Geothermal energy is the energy naturally present inside the earth crust. When a large volume of hot water and steam is trapped in subsurface porous and permeable rock structure and a convective circulating current is set up, it forms a geothermal reservoir. A geothermal system can be defined as - convective water in the upper crust of earth, which transfers heat from a heat source (in the reservoir) to a heat sink, usually the free surface. A geothermal system is made up of three main elements: a heat source, a reservoir and a fluid, which is the carrier that transfers the heat. As an alternative source of energy geothermal energy has been under attention of the researchers for quite some time. The reason behind this is the existence of several benefits like clean and renewable source of energy which has considerable environmental advantage, with no chemical pollutants or wastes are generated due to geothermal emissions, and the reliability of the power resource. Hence research has been directed in several directions like exploration of geothermal resources, modeling the characteristics of different types of geothermal reservoirs and technologies to extract energy from them. The target of these models has been the prediction of the production of the hot water and steam and thus the estimation of the electricity generating potential of a geothermal reservoir in future years. In a geothermal power plant reinjection of the heat depleted water extracted from the geothermal reservoir has been a common practice for quite some time. This started for safe wastewater disposal and later on the technology was employed to obtain higher efficiency of heat and energy extraction. In most of the cases a very small fraction of the thermal energy present in the reservoir can be recovered without the reinjection of geothermal fluid. Also maintaining the reservoir pressure is essential which gradually reduces due to continuous extraction of reservoir fluid without reinjection, especially for reservoirs with low permeabilities. Although reinjection of cold-water has several benefits, the possibility of premature breakthrough of the cold-water front, from injection well zone to production well zone, reduces the efficiency of the reservoir operation drastically. Hence for maintaining the reservoir efficiency and longer life of the reservoir, the injectionproduction well scheme is to be properly designed and injection and extraction rates are to be properly fixed. Modeling of flow and heat transport in a geothermal reservoir due to reinjection of coldwater has been attempted by several researchers analytically, numerically and experimentally. The analytical models which exist in this field deal mostly with a single injection well model injecting cold-water into a confined homogeneous porous-fractured geothermal reservoir. Often the thermal conductivity is neglected in the analytical study considering it to be negligible which is not always so, as proved in this study. Moreover heterogeneity in the reservoir is also a major factor which has not been considered in any such analytical study. In the field of numerical modeling there also exists a need of a general coupled three-dimensional thermo-hydrogeological model including all the modes of heat transport (advection and conduction), the heat loss to the confining rocks, the regional groundwater flow and the geothermal gradient. No study existing so far reported such a numerical model including those mentioned above. The present study is concerned about modeling the non-isothermal flow and heat transport in a geothermal reservoir due to reinjection of heat depleted water into a geothermal reservoir. Analytical and numerical models are developed here for the transient temperature distributions and advancement of the thermal front in a geothermal reservoir which is generated due to the cold-water injection. First homogeneous geothermal aquifers are considered and later heterogeneities of different kinds are brought into picture. Threedimensional numerical models are developed using a software code DuMux which solves flow and heat transport problems in porous media and can handle both single and multiphase flows. The results derived by the numerical models have been validated using the results from the analytical models derived in this study. Chapter 1 of the thesis gives a brief introduction about different types of geothermal reservoirs, followed by discussion on the governing differential equations, the conceptual model of a geothermal reservoir system, the efficiency of geothermal reservoirs, the modeling and simulation concepts (models construction, boundary conditions, model calibration etc.). Some problems related with geothermal reservoirs and geothermal power is also discussed. The scenario of India in the context having a huge geothermal power potential is described and different potential geothermal sites have been pointed out. In Chapter 2, the concept of reinjection of the heat depleted (cold) water into the geothermal reservoir is introduced. Starting with a brief history of the geothermal reinjection, the chapter describes the purpose and the need of reinjection of geothermal fluid giving examples of different geothermal fields over the world where reinjection has been in practice and benefitted by that. The chapter further discusses on the problems and obstacles faced by the geothermal projects resulting from the geothermal reinjection, most important of which is the thermal-breakthrough and cooling of production wells. Lastly