Turning Heat into Power: Thermoelectric Generators Harness Electricity from Your Cup of (Hot) Coffee
So-called thermoelectric generators convert ambient heat into electricity. They can be used to supply power to a wide range of applications, from wearable devices and sensors to remote electronics systems such as satellites. But to put these applications into practice, these generators must also be flexible and available in many different forms. Moreover, they should be relatively easy to make. Materials researchers at KU Leuven have addressed these requirements by developing an innovative production process.
A temperature difference of a few degrees is sufficient to produce a usable amount of electrical current. For example, to turn on a lamp or to make a portable device or sensor work. Materials that convert heat into electricity are called thermoelectric generators. They can be made of metal or semiconductor material and are usually small in size. These generators therefore allow us to utilize the ambient heat that is available all around us, even on our bodies – which of course continually produce heat. The big advantage of thermoelectric generators? They can power devices without power wires or batteries.
But before such devices can be built without wires or batteries, the right thermoelectric generators must first be available. Flexibility and versatility are important here. Thermoelectric generators must be made in many different shapes and be flexible. The latter in particular is currently still a significant challenge: existing thermoelectric generators are often very stiff and break at the slightest deformation.
But that is not the case with the generators made by the group of Francisco Molina-Lopez, a lecturer at the materials science department of KU Leuven. The materials researchers devised a new method to produce thermoelectric generators quickly and relatively easily. To this end, they melt metal powder on a plastic film through layer-by-layer production using laser light. The thermoelectric generators are ‘printed’ in this way in the form of flat strips. The strips can be applied with adhesive tape to surfaces that emit heat. Because they are pliable, these surfaces may also be curved. The researchers demonstrated this by taping their thermoelectric generators to a coffee mug: its contents gave 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 applied strips to his arm, after which he generated his ‘own’ electricity.
The strips are made of bismuth telluride, a material known for its thermoelectric properties. “We use this material as a standard,” Molina-Lopez explains, “because it is by far the best for thermoelectric generators at room temperature. But we hope to expand the production process to other materials in the future.’
One of the advantages of the new process is that large pieces of thermoelectric material can be printed in a relatively short time.
Ultimately, we want to achieve cuttable energy plates and then we can simply cut strips from the printed material. You can compare it with a slab of dough from which you press cookies with a mold.
Professor Francisco Molina-Lopez
In addition, the remaining printed material on the plastic film can be reused. ‘So we don’t waste this valuable material during the production process.’ Molina-Lopez and his colleagues showed that they can reuse the remains of the material several times without loss of performance. In a next step, they want to improve their method and design to also recycle the material from decommissioned thermoelectric generators.