7.5 Fuel cell vehicles
A fuel cell vehicle (FCV) is a hybrid electric car, but instead of using an internal combustion engine, energy is stored as hydrogen and a fuel cell converts that hydrogen into electricity to run the electric motor. A small rechargeable battery is also used as an energy store for regenerative braking.
Hydrogen fuel cells can be thought of as a form of battery, but where hydrogen and oxygen (from the air) are used instead of the lead and lead dioxide in the plates of a lead-acid car battery. Hydrogen can be manufactured in a wide variety of ways, but the fuel cells used in road vehicles require high purity hydrogen that is best produced by the electrolysis of water.
Hydrogen produced from electricity from renewable energy sources will have low associated CO2 emissions and may be described as ‘green’ hydrogen. That from nuclear electricity has been called ‘pink’ hydrogen.
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Hydrogen as a fuel
Hydrogen can be conveniently distributed and stored as a high pressure gas, typically at 700 bar (atmospheres) pressure.
The energy densities of petrol, DERV and batteries have been discussed in Section 7.1. Table 6 compares the densities of petrol and hydrogen.
| Fuel | kWh per litre | kWh per kg |
|---|---|---|
| Petrol (no blended biofuel) | 9.1 | 12.4 |
| Hydrogen at 700 bar | 1.4 | 33.3 |
Hydrogen would appear to have a considerable advantage over petrol in that its energy density per kilogram is three times higher. However, its volumetric energy density is considerably worse. In practice, these figures are masked by the need to contain the hydrogen in suitably strong (and heavy) storage tanks. A practical vehicle hydrogen tank is likely to be far heavier than the hydrogen it contains.
High pressure hydrogen can be rapidly dispensed from filling pumps in a similar manner to petrol or DERV. However, the infrastructure problems of setting up a new network of hydrogen filling stations are even larger than those for battery electric vehicles.
Hydrogen remains an attractive option for forms of transport that undertake long journeys but require a short refuelling time and for which a limited number of refuelling points would be acceptable. Examples are long distance heavy goods vehicles, coaches, trains and even aircraft.
FCVs in practice
Interest in fuel cells for road transport developed with the rise in environmental concerns around transport in the 1980s and 1990s.
Fuel cell cars have been produced but sales worldwide have only been a tiny fraction of those of BEVs. The life expectancy of fuel cells has improved and is now considered sufficient to last the whole life of a car.
Fuel cell buses have been in use for over fifteen years in various European cities (see Figure 15) and much operational experience has been gathered for comparison with the rival low-carbon technology of battery-electric buses.
