The expanding use of renewable and sustainable energy systems is at the forefront of the global effort to reduce CO2 emissions and mitigate climate change. Thermal energy storage has become a critical component of many of these new and innovative systems, and research in this field has expanded to meet their requirements. Water has been traditionally used as a storage medium because of its high heat capacity and low cost, but depending on the application, the storage volume requirements may be excessively large. Phase Change Materials (PCMs) offer an opportunity to reduce the storage volume through latent energy storage. However, energy storage in PCM presents new challenges, and careful design of thermal storage is required to realize the benefits. The design of PCM storage must consider the system operation, operating temperature range, PCM properties, encapsulation, and the heat transfer fluid. In the current state-of-the-art literature, there is no standard method for designing PCM thermal storage based on system requirements. The objective of this thesis is to deliver a methodology to assess the feasibility of using PCM for thermal energy storage in place of water. This is done by identifying which applications benefit from PCM, comparing the analytical and numerical performance of water-only to hybrid water-PCM storage, and developing a method to size PCM containment to achieve theoretical performance when PCM is beneficial. This research study develops analytical solutions for sizing PCM thermal energy storage based on system boundary conditions. These boundary conditions consist of the system itself (e.g. heat pump, absorption chiller), the energy source into the system, and the required load from the system (e.g. a building). The PCM is incorporated into a water tank such that the water acts as both a heat transfer fluid and an energy store. Analytical predictions of the total energy storage capacity in this hybrid water-PCM thermal storage unit are coupled to analytical predictions of the rate of melting and solidification to appropriately determine the required volume and encapsulation thickness of PCM thermal storage based on the system requirements. The results are verified against full-system numerical simulations based on case studies of solar absorption cooling and heat-pump heating. It is shown in this study that the total required volume of storage is a function of the temperature differential of the system, and the total mismatch in time between when energy is available and when it is required. A mathematical formulation is proposed which quantifies the required storage volume based on the temperature differential, the source and load profiles, and the percentage of PCM in the hybrid water-PCM storage unit. Furthermore, the rate of melting and solidification of the thermal storage is coupled to the overall storage size and required time for charging, and a mathematical formulation is proposed which solves for the PCM encapsulation thickness. The method assumes a conservative conduction-dominated domain and demonstrates how complete melting can be ensured before the system reaches its maximum allowable temperature. The map the region of applicability of PCM thermal storage is also presented which is defined in terms of the non-dimensional Biot and Stefan numbers, in which systems utilizing PCM thermal storage will benefit from volume reduction when compared to using water only. This region is characterized with a low Biot number, corresponding to a slender geometry acting as a lumped system, as well as a low Stefan number, corresponding to limited temperature differential and limited sensible energy storage. These characteristics favor the use of PCM thermal storage instead of water only. This thesis presents a novel contribution to the state-of-the-art literature in PCM thermal storage, which is established through the analytical methodology for sizing PCM thermal storage based on system boundary conditions. The details of the contribution are presented in the form of three journal publications that have been integrated into this sandwich Ph.D. thesis on PCM thermal energy storage. / Thesis / Candidate in Philosophy

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All energy, wind, water, biofuel and fossil fuel besides nuclear- and tide power originates from the sun. It’s very hard to take full advantage of the huge amount of energy hitting the earth each day from the sun. The suns highest radiation appears often when the energy need reaches its lowest. That’s why it’s very important to be able to store energy over time when the sun doesn’t shine. A large part of energy storage is thermal energy storage, which can either be done sensible, latent or chemical. Another possible thermal storage is a combination of sensible and latent. This exam was aiming to investigate different types of energy storage methods available on the market and a much more detailed analysis for different storage methods with phase change materials (PCM). A new method was designed for a new storage tank suitable for Absolicon Solar Collector AB and their energy park in the city of Härnösand. The methods for this exam were to create a theoretical storage tank suitable to Absolicons Energy Park with some simple calculations. The criteria for the storage tank was to create a storage tank that could provide the district heat in Härnösand with 160 degrees pressurized water and create 160 degrees steam to the industry. The dimensions of the storage tank where chosen by the conditions in Härnösand and from the specific data of Härnösands district heat and from Absolicons new solar collectors. The work temperature of the system were set to 160 degrees which meant that the storage tank would be able to work in those conditions with high temperature. A suitable phase change material and methods for encapsulation of the phase change material suitable for this system was to be found. Small tests were made with a new type of encapsulation for phase change materials in higher temperature. Simple calculations of two types of storage tanks were made. The first storage tank was made with a PCM from PCM products named A164. This PCM was encapsulated with special bags that could handle temperature up to 200 degrees with surrounding rapeseed oil and a copper loop that handled the heat transfer. The second thank was made with the same PCM and encapsulation but with water glycol surrounding the PCM and two types of heat exchangers for the heat transfer. The results from the first tank were that it didn’t work with the district heat. Because a wrong calculation with the schematic of the system made it impossible to connect into the district heat of Härnösand. The only good thing was that it didn’t need to be pressurized because of the rapeseed oil but the bad heat transfer between oil and water made a pressurized tank of water more profitable. The results from the second tank showed that it could produce 160 °C to the district heat for 2 h and 7 minutes. The schematic connection worked and the tank would in the near future be able to connect into the district heat. The result for the encapsulation showed that the bags were able to stand temperatures up to 190 degrees for a short period of time.

