PUBLICATIONS - 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

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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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Airbrushing: A Novel Method for Preparation of High‐Emissivity Black Coating for Infrared Measurements

admin — Fri, 05 Sep 2025 17:52:59 +0000

The article “Airbrushing: A Novel Method for Preparation of High‐Emissivity Black Coating for Infrared Measurements“, published in the journal Advanced Materials Interfaces (IF = 4.4), presents a new method for applying high-emissivity black coatings intended for infrared (IR) temperature measurements.

Researchers from the Laboratory for Refrigeration and District Energy (Process Engineering) collaborated on the study with colleagues from the Jožef Stefan Institute, the National Institute of Chemistry, the Colorado School of Mines, and the University of Barcelona.

Using a simple and cost-effective airbrush system, we developed a procedure for applying a homogeneous and thin black coating layer (minimum thickness of 3 µm), which can be deposited on various shapes and materials—including curved surfaces. The average emissivity of the coating is approximately 0.95 in the 0.5–2.5 µm spectral range, enabling high-precision IR temperature measurements even on surfaces with naturally low emissivity (e.g., metals).

The effectiveness of the method was demonstrated by direct IR temperature measurements of the magnetocaloric effect on a LaFeCoSi magnetocaloric material sample, where the results showed excellent agreement with the manufacturer’s technical data. The coating is fully removable with standard laboratory solvents, allowing temporary application without affecting the sample properties.

This new method combines the advantages of conventional painting and printing techniques and opens up possibilities for more precise thermal characterization of caloric materials and other samples where high emissivity is essential for IR measurements.

Figure: a) Photograph of the airbrush used in this work; b) Schematic representation of coating procedure; c) Average total coating thickness as a function of number of layers; d) Temperature profile of a coated magnetocaloric plate under a cycling applied magnetic field 0 – 1.1 T.

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TCCbuilder: an open-source tool for the analysis of thermal switches, thermal diodes, thermal regulators, and thermal control circuits

admin — Mon, 11 Nov 2024 07:21:40 +0000

Researchers of the LAHDE laboratory of the Faculty of Mechanical Engineering, University of Ljubljana, have developed an open-source tool for 1D modeling of thermal control circuits, TCCbuilder. It is the first tool of its kind that enables quick and easy modeling of thermal control circuits thanks to its graphical user interface and thr associated library of materials used to create thermal control elements.

In addition to the library of materials, there is also a library of already designed thermal control elements from the literature. Researchers from various fields are invited to complement both libraries as well as to add functionality to the tool. Thus, TCCbuilder is not only an indispensable tool for simulations, but also a platform for the exchange of information and networking of researchers in the field of heat transfer and materials.

The tool is presented in the article TCCbuilder: an open-source tool for the analysis of thermal switches, thermal diodes, thermal regulators, and thermal control circuits, which describes the validation and examples of the use of the tool. The article is published in the journal iScience (IF = 4.6), Special Issue: Advanced thermal control: fundamentals and applications.

The article is freely accessible under the following link: https://doi.org/10.1016/j.isci.2024.111263

Summary:

In the area of thermal management, thermal control elements (TCEs) and thermal control circuits (TCCs) are proving to be innovative solutions to the challenges of temperature control and heat dissipation in various applications, ranging from electronic cooling to energy conversion and temperature control in buildings. Their integration promises to improve power density, energy efficiency, system reliability and system life expectancy. With the aim of connecting researchers in the field of thermal management and accelerating the development of TCEs and TCCs, we have developed an open-source TCC simulation tool called TCCbuilder that enables a quick and easy time-dependent 1D numerical analysis of the behavior of TCEs and TCCs. It uses the heat conduction equation to solve temperature profiles in different devices. The TCCbuilder application offers features not previously available with any other TCC modelling tool: a large adjacent library of materials and TCEs as well as a user-friendly graphical user interface (GUI).

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Perspectives and Energy Applications of Magnetocaloric, Pyromagnetic, Electrocaloric, and Pyroelectric Materials

admin — Mon, 16 Sep 2024 05:07:30 +0000

Researchers from the Laboratory for Refrigeration and District Energy (LAHDE), in collaboration with the University of Seville and the University of Cambridge, have published a paper in the journal Advanced Energy Materials (IF=27.6), presenting a perspective on the use of magnetocaloric, pyromagnetic, electrocaloric, and pyroelectric materials for energy conversion. These related technologies, as alternatives to existing ones, have significant potential for improving energy efficiency and the use of environmentally friendly materials and fluids, particularly in the fields of cryogenics, refrigeration, heat pumps, and efficient conversion of low-temperature waste heat into useful work.

