A low-carbon transport system for conventional sized cars
Whatever the benefits and incentives for using much smaller vehicles for private transport, it remains likely that
the market for cars with room for 4 to 5 passengers will continue to remain popular. Whilst some reductions in
CO2 emissions per km can be achieved through the use of advanced technologies, these reductions will be
limited by the law of diminishing returns both in thermodynamic and economic terms.
overall CO2 reductions achieved this way will be quickly swamped by the sheer increase in car use
worldwide. Therefore, some other solution other than that of making them radically smaller will have to be
found to minimise these impacts.
Another approach is to ensure that cars are powered from non-carbon, non-polluting energy sources. This can
theoretically be achieved by using:
fuels derived from biomass
hydrogen based fuels refined from a low or zero carbon emitting primary energy source, in conjunction
with fuel cells or an IC engine or
battery electric vehicles (BEVs) charged from a low or zero carbon emitting primary energy source
The advantages and disadvantages for each of these methods will now be discussed.
Using fuels derived from biomass
Most current automotive liquid fuels derived from biomass such as biodiesel and ethanol are refined from the oil
or sugars produced from food crops such as, rapeseed, wheat or sugar beet. Whilst the carbon absorbed by the
plant feedstock compensates for emissions during the combustion of fuel, there are numerous other energy
inputs required to refine them, and this energy is currently sourced mainly from fossil fuel power stations.
Intensive farming of the biomass feedstock also requires fertilisers that release Nitrous oxide, another powerful
greenhouse gas. Worse still, new land allocated for growing biofuels can release substantial carbon to the
environment due to the tilling of soils and deforestation.
There are also other controversial issues concerned
with the production of biofuels such as the diversion of cropland away from growing food.
One reason why conventional refining techniques for liquid biofuels have limited environmental benefits is
because the cellulose and lignin component of the plant goes to waste. One solution is to use Second
generation biofuels, which utilise this plant material during production and can use a wider range of feedstocks
such as farm residues and wood which have less impact on food crops. However, the removal of residues and
wastes can affect the long-term fertilisation of crops and can potentially result in damage to the natural ecology
of the area unless the whole process is carefully managed.
Other efficient methods of using biomass for
energy include anaerobic digestion where methane is produced from decomposing vegetation and the direct
combustion of biomass to generate electricity which can be potentially used to charge battery electric vehicles
The availability of biomass is still substantial; for example, it is estimated that the biomass resources in the US
could contribute typically 20% of the primary energy contained in the petroleum used in that country
27 Safety legislation and the increased use of accessories such as air conditioning have resulted in greater vehicle body weight. Although the
EU set vehicle manufacturers non-obligatory CO2 targets of 120 g/km for the average car by 2012 (Commission Directive 93/116/EC.)
these weight increases have probably placed even these modest targets beyond reach
28 If long term fallow land is used for the production of energy crops or intensive food production in order to meet increased demand for
land, signifi cant quantities of CO2 will be released possibly enough to negate for many years the CO2 benefi ts from switching to
biofuels. This is because CO2 is released from soil when organic matter is mineralised, a process that is speeded up by ploughing. More
CO2 is released from soils with large quantities of organic matter such as fallow land or grassland European Energy Agency Briefing
Unfortunately most agricultural waste is nothing of the kind. It is the organic material which maintains the soils structure, nutrients
and store of carbon. A paper commissioned by the US government proposes that, to help meet its biofuel targets, 75% of annual crop
residues should be harvested. According to a letter published in Science last year, removing crop residues can increase the rate of soil
erosion 100-fold. The Last Straw Monboit.com
30 Oak ridge laboratory estimates that the US has resources for an annual total of 512 million dry tons of biomass equivalent to 8.09 Quads
of primary energy which could initially be available at less than $50/dry delivered. 8.09 quads is 20% of the 39.2 quads of primary energy
contained in the petroleum used in the US in 2002. A Quad is a unit of energy equal to 1015 (a short-scale quadrillion) BTU or 1.055 × 10
joules (1.055 exajoules or EJ) in SI units.