Office-style shirt with hidden thermoelectric generator to power low-energy wearable electronics

Holst Centre and imec present a new shirt with completely hidden integrated thermoelectric generator (TEG). The TEG produces an average power of 1mW when sitting in the office at 22°C, and 2mW when walking around. These values double at 17°C. As such, the device is an ideal power supply for low-power wearable electronics such as health-monitoring devices. Because the TEG is now completely hidden in textile, the shirt has greatly improved on a previous model in terms of comfort and potential market acceptance.

A TEG with 16 one-stage thermopiles has been designed, fabricated and integrated into an office-style shirt. The thermopiles are sandwiched between a hot plate (3cm in diameter) and a cold plate (sized 3cm x 3,5cm), resulting in an overall thickness of 5mm. They are mounted on a piece of cotton and sewed to the shirt from the inside. The cold plates of the thermopiles are glued to carbon fabric which has been sewed on the inner side of the shirt. As a result, the device and wiring are hidden between two cotton layers and invisible from either side of the shirt. In contrast to an earlier concept, the new TEG does not require any modifications to existing garments and is very comfortable to wear. The shirt can be washed and ironed. This new design greatly improves the potential acceptance of the energy harvester on the market.

The TEG has been tested on a standing and on a sitting person, and at different ambient temperatures. The measurements show an average power of 1mW in the office at 22°C, and this value doubles if the person walks. The power also doubles at an ambient temperature of 17°C, i.e., it increases to 2mW on a standing or sitting person, and to 4mW on a walking person. In practical applications, the power available for applications is limited by the presence of energy storage elements. Nevertheless, the TEG is an ideal power supply for low-power wearable electronics such as health-monitoring devices. For example, wireless electrocardiography requires about 0.4mW today and is expected to require only 0.1mW in the near future.

The new thermoelectric shirt is a good compromise between optimal comfort and power output. The power would have been slightly higher if the thermopiles were always tightly pushed to the skin, as in a previously developed slim-fit-style shirt. In this earlier design, the TEG was pushed to the skin by using elastic bands, and the cold plates were visibly located above the textile. For this shirt, only 14 thermopiles were needed to obtain the same power output. But in terms of comfort, elastic bands are not a preferable solution. Also, in the new design, the inner cotton layer located between the skin and the hot plate adversely affects the power, but it contributes to a better comfort. On the other hand, an improvement in power generation was provided by the carbon fabric to which the cold plates were glued. This carbon fabric acts like an easy-to-integrate flexible heat spreading layer and replaces the rigid aluminum plates used in previous concepts. Measurements with an 8cmx8cm carbon fabric have shown that the power generation improved by 30% as compared to an Al heat spreading plate of 3cmx4cm.

More detailed results have been presented at the 9th European Conference on Thermoelectrics, September 2011

See also:

• Micro-power generation and storage