ICRC 2024 - Session 2

Prof. Dr. Oliver Gutfleisch
Technische Universität Darmstadt

Jürgen Gassmann, Fraunhofer IWKS 

Dr. Wenjie Xie, Fraunhofer IWKS 

Cooling and refrigeration accounts for about 20% of global electricity demand and 8% of greenhouse gas emissions. Current vapor compression technology is inefficient and uses refrigerant gases with a high global warming potential. Magnetocaloric cooling is a solid-state cooling technology that uses the cyclic magnetization and demagnetization of solid-state magnetocaloric materials instead of high- global warming potential refrigerants. The application temperature depends on the class of magnetocaloric material used, and magnetocaloric cooling applications range from room temperature cooling to hydrogen liquefaction. Good magnetocaloric materials exhibit high magnetization change, large entropy change, and adiabatic temperature change. Besides these basic material properties, secondary functionalities such as mechanical stability, cyclic performance, criticality, and non-toxicity are required along with the fact that the material should be easy to shape in a scalable process. In our research we investigate several magnetocaloric materials and mulitcaloric materials regarding there caloric performance including the secondary functionalities and different scalable processing techniques to enable the development of magnetocaloric materials from basic mechanism to application. This work was supported by the ERC Advanced Grant "CoolInnov" (No. 743116) and the CRC/TRR 270 "HoMMage" (DFG).

The global cooling industry accounts for over 10% of the global green house gas emissions and was only recently recognized as a major contributor to global warming in COP23. In my talk I will give an overview of the magnetic cooling technology, its fundamental principles and its benefits to our society. Furthermore, I will also highlight the importance of utilizing re-used/recycled magnets and our recent progress with our cooling machines.

The applications of thermoelectric (TE) technology around room temperature are monopolized by bismuth telluride (Bi2Te3). However, due to the toxicity and scarcity of tellurium (Te), it is vital to develop a next-generation technology to mitigate the potential bottleneck in raw materials supply for a sustainable future. Hereby, we develop a Te-free n-type compound Mg3Sb0.6Bi1.4 for near-room-temperature applications. A higher sintering temperature up to 1073 K is found beneficial in reducing the electrical resistivity, but only if the Mg is heavily overcompensated in the initial stoichiometry. The optimizations of processing and doping yield a high average zT of 1.1 in between 300 K and 573 K. Together with the p-type MgAgSb, we demonstrate module-level conversion efficiencies of 3% and 8.5% under temperature differences of 75 K and 260 K, respectively, and concomitantly a maximum cooling of 72 K when used as a cooler. Besides, the module displays exceptional thermal robustness with a < 10% loss of the output power after thermal cycling for ~32000 times between 323 K and 500 K. These proof-of-principle demonstrations will pave the way for robust, high-performance, and sustainable solid-state power generation and cooling to substitute the highly scarce and toxic Bi2Te3.

Thermoelectric cooling relies on the Peltier effect, which is the electrical induction of a thermal current, in an all-solid-state device without moving parts. A thermoelectric cooler converts electrical energy into directed thermal energy (directed heat), lifting entropy from a low temperature compartment to a higher temperature. It works against the tendency of entropy to flow from a high temperature to a low temperature. By alternately connecting n- and p-semiconductor materials in electrical series and thermally in parallel, such a device can be realized. Currently, thermoelectrics are dominant in small-scale cooling, i.e. below 25 W thermal power, where conventional vapor compression refrigeration systems are not competitive in terms of cost and efficiency. For more than six decades, the technology has been based on bismuth telluride thermoelectric materials, which are evaluated in terms of resource chemistry. Materials that perform better at room temperature are needed to make thermoelectric cooling economically feasible for large-scale cooling systems. The full potential of p-type misfit layered calcium cobaltate has not yet been realized. Its highly anisotropic thermoelectric properties are consistent with the crystal structure, and the anisotropic enhancement by grain alignment in polycrystalline ceramic appears to be the most favorable. Resource chemistry assessment is provided.

Thermoelectric technology is pivotal for a sustainable future, providing a solution to one of the most persistent challenges of modern society: energy efficiency. By directly converting heat into electricity, thermoelectric generators enable the recovery of energy that would otherwise dissipate uselessly into the environment. Oxide materials, composed of naturally abundant, non-toxic elements, are recognised as a viable alternative to conventional thermoelectrics. They offer the capability to operate at elevated temperatures, which leads to enhanced Carnot efficiency, and exhibit remarkable structural and microstructural versatility. Strontium titanate emerged as a front-runner among n-type oxide thermoelectrics owing to its promising electronic properties, phase stability, and exceptional flexibility for tuning by substitution and incorporating A-site and oxygen deficiency. In addition, strontium titanate is chemically compatible with many other materials, enabling a variety of thermoelectric composite approaches. A concise overview of the underlying mechanisms responsible for promising thermoelectric performance will be presented, together with current trends in the design of thermoelectric materials based on strontium titanate.