PUBLICATIONS - Laboratory for refrigeration and district energy

Thermomagnetic conversion of low-grade waste heat: the role of permanent magnet selection

admin — Mon, 31 Aug 2026 08:39:29 +0000

A substantial amount of energy used in industrial and other thermal processes is ultimately released into the environment as low-grade waste heat. Recovering even a fraction of this energy and converting it into useful electricity could contribute to improved overall energy efficiency and a more sustainable use of energy resources. However, efficient conversion of heat available at relatively low temperatures remains a significant technological challenge.

Thermomagnetic energy conversion offers an alternative approach for harvesting low-grade waste heat. It exploits the strong temperature dependence of the magnetic properties of thermomagnetic materials. By cyclically heating and cooling the material around its magnetic transition temperature, changes in magnetization can be used to induce variations in magnetic flux and ultimately generate electrical power.

In our new study, published in iScience (Cell Press), we investigate an important component of a thermomagnetic generator (TMG) that has received comparatively little attention: the permanent magnets used as the magnetic-field source. Their properties directly influence the magnetic field available to the thermomagnetic material and therefore the performance of the complete energy-conversion system. At the same time, permanent-magnet selection also involves important considerations related to material availability, cost, thermal stability, and the use of critical raw materials.

A numerical model of the thermomagnetic generator was used to systematically compare configurations employing three technologically relevant permanent-magnet materials: neodymium-iron-boron (NdFeB), Alnico, and Ferrite. The analysis considers the interaction between the magnetic field source and the thermomagnetic material and evaluates the resulting electrical response of the generator, including output power, voltage, and current during cyclic operation.

The results demonstrate how strongly the choice of permanent magnet can influence the performance of a thermomagnetic generator. While high-performance permanent magnets can provide stronger magnetic fields and consequently higher electrical output, rare-earth-free alternatives such as Alnico and Ferrite offer different combinations of magnetic properties, temperature stability, material availability, and cost. The comparison therefore highlights that the optimal magnetic-field source cannot be selected solely on the basis of magnetic strength, but should be considered as part of the overall thermomagnetic-generator design.

The study contributes to the development of more efficient and practically viable thermomagnetic energy-harvesting systems and provides guidelines for selecting permanent magnets according to the requirements of a specific application. In the longer term, such systems could provide an additional pathway for converting currently unused low-temperature waste heat into electricity, particularly in applications where conventional heat-to-power technologies are difficult to implement.

The article is available in iScience: link to article.

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Phase change materials for more efficient household refrigeration

admin — Tue, 18 Aug 2026 08:27:13 +0000

A new review paper, Phase Change Materials for Performance Enhancement in Household Refrigeration: A Review, has been published in Strojniški vestnik – Journal of Mechanical Engineering. The paper explores the use of phase change materials (PCMs) to improve the performance of household refrigeration systems.

PCMs can store and release thermal energy during a phase transition, thereby increasing the thermal inertia of a refrigeration system. When properly selected and integrated, they can contribute to more stable temperatures, improved system efficiency, reduced peak loads, and longer preservation of suitable temperatures during power outages.

The paper presents key criteria for PCM selection and reviews different integration strategies, including PCM placement at the evaporator, condenser, inside the refrigerated compartment, and in combined configurations. The review also shows that commercially available PCMs already cover temperature ranges relevant to household refrigeration. Achieving the desired benefits, however, strongly depends on selecting an appropriate phase-change temperature and integrating the material correctly into the refrigeration system.

The work was carried out within the GreenTech project and is the result of collaboration between researchers from the Faculty of Mechanical Engineering, University of Ljubljana, and Gorenje, Hisense Europe.

The paper was authored by Urban Tomc, Katja Klinar, Luka Porenta, Marko Kralj, Tomaž Bregar and Andrej Kitanovski.

Paper: Phase Change Materials for Performance Enhancement in Household Refrigeration: A Review
DOI: 10.5545/sv-jme.2026.1670

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Comprehensive thermodynamic screening of low-GWP refrigerant mixtures for micro-scale vapour compression refrigerators and heat pumps

admin — Tue, 14 Jul 2026 18:19:52 +0000

As the demand for high-performance micro and power electronics grows, micro-scale vapour compression refrigeration (VCR) systems and heat pumps (VCHP) have emerged as critical technologies for active heating and cooling. However, the progressive phase-down of hydrofluorocarbon refrigerants under international environmental regulations, necessitates the identification of low-global warming potential alternatives specifically optimized for the unique constraints of small-scale hardware, such as limited compressor displacement and low refrigerant charge.

