New twist on old technique could extract electricity from low-grade heat

Pyroelectricity could extract energy from heat sources below100°C, offering an alternative to thermo-electric generators (TEGs).

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TEGs make electricity through the Seebeck effect, where different conductive materials in contact develop a potential difference across the contact point.

Pyroelectric materials, in contrast, develop a potential across a crystal of one material due to electrons shifting as a consequence of heat-induced crystal lattice changes – the effect was first seen in the mineral tourmaline in the 18th century.

Engineers at the University of California, Berkeley, have developed a thin-film pyroelectric device from the ferro-electric 0.68Pb(Mg1/3Nb2/3)O3 – 0.32PbTiO3 – a relaxor ferroelectric with strong field- and temperature-induced polarisation susceptibilities, according to ‘Pyroelectric energy conversion with large energy and power density in relaxor ferroelectric thin films‘, a paper published in Nature Materials.

“We know we need new energy sources, but we also need to do better at utilising the energy we already have,” said researcher Lane Martin. “These thin films can help us squeeze more energy than we do today out of every source of energy.”

The energy converter was fabricated from films 50-100nm thick.

“By creating a thin-film device, we can get the heat into and out of this system quickly, allowing us to access pyroelectric power at unprecedented levels for heat sources that fluctuate over time,” said Martin. “All we’re doing is sourcing heat and applying electric fields to this system, and we can extract energy.”

The team is claiming record pyroelectric energy conversion: 1.06J/cm3, 526Watt/cm3 and 19% Carnot efficiency “equivalent to the performance of a thermoelectric with an effective Zt ≈ 1.16 for a temperature change of 10K”, according to the paper. “Our findings suggest that pyroelectric devices may be competitive with thermoelectric devices for low-grade thermal harvesting.”

“Part of what we’re trying to do is create a protocol that allows us to push the extremes of pyroelectric materials so that you can give me a waste-heat stream and I can get you a material optimized to address your problems,” said Martin.


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