Experimental study and thermo-hydraulical modelling of a heat and cold storage prototype coupling thermocline and latent heat technologies

Heating and cooling in residential buildings hold a 28% share in the total energy consumption of Europe, out of which 75% of the energy is still generated from fossil fuels, while only 19% comes from renewable energy sources. To increase the share of renewable energy in the near future, the French energy commission has identified 4th generation district heating networks as a plausible option. Key hardware components for next generation smart urban heating networks are heat and cold storages, which allows a shift between production and consumption as necessary.
The prototype that will be studied in this thesis couples in a same component heat and cold storage, in order to obtain significant gains in terms of compactness and cost. Cold storage is based on ice-water phase change transition around finned tubes in charging mode (-6°C), and on direct contact between water and ice in discharging mode (direct contact = water flow through the ice and directly exchange heat without any wall between water and ice). Heat storage is based on thermocline technology with water (60-70°C) as coolant.
The prototype is currently under manufacturing in the framework of a European Project and will be operational at the beginning of the thesis. The objective of the thesis is on the one hand to experimentally characterize the performances of the storage, on the other hand to work on a numerical modelling of the storage. The thermos-hydraulical modeling of the discharge in cold mode, with direct contact between ice and water, is particularly challenging. The study of the addition of Phase Change Material capsules for heat storage (50-60°C), in order to boost energy and power, will also be studied with potential implementation in prototype during the thesis.

Reducing the impact of uncertainties in the optimization of low-carbon energy system at district level

Energy system optimization models (ESOM) are powerful tools for improving decision making in the transition towards carbon-free energy systems.

The results provided by ESOMs are greatly influenced by data uncertainty since they are considered on a future time horizon. For instance, the possible evolution of energy prices, energy production and demand or the efficiency of technologies must be taken into account. Although many works have started in recent years to study the impact of these uncertainties on the solutions, it has been pointed out that modeling simplifications may induce significant bias in the obtained results.

The work proposed in this new PhD topic aims at studying the response of ESOM along energy system design and transformation steps, and reducing or assessing the impact of uncertainties as early as possible in the process. It will especially aims at limiting the bias related to model simplification, by systematically propagating relevant information from more detailed models towards simplified models used for sensitivity analysis and optimization under uncertainty. To this aim, the currently envisioned path is to leverage techniques such as machine learning, and in particular the constraint learning approach, to extract relevant information from simulation and inject back into the simplified optimization models.

As a result, the work is expected to improved the methods currently in use for designing and improving energy systems at local level, in order to favor energy savings, and limit CO2 emissions as well as other environmental impacts.

High yield strength austenitic stainless steels for nuclear applications: numerical design and experimental study

The PhD thesis is part of a project that aims at designing new austenitic stainless steels grades for nuclear applications, which are specifically suitable to in-service conditions encountered by the components and to the manufacturing process. More precisely, the subject deals with bolt steels achieved by controlled nitriding of powders which are then densified by hot isostatic pressing. Indeed, current bolt steel grades may suffer from stress corrosion cracking, while nitriding allows to increase the chromium content, which is beneficial from that point of view.
The study will start by the definition of specifications and associated criteria, then CALPHAD calculations in the Fe-Cr-Ni-Mo-X-N-C system will be done to define promising compositions. Then, selected compositions will be supplied as powders. The behaviour of powders during nitriding will be studied and modelled. Samples will be nitrided, densified and heat treated. One grade will be then selected and fully characterised: mechanical properties and deformation mechanisms, corrosion behaviour. One important objective is to demonstrate the advantages of the new grade compared to the industrial solution.

Numerical and experimental studies of an ejector designed for a cold or heat production cycle

The ejector has been the research subject in the literature as the main component of refrigeration cycles using “thermal compression” thanks to its simplicity without moving parts. It uses a high-pressure fluid called “primary fluid” to drive and compress a low-pressure fluid, which is called “secondary fluid”. The performance of the ejector is defined by the entrainment ratio, which is the mass-flow ratio between the secondary and primary flows; as well as the critical pressure, which limits the operating range of the ejector. Most of the numerical and experimental studies have been conducted on water vapor ejectors. The studies showed that the geometry optimization is crucial in order improve the ejector performance. Moreover, experiments showed that the flow inside an ejector is often supersonic and highly compressible therefore inducing strong pressure variation. This can induce strong temperature variations and the apparition of liquid water and ice in ejectors have already been witnessed.

Numerical studies carried out previously have shown the importance of accurately modeling the liquid-vapor phase changes in order to establish consistent and accurate numerical models for flows hydrodynamics within the ejector. However, these studies give little or no consideration to the temperature field distribution within the ejector. The main difficulty here are the huge pressure variations that happen inside the ejector which lead to liquid vapor phase changes in a highly compressible flow. In this PhD project, we aim to investigate innovative solutions with ejector integrated into thermodynamic cycles working with natural fluids (ammonia, water, CO2 …) in order to improve the global performances. For this, it is important to understand the local physical phenomena of the flows inside an ejector, especially the impact of liquid-vapor phase change as well as the impact of the operating conditions.

