Potential, problem, project - the three P's of TEGs

Waste heat is an unintentional by-product of many everyday processes: In combined heat and power plants, through district heating, in data centers or biogas plants, to name just a few examples. Engines and machines also emit heat. Electrical energy is converted into thermal energy in these processes. "Most of the energy used every day is released as thermal energy. Energy consumption in the EU amounts to around 17,000 terawatt hours per year. If we were to use just one percent of this, we could generate 170 terawatt hours of electricity from waste heat and thus cover around one third of the total annual electricity demand in Germany," says Prof. Dr. Uli Lemmer, Head of the Light Technology Institute (LTI) at KIT, demonstrating the enormous potential of generating electricity from waste heat. The basis for this would be thermoelectric generators, as they can generate electricity from unused heat by means of the thermoelectric effect using positive and negative semiconductors. The problem: their production is too complex. "Thousands of semiconductors are placed next to each other piece by piece by machine. This makes production expensive and the widespread use of TEGs has been impossible up to now," says Lemmer, describing the problem. The research work in the project "Origami inspired thermoelectric generators by printing and folding" (ORTHOGONAL for short) aims to change this.

Thermoelectrics meets Japanese folding art

A fundamental milestone that the researchers at the LTI have developed and which also forms the basis of the project is 2D printing with subsequent 3D folding. "We have done a lot of research in the field of thermoelectrics in recent years, including analyzing and further developing the printing process. The materials we use are a decisive step forward in the cost-effective and flexible production of TEGs," explains Lemmer. "Dr. Mofasser Mallick, a research associate at the Light Technology Institute (LTI), convinced us that it makes more sense to work with inorganic rather than organic materials. We combined his knowledge of classic thermoelectrics with our printing techniques," continues Lemmer. The result is the use of highly efficient inorganic nano-composite materials for printed electronics. The researchers ground up classic thermoelectric materials, processed the pulverized nanoparticles into printable inks using screen printing, applied them to ultra-thin substrates, fused them and cured them using photonic sintering. Inspired by the Japanese art of origami, they then mechanically folded the resulting 2D film into the desired 3D geometry.

Understanding and scaling the thermoelectric art

The aim now is to further develop this process and make it usable for a wide range of applications by scaling it up. "In order to scale up the process and make the technology marketable, we need to understand how the materials work and how we can improve stability," says Lemmer. The required layer thickness of the TEGs depends on the subsequent application. A distinction is made between heat dissipation against air and heat dissipation against liquid. "A layer thickness of a few millimetres is required for heat dissipation against air. As we can't print this, we came up with the idea of folding. However, folding doesn't work for heat dissipation against liquids. We have to stay in the plane, but we need thicker layers for greater efficiency," explains Lemmer. The research team is looking at various ways to adjust the thickness of the layers. "We are trying out an incredible amount and simulating a wide variety of applications," says Lemmer.

From smartwatches to thermal power plants

The aim of the five-year ERC funding is not only to deepen understanding at the material level, but also to develop two demonstrators - one for small-scale use and one for industrial use. As heat is generated in countless processes, the possible application scenarios are just as diverse. However, the greater the temperature difference, the more efficient a TEG. In the long term, the researchers are therefore focusing on the topic of waste heat recovery in order to increase energy efficiency in industry and society and thus contribute to the energy transition. "Thermal power plants are a good example of waste heat recovery. Temperature differences of up to 300 degrees Celsius prevail here, resulting in high efficiency for TEGs. With district heating, the temperature difference is 30 to 40 degrees Celsius, so the efficiency is already significantly lower. With wearables, such as a smartwatch, the efficiency is then less than one percent. Nevertheless, you have to keep an eye on the target application and the potential it offers: In the case of a smartwatch, it is the extended battery life and therefore convenience for the end customer; in the case of power plants, it is the commercial generation of electricity. The decisive factor is that an unused product is made usable in order to generate energy from it," explains Lemmer. In terms of social benefits, industrial applications make sense, but from an economic point of view they place high demands on cost-effective production. The cost-effective production of TEGs is therefore a further concern of the researchers in order to create a competitive product.

 

An impactful outlook

The researchers' work is already very promising. "There is interest in the technology, as we can see from the many different inquiries and discussions from watch manufacturers to steelworks. The core technology for the process is printing and, thanks to our years of research, we can do this better than the competition. I see potential here for a spin-off in order to bring the technology into widespread use and thus make a contribution to the energy transition," says Lemmer, looking optimistically to the future.

Further links

• KIT press release

Images: KIT