Comparing Alternative Fuels For Cars

Bottom of the table, unsurprisingly, are petrol vehicles, but coming in a close second last are hybrid vehicles that can run on compressed natural gas or petrol. Top of their league are fuel cell powered vehicles using hydrogen gas obtained from natural gas methane.

Karl Høyer of Oslo University College and Erling Holden of Western Norway Research Institute, Norway, and reported in Inderscience's International Journal of Alternative Propulsion their analysis of fuel chains including petrol, natural gas, and alternative fuel sources such as methanol and ethanol, hydrogen and biofuels.

"Alternative fuels are not in themselves a road towards sustainable mobility," the researchers say. However, their analysis places petrol and hybrid vehicles firmly at the bottom of the league table when all energy factors from source to consumer use are taken into account. "Any alternative fuel we considered is better than the cars that are used mostly today," they add.

Currently, there is no consensus regarding sustainable transport development. Even if a particular energy reduction goal is set for the transport sector there is no agreement on actions that should be taken to achieve this goal. Different lobbying groups have different approaches to the problem and opponents of any particular approach can usually find evidence to suggest a particular approach is not sustainable. The ongoing biofuel debate is a case in point.

Nevertheless, there are several factors that must be considered in assessing a particular alternative fuel: the efficiency route, the substitution route and the reduction route. Each has its strong defenders, say Høyer and Holden, but in reality there are substantial grey areas between them. One important facet of the debate that is often overlooked, is to ensure the three impact categories - energy use, carbon emissions, and nitrogen oxide pollution - are considered together. "This is a highly problematic task and should only be carried out with great care," the team adds.

They used a simple ranking system to create their league table, based on giving each energy form a weight from 1 to 16 depending on its impact in these three areas. When the weights are added up from well to wheel, they provide an overall value for each energy chain. For example, extraction of natural gas, processing into liquefied hydrogen, storage, and end use in a fuel cell car. The energy chains giving the lowest sum-figures are the highest in the league table and those with largest figures are considered potentially the most environmentally harmful.

Interestingly, the team's analysis puts a fossil-based alternative, natural gas conversion into hydrogen for fuel cells at the top of the list. In contrast the direct use of natural gas in hybrid cars is lower down the list in terms of efficiency, energy, and pollution. Biological methanol for use in fuel cell vehicles is way down the list despite biomethanol being a potentially renewable resource unlike natural gas. "It must be emphasised that no single chain comes out with the best score on all impact categories," the researchers say, "There are always some sorts of trade offs involved. Thus, there are no obvious winners; only good or bad trade offs between different impact categories."

Reference: Int. J. Alternative Propulsion, Vol. 1, No. 4, 2007 pp 352-368