Unstructured modelling of solid-liquid phase change using parallel computing. Application to the analysis of thermal energy storage systems with encapsulated phase change materials

The main goal of this work is to numerically simulate convection phenomena in solid-liquid phase change processes and its application in the analysis of a thermal energy storage system with phase change materials. First two chapters consist in the resolution of two well-known numerical problems used as an introduction to the governing equations of uid ows and heat transfer. Here, the necessary tools for discretizing and solving these equations are brie y described. Third chapter speci cally deals with the problem of modelling solid-liquid phase change and its application into the simulation of a special thermal storage system

Numerical simulations of thermal storage systems : emphasis on latent energy storage using phase change materials (PCM)

The present thesis aims at studying the use of phase change materials (PCM) in thermal energy storage (TES) applications and to develop and implement numerical tools for their evaluation. Numerical analysis is nowadays an indispensable tool for the design, evaluation and optimization of thermal equipment, complementing the experimental techniques. Two levels of analysis are carried out, one in the field of Computational Fluid Dynamics, allowing the accurate simulation of the complex heat transfer and fluid dynamics phenomena present in solid-liquid phase change problems; and another one in which the governing equations are treated assuming several suitable simplifications and integrating empirical correlations, intended for the study of whole thermal storage systems throughout several charge/discharge cycles. Furthermore, the specific application of thermal storage in concentrated solar power (CSP) stations is studied. Different single-tank systems, making use of both sensible and latent energy capacities of the materials, are evaluated and compared against the two-tank molten-salt systems used in current CSP plants. Moreover, a new single-tank TES concept which combines the use of solid and PCM filler materials is proposed, with promising results for its utilization in CSP. In chapters 2 and 3, a numerical fixed-grid enthalpy model for the simulation of the solid-liquid phase change is developed. This technique is implemented using the Finite Volume Method in a collocated unstructured domain discretization and using explicit time integration schemes. Issues regarding the form of the energy equation, the treatment of the pressure equation as well as the momentum source-term coefficient introduced by the enthalpy-porosity method, are described in detail in the first chapter. In the second, the possibility of taking into account the variation of the different thermo-physical properties with the temperature is dealt with. Thermal expansion and contraction associated to the phase change are taken into account in the conservation equations and different strategies for the numerical treatment of the energy equation are discussed in detail. Furthermore, simulations of an interesting case of melting of an encapsulated PCM are carried out using two and three-dimensional meshes, and the results are compared against experimental results from the literature. In the next two chapters, the issue of numerically simulating whole single-tank TES systems is developed. These systems are composed of a single tank filled with solid and/or PCM materials, forming a packed bed through which a heat transfer fluid flows. Thermal stratification separates the fluid layers at different temperatures. The zone in which a steep temperature gradient is produced is called "thermocline", and it is desirable to maintain it as narrow as possible in order to keep a high stored exergy. Different designs of single-tank TES systems ¿classified according to the filler material/s used¿ are evaluated for CSP plants. The analysis is performed evaluating different aspects, as the energy effectively stored/released and the efficiency in the use of the theoretical capacity after several charge/discharge cycles, obtaining results independent of the initial thermal state. The operating time is not fixed, but depends on the temperature of the fluid coming out of the tank, limited by the restrictions of the receiving equipment (solar field and power block). Degradation of the stratification is observed to occur after several cycles, due to the temperature restrictions. In this context, a new concept of single-tank TES is presented, which consists of the combination of different layers of solid and PCM filler materials in a suitable manner, resulting in a lower degradation of the thermocline and increasing the use of the theoretical capacity. This concept, called Multi-Layered Solid PCM (MLSPCM), is demonstrated as a promising alternative for its use in CSP plants.Esta tesis se centra en el estudio del uso de materiales de cambio de fase (PCM) en el almacenamiento de energía térmica (TES) y en el desarrollo de herramientas numéricas para su evaluación. El análisis numérico es hoy en día una herramienta indispensable para el diseño, evaluación y optimización de equipos térmicos, complementando las técnicas experimentales. Se realizan dos niveles de análisis, uno en el campo de la dinámica de fluidos computacional, permitiendo la simulación precisa de fenómenos complejos de transferencia de calor y dinámica de fluidos presentes en los problemas de cambio de fase sólido-líquido; y otro en la que las ecuaciones gobernantes son tratadas mediante simplificaciones razonables e integrando correlaciones empíricas, destinado al estudio de sistemas TES en varios ciclos de carga/descarga. Por otra parte, se estudia el almacenamiento térmico para plantas de generación termosolar (CSP). Se evalúan diferentes sistemas de un solo tanque, utilizando tanto las capacidades de energía sensible como latente de los materiales, y se comparan con los sistemas de sales fundidas de doble tanque utilizados actualmente. Además, se propone un concepto novedoso de TES de un único tanque que combina el uso de materiales de relleno sólidos y PCM, con resultados prometedores para su utilización en CSP. En los capítulos 2 y 3, se desarrolla un modelo de entalpía de malla fija para la simulación de la fusión y solidificación. Se utiliza una discretización por volúmenes finitos en mallas no estructuradas en un esquema colocado, y esquemas de integración temporal explícitos. En el primer capítulo, se discuten cuestiones relativas a la forma de la ecuación de energía, el tratamiento de la ecuación de presión, así como el coeficiente de término fuente en la ecuación de momentum introducido por el método de entalpía-porosidad. En el segundo, se trata la posibilidad de tener en cuenta la variación de las propiedades termofísicas con la temperatura. La expansión/contracción térmica asociada al cambio de fase se tiene en cuenta en las ecuaciones de conservación y se tratan en detalle diferentes estrategias para el tratamiento numérico de la ecuación de la energía. Además, se realizan simulaciones de un caso interesante de fusión de un PCM encapsulado, utilizando mallas bi y tridimensionales, y los resultados se comparan con otros de la literatura. En los dos capítulos siguientes, se desarrolla el tema de la simulación numérica de sistemas TES de un único tanque. Estos sistemas están compuestos de un tanque relleno de materiales sólidos y/o PCM, formando un lecho poroso a través del cual circula un fluido de transferencia de calor. La estratificación térmica separa las capas de fluido a diferentes temperaturas. La zona en donde se da el mayor gradiente de temperaturas vertical se conoce generalmente como "termoclina", la cual es deseable mantenerla lo más angosta posible, con el fin de mantener una mayor exergía almacenada. Diferentes diseños de sistemas de un solo tanque -clasificados de acuerdo con el/los material/es de relleno utilizado/s- se evalúan para plantas de CSP. El análisis se realiza evaluando diferentes aspectos, como la energía efectivamente almacenada/liberada y la eficiencia en el uso de la capacidad teórica luego de varios ciclos de carga/descarga, obteniendo resultados independientes del estado térmico inicial. El tiempo de operación no es fijo, sino que depende de la temperatura del fluido de salida, limitada por las restricciones de los equipos que lo reciben (campo solar y bloque de potencia). Se observa una degradación de la estratificación a lo largo de los ciclos debido a las restricciones de temperatura. En este contexto, se presenta concepto de TES novedoso, combinando de diferentes capas de materiales de relleno sólidos y PCM de una manera adecuada. Este concepto, llamado "multi-layered solid-PCM" (MLSPCM) resulta ser una alternativa prometedora para su uso en plantas de CS

