114 Fuel Cell Powered

In Chapters 1 and 4 we introduced various fuel cell powered vehicles. However, for a case study we will present the type of fuel cell vehicle that is most likely to make a commercial impact in the medium term, the fuel cell powered bus.

There are several important reasons why fuel cell systems are even more promising in city bus applications than for other types of vehicle. The three most important are as follows.

1. Fuel cells are expensive, so it does not make sense to buy one, and then leave it inactive and out-of-use for most of the day and night, which is the state of most cars. Buses, on the other hand, are in use for many hours each day.

2. The supply of hydrogen for fuel cell vehicles is such a difficult problem that we devoted a whole chapter of this book to it. Buses, on the other hand, refuel in one place, so only one refilling point needs to be supplied.

4 Taken from Toyota sales information at www.toyota.com.

3. The advantages of zero emissions at the point of use are particularly important for city vehicles, which is exactly what this type of bus is, all its life.

It is not surprising then that buses feature strongly among the most exciting demonstration fuel cell vehicles. A modern design was shown in Figure 1.16. Figure 11.6 shows an earlier example that was used between 1998 and 2000 in Vancouver and Chicago. Some high altitude trials were also carried out in Mexico City. The layout of the fuel cell engine is shown in Figure 11.7, and had a maximum power of 260 kW. Ballard made a good deal of data on this system available, and further information can be deduced by calculations from the given data, as presented in Chapter 11 of Larminie and Dicks (2003).

Referring to Figure 11.7, the system consisted of two fuel cell units (5), each consisting of 10 stacks, each of about 40 cells in series. So the total number of cells was about 800, giving a voltage of about 750 V. In use the voltage fell to about 450 V at maximum power. The voltage was stabilised to between 650 and 750 V using a DC/DC converter, as outlined in Section 6.2. There were several step-down DC/DC converters to provide lower voltages for the various subsystems, such as the controller (2), and to charge a 12 V battery used when starting the system. These voltage conversion circuits are unit (1) in Figure 11.7.

The fuel cell system is water cooled, with a 'radiator' and electrically operated fan (3). This could dispose of heat at the rate of 380 kW. As was explained in Section 4.6, fuel cell systems need to get rid of more heat than IC engines of equivalent power. This cooling system also removes heat from the motor (4) and the power electronics (1).

Figure 11.6 Fuel cell powered bus, 1998 model (reproduced by kind permission of Ballard Power Systems.)
Figure 11.7 Fuel cell engine for buses based on 260 kW fuel cell (diagram reproduced by kind permission of Ballard Power Systems.)

An ion exchange filter was used to keep the water pure, and prevent it from becoming an electrical conductor. Clearly then, no anti-freeze could be used, and so the system had to be kept from freezing, which was done using a heater connected to the mains when not in use. This is one important improvement seen on the more modern fuel cell buses such as that of Figure 1.16. All the losses are dealt with by this cooling system, so we note that 380 kW seems an appropriate value for a 260 kW fuel cell, and suggests an efficiency for the fuel cell of about 41% at maximum power, from the calculation:

output 260

output + losses 260 + 380

The efficiency at lower powers will be a little higher than this. The motor (4) is rated as 160 kW continuous, which means that for short periods it could operate at about 200kW. The motor was normally of the BLDC type explained in Section 6.3.2. There is evidence (Spiegel et al. 1999) that some models of this bus used induction motors, which illustrates very well what we said in Chapter 6 about the type of motor used being relatively unimportant. Induction motors are rugged and lower in cost, BLDCs are slightly more efficient and compact. Dynamic braking was used to reduce wear on the friction brakes, but not regenerative braking (see Section 6.1.7). The motor is coupled to the forward running drive shaft via a 2.42:1 fixed gear, and to the rear axle via a differential, which will have a gear ratio of about 5:1, as in Figure 8.6 and Section 8.4.

If the fuel cell output is 260 kW, and the maximum motor power is about 200 kW, where does the remaining 60 kW go? The major 'parasitic' power loss is the air compressor (6), which is needed as the fuel cell operates at up to about 3 bar (absolute). As was explained in Chapter 4, this increase in pressure brings performance benefits, but takes energy. Even using a turbine, which extracted energy from the exhaust gas, the electrical power required to drive the compressor will have been about 47 kW. The other major power losses will have been in the power electronics equipment, about 13 kW assuming 95% efficiency, and for the fan to drive the cooling system, probably about 10 kW. These three loads explain the 'missing' 60 kW.

This bus used compressed hydrogen tanks stored on the roof of the bus. These posed no greater safety problems than those present in a normal diesel fuelled bus. Any rupture of the tank, and the hydrogen would rapidly dissipate upwards and out of harm's way. The pressure of the tanks was about 250 bar when full, which was reduced to the same pressure as the air supply, about 3 bar. Usually when the pressure of a gas is reduced greatly, there is usually a cooling effect, but this does not happen with hydrogen. The hydrogen behaves very differently from an ideal gas, and the so-called Joule-Thompson effect comes in to play and there is actually a very modest temperature rise of about 7°C in the pressure regulation system.

Much was learnt from the generally successful trials with these buses over several years. This information has been incorporated into the new design of buses, such as those of Figure 1.16, and those from other non-Ballard companies such as the MAN bus of Figure 4.2. People are more likely to take a ride in a fuel cell bus before they go for a drive in a fuel cell car.

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