Enhanced heat transfer for high-performance electrocaloric cooling

Research topic:

Conventional refrigeration systems use refrigerant gases, whose choice results from a trade-off between efficiency, safety, availability, and environmental impacts. In particular, the third-generation refrigerants (hydrofluorocarbons or HFCs) constitute significant greenhouse gases contributors (GHG), alone responsible for approximately 3% to 6% of global GHG emissions. The 4^th generation of refrigerant gas remains awaited; the current trend is to return to the 1^st generation (gas called natural refrigerant gas) with their knowns drawbacks, or to use hydrofluoroolefins (HFO) whose environmental impact is already documented.

Changing completely the paradigm, solid refrigerants may offer an elegant alternative. It is based on materials called “caloric materials”, in the sense that their entropy may be driven by an external loading like an electromagnetic field or a mechanical stress or pressure.

In this category, CooliSH project focuses on the use of electrocaloric materials for refrigeration based on lead-free formulations using abundant elements. The working principle is based on a regenerator, made of electrocaloric plates, through which a fluid circulates periodically between a hot reservoir and a cold reservoir. The oscillation of the fluid, combined with periodic excitation of the regenerator, creates a temperature gradient between the two sides of the regenerator. Therefore, connecting the hot reservoir to a heat sink at a constant temperature allows to cool the cold reservoir. This refrigeration system faces three major challenges: (i) the complex problem of heat and mass transport, (ii) electrical energy management, and (iii) the difficulties of scaling up and integrating the functions required for refrigeration into a holistic approach.

This PhD offer addresses the first challenge of the heat transfer enhancement, and the thermodynamic cycles involved in electrocaloric refrigeration. The refrigeration process using caloric materials includes a heat transfer problem in periodic regime with body sources. The physical mechanisms behind the temperature span generation (temperature difference between heat sink and heat source) remain unclear. To go beyond current empirical approaches, a fast-computing analytical model of the solid-liquid heat exchange and longitudinal heat flux generation is one of the objectives of the project, by extending previous model [3] to the electrocaloric case with arbitrary waveforms. It will guide the research in drawing up the relevant design guidelines and in assessing the impact of the operating conditions.

The most critical aspects that come next are the enhancement of the cooling power. In fact, from a theoretical point of view, defining a convective heat transfer coefficient along a plate with unsteady or inhomogeneous boundary conditions required to go behind the classical ratio between heat flux and temperature difference [4]. But, this method is no longer valid in presence of an unsteady fluid flow as it is the case in this work. Therefore, the first question is how: to define the convective heat exchange between the electrocaloric palate and the periodic flow ? The second question concerns the increase in heat transfer between the laminar flow and the plates by controlling the geometry, the frequencies and the phase shift between the fluid flow and the heat source in the plates.

The PhD work will include the development of a test bench that mimic the refrigeration system in order to analyze the heat exchange between the electrocaloric plates and the fluid flow. An numerical study including 2D-semi-analytical modeling and 3D numerical modeling will be carried out to guide the design of the bench and to analyze specific phenomena as well. s Finally, the PhD work will define the most relevant configurations that will be implemented into an electrocaloric cooler by another partner of the CooliSH project.

References

[1] M. Ozbolt, A. Kitanovski, J. Tusek, et A. Poredos, « Electrocaloric refrigeration: Thermodynamics, state of the art and future perspectives », International Journal of Refrigeration-Revue Internationale Du Froid, vol. 40, p. 174-188, 2014, doi: 10.1016/j.ijrefrig.2013.11.007.

[2] J. Li et al., « High cooling performance in a double-loop electrocaloric heat pump », Science, vol. 382, no 6672, p. 801 805, 2023, doi: 10.1126/science.adi5477.

[3] G. Sebald, A. Komiya, J. Jay, G. Coativy, et L. Lebrun, « Regenerative cooling using elastocaloric rubber: Analytical model and experiments », Journal of Applied Physics, vol. 127, no 9, p. 094903, 2020, doi: 10.1063/1.5132361.

[4] A. Degiovanni, and B. Rémy. « An Alternative to Heat Transfer Coefficient: A Relevant Model of Heat Transfer between a Developed Fluid Flow and a Non-Isothermal Wall in the Transient Regime ». International Journal of Thermal Sciences, vol. 102, p. 62‑77, 2016, doi: 10.1016/j.ijthermalsci.2015.10.036.

Working conditions:

Working place

The PhD will take place at INSA Lyon at LGEF and CETHIL laboratories. In the frame of the double degree program, a mobility of 1 year to Tohoku University (Japan) is planned.

Tentative schedule

October 2026 – September 2027 (INSA Lyon, France)

• Bibliographic survey on caloric cooling with a focus on heat transfer issues
• Developpement of experimental techniques to quantify heat transfer effectiveness of in a parallel plate configuration of solidliquid heart transfer in periodic regime.
• 3D simulation of the heat transfer process using finite element and computational fluid dynamics solvers.

October 2027 – September 2028 (Tohoku University, Japan)

• Experimental quantification of the heat transfer effectiveness, spatial determination of temperature and fluid velocity fields

October 2028 – September 2029 (INSA Lyon, France)

• Finalization of the experimental and theoretical work
• Articles and PhD writing

Financial conditions

The PhD candidate will be basically based in Lyon and will have to move to Japan for the 12 months research work at Tohoku University in the form of one long-term business trip. The incurred extra expenses (transportation and lodging in Japan) will be fully covered by the laboratory.

During the stay in Japan, the PhD candidate will benefit from the international student installation assistance from Tohoku University (visa application, lodging finding…).

According to the double-degree agreement between INSA Lyon and Tohoku University, the PhD candidate will be totally exonerated from Tohoku University tuition fees for the whole duration of the PhD.

The LGEF (Laboratory of Electrical Engineering and Ferroelectricity) is an associated research team (EA682) of INSA Lyon (National Institute of Applied Sciences). It comprises around thirty people, including 17 permanent staff. It occupies the third floor and part of the basement of the Gustave Ferrié building. Part of the premises is classified as a restricted access zone (ZRR).

The laboratory conducts research focused on "multiphysics coupling in conversion materials and their use in dedicated systems." Its activities draw on several disciplines, ranging from electroactive materials (ceramics, single crystals, polymers, composites, etc.) to the systems and functions developed: energy harvesting, vibration control, autonomous and self-powered devices, flexible actuators, and much more.

By the nature of its research, the laboratory contributes to the major societal challenges of the Institute: "Energy for sustainable development", "Transport: structures, infrastructure and mobility" and "Global health and bioengineering".

The PhD candidate must hold a master’s degree or equivalent, preferably in the field on energy, mechanics or heat transfer.