Fuel cells directly convert chemical energy into electric energy. The conversion does not create any noise or vibrations. Whereas the efficiency of engines is limited by the Carnot process, the efficiency of a fuel cell is not.

Types of fuel cells

The following table gives an overview of different existing fuel cell types. Due to high power density, low process temperature and flexible use, the PEM type is seen as the most promising candidate for mobile applications. The following evaluation therefore refers to this type.

Source: Hörl et al. 2001

Principle of PEM fuel cell

A PEM fuel cell consists of two reaction chambers separated by an electrolyte. The two reaction chambers are fed with oxygen (or ambient air) and hydrogen (or hydrogen containing gas) respectively.

The electrolyte is realised by a polymer membrane plated with a thin platinum coating containing a catalyst. This membrane conducts only protons.

The power generating process is based on the following overall reaction:

2 H2 O2 ? 2 H20

This reaction can be split up into two sub-processes:

1. Hydrogen is ionised (split into electrons and protons) on the anode-side by a catalytic process according to the following scheme:

H2 ? 2 H 2 e-

2. The protons pass the membrane and react on the cathode-side with oxygen to form water:

½ O2 2 H 2 e- ? 2 H20

These reactions create an electron excess on one side and a corresponding lack on the other side thus causing a potential difference or voltage.

This process can be seen as a cold combustion of hydrogen with oxygen. This yields the following advantages over conventional combustion in an internal combustion engine:

• efficiency not limited by Carnot process
• no noise generated
• no harmful emissions
• process is easier to control

Voltage and power density

At its working point a single cell only produces a voltage of 0,7 V. In order to reach operating voltage desired for a given system, several cells have to be combined to form a fuel cell stack. The effective current of a cell is proportional to the membrane area. The power density is a function of current density and reaches a peak of 0,5 W/cm² at 900mA/cm².

Auxiliary devices

Apart from the fuel cell stack, a fuel cell system requires several auxiliary devices for fuel supply and heat removal.

The commercially available 230 kW fuel cell system P4/P5 (from XCellsis) includes the following components:

• Fan/radiator module
• Water/glycol cooling system system module
• Engine inverter/Controller Module
• Fuel Cell stack module
• fuel cell air system module
• fuel cell hydrogen regulating system and diffuser module
• P4VA auxiliary drive & electric motor
• P4T Auxiliary drive & electric motor including traction
• major pipe interconnection system
• major electric power interconnection system
• interfaces to the vehicles
• dynamic braking

Hydrogen supply

Fuel cell powered vehicles can be either supplied directly by hydrogen or by other fuels (especially methanol) to be reformed on-board before injection into fuel cell.

1) Hydrogen stored on-board: Hydrogen can be either stored in a pressurized or in a liquefied form to save volume.

2) Hydrogen can be generated from methanol or hadrocarbons by an on-board reformer. See below.

Fuel reforming to obtain hydrogen

The main advantage of an on-board reformer lies in the higher energy density of methanol and other "indirect" fuels. This leads to higher range compared to hydrogen storage.

A methanol reformer produces hydrogen and CO2 from methanol and water. In the process a number of by-products are produced some of which have a negative effect on the fuel cell catalyst. Therefore the reforming gas has to be cleaned before use inaside the fuel cell.

Types of fuel production and transport

Figure 1 gives an overview of the most important fuel supply chains for fuel cells.

Figure 1: Fuel supply chains for PEM fuel cells

Source: IZT

Layout of a fuel cell driven rail vehicle

A study by DB AG examined the layout of a fuel cell driven MU (DMU of type VT 610). The results for the fuel cell system NEBUS (by XCellsis) are given in the following table: 

Source: Hörl et al. 2001

Fields of application

Stationary: decentralised power generation

Mobile devices: energy supply for notebooks, camcorders etc.

Transportation: cars, busses, railways, vessels

Railway applications

The following areas of application for fuel cells in railways are conceivable:

• traction
• energy supply for auxiliaries and/or comfort functions
• stationary applications

The evaluation focuses on an application for traction.

Manufacturers

Ballard Power Systems (Canada)

Siemens

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.