47 A Complete Fuel Cell System

In Section 4.2 we explained how a fuel cell worked. We saw that, in essence, it is very simple. Hydrogen is supplied to one electrode, oxygen to the other, and electricity is produced. Pure water is the only by-product. However, in the following sections we went on to show that in practice a fuel cell is a complex system. They are difficult to make. The water balance and temperature require careful control. They consist of much more than just electrodes and electrolyte. These 'extras' are sometimes called the balance of plant (BOP).

On all but the smallest fuel cells the air and fuel will need to be circulated through the stack using pumps or blowers. In vehicles compressors will be used, which will be

9 Note that this vehicle can also be seen in Figure 8.16, but this is a somewhat different version, which has more conventional cooling arrangements.

linked with the humidification system (Section 4.5). To keep this working properly, there will need to be a water recovery system. A cooling system will be needed (Section 4.6).

The DC output of a fuel cell stack will rarely be suitable for direct connection to an electrical load, and so some kind of power conditioning is nearly always needed. This may be as simple as a voltage regulator, or a DC/DC converter.10 Electric motors too will nearly always be a vital part of a fuel cell system, driving the pumps, blowers and compressors mentioned above.

Various control valves will usually be needed, as well as pressure regulators. An electronic controller will be needed to co-ordinate the parts of the system. A special problem the controller has to deal with is the start-up and shut-down of the fuel cell system, as this can be a complex process.

This very important idea of the 'balance of plant' is illustrated in Figure 4.23, which is the fuel cell engine from a car. It uses hydrogen fuel, and the waste heat is only used to warm the car interior. The fuel cell stacks are in the rectangular block to the left of the picture. The rest of the unit (pumps, humidifier, power electronics, compressor) takes up well over half the volume of the whole system.

The presence of all this balance of plant has important implications for the efficiency of a fuel cell system, as nearly all of it requires energy to run. Back in Section 4.3.2 we saw that the efficiency of a fuel cell rises substantially if the current falls, as it is proportional to the operating voltage. However, when the balance of plant is included, this effect is largely wiped out. The power consumed by the ancillaries does not usually fall in proportion to the current, and in some cases it is fairly constant. The result is that

Figure 4.23 The 75 kW (approx.) fuel cell system used, for example, in the Mercedes A Class shown in Figure 1.14 (Reproduced by kind permission of Ballard Power Systems.)

10 Together with electric motors, these circuits are explained in Chapter 6.

over a very broad range of operating powers the efficiency of most fuel cell systems, such as that of Figure 4.23, is more-or-less constant.

One aspect of fuel cells that we have not addressed so far is the very important question of 'Where does the hydrogen come from?' This is an important and wide ranging topic, and will be explored in the next chapter.

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