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.

Actuellement, les codes de calcul de composants thermohydrauliques de réacteurs nucléaires du CEA et d'EDF utilisent des modèles physiques diphasiques de mélange à 3 ou 4 équations. Or, il existe un fort besoin industriel pour des modèles physiques plus sophistiqués tels que le modèle diphasique à 6 équations voire des modèles multichamps. Par ailleurs, le code système CATHARE du CEA, un des codes systèmes les plus utilisés aujourd'hui sur le plan international, utilise un modèle physique diphasique à 6 équations et un schéma numérique semi-implicite de type ICE à maillages décalés. Le schéma de CATHARE est connu pour sa robustesse dans une large gamme de configurations d'écoulement. En s'inspirant de l'expérience de CATHARE, on propose de mettre en œuvre un schéma volumes finis colocalisés de type « pressure based ». Le but est d'obtenir un décentrement des flux qui assure la robustesse du schéma tout en gardant une bonne précision. De plus, le fait de pouvoir utiliser des maillages colocalisés (structurés ou non-structurés) permet de traiter de différentes configurations complexes de cœur de réacteurs et de réaliser des calculs fin d'inter assemblage. Le schéma doit conserver exactement la masse et l'énergie et la solution numérique doit converger lorsque l'on raffine le maillage. Le schéma doit être capable de traiter des cas d'apparition et de disparition des phases, par exemple le cas de la colonne bouillante où il y a changement de phase dû au transfert de chaleur, des cas de tuyères avec changement de phase dû à un élargissement ou un rétrécissement brusque, ou de séparation de phase par gravité. En outre le schéma doit être capable de traiter des configurations de calcul à faible nombre de Mach, par exemple le cas du renoyage d'un cœur de réacteur. L'objet de la thèse consistera à développer une méthode Volumes Finis co-localisés (dans l'esprit de Ghidaglia et al.) et la direction d'investigation s'inspirera des travaux de Jeong et al. qui a conduit au code CUPID. Références. Ghidaglia, J. M., Kumbaro, A., & Le Coq, G. (2001). On the numerical solution to two fluid models via a cell centered finite volume method. European Journal of Mechanics-B/Fluids, 20(6), 841-867. Jeong, J. J., Yoon, H. Y., Cho, H. K., Kim, J., & Park, I. K. (2008). A semi-implicit numerical scheme for a transient two-fluid three-field model on an unstructured grid. International Communications in Heat and Mass Transfer, 35(5), 597-605. / Currently, there is a strong industrial need for sophisticated physical models such as the two-phase model with 6 equations or multi-field models for the thermo-hydraulic calculation of nuclear reactor components. In addition, the system code CATHARE of CEA, one of the most utilized system codes at the international level, employs a two-phase 6 equations and a semi-implicit numerical scheme of ICE type on staggered grids. The code CATHARE is known for its robustness in a wide range of flow configurations. Drawing on the experience of CATHARE, we propose to implement a collocated finite volume pressure based scheme. The aim is to obtain a decentering of flux which ensures the robustness of the scheme while keeping good accuracy. In addition, being able to use collocated grids (structured or unstructured) can handle different complex configurations. The scheme should conserve exactly the mass and energy and the numerical solution needs to converge when the mesh is refined. The scheme should be able to handle cases of phase appearance and disappearance, for example in the case of boiling column where phase change is due to heat transfer, the case of nozzles with phase change due to a widening or abrupt narrowing, or phase separation by gravity. In addition the scheme should be capable of calculation configurations at low Mach number. The purpose is to develop a co-located Finite Volume method (in the spirit of Ghidaglia et al.) And direction of investigation is build on work of Jeong et al. which led to CUPID code. References: Ghidaglia, J. M., Kumbaro, A., & Le Coq, G. (2001). On the numerical solution to two fluid models via a cell centered finite volume method. European Journal of Mechanics-B/Fluids, 20(6), 841-867. Jeong, J. J., Yoon, H. Y., Cho, H. K., Kim, J., & Park, I. K. (2008). A semi-implicit numerical scheme for a transient two-fluid three-field model on an unstructured grid. International Communications in Heat and Mass Transfer, 35(5), 597-605.

No description available.

No description available.

In the present study, a finite volume method is employed to modelthe advection-diffusion phenomenon during a pure substance meltingprocess. The exercise is limited to a benchmark problem consisting ofthe 2D melting from a vertical wall of a PCM driven by natural convectionin the melt. Numerical results, mainly the temporal evolutionof average Nusselt number at the hot wall and the average liquid fraction,are validated by available literature data and the effect of thermalinertia in the heat transfer is considered as well. Finally, motivatedby recent publications and the model presented here, possible new researchtopics are proposed.

