Scientific direction Development of key enabling technologies
Transfer of knowledge to industry

PhD : selection by topics

Engineering science >> Mechanics, energetics, process engineering
3 proposition(s).

Thermal modelling of sugar alcohol cristallisation for energy storage systems based on phase change material

Département Thermique Biomasse et Hydrogène (LITEN)

Laboratoire Stockage Thermique

01-09-2018

SL-DRT-18-0650

fabrice.bentivoglio@cea.fr

Heat represents 50% of the finale energy consumption in the France energetic mix. It has been identified, in the framework of a general orientation law in 2015 (Loi TECV), as a major source of CO2 emission reduction in particular through the development of urban heating networks that allow massive integration of renewable energy such as biomass, solar or waste incineration. In order to adapt the fluctuant consumption of a urban heating network to renewable energy with low flexibility, next generation urban heating network will combined smart meters for precise diagnostic, smart management systems for a better decision process and a key technological component to shift production and match with consumption: heat storage. Phase Change Material (PCM) heat storage, that allows higher storage density (kWh.m3) than classical hot water tanks, has been identified as a promising concept in particular for heating network sub-station, located inside residential buildings then requiring low volume storage. Current studies on PCM storage for urban heating networks mainly consider paraffin, fatty acids and fatty alcohols as PCM. These PCM families can reach a storage density 50% higher than water. Using Sugar alcohol family (or polyols), a storage density 2 or 3 times higher than water can be reached. In addition sugar alcohol are cheap, safe (even edible) and non-corrosive. These ?ideal? PCM only show one major drawback: a very low nucleation rate and and a very low crystallisation speed that make them unusable in a storage system without adding specific system to help crystallisation. The Laboratoire de Stockage Thermique (LITEN ? CEA Grenoble) develops PCM storage systems for urban heating application, using sugar alcohol and based on tube and shell heat exchangers technology. The specific system used to force crystallisation is based on bubbling that generate mechanical shear stress. The principle has been successfully tested at laboratory scale (500g), around a single finned tube (1kg) and finally in a pre-industrial scale prototype (400kg). The results obtained are very promising. However, complex phenomenon have been brought out, such as a high crystallization delay or a strong coupling between thermal and statistical aspects. The objective of this PhD is, based on new experimental results that will be obtained in the facilities yet available in the laboratory, to propose a model of the heat released by a sugar alcohol in a tube and shell PCM heat storage using bubbling to activate the crystallisation. The model will be based on CFD and 2D models that have yet been developed in the laboratory by previous PhD student but for classical PCM (PCM with high crystallization rate and crystallisation speed such as paraffin). These models are based on enthalpy-porosity Voller formulation. The PhD work will starts with a bibliographic study on the crystallization of sugar alcohol and about the effect of bubbling on this crystallisation, in order to develop a model coupling thermal and kinetic aspect. Then the model will be implemented in a 2D or a CFD code and validated against experimental measurements, first at laboratory scale and then at the scale of a pre-industrial prototype.

Recycling of polymer composites by means of a supercritical fluid

Département Thermique Biomasse et Hydrogène (LITEN)

Laboratoire de ThermoConversion de la Bioressource

01-10-2018

SL-DRT-18-0660

anne.roubaud@cea.fr

The subject of this PhD is to study and develop a supercritical fluid process for the recycling of carbon reinforced plastic composite, both resins and fibers. Use of composite materials is increasing in a wide range of applications : industrials, sporting, automotive, aeronautical, marine?but the lack of recycling for those non biodegradable materials is an environmental burden. Since 20 years several treatments have been developed for composites wastes, mechanical, thermal processes like pyrolysis and thermochemical like solvolysis. This process is able to breakdown the composite polymer matrix and hence allow the recovery of the fibers. Supercritical fluids are used due to their high diffusivity in porous materials combined to their chemical reactivity. The objective of this work is to define the necessary process conditions for deconstruction of composite resins of carbon fibers reinforced plastic composites. The aim is to recycle the fibers and also to allow a further chemical valorization of resins decomposition products. This work include a detailed analysis of the organic molecules produced and the development of a chemical mechanism for this depolymerisation. Mechanical properties of fibers after treatment will be determined to validate the recycling interest. This work will provide finally a first technico-economical evaluation of the proposed process.

Development of a drip process for the elaboration of silicon milli-balls.

Département des Technologies Solaires (LITEN)

Laboratoire Matériaux et Procédés Silicium

01-10-2018

SL-DRT-18-0903

malek.benmansour@cea.fr

The photovoltaic industry knows a strong growth and an important reduction of the manufacturing costs of the solar devices. One of the reasons is the industry's adoption of the diamond wire technology for silicon blocks cutting. This technology besides allowing a reduction of the core wires, allows the feasibility of the silicon wastes (named kerf-Si or powders) which represent approximately 40 % of losses. Those powders have a granulometry centered around the micron and an apparent density close to 0.7 g.cm-3 when the density of the polysilicon is at 2.33 g.cm-3. This last point is an important issue for the implementation of a process of silicon powders recycling. Thus, the aim of this work is to develop a solution allowing the shaping and the densification of the fine silicon powder in order to reuse it in a directional solidification process. The studied solution in the present thesis, co-financed by the CEA (French Atomic Energy Commission), is to develop a synthesis process of milli-balls of silicon by a flow of drops. The work will start by the conception, the realization and the optimization of the pilot furnace dedicated to the flow of drops as well as the development of an on-line monitoring system. The control of the powder densification requires the study of numerous process parameters and the characteristics of the raw material. The obtained milli-ball are finally validated as a raw material in a standard crystallization process in order to elaborate silicon ingots for photovoltaic applications.

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