There is a current requirement for technologies that store heat for both domestic and industrial applications. Phase-change materials (PCMs) represent an important class of materials that offer potential for heat storage. Heat-storage systems are required to undergo multiple melt/freeze cycles without any change in melting-crystallisation point and heat output. Salt hydrates are attractive candidates on account of their high energy densities, but there are issues associated with potential crystallisation of lower-hydrates, long-term stability, and reliable nucleation. An extensive review of the PCMs in the literature, combined with an evaluation of commercially available PCMs led to the conclusion that many of the reported PCMs, lack at least one of the key requirements required for use as a heat-storage medium. The focus of this research was therefore to identify and characterise new PCM compositions with tailored properties. New PCM compositions based of sodium acetate trihydrate were developed, which showed improved properties through the use of selective polymers that retard the nucleation of undesirable anhydrous sodium acetate. Furthermore, the mechanism of nucleation of sodium acetate trihydrate by heterogeneous additives has been investigated using variable-temperature powder X-ray diffraction. This study showed that when anhydrous Na2HPO4 was introduced to molten sodium acetate trihydrate at 58°C the hydrogenphosphate salt is present as the dihydrate. On heating to temperatures in the range 75-90°C the dihydrate was observed to dehydrate to form anhydrous Na₂HPO4. This result explains the prior observation that the nucleator is deactivated on heating. The depression of melting point of sodium acetate trihydrate caused by the addition of lithium acetate dihydrate has also been investigated using differential scanning calorimetry and powder X-ray diffraction. It has been possible to tune the melting point of sodium acetate trihydrate thereby modifying its thermal properties. Studies of the nucleation of sodium thiosulfate pentahydrate, a potential PCM, led to the structural characterisation of six new hydrates using single crystal Xray diffraction. All of these hydrates can exist in samples with the pentahydrate composition at temperatures ranging from 20°C to 45°C. These hydrates are: α-Na₂S₂O₃·2H₂O, which formed during the melting of α-Na₂S₂O₃·5H₂O; two new pentahydrates, β-Na₂S₂O₃·5H₂O and γ-Na₂S₂O₃·5H₂O; Na₂S₂O₃·1.33 H₂O, β-Na₂S₂O₃·2H₂O and Na₂S₂O₃·3.67 H₂O, which formed during the melting of β- Na₂S₂O₃·5H₂O. Furthermore, new PCMs in the 75-90°C range were identified. The commercial impact and route to market of several of the PCMs are discussed in the final chapter.

Recently, buildings have been receiving more serious attention to help reduce global energy consumption. At the same time, thermal comfort has become an increasing concern for building occupants. Phase Change Materials (PCMs), which are capable of storing and releasing significant amounts of energy by melting and solidifying at a given temperature, are perceived as a promising opportunity for improving the thermal performance of buildings. This is because they use their thermophysical properties and latent heat while transforming state (or phase) as a feature for thermal energy storage systems to reduce overall energy demand, specifically during peaks hours, as well as to improve thermal comfort in buildings. This research aims to provide an overview of opportunities and challenges for the utilization of PCMs in the Architecture, Engineering, and Construction (AEC) sector, a broader understanding of specifically promising technologies, and a clarification of the effectiveness of different applications in building enclosures design especially in exterior walls. The research discusses how PCMs can be incorporated within building enclosures effectively to enhance building performance and improve thermal comfort while reducing heating and cooling energy consumption in buildings. The major objectives of the research include studying the properties of PCMs and their potential impact on building construction, clarifying PCMs selection criteria for building application, identifying the effectiveness of utilizing PCMs on saving energy, and evaluating the contribution of utilizing PCMs in building enclosures to thermal comfort. The research uses an exploratory quantitative approach that contains three main stages: 1) a systematic literature review, 2) laboratory experiments, and 3) validation to meet the goal of the research. Finally, by extrapolating results, the research ends with a practical assessment of application opportunities and how to effectively utilize PCMs in exterior walls of buildings. / PHD

