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Jun 18, 2023

Thermoelectric technologies emerge from US

Two radically different thermo-electric energy harvesters emerged from the US

Two radically different thermo-electric energy harvesters emerged from the US this week.

One is a thermo-acoustic proof-of-concept from Penn State University, the other the latest iteration of Nextreme's production Seebeck generator.

The thermo-acoustic device is an attempt to get enough electricity from a developing world cooking fire to drive a fan.

The idea is that the fan which will increase the efficiency of the fire while reducing emissions – cooking smoke kills 1.9 million people per year, estimates the UN.

Reliability is key, so researcher Paul Montgomery decided there should be few or no moving parts to wear out, hence the choice of acoustic conversion – and Penn State is renown for acousto-thermodynamics.

It comes in two parts, thermo-acoustic oscillator to create a vibrating column of air, and a vibration to electrical converter – a loudspeaker working in reverse in the demonstrator.

The oscillator is a tube with one closed end and heat transfer material just inside the open end.

With the right dimensions, conditions, and heat exchanger orientation; a small part of the air is heated up, and its expansion causes it to move to a cold part of the chamber. Here it cools, contracts, and is drawn back to the hot part.

This cycle repeats, at a frequency determined by the bounce of the elastic air column and the length of the tube.

The effect has been know for a century since Victorian scientist Lord Rayleigh recognised that when heat is added to a sound wave during compression and removed during expansion, the amplitude of the sound wave grows.

"A very high-tech thermo-acoustic generator has been built for space applications, it employed pressurised helium gas and a very expensive linear alternators," said Montgomery. "For the cook stove application, we attempted a much simpler prototype."

He uses atmospheric air in a rectangular duct folded from sheet metal.

Heat is transferred to the gas by a ceramic honeycomb with its holes running along the duct.

"The ceramic is actually a material that is mass-produced as a substrate for catalytic converters used in automotive exhaust systems," Penn State Professor Steven Garrett told Electronics Weekly. "Until the ceramic is coated with the catalyst, usually platinum, it is quite inexpensive, so cheap in fact that it is used as fire brick in barbeques."

One end of the ceramic is blackened and absorbs 20W of heat by electromagnetic radiation from the hot end of the resonator that is in the stove.

The other is attached to an external computer-style heatsink for cooling. The demonstrator produces 25mW, although Montgomery estimates that the finished product would produce 10W from a 4-8kW stove and cost $25.

"Even a relatively inefficient heat engine would be capable of generating enough electricity from small amounts of waste heat to power a fan and possibly have sufficient excess capacity to charge a phone or a battery that could provide lighting at night," he said.

The findings were presented to the 2nd Pan-American/Iberian meeting on acoustics, arranged by the American Institute of Physics , and Montgomery has written a background document.

The other development, from semiconductor Peltier/Seebeck device maker Nextreme, is aimed at the lower end of the power and temperature difference scales.

eTEG HV37 is a ‘high voltage’ device in that is generates 170mV open circuit from a 10K temperature difference, at which point it can deliver 1mW with the right loading.

At 50K difference, it can deliver 24mW or an open-circuit voltage of 850mV.

It is 6mm2, 0.6mm high, and joins two other generators: HV56 and HV14.

"Deployment of distributed sensors and sensor networks have led to an increased interest in renewable and autonomous power sources," said Dave Koester, v-p of engineering at Nextreme. "The use of waste heat is an attractive source of energy for many applications where power on the order of µW-mW is required."

The relatively high output voltage comes from connecting many junctions in series using eutectic gold-tin solder, and a patented thermal bump fabrication process that can achieve thousands of elements per square centimetre.

HV37 replaces the firm's original thermoelectric generator – UPF40 – with something smaller that produces the same power at higher voltage.