Le récepteur d’une centrale solaire à tour est l’élément clé de la conversion du rayonnement en chaleur. Dans le cadre de la thèse, il s’agit d’un récepteur métallique dans une centrale de type HSGT (Turbine hybride solaire gaz) refroidi par air pressurisé. En condition normale de fonctionnement, le récepteur chauffe l’air de 350 à 750°C. La température de l’air en sortie chute à 400°C en moins de 15 minutes si le soleil est masqué, par un nuage par exemple. L’objectif est de maintenir la température de l’air en sortie supérieure à 600°C durant 15 minutes sans ensoleillement. Pour parvenir à cet objectif, un stockage thermique intégré au récepteur est envisagé. Parallèlement le stockage de chaleur doit prolonger la durée de vie du récepteur en lui évitant de subir d’intenses chocs thermiques. L’étude porte sur la zone la plus chaude du récepteur, atteignant 800°C. Lorsque le soleil brille (le récepteur est insolé), une partie de la chaleur est stockée dans un matériau qui passe de l’état solide à liquide. Cette chaleur est restituée au récepteur lors de la transformation inverse (liquide à solide) si le soleil est masqué. Les variations de la température du récepteur sont ainsi plus douces et le récepteur est épargné des chocs thermiques. L’utilisation d’un matériau à changement de phase tel que le carbonate de lithium (fusion à 723°C) réduit le volume et la masse du stockage installé directement à l’arrière du récepteur. Ce matériau stocke une grande quantité de chaleur sur une gamme de température peu étendue. Cependant les matériaux à changement de phase ne permettent pas de transférer la chaleur rapidement à cause de leur faible conductivité thermique. C’est la raison pour laquelle l’intensification de ces transferts est étudiée. La mise en place d’ailettes en cuivre à l’intérieur du stockage améliore les transferts de chaleur, grâce à la conductivité thermique élevée du métal. Un modèle numérique représentatif du comportement thermique du stockage est développé. Le travail de conception du stockage aboutit à la fabrication d’un banc expérimental. Les résultats obtenus sont comparés au modèle afin de le critiquer. Les conclusions permettent d’envisager la conception d’un stockage thermique de protection à l’échelle du récepteur. / The thesis deals with the problem of thermal inertia and life time of the solar receiver of a Concentrated Solar Power tower plant. A specific attention is paid to the situation of HSGT (Hybridized Solar Gas Turbine) systems using pressurized air as HTF (Heat Transfer Fluid). The intermittence of solar radiation, mainly resulting from cloudy events, causes important temperature fluctuations that contribute to the premature aging. Therefore, a Thermal Energy Storage (TES) is developed for the protection of the receiver. The design focuses on the high temperature section of the receiver. As a consequence of the elevation of temperature in this stage, the expected temperature of the receiver ranges between 600°C and 800°C. Once the receiver is no longer irradiated, the temperature of the outlet air of the receiver, which is 750°C at designed point, decreases below 400°C in less than 15 minutes. The objective is to integrate the TES into the solar receiver to maintain this air temperature higher than 600°C after 15 minutes of discharge. A low capacity TES is targeted. Besides, the storage should enhance the lifetime of the receiver during the operation, by avoiding temperature drops. A test bench is designed based on a technology using both Phase Change Material (PCM) and metallic fins in order to enhance charge and discharge power of the storage unit. The selected metal is copper, because of its great thermal conductivity. The thermal storage medium must operate in the range 600°C – 800°C. The lithium carbonate has been selected mainly because of its phase change temperature, 723°C. A numerical model is developed in order to help the design of the test bench and compare experimental results. The conclusions lead to one-scale design of the thermal storage integrated to the solar receiver.

abstract: Thermal Energy Storage (TES) is of great significance for many engineering applications as it allows surplus thermal energy to be stored and reused later, bridging the gap between requirement and energy use. Phase change materials (PCMs) are latent heat-based TES which have the ability to store and release heat through phase transition processes over a relatively narrow temperature range. PCMs have a wide range of operating temperatures and therefore can be used in various applications such as stand-alone heat storage in a renewable energy system, thermal storage in buildings, water heating systems, etc. In this dissertation, various PCMs are incorporated and investigated numerically and experimentally with different applications namely a thermochemical metal hydride (MH) storage system and thermal storage in buildings. In the second chapter, a new design consisting of an MH reactor encircled by a cylindrical sandwich bed packed with PCM is proposed. The role of the PCM is to store the heat released by the MH reactor during the hydrogenation process and reuse it later in the subsequent dehydrogenation process. In such a system, the exothermic and endothermic processes of the MH reactor can be utilized effectively by enhancing the thermal exchange between the MH reactor and the PCM bed. Similarly, in the third chapter, a novel design that integrates the MH reactor with cascaded PCM beds is proposed. In this design, two different types of PCMs with different melting temperatures and enthalpies are arranged in series to improve the heat transfer rate and consequently shorten the time duration of the hydrogenation and dehydrogenation processes. The performance of the new designs (in chapters 2 and 3) is investigated numerically and compared with the conventional designs in the literature. The results indicate that the new designs can significantly enhance the time duration of MH reaction (up to 87%). In the fourth chapter, organic coconut oil PCM (co-oil PCM) is explored experimentally and numerically for the first time as a thermal management tool in building applications. The results show that co-oil PCM can be a promising solution to improve the indoor thermal environment in semi-arid regions. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2020