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Au cours des deux dernières décennies, le contexte économique a changé de manière significative en raison de la hausse des prix de l'énergie. Le bâtiment étant devenu le principal secteur consommateur d'énergie, la réduction de celle-ci est devenue un objectif économique, sociétal et environnemental. Ce sujet mobilise de nombreux travaux de recherche. Les Matériaux à Changement de Phase (MCP) représentent une solution innovante qui pourrait contribuer à améliorer la performance énergétique des bâtiments. Ils sont principalement utilisés pour la régulation de température, et leur forte capacité de stockage est un moyen de réduire la consommation d'énergie. Notre étude vise à caractériser, via une approche expérimentale et numérique, le comportement d'un PCM (l’Octadécane). Pour cela, nous avons développé et mis en œuvre un modèle numérique qui corrobore les résultats expérimentaux, et ainsi améliore la prédiction de la performance du MCP considéré.Dans ce travail, notre principale préoccupation est de mettre en évidence les erreurs ou simplifications présentes dans le modèle numérique traditionnel pouvant entraîner un écart global par rapport au comportement réel du MCP. Ces différences conduisent à une estimation erronée des temps de fusion et de la quantité d'énergie stockée. L'amélioration significative de notre modèle est la prise en compte de la convection naturelle, de la surfusion, et l'utilisation des courbes réelles d'enthalpie du MCP considéré. La relation température-enthalpie réelle tient compte de la présence d'une fraction d'impuretés solubles dans le matériau. L’originalité de ce travail est de traiter ces phénomènes physiques via la méthode de Boltzmann réseau (connue sous l'acronyme LBM) avec des fonctions de distribution doubles couplée à une formulation enthalpique. Une telle approche permet de passer outre la non-linéarité des équations régissant l'écoulement et le transfert de chaleur. Sa simplicité de mise en œuvre et son caractère local permettent d'affiner le modèle. Ainsi, on peut couvrir les problèmes de changement de phase, y compris ceux pouvant avoir lieu dans des matrices poreuses ou fibreuses. Ce dernier point a été couvert dans cette thèse.Enfin, il s'est avéré que l'approche numérique adoptée ici pour traiter les problèmes de changement de phase corrobore à la fois nos résultats expérimentaux et ceux disponibles dans la littérature. / Over the past two decades, the economic context has changed significantly due to the rise in energy prices. The building sector has become the main consumer of energy. Thereby, reducing the latter is now an economic, societal and environmental necessity. Accordingly, this topic mobilizes many researches. Phase Change Materials (PCMs) represent an innovative solution, which could improve buildings' energy performance. They are primarily used for temperature regulation, and their high storage capacity can reduce energy consumption.Our study aims at characterizing, via a complementary approach of experimental and numerical simulation, the behavior of a PCM (n-Octadecane). For this, we have developed and implemented a numerical model that corroborates the experimental results, and hence improves the prediction of the PCM performance.In this work, our main concern is to highlight the common errors or simplifications taken in the traditional numerical model, which can result in an overall discrepancy compared to the actual behavior of PCMs. Those discrepancies lead to wrong estimation of the fusion times and amount of energy stored. The major improvement of our model is the consideration of the natural convection, the supercooling, and the use of real enthalpy curves of the considered PCM. The actual temperature-enthalpy relationship takes into account the presence of a fraction of soluble impurities in the material. The originality of this work is to handle these physical phenomena via a lattice Boltzmann method (known by the acronym LBM), which leans on double distribution functions and coupled with the enthalpy formulation. Such an approach overcomes the non-linearity in the governing equations of fluid flow and heat transfer. Its simplicity and local character allow adding complexity to the model. Thereby, one can cover up the phase change problems, including those, which may occur in heterogeneous matrices. This last point has been also covered in this thesis.Finally, it turned out that the approach implemented here for phase change problems supports both, our experimental results and those available in the literature.

