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Thermoelectrics：Every little helps
HERE is a thought: approximately 60% of the energy converted in power
generation is wasted. The price of energy is high, both in terms of the actual
cost to the consumer and the consequences of the climate change that generating
power from fossil fuels causes. If even a small proportion of this wasted heat
could be converted to useful power, it would be a good thing.
At this week’s meeting of the American Physical Society, in Baltimore,
Mercouri Kanatzidis of Michigan State University proposed such a scheme. He
advocates attaching thermoelectric devices that convert heat into electricity to
chimney stacks and vehicle exhausts, to squeeze more useful energy from power
generation.
The technology to do so has existed for years. If one end of an electrical
conductor is heated while the other is kept cool, a small voltage is created
between the two. Placing two dissimilar metals, or other electrically conductive
materials, in contact with each other and then heating them also generates a
voltage. Such devices, called thermocouples, are nowadays usually made using
semiconductors. They are widely used as thermometers. But if they could be made
cheaper, or more efficient, or both, they could also be employed to generate
power.
Dr Kanatzidis is developing new thermoelectric materials designed to be
capable of converting up to 20% of the heat that would otherwise be wasted into
useful electricity. The challenge lies in finding a substance that conducts
electricity well and heat badly. These two properties define what physicists
call the “figure of merit” of a thermoelectric substance, which describes the
power a device made of that substance could generate. Dr Kanatzidis’s group aims
to make materials with higher figures of merit than those attainable with
today’s semiconductors.
Since the electrical properties of solids depend on their crystal
structures, his group is experimenting with new atomic lattices. In particular,
they are working on a group of chemicals called chalcogenides. These are
compounds of oxygen, sulphur, selenium and tellurium that are thought to be
particularly suitable for thermoelectric applications because their structure
allows electric currents to flow while blocking thermal currents. They thus have
a high figure of merit. Dr Kanatzidis’s group is developing new ways of making
these compounds crystallise correctly.
But even existing devices could become economically useful as fuel prices
rise, Dr Kanatzidis argues. In America, transport accounts for a quarter of the
energy used. Fitting small thermoelectric devices to the exhaust pipes of
vehicles could squeeze another 10% from the fuel—a saving that would be
especially relevant in hybrid petrol/electric devices where the battery is
recharged in part by recycling energy that would otherwise be dissipated by
energy-draining activities such as braking. Similarly, attaching thermoelectric
devices to the flues of power plants could generate more useful power.
And thermoelectric devices could be used in other areas. They could work
alongside solar cells and solar heating systems. They could also be used in
geothermal and nuclear power plants. Dr Kanatzidis argues that wherever heat is
generated as part of power generation, thermoelectric devices could help extract
more useful energy. Waste not, want not.
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发表于 2017-8-6 15:52:41