Numerical simulation of sensible and latent thermal energy storage systems

El objetivo principal de esta tesis es la resolución numérica de problemas de transferencia de calor y dinámica de fluidos y su aplicación para el estudio del comportamiento transitorio de sistemas térmicos de acumulación de energía (TES). Tres diferentes sistemas han sido considerados, cubriendo un amplio rango de condiciones de trabajo (desde muy baja de temperatura criogenica hasta muy alta temperatura en plantas CSP) y aplicaciones (desde domestica/residencial hasta energía renovables o de la industria aeroespacial). En este sentido, i) sistemas recuperadores de energía térmica para residencias en el rango de bajo a media temperatura; ii) un acumulador térmico usado un sistema de propulsión criogenica en el espacio en el rango de baja temperatura y; iii) sistema de acumulación térmica del tipo dos tanque para plantas solares de alta concentración en el rango de alta temperatura. La tesis esta dividida en cinco capítulos. El capitulo 2, esta dedicado en presentar la metodología empleada para la resolución computacional de la dinámica de fluidos y problemas de transferencia de calor en un dispositivo con almacenamiento para la recuperación de energía térmica de agua que se vierte al drenaje para viviendas residenciales. El estudio de las características del dispositivo fue realizado usando herramientas numéricas y experimentales. La simulación numérica fue realizada usando la plataforma NEST. La discretización de la ecuaciones de gobierno basadas en técnicas de volúmenes finitos. Correlaciones empíricas especiales han sido implementadas para ser usadas en la resolución numérica del flujo de fluido dentro de una tubería en espiral. Una infraestructura experimental ha sido desarrollada para el análisis del sistema. Diferentes flujos másicos y temperaturas de operación han sido estudiados. Los resultados numéricos han sidos comparados con resultados experimentales. La simulación numéricas realizadas predicen razonablemente bien el comportamiento transitorio de estos dispositivos. El capitulo 3, enfoca su atención en un prototipo de acumulador para baja temperatura usado para sistemas de propulsión criogenica en el espacio. Las simulaciones numéricas fueron realizadas usando la plataforma NEST. En este capitulo, dos modelos numéricos son adaptados, uno para resolver el flujo bifásico a través de tuberías bajo condiciones criogenicas, y otra para solucionar el material de cambio de fase usando un modelo entalpico de malla fija. El análisis numérico se basa en: i) la resolución unidimensional y transitoria de las ecuaciones gobernantes del fluido propulsor; ii) resolución multidimensional y transitoria de las ecuaciones gobernantes en la región ocupada por el material de cambio de fase, incorporando modelo de turbulencia para solucionar el fenómeno de convección que se produce; iii) los elementos solidos son modelados considerando un tratamiento multidimensional y transitorio de la ecuación de la energía. Los resultados numéricos son comparados con resultados experimentales de la literatura. La validación experimental bajo diferentes condiciones de trabajo de fluido criogenico y/o del material de cambio de fase muestra las posibilidades de este modelo para fines de optimización del diseño y de predicción. El capitulo 4, esta enfocado en el desarrollo de modelos numéricos para la simulación de sistemas de acumulación de energía térmica de dos tanques en centrales solares de alta concentración. La simulación numérica fue desarrollada dentro de la plataforma NEST, donde los diferentes elementos que componen el sistema son asociados para solucionar todo el sistema. Algunos elementos del sistema han sido especialmente desarrollados. Los modelos matemáticos consideran el comportamiento transitorio de la sal fundida, el gas de la cavidad, las paredes del tanque y sus aislantes, diferentes configuraciones de cimientos, la radiación entre la sal y las paredes del tanque en la zona de la cavidad del gasThe main objective of this thesis is the numerical resolution of heat transfer and fluid flow problems and its application to study the unsteady behaviour of thermal energy storage (TES) systems. Three different systems have been considered, covering a wide range of working conditions (from very low cryogenic temperature to very high temperature CSP plant) and applications (from domestic/residential to renewable or aerospace industry). In that sense, i) for residential heat recovery systems in the low-to-medium temperature range; ii) a thermal accumulator used in a in-space cryogenic propulsion system in the low temperature range and; iii) the two-tank thermal energy storage system in concentrating solar power (CSP) plants in the high temperature range. The thesis is divided into five chapters. Chapter 2 is devoted to present the methodology employed for the resolution of Computational Fluid Dynamics (CFD) and Heat Transfer problems in a drain water heat recovery (DWHR) storage-type unit for residential housing. The study of the performance of this device has been carried out using both numerical and experimental tools. The numerical simulation has been performed using the NEST platform, where the different elements of the DWHR storage are linked to solve the system. The discretisation of the governing equations based on finite volume techniques (FVM) is presented. Numerical techniques such as the discretisation schemes, boundary conditions implementation and solution procedure for incompressible and transient flow problems are shown. Special empirical correlations have been implemented to be used in the numerical resolution of the single-phase flow inside the coiled pipe. An experimental infrastructure has been developed to analyse the whole system. Different internal flow rates and operational temperatures have been studied. The numerical results are compared against experimental results. The numerical simulations performed predict reasonably the transient behaviour of the DWHR storage device. Chapter 3 focuses the attention on a Low Temperature Accumulator prototype used in an in-space cryogenic propulsion system. The numerical simulation has been carried out using the NEST platform, where the different elements of the thermal accumulator are linked to solve the whole system. In this chapter, two numerical models have been adapted, one used to solve the two-phase flow inside ducts working under cryogenic conditions, and another to solve the PCM using a fixed-grid enthalpy model. The numerical analysis is based on: i) a one-dimensional and transient resolution of the governing equations for the fluid flow of propellant; ii) a multi-dimensional and transient resolution of the governing equations in the region occupied by the PCM, incorporating a model for the turbulence to solve the convection phenomena involved; iii) the solid elements are modelled considering a multidimensional and transient treatment of the thermal conduction equation. The numerical results are compared against experimental results from the literature. The experimental validation under different working conditions of the cryogenic flow and/or the PCM material shows the possibilities of this model for design optimization and prediction purposes. Chapter 4 is focused on the development of a numerical model for the simulation of the two-tank thermal energy storage system in concentrating solar power (CSP) plants. The numerical simulation has been performed using the NEST platform, where the different elements of the two-tank storage are linked to solve the system. Some elements of the system have been specifically developed. The mathematical model considers the transient behaviour of the molten salt fluid, the gas ullage, the tank walls and insulation, different configuration of the foundation, radiation exchange between the salt and the tank walls in the ullage. A parametric study of the two-tank storage system has been done, in order to identifyPostprint (published version

