Fuel cells and electrolysers in future energy systems - VBN

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. In PAFCs, the phosphoric acid becomes solid at a temperature of 40 °C [42]. Unless the
PAFCs are either operated continuously or insulated, the start‐up time may be hours.
In high temperature FCs, such as MCFCs and SOFCs, the situation is somewhat different. Here, the tempera‐
ture poses a challenge because of problems with temperature gradients within the cells. Because of their high
operating temperature and the temperature 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 hori‐
zontally along the stacks or making hexagonal designs; fitting the cells with start‐up burners; introducing buff‐
ers enabling a smooth start‐up load; or, as a potentially more promising alternative, keeping the cells at a high
temperature by operating them periodically and with insulation. Insulation has been investigated for SOFCs
[43]. It was found that SOFCs can be operated 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 tem‐
peratures are high, the regulation ability of the FC is system‐dependent, i.e. depending on the supply and
support system and gas turbine in hybrid systems. Hence, it is possible to eliminate 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 vo‐
lume of the FC especially in small‐scale systems, like e.g. micro‐CHP [27]. In combination with higher losses
relating to the balancing of the plant with small‐scale applications, the insulation required makes larger CHP
plant applications more likely, no matter if this insulation increases the possibility of load following or not.
Good regulation may also be possible for MCFC and PAFC, but no analyses of this ability have been identified.
For SOFCs, metal‐supported cells may improve the rapid start‐up ability, because the tolerance to thermal
gradients in the cells is improved [18;35;44].
The FC systems also have to be designed for the load changes in the applications in which they are potentially
used. While all low temperature FCs can follow load changes rapidly, thermal cycling and gas flow handling
pose a challenge to high temperature FCs, such as SOFC and MCFC with load changes, and thus have to be
handled carefully in the FC system control and design. The electrochemical processes and gas transport can
respond quickly. The temperature changes in the cell take place at a slower rate, because of the density of the
cells [45]. The temperatures in the cell affect the current and voltage and, thus, the operational characteris‐
tics. Since the load response itself is quick, while the temperature changes have longer responses, it is possi‐
ble to make control systems which can regulate the temperature, in order to achieve the desired responses
[20;43;46;47].
In the high temperature FCs, good load‐following abilities have been accomplished in the cases of both MCFCs
and SOFCs. In a MCFC, the thermal transients normally have long time constants, e.g. from 100 to 1000 sec‐
onds, which is due to the relatively large mass of the cell. However, thermal cycling affects the performance
of high temperature FCs. This indicates that a good FC control system is important both in terms of dynamic
responses due to load changes and to the lifetime of the cell. For MCFCs, experiments with stepwise changes
in applied load resistance show that a dynamic response is possible by use of controllable heaters, thermo‐
couples, and insulating materials. Also control systems have been proposed to efficiently avoid thermal cy‐
cling. [20;45]
Normally, 20 per cent of full load is the minimum load for FCs. This is a not technical limit but the level at
which efficiencies are significantly reduced. The data on load balancing and response times are still limited for
both MCFCs and SOFCs. AFC, PAFC and PEMFC have good load‐following characteristics, and HT‐PEMFC may
solve some of the problems for this type of cells. For SOFCs, good load‐following abilities have been achieved
with existing technology and also good efficiencies at part load [48;49]. Different operation strategies can be
applied; however, research into the balance of plant systems is still required for high efficiencies in SOFC. New
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