the problem of this thesis is discussed which is to model the transient temperature distribution and the movement of the cold-water thermal front generated due to the reinjection. The need of this modeling is elaborated which represents the motivation of taking up the problem of the thesis. Chapter 3 describes an analytical model developed for the transient temperature in a porous geothermal reservoir due to injection of cold-water. The reservoir is composed of a confined aquifer, sandwiched between rocks of different thermo-geological properties. The heat transport processes considered are advection, longitudinal conduction in the geothermal aquifer, and the conductive heat transfer to the underlying and overlying rocks of different geological properties. The one-dimensional heat transfer equation has been solved using the Laplace transform with the assumption of constant density and thermal properties of both rock and fluid. Two simple solutions are derived afterwards, first neglecting the longitudinal conductive heat transport and then heat transport to confining rocks. The analytical solutions represent the transient temperature distribution in the geothermal aquifer and the confining rocks and model the movement of the cold-water thermal front in them. The results show that the heat transport to the confining rocks plays an influential role in the transient heat transport here. The influence of some parameters, e.g. the volumetric injection rate, the longitudinal thermal conductivity and the porosity of the porous media, on the transient heat transport phenomenon is judged by observing the variation of the transient temperature distribution with different values of the parameters. The effects of injection rate and thermal conductivity have been found to be high on the results. Chapter 4 represents another analytical model for transient temperature distribution in a heterogeneous geothermal reservoir underlain and overlain by impermeable rocks due to injection of cold-water. The heterogeneity of the porous medium is expressed by the spatial variation of the flow velocity and the longitudinal effective thermal conductivity of the medium. Simpler solutions are also derived afterwards first neglecting the longitudinal conduction, then the heat loss to the confining rocks depending on the situation where the contribution of them to the transient heat transport phenomenon in the porous media is negligible. Solution for a homogeneous aquifer with constant values of the rock and fluid parameters is also derived with an aim to compare the results with that of the heterogeneous one. The effect of heat loss to the confining rocks in this case is also determined and the influence of some of the parameters involved, on the transient heat transport phenomenon is assessed by observing the variation of the results with different magnitudes of those parameters. Results show that the heterogeneity plays a major role in controlling the cold-water thermal front movement. The transient temperature distribution in the geothermal reservoir depends on the type of heterogeneity. The heat loss to the confining rocks of the geothermal aquifer also has influence on the heat transport phenomenon. In Chapter 5 another analytical model is derived for a heterogeneous reservoir where the heterogeneous geothermal aquifer considered is a confined aquifer consisted of homogeneous layers of finite length and overlain and underlain by impermeable rock media. All the different layers in the aquifer and the overlying and underlying rocks are of different thermo-hydrogeological properties. Results show that the advancement of the cold-water thermal front is highly influenced by the layered heterogeneity of the aquifer. As the cold-water thermal front encounters layers of different thermo-hydrogeological properties the movement of it changes accordingly. The analytical solution derived here has been compared with a numerical model developed by the multiphysics software code COMSOL which shows excellent agreement with each other. Lastly it is shown that approximation of the properties of a geothermal aquifer by taking mean of the properties of all the layers present will lead to erroneous estimation of the temperature distribution. Chapter 6 represents a coupled three-dimensional thermo-hydrogeological numerical model for transient temperature distribution in a confined porous geothermal aquifer due to cold-water injection. This 3D numerical model is developed for solving more practical problems which eliminate the assumptions taken into account in analytical models. The numerical modeling is performed using a software code DuMux as mentioned before. Besides modeling the three-dimensional transient temperature distribution in the model domain, the chapter investigates the regional groundwater flow has been found to be a very important parameter to consider. The movement of the thermal front accelerates or decelerates depending on the direction of the flow. Influence of a few parameters involved in the study on the transient heat transport phenomenon in the geothermal reservoir domain, namely the injection rate, the permeability of the confining rocks and the thermal conductivity of the geothermal aquifer is also evaluated in this chapter. The models have been validated using analytical solutions derived in this thesis. The results are in very good agreement with each other. In Chapter 7 the main conclusions drawn from the study have been enlisted and the scope of further research is also pointed out.

