The advantage of hydrogen as a fuel in the internal combustion and gas turbine engine is the product of combustion is only water and the exhaust contains no carbon dioxide, sulfur dioxide or carbon monoxide. Burning hydrogen does however pro­duce NOx which is a function of the temperature where hydrogen burns at a little higher temperature than petrol. This is not a problem encountered with fuel cells. Research is under way using recirculation of exhaust gas to reduce NOx emissions (Heffel, 2003). Both liquid hydrogen and fuel cell vehicles have been developed by a number of automotive companies. The problems with hydrogen are its supply to vehicles and storage on board.

If we consider that hydrogen may be used as a universal transport fuel either in an internal combustion engine or in a fuel cell, then a new infrastructure will be required. The main question is what needs to be developed first, the hydrogen power vehicles or the infrastructure? The widespread use of hydrogen-powered vehicles will need a network of filling stations but these will probably not be built until there is a number of vehicles to justify the cost of construction. The most likely sequence will be the construction of the infrastructure prior to the widespread introduction of hydrogen-powered vehicles. A study of the early development of the fossil fuel automotive industry illustrates the same problems (Melaina, 2007). The car was introduced via mass production but in the beginning there were few petrol stations, as is the case for hydrogen at present. However, motorists could obtain fuel in cans from shops and repair garages. This supply system had developed for the sales of paraffin (kerosene) for lighting and heating before the introduction of cars. As the car ownership increased there was a demand for filling stations which were built in increasing numbers. Such is the nature of compressed or liquid nitro­gen that small-scale supplies cannot be obtained without the development of a widespread infrastructure.

Some of the options for the supply of hydrogen to filling stations are shown in Fig. 5.14. The first option is to produce hydrogen, in this case from natural gas, at a central production unit. The hydrogen is liquefied and transported to the filling sta­tion by tanker where it is stored until required. This is very much the system that is

Reforming Compressor

Fig. 5.14. Some of the possible methods of supplying hydrogen to petrol stations.

used at present for petrol and diesel. In the second option the hydrogen is generated at the filling station either by electrolysis or from natural gas and stored. In the third option hydrogen is supplied to the filling station by pipeline where it is compressed or liquefied. All these options have some of the problems outlined above, including the energy required to compress or liquefy the hydrogen, its storage and its dispen — sion. A well-to-wheel assessment of the supply of hydrogen and storage on the vehicle concluded that the energy used to deliver hydrogen in a liquid form was similar to that for petrol and diesel.

The on-site generation was much more costly in terms of energy. In both cases 80% of the energy use was expended on hydrogen production and liquefaction or compres­sion (de Wit and Faaij, 2007). When the fuel delivery and driving costs were considered, on-site generation was again the most expensive. The driving costs were higher than petrol and diesel in all cases.

Once the hydrogen has been made available there are a number of options for its use in the vehicle for both internal combustion engines and fuel cells (Ogden et al., 1999). Compressed hydrogen or liquid hydrogen will have to be stored in

cylinder/tanks at pressure of up to 600 psi or at -253°C in insulated tanks. Either method of storage will pose problems when filling the vehicle. There are options to avoid the on-board storage of hydrogen which includes the on-board produc­tion of hydrogen from methanol or petrol (Fig. 5.15). Both methanol and petrol are liquid and can easily be supplied by the present infrastructure, but petrol is non-sustainable and methanol would need to be produced in a sustainable manner. Both systems would add weight, complication, increase fuel consumption and cost to the vehicle.