Waste heat could be used to power electronics
Japanese researchers have developed an energy-harvesting device that can convert environmental waste heat (such as that lost from the human body) into electricity — an attractive prospect for powering small electronics sustainably. Their findings have been published in the journal Scientific Reports.
A thermocell is a type of energy-harvesting device that converts environmental heat into electricity through the thermal charging effect. Although thermocells are inexpensive and efficient, so far only low output voltages — just tens of millivolts (mV) — have been achieved and these voltages also depend on temperature. These drawbacks need to be addressed for thermocells to reliably power electronics; now, a research team led by the University of Tsukuba has brought the technology a step closer to commercialisation.
The team developed a thermocell containing a material that exhibited a temperature-induced phase transition of its crystal structure. Just above room temperature, the atoms in this solid material rearranged to form a different crystal structure. This phase transition resulted in an increase in output voltage from zero to around 120 mV, representing a considerable performance improvement over that of existing thermocells.
“The temperature-induced phase transition of our material caused its volume to increase,” explained Professor Yutaka Moritomo, senior author of the study. “This in turn raised the output voltage of the thermocell.”
The researchers were able to finely tune the phase transition temperature of their material so that it lay just above room temperature. When a thermocell containing this material was heated above this temperature, the phase transition of the material was induced, which led to a substantial rise of the output voltage from zero at low temperature to around 120 mV at 50°C.
As well as tackling the problem of low output voltage, the thermocell containing the phase transition material also overcame the issue of a temperature-dependent output voltage. Because the increase of the output voltage of the thermocell induced by the thermal phase transition was much larger than the temperature-dependent fluctuations of output voltage, these fluctuations could be ignored.
“Our results suggest that thermocell performance can be strongly boosted by including a material that exhibits a phase transition at a suitable temperature,” Prof Moritomo said. “This concept is an attractive way to realise more efficient energy-harvesting devices.”
The team’s efforts combining thermocell technology with an appropriately matched phase transition material resulted in an increased ability to harvest waste heat to power electronics, which is an environmentally sustainable process. Their design thus has the potential to provide independent power supplies for advanced electronics.
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