Les travaux présentés dans ce manuscrit portent sur l’optimisation d’une chaufferie collective multi-énergie en l’équipant d’un système de stockage thermique de type hydro-accumulation. L’efficacité de ce dernier dépend de sa capacité à conserver son énergie initiale. Ainsi, pour minimiser les pertes thermiques, le système de stockage doit être correctement isolé. Un modèle de ballon d’hydro-accumulation stratifié est développé et validé expérimentalement. Une étude paramétrique est menée afin d’étudier l’impact, sur les pertes thermiques, de paramètres géométriques et météorologiques. Ensuite, une approche de dimensionnement, fondée sur une stratégie de gestion séquentielle et une étude paramétrique est proposée. Plusieurs critères énergétiques et économiques peuvent ainsi être évalués pour différents volumes de stockage thermique. L’approche de dimensionnement proposée a été appliquée à plusieurs sites exploités par Cofely GDF-Suez, notre partenaire industriel. Les résultats obtenus montrent que le dimensionnement optimal du système de stockage et la gestion intelligente du procédé permettent d'optimiser le fonctionnement d’un site. Des économies énergétiques et financières importantes peuvent ainsi être réalisées. La stratégie de gestion séquentielle proposée n’anticipe pas les besoins du réseau de chaleur. C’est pourquoi une stratégie fondée sur la commande prédictive (MPC pour Model Predictive Control) est susceptible d’améliorer le fonctionnement et les performances d’une chaufferie collective équipée d’un système d’hydro-accumulation. Afin de mettre en œuvre un tel contrôleur, la puissance thermique demandée par le réseau de chaleur doit être correctement prédite. Par conséquent, une méthode de prédiction a été développée. Elle s’appuie sur une analyse multi-résolution par transformée en ondelettes discrète et sur l’utilisation de réseaux de neurones artificiels de type perceptron multicouche. La stratégie séquentielle (non prédictive) et la stratégie prédictive ont été appliquées à une chaufferie collective mixte située dans une commune du nord de la France. Pour ce cas d’étude, la stratégie prédictive est plus efficace. De plus, les résultats montrent que, quelle que soit la stratégie utilisée, équiper la chaufferie d’un système d’hydro-accumulation correctement dimensionné est un investissement rentable. Il permet de minimiser la consommation d’énergie fossile et, par conséquent, le coût économique et les émissions de CO2. / The present work deals with optimizing a multi-energy district boiler by adding to the plant a thermal water storage tank. The effectiveness of such a system depends on how long the stored energy can be kept without considerable degradation. The storage tank should be properly insulated to reduce the rate of heat loss. Thus, firstly, a stratified water thermal storage model is developed and experimentally validated. A parametric study is carried out to determine the influence of geometric and meteorological parameters on heat loss. Next, a reliable sizing method based on a sequential management strategy and a parametric study is proposed. Various energy and economic criteria have been evaluated for a range of thermal storage sizes. The proposed methodology has been applied to many plants managed by Cofely GDF-Suez, our industrial partner. Results highlight the ability of a thermal storage tank (optimally sized and managed) to improve the operation of a multi-energy district boiler and realize significant energy and economic savings. The main drawback of the proposed sequential management strategy lies in not taking into account the future power demand. That is why a strategy based on a Model Predictive Controller (MPC) is likely to improve operation and performance. In order to implement such a controller, the power demand has to be accurately forecasted. As a consequence, a short-term forecast method, based on wavelet-based Multi-Resolution Analysis (MRA) and multilayer Artificial Neural Networks (ANN) is proposed. Both the sequential and the predictive strategies are applied to a northeast France multi-energy district boiler selected as a case study. The main result to retain is that the efficiency of water thermal storage tank is mainly related to its design and the way it is managed. For this case study, the predictive strategy regardless the size of the storage tank, the predictive strategy is more reliable. Furthermore, in all cases an adequately sized and managed thermal storage tank is a profitable investment. It allows the fossil energy consumption to be significantly reduced. The same remark applies to the functioning costs and CO2 emissions.

Presented work deals with appropriate ways of storing thermal energy and selection of materials for a high capacity storage unit. The thesis describes technologies currently used for long-term storage of thermal energy, the structural arrangement of the thermal unit, physical properties of the materials usable for construction of high-temperature storage units and the utilization of these materials. The thesis also describes the measuring procedure of the material characteristics and the method of evaluation. Size of the solar collector and the required volume of the storage for heat supplying of the selected building in the climatic conditions of the Czech Republic were designed.