This study presents a comprehensive and systematic thermodynamic screening of approximately 30,000 refrigerant compositions, including pure fluids, binary, and ternary mixtures, specifically aimed at three key application scenarios: domestic and commercial micro-appliances, personal cooling systems for hostile environments, and reversible heat pumps for electric vehicle cabin and battery management. Using a multi-criteria decision-making approach based on the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) method to maximize simultaneously coefficient of performance and volumetric heating and cooling capacity, optimal candidates were identified across four practical scenarios: high-performance, safety-critical – low-flammability (A1), low-pressure, and a combined low-pressure/non-flammable scenario.

Findings demonstrate that added complexity in ternary mixtures offer no significant thermodynamic advantage over simpler binary blends for micro-scale applications. Final recommendations for next-generation micro-VCR/VCHP systems include: R1270 and R290/RE170 (60/40)% for maximum performance; R1234yf/R1224yd(Z) (72/28)% for safety-critical (A1) applications; RE170/R600 (65/35)% and R600/R1270 (28/72)% for natural, low-pressure operation; and R1234ze(E)/R1233zd(E) (72/28)% or R1234ze(E)/R1224yd(Z) (68/32)% for combined constraints. This study establishes a structured roadmap to navigate the complex landscape of sustainable refrigerant selection in the micro-VCR/VCHP domain.

Link: https://doi.org/10.1016/j.applthermaleng.2026.132282

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Direct Characterization of Electrocaloric and Magnetocaloric Responses in Multicaloric Composite Films

admin — Wed, 01 Jul 2026 05:57:47 +0000

Researchers from the Jožef Stefan Institute, the University of Ljubljana, the University of Barcelona, IFW-Dresden and collaborating institutions have published the article Direct observation of room-temperature electrocaloric and magnetocaloric responses in multicaloric composite films.

The study combines the electrocaloric ceramic PMN-10PT with the magnetocaloric La-Fe-Si-Co (LFSC) alloy. Using Powder Aerosol Deposition (PAD), the team fabricated dense multicaloric composite films approximately 5 μm thick. PAD enables the fabrication of functional thick films entirely at room temperature and offers an attractive route for integrating advanced caloric materials into future cooling devices.
A key achievement of the work is the direct characterization of both electrocaloric and magnetocaloric responses in the same multicaloric composite film at room temperature. The composite films exhibited an approximately 32 % higher electrocaloric temperature change compared to pure PMN-10PT films. The enhancement is attributed to Maxwell–Wagner polarization effects induced by the embedded magnetocaloric particles.
In addition, the magnetocaloric response was directly measured in the same composite films, experimentally confirming their multicaloric functionality and demonstrating the feasibility of combining electrocaloric and magnetocaloric effects within a single aerosol-deposited structure.
The results provide new insights into the development of multifunctional caloric materials and demonstrate the potential of multicaloric composite films for future solid-state cooling and micro-cooling technologies targeting electronics, sensors, and other miniaturized systems.

Link: https://www.sciencedirect.com/science/article/pii/S0955221926004607

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New article: Roadmap for electrocaloric films characterization

admin — Thu, 28 May 2026 08:06:32 +0000

The article Roadmap for electrocaloric films characterization, published in iScience (Cell Press), presents a numerical investigation of substrate and infrared (IR) coating effects on the characterization of electrocaloric thick-film structures for future cooling applications.

Researchers from the Laboratory for Refrigeration and District Energy at the Faculty of Mechanical Engineering, University of Ljubljana, in collaboration with the Jožef Stefan Institute, investigated how substrate thermal properties and high-emissivity black coating thickness influence the accuracy of infrared temperature measurements of electrocaloric materials. The aim of the study was to establish guidelines for optimized characterization of electrocaloric structures at the micro- and nano-scale.

