In this section we will look at the main types of alternative fuels. We start with Biofuels as this constitutes probably the most popular AF currently in use.
Much recent attention has been focused on biofuels. This is highlighted economically by the fact that worldwide investment in biofuels rose from US$5bn in 1995 to US$38bn in 2005, owing to substantial investments by companies such as BP, Shell and Ford, and by Richard Branson (Grunwald, 2008).
Biofuels are essentially fuels produced from renewable plant material and oils. The International Energy Agency (IEA, 2004: 26) defines biofuels in the following way: 'Either in liquid form such as fuel ethanol or biodiesel or gaseous form such as biogas or hydrogen, biofuels are simply transportation fuels derived from biological (eg agricultural) sources.'
There are two main types of biofuel:
Biodiesel (or Alkyl Esters)
Biodiesel is made from plant and animal oils through a process called transesterification (ie the production of esters from oil or fat). In this process, the fat or oil is reacted with alcohol in the presence of a catalyst to produce biodiesel and glycerine (www.biodiesel.org). The main sources of oil used in the production of biodiesel vary according to country, depending on local growing conditions. In Asia palm oil is the norm, in the United States it is soybean oil and in Europe the norm is rapeseed oil (or canola). Other plant oils that can be used include sunflower oil, cottonseed oil, mustard seed oil, coconut oil and hemp oil. In 2006, the United States produced 250 million gallons of biodiesel, up from 2 million gallons in 2000, but this still only represented less than 1 per cent of total highway diesel fuel used (Union of Concerned Scientists, 2007).
Bioethanol can be produced from any biological foodstock that contains sugar, or materials such as starch or cellulose that can be made into sugar (IEA, 2004). The main sources of sugars for bioethanol are wheat, corn, sugar beet, straw, maize, reed canary grass, cord grass, Jerusalem arti-chokes, myscanthus, sorghum, sawdust and willow and poplar trees (ESRU, 2007), although sugar beet and corn account for 80 per cent of all bioethanol produced in the world in 2007 (Sperling, 2008). Bioethanol has been used as a fuel for decades. Brazil has been using bioethanol made from sugar cane since the 1930s, and indeed in the 1980s was selling cars that ran exclusively on such fuel (Sperling, 2008). The United States has also been using bioethanol (produced from corn) for many decades (not so much for environmental reasons as to reduce its dependence on imported conventional oil).
Both biodiesel and bioethanol are usually blended with existing fuel to make them usable. Biodiesel is usually blended with conventional diesel and bioethanol is usually blended with conventional petrol ('gasoline' in the United States), although it can be blended with diesel after some modification (IEA, 2004). Thus, B20 means there is a 20 per cent blend of biodiesel with conventional diesel and similarly E20 means that there is a 20 per cent blend of bioethanol with conventional petrol. As the percentage blend of ethanol increases, so its corrosive impact increases, and over about 10 per cent susceptible conventional vehicle components (particularly the rubber elements) need to be replaced by ethanol-resistant components. However, with biodiesel this problem is reduced. In the United States, the most common blend is B20, but in Germany, Austria and Sweden 100 per cent pure blended biodiesel is used in goods vehicles and buses with only very minor engine modifications (IEA, 2004). Vehicles that can use conventional fuel or any blend of biofuels are known as flexible-fuel vehicles (sometimes called flex-fuel vehicles).
One of the main reasons why biofuels have gained so much attention is that low blends (generally agreed to be up to about 10 per cent) can be used directly in existing cars with no engine modifications, and the refuelling infrastructure is exactly the same as for conventional fuel (ie through fuel pumps). In early 2008, there were 165 biodiesel and 16 bioethanol stations around the UK (Anon, 2008). This makes it very convenient and cheap compared with the development of other renewable fuel alternatives (such as hydrogen, electric power or LNG/CNG), which require major modifications to both vehicles and refuelling distribution systems.
Attention on the environmental impacts of transport is not new. In the 1970s and 1980s the focus was on the use of non-renewable resources (ie oil) following the OPEC oil crisis and an increasing understanding of the effects of transport on the local environment (particularly the health impacts of sulphur and lead). Since the 1990s, however, attention has been focused on the global impacts of pollution, and in particular on the impact of greenhouse gas emissions (particularly CO2) on climate change. The EU Biofuels Directive was adopted in May 2003. Its aim was to promote the use of transport fuels made from biomass and other renewable sources. The directive sets a reference value of 5.75 per cent (by energy) for the market share of biofuels by 2010. In the case of the UK, a conditional target of 10 per cent for the energy content share of biofuels in petrol and diesel was set for 2020. As part of the UK's 2006 Climate Change Programme, a further target of 5 per cent (by volume) was set for the proportion of road transport fuel to be derived from renewable sources by 2010. To aid in the achievement of this target, a Renewable Transport Fuel Obligation (RTFO) was established for fuel suppliers (which started in April 2008). Under the RTFO, companies are required to measure and report on how much carbon their fuel has saved on a life cycle basis (including land-use changes) (DfT, 2007). In 2008, the government announced that from 2010, the RTFO will reward fuels according to their carbon savings in order to encourage technological advances.
