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There are no totally satisfactory
competitors at the moment to oil based fuels but below we discuss some
imminent developments.
Oil and gas are hidden treasures buried in the ground, so oil and gas
raw materials are intrinsically cheap, requiring only the costs of finding
and taking; and that is not all. There are three other features that make
derivatives such as petrol, diesel and kerosene uniquely attractive: their
very high energy densities, the speed of recharging and the existing world-wide
distribution network.
Look at these three features in more detail:
- Energy densities are of prime importance because the fuel has to be carried in the
vehicle if it is to have mobility in 2 or 3 dimensions. Ground based
transport can move in 2 dimensions and aircraft in 3. If the fuel
is not carried on the vehicle, an umbilical link ties it effectively
to a 1 dimensional fixed path, for example an electric train or tram.
To illustrate the difference in energy densities let's take three
alternative and new clean energy sources which could give 2 dimensional
flexibility: rechargeable electric batteries, hydrogen gas and Fuel
Cells (which are batteries energised by some form of hydrogen). Rechargeable
batteries are relatively expensive and heavy (due to their low energy
densities) and so, currently, they are impractical in many cases.
If you take a comparison between the energy stored per unit weight
of petrol and lead-acid batteries the ratio is about 500:1; even with
nickel-metal hydride batteries (currently a popular contender), the ratio
approaches 300:1. Lithium-ion is becoming a practical, commercial
energy storage device for use in motor vehicles, it has an energy
density some 30% to 60% higher than Ni-MH but the supply of Lithium
can make the cost volatile.
Pure Hydrogen would be ideal, if sustainably derived, but unfortunately
this is a gas and so by definition has a very low density. Extreme
compression or cryogenic temperatures are needed to overcome this
problem which poses technological problems and adds safety concerns,
although the latter are exaggerated because the techniques are well
tried and proven.
Fuel Cells are based on hydrogen but liquid compounds containing hydrogen
can be used instead of pure hydrogen. Such a system can, theoretically,
have energy densities approaching those of the conventional combustion
engine but often this means using petroleum compounds and then its main advantage
is lost.
[Our reference on Energy Densities was the pdf article:
"Fuels of the Future for Cars and Trucks", Dr. James J.
Eberhardt, Energy Efficiency and Renewable Energy, U.S. Department
of Energy, 2002 Diesel Engine Emissions, Reduction (DEER) Workshop,
San Diego, California, August 25 - 29, 2002. This pdf no longer seems to be available.]
- Speed of recharging is a crucial factor and the comparisons are easy to make.
How long does it take to recharge your car (in other words fill the tank with petrol), about
three minutes? Even allowing for the further three minutes it takes queuing to pay, that
is incredibly fast and by comparison it might take half a day to recharge a set of batteries
using today's technology.
- Distribution networks of conventional oil-based fuels are now established world-wide. For
alternative fuels it might take a long time to build up even a fractional infrastructure
to compete with existing service stations. However that is not necessary if the
alternative fuel could be supplied and handled within the existing infrastructure.
Are there any competitors at all in practice? Well, when it
comes to cars, buses and lorries there are some developments. Private
and fleet cars are so numerous that they contribute significantly to
the greenhouse effect so they form an important group.
We can identify several possible alternatives which go some way towards
reducing pollution:
[Note: access to our Reference articles for more information
on each topic is available below.] |
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- Biodiesel (a Biofuel), a Renewable Alternative to Petro-diesel for Motor Vehicles
Compression ignition internal combustion engines are common and
are traditionally fuelled by diesel derived from petroleum oil.
Thankfully diesel is a compound which can be replaced with biodiesel
which is an organically based product and is renewable. It is relatively
easily produced from plant and animal oils, fats and greases. Biodiesel
also gives other benefits in the pollution stakes. It should be noted that biodiesel cannot be sustainably produced in the quantities demanded by the transport industry but in so far as it can contribute fractionally to a diminished carbon footprint it is welcome.
- Liquefied Petroleum Gas (LPG)
An alternative to petrol (gasoline) it offers lower local pollution
levels than normal fuels and vehicles that run on LPG (or are converted
to) receive some subsidies in the UK. This fuel is compatible with
petrol and many vehicles can run on either (dual-fuel vehicles)
so the limited distribution of LPG is not a problem. Its main claims
to fame are its reduced local pollution and it is also cheaper to
run. Since it is petroleum based (oil or natural gas) it is not
renewable and does not qualify for tackling climate change. Now
if we could have a Liquid Renewable Gas that would be worth shouting
about.
- Liquefied Natural Gas (LNG)
and Compressed Natural Gas (CNG)
In practice, LNG and CNG are replacements for petro-diesel and suitable
for heavier freight vehicles. Natural gas is intrinsically cleaner
than petroleum gas but since it is a fossil fuel it is not renewable
and contributes to global warming. At a local level it produces
much less pollution than petrol or diesel and its use attracts financial
incentives. The fuel tanks are specially designed for intense refrigeration
(LNG) or high pressure (CNG) which makes them larger and heavier.
