Thermal energy storage (TES) is an innovative technology that can help mitigate environmental problems and make energy consumption in air conditioning systems more efficient. TES also helps to decouple the production and use of cooling. In this work, a mathematical model was used to obtain the thermal loads of the environment based on Brazilian standards and to simulate the operation of an air conditioning system integrated with TES. A refrigeration system capable of providing cooling capacity for the selected environment was used. It simulated the operation of refrigeration systems to evaluate the coefficient of performance of the refrigeration cycle and the refrigeration system, including the TES, for the city of Teresina, in the northeast region of Brazil. On each day, the efficiency of the air-cooled chiller coupled with a TES that used paraffin with phase change material was verified. Based on the data obtained from the mathematical model, the reduction in energy consumption achieved by coupling TES with the air conditioning system varied between 10.6% and 1.3%.

Many efforts are being made to mitigate the main disadvantage of most phase change materials - their low thermal conductivities - in order to deliver latent heat energy storage systems (LHESS) with adequate perfor-mance. However, the effect of applied methods is difficult to compare as they are mostly tested for different storage types and sizes and/or different boundary and initial conditions, which hinders rapid progress in the optimization of these approaches. In this work, a previously developed method for comparing the performance of LHESS is applied to experimental results of different storage systems under different conditions and subsequently analyzed and further refined. The main idea of the method is to normalize the power with the volume and a reference temperature difference and compare its mean value plotted over the normalized mean capacity flow of the heat transfer fluid (HTF). This enables the presentation of the results in a compact and easily comparative way. Attention has to be paid when it comes to the choice of the reference temperature difference, the reference volume and the method for calculating the mean value. Two variants of calculating the mean value (time-weighted and energy-weighted) and two variants of reference temperatures for determining the temperature difference to the inlet temperature of the HTF (initial temperature and melting temperature) are applied and discussed in detail. While the method significantly increases the comparability of results, none of the options listed above are without drawbacks. Approaches are shown to reduce or eliminate these drawbacks in the future. The recommendation for comparing different LHESS under different conditions is to use the method described here and clearly state the chosen reference temperature, reference volume and method for calculating the mean value.

This paper investigates ellipsoid-shaped macro-encapsulated phase change material (PCM) on a component scale. The selected PCM is a paraffin-based commercial material, namely ATP60; differential scanning calorimetry and transient plane source method are used to measure ATP60's thermo-physical properties. A 0.382 m(3) latent heat thermal energy storage (LHTES) component has been built and experimentally characterized. The temperature measurement results indicate that a thermocline was retained in the packed bed region during charging/discharging processes. The experimental characterization shows that increasing the temperature difference between the heat transfer fluid (HTF) inlet temperature and phase-change temperature by 20 K can shorten the completion time of discharge by 65%, and increasing HTF inlet flowrate from 0.15 m(3)/h (Re = 77) to 0.5 m(3)/h (Re = 256) can shorten the completion time of charge by 51%. Furthermore, a one-dimensional packed bed model using source-based enthalpy method was developed and validated by comparison to experimental results, showing discrepancies in the accumulated storage capacity within 6.6% between simulation and experiment when the Reynolds number of the HTF inlet flow ranges between 90 and 922. Compared with a conventional capsule shaped in 69-mm-diameter and 750-mm-long cylinders, the ellipsoidal capsule shows 60% less completion time of discharge but 23% lower storage capacity. Overall, this work demonstrates a combined experimental and numerical characterization approach for applying novel macro-encapsulated PCM geometries for heating-oriented LHTES.

Latent heat thermal energy storage (LHTES) systems can be coupled with heat pump (HP) systems to realize heat load shifting on demand side. With phase change material (PCM), well designed LHTES components exhibit high storage energy density and thus have large potentials to be integrated in residence where a compact energy storage solution is needed. However, real installations of LHTES-HP integrated systems are still rare nowadays; feasibility of this technology in achieving technically sound and economically viable load shifting operations should be demonstrated and understood by stakeholders to promote its implementation. Therefore, this thesis presents an exemplary feasibility study for three selected off-the-shelf macro-encapsulated PCM products, encompassing in-depth experimental and numerical modelling investigations on three levels—material, component, and system. The feasibility is studied with a specific scenario where the macro-encapsulated LHTES systems are designed to integrate with HP-based heating systems in common Swedish residential buildings. 

On the material level, three commercial PCMs (C48, C58, and ATP60) are selected by the operating temperature levels in typical HP-based space heating systems. Differential Scanning Calorimetry and Temperature-History method are employed to measure PCM enthalpy-temperature profiles; Transient Plane Source method is used to measure the thermal conductivity of ATP60. C58 based on sodium acetate trihydrate is prioritized for in-depth feasibility analyses because of its highest volumetric heat storage capacity.

