Thermoelectrics make electricity from your daily (hot) coffee

So-called thermoelectric generators (’thermoelectrics’) convert ambient heat into electricity. They can be used to power a broad range of applications, from wearables and sensors to remote electronic systems like satellites. But to realize all these applications, thermoelectrics must also be flexible and be available in many different shapes. Moreover, they must be relatively easy to make. Materials researchers at KU Leuven tackled these demands by developing an innovative manufacturing process.

A temperature difference of a few degrees is enough to generate a usable amount of electric current. To turn on a light, for example, or make a small device such as a wearable or a sensor run. Materials that convert heat into electricity are called thermoelectric generators, or thermoelectrics for short. They can be made out of metal or semiconductor material, and they are usually small in size. As a result, thermoelectrics can be used to harness ambient heat that is available all’around us, right down to our bodies - which, of course, constantly produce heat. The big advantage of thermoelectrics? They can power devices without the need for power wires or batteries.

But before such wireand battery-free devices can be built, the right thermoelectrics must first be available. Flexibility and versatility are important here. Thermoelectrics must be made in many different shapes, and they must also be pliable. In particular, the latter is still quite a challenge today: existing thermoelectrics are often very rigid, causing them to break at the slightest deformation.

Not so with the thermoelectrics manufactured by the group of Francisco Molina-Lopez, assistant professor at the KU Leuven’s Department of Materials Engineering. The materials researchers have devised a new method to produce flexible thermoelectrics quickly and relatively easily. They do this by melting metal powder onto a plastic film via additive manufacturing, using laser light. The thermoelectrics are ’printed’ in this way in the form of flat strips. The strips can be applied with adhesive tape to surfaces that give off heat, and thanks to their pliability, those surfaces may also be curved. The researchers demonstrated this by sticking their thermoelectrics to a coffee mug, with the content giving off enough heat to produce several microwatts of electricity - enough to power a calculator, an electronic watch or an RFID tag. In another demonstration, a researcher attached the strips to his arm, after which he was generating his ’own’ electricity.

The strips are made out of bismuth telluride, a material that’s known for its thermoelectric properties.

We use this material as a benchmark because it’s by far the best for thermoelectrics at room temperatures. But we hope to extend the manufacturing process to other materials in the future.

Professor Francisco Molina-Lopez
One of the advantages of the new process is that large areas of the thermoelectric material can be printed in a relatively short time. ’Eventually, we want to obtain so-called cuttable power arrays, where we can simply cut the strips from the printed material’, says Molina-Lopez. ’It’s a bit like pushing out biscuits from a layer of dough.’ Moreover, the remaining printed material on the plastic film can be re-used. ’So we are not wasting this precious material in the fabrication process.’ Molina-Lopez and his colleagues have shown that they can re-use the leftover material several times without loss in performance. In a next step they want to improve their method and design to even recycle the material from decommissioned thermoelectrics.