Global results for the total life cycle of each combination of fuel/vehicle technology are presented in Table 16.
Table 16. Comparative Well-to-Wheel results.
In addition to energy consumption and GHG emission, values where calculated for the emission of pollutants (Table 17).
It must be noted that the pollutant emissions from hydrogen vehicles occur exclusively in the production of hydrogen, and thus have minimal or virtually no local effect, while most of the emissions of conventional vehicles occur locally.
Table 17. Well-to-Wheel emission of pollutants (kg/km).
Since the early 19th century, scientists have recognized hydrogen as a potential source of fuel. Current uses of hydrogen are in industrial processes, rocket fuel, and spacecraft propulsion. With further research and development, this fuel could also serve as an alternative source of energy for several purposes and fueling motor vehicles. However it is an intermediate fuel, as there is no natural resource of hydrogen of sufficient dimension and intensity. It must be produced from other resources. When produced from renewable resources and technologies, such as hydro, solar, and wind energy, hydrogen becomes a renewable fuel.
The main result of this work shows that public transport by buses consuming hydrogen produced by conventional technologies does not as yet present a solution for the reduction of GHG emissions. Conventional technologies are still less polluting and more efficient, when the global process is appraised (life cycle analysis). This stems from the inefficiencies of the conventional processes for hydrogen production.
However, hydrogen from renewable energy resources shows a clear benefit, but it must be noted that this solution seems to stumble on economic barriers, as renewable power generation does not stand alone, at least without subsidies. The production of hydrogen from grid electricity is clearly an intermediate solution, as it is currently the most practical way of producing low amounts of hydrogen, but in the long term it is not a sustainable strategy.
The disadvantages associated with hydrogen can be greatly reduced if the local effect of the less polluting vehicle is taken into consideration. Whether this is enough to offset the disadvantages is a matter for further studies.
The energy consumption of each vehicle reflects the technological development of each technology. Diesel and CNG vehicles (with a slight advantage for Diesel) are currently very efficient; the technology is well dominated but has currently reached a point where large efficiency increments are not expected. On the other side, fuel cell technology has the potential to be much more efficient, possibly double the actual values, and much experience will be gathered during the operation of the vehicles in Porto for two years. This will help reduce the fuel consumption of hydrogen vehicles to a point where the extra energy used in the production of the fuel could be compensated.