10 mai 2017

Future high-efficiency plants converting concentrated solar energy into electricity or solar fuels will require a heat receiver in which thermal transfers will be optimized to achieve maximum conversion efficiency and high temperatures (> 700 ° C). Among the technologies studied, porous ceramic receivers and exchangers have the advantage of resisting high temperatures and of increasing transfers between the heat transfer fluid and the solid matrix. In order to achieve high thermal efficiency and to improve the understanding of limiting phenomena, the optimization and intensification of heat transfer requires an accurate model of the conversion efficiency dependencies to the geometric, thermophysical and thermoradiative properties of the porous solid phase. The thesis aims to develop a reduced model of the porous medium whose parameters will be obtained through the analysis of the random paths used by the Monte Carlo Symbolic (MCS) statistical method to solve coupled thermal transfers. The model will then be used to optimize the receivers and the porous exchangers according to the operating conditions and their geometries.

The conversion of concentrated solar energy into domestic or industrial electricity is one of the most promising routes for the production of renewable energy in the 21st century. Future high-efficiency solar power plants will have a heat receiver (or thermochemical reactor) installed at the top of a 100-meter-high tower, surrounded by tens of thousands of segmented mirrors at ground level (the heliostats), tracking the sun and concentrating the rays inside the receiver. Among the remaining scientific and technological challenges, the optimization of heat transfers in the receiver to achieve maximum conversion efficiency is currently a major challenge. For solar thermal power plants, the current trend (e.g. the US SunShot research program) is to increase the temperature of the heat transfer fluid (> 500 ° C) in the receivers (Figure 1) and searching for storage devices at high temperature and low cost. For thermochemical solar facilities, which have cycles with high-temperature chemical reactions to produce hydrogen or synthesis gas, cycling and material resistance together with the thermal efficiency of the cycle are the main current research topics.

In order to increase the thermal efficiency, the challenges associated with the modeling and simulation of such high temperature receivers and reactors include taking into account energy and species transfer processes that are coupled at different scales of complex 3D geometry of receivers and reactors.

The applications targeted by this thesis are atmospheric air volumetric receivers [1] (Figure 1) and storage exchangers [2] (Figure 2) for thermochemical cycles, both of which involve porous (open-cell) medium of ceramic foam.

Currently, coupled transfer models using the hypothesis of equivalent porous media (and defining effective properties difficult to know) are used outside their domain of validity. Indeed, the effective properties used are only valid from a certain scale, beyond the representative elementary volume, which extends over 5 to 10 diameters of porous cells [3]. For volumetric air receivers and storage exchangers, the predominant phenomena occur on a smaller scale in the first 5 pores. For example, for the storage exchanger of FIG. 2, the porous medium has a thickness in the annular space which is smaller than the thickness required for the validity of the effective properties (3 thickness diameters between the two tubes). The objective of the thesis is therefore to answer this shortcoming by studying finely the coupled transfers thanks to their detailed resolutions in representative 3D geometry (for example the one represented in FIG. 3). After a study of the predominant transfer mechanisms (which can be controlled by, for example, a change in geometry or materials), a reduced model of coupled transfers in the porous material will be developed for use in a performance optimization study. The use of the Symbolic Monte Carlo Method [4] will help to achieve this goal and will provide an understanding of the mechanisms of heat transfer and material limiting the conversion efficiency.

A simulation tool for the heat transfer in complex 3D geometry using the Monte-Carlo method will be used. This tool was partly developed by a Toulouse based company, MESO-STAR SAS, for PROMES during a previous project (ANR SEED OPTISOL) whose use and valorization will be effective thanks to this thesis work (code Star-Therm Monte Carlo).

The first objective of the thesis is to develop a reduced model of coupled thermal heat transfer in a porous medium using an accurate statistical model using the Symbolic Monte Carlo method.

The second objective is the optimization of receivers and porous exchangers according to their operating conditions and their geometries.

This thesis topic will bring new knowledge and new numerical tools for the study and identification of the dominant mechanisms limiting the conversion efficiency in porous receivers and exchangers.