Thermal energy storage (TES) modules are specifically designed to respond to transient thermal loading. Their dynamic response depends on the overall structure of the module, including module geometry and dimensions, the internal spatial distribution of phase change material (PCM) and conductive heat-spreading elements, and the thermophysical properties of the different materials composing the module. However, due to the complexity of analyzing a system’s dynamic thermal response to transient input signals, optimal design of a TES module for a particular application is challenging. Conventional design approaches are limited by (1) the computational cost associated with high fidelity simulation of heat transfer in nonlinear systems undergoing a phase transition and (2) the lack of model integration with robust optimization tools. To overcome these challenges, I derive reduced-order dynamic models of two different metal-PCM composite TES modules and validate them against a high fidelity CFD model. Through simulation and validation of both turbulent and laminar flow cases, I demonstrate the accuracy of the reduced-order models in predicting, both spatially and temporally, the evolution of the dynamic model states and other system variables of interest, such as PCM melt fraction. The validated models are used to conduct univariate and bivariate parametric studies to understand the effects of various design parameters on different performance metrics. Finally, a case study is presented in which the models are used to conduct detailed design optimization for the two HX geometries.

This dissertation presents the development of two unique measurement platforms. <br><br>The first system is a high-temperature Z-Meter. This system is designed to simultaneously measure the electrical resistivity, Seebeck coefficient, and thermal conductivity of a thermoelectric sample to accurately determine the figure of merit, ZT, for that material. It is designed to operated at sample temperatures of up to 1000C, and with temperature gradients on the order of 500C across the sample. This system also provides <i>in situ</i> load monitoring for contact pressure and allows the user to adjust loading during the experiment. <br><br>The second part of this dissertation focuses on the development of enhanced composite phase change material (PCM) heat sinks to improve passive thermal management in mobile electronics. We present a new design for a composite PCM heat sink and utilize off-the-shelf PCMs to show characterize the performance. In order to accurately investigate the performance enhancement of these designs, we develop a turn-key thermal management evaluation platform to allow the user complete control over the power profiles and cycling applied to the test chip, as well as providing <i>in situ</i> temperature monitoring within the chip. The proposed package designs show significant improvement in the length of time extended before reaching the cut-off temperature within the heatfluxes tested, 6 - 14 W/cm^2, and accomplish this while weighing less than the equivalent sensible heat storage design.<br><br><br><br>

Thermal energy storage (TES) in a concentrating solar power (CSP) plant allows for continuous operation even during times when solar radiation is not available, thus providing a reliable output to the grid. Energy can be stored either as sensible heat or latent heat, of which latent heat storage is advantageous due to its high volumetric energy density and the high Rankine cycle efficiency owing to the isothermal operation of latent thermal energy storage (LTES) system. Storing heat in the form of latent heat of fusion of a phase change material (PCM), in addition to sensible heat, significantly increases the energy density, thus potentially reducing the storage size and cost. However, a major technical barrier to the use of latent thermal energy of PCM is the high thermal resistance to energy transfer due to the intrinsically low thermal conductivity of PCMs, which is a particularly acute constraint during the energy discharge. Secondly, for integration of TES in CSP plants, it is imperative that the cyclic exergetic efficiency be high, among other requirements, to ensure that the energy extracted from the system is at the maximum possible temperature to achieve higher cycle conversion efficiency in the power block. The first objective is addressed through computational modeling and simulation to quantify the effectiveness of two different approaches to reduce the thermal resistance of PCM in a LTES, viz. (a) developing innovative, inexpensive and passive heat transfer devices that efficiently transfer large amount of energy between the PCM and heat transfer fluid (HTF) and (b) increase the heat transfer area of interaction between the HTF and PCM by incorporating the PCM mixture in small capsules using suitable encapsulation techniques.   The second portion of the research focuses on numerical modeling of large scale latent thermal storage systems integrated to a CSP plant with the aforementioned enhancement techniques and cascaded with more than one PCM to maximize the exergetic efficiency. Based on systematic parametric analysis on the various performance metrics of the two types of LTES, feasible operating regimes and design parameters are identified to meet the U.S. Department of Energy SunShot Initiative requirements including storage cost < $15/kWht and exergetic efficiency > 95%, for a minimum storage capacity of 14 h, in order to reduce subsidy-free levelized cost of electricity (LCE) of CSP plants from 21¢/kWh (2010 baseline) to 6¢/kWh, to be on par with the LCE associated with fossil fuel plants. / Ph. D.

