4 T HE NATURE OF FUEL CELLS
4.5 S TART ‐ UP , OPERATION AND REGULATION ABILITIES OF GRID ‐ CONNECTED FUEL CELLS
The operation and regulation abilities of fuel cells have to be addressed from a system per‐
spective, since factors such as electrochemical reactions, current and voltage change, gas flow controls, fuel processing, pressure, and water management interact with changing loads. Demands are made on fuel cells in order to meet certain requirements for power quality. In general, all types of fuel cells can respond quickly to an experienced load change;
however, differences exist in the start‐up performances of the fuel cells. While stacking the cells increases the voltage of the direct current from the fuel cell; voltage decreases as the amount of power drawn from the cell increases. This is solved by use of appropriate power electronics. A DC/DC converter, and possibly also a DC/AC inverter, can convert the output voltage DC to the voltage DC or AC required for the specific application. When converting output DC to AC, the voltage peaks of the cell as well as the voltage and current relation can be regulated. The grid‐connected fuel cells have to meet certain requirements if they are to support the electricity grid stability. A battery can supply start‐up power and assist in the power conditioning. Furthermore, a transformer can convert lower voltage power into higher voltages, if needed for the supply to the electricity grid, at the distribution or trans‐
mission level. An example of the principle of such a grid‐connected system is illustrated in Fig. 9. Stand‐alone systems also have to meet the requirements of the applications which
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
insulation required makes larger CHP plant applications more likely, no matter if this insula‐
tion 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 [40;50;56].
The fuel cell systems also have to be designed for the load changes in the applications in which they are potentially used. While all low temperature fuel cells can follow load changes rapidly, thermal cycling and gas flow handling pose a challenge to high tempera‐
ture fuel cells, such as SOFC and MCFC with load changes, and thus have to be handled carefully in the fuel cell 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 [57]. The temperatures in the cell affect the current and voltage and, thus, the operational characteristics. Since the load response itself is quick, while the temperature changes have longer responses, it is possible to make control systems which can regulate the temperature, in order to achieve the desired responses [42;55;58;59].
In high temperature fuel cells, 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 seconds, which is due to the relatively large mass of the cell. However, thermal cycling affects the performance of high temperature fuel cells. This indicates that a good fuel cell control system is important both in terms of dynamic re‐
sponses 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, thermocouples, and insulating materials. Also control systems have been proposed to efficiently avoid thermal cycling. [42;57]
Normally, 20 per cent of full load is the minimum load for fuel cells. 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 [60;61]. Different operation strategies can be applied; however, research into the balance of plant systems is still required for high efficiencies in SOFC. New systems need to be developed in order to achieve faster start‐up and to maintain good lifetimes in load‐following applications of SOFC.
In CHP applications, heat storages and boilers can reduce the requirements for start‐up and shutdown as well as load following. However, good load‐following abilities are still required in order to reduce the peak load capacity installed.
Thus, when in operation, all types of fuel cells may have very fast regulation abilities, pro‐
viding them with properties similar to those of batteries. With the right control systems, start‐up times can be reduced significantly or almost eliminated for high temperature fuel cells.