Energy efficient cooling will be one of the most important challenges for our future society. Energy demands for refrigeration and air conditioners are expected to grow dramatically in the coming decades. All refrigeration technologies have one thing in common: they contain a refrigerant which changes state, and in doing so, changes temperature. Within the global climate change goals, various regulations like the EU f-gas regulation and the Kigali Amendment of the UN environmental program have been introduced that limit the future use of refrigerants for conventional gas-compression technology with high global warming potentials. The alternatives are often toxic, explosive or inefficient. Magnetic refrigeration is the only alternative cooling technology which would simultaneously eliminate the need for harmful refrigerant gases and reduce the energy requirements, and hence carbon dioxide emissions.

The magnetocaloric effect

The magnetocaloric effect (MCE) originates in the partial alignment of the magnetic moments of the material by an external magnetic field. Therefore a decrease in the entropy of the magnetic moments is observed. When applying a magnetic field under adiabatic conditions, the reduction of the magnetic entropy is compensated by an increase of the entropy of the crystal lattice. This transfer of entropy is causing the warming of the material (step 1). Subsequently, the excessive heat is removed and the material is at the starting temperature again, but in a magnetized state (step 2). By removing the magnetic field again, the opposite effect is observed and the material cools down (step 3). Now, heat can be absorbed from the cooling target (step 4) and the cycle starts from the beginning. A cooling machine can be designed by utilizing the magnetocaloric effect in a cyclic operation.

Materials choice

In order to make this technology ready for a market entry, the development focusses on two fundamental aspects that go hand-in-hand: the development of a well-performing material and the technical implementation of a magnetocaloric cooling cycle in a reliably working device. The main requirements for both sides of our research focus is depicted in the scheme.

The materials considered as promising for magnetocaloric cooling show the largest effects around a phase transition. Second-order transitions like a pure Curie-Temperature show moderate but fully reversible temperature changes over a large temperature regime. Materials with a coupled first-order magnetostructural phase transition exhibit much larger magnetocaloric effects, but due the inherent thermal hysteresis, the cyclic response under frequent magnetic field application is lowered. Our research focusses on the conventional first-order material La-Fe-Si with a small thermal hysteresis and large effects in cyclic magnatic field changes and on the family of Heusler compounds, which exhibit an inverse magnetocaloric effect with a larger thermal hysteresis but also better mechanical and chemical stability.

In order to use the thermal hysteresis in Heusler alloys, a novel concept of alternating magnetic field and pressure application has been proposed, that exploits the full potential of the magnetostructural phase transition and reduces the amount of magnetic field source needed for a cyclic operation. Details can also be found here: www.coolinnov.eu/

Heusler Alloys

Multi-stimuli cooling cycle

Magnetocaloric materials for low temperature cooling

LaFeSi
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