With global warming, an inevitable threat to humanity, significant efforts in all carbon emitting industries are required. Electric vehicles are a suitable alternative to the petroleum dominated automotive industry. However, obstacles like charging infrastructure and limited range still stand in the way of their continued acceptance. This limited driving range can be further reduced in cold weather due to decreased battery efficiency and increased heating load. The heating in most electric vehicles is provided by an electrical positive temperature coefficient resistor. This architecture can lead to reductions in range of over 50 %. A thermal storage system has been devised and presented in this thesis which can partially or fully offset the thermal requirements. This is accomplished by pre-heating a thermal storage tank which then uses sensible energy to provide the heat for the cabin and battery pack. The system has been shown to reduce consumption and improve driving range in low ambient temperature conditions. This system successfully offers a potential solution to the concern of large range fluctuations due to different ambient temperatures. After producing a representative electric vehicle model in AMESim, it was compared to the Nissan Leaf with acceptable errors. The range implications for this baseline electric vehicle are then presented. A coolant based, thermal storage tank is then added to the model and simulated across a variety of temperatures and thermal storage masses. The results show that an 80 kg, 80 °C coolant tank can provide all the heating requirements for a 36 km, hour and 9 minute city drive cycle. Offering a calculated consumption reduction of up to 36 % at -30 °C as compared to the baseline electric vehicle model. Furthermore, a yearly analysis was performed based on this cycle and the results have shown that an optimal 30 kg thermal storage tank can decrease the yearly average consumption by up to 20 Wh/km or 12 %. / Thesis / Master of Applied Science (MASc)

In this investigation an electric field was applied to a phase change thermal storage system while it was discharging energy. The phase change material used was octadecane. Octadecane is a high purity dielectric material that has a melting temperature close to room temperature. The material was forced to solidify using a heat exchanger mount below the phase change material, cold water flowed through the heat exchanger to ensure it maintained a constant temperature below the melting temperature of the phase change material. By applying -8kV to 9 electrodes – positioned in the phase change material – and by using the heat exchanger as an electrical ground – an electric field was generated in the phase change material. The electric field caused unbalanced body forces in the fluid which generated electro-convection in the fluid. The system was designed such that electro-convection is the only source of convection in the system to isolate the effects of electro-convection, allowing for the underlying physics of electro-convection to be studied easier. To understand the effects of applying electro-convection, a case where there is no applied voltage on the electrodes was compared to a case where there was -8 kV applied to the electrodes. Experiments showed that the effect of applying electro-convection depends on the initial temperature; however, it was found that the improvement after two hours was less than 10%. For a wall temperature of 8.5℃ and an initial temperature of 50℃ - the melting temperate of octadecane is 28℃- then the maximum enhancement of the energy extracted is 50%, but two hours after the start of the test the enhancement approached zero. For a wall temperature of 8.5℃ and an initial temperature of 30℃, the maximum enhancement is 10% and similarly fall to zero after a few hours of application. A simple analytical model was developed. The experimental and numerical results showed that at the early stages of energy discharge the electro-convection case had a large improvement compared to a pure conduction case, however as time progresses this improvement decreases. The explanation for the trend is that adding convection only increases the rate that energy is taken out of the liquid, thus the maximum improvement is bounded by the amount of sensible energy in the liquid phase change material, once this sensible energy is removed applying electrohydrodynamics is no longer beneficial. / Thesis / Master of Applied Science (MASc)