<p>  </p> <p>Thermal energy storage (TES) is used to reduce the operating costs of heating, ventilation, and air conditioning (HVAC) systems by shifting loads away from on-peak periods, to reduce the maximum heating or cooling capacity needed from the HVAC system, and to store excess energy generated by on-site solar power. The most commonly-used form of TES is ice storage with air conditioning (A/C) systems in commercial buildings. There has been extensive research into many other forms of TES for use with HVAC systems, both in commercial and residential buildings. However, this research is often limited to use with either heating or cooling systems.</p> <p>Year-round, high-density storage for both heating and cooling would yield significantly larger cost savings than existing TES systems, particularly for residential buildings, where heating loads are often larger than cooling loads. This dissertation examines the feasibility of using metal hydrides for year-round storage, as well as analyzing the potential of variable-temperature energy storage for optimizing system performance beyond allowing for year-round use.</p> <p>Metal hydrides are metals that exothermically absorb and endothermically desorb hydrogen. Since the temperature this reaction occurs at depends on the hydrogen pressure, hydrides can be used for energy storage at varying temperatures. System architecture for using metal hydrides with an HVAC system is developed. A thermodynamic model which combines a dynamic model of the hydride reactors with a static model of the HVAC system is used to calculate operating costs, compared to a conventional HVAC system, for different utility rates and locations. The payback period of the system is unacceptably high, due to the high initial cost of metal hydrides and the operating costs of compressing hydrogen to move it between hydride reactors.</p> <p>In addition to the metal hydride system model, a generalized model of a variable-temperature TES system is used to determine the potential cost savings from dynamically altering the storage temperature to achieve optimal cost savings. Dynamic tuning does result in cost savings but is most effective for storage tank sizes significantly smaller than the optimal tank size. An alternate system design where the storage tank is charged with the outlet flow from the house achieves larger cost savings even for the optimally-sized tanks. Payback periods calculated for optimal sizing show that year-round storage has a lower payback period than separate cold and heat storage if the year-round storage system is not more expensive than two separate storage tanks. </p>

The trend in the electronic and electrical equipment industry towards denser and more powerful product requires a higher level of performance from cooling devices. In this context, passive cooling techniques such as latent heat storage systems have attracted considerable attention in recent years. Phase change materials (PCMs) have turned out to be extremely advantageous in this regard as they absorb high amount of latent heat without much rise of temperature. But unfortunately, nearly all phase change materials (PCMs) with high latent heat storage capacity have unacceptably low thermal conductivity, which makes heating and cooling processes slow during melting and solidification of PCMs. Augmentation of heat transfer in a PCM is achieved by inserting a high thermal conductivity material, known as thermal conductivity enhancer (TCE), into the PCM. The conglomeration of PCM and TCE is known as a thermal storage unit (TSU). In this thesis, detailed and systematic analyses are presented on the thermal performance of TSUs subjected to two types of thermal loading- (a) constant power loading in which a constant power level is supplied to the chip (heater) for a limited duration of time, and (b) cyclic loading. Eicosane is used as the PCM, while aluminium pin or plate fins are used as TCEs. First, a 1-D analytical model is developed to obtain a closed-form temperature distribution for a simple PCM domain (without TCE) heated uniformly from the bottom. The entire heating process is divided into three stages, viz. (a) sensible heating period before melting, during which heat is stored in the solid PCM in the form of specific heat, (b) melting period, during which a melt front progresses from the bottom to the top layer of the PCM and heat is stored in latent as well as in sensible forms, and (c) post melting period, during which energy is stored again in the form of sensible heat. For each stage, conduction energy equation is solved with a set of initial and boundary conditions. Subsequently, a resistance capacitance model of phase change process is developed for further analysis. For transient performance under constant thermal loading, experimental investigations are carried out for TSUs with different percentages of TCE. A numerical model is developed to interpret the experimental results. The thermal performance of a TSU is found to depend on a number of geometrical parameters and boundary conditions. Hence, a systematic approach is desirable for finding the best TSU design for which the chip can be operated for a longer period of time before it reaches a critical temperature (defined as the temperature above which the chip starts malfunctioning). As a first step of the approach, it is required to identify the parameters which can affect the transient process. It is found that the convective heat transfer coefficient, ‘h’ and the exposed area for heat transfer have little effect on the chip temperature during the constant power operation. A randomized search technique, Genetic Algorithm (GA), is coupled with the CFD code to find an optimum combination of geometrical parameters of TSUs based on the design criteria. First, the optimization is carried out without considering melt convection within the PCM. It is found that the optimum half-fin width remains fixed for a given heat flux and temperature difference. Assuming a quasi steady process, the results of optimization are then explained by constructing and analyzing a resistance network model. The resistance network model is then extended to include the effect of melt convection, and it is shown that the optimum pitch changes with the strength of convection. Accordingly, numerical analysis is carried out by considering the effect