Periodic Reporting for period 4 - cool innov (Turning the concept of magnetocaloric cooling on its head)
Reporting period: 2022-04-01 to 2022-12-31
The overall contribution of the refrigeration sector to climate change is frequently overlooked and according to the IEA in 2018 “Growing demand for air conditioners is one of the most critical blind spots in today’s energy debate”. Looking at the identified nine selected key technologies (EU commission, 2020), cooling again is not listed. The trajectory of cooling is changing dramatically BUT is not on the agenda. Approximately 10% of total greenhouse gas emissions stem from cooling alone, 20% of the global electricity is used. Roughly 3 billion cooling devices are used for household, commercial and transport refrigeration, air conditioning and heat pumping and the amount in increasing rapidly. Facing this development, it becomes clear that our current cooling technology based on vapour compression cycles needs to be changed.
New solid-state based cooling technologies provide higher energy efficiencies using materials with a phase transition, for which a driving stimulus such as magnetic field, electric field or pressure is applied. During this process the material can heat up or cool down, when exposed by a magnetic field, this is called the magnetocaloric effect (MCE).
Solid refrigerants are non-explosive, non-toxic and easier to recycle. However, the problem of today's magnetocaloric refrigerants is that the largest effects are shown by materials with first-order phase transitions. While they exhibit significant changes in temperature, their inherent hysteresis gives rise to irreversibility and energy losses during cyclic operation hindering full exploitation of the material’s caloric potential.
Magnetocaloric refrigeration has struggled to break into mass markets because it is a relatively expensive technology resulting from the large volume of high-performance permanent magnets required for this technology. Cool Innov rethinks the whole concept of the conventional caloric cooling. Instead of trying to squeeze the best out of magnetocaloric materials in relatively low magnetic fields, we introduce a second stimulus in the form of mechanical load. This allows a significantly improved exploitation of the material’s caloric potential and a reduction of the permanent magnet volume at the same time. A comparison between the conventional and multi-stimuli solid-state cooling cycle is shown in Fig. 1.
One important objective of Cool Innov is the development of existing and discovery of new refrigerant materials that show caloric effects both under the magnetic field and under mechanical load, requiring high magneto- AND elastocaloric response under ideally small stimuli. Consequently, the material needs to be mechanically and chemically stable and composed of non-resource-critical elements.
The next objective is the shaping of the materials into geometry that will allow the most efficient heat exchange during the operation of the thermodynamic cooling cycle, which we tackled by using the concept of additive manufacturing.
Finally, combining our breakthroughs in material physics & science with advanced engineering and processing, we built a worldwide unique testbed that allows the validation of multi-caloric materials in a multi-stimuli cooling cycle.
New solid-state based cooling technologies provide higher energy efficiencies using materials with a phase transition, for which a driving stimulus such as magnetic field, electric field or pressure is applied. During this process the material can heat up or cool down, when exposed by a magnetic field, this is called the magnetocaloric effect (MCE).
Solid refrigerants are non-explosive, non-toxic and easier to recycle. However, the problem of today's magnetocaloric refrigerants is that the largest effects are shown by materials with first-order phase transitions. While they exhibit significant changes in temperature, their inherent hysteresis gives rise to irreversibility and energy losses during cyclic operation hindering full exploitation of the material’s caloric potential.
Magnetocaloric refrigeration has struggled to break into mass markets because it is a relatively expensive technology resulting from the large volume of high-performance permanent magnets required for this technology. Cool Innov rethinks the whole concept of the conventional caloric cooling. Instead of trying to squeeze the best out of magnetocaloric materials in relatively low magnetic fields, we introduce a second stimulus in the form of mechanical load. This allows a significantly improved exploitation of the material’s caloric potential and a reduction of the permanent magnet volume at the same time. A comparison between the conventional and multi-stimuli solid-state cooling cycle is shown in Fig. 1.
One important objective of Cool Innov is the development of existing and discovery of new refrigerant materials that show caloric effects both under the magnetic field and under mechanical load, requiring high magneto- AND elastocaloric response under ideally small stimuli. Consequently, the material needs to be mechanically and chemically stable and composed of non-resource-critical elements.
The next objective is the shaping of the materials into geometry that will allow the most efficient heat exchange during the operation of the thermodynamic cooling cycle, which we tackled by using the concept of additive manufacturing.
Finally, combining our breakthroughs in material physics & science with advanced engineering and processing, we built a worldwide unique testbed that allows the validation of multi-caloric materials in a multi-stimuli cooling cycle.
A first breakthrough was the demonstration of the proof of concept of the multi-stimuli cooling cycle (Gottschall et al., Nature Materials 17, 2018). By alternatingly applying a magnetic field of 1.8 T and a uniaxial stress of 80 MPa to a Ni-Mn-In Heusler alloy we could achieve a cyclic multicaloric effect of -1.2 K. In order to develop materials with a largely enhanced cyclic multicaloric performance, we followed a two-way strategy: optimizing the microstructure of known Ni-Mn-based Heusler alloys to provide higher mechanical stability and exploring novel all-d-metal Heusler alloys with largely enhanced intrinsic mechanical strength by high-throughput screening and process development.