The first part of the paper describes the main challenges faced by these materials and their applications, while the second part proposes specific solutions, especially in the area of improving heat transfer between these materials and the working medium in devices. Among these, the authors have introduced a new operational concept for the first time, for which LAHDE holds U.S. and Chinese patents. This concept enables a significant increase in power density, not only for caloric devices but also for devices based on thermoacoustic technologies and mechanical Stirling principles. The article concludes by addressing and opening up entirely unexplored areas, ranging from ultra-high temperature heat pumps to applications beyond our planet.

Open-access paper is available here: https://onlinelibrary.wiley.com/doi/10.1002/aenm.202401739

Abstract:

This perspective provides an overview of the state of research and innovation in the areas of magnetocaloric and pyromagnetic materials, and electrocaloric and pyroelectric materials, including the overlapping sub-areas of multicaloric and multipyro materials that can operate simultaneously under the application of magnetic and electric fields. These materials are critically examined for their potential to revolutionize cooling, heating, and energy-harvesting applications. This perspective first summarizes the state-of-the-art advancements and highlights recent significant developments. Then, it is identified and discussed that the prevailing challenges hindering the widespread adoption of technologies based on these materials. In this context, after consulting with members of the caloric and pyro communities, a technology roadmap is outlined to guide research efforts in overcoming current barriers to applications, with the goal of achieving impactful results by 2040. This roadmap emphasizes the importance of focusing on under-researched materials, novel devices, and application spaces, paving the way for interdisciplinary efforts that can lead to significant reductions in carbon dioxide emissions.

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Response of energy solutions to climate change

lahde — Thu, 06 Jun 2024 06:08:51 +0000

The Slovenian Energy Association (SZE) successfully hosted the SZE 2024 International Conference from June 2 to June 4, 2024, at the Grand Hotel Bernardin in Portorož, Slovenia. This prestigious event was organized in collaboration with Kronoterm, the main sponsor.

Our group presented the latest findings from the 3Diverse project, which focused on reducing the supply and return temperatures within an isolated branch of a district heating network.

The 3DIVERSE project (LIFE21-CET-PDA-3DIVERSE) has received funding from the European Union’s LIFE programme under the grant agreement Nº 101077343.

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Small demonstrator of a thermoelectric heat-pump booster for an ultra-low-temperature district-heating substation

lahde — Sat, 16 Mar 2024 19:57:01 +0000

The article Small demonstrator of a thermoelectric heat-pump booster for an ultra-low-temperature district-heating substation represents, as first in the world, experimental proof which demonstrates the effective use of thermoelectric heat pumps in a 4th or 5th generation district heating systems. The article was published in the journal Applied Energy (IF=11.2).

The investigation was carried out by the researchers of the Laboratory for refrigeration and district energy (Process Engineering) in cooperation with the company Danfoss Trata d.o.o.

One of the most important measures for the energy efficiency of district heating systems is to reduce the supply and return temperatures of the system. The 4th generation district heating systems operate at supply temperatures of 50 to 60 °C, which enables better utilization of renewable and waste heat sources. In the 5th generation, supply/return temperatures are significantly lower (< 30°C). These temperature levels lead to the need for a local increase in the heating temperature (e.g. for the sanitary hot water), which is solved by using auxiliary heat pumps or so-called ˝booster’s˝. Energy-consuming electric heaters or fuel-burning boilers are most often used to raise the temperature, while heat pumps are an alternative. Among the latter, vapor-compression heat pumps are in use, but they use environmentally unfriendly or toxic and flammable refrigerants, are noisy and cause vibrations, which negatively affects the comfort of living in the living environment.

In the published research, the researchers present the development and operation of a demonstration thermoelectric (Peltier) heat pump booster (TE-HP), which can serve as an auxiliary heat pump in the substation of the 4th or 5th generation district heating system. An extensive parametric experimental analysis is presented in terms of heating capacities, flow rates, supply water temperature increase and efficiencies.

The developed TE-HP demonstrates many advantages over the comparable state-of-the-art, namely extreme compactness, silent operation without moving parts and vibrations, high efficiency of operation and the use of environmentally friendly recyclable refrigerants without discharges into the environment. It operates in the range of heating powers up to 650 W and the highest temperature increase of the supply water up to 50 K. At a temperature increase of the supply water by 10-15 K, it operates with a heating coefficient of performance (COP) between 2 and 2.4, which represents the ratio between the obtained heat and the electrical input energy, which significantly outperforms any direct conversion of fuels or electricity to heat, which has a COP ratio < 1.

The article is available at: https://doi.org/10.1016/j.apenergy.2024.122899

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