Using numerical simulations, the researchers demonstrated that substrates with low thermal effusivity and low thermal conductivity significantly reduce heat losses during measurements, enabling more accurate determination of the electrocaloric response. In addition, the influence of electrocaloric film thickness and black coating thickness on the IR correction factor was systematically analyzed. The results show that low single-digit correction factors can be achieved when the electrocaloric layer is thicker than the coating.

The study provides important guidelines for reliable electrocaloric material characterization and offers a roadmap for the design of future micro-scale caloric cooling systems..

Link: https://www.cell.com/iscience/fulltext/S2589-0042(26)01340-4

The post New article: Roadmap for electrocaloric films characterization first appeared on Laboratory for refrigeration and district energy.

Ultra-high-temperature electrocaloric heat pump

admin — Mon, 05 Jan 2026 09:01:01 +0000

Researchers from the Laboratory for Refrigeration and District Energy (LAHDE) have published first study on high-temperature electrocaloric heat pump in Journal of Energy conversion and management, one of the world’s leading scientific journal in the field of energy.

Nearly a quarter of waste heat is generated at temperatures exceeding 300 °C, yet conventional heat pump technologies remain inefficient in recovering and utilizing heat from such high temperature sources. In this study, we introduce a novel electrocaloric heat pump designed to operate with heat sources and sinks at ultra-high temperatures. The proposed heat pump system incorporates an active electrocaloric regenerator featuring PbZr_0.52Ti_0.48O₃ (PZT) epitaxial thin-film multilayers, which exhibit a superior electrocaloric adiabatic temperature change of 11.03 K at 402 °C (675 K), and pressurized helium gas as the working fluid. We conducted a parametric analysis to simulate the performance of the ultra-high-temperature electrocaloric heat pump and identified the optimal conditions for single- and multi-stage configurations. The single-stage setup achieved a maximum heating COP of 7.8 at a 30 K temperature span across the regenerator at a heat source temperature of 660 K. In comparison, the multi-stage configuration yielded the highest heating power 113.3 W/kg_EC and a max. COP 2.7 between 630 and 690 K. This study established a foundation for notable advancements in ultra-high-temperature heat pumps by employing electrocaloric energy conversion.

Link: https://www.sciencedirect.com/science/article/pii/S019689042501338X

The post Ultra-high-temperature electrocaloric heat pump first appeared on Laboratory for refrigeration and district energy.

Emerging opportunities for high-temperature solid-state and gas-cycle heat pumps

admin — Sun, 21 Dec 2025 12:51:38 +0000

Researchers from the Laboratory for Refrigeration and District Energy (LAHDE), together with an international team, have published a study in Nature Energy (Springer Nature), the world’s leading scientific journal in the field of energy. The paper offers the first comprehensive assessment of an often-overlooked opportunity: alternative high-temperature heat-pump technologies that could replace inefficient fossil-fuel combustion and direct electric heating in industry and the energy sector. Doing so could significantly reduce energy use, greenhouse-gas emissions, and thermal pollution.

Heat lies at the core of the energy challenge. Around 50% of final energy consumption is used for heating and cooling, and roughly 50% of final energy is ultimately released as waste heat—heat that is frequently discharged into the environment via cooling towers, the warming of rivers or seawater, and other outlets. The study shows that high-temperature heat pumps can capture this waste heat and upgrade it efficiently to much higher temperature levels, suitable for demanding industrial processes.

Today’s commercial high-temperature heat pumps are typically limited to about 250 °C, whereas many industrial processes require higher temperatures—up to, or even beyond, 1000 °C. The newly published research therefore systematically presents and compares alternative approaches that could enable a step change: caloric, thermoelectric and thermoacoustic technologies, as well as mechanical processes based on Stirling and Brayton cycles.

Link: https://www.nature.com/articles/s41560-025-01908-4

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Numerical study of macroscopic thermal diodes: influence of interface topography and contact resistance

admin — Mon, 13 Oct 2025 13:02:34 +0000

We continue our exploration of solid-state thermal diodes – a compact, fully passive asset for next-generation thermal management.

Our latest paper, “Numerical study of macroscopic thermal diodes: influence of interface topography and contact resistance,” has just been published in iScience (IF 4.1).

