University of Tsukuba researchers in Japan have developed an improved thermocell design to convert heat into electricity. The thermocell contained a material that exhibited a phase transition from one arrangement of atoms to another when heated to 50° C (122° F).. The phase transition caused the output voltage of the thermocell to increase substantially to a level sufficient to power electronics. This thermocell shows promise as a way to use waste environmental heat to power electronic devices.

Collecting energy from environmental waste heat such as that lost from the human body is an attractive prospect to power small electronics sustainably. 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 and contribute to the development of a sustainable society.

A University of Tsukuba-led research team recently improved the energy-harvesting performance of thermocells, bringing this technology a step closer to commercialization.

Their findings have been published in Scientific Reports.

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.

Schematic structure of NaxCo[Fe(CN)6]y (Co-PBA) in the (a) low-spin (LS) and (b) high-spin (HS) phases. For simplicity, guest ions (Na+) are omitted. The LS–HS phase transition is triggered by cooperative charge transfer from Fe2+ to Co3+, which causes spin state transition of Co from LS Co3+ to HS Co2+. Image Credit: University of Tsukuba. Click image for the largest view.

Professor Yutaka Moritomo, senior author of the study explained, “The temperature-induced phase transition of our material caused its volume to increase. 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 (122° F).

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.

Professor Moritomo added, “Our results suggest that thermocell performance can be strongly boosted by including a material that exhibits a phase transition at a suitable temperature. This concept is an attractive way to realize more efficient energy-harvesting devices.”

The research team’s design combining thermocell technology with an appropriately matched phase transition material leads to increased ability to harvest waste heat to power electronics, which is an environmentally sustainable process. This design has potential for providing independent power supplies for advanced electronics.

This is progress! 50° C or 122° F is not very hot and 120 mVolts is a giant stride forward. One can imagine that 90° F is attainable and maybe a doubling of voltage would put this technology squarely into the mass market. Congratulations are in order – go Tsukuba U. Go!


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