No description available.

No description available.

No description available.

The latent-heat-storage capacity in Phase-Change Materials can be used for storing or releasing energy within a small temperature interval. Upon the phase transition taking place in a narrow temperature span, the material takes up or releases more energy compared to sensible heat storage. For an ideal phase-change material, the transition temperature is a single value, but for the most common phase-change materials on the market, used in building applications, the transition temperature is distributed within a temperature range of several degrees. Integration of phase-change materials in building applications can be effected in several ways, for example by impregnating phase-change materials into porous building materials like concrete, wallboards, bricks or complements of the building structure. Integrating storages filled with phase-change materials makes other implementations, for instance accumulating tanks or envelopes as presented in this thesis, in an air heat exchanger. An appropriate phasetransition temperature of the supposed application is critical to the functionality of the material. For example, in cooling applications, the transition temperature of the material should be a few degrees lower than the requested comfort temperature in the building, and the opposite for heating applications. In order to assess the thermal properties and the durability of the material, a watercalorimetric equipment was developed and employed in an accelerated testing programme. The heat capacity of the material and in particular possible change in the heat capacity over time, after thermal cycling of the material, were measured. In the thermal cycling of the material from solid to liquid phase, the temperature rise and required energy supply were recorded. The testing programme was undertaken according to control procedures and documents. In order to be able to utilize the heat-storage capacity in the best way, it is necessary to gain knowledge about thermal properties of the material, especially the long-term behaviour of the material and the deterioration rates of the thermal properties. A semi-full-scale air heat exchanger based on phase-change material was developed and tested under real temperature conditions during the summer of 2004. The test results were used to compare and verify computer simulations made on a similar plant. The air heat exchanger utilises the ambient diurnal temperature swing to charge and discharge the phasechange material. The material tested in the calorimeter and in the air heat exchanger has an estimated phase-change temperature of about 24 °C. / QC 20101123

L'objectif de cette thèse est d'étudier un système énergétique intégré dans l'enveloppe des bâtiments permettant de préchauffer l'air neuf.L'originalité du travail repose sur le fait que ce mur est équipé de matériau à changement de phase (MCP).Celui-ci a pour rôle de stocker l'énergie solaire captée en façade puis de la déstocker en préchauffant l'air neuf de ventilation. Notre étude est constituée de deux grandes phases, une phase expérimentale et une phase numérique.La phase expérimentale consiste à effectuer des essais en laboratoire, afin de connaître le comportement du système étudié sous sollicitations thermiques. Ces essais ont été réalisés grâce à un prototype du mur, instrumenté et installé entre deux cellules avec des conditions climatiques contrôlées.Le but de la phase numérique est de mettre en place un modèle ID permettant de simuler le comportement thermique du mur et en particulier celui du MCP. Ce modèle a été validé en comparant les résultats numériques avec ceux obtenus expérimentalement.Pour modéliser le changement de phase nous avons utilisé les paramètres thermophysiques du matériau obtenu par caractérisation expérimentale réalisée dans notre laboratoire. Nous avons ensuite utilisé la méthode de la capacité variable pour simuler le comportement de la paroi stockeuse du mur. Dans la dernière partie du travail numérique le modèle a été utilisé afin de montrer l'influence de quelques paramètres permettant d'optimiser les gains énergétiques.La simulation dynamique du système a été effectuée grâce au logiciel TRNsys, qui nous a permis d'effectuer des bilans énergétiques et d'estimer l'efficacité du système pour des climats variés. / The objective of this thesis is to study an integrated energy system in the building envelope for fresh air preheating. The originality of the work is that the wall is equipped with phase-change material (MCP) packed into briquettes. The main role ofthe wall is to preheat the fresh air (coming from outside) by destocking the solar energy captured in sunny periods.Our study consists of two phases, an experimental phase and a numerical one. The experimental phase involves a series of tests that allow studying the thermal behaviour of the system under thermal stress. These tests were done in laboratory through a prototype of the wall which is instrumented and installed between two airconditioned cells.The purpose of the numerical phase is to develop a one-dimensional model to simulate the thermal behaviour of the wall and especially that of MCP. This model has been validated by comparing numerical results with those obtained experimentally. To model the phase changing we used the thermophysical parameters of the same material obtained by experimental characterization conducted in our laboratory.We used a variable capacity method whose principle is to vary the heat capacity as a function of temperature in order to simulate the phase changing of the wall. The last part of the numerical work is the exploitation of the model, the aim is to determine the optimal configuration of the wall that provides maximum energy savings. Dynamic simulation of the system was performed using the TRNSYS. This one is equipped with weather files which allow carrying out heat balances and the estimation of the system efficiency for various climates.