- Ethanol and Methanol
Can be used as alternatives or complements to petrol (gasoline)
and can give less local pollution. If the raw source is petroleum
then they are not renewable but they can be produced organically
(eg from sugar cane) and then they can contribute real advantages
in reducing climate change. Producing these alcohols organically
can also bring economic benefits, for example, to farmers. Due to the large areas of agricultural land needed, like biodiesel, the renewable alcohols can only be expected to supply a small part of the global demand.
- Fuel Cells
Fuel cells are not, strictly speaking, renewable or alternative
energy, they are engines which convert energy; the energy source
is actually hydrogen. We include them here because potentially they
could be so important in the battle for clean and efficient energy.
They are not new but the technology is complex and research and
development is needed to make them more accessible. The hydrogen
fuel can be derived from a variety of sources. Ideally they could
be fuelled by pure hydrogen, manufactured by a renewable process,
but on the other hand, they might be fuelled by some hydrocarbon
compound. If the source of energy is renewable then we have a desirable
situation but if it is petroleum derived, for example, then it is
not a renewable system. Energy density is an intrinsic problem (see above)
but there are developments which indicate that fuel cells could, in future, provide
an important source of energy in transport applications.
- Hybrid engine systems
Similarly to fuel cells these are not energy sources but machines. They use internal combustion engines in tandem with battery-driven
electric motors to conserve energy, and several domestic cars are
now in production. The batteries are charged from the kinetic energy
of the vehicle (eg when braking) and/or the ic engine. Japan led the way with
a selection of production models and they are available now. The Toyota Prius
and the Honda Insight are cars which pioneered the technology using Nickel-metal Hydride
batteries (designed, it is claimed, to last as long as the car)
which are charged via an alternator during normal driving. Power output is delivered
to the road wheels by either the petrol engine or electric motor, or both, depending on
the demands such as acceleration or during cruising. Individual vehicles use slightly
different power sharing systems but the outcome is similar. Currently the vehicles
are dearer, for their class, than petrol and diesel cars (even taking
into account subsidies in the UK) and they only mitigate the problem
of carbon emissions, they don't solve it. Nevertheless much of the energy
that is regenerated is truly green.
One advantage of the electric system over the petrol engine is the
torque available over a wide speed range, a normal car has several
gears to narrow the speed range in use.
[Further information on Hybrid Cars is available on the main Motor Vehicle page.]
In summary:
Petrol and diesel fuelled vehicles have a strong track record and
a very strong popularity because of the cheap and flexible fuel characteristics.
It seems unlikely that they will be replaced in their entirety for the
foreseeable future. Unfortunately this state of affairs is going to
continue the trend to change the climate detrimentally. On the other
hand there are development programmes which address the problem, if
only in a small way at the moment.
Biodiesel is a green fuel that deserves more attention
and better concessions than it currently gets, in the UK.
LPG, as a fuel, appears currently to be a relatively wide-scale
method of reducing local pollution, and it also makes for cheaper driving
with the large tax concessions. It is primarily suited to cars and small
petrol vehicles and the cost benefits are mainly applicable where high
mileages are the norm. Fleet vehicles, local authorities and some small
road transport utilities are well suited to the technology. More filling
stations and purpose built vehicles are coming on stream.
LNG and CNG attract similar comments as LPG but the
fuel is suited to large diesel vehicles. We think that the money used
to subsidise these fuels and LPG could better be spent on encouraging
the use of other fuels such as biodiesel and organically derived ethanol
and methanol.
Ethanol and Methanol can be green fuels if only they
are derived from sustainable energy crops. Despite their lower energy
densities they hold modest potential for low global pollution.
Fuel Cells are high profile devices hyped for their
promise. Technologically they are indeed very interesting and hold great
potential for flexibility. The bottom line is, however, that development
is needed and they must be engineered to be charged with green fuel.
Hybrid vehicles are still relatively novel, but they can be
bought now. The proportional saving of energy (more mpg) is so far quite
small in practice but what there is, is truly green.
If we are optimistic perhaps the hybrid cars will start an alternative
approach to transport, increasingly using efficient electrical motors.
Aircraft Transport seems to be tied irrevocably to
petroleum fuels for the foreseeable future hence they will remain global-warming
behemoths. There have been occasional experiments with green fuel but the main advances seem to have been in increased efficiency rather than a fundamental change of fuel. One way to reduce their detrimental effect would possibly be a special taxation with the proceeds being used to offset their
polluting contribution. Since governments are reluctant to do this we
suggest paying a voluntary tax each time you fly by planting a few trees
via, for example, The Carbon Neutral Company or Future Forests schemes. |