Dynamic simulation of indirect air conditioning systems with optimized computational time

Peer ReviewedPostprint (published version

Thermal System Oriented Simulation of Aircraft Electrical Environmental Control Systems Including its Electric Coupling

A flexible numerical platform based on libraries has been developed within the Dymola/Modelica framework to simulate Environmental Control Systems (ECS). The goal was to build up a flexible tool to analyse complex systems including their thermal and electrical perimeters at both steady and transient conditions focusing on three key characteristics: numerical robustness, optimal time consumption, and high accuracy. This document aims to underline both the most relevant features of the numerical tool and the main challenges addressed during its development. Some illustrative simulations are shown in order to highlight the tool capabilities.Peer ReviewedPostprint (published version

Thermo-mechanical parametric analysis of packed-bed thermocline energy storage tanks

© 2016. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/A packed-bed thermocline tank represents a proved cheaper thermal energy storage for concentrated solar power plants compared with the commonly-built two-tank system. However, its implementation has been stopped mainly due to the vessel’s thermal ratcheting concern, which would compromise its structural integrity. In order to have a better understanding of the commercial viability of thermocline approach, regarding energetic effectiveness and structural reliability, a new numerical simulation platform has been developed. The model dynamically solves and couples all the significant components of the subsystem, being able to evaluate its thermal and mechanical response over plant normal operation. The filler material is considered as a cohesionless bulk solid with thermal expansion. For the stresses on the tank wall the general thermoelastic theory is used. First, the numerical model is validated with the Solar One thermocline case, and then a parametric analysis is carried out by settling this storage technology in two real plants with a temperature rise of 100 °C and 275 °C. The numerical results show a better storage performance together with the lowest temperature difference, but both options achieve suitable structural factors of safety with a proper design.Peer ReviewedPostprint (author's final draft

Parametric study of two-tank TES systems for CSP plants

The two-tank thermal energy storage (TES) system is the most used technology for storage in concentrating solar power (CSP) plants. This work focuses on a parametric study, which aims to identify the most important parameters on TES system, in order to improve the design and increase the performance of the plant. Three parameters have been considered: meteorological data, insulation thickness of the storage tank, and configuration of the foundation of the storage tank. The effect of each parameter is evaluated using numerical simulations based on a modular object-oriented methodology. The main issues related to the mathematical models and its numerical methodology are also presented in this paper.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

Thermal Systems Oriented Two-Phase Heat Exchanger Models. Focus on Numerical Robustness.