Today's energy production is changing from scheduled to intermittent generation due to the increasing energy injection from renewable sources. This alteration requires flexibility in energy generation and demand. Electric heat pumps and thermal storages were found to have a large potential to provide demand flexibility which is analysed in this work. A three-fold method is set up to generate thermal load profiles, to simulate heat pump pools and to assess heat pump flexibility. The thermal profile generation based on a combination of physical and behavioural models is successfully validated against measurement data. A randomised system sizing procedure was implemented for the simulation of heat pump pools. The parameter randomisation yields correct seasonal performance factors, full load hours and average operation cycles per day compared to 87 monitored systems. The flexibility assessment analysis the electric load deviation of representative heat pump pool in response to 5 different on / off signals. The flexibility is induced by the capacity of thermal storages and analysed by four parameters. Generally, on signals are more powerful than off signals. A generic assessment by the ambient temperature yield that the flexibility is highest for heating days and the activated additional space heating storage: Superheating of the storage to the maximal temperature provides a flexible energy of more than 400 kWh per 100 heat pumps in a temperature range between -10 and +13 °C.

[pt] O presente trabalho versa sobre o estudo e modelagem de um sistema de refrigeração para ar condicionado de edifícios. O modelo considera um sistema de compressão de vapor de grande porte, chamado habitualmente chiller; um tanque de armazenamento de água gelada e uma torre de resfriamento.Para o desenvolvimento do modelo utilizam-se as equações constitutivas e as equações de conservação da massa e energia em todos seus componentes.O modelo permite obter as condições de funcionamento ótimo do sistema de refrigeração reduzindo o tamanho de seus componentes (menor custo de investimento) e o consumo de energia (custo de operação) em correspondência com o diagrama de carga térmica do edifício.Consideram-se as variações da temperatura do meio ambiente ao longo do dia (transiente horário) para estudar a influência destas variações no desempenho global do sistema de refrigeração.Aplica-se o modelo à obtenção das características dos componentes do sistema de refrigeração para condicionamento de ar de um dos blocos do prédio Cardeal Leme da PUC-Rio.No presente estudo, a estratégia de operação usada é um fator decisivo na seleção da melhor alternativa econômica. / [en] The present work aims the study and modeling of a system of refrigeration systems for air-conditioning in buildings. The model considers a high capacity vapor-compression refrigeration system, for water cooling (chiller); a tank of thermal storage tank and a cooling tower.For the development of the model the constitutive equations and the equations of conservation of mass and energy are used over all its components.The model provides the optimal operating conditions of the refrigeration system to reduce the size of its components (lesser cost of investment) and the energy consumption (operation cost) according to the thermal load of the building.Daily temperature variations of the environment are taken into account (hourly transient) in order to study the influence of these variations over the global performance of the refrigeration system.The model is applied to the study of the air conditioning system of one block of the Cardeal Leme building, at PUC-Rio.In the present study, the strategy employed is a keye factor in the selection of the best economical alternative.

Sustainable energy solutions are necessary in developing nations as current food preparation practices are becoming harmful to the environment, economic development and the overall health of the population. The purpose of this study was to create a Scheffler reflector-based system prototype, experimentally analyze the system and to predict its behavior when subjected to the solar conditions of Rajasthan, India. Former designs from India, the University of Iowa and several other institutions were consulted during the formulation of the prototype design. While consulting a specific set of design constraints, pertinent to developing counties, a Scheffler reflector and tracking stand were fabricated. Solutions for a thermal storage unit were investigated for eventual integration with the prototype. Solar flux data for Iowa and India was used to predict the amount of energy transmitted by the reflector. Experiments were designed and completed to observe the temperatures experienced at the focal point of the reflector and estimate the energy stored by a steel mass. A series of sun angles, monthly solar flux data and experimental data were used to predict the performance of the storage unit, over a three day span, in Rajasthan. Aspects of the system were then modified to investigate their effects on the temperature of the storage unit.

A prototype thermal storage system, using phase change materials, was developed for a novel electric vehicle climate conditioning application. The proposed system consists of a heat transfer fluid circulating between either an on-board hot or cold thermal storage unit, which we refer to as thermal battery, and a liquid-air heat exchanger that provides heat exchange with the incoming air to the vehicle cabin. The research presented herein focuses primarily on the development of the on-board system and hot battery. While the air conditioning system was developed strictly for laboratory use, it was designed to meet application realistic performance metrics, e.g., a heat dissipation rate of 2 kW. The prototype was tested with three phase change materials: paraffin wax, xylitol, and erythritol. Furthermore, a full system thermodynamic model was developed to predict thermal performance that features semi-analytic solution to the coupled forced convection and phase change conduction heat transfer. Modeling results are compared against a numerical benchmark as well as our own experimental data. / text

A thermal energy store corrects the misalignment of heating demand in the winter relative to solar thermal energy gathered in the summer. This thesis reviews the viability of a solar charged hot water tank thermal energy store for a school at latitude 56.25N, longitude -120.85W