46 Thermal Management of the PEM Fuel Cell

It might be supposed that the cooling problem of a fuel cell would be simpler than for IC engines. Since they are more efficient, then less heat is generated, and so there is less heat to dispose of. Unfortunately however, this is not the case.

It is true that there is somewhat less heat energy produced. A fuel cell system will typically be about 40% efficient, compared to about 20% for an IC engine. However, in an IC engine a high proportion of the waste heat simply leaves the system in the exhaust gas.

With a fuel cell the oxygen depleted and somewhat damper air that leaves the cell will only be heated to about 85°C, and so will carry little energy. In addition, compared to an IC engine, the external surface is considerably cooler, and so far less heat is radiated and conducted away through that route.

The result is that the cooling system has to remove at least as much heat as with an IC engine, and usually considerably more.

In very small fuel cells the waste heat can be removed by passing excess air over the air cathode. This air then supplies oxygen, carries away the product water, and cools the

8 Larminie and Dicks (2003), Chapter 4.

cell. However, it can be shown that this is only possible with fuel cells of power up to about 100 W. At higher powers the airflow needed is too great and far too much water would be evaporated, and the electrolyte would cease to work properly, for the reasons outlined in the previous section. Such small fuel cells have possible uses with portable electronics equipment, but are not applicable to electric vehicles.

The next stage is to have two air flows through the fuel cell. One is the 'reactant air' flowing over the fuel cell cathodes. This will typically be at about twice the rate needed to supply oxygen, so it never becomes too oxygen-depleted, but does not dry out the cell too much. The second will be the 'cooling air'. This will typically blow through channels in the bipolar plates, as shown in Figure 4.21.

This arrangement works satisfactorily in fuel cell of power up to 2 or 3 kW. Such fuel cells might one day find use in electric scooters. However, for the higher power cells to be used in cars and buses it is too difficult to ensure the necessary even air flow through the system. In this case a cooling fluid needs to be used. Water is the most common, as it has good cooling characteristics, is cheap, and the bipolar plates have in any case to be made of a material that is corrosion-resistant.

The extra cooling channels for the water (or air) are usually introduced into the bipolar plate by making it in two halves. The gas flow channels shown in Figure 4.21 are made

Figure 4.21 Three cells from a PEM fuel cell stack where the bipolar plated incorporate channels for cooling air, in addition to channels for reactant air over the electrodes
Figure 4.22 Solid metal cooling fins on the side of a GM Hy-wire demonstration fuel cell vehicle

on one face, with the cooling water channels on the other. The two halves are then joined together, giving cooling fluid channels running through the middle of the completed bipolar plate. The cooling water will then need to be pumped through a conventional heat exchanger or 'radiator', as with an IC engine. The only difference is that we will need to dispose of about twice as much heat as that of the equivalent size of IC engine.

Because larger 'radiators' are sometimes needed for fuel cells, some imagination is sometimes needed in their design and positioning. In the ground-breaking General Motors Hy-wire design, which can also be seen in Figure 4.1, large cooling fins are added to the side of the vehicle,9 as shown in Figure 4.22.

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