On the component level, three full-scale macro-encapsulated LHTES components (Component 1: cylindrical encapsulation of C48; Component 2: cylindrical encapsulation of C58; Component 3: ellipsoidal encapsulation of ATP60) are developed for integration in single-family houses to achieve full peak load shifting. A test rig is built for characterizing the three components under possible operational conditions in practical systems. The heat transfer enhancing effects from increasing the temperature difference between heat transfer fluid (HTF) inlet temperature and phase-change temperature as well as from increasing the HTF inlet flowrate are quantified. Performance indicators, such as completion time of charge/discharge, energy storage density, and capacity enhancement factor, are evaluated at different operating temperature ranges. Overall, Component 2 is feasible in delivering around 90% of storage capacity (the capacity loss is due to phase separation). However, storage design and control improvements are still needed for realizing full peak load shifting over a three-hour discharging process. For Component 2, an improved storage solution with a reduced capsule diameter and time-increasing HTF flowrate profiles is developed through numerical simulation using an experimentally-validated two-dimensional heat transfer model. Furthermore, a one-dimensional model is developed and validated for simulating storage thermal output of Components 2 and 3. 

On the system level, a numerical model is developed to calculate electricity input to the LHTES-HP integrated systems for technical, economic, and environmental load shifting evaluation. Three new integration layouts are developed to charge scaled-up Component 2 with a de-superheater and/or a booster heat pump cycle. The new layouts can improve the weekly heating performance factor by 22%–26%, compared with a conventional layout using the condenser for charging. Savings in operational expenses can justify a capital expense of 25,000 Swedish Krona (about 2,500 €) for the LHTES system with a 15-yr operation. Although this justifiable capital expense is lower than the storage component cost alone estimated with the cost of Component 2, it is anticipated that similar LHTES solutions may gain more economic feasibilities with larger peak-valley electricity price differences foreseeable in the future. 

Through presentation of the multi-level feasibility evaluation, this thesis identifies key design and operational issues which might be neglected in single-level investigations. Furthermore, the thesis develops two new LHTES-HP integrated solutions with improved storage design/control strategies and enhanced system coupling methods from the existing solutions. This provides application-oriented insight for design and operation of the load-shift based LHTES installations in residential buildings, potentially contributing to decarbonisation of the increasingly electrified heating sector. 

Latenta värmeenergilagringssystem (eng. LHTES) kan kopplas till värmepumpsystem (eng. HP) för att uppnå en förskjutning av värmebehovet. Med fasförändringsmaterial (eng. PCM) uppvisar väldesignade LHTES-komponenter hög lagringsenergitäthet och har således stora möjligheter att integreras i bostäder där en kompakt lagringslösning efterfrågas. Men installationer av LHTES-HP-integrerade system är fortfarande knappa nuförtiden eftersom att det är nödvändigt att demonstera möjligheten hos denna teknik att uppnå tekniskt och ekonomiskt sund förskjutning av värmelaster samt öka föreståelsen hos intressenter för att främja implementering av tekniken. Således presenterar denna avhandling en studie på implementeringen av tre utvalda och tillgängliga makroinkapslade PCM-produkter. Studien omfattar djupgående undersökningar med experimentella och numeriska modellering på tre nivåer – material-, komponent- och system-nivå. Implementeringen studeras utifrån ett specifikt scenario där de makroinkapslade LHTES-systemen är utformade för att integreras med HP-baserade värmesystem i typiska svenska bostadshus.

På materialsidan väljs tre kommersiella PCM:er (C48, C58 och ATP60) utifrån arbetstemperatursnivån i typiska HP-baserade värmesystem. Metoderna Differential Scanning Calorimetry och Temperature History används för att mäta PCM:ens entalpi-temperaturprofiler; Metoden Transient Plane Source används för att mäta ATP60:s värmeledningsförmåga. C58, som är baserat på natriumacetat trihydrat, prioriteras för noggranna genomförbarhets analyser på grund av sin högsta värmelagringskapacitet.

