Thermal energy storage (TES) systems play a crucial part in the success of concentrated solar power as a reliable thermal energy source. The economics and operational effectiveness of TES systems are the subjects of continuous research for improvement, in order to lower the localized cost of energy (LCOE). This study investigates the use of spherical tanks and their role in sensible heat storage in liquids. In the two tank system, typical cylindrical tanks were replaced by spherical tanks of the same volume and subjected to heat loss, stress analysis, and complete tank cost evaluation. The comparison revealed that replacing cylindrical tanks by spherical tanks in two tank molten salt storage systems could result in a 30% reduction in heat loss from the wall, with a comparable reduction in total cost. For a one tank system (or thermocline system), a parametric computational fluid dynamic (CFD) study was performed in order to obtain fluid flow parameters that govern the formation and maintenance of a thermocline in a spherical tank. The parametric study involved the following dimensionless numbers: Re (500-7500), Ar (0.5-10), Fr (0.5-3), and Ri (1-100). The results showed that within the examined range of flow characteristics, the inlet Fr number is the most influential parameter in spherical tank thermocline formation and maintenance, and the largest tank thermal efficiency in a spherical tank is achieved at Fr = 0.5. Experimental results were obtained to validate the CFD model used in the parametric study. For the flow parameters within the current model, the use of an eddy viscosity turbulence model with variable turbulence intensity delivered the best agreement with experimental results. Overall, the experimental study using a spherical one tank setup validated the results of the CFD model with acceptable accuracy.

ABSTRACT HEAT REMOVAL FROM A LARGE SCALE WARM WATER STORAGE Kayserilioglu, Yavuz Selim M.S., Department of Mechanical Engineering Supervisor: Prof. Dr. R&uuml / knettin Oskay August 2004, 88 Pages A preliminary experimental study was performed in order to investigate the charging and heat removal characteristics of a sensible heat storage. Two sets of experiments were performed at two aspect ratios. Heat removal processes of these two sets were different while the charging processes were similar. In the first set of experiments, after the charging of the storage unit with relatively warm water was complete, heat removal process was started with simple heat exchangers from different elevations within the storage while the charging of the storage unit was continued. In the second set of experiments, after the charging of the storage unit was complete, heat removal from the storage unit was started without further charging of the storage unit. Charging water was fed into the storage from the top of one side and relatively colder water was drained from the bottom of the opposite side. Internal heat exchangers were used for the heat removal. Vertical temperature profile developments during the charging and heat removal periods were investigated. Thermal stratification was observed in all experiments. Heat exchangers extracted heat from different elevations in different experiments and the trend was that more heat can be extracted in upper elevations. Comparable heat can be extracted from the same elevation of lower and higher aspect ratio. Keywords: Sensible Heat Storage, Heat Removal, Thermal Stratification, Warm Water Storage

Cette étude porte sur les systèmes de stockage thermique régénératif dont le principe consiste à stocker de l’énergie sous forme de chaleur sensible dans un lit fixe. Le système est chargé et déchargé à l’aide d’un fluide caloporteur circulant à travers le lit fixe. Ce type de système est prometteur pour réduire le coût des infrastructures de stockage, par exemple dans les centrales solaires thermodynamiques. Cependant, le pilotage de ces systèmes est relativement complexe car leur fonctionnement est régi par divers phénomènes et met en jeux plusieurs modes de transfert de chaleur. Leur identification a permis de développer un modèle numérique monodimensionnel constitué d’une équation de continuité et de trois équations d’énergie : une pour le fluide caloporteur, une pour le solide du lit fixe et une pour les parois du réservoir. Les études expérimentales réalisées sur trois systèmes différents (un système huile/roches+sable, un système gaz/roches et un système gaz/céramique structuré en canaux), ainsi que des résultats issus de la littérature ont permis de valider le modèle proposé dans une large gamme de configurations. Le modèle s’avère notamment capable de traiter les fluides caloporteurs liquides ou gazeux et les lits fixes structurés en canaux ou constitués de milieux granulaires à granulométrie simple ou double. / This study deals with regenerative heat storage systems which aim to store sensible heat in a packed bed. The system is charged and discharged thanks to a heat transfer fluid which circulates through the packed bed. This kind of system is promising to reduce the cost of heat storage facilities, for example in concentrated solar power plants. However, the operation of these systems is relatively complex because their thermal behavior is governed by several phenomena and heat transfer modes. Thanks to the identification of these mechanisms, a one-dimensional numerical model consisting of one continuity equation and three energy conservation equations has been developed. There is one energy equation for the heat transfer fluid, one for the packed bed and one for the walls of the tank. The experimental studies carried out on three different systems (an oil/rock+sand system, a gas/rock system and a structured gas/ceramic system) and experimental results from the literature have enabled to validate the model in various configurations. The model is able to deal with liquid or gaseous heat transfer fluids and with structured packed beds with channels or granular packed beds with uniformly sized particles or particles of two different sizes.

