(Image: Rex Features)
A new breed of structures called skutterudites could finally tap the floods of energy our machines waste as heat
Thermodynamics will always take its pound of flesh. Its laws ultimately dictate that more than half of the energy we use in cars, dishwashers, factories and elsewhere is lost as waste heat (see also 鈥�Wonder stuff: Electron freeway for cool gadgets鈥�). That鈥檚 just an average: for car engines, the proportion is more like two-thirds.
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Reclaim even a small amount of that lost heat as electricity, and that would massively boost energy efficiency. Thermoelectric materials allow us to do just that, by coaxing a current from a temperature difference. Wrap a thermoelectric substance around your car鈥檚 exhaust, and its waste heat could power the electrics. Incorporate thermoelectric elements into a refrigerator, and heat extracted from its interior could power it. Add them to solar panels and the sun鈥檚 heat, not just its light, could be used to make electricity.
Good harvest
Too good to be true? So far, yes. The most efficient thermoelectric material yet discovered, lead telluride, was benched because of fears, and legislation, about the use of lead in electrical equipment. Replace the lead with its less toxic cousin bismuth and you are still at the mercy of the fluctuating price of tellurium, a metallic by-product of copper mining. Decades of research have failed to identify an alternative thermoelectric material to these that can cover its costs commercially.
Even more damningly, the already-mediocre efficiency of most thermoelectric substances drops off still further at high temperatures, making them useless for things like car engines. In thermodynamic terms, this is where skutterudites could make the difference. These are compounds in which atoms of rare earth elements such as cerium or ytterbium rattle around a cage-like structure of cobalt and antimony 鈥� an architecture peculiarly suited to trapping heat while letting electricity flow. What鈥檚 more, the hotter things get, the better they work.
Gregory Meisner heads a group at General Motors that is developing skutterudites with funding from the US Department of Energy. By 2016, they hope to finish a prototype pickup truck with a thermoelectric generator to harvest the energy from the exhaust to power the vehicle鈥檚 radio, headlights and other accessories. 鈥淵ou could also use a thermoelectric generator to help charge the battery in a hybrid vehicle,鈥� says Meisner.
The material mix still isn鈥檛 ideal: China currently has a near-monopoly on many rare earth elements, for example, and cobalt is expensive and relatively hard to come by. Earlier this year, researchers at the University of Osaka in Japan reported promising results by using cheaper nickel and iron 鈥� albeit while adding in thallium, another toxic element (). Meisner鈥檚 team also uses some iron to keep the costs down, and is investigating whether more widely available calcium could replace some or all of the rare earth elements.
Tweaks to these structures are also making better use of their ability to endure temperatures up to 550 掳C, for example to reclaim heat from very-high-temperature processes such as steel milling. Meanwhile other concoctions, such as the exotically named lithium purple blue, are showing properties at lower temperatures.
鈥�70% of fuel is wasted as heat in petrol cars鈥�
It is slow progress, and we are still looking for the killer material 鈥� and the killer app 鈥� says of the California Institute of Technology in Pasadena. But in the uphill battle against thermodynamics, every little helps.
Read more: 鈥�Wonder stuff: Seven new materials to change the world鈥�
This article appeared in print under the headline 鈥淔uture stuff: Skutterudites鈥�
