Solid-state cooling with pressure: development of barocaloric cooling device (COOL PRESS)

Objective 1: Investigation of principles for hydrostatic pressure generation and pressure work recovery for barocaloric (BC) technology

Activities performed:

Within Work Package 1 (WP1), we conducted a comprehensive review and analysis of pressure generation systems and pressure work recovery mechanisms. Two most promising approaches were identified: (i) an oscillating pressure intensifier and (ii) a mechanical piston-based pressure generator.

The oscillating pressure intensifier, based on a hydraulic pump, was theoretically analyzed and found to have limited efficiency due to losses in the primary pump and limited fluid displacement at high pressures (~2000 bar). A mechanical piston-based pressure generator proved to be a more suitable solution. Several prototypes were developed, capable of reaching pressures up to ~4000 bar and compatible with use in a universal testing machine for direct BC material characterization (WP2).

Based on an analytical model (Mathematica), we also developed an innovative camshaft-based drive system powered by an electric motor, enabling synchronized loading of phase-shifted piston generators and BC regenerators. The key innovation is the optimized cam profile adapted to the mechanical response of the materials, enabling efficient pressure work recovery and approximately constant torque operation.

Results:
A mechanical system for high-pressure generation and efficient pressure work recovery was developed and experimentally validated. The system was tested using springs simulating BC material response. Results show near-constant torque operation and high work recovery efficiency (>70%), representing a significant advancement for high-power applications.

The developed solution represents the first known drive system for mechanocaloric devices enabling efficient operation under very high forces with high energy efficiency. A patent application (PCT) [1] has been filed, and a scientific paper is under review in Nature Communications [2].

[1] A. Žerovnik, J. Tušek; DRIVING SYSTEM FOR MECHANOCALORIC HEATING OR COOLING APPARATUS, PCT Application No. PCT/EP2026/051802

[2]A. Žerovnik, S. Dall’Olio, S. Krašna, Ž. Ahčin, J. Tušek; Efficient Mechanocaloric Drive System Enabled by Constant-Torque and Work-Recovery Design; Under review in Nature Comunications, 2026

Objective 2: Design of an experimental setup for direct BC effect measurement and characterization of promising BC materials

Activities performed:

Within WP2, we reviewed existing BC materials and identified the most promising candidates based on BC effect magnitude, pressure range, hysteresis, and stability.

Several experimental systems were developed for direct BC effect measurements (partly in WP1), including piston-based and mechanical pressure generator systems. A final experimental setup was developed enabling direct characterization through simultaneous measurement of pressure, deformation (LVDT), and adiabatic temperature change (thermocouples). The system was calibrated and validated.

A mechanical model of the setup (Matlab) was also developed for measurement correction. Due to cyclic operation at high pressures (>2 kbar), sealing challenges were encountered (piston, thermocouples), which have largely been resolved, though full testing on all materials is still pending.

Therefore, indirect measurements and specific heat measurements were conducted in collaboration with the Polytechnic University of Barcelona on selected materials (silicone rubber, tris-hydroxymethylethane, 1,3-dimethyladamantane). A paper comparing direct and indirect methods is in preparation.

Additionally, the potential of BC materials for solid-state thermal energy storage was identified and studied in Cell Reports Physical Science [3].

Results:
An advanced experimental platform for direct BC characterization was established, enabling simultaneous measurement of pressure, deformation, and adiabatic temperature change.

Pressures up to 4 kbar were achieved. Silicone rubber showed reversible adiabatic temperature changes of ~15 K at 1.5 kbar and up to ~23 K at 4 kbar, among the highest reported values.

Based on combined direct and indirect measurements, key thermodynamic properties were determined (also using a phenomenological model in WP3) and used as input for numerical modeling of the BC regenerator (WP4).

[3] Ž. Ahčin, A. Kitanovski, J. Tušek; Latent thermal energy storage using solid-state phase transformation in caloric materials; Cell Reports Physical Science, 2024; 5

Objective 3: Analysis of mechanical behavior of BC materials under hydrostatic pressure

Activities performed:

Within WP3, visco-hyperelastic models were reviewed, focusing on silicone rubber. A general 3D visco-hyperelastic model based on multiplicative decomposition was selected.

Material parameters were obtained experimentally (creep, stress relaxation, shear modulus). A nonlinear continuum mechanics model was developed for large deformations and real geometries.

Additionally, a phenomenological viscoelastic model based on the Kelvin–Voigt model was developed using experimental data (WP2), capturing hysteresis and energy dissipation as functions of pressure and temperature. Combined with Maxwell relations, it enables calculation of full BC properties for use in WP4.

Results:
An advanced numerical model for mechanical response at high pressures and large deformations was developed. It enables prediction of stress distributions and optimization of loading conditions.

The phenomenological model allows fast engineering predictions. Results provide a basis for optimizing BC element geometry and serve as input for regenerator modeling (WP4).

Objective 4: Numerical analysis of different active BC regenerators

Activities performed:

Within WP4, a numerical model of the BC regenerator was developed for both direct and indirect heat transfer configurations.

The model is based on energy conservation and includes governing equations for BC material, heat transfer fluid, and (for indirect transfer) the housing. It uses input data from WP2 and enables analysis of materials, geometry, and operating conditions.

Due to delays in WP2, optimization is still ongoing. For the selected indirect configuration, a regenerator housing was designed in Ansys Multiphysics, capable of withstanding up to 2 kbar.

Results:
A numerical model enabling performance prediction (power, temperature span, COP) was developed. Structural design of the housing was validated.

Objective 5: Design, construction and testing of an active BC regenerative device

Activities performed:

Within WP5, a BC device was designed based on WP1–WP4 results. The experimental setup was upgraded with a new actuator enabling larger displacements.

Prototype housings were developed. Due to sealing challenges, a stepwise approach was used, first validated in WP2. Final system assembly is ongoing.

Results:
A complete BC device concept (indirect heat transfer) was developed. Testing is pending due to sealing challenges, but the system is in final manufacturing stage.

Conclusion and project impact

Most project objectives have been successfully achieved, with results exceeding expectations, particularly in experimental validation and drive system development. Objectives 4 and 5 are ongoing with minor delays, but key foundations are established.

The COOL PRESS project was conceived as a foundational step toward next-generation cooling technologies based on the BC effect. A key strategic goal—positioning the research group internationally—was fully achieved.

Based on strong results and active dissemination, the team was invited to join the EIC Pathfinder Challenge project FROSTBIT (~€4M total, ~€500k for the team), where it leads the development of a BC regenerator based on advanced spin crossover (SCO) materials.

This confirms that COOL PRESS results are recognized as high-impact at the European level and represent a strong foundation for further development toward higher TRL and potential industrial implementation.