The research will begin with a bibliographic study on the use of statistical methods in thermal heat transfer (conduction, convection and radiation). Training on the Star- Therm code developed by MESO-STAR SAS will be followed in their premises in Toulouse to ensure an effective learning.

The type of porous medium (receivers and porous exchangers), operating conditions and relevant observables to be calculated will first be defined and the associated 3D geometries will be constructed (with CAD software and technical service support if necessary) .

A statistical study of the influence of radiative transfers on coupled transfers will be carried out using a statistic on Monte-Carlo paths from which the sources contribute to the observable. The interpretation and analysis of the statistical information will be made possible by the implementation of the Symbolic Monte Carlo method whose objective will be to calculate the parameters of the reduced model of the coupled energy transfers in the porous medium. This study will require adapting the Star-Therm code to provide the necessary statistical information. The modifications of Star-Therm will be carried out in partnership with the engineers of MESO-STAR.

In order to significantly improve the thermal efficiency, and based on the statistical tool, an optimization of the process parameters will be carried out using the analysis tool developed, after, for example, a modification of the boundary conditions or thermophysical or thermoradiative properties and possibly local geometry. This part consists of interpreting the information obtained from the simulations and analyzing the mechanisms of the preponderant heat transfers related to an application and proposing an optimal solution enabling to increase the thermal efficiency. The solution will be validated by a simulation (and possibly by an experimental measure).

[1] S. Mey-Cloutier, C. Caliot, A. Kribus, Y. Gray, G. Flamant, Experimental study of ceramic foams used as high temperature volumetric solar absorber, Solar Energy, 136, pp 226- 235, 2016.

[2] A. Banerjee, R. Bala Chandran, J.H. Davidson, Experimental investigation of a reticulated porous alumina heat exchanger for high temperature gas heat recovery, Applied Thermal Engineering, 75, pp 889-895, 2015.

[3] A. Kribus, Y. Gray, M. Grijnevich, G. Mittelman, S. Mey-Cloutier, C. Caliot, The promise and challenge of solar volumetric absorbers, Solar Energy, 110, pp 463-481, 2014.

[4] M. Galtier, M. Roger, F. André, A. Delmas. A Symbolic approach for the identification of radiative properties. J. of Quantitative Spectroscopy and Radiative Transfer, 196, pp 130-141, 2017.

Co-supervision (direction) of the thesis:

Cyril Caliot (Chargé de Recherche CNRS, Habilité à Diriger des Recherches)

+33 4 68 30 77 44,

Laboratoire Procédés, Matériaux et Energie Solaire (PROMES), 7 rue du Four Solaire, 66120 Font-Romeu-Odeillo-Via.Mouna El Hafi (Maître-Assistante, Habilité à Diriger des Recherches) +33 5 63 49 31 49,

Recherche d’Albi en génie des Procédés des Solides Divisés, de l’Energie et de l’Environnement (RAPSODEE), Université Fédérale de Toulouse, Ecole des Mines d’Albi, 81013 ALBI CT Cedex 09.Supervisor:

- Stéphane Abanadès (Directeur de Recherche CNRS)

+33 4 68 30 77 30,

Laboratoire Procédés, Matériaux et Energie Solaire (PROMES), 7 rue du Four Solaire, 66120 Font-Romeu-Odeillo-Via.

The thesis will take place at the RAPSODEE laboratory in Albi.

The thesis proposed is a thesis in modeling of coupled heat transfers, the candidate must have competences:

1. in applied mathematics and / or thermal heat transfer (including radiative transfer), 2. Computer programming (C ++ langages, etc.),

3. Written and oral expression in French and English.

He will be motivated by modeling and numerical simulation.

The candidate will have to send a CV, a letter of motivation highlighting the competences in line with the proposed subject, as well as transcripts of the grades obtained during Master 1 and 2.

The thesis will be co-financed by the Labex Solstice (University of Perpignan Via Domitia) and the Ecole des Mines d'Albi for approximately € 1750 gross monthly.

*Documents for enrollment in thesis at the Ecole Doctorale Energie Environnement E2:*

*- registration form** - grades of Master 1 and Master 2 (translated if necessary, in English or French) - The present summary of the thesis subject** - This timing of the thesis *