Harvesting body heat from wearables

Materials World magazine
10 Feb 2020

A new flexible device integrated in wearables can convert body heat into power. Shardell Joseph reports. 

A flexible thermoelectric generator (TEG) can create electricity by harvesting and converting body heat. According to researchers at North Carolina State University (NC State), USA, the prototypes are lightweight, conform to bodyshapes, and are capable of generating much more power than existing lightweight heat harvesting technologies.

Reported in the paper, Flexible thermoelectric generators for body heat harvesting – enhanced device performance using high thermal conductivity elastomer encapsulation on liquid metal interconnects, published in Applied Energy in this month, TEGs have a 1.7 times higher output power density compared to conventional devices, and a six times higher heat rate transfer.

‘Wearable thermoelectric generators generate electricity by making use of the temperature differential between your body and the ambient air,’ said NC State Electrical and Computer Engineering Associate Professor, Daryoosh Vashaee.

‘Previous approaches either made use of heat sinks – which are heavy, stiff and bulky – or were able to generate only one microwatt or less of power per centimetre squared – µW/cm2. Our technology generates up to 20µW/cm2 and does not use a heat sink, making it lighter and much more comfortable.’

To electrically connect the semiconductor elements, the team used liquid metal interconnects made of EGain – a non-toxic alloy of gallium and indium – offering electrical conductivity and flexibility. Measured at 2mm-thick, the conductive layer is topped with a polymer layer to prevent heat from escaping the device.

‘The key here is using a high thermal conductivity silicone elastomer doped with graphene flakes and EGain ,’ said NC State Electrical and Computer Engineering Professor, Mehmet Ozturk. ‘Using this elastomer allowed us to boost the thermal conductivity – the rate of heat transfer – by six times, allowing improved lateral heat spreading.

‘The flexible device reported in this paper is significantly better than other flexible devices reported to date and is approaching the efficiency of rigid devices, which is very encouraging.’

According to the paper, the high thermal conductivity elastomer not only improves the output power density of TEGs, utilised as a heat spreader, but it also reduces the parasitic thermal resistance of the encapsulated layer. The elastomer, doped with both graphene nanoplatelets and EGain to further increase its thermal conductivity.

The performance of the device is further enhanced with an additional thin copper layer that also spreads the heat, further enhancing the output power by 1.3 times at 1.2m/s air velocity – average walking speed.

‘Worn on the wrist, our best devices achieve power levels in excess of 30μW/cm2 at an air velocity of 1.2m/s outperforming previously reported flexible TEGs,’ the paper read.

Testing the device, the researchers incorporated the TEG into tshirts, discovering that the tshirt TEGs were still capable of generating 6µW/cm2, or as much as 16µW/cm2 if a person is running.

If commercialised, the technology can be expected to eliminate the need for the manufacturing of new flexible and thermoelectric materials, as it incorporates the same semiconductor elements used in rigid devices. From now, the team will work on improving the efficiency of the flexible devices.