🔍 Key insights:

Interface topography alone has little effect on the rectification factor when contact resistance is negligible. However, it can extend the temperature range of rectification by up to 10 K.

Constant contact resistance tends to reduce rectification. However, when temperature-dependent contact resistance is introduced at low temperatures, rectification can be boosted significantly. In one case, from nearly zero to nearly unity.

These findings provide new guidelines for optimizing solid-state thermal diodes and open up exciting possibilities for practical thermal management applications.

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AI-Driven Monte Carlo Uncertainty Analysis of Curie Temperature Effects on Active Magnetic Regenerator Performance

admin — Mon, 06 Oct 2025 08:51:19 +0000

In the paper AI-Driven Monte Carlo Uncertainty Analysis of Curie Temperature Effects on Active Magnetic Regenerator Performance, published in the International Journal of Refrigeration (IF = 3.8), researchers from the Laboratory for Refrigeration and District Energy revisited the methodology for analyzing the impact of statistical variations in Curie temperature on the performance of multilayer magnetocaloric regenerators, as previously established in a recent publication [link].

The study was conducted in collaboration with colleagues from the Federal University of Santa Catarina (Brazil), this time using their numerical model of a multilayer active magnetic regenerator (AMR) combined with machine learning. Instead of second-order materials, the study focused on first-order LaFeSiH magnetocaloric materials, which are even more sensitive to Curie temperature distributions.

Once again, the study revealed that Curie temperature deviations above 1 K drastically reduce the probability of achieving the target cooling power. Given current margins provided by MCM suppliers (standard deviations between 1.5 and 2 K) and a typical AMR layer count (10 to 15), the cooling power would need to be oversized by 30% to 80% to ensure a 90–95% probability of meeting performance targets. In practice, this would require oversized magnets and regenerators, significantly increasing production costs.

Thus, large-scale production of magnetic refrigeration devices with current LaFeSiH materials is unfeasible, as quality assurance standards would require much tighter control over Curie temperature uncertainty than currently offered by manufacturers. For a successful market deployment of magnetocaloric technology, MCM manufacturers will need to prioritize reducing Curie temperature variability in their materials.

Figure: a) Schematic representation of a multilayer AMR and the Curie temperature deviation. Solid lines represent the original curves and dashed lines represent the ones achieved by the manufacturing process, b) Effect of the uncertainty of the Curie Temperature on the performance of the 10 layer AMR, c) Cumulative Distribution Function for the 10 layer AMR, d) Effect of the certainty level on the achieved performance of the 10 layer AMR.

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Influence of Layering and Curie Temperature Uncertainty on the Performance of Magnetocaloric Regenerators

admin — Fri, 19 Sep 2025 14:42:37 +0000

In the paper Influence of Layering and Curie Temperature Uncertainty on the Performance of Magnetocaloric Regenerators, published in Advanced Functional Materials (IF = 19.0), researchers from the Laboratory for Refrigeration and District Energy (Faculty of Mechanical Engineering, University of Ljubljana) investigate how statistical variations in Curie temperature affect the performance of multilayer active magnetic regenerators (AMRs). We conducted the research in collaboration with colleagues from the University of Santa Catarina (Brazil) and the Technical University of Denmark.

Magnetic refrigeration is a promising alternative to vapor-compression cooling, offering better energy efficiency and lower environmental impact. However, the narrow operational temperature window of magnetocaloric materials (MCMs) and the use of rare-earth elements are major limitations. Layering several MCMs with different Curie temperatures can help span broader temperature ranges—but introduces sensitivity to material variability.

Using a 1D numerical model and radial basis function neural networks, the team analyzed how Curie temperature uncertainties affect the cooling power of multilayer AMRs made from La-Fe-Co-Si alloys. Results reveal that while more layers improve peak performance, they also increase sensitivity to manufacturing inconsistencies. Even standard deviations above 1 K can significantly reduce the probability of reaching desired cooling targets.

This work underscores the need for precise control over MCM properties and contributes essential insights into designing robust and commercially viable magnetocaloric systems.

Figure: Probability of all the Curie temperatures being distributed such that the normalized cooling power would be equal to or greater than 0.9 relative to the standard deviation and at three different temperature spans for different numbers of MCM layers.

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