The thesis is focused on cogeneration units on vegetable oil. The first chapter contains basic information about the production, supply and consumption of electric energy in the Czech Republic and about renewable energy sources. The following chapter is devoted to possibilities of use oil kinds of phytomass in the Czech Republic, or products made from them. Next chapter is mentioned on legislation allowing the use of used cooking oils for energy purposes. It contains a summary of the laws and regulations and options regarding subsidies for electric and heat power. The main objective of these chapters is to explain the issue of the establishment and operation of cogeneration units on vegetable oil. The following chapters are devoted to the design and study of simultaneously operated cogeneration unit on the vegetable oil with a combustion engine and the calculation of economic operation running. The reason of study is to evaluation, which on based of the input values and calculation methods determine the economic efficiency of the project cogeneration unit on vegetable oil under mode purchase price and mode green bonus. Appreciation is also given to the overall legal review and responsibilities of cogeneration units on vegetable oil.

This study has examined how an oil storage could be converted into a thermal heat storage (TES). Focus was put on the transient thermal heat flow that occurs during the early years when using a rock cavern as a TES. First existing literature were studied to learn from earlier experiences. Crucial steps of a conversion were identified as well as important mistakes that have been made in the past. Simulations of Sundsvall’s district heating (DH) system were made to see what impact a large TES would have. These simulations showed the importance of being able to transfer enough amount of heat. Then heat simulations were preformed to study the transient heat flow. This showed that much of the heat will be heating the rock around the cavern. Over time, the losses decrease as the rock around the cavern remains heated, due to its thermal inertia. This means that some energy needs to be considered an investment cost as it will not be used in the DH-grid but will increase the efficiency of the TES. 4 different heating strategies were analysed and the heat losses during 25 years were measured. The results showed that a conversion would save between 0,7 – 1,55 million SEK annually depending on how many caverns were converted and cost approximately 6 million SEK for one cavern, 10,5 million SEK for two caverns and 15 million SEK for three caverns. / I denna studie har det undersökts hur ett oljelager kan omvandlas till ett termisk värmelager. Fokus låg på det transienta värmeflödet som inträffar under de första åren när ett bergrum används som värmelager. Först studerades litteratur för att lära av tidigare erfarenheter. Avgörande steg för en konvertering identifierades liksom viktiga misstag som har gjorts tidigare. Simuleringar av Sundsvalls fjärrvärmesystem gjordes för att se vilken påverkan ett stort värmelager skulle ha. Dessa simuleringar visade vikten av att kunna överföra tillräcklig mängd värme. Därefter genomfördes värmesimuleringar för att studera det transienta värmeflödet. Detta visade att mycket av värmen kommer att värma berget runt bergrummet. Med tiden minskar förlusterna när berget runt rummet förblir uppvärmd på grund av dess termiska tröghet. Detta innebär att en del energi måste betraktas som en investeringskostnad eftersom den inte kommer att användas i fjärrvärmesystemet utan kommer att öka effektiviteten hos lagret. Fyra olika uppvärmningsstrategier analyserades och värmeförlusterna under 25 år mättes. Resultaten visade att en omvandling skulle spara mellan 0,7 - 1,55 miljoner SEK årligen med en trivial driftstrategi beroende på hur många bergrum som konverterades och kosta cirka 6 miljoner SEK för ett bergrum, 10,5 miljoner SEK för två bergrum och 15 miljoner SEK för tre bergrum. I framtida studier bör en optimal driftstrategi tas fram utifrån det aktuella systemet.

The subject of the diploma thesis is accumulation tanks in heating systems. The first part introduces the topic of thermal energy storage and types of accumulation tanks. The second part describes two specific options of heating the building of the boarding house and the restaurant with the use of storage tanks in the heating system. A gas condensing boiler is designed as a heat source in one variant and a pellet boiler in the other variant. The third part deals with experimental measurements, in which the accumulation tank with the installation for better stratification of hot water and the accumulation tank without the installation are compared.