The simulation of complex refrigeration architectures (that usually include vapor compression cycles) provides useful information for design, study and optimization purposes. Such arrangements may include several interconnected systems and a large variety and quantity of components. All components must meet two crucial requirements, namely, low CPU resolution time and high numerical robustness, in order to achieve relatively fast simulations and to prevent solver resolution issues at the architecture level. Among the usual components present, heat exchangers are the most challenging to address considering both the phenomenological and the numerical point of views. A generic heat exchanger model oriented for flexible purposes and meeting the aforementioned requirements has been developed under the Modelica programming language. The model can handle both single-phase and two-phase flows based on a simplified approach that considers three different zones for the refrigerant phase. Its numerical robustness has been extensively tested focusing on different boundary characteristics (definition, values, and signal types) and on demanding operating conditions (null mass flow rate, reversed flow, and reversed heat direction). This document presents the main characteristics of the model and a complete assessment of its numerical behaviour in terms of robustness and CPU time consumption.This project has received funding from the Clean Sky 2 Joint Undertaking (JU) under grant agreement No 886533. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union. Carles Oliet, as a Serra Húnter lecturer, acknowledges the Catalan Government for the support through this Programme.Postprint (author's final draft

Heat exchangers modelling and calibration for complete ECS architectures simulations

The aeronautical industry is currently designing new Environmental Control System (ECS) architectures based on the More Electrical Aircraft (MEA) concept in order to improve the energy consumption efficiency inside commercial airplanes. The support of appropriate simulation tools is critical for this task. Within the ECS thermal perimeter, heat exchangers are arguably the most challenging components to model from the numerical and phenomenological point of views. The present work details a heat exchanger model that has been developed to fit the requirements for simulations at the architecture level: fast resolution, good accuracy and numerical robustness.Peer ReviewedPostprint (published version

Drain water heat recovery storage-type unit for residential housing

© 2016. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The drain water heat recovery (DWHR) system is an interesting household technology to reduce energy costs and environmental impact. The objective of the utilisation of these devices is the recovery of the waste heat from domestic warm drain water, and transferring it to cold water entering the house. A drain water heat recovery unit has been built in this work. The authors are using both numerical and experimental tools to design and study the performance of this device, focusing on the analysis of a specific drain water heat recovery storage-type based on a cylindrical tank with an internal coiled pipe. The numerical simulation has been performed using an in-house platform, where the different elements of the DWHR storage are linked to solve the system. On the other hand, an experimental infrastructure has been developed to analyse of the system, which has been instrumented to provide detailed information of its heat recovery and storage capacities and temperature map. Different internal flow rates and operational temperatures have been studied. From the results obtained it can be said that the device shows interesting heat recovery and storage capacities, while the numerical platform shows promising comparison results against the experiments.Peer Reviewe

Drain water heat recovery storage-type unit for residential housing

© 2016. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The drain water heat recovery (DWHR) system is an interesting household technology to reduce energy costs and environmental impact. The objective of the utilisation of these devices is the recovery of the waste heat from domestic warm drain water, and transferring it to cold water entering the house. A drain water heat recovery unit has been built in this work. The authors are using both numerical and experimental tools to design and study the performance of this device, focusing on the analysis of a specific drain water heat recovery storage-type based on a cylindrical tank with an internal coiled pipe. The numerical simulation has been performed using an in-house platform, where the different elements of the DWHR storage are linked to solve the system. On the other hand, an experimental infrastructure has been developed to analyse of the system, which has been instrumented to provide detailed information of its heat recovery and storage capacities and temperature map. Different internal flow rates and operational temperatures have been studied. From the results obtained it can be said that the device shows interesting heat recovery and storage capacities, while the numerical platform shows promising comparison results against the experiments.Peer Reviewe