Since fuel cells run on a different fuel, an assessment of the direct energy efficiency effects is difficult. There are however two quantities directly related to energy efficiency that may be used for a comparison between fuel cell and diesel-electric vehicles:

• Efficiency of the conversion chain
• Greenhouse gas emissions (in CO₂ equivalents)

Efficiency of the conversion chain

Since fuel cells do not involve a combustion process, their efficiency is not limited by the Carnot process and may be well over 50 % (the manufacturer XCellsis even gives an efficiency rate as high as 80 %, a figure which is hardly reached in practice).

The efficiency of the fuel cell itself however does not tell much about the energy performance of the system. It is essential to look at the entire energy chain rather than the on-board conversion processes only.

A comparison between a fuel cell and a conventional diesel-electric vehicle performed by DB AG (Hauser, Kleinow, Ponholzer 1999) yielded the following results:

Whereas diesel-electric traction has an overall efficiency of 31,7 %, the efficiency of fuel cell traction lies between 6,1 % and 31,1 % depending on fuel supply chosen. Details are shown in Figure 2. Supply chain 1 refers to H2 production through electrolysis (power from public grid) and storage and transport in liquid state. Supply chain 2 refers to H2 production through natural gas reforming and storage and transport in gaseous state.

Figure 2: Comparison of overall efficiencies between diesel-electric and fuel cell traction (two supply chains)

Source: IZT, data from Hauser, Kleinow, Ponholzer 1999.

Greenhouse gas emissions

Just like the overall efficiency, the GHG balance of the fuel cell vitally depends on the prechain. A study by the IFEU institute (Patyk 2000) gives a comparison between the GHG balance of different fuel cell solutions and conventional diesel engines (referring to road transport). The following table gives the results of this study:

Source: Patyk 2000, IZT calculations

These figures show that only the fuel chains starting out from renewable sources offer substantial advantages over diesel propulsion. The only fuel chain slightly better than conventional diesel chain is hydrogen production from natural gas in big centralised reforming plants.

Conclusion

If fossil fuels are used for hydrogen production, then the fuel cell only offers an advantage if the energy efficiency over the whole chain is as good as for modern diesel-electric chain. This is roughly the case for natural gas reforming.

If renewable energy is used for hydrogen production, then fuel cells operate with almost no (fossil) carbon dioxide emission. This however is true for the use of H2 not only in fuel cells but also in internal combustion engines. Therefore the overall efficiency will decide on the most environmentally favourable technology to be used.

Local emissions

The local emissions of a fuel cell powered traction are close to negligible.

Global emissions

A study by the IFEU institute (Patyk 2000) examined the pollutant emission along the whole chain for different fuel chains for fuel cells. This included the two impact categories

• Acidification (with the balancing parameters SO2, NOX, NH3 and HCl)
• Eutrophication (with the balancing parameters NOX and NH3)

Source: Patyk 2000, IZT calculations

These data show that as far as emission control is concerned, most fuel cell options offer advantages over diesel propulsion. However, fuel cells with biofuels have an emission performance considerably worse than diesel.