Alternative Cars, Part 7 - Fuel Cells

Last week in Alternative Cars Part 6 we looked at hybrid cars. Unlike many other of the alternative powertrains covered in this series, hybrid cars are currently available to the public. Also, available, but only on a very selective basis, are fuel cell cars. Almost every major car company worldwide is working on fuel cell cars and many have issued them in a boutique fashion for publicity and pseudo-normal consumer testing.

So what are fuel cell cars and what are their advantages and disadvantages?

Function

A fuel cell car is also an electric car. Most fuel cell cars use a battery (and/or super capacitor) and drive the wheels via an AC electric motor. Regenerative braking is used to improve energy efficiency by retrieving energy that would otherwise be lost through heating the brakes. All this technology is much the same as in hybrid petrol/electric cars, or pure electric cars.

However, the source of energy is not an internal combustion engine or a storage battery – instead, it is a fuel cell. A fuel cell is a device that generates electricity through an internal chemical reaction. A fuel – usually hydrogen - is fed to the fuel cell along with oxygen derived from the atmosphere. The fuel reaches a catalyst and is split into positively and negatively charged atomic components. A special membrane then allows only the positive components to reach one side of the battery. The negatively charged components must travel along an external conductor, so creating a voltage difference between the two sides of the battery. This process can be thought of as being the opposite of electrolysis, where electricity is used to split water into hydrogen and oxygen.

The voltage generated by a fuel cell is relatively small, so rather like a normal car battery or a hybrid’s high voltage battery, a stack of cells must be used to generate sufficient voltage to be useful.

In a hydrogen fuel cell the waste product is just water. Other fuels can also be used - for example, hydrocarbons like diesel and methanol. In these cells, carbon dioxide is produced in addition to water.

Advantages

A fuel cell is highly efficient at converting the energy contained within the fuel into electrical energy. It is generally stated than a hydrogen fuel cell vehicle is about twice as efficient as a traditional petrol engine car. However, this is a ‘tank-to-wheel’ calculation, not a ‘well-to-wheel’ figure (see below).

Fuel cell cars have the potential for reducing C02 emissions. However, this depends on how the hydrogen is produced. If renewable energy sources (eg solar or wind) are used to produce the hydrogen, no carbon emissions are produced. However, most hydrogen is produced from natural gas, where there are carbon emissions (albeit lower than those from a comparable petrol vehicle) and non-carbon neutral energy is usually consumed.

Unlike batteries used in electric and hybrid vehicles, a fuel cell has the potential to have a life expectancy as long as the car.

Fuel cell cars have low or zero emissions.

Disadvantages

The disadvantages need to be carefully categorised as being intrinsic or likely to be addressed by ongoing advances in technology.

A fuel cell must be managed carefully to control heat build-up (the internal chemical reaction generates heat), fuel and internal water flows. The cost of the membrane and catalysts are currently high. The ability of a fuel cell to provide power at low ambient temperatures is still problematic. However, it’s likely that these issues will all be able to be overcome in the near to mid-term future.

Fuel cell vehicles currently have a range smaller than petrol engine vehicles, and much smaller than the best commercially available diesel and hybrid cars. The fuel economy of fuel cell cars, measured on the basis of litres/100km (rather than energy use, for example) is poor. For their size, fuel cell cars are also heavy.

But the provision of the fuel itself is a much more major problem. While fuel cells are not required to run on hydrogen, most automotive research is being directed at fuel cells using this fuel. (In other words, the most promising fuel cells for automotive use are hydrogen fuelled.) Fuel cell vehicles use hydrogen stored under great pressure (eg 5000 psi) and dedicated hydrogen re-fuelling stations are required. The lack of hydrogen refuelling infrastructure is a massive disincentive to the public adoption of hydrogen-fuelled vehicles – fuel cell or otherwise.

In addition to refuelling infrastructure, hydrogen must be produced from other compounds, a process that requires energy and produces emissions.

Current fuel cell vehicles have a ‘well to wheel’ efficiency of only 29 per cent, compared with 32 per cent for currently available hybrid cars (and 14 per cent for conventional petrol engine cars). The relatively poor performance of hydrogen fuel cell vehicles is because of their 58 per cent ‘well to tank’ figure versus the petrol ‘well to tank’ figure of 88 per cent.

Solution

The thought of zero emissions from a car that generates its own electricity is very attractive. However, the requirement for a fuel that is effectively completely unavailable to the vast majority of car owners makes the concept infinitely less viable.

A widespread adoption of hydrogen fuel cell vehicles requires not only analysis of the emissions and energy costs of the production of the hydrogen, but also the associated energy and emissions cost of developing a whole new worldwide re-fuelling infrastructure! (However, it does need to be kept in mind that hydrogen can be produced just by using electricity and water – and these commodities are very widely available.)

Despite many manufacturers – including Daimler, Ford, GM, Honda, Hyundai, Nissan, Toyota and Volkswagen – feverishly working on hydrogen fuel cell vehicles, its immediate use would appear to be limited to fleets that can be refuelled at a single location, for example, taxis, buses and emergency vehicles.

Conclusion

The enthusiasm surrounding the near future release of fuel cell vehicles appears to be misplaced. In the long term, when oil supplies dwindle and oil costs rise, cars that do not run on oil or its refinements will be very attractive. However, apart from specialised uses like short trips in cities with bad air pollution, and for centralised fleets, widespread adoption of fuel cell cars is likely to be limited.

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