Dans les années récentes, des politiques visant à promouvoir l’efficacité énergétique ont été instaurées en réponse aux obligations réglementaires européennes et internationales. Le stockage d’énergie thermique s’est révélé être une technologie qui permet une amélioration de l’efficacité énergétique, en particulier celle des installations techniques pour le conditionnement d’air, le chauffage et l’eau chaude sanitaire pour le bâtiment. Parmi les différents types existants, le stockage thermique par chaleur sensible est le plus ancien et le plus répandu sur le marché. Or, l’intégration du ballon de stockage dans les installations énergétiques s’avère délicate tant dans la phase de conception que de l’exploitation de ces installations. Par ailleurs, il convient d’évaluer – pour les systèmes et équipements techniques du bâtiment – leurs consommations énergétiques annuelles (ou saisonnières). Pour répondre à l’ensemble de ces exigences, le recours à la modélisation et simulation dynamique des composants et systèmes énergétiques devient indispensable. Le travail de la présente thèse présente une approche de modélisation et de simulation dynamique d’un ballon de stockage d’eau par chaleur sensible qui répond aux contraintes particulières suivantes : assurer une modélisation fine à partir de la résolution des équations de Navier-Stokes d’un composant – le ballon de stockage – dans lesquels les mécanismes de transfert et d’écoulement sont complexes et réaliser une modélisation dynamique d’un système thermique associant des divers composants techniques d’un circuit et ceci avec des temps de calcul raisonnables, compatibles avec les pratiques courantes des bureaux d’étude spécialisés en conception d’installations . Le travail réalisé associe donc une analyse fine du comportement dynamique du ballon grâce au développement d’un modèle CFD, la détermination d’un modèle réduit à partir de ce modèle – qui permet la construction d’un champ dynamique de température – et enfin une modélisation sous Modelica adaptée à la simulation d’un système énergétique complexe. Dans les différentes phases de cette étude, les résultats issus de la simulation sont alors confrontés aux résultats déduits de divers travaux expérimentaux. La validation de la démarche suite à cette confrontation calculs/expériences permet d’envisager l’application des outils présentés à des projets techniques notamment au projet « PV cooling » de climatisation des bâtiments avec une ressource solaire photovoltaïque, projet réalisé en parallèle de ce projet de thèse et porté par les acteurs industriels qui soutiennent cette recherche. / In recent years, policies to promote energy efficiency have been introduced in response to European and International regulatory obligations. Thermal Energy Storage has proven to be a technology that improves energy efficiency, particularly for the air conditioning, heating and domestic hot water utilities in buildings. Among the existing types, sensible heat storage is the oldest and most widespread on the market. The integration of the storage tank into energy installations may be tricky in both the design and operation phases of these installations. Moreover, the annual (or seasonal) energy consumption of the building's technical systems and equipment should be evaluated. To meet all these requirements, dynamic modeling and simulation of energy components and systems becomes essential. The work of this thesis presents a dynamic modeling and simulation approach of a sensible heat water storage tank which respond to the following particular constraints: To ensure a fine modeling based on the resolution of the Navier-Stokes equations of a component – the storage tank – in which the flow and transfer mechanisms are complex, and to carry out a dynamic modeling and simulation, with reasonable computational time, of a thermal energy system associating various technical components of a circuit and compatible with the usual practices of the specialized system design offices. Thus, the carried out work combines a detailed analysis of the dynamic behavior of the storage tank through the development of a CFD model, the development of a reduced model from the previous CFD model that allows the construction of temperature dynamic fields and finally a Modelica modeling adapted to the simulation of a complex energy system. In the different phases of this study, the results from the simulation are compared to the results deduced from various experimental works. The validation of the approach following this comparison between calculations and experimental results makes it possible to consider the application, of the presented tools, in technical projects and in particular the project “PV cooling” for buildings air conditioning with a photovoltaic solar resource, a project that is carried out in parallel with this thesis by the industrial players supporting this research.

In recent years we may observe a growth in construction of passive houses and low energy houses using lightweight constructions such as modern wooden houses. It is assumed that wooden houses keep overheating more comparing to brick houses during summer period. Due to the lack of research in this field the paper investigates the influence of internal thermal storage mass in passive houses constructions on their summer thermal stability under the Czech climatic conditions. Only sensible heat accumulation without a usage of phase change materials is examined. Differences between wooden houses comparing to brick-built houses are emphasized. Objects of research are mostly residential passive houses in low energy building standards. However, the results of research might be applied to different types of buildings as well. The first section outlines theoretical fundamentals. For the research itself various scientific research methods were used, such as basic mathematical calculations, experimental temperature measurement of two buildings (detached house in Dubňany and in Moravany) and numerical simulations. Own tribute to the research was first of all discussion on the topic of thermal accumulation and structures heat capacity calculation. Experimental measurements outlined conclusive evidence about the importance of internal thermal storage mass in respect of interior summer overheating. The research confirmed that the highest interior temperature reached is mostly influenced by solar gains through unshaded windows. However, the influence of internal thermal storage mass is not remote. If we compare standard timber-framed wooden house to the hole ceramic bricks-built house, the wooden house will overheat by 0,5°C more during a standard day. Wider spread in the maximum temperature reached was measured for lightweight consturctions wooden houses without any internal thermal storage mass. Therefore, such structures should have an additional layer of thermal storage mass.

The use of thermal storage energy, using phase change materials appears to be an effective way to store thermal energy storage with the benefits of the high amount of energy while maintaining isothermal nature of the process. PCM methods are used in latent thermal storage systems for heat pumps, as well as in solar engineering or for temperature control in spacecraft. The past decade has extended these principles for cooling and heating in the building. There are a number of PCM systems, which operate over a wide temperature range, are used in various applications. This document includes a brief overview of the development and analysis of available thermal storage working mainly on the principle of PCM.