Utilizing Phase Change Materials (PCM) for Latent Thermal Energy Storage (LTES) applications have previously been extensively researched as a measure to reduce greenhouse gas emissions from energy consumption. In order to make use of the waste heat from industrial processes for LTES purposes, a new demand emerged for PCMs capable of phase change in mid-temperature ranges of 100 °C - 200 °C. This higher temperature requirement made most of the previously studied material inapplicable as they had much lower melting and solidification temperatures. With this in mind, a new generation of PCMs consisting of Sugar Alcohols (SA) has been proposed. Erythritol is seen as an especially promising SA with good thermophysical properties for LTES purposes. However, it has been shown to suffer from severe supercooling, which makes it unreliable in real applications. To eradicate this issue, two additives, Graphene Oxide (GO) and Polyvinylpyrrolidone (PVP) at varying mass fractions were mixed with pure erythritol to form a composite which was studied using the Temperature-history (T-history) method to determine its effectiveness in reducing supercooling. Results show that at its most effective mass fraction, GO reduces supercooling by 28 o C and a 31 o C reduction is seen by the addition of PVP. The impacts on the duration of crystallization was also documented and analyzed using the same method. It was observed that the duration of crystallization was increased with increasing mass fractions of the additives. Other important properties of the composites were also studied in order to determine the overall feasibility for industrial applications. It includes analysis of the storage capacity through latent heat, changes in viscosity along with impacts on thermal diffusivity of the composites. / Att använda fasändringsmaterial (PCM) för termisk energilagring i form av latent värme (LTES) har tidigare extensivt forskats och undersökts som en lösning för att minska utsläppen av växthusgaser från energiförbrukning. För att utnyttja spillvärme från industriella processer för LTES-ändamål uppstod en efterfrågan på PCM som ändrar fas i temperaturer mellan 100 °C - 200 °C. Detta krav på högre temperatur gjorde att de flesta av de tidigare aktuella materialen inte kunde tillämpas eftersom de hade mycket lägre smält- och kristalliseringstemperaturer. Med detta i åtanke har en ny generation av PCM bestående av sockeralkoholer (SA) föreslagits. Erytritol ses som ett särskilt lovande SA med goda egenskaper för LTES-ändamål. Den har dock visat sig drabbas av svår underkylning, vilket gör den opålitligt i verkliga tillämpningar. För att utrota detta problem blandades två tillsatser, Graphene Oxide (GO) och Polyvinylpyrrolidone (PVP) vid olika massfraktioner med ren erytritol för att bilda en komposit som studerades med metoden Temperature-history (T-history) för att bestämma dess effektivitet på att minska underkylningen. Resultaten visar att GO på sin mest effektiva massfraktion minskar underkylningen med 28 o C och tillsats av PVP lyckats minska den med som mest 31 o C. Påverkningarna på varaktighet av kristallisering dokumenterades och analyserades med samma metod. Det var observerad att varaktigheten av kristallisering ökades med ökande massfraktioner av tillsatserna. Även andra viktiga egenskaper hos kompositerna studerades för att avgöra rimligheten att använda dessa för industriella tillämpningar. Det inkluderar analys av lagringskapaciteten genom latent värme, förändringar i viskositet tillsammans med påverkan på kompositernas termiska diffusivitet.

Bei oberflächennahen geothermischen Anwendungen kommt es zu Temperaturveränderungen im Untergrund, um Wärme zu gewinnen oder zu speichern. Die dadurch verursachten thermischen Auswirkungen können die thermischen, hydraulischen und chemischen Bedingungen des Untergrunds verändern und sich auf die Systemleistung auswirken. Die Situation wird noch komplizierter, wenn die Grundwasserleiter unterschiedlich gesättigt sind und eine Verunreinigung des Untergrunds vorliegt. In dieser Arbeit werden vollständig gekoppelte Wärme- und Stofftransportmodelle angewandt, um die Wechselwirkung zwischen oberflächennahen geothermischen Anwendungen und der unterirdischen Umgebung besser zu charakterisieren. Es werden drei verschiedene Szenarien untersucht. Zunächst werden die thermischen Auswirkungen und die Nachhaltigkeit einer intensiven oberflächennahen geothermischen Nutzung in einem Wohngebiet über einen Zeitraum von 24 Jahren bewertet. Überwachungsdaten des Standorts werden in das Modell integriert, und die Bedeutung standortspezifscher Kenntnisse für die Planung wird hervorgehoben. Zweitens wird der gekoppelte Feuchtigkeits- und Wärmetransport eines Erdwärmespeichersystems (BTES) untersucht. Es wird eine Sensitivitätsanalyse durchgeführt