By tailoring microstructures, we found that that a <001> texture is preferable to minimize the critical stresses required. The highest cyclic stability has be achieved by Fe-doping in combination with grain refinement increasing the current values by more than three orders of magnitude. In comparison to the proof-of-concept, we achieved compressive stresses of 300 MPa and 16000 cycles with an elastocaloric effect of -4.5 K without mechanical degradation (Pfeuffer et al., Acta Mater. 221 (2021) 117390).
Going beyond material development, we built a testbed for multi-stimuli cooling cycle including a pulsed magnetic fields source up to 9 T. In consequence, the expensive permanent magnets could be fully replaced by a solenoid which can provide significantly enhanced magnetic fields and therefore higher multicaloric effects. We showed that the transition can follow the high field change rate (Pfeuffer et al., Phys. Rev. Mater. 4, 111401(R) (2020)) and we achieved large effects of 20 K in a magnetic field change of 15 T in Ni-Co-Mn-Ti.
Bridging micro-magnetic simulations and experimental science, we developed a phase field model for known multicaloric Heusler alloys to simulate the thermal hysteresis of magnetic materials in order to evaluate best compounds with tailored hysteresis for the multi-stimuli cooling cycle. To simulate the stress- and field-induced transformation of Heusler alloys with complex microstructures, we implemented a model capable of describing additive manufacturing processes (Yang et al., npj Computational Materials 5, 81 (2019)). We were able to experimentally validate this by successfully producing Ni-Mn-Sn Heusler alloys by direct energy deposition.
By tailoring microstructures, we found that that a <001> texture is preferable to minimize the critical stresses required. The highest cyclic stability has be achieved by Fe-doping in combination with grain refinement increasing the current values by more than three orders of magnitude. In comparison to the proof-of-concept, we achieved compressive stresses of 300 MPa and 16000 cycles with an elastocaloric effect of -4.5 K without mechanical degradation (Pfeuffer et al., Acta Mater. 221 (2021) 117390).
Going beyond material development, we built a testbed for multi-stimuli cooling cycle including a pulsed magnetic fields source up to 9 T. In consequence, the expensive permanent magnets could be fully replaced by a solenoid which can provide significantly enhanced magnetic fields and therefore higher multicaloric effects. We showed that the transition can follow the high field change rate (Pfeuffer et al., Phys. Rev. Mater. 4, 111401(R) (2020)) and we achieved large effects of 20 K in a magnetic field change of 15 T in Ni-Co-Mn-Ti.
Bridging micro-magnetic simulations and experimental science, we developed a phase field model for known multicaloric Heusler alloys to simulate the thermal hysteresis of magnetic materials in order to evaluate best compounds with tailored hysteresis for the multi-stimuli cooling cycle. To simulate the stress- and field-induced transformation of Heusler alloys with complex microstructures, we implemented a model capable of describing additive manufacturing processes (Yang et al., npj Computational Materials 5, 81 (2019)). We were able to experimentally validate this by successfully producing Ni-Mn-Sn Heusler alloys by direct energy deposition.
The fundamental part of research during the project dealt with the discovery of new materials in the known class of Heusler alloys. Instead of searching by experimental trial and error, high-throughput screening for new compounds was developed. These calculations help optimizing the material itself at the atomistic level (Fortunato et al., accepted for publication in Adv. Sci. (2023) & Tanzim et al., accepted for publication in Adv. Funct. Mater. (2023)).
We found that the central part of a multi-stimuli cooling device is a novel mechanically stable heat exchanger fabricated from the best optimized materials by 3D printing. Realizing additively manufactured Heusler alloys has a high impact on both the engineering of more efficient heat exchangers with fine structures and the material science of tuning a magnetocaloric material by different methods affecting the microstructure.
We developed and implemented a phase field model coupled with micromagnetism in a finite-element framework for simulating the magnetocaloric effect and the multi-stimuli concept proposed. This model is a great progress in the theoretical simulation of martensitic phase transitions under different external stimuli. It will in general help to optimize the construction and design of the active material and therefore the overall performance of a magnetic cooling device.
Supported by advanced technologies in the field of engineering and material science, we were able to construct a worldwide unique testbed that can apply pulsed magnetic field and uniaxial stress simultaneously or subsequently in cyclic manners. The direct experimental simulation of the multi-stimuli cycle enables to precisely determine the multicaloric performance of various materials and prompts the use of pulsed fields as magnetic stimulus.
We found that the central part of a multi-stimuli cooling device is a novel mechanically stable heat exchanger fabricated from the best optimized materials by 3D printing. Realizing additively manufactured Heusler alloys has a high impact on both the engineering of more efficient heat exchangers with fine structures and the material science of tuning a magnetocaloric material by different methods affecting the microstructure.
We developed and implemented a phase field model coupled with micromagnetism in a finite-element framework for simulating the magnetocaloric effect and the multi-stimuli concept proposed. This model is a great progress in the theoretical simulation of martensitic phase transitions under different external stimuli. It will in general help to optimize the construction and design of the active material and therefore the overall performance of a magnetic cooling device.
Supported by advanced technologies in the field of engineering and material science, we were able to construct a worldwide unique testbed that can apply pulsed magnetic field and uniaxial stress simultaneously or subsequently in cyclic manners. The direct experimental simulation of the multi-stimuli cycle enables to precisely determine the multicaloric performance of various materials and prompts the use of pulsed fields as magnetic stimulus.