of melt convection, and a correlation for optimum pitch is developed. Having established the role of melt convection on the thermal performance of TSUs, rigorous computational and experimental studies are performed in order to develop correlations among different non-dimensional numbers, such as Nusselt number, Rayleigh number, Stefan number and Fourier number, based on a characteristic length scale for convection. The enclosures are classified into three types, depending on the aspect ratio of cavity, viz. shallow, rectangular and tall enclosures. For a shallow enclosure, the characteristic length is the height of cavity whereas for a tall enclosure, the characteristic length is the fin pitch. In case of rectangular enclosure, both pitch and height are the important characteristic lengths. For cyclic operation, it is required that the fraction of the PCM melting during the heating cycle should completely solidify back during the cooling period, in order that that TSU can be operated for an unlimited number of cycles. If solidification is not complete during the cooling period, the TSU temperature will tend to rise with every cycle, thus making it un-operational after some cycles. It is found that the solidification process during the cooling period depends strongly on the heat transfer coefficient and the cooling surface area. However, heat transfer coefficient does not play any significant role during the heating period; hence a TSU optimized for transient operation may not be ideal for cyclic loading. Accordingly, studies are carried out to find the parameters which could influence the behaviour of PCM under cyclic loading. A number of parameters are identified in the process, viz. cycle period and heat transfer coefficient. It is found that the required heat transfer coefficient for infinite cyclic operation is very high and unrealistic with air cooling from the surface of the TSU. Otherwise, the required cooling period for complete re-solidification will be very high, which may not be suitable for most applications. In an effort to bring down the cooling period to a duration that is comparable to the heating period, a new design is proposed where both ‘h’ and area exposed to heat transfer can be controlled. In this new design, the gaps between the fins in a plate-fin TSU are alternately filled with PCM, such that only one side of a fin is in contact with PCM and the other side is exposed to the coolant (air). In this arrangement, the same heat flow path through the fin which is used for heating the PCM (during the heating stage) can also be used for cooling and solidifying the PCM during the cooling part of the cycle. Natural or forced air cooling through the passages can be introduced to provide a wide range of heat transfer coefficient which can satisfy the cooling requirements. With this arrangement, the enhanced area provided for cooling keeps the ‘h’ requirement within a realistic limit. This cooling method developed is categorized as a combination of active and passive cooling techniques. Analytical and numerical investigations are carried out to evaluate the thermal performance of this modified PCM-based heat sink in comparison to the ones with conventional designs. It is found that, the performance of new PCM-based heat sink is superior to that of the conventional one. Experiments are performed on both the conventional and the new PCM-based heat sinks to validate the new findings.

<p>An emerging technology in the field of transient thermal management is thermal energy storage, or TES, which enables temporary, on-demand heat rejection via storage as latent heat in a phase-change material.  Latent TES devices have enabled advances in many thermal management applications, including peak load shifting for reducing energy demand and cost of HVAC systems and providing supplemental heat rejection in transient thermal management systems.  However, the design of a transient thermal management system with integrated storage comprises many challenges which are yet to be solved.  For example, design approaches and performance metrics for determining the optimal dimensions of the TES device have only recently been studied.  Another area of active research is estimation of the internal temperature state of the device, which can be difficult to directly measure given the transient nature of the thermal storage process.  Furthermore, in contrast to the three main functions of a thermal-fluid system--heat addition, thermal transport, and heat rejection--thermal storage introduces the need for active, real-time control and automated decision making for managing the operation of the thermal storage device. </p> <p>In this thesis, I present the design process for integrating thermal energy storage into a single-phase thermal management system for rejecting transient heat loads, including design of the TES device, state estimation and control algorithm design, and validation in both simulation and experimental environments. Leveraging a reduced-order finite volume simulation model of a plate-fin TES device, I develop a design approach which involves a transient simulation-based design optimization to determine the required geometric dimensions of the device to meet transient performance objectives while maximizing power density.  The optimized TES device is integrated into a single-phase thermal-fluid testbed for experimental testing.  Using the finite volume model and feedback from thermocouples embedded in the device, I design and experimentally validate a state estimator based on the state-dependent Riccati equation approach for determining the internal temperature distribution to a high degree of accuracy.  Real-time knowledge of the internal temperature state is critical for making control decisions; to manage the operation of the TES device in the context of a transient thermal management system, I design and test, both in simulation and experimentally, a logic-based control strategy that uses fluid temperature measurements and estimates of the TES state to make real-time control decisions to meet critical thermal management objectives. Together, these advances demonstrate the potential of thermal energy storage technology as a component of thermal management systems and the feasibility of logic-based control strategies for real-time control of thermal management objectives.</p>