und die Wärmeentzugsefzienz zwischen verschiedenen Szenarien verglichen. Bei Austrocknung oder starker Gasphasenkonvektion werden signifkante Änderungen im Wärmetransportverhalten erwartet. Drittens wird ein nichtisothermes Drei-Komponenten-Zweiphasenströmungsmodell abgeleitet und mit der Finite-Elemente-Methode implementiert. Die Validierung des numerischen Modells bestätigt seine Fähigkeit, die gekoppelte Strömung sowie den Wärme- und Stofftransport in einem ungespannten Grundwasserleiter zu simulieren, der einem BTES-Betrieb ausgesetzt ist. Insbesondere wird das Potenzial für die thermisch verstärkte Verflüchtigung von Trichlorethylen (TCE) in wässriger Phase bewertet. Da durch die Wärmezufuhr eine Auftriebsströmung induziert wird, nimmt die Verringerung der Schadstoffmasse erheblich zu und erreicht nach fünf Jahren mehr als 70 %. Die in dieser Arbeit gewonnenen Erkenntnisse können durch die Verbesserung ihrer ökologischen und ökonomischen Leistungen zu einer breiteren Akzeptanz von Technologien der flachen Geothermie beitragen. / Shallow geothermal applications induce temperature changes in the subsurface for heat extraction or storage purposes. Their induced thermal impacts may alter the thermal, hydraulic, and chemical conditions of the subsurface and feedback on the system performance. The situation is further complicated in variably saturated aquifers and when subsurface contamination exists. In this work, fully coupled heat and mass transport models are applied to improve the characterization of interaction between shallow geothermal applications and the subsurface environment. Three different scenarios are investigated. First, the thermal impact and sustainability of intensive shallow geothermal exploitation in a residential area is evaluated over a 24-year period. Monitoring data from the site is integrated into the model and the importance of site-specifc knowledge for planning is highlighted. Second, the coupled moisture and heat transfer of a soil borehole thermal energy storage (BTES) system is explored. A sensitivity analysis is performed and the heat extraction efciency is compared among various scenarios. Signifcant changes in the heat transport behavior are expected when drying out or strong gas phase convection occurs. Third, a non-isothermal three-component two-phase flow model is derived and implemented with the fnite element method. Validation of the numerical model confrms its ability to simulate the coupled flow, heat and mass transport in an unconfned aquifer subject to BTES operation. In particular, the potential for thermally-enhanced volatilization of aqueousphase trichloroethylene (TCE) is assessed. As buoyant flow is induced due to heat injection, reduction of the contaminant mass grows considerably, reaching more than 70% after fve years. The fndings obtained from this thesis can contribute to a wider adoption of shallow geothermal technologies through the enhancement of their environmental and economical services.

Utilizing Phase Change Materials (PCM) for Latent Thermal Energy Storage (LTES) applications have previously been extensively researched as a measure to reduce greenhouse gas emissions from energy consumption. In order to make use of the waste heat from industrial processes for LTES purposes, a new demand emerged for PCMs capable of phase change in mid-temperature ranges of 100 °C - 200 °C. This higher temperature requirement made most of the previously studied material inapplicable as they had much lower melting and solidification temperatures. With this in mind, a new generation of PCMs consisting of Sugar Alcohols (SA) has been proposed. Erythritol is seen as an especially promising SA with good thermophysical properties for LTES purposes. However, it has been shown to suffer from severe supercooling, which makes it unreliable in real applications. To eradicate this issue, two additives, Graphene Oxide (GO) and Polyvinylpyrrolidone (PVP) at varying mass fractions were mixed with pure erythritol to form a composite which was studied using the Temperature-history (T-history) method to determine its effectiveness in reducing supercooling. Results show that at its most effective mass fraction, GO reduces supercooling by 28 oC and a 31 oC reduction is seen by the addition of PVP. The impacts on the duration of crystallization was also documented and analyzed using the same method. It was observed that the duration of crystallization was increased with increasing mass fractions of the additives. Other important properties of the composites were also studied in order to determine the overall feasibility for industrial applications. It includes analysis of the storage capacity through latent heat, changes in viscosity along with impacts on thermal diffusivity of the composites. / Att använda fasändringsmaterial (PCM) för termisk energilagring i form av latent värme (LTES) har tidigare extensivt forskats och undersökts som en lösning för att minska utsläppen av växthusgaser från energiförbrukning. För att utnyttja spillvärme från industriella processer för LTES-ändamål uppstod en efterfrågan på PCM som ändrar fas i temperaturer mellan 100 °C - 200 °C. Detta krav på högre temperatur gjorde att de flesta av de tidigare aktuella materialen inte kunde tillämpas eftersom de hade mycket lägre smält- och kristalliseringstemperaturer. Med detta i åtanke har en ny generation av PCM bestående av sockeralkoholer (SA) föreslagits. Erytritol ses som ett särskilt lovande SA med goda egenskaper för LTES-ändamål. Den har dock visat sig drabbas av svår underkylning, vilket gör den opålitligt i verkliga tillämpningar. För att utrota detta problem blandades två tillsatser, Graphene Oxide (GO) och Polyvinylpyrrolidone (PVP) vid olika massfraktioner med ren erytritol för att bilda en komposit som studerades med metoden Temperature-history (T-history) för att bestämma dess effektivitet på att minska underkylningen. Resultaten visar att GO på sin mest effektiva massfraktion minskar underkylningen med 28 oC och tillsats av PVP lyckats minska den med som mest 31 oC. Påverkningarna på varaktighet av kristallisering dokumenterades och analyserades med samma metod. Det var observerad att varaktigheten av kristallisering ökades med ökande massfraktioner av tillsatserna. Även andra viktiga egenskaper hos kompositerna studerades för att avgöra rimligheten att använda dessa för industriella tillämpningar. Det inkluderar analys av lagringskapaciteten genom latent värme, förändringar i viskositet tillsammans med påverkan på kompositernas termiska diffusivitet.

<p>The use of renewable energy systems is ever-growing in today's electricity grid to reduce the carbon footprint on the environment. However, a problem with wind and solar renewable energy systems is availability. Wind and solar energy production are entirely dependent on the weather, whereas global electricity demands have no such limitation. A cost-effective solution to the energy availability problem is to incorporate energy storage systems. The Economic Long-Duration Electricity Storage by Using Low-Cost Thermal Energy Storage and High-Efficiency Power Cycle (ENDURING) system developed at the National Renewable Energy Laboratory (NREL) is a potential  energy storage system. In the ENDURING system, particles are heated via renewable energy or off-peak grid electricity and stored in large silos. When the electricity needs to be regenerated, the hot particles are passed to a Pressurized Fluidized Bed Heat Exchanger (PFB-HX), which heats air, and the hot pressurized air flows to a turbine and generator to produce electricity. The focus of this dissertation is on two components within the ENDURING system: the particle heater and the PFB-HX.</p> <p>First, the heat transfer within the particle heater is investigated numerically via Computational Fluid Dynamics (CFD) coupled with Discrete Element Modeling (DEM). Although heat transfer to traditional molecular fluids such as liquids and gases are well characterized, the heat transfer to flowing particles is less understood. The heater surface angle, particle-particle and particle-wall friction coefficients, and contact resistance are parametrically varied to discover their individual effects on the heat transfer process. A separate set of simulations is conducted to compare against an experimental particle heater built at NREL. In addition to elucidating the heat transfer performance, the simulations also reveal oscillatory flow patterns. It is discovered that such turbulent behavior is related to the geometry of the heater elements.<br> </p> <p>Second, a laboratory-scale experimental setup of the PFB-HX is built. The temperature, pressure drop, and minimum fluidization velocity are used to characterize the heat transfer and assess the capabilities of the PFB-HX. High-temperature fluidized bed experiments with an initial temperature gradient are performed. The bed becomes fluidized, but temperature gradients remain, and the bed is not fully mixed. At sufficient superficial velocity, the bed temperature becomes uniform. CFD-DEM coupled simulations are performed to investigate the temperature distributions more precisely. Initial bed temperature differences of 100, 300, and 500K are simulated with varying superficial velocities to create a regime map. The purpose of the regime map is to determine when the fluidized bed temperature becomes fully mixed for different initial conditions and gas velocities. The overall goal of this work is to understand the heat transfer processes of the flowing particles in both the particle heater and the PFB-HX to aid in the design of the ENDURING system.</p>