The significant increase in energy requirements across the world, provides several opportunities for innovative methods to be developed to facilitate the storage and utilization of energy. The major energy demand is in the form of electrical energy for domestic as well as industrial sectors, a large part of which are the heating and cooling requirements. Appropriate utilization of thermal energy storage can effectively aid in reducing the electrical demand by storage and release of this thermal energy during peak hours. Thermal Energy Storage using Phase Change Materials (PCMs) is an attractive method of energy storage, with a wide variety of potential applications. Several configurations have been tested by researchers to develop energy storage devices with PCMs. The cycling of melting and solidification of PCMs results in storage and release of heat at a relatively small temperature difference. Design and deployment of these storage systems have certain challenges and considerations associated to them for instance, when used in buildings, PCMs should be non-toxic, non-corrosive, and others. In this thesis, we aim to provide models for designing Latent Thermal Energy Storage (LTES) devices with PCMs, based on their operating conditions, thermophysical properties of materials, and geometric parameters. The models are developed considering fluid dynamics and heat transfer involved in melting and solidification of PCMs. Parameters like inlet temperature and velocity, and volume of storage container are varied to determine the time taken for melting or solidification. For sizing and predicting performance of the storage devices we aim at presenting an analytical correlation, with time taken for melting as the variable defining the ‘charging/discharging time’ of storage device. Along with this, a transient model is developed to predict amount of PCM melted/solidified, along with rate of latent energy storage in defined time period intervals.

Droplet evaporation (and condensation) is one of the most common instancesof multiphase flow with phase change, encountered in nature as well as intechnical and industrial applications. Examples include falling rain drops, fogsand mists, aerosol applications like electronic cigarettes and inhalation drugdelivery, engineering applications like spray combustion, spray wet scrubbing orgas absorption, spray drying, flame spray pyrolysis.Multiphase flow with phase change is a challenging topic due to the inter-twined physical phenomena that govern its dynamics. Numerical simulation isan outstanding tool that enables us to gain insight in the details of the physics,often in cases when experimental studies would be too expensive, impracticalor limited.In the present work we focus on simulation of the evaporation of smalldroplets. We perform simulation of evaporation of a pure and two−componentdroplet, that includes detailed thermodynamics and variable physical andtransport properties. Some of the conclusions drawn include the importance ofenthalpy transport by species diffusion in the thermal budget of the system, andthe identification and characterization of evaporating regimes for an azeotropicdroplet.In the second part we develop a method based on the immersed boundaryconcept for interface resolved numerical simulation of laminar and turbulentflows with a large number of spherical droplets that undergo evaporation orcondensation. / Droppförångning (och kondensation) är en av de vanligaste fallen av flerfasflöde med fasförändring, både i naturen och i tekniska och industriella tillämpningar. Exempel är fallande regndroppar, dimma, aerosol-tillämpningar som elektroniskacigaretter och läkemedelsleverans via inandning, tekniska tillämpningar som sprayförbränning, våtskrubbning med sprayning, gasabsorption, spraytorkning samt flammsprayspyrolys. Flerfasflöde med fasförändring är ett utmanande ämne på grund av de sammanflätade fysikaliska fenomen som styr dess dynamik. Numerisk simulering är ett utmärkt verktyg som gör det möjligt för oss att få insikt i detaljerna i fysiken, ofta i fall då experimentella studier skulle vara för dyra, opraktiska eller begränsade. I det nuvarande arbetet fokuserar vi på simulering av förångning av små droppar. Vi utför simulering av förångning av en ren och två−komponentdroppe, som inkluderar detaljerad termodynamik samt varierande fysikaliska och transportegenskaper. Några av de slutsatser som dras inbegriper betydelsen av entalpitransport genom diffusion av olika ämnen i systemets termiska budget samt identifieringen och karakterisering av förångningsregimer för en azeotropiskdroppe. I den andra delen utvecklar vi en metod baserad på det nedsänkta rand konceptet för gränssnittskompletterad numerisk simulering av laminära och turbulenta flöden med ett stort antal sfäriska droppar som genomgår förångning eller kondensering. / <p>QC 20171117</p>

A prospect in utilizing thermal energy in development of energy systems is by the useof phase change materials (PCMs). PCMs are materials that can store and releaseenergy during phase changes, e.g. from solid to liquid. By-products from the woodand pulping industry could be of interest in this area, in part to add value to theby-products, enhance the yield of the raw product of wood and become less dependenton fossil based fuels. Capsules of lignin and tall oil pitch/tall oil fatty acids weresuccessfully produced with a straightforward coprecipitation method. The solventused was acetone and the antisolvent was distilled water. Dynamic light scatteringanalysis showed average particle diameters of 300 to 500 nm and fairly lowpolydispersity, between 0.2 to 0.3, indicating spherical particles. Scanning electronmicroscopy confirmed particle size and the formation of capsules with shell thicknessless than 100 nm. The particle dispersions showed sufficiently high zeta potential tomaintain a stable colloidal system. Thermal analysis confirmed stability in atemperature range of at least -40 °C to +50 °C, and resistance to decomposition at leastup to around 200 °C. There were also indications of enhanced thermal stability of talloil pitch due to encapsulation. A desired feature for a phase change material is theability to keep a constant temperature during phase change. Regarding using thesynthesized material as phase change material, the results show that the temperaturerange in which phase change, such as melting, occurs is too broad. Latent heat of fusionof 4.7 J/g for the material is also very low compared with commercial phase changematerials. Further studies in modifying the system to impact melting point andenhancement of latent heat is needed if the material should be applicable as an efficientand competitive phase change material.

No description available.

No description available.