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16/09/2011
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Tom Shelley reports on some of the developments aimed at producing electricity directly from car exhausts and other waste heat sources.
 While solid state devices are well established for using electricity to directly cool electronic devices, and for turning heat from nuclear sources into power for deep space probes and military satellites, the move in the UK, Europe and the US is to dramatically improve efficiencies and cut costs with a view to producing electric power from car exhausts.
It is well known that car companies such as BMW and Fiat are heavily engaged in developing solutions and that NASA has set itself the goal of coming up with materials that perform the conversion process more than five times better than what is available today. At a seminar organised by the NanoKTN, Professor Douglas Paul at the University of Glasgow referred to the use of bismuth telluride and antimony telluride in most commercial Peltier coolers, the reverse process of turning heat into power, and silicon-germanium formulations in most space applications.
A lot of research is going into using bismuth telluride as a waste heat or even human body heat as a power source but as Professor Anthony Powell of Heriot Watt University pointed out that tellurium makes up only about one part per billion of the Earth's crust similar to the abundance of gold and platinum, and there is no natural tellurium ore. Hence, for this reason, outside NASA, the way forward has to be to design materials that will do the job without using tellurium.
Relative performance is decided by a 'Figure of merit', which has the Seebeck coefficient squared, multiplied by electrical conductivity, divided by thermal conductivity. Most substances with good electrical conductivities tend to have good thermal conductivities, but big improvements in the relevant ratio can be made by nanostructuring to take advantage of quantum effects in wells, wires, dots and other configurations.
The NanoKTN event was sponsored by Johnson Matthey, and it did not deny that it hoped to turn up something that could be added onto car catalytic converters with materials supplied by the company. The working temperature should be around 1000°C.
Dr Gao Min from the Cardiff Thermoelectric Centre at the Cardiff School of Engineering was optimistic about the near term commercial prospects for solid state heat to power conversion. He considered that it should be viable even to tap heat from the human body to power sensors, arguing that even at an efficiency of only 0.3%, and a temperature difference of 5K, the conversion of the available 6mW/cm2 to 20µW/cm2 should be more than adequate to power devices.
The only completely different approach put forward at the meeting was by Dr Neil Fox of the University of Bristol, who was advocating the use of diamond thermionics, based on the physical principles used for decades in vacuum tube electronics. A prototype unit has been developed for E.ON with inkjet printed, lithiated, nanocrystalline diamond electrodes. The intended application is as the power generator in a solar concentrator system, instead of using steam raising boilers.
Design Pointers
• Direct solid state conversion of heat to electric power is already well established in niche applications
• Efforts are now being devoted to raise efficiencies and reduce costs with the aim of economically producing electric power from waste heat sources
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Author
Tom Shelley
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