Multi-layered solid-PCM thermocline thermal storage concept for CSP plants. Numerical analysis and perspectives

Thermocline storage concept has been considered for more than a decade as a possible solution to reduce the huge cost of the storage system in concentrated solar power (CSP) plants. However, one of the drawbacks of this concept is the decrease in its performance throughout the time. The objective of this paper is to present a new thermocline-like storage concept, which aims at circumventing this issue. The proposed concept consists of a storage tank filled with a combination of solid material and encapsulated PCMs, forming a multi-layered packed bed, with molten salt as the heat transfer fluid. The performance evaluation of each of the prototypes proposed is virtually tested by means of a detailed numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. The virtual tests carried out are designed so as to take into account several charging and discharging cycles until periodic state is achieved, i.e. when the same amount of energy is stored/released in consecutive charging/discharging cycles. As a result, the dependence of the storage capacity on the PCMs temperatures, the total energy and exergy stored/released, as well as the efficiencies of the storing process are compared for the different thermocline, single PCM, cascaded PCM and the proposed multi-layered solid-PCM (MLSPCM) configurations. The analysis shows that the multi-layered solid-PCM concept is a promising alternative for thermal storage in CSP plants.Peer ReviewedPostprint (author’s final draft

A new thermocline-PCM thermal storage concept for CSP plants. Numerical analysis and perspectives

Thermocline storage concept has been considered for more than a decade as a possible solution to reduce the huge cost of the storage system in CSP plants. However, one of the drawbacks of this concept is the decrease in its performance throughout the time. The objective of this paper is to present a new thermocline-PCM storage concept which aims at circumventing this issue. The concept proposed is built of different solid filler materials and encapsulated PCMs combined into a multi-layer storage tank with molten salt as heat transfer fluid. The performance evaluation of each of the prototypes proposed is virtually tested by means of a detailed numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. The virtual tests carried out are designed so as to take into account several charging and discharging cycles until equilibrium is achieved, i.e. the same amount of energy stored in the charging phase is delivered in the discharge. As a result, the dependence of the storage capacity on the PCMs temperatures, the total energy stored/released, as well as the efficiencies of the storing process have been compared for the different thermocline, PCM-only and multi-layered thermocline-PCM configurations. Based on this analysis the selection of the best option for a given case/plant is proposed.Peer ReviewedPostprint (published version