Based on the strong research background of both CEA and INSA Lyon, we will conduct numerical and experimental works about the ejector and the thermodynamic cycles with the following research plan:
* Numerical work:
_ Development of a 1D model and perform the CFD simulations for comparison
_ Modelling and simulations of the identified thermodynamic cycles integrated the appropriate ejector
_ Design of ejector for tests
*Experimental work : fabrication of test ejector and perform measurements for model validation and analysis

For more than 15 years, CEA has conducted extensive research on thermodynamic cycles in order to develop innovative solutions for production of heat, cold and electricity. Recently, we have developed a new model of ejector for integration into a thermodynamic cycle . To bring new insight about the local phenomena of the flows inside an ejector considering the liquid-vapor phase, we have investigated and performed CFD simulations. INSA Lyon has strong research background on the topics related to CO2 such as heat pump cycles, heat exchangers as well as ejector. The test bench of ejector at INSA Lyon together with the INES platform at CEA will be served for the experimental work of this project.

Study of the thermoconversion and de-polymerization mechanisms of plastic wastes in supercritical water conditions

The waste valorization is a hot topic that has attracted great interest in the Circular Carbon Economy. Substantial efforts have been devoted to strengthening sustainable processes in recent years. These are based on the development of systems to improve carbon circularity (material and energy recycling).Global production of plastics doubled from 230 million tons in 2000 to 460 million tons in 2019. This exponential production/consumption has significant consequences on the environment. Despite the existence of recycling methods, only 9% of global plastic production is currently recycled, and the remaining quantity (not valorized) represents a real source of pollution [1].
Mixtures of different types of plastics make sorting stages difficult, which represents the main disadvantage for material recycling systems. An interesting application recently reported in the literature is the use of the hydrothermal gasification process to treat waste (and mixtures of difficult-to-sort) plastics to produce a gas rich in CH4 and H2 [2]. Hydrothermal gasification (HTG) is a thermochemical process which employs the supercritical conditions of water (T > 374 ° C, P > 221 bar), in order to convert the organic carbon contained in the wet feedstock into a gaseous phase (which contains CH4, H2, CO and CO2, mainly). In addition, the flexibility of the process also allows the study of de-polymerization of these wastes in conditions close to the critical point of water, which facilitates the production of chemical intermediates (and their reuse) in the chemical industry.
Thus, the understanding of the conversion mechanisms of different types of plastics (and their mixtures) seems essential to valorize these wastes. However, the identification of reaction pathways is still a major scientific obstacle. The objective of the thesis is the study of the reaction mechanisms of transformation of model plastics (and their mixtures) in supercritical water conditions. Understanding the phenomena will lead to the optimization of the HTG process (with and without catalysts) to facilitate the production of a gas rich in CH4/H2 and the production of intermediates for the chemical industry. The focus of this PhD work will involve: i) the study of thermo-conversion and de-polymerization of plastics; ii) the study of the behavior of catalysts in the supercritical water environment (activation/deactivation); iii) the study of selectivity towards the production of a gas containing CH4/H2 and the production of chemical intermediates.

Understanding the Impact of Operating Conditions and Utilization Profiles on Solid Oxide Electrolysis Stacks Lifetime

The shift to a low-carbon European Union (EU) economy raises the challenges of integrating renewable energy sources (RES) and cutting the CO2 emissions of energy intensive industries (EII). In this context, hydrogen produced from RES will contribute to decarbonize those industries, as feedstock/fuel/energy storage. Among the different technologies for low carbon H2 production, high temperature electrolysis (HTE) enables the production of green hydrogen with extremely high efficiency. The solid oxide cells (SOC) are typically operated in the 650-to-850°C temperature range, and arranged in pile-ups or stacks to increase the overall power density and address (pre-) industrial markets.
The technology has recently entered a phase of aggressive industrialization. However, significant efforts are still required to turn the high efficiencies into a competitive levelized cost of H2. As long as such cost remains largely controlled by that of stack manufacturing, stack degradation and the relationship with operating conditions remain a crucial subject of research and development. Moreover, recent advances have shown that to properly evaluate stack lifetimes, actual testing beyond 5 kh is critical [1,2]. A better understanding of degradation over the 5-to-10 kh range [3–5] could thus enable the development of both accelerated stress tests (AST) to reduce the necessary test duration, as well as optimized operational strategies to extend stack lifetimes.

Deployment strategy for energy infrastructures on a regional scale: an economic and environmental optimisation approach

The CEA develops a software to optimize dimensioning and control of energy systems, in order to conduct tec-eco studies, including an environmental part, for industry and territories. The optimization is run by a MILP solver.

We want to go further by optimizing the deployment of infrastructure over time and space. Indeed, changes in demand, economic environment, and technological performance need to be taken into account from the beginning of an energy system deployment. The spatial dimension is also important, to make the good choice between centralizing production to make economies of scale, or dispatch the production resources across a territory and ensuring transportation.

Addressing these broader issues leads to more complex calculation with higher times of resolution.

The goals of the PHD will therefore be as follows:
- Establish a generic formalism to describe this type of problem and make it easily modelable, taking into account economic and environmental aspects, as well as the associated uncertainties.
- Compare, select and improve methods of optimization and artificial intelligence allowing to deal with the complexity of the problem.
- Apply this algorithm on concrete case studies.
We are looking for a candidate with a background in applied mathematics. They should be interested in the energy transition.

A better understanding of diffusion welding in a+ß titanium alloy

As part of a short-term nuclear project, the CEA/LITEN is supporting the manufacturing activities of a titanium alloy steam generator by HIP (Hot Isostatic Pressing). Depending on its thermal and/or thermomechanical history, the alloy Ti64 presents phases in different proportions, chemical compositions and crystallographic structures.
How does diffusion welding take place between two different phases? Is there one that cross the interface preferentially and if yes, why? Which HIP parameters have a real influence? What starting microstructure allows optimal diffusion welding?
These are the questions that the thesis should answer.

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