supply system, anode recirculation to maintain high efficiencies, and a better stack design
to increase lifetime. The insulation of such high temperature cells also increases the volume
of the fuel cell, especially in small‐scale systems, like e.g. micro‐CHP [44]. In combination
with higher losses relating to the balancing of the plant with small‐scale applications, the
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