Environmental Impacts of Biofuels
Hydrogen is a second key potential alternative energy source for transport. In the early 2000s, hydrogen was being viewed as a panacea for the future and in 2003 the International Partnership for the Hydrogen Economy, established by the US Department of Energy with signatories from around the world, aimed to accelerate the transition to a hydrogen economy (see www.iphe.net). The impetus towards this shift has, however, slowed as problems have emerged.
To date, much of the research into hydrogen as an AF has focused on passenger cars and buses rather than freight vehicles, although there is considerable interest in the potential for hydrogen use in the light goods vehicle (LGV) sector. As the technology improves, and as long as it is viewed as being successful, transferral of this energy source to larger vehicles is likely.
The main form of hydrogen to be used in transport is the hydrogen fuel cell. This is a device that converts hydrogen gas and oxygen into water via a process that generates electricity. Fuel cell vehicles are generally powered by pure hydrogen which comes in the form of compressed hydrogen gas, metal hydrides stored in cylinders or as liquid hydrogen, though any hydrogen-containing feedstock (such as petrol and diesel oil) could be used (DfT, 2000). Proton exchange membrane fuel cells (PEMFC) are being developed for both transport and stationary applications (such as power for warehouses). PEMFCs are not new; they were invented in the 1950s by General Motors and were used by NASA in the Gemini space project. The PEMFC works by harnessing the chemical energy that results from the reaction of hydrogen and oxygen and transforming it into electrical energy. It is very efficient at energy production and is almost totally recyclable.
The main environmental benefit of hydrogen is that its only real tailpipe emission is water vapour. For use in cities this can be very beneficial and it is for this reason that bus companies all over the world are currently trialling them (for instance through the Clean Urban Transport in Europe (CUTE) initiative).
As more research into the use of hydrogen is carried out, however, major doubts have crept in concerning its environmental credentials. The principle issues of contention are fivefold:
- Hydrogen is 'an energy carrier not an energy source' (EurActiv, 2006). This means that it has to be produced from other sources (coal, nuclear etc), so it is only as clean as these source fuels. It can be made from renewable energy sources, such as wind power, but there is concern that if there is a global switch to the use of hydrogen, there will be insufficient supplies of renewables, whose price will increase as a result, encouraging the use of non-renewables again. Even if renewables can be used, in a major study of the benefits of hydrogen fuel for the DfT, Eyre, Fergusson and Mills (2002: 6) concluded: 'until there is a surplus of renewable electricity it is not beneficial in terms of carbon reduction to use renewable electricity to produce hydrogen — for use in vehicles or elsewhere.' They suggest that it is more efficient to use renewables for purposes other than hydrogen formation.
- The pollutant emissions from hydrogen have also been challenged. A report to the DfT (2002: 4) by AEA Technology suggested that 'direct emissions of hydrogen to the atmosphere from human activity may alter the natural chemistry of the atmosphere and exacerbate problems relating to the impacts of photochemical pollution (ozone) and climate change.' Hydrogen is an indirect greenhouse gas with a potential global warming effect, because emissions of hydrogen lead to increased burdens of methane and ozone (Collins, Derwent and Johnson, 2002). It appears that the precise impact of hydrogen on the environment is not yet clear.
- In order to be able to produce hydrogen fuel cells, a small amount of platinum is required (to act as a catalyst). There are substantial negative environmental effects associated with the mining and refining of platinum, including atmospheric emissions of SO2, ammonia, chlorine and hydrogen chloride (estimated to be around 180 kg of carbon per ounce), but also long-term groundwater and disposal problems (DfT, 2002). If recycled platinum can be used, this reduces the environmental footprint significantly.
- A whole new refuelling infrastructure needs to be developed. Hydrogen filling stations need to be set up globally, requiring a considerable investment and a great deal of environmental pollution. For vehicles, hydrogen would be purchased in liquid form and the oxygen would be obtained from the air. However, because of its low energy-to-volume ratio, hydrogen is difficult to carry in vehicles as well as to store and distribute (NREL, 2003).
- At present the fuel cells do not allow long-distance travel (ie their range is limited).
In conclusion, hydrogen does not appear to be the 'dream ticket' it was expected to be. Until there is a surplus of renewables from which it can be produced, and until the platinum problem is dealt with, it seems that hydrogen merely transfers the environmental effects from the tailpipe to the electricity generation. In the future, it may be possible to produce hydrogen by 'splitting' water (ie by electrolysis). If this can be done using sunlight, either through photoelectrochemical or photobiological processes, the lifecycle impact of hydrogen production is virtually nothing (NREL, 2003). At present, this technology is not well understood (or some would say that the big oil producers are not in favour of it, so less investment is being made in it). It seems likely that the majority of hydrogen energy will continue to be produced from non-renewables in the foreseeable future.