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Forget cars fuelled by alcohol and vegetable oil. Before long, you
might be able to run your car with nothing more than water in its fuel
tank. It would be the ultimate zero-emissions vehicle.

While water, plain old H2O, is not at first sight an obvious power
source, it has a key virtue: it is an abundant source of hydrogen, the
element widely touted as the green fuel of the future. If that hydrogen
could be liberated on demand, it would overcome many of the obstacles
that till now have prevented the dream of a hydrogen-powered car
becoming reality. Producing hydrogen by conventional industrial means
is expensive, inefficient and often polluting. Then there are the
problems of storing and transporting hydrogen. The pressure tanks
required to hold usable quantities of the fuel are heavy and
cumbersome, which restricts the car's performance and range.

Tareq Abu-Hamed, now at the University of Minnesota, and colleagues at
the Weizmann Institute of Science in Rehovot, Israel, have devised a
scheme that gets round these problems. By reacting water with the
element boron, their system produces hydrogen that can be burnt in an
internal combustion engine or fed to a fuel cell to generate
electricity. "The aim is to produce the hydrogen on-board at a rate
matching the demand of the car engine," says Abu-Hamed. "We want to use
the boron to save transporting and storing the hydrogen." The only
by-product is boron oxide, which can be removed from the car, turned
back into boron, and used again. What's more, Abu-Hamed envisages doing
this in a solar-powered plant that is completely emission-free.
Simple chemistry

The team calculates that a car would have to carry just 18 kilograms of
boron and 45 litres of water to produce 5 kilograms of hydrogen, which
has the same energy content as a 40-litre tank of conventional fuel. An
Israeli company has begun designing a prototype engine that works in
the same way, and the Japanese company Samsung has built a prototype
scooter based on a similar idea.

The hydrogen-on-demand approach is based on some simple high-school
chemistry. Elements like sodium and potassium are well known for their
violent reactions with water, tearing hydrogen from its stable union
with oxygen. Boron does the same, but at a more manageable pace. It
requires no special containment, and atom for atom it's a light
material. When all the boron is used up, the boron oxide that remains
can be reprocessed and recycled.

Abu-Hamed and his team are not the first to investigate
hydrogen-on-demand vehicles. The car giant DaimlerChrysler built a
concept vehicle called Natrium (after the Latin word for sodium, from
which the element's Na symbol is drawn), which used slightly more
sophisticated chemistry to generate its hydrogen. Instead of pure water
as the source of the gas, it used a solution of the hydrogen-heavy
compound sodium borohydride. When passed over a precious-metal catalyst
such as ruthenium, the compound reacts with water to liberate hydrogen
that can be fed to a fuel cell. It was enough to give the Natrium a top
speed of 130 kilometres per hour and a respectable range of 500
kilometres, but DaimlerChrysler axed the project in 2003 because of
difficulties in providing the necessary infrastructure to support the
car in an efficient, environmentally friendly way.

Engineuity, an Israeli start-up company run by Amnon Yogev, a former
Weizmann Institute scientist, is working on a similar strategy, but
using the reaction between aluminium wire and water to generate
hydrogen. In Engineuity's design, the tip of the metal wire is ignited
and dipped into water to begin splitting the water molecules. The
liberated hydrogen is piped into the engine alongside the resulting
steam, where it is mixed with air and burnt. Engineuity is looking for
investors to pay for a prototype, and claims it will be able to
commercialise its idea "in a few years' time". The US company PowerBall
Technologies envisages a hydrogen-on-demand engine containing plastic
balls filled with sodium hydride powder that are split to dump the
contents into water, where it reacts to produce hydrogen.

Abu-Hamed says the generation of hydrogen for his team's engine would
be regulated by controlling the flow of water into a series of tanks
containing powdered boron. To kick-start the reaction, the water has to
be supplied as vapour heated to several hundred degrees, so the car
will still require some start-up power, possibly from a battery. Once
the engine is running, the heat generated by the highly exothermic
oxidation reaction between boron and water could be used to warm the
incoming water, Abu-Hamed says. Alternatively, small amounts of
hydrogen could be diverted from the engine and stored for use as the
start-up fuel. Water produced when the hydrogen is burnt in an internal
combustion engine or reacted in a fuel cell could be captured and
cycled back to the vehicle's tank, making the whole on-board system
truly zero-emission.

Hydrogen-on-demand, whether from water or another source, could address
two of the big problems still holding back the wider use of hydrogen as
a vehicle fuel: how to store the flammable gas, and how to transport it
safely. Today's hydrogen-fuelled cars rely on stocks of gas produced in
centralised plants and distributed via refuelling stations in either
liquefied or compressed form. Neither is ideal. The liquefaction
process eats up to 40 per cent of the energy content of the stored
hydrogen, while the energy density of the gas, even when compressed, is
so low it is hard to see how it can ever be used to fuel a normal car.

Hydrogen-on-demand would not only remove the need for costly hydrogen
pipelines and distribution infrastructure, it would also make hydrogen
vehicles safer. "The theoretical advantage of on-board generation is
that you don't have to muck about with hydrogen storage," says Mike
Millikin, who monitors developments in alternative fuels for the Green
Car Congress website. A car that doesn't need to carry tanks of
flammable, volatile liquid or compressed gas would be much less
vulnerable in an accident. "It also potentially offsets the
requirements for building up a massive hydrogen production and
distribution infrastructure," Millikin says.

There is a potentially polluting step that has to be tackled. "You'll
need an infrastructure to produce and distribute whatever the key
elements of the generation system might be," Millikin warns. While
Abu-Hamed's scheme still requires a distribution network and
reprocessing plant, he has devised an ingenious plan that will allow
the spent boron oxide to be converted back to metallic boron in a
pollution-free process that uses only solar energy (see Diagram).
Heating the oxide with magnesium powder recovers the boron, leaving
magnesium oxide as a by-product. The magnesium oxide can then be
recycled by first reacting it with chlorine gas to produce magnesium
chloride, from which the magnesium metal and chlorine can then be
recovered by electrolysis.
Solar source

The energy to drive these processes would ultimately come from the sun.
The team calculates that a system of mirrors could concentrate enough
sunlight to produce electricity from solar cells with an efficiency of
35 per cent. Overall, they say, their system could convert solar energy
into work by the car's engine with an efficiency of 11 per cent,
similar to today's petrol engines.

Experts are sceptical that we'll be seeing cars running on water any
time soon. "It's not the kind of thing you're going to see appearing in
a car in five or even ten years' time," says Jim Skea, research
director at the UK Energy Research Centre in London. For example,
DaimlerChrysler is now focusing its efforts on cars running on
compressed hydrogen because filling stations that supply it already
exist in some places.

Proponents of cars that run on water are banking that long term the
idea will win out. Engineuity's Yogev claims the running costs will be
comparable to those of today's petrol engines and expects to have a
prototype built within three years.