Multi-layered solid-PCM thermocline thermal storage for CSP. Numerical evaluation of its application in a 50 MWe plant

Thermocline storage concept is considered as a possible solution to reduce the cost of thermal storage in concentrated solar power (CSP) plants. Recently, a multi-layered solid-PCM (MLSPCM) concept—consisting of a thermocline-like tank combining layers of solid and phase change filler materials—has been proposed. This approach was observed to result in lower thermocline degradation throughout charge/discharge cycles, due to the thermal buffering effect of the PCM layers located at both ends of the tank. MLSPCM prototypes designed for a pilot scale plant were numerically tested and compared against other designs of single-tank thermocline systems, such as: solid-filled thermocline, tanks filled with a single encapsulated PCM and cascaded-PCM configurations. Results showed promising results of the MLSPCM configurations for their potential use in CSP plants. In this work, the MLSPCM concept is used for designing a thermal energy storage (TES) system for a CSP plant with the dimensions and operating conditions of a parabolic trough plant of 50 MWe, similar to Andasol 1 (Granada, Spain). The performance evaluation of each of the proposed prototypes is virtually tested by means of a numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. Two sets of cases are considered, one with the objective of testing the TES systems individually, by defining specific operating conditions and taking the systems to a periodic steady state; and another, aiming to evaluate their performance after several days of operation in a CSP plant, in which the weather variability and the thermal behavior of the tank walls and foundation are simulated. Thermal performance parameters, such as total energy and exergy stored/released and the efficiency in the use of the storage capacity, are calculated and compared with those obtained by other thermocline-like configurations (single-solid and single-PCM), and with a reference 2-tank molten-salt system. Obtained results allow to continue considering the MLSPCM concept as an interesting alternative for thermal storage in CSP facilities.Peer ReviewedPostprint (author’s final draft

Unstructured modelling of solid-liquid phase change using parallel computing. Application to the analysis of thermal energy storage systems with encapsulated phase change materials

The main goal of this work is to numerically simulate convection phenomena in solid-liquid phase change processes and its application in the analysis of a thermal energy storage system with phase change materials. First two chapters consist in the resolution of two well-known numerical problems used as an introduction to the governing equations of uid ows and heat transfer. Here, the necessary tools for discretizing and solving these equations are brie y described. Third chapter speci cally deals with the problem of modelling solid-liquid phase change and its application into the simulation of a special thermal storage system

Unstructured modelling of solid-liquid phase change using parallel computing. Application to the analysis of thermal energy storage systems with encapsulated phase change materials

The main goal of this work is to numerically simulate convection phenomena insolid-liquid phase change processes and its application in the analysis of a thermalenergy storage system with phase change materials.First two chapters consist in the resolution of two well-known numerical problemsused as an introduction to the governing equations of uid ows and heat transfer.Here, the necessary tools for discretizing and solving these equations are brieydescribed.Third chapter speci cally deals with the problem of modelling solid-liquid phasechange and its application into the simulation of a special thermal storage system

Hybrid navigation by Decca and dead reckoning

Translated from Japanese (J. Fac. Mar. Sci. Tech. Tokai Univ. 1984 (19) p. 59-76)SIGLEAvailable from British Library Document Supply Centre- DSC:3623.66(DRIC-Trans--7549)T / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Fixed-grid numerical modeling of melting and solidification using variable thermo-physical properties - Application to the melting of n-Octadecane inside a spherical capsule

A fixed-grid enthalpy model for unstructured meshes and explicit time integration schemes (Galione et al., 2014) is here extended for taking into account the change in density and other thermo-physical properties with the temperature and phase. Thermal expansion and contraction associated to the phase change are taken into account in the conservation equations, and different strategies for the numerical treatment of the energy equation are discussed in detail. Further modifications to the original model are also presented.; The proposed model is used for simulating a case of melting of n-Octadecane inside a spherical capsule. Two and three-dimensional simulations are performed using constant and variable properties. The effect of adopting two different numerical schemes for the convective term of the energy equation is evaluated. A comprehensive examination of the thermo-physical properties is performed, and the different values and correlations used are here presented and criticized. Differences in the flow patterns are encountered between two and three-dimensional simulations. The effects of considering constant or variable properties are discussed. Two different thermal boundary conditions are tested and the results are compared against experimental data obtained from the literature.Peer Reviewe

Unstructured 3D numerical modeling of the melting of a PCM contained in a spherical capsule

Peer ReviewedPostprint (published version