THERMOELECTRICITY
Hot prospect
Converting residual heat from car exhaust fumes into electricity could cut fuel
consumption and reduce carbon emissions. Siobhan Wagner reports
RESEARCHERS in Germany are develop-
ing a thermoelectric generator to convert
the heat from car exhaust fumes into
electricity. The device will feed energy
into the car’s electronic systems and
could cut fuel consumption and help
reduce the vehicle’s CO 2 emissions.
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A thermoelectric generator creates
voltage when there is a different tempera-
ture either side of it. So the greater the
temperature difference, the more current
the generator can produce.
A team from the Fraunhofer Institute
for Physical Measurement Techniques is
developing new thermoelectric materials,
modules and systems to harness this kind
of residual heat.
‘The temperature in the exhaust pipe
can reach 700ºC or more,’ said Harald
Böttner, head of the Thermoelectric
Systems department. ‘The temperature
difference between the exhaust and a pipe
carrying engine cooling fluid can thus be
several hundred degrees,’ he added.
The thermoelectric converter will
make use of this huge differential. With
the flow of heat from the exhaust fumes
on one side and the cold side of a coolant
pipe on the other, the charge carriers will
pass through special semiconductors and
produce a current similar to a battery.
Böttner said thermoelectric effects
were first recorded in the 1800s, but
thermoelectric materials have not signifi-
cantly improved in efficiency since the
1950s. ‘During the last decade there have
been many efforts to increase the quality
of materials, using new developments in
nanotechnology and additional
knowledge about the physics behind
thermoelectric material,’ he said.
Inadequate efficiency was one of the
key factors preventing such materials
from being used in cars, but new
developments mean this is no longer the
case.
Earlier this year, for example,
researchers at Boston College and MIT in
the US demonstrated a boost in the
efficiency of bismuth antimony telluride
— a semiconductor alloy commonly used
for thermoelectric cooling. By breaking
‘Most drivers do not realise that two-thirds
of the fuel consumed in their car is emitted,
unused, as heat’ Harald Böttner, Fraunhofer Institute
the bulk material into tiny chunks —
from five to 50nm across — the
researchers increased a key measure of
thermoelectric conversion, called the
ZT of the alloy, from one to 1.4.
Böttner said the thermoelectric
generator technology will not be ready
commercially for another five years.
But with many people concerned about
rising fuel costs and CO 2 emissions, he
added, it cannot come fast enough.
Böttner said most drivers do not
realise that two-thirds of the fuel
consumed in their car is emitted,
unused, as heat. Approximately 30 per
cent is lost through the engine block
and a further 30 to 35 per cent as
exhaust fumes. Any way to harness the
unused waste heat could significantly
lower the car’s fuel consumption. The
technology could also be applied to
other machines and even power
stations.
The long-term objective for Böttner
and his team is to reduce dependence
on alternators — the generators
currently used to provide electricity in
cars — and to use thermoelectricity to
supply energy to the ever-rising
amount of electronic gadgetry.
Thermoelectric generators could
step in and cover a significant
proportion of these requirements,
but in no way replace alternators
entirely.
‘It would not be possible to get rid of
the alternator,’ said Böttner. ‘A
standard car consumes about 5kW of
electrical energy while driving, while a
small car will consume roughly about
1kW. We hope to produce a generator
that could produce about 1kW.’
The most important feature of
thermoelectric generator technology,
however, could be the fuel savings.
‘This could make it possible to cut fuel
consumption by between five and seven
per cent,’ claimed Böttner.
the EnGIneeR 30 JUNE–13 JULY 2008
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