På komponentnivå utvecklas tre fullskaliga makroinkapslade LHTES-komponenter (Komponent 1: cylindrisk inkapsling av C48; Komponent 2: cylindrisk kapsling av C58; Komponent 3: ellipsoid inkapsling av ATP60) för integrering i enfamiljshus för att uppnå full Peak Load Shifting (förskjutning av värmelasten under toppeffekttimmarna). En testprototyp byggs för att testa dessa tre komponenter under olika driftförhållanden som kan förekomma i typiska värmesystem. Effekten av att öka temperaturskillnaden mellan värmeöverföringsfluidens (eng. HTF) inloppstemperatur och smältpunkt samt att öka HTF-inloppsflödeshastigheten är en ökade värmeöverföring, vilken kvantifieras. Prestandaindikatorer såsom tid för total laddning / urladdning av prototypen, energilagringstäthet och kapacitetsförbättringsfaktor utvärderas vid olika driftstemperaturområden. Sammantaget kan Komponent 2 leverera cirka 90% av lagringskapaciteten (kapacitetsförlusten beror på fasseparation). Dock behövs det fortfarande förbättring i lagringsdesign och styrstrategin för att uppnå total Peak Load Shifting under en 3-timmars urladdningsprocess. För komponent 2 har det utvecklats en förbättrad lagringslösning med reducerad kapseldiameter och tids-ökande HTF-flödesprofiler genom numerisk simulering med en experimentellt validerad tvådimensionell värmeöverföringsmodell. Vidare har det utvecklats en endimensionell modell för att simulera värmelagringskapacitet för Komponent 2 och 3.

På systemnivån har det utvecklats en numerisk modell för att beräkna elektricitetskonsumption hos de LHTES-HP integrerade systemen för teknisk, ekonomisk och miljömässig utvärdering av förskjutning av värmelasterna. Tre nya integrationslayouter har utvecklats för att ladda Komponent 2 med en de-superheater och/eller en booster-värmepumpcykel. De nya layouterna kan förbättra den värmeprestandafaktorn (som beräknas för en vecka) med 22%–26% jämfört med en vanlig layout där kondensorn används för laddning av Komponent 2. Besparingarna i driftskostnaderna kan motivera en kapitalkostnad på 25 k SEK för LHTES-systemet med 15-årig teknisk livslängd. Även om denna motiverade kapitalkostnaden är lägre än kostnaden för enbart LHTES-komponenten under de nuvarande marknadsförhållandena, förväntas sådana LHTES-lösningar bli ekonomiskt genomförbara med större prisskillnader i elpriser i framtida marknaden.

Genom utvärdering av implementeringen på flera nivåer identifierar denna avhandling avgörande design- och driftsproblem som möjligtvis blir försummade i en studie som genomförs på enbart en nivå. Dessutom utvecklar avhandlingen nya LHTES-HP-integrerade lösningar med förbättrade lagringsdesigner, styrstrategier och systemkopplingsmetoder jämfört med befintliga lösningar. Detta ger en applikationsorienterad insikt för design och drift av framtida LHTES-installationer, vilket kan bidra till dekarbonisering av den allt mer elektrifierade uppvärmnings sektorn.

Nyckelord: fasförändringsmaterial, värmeenergilagring, värmepump, lastförskjutning, rymdvärme.

Integrating latent heat thermal energy storage (LHTES) units into building heating systems has been increasinglyinvestigated as a heat load management technology. A conventional LHTES integration method for heat pumpbased heating systems is to connect the heat pump’s condenser for charging the LHTES unit. This integratinglayout however usually leads to increased electricity input to the heating system. To underline this issue andprovide solutions, this paper presents three new LHTES integrating layouts where the LHTES unit is connectedwith the de-superheater of the main heat pump (Case 2), the condenser of a cascaded booster heat pump cycle(Case 3), or a combination of using both the de-superheater and the booster cycle (Case 4). In the context of amulti-family house in Stockholm, a quasi-steady state heating system model was developed to evaluate the newintegrating layouts, which were benchmarked against the baseline heating system without storage (Case 0) andthe conventional integrating layout using the main heat pump condenser (Case 1). Hourly electric power input tothe heating system was modelled for calculating the performance indicators including the heating performancefactor, the operational expense and justifiable capital expense, and the indirect CO2 emissions. Two load shiftingstrategies were simulated for an evaluation period of Week 1, 2019: 1) charge during off-peak hours (8 pm to 6am) and 2) charge during daytime hours (10 am to 7 pm). The simulation results of the off-peak charging strategyshow that, in Cases 2–4, the heating performance factor is 22%-26% higher than Case 1 and the operational expense can be reduced by 2%-5% as compared with Case 0. The savings in the operational expense can justifythe capital expense of 11 k-25 k Swedish Krona (SEK) for the LHTES systems in Cases 2–4 assuming a 15-yearoperation. Furthermore, the advantage of using the daytime charging strategy is principally the mitigation of CO2 emissions, which is up to 14% lower than the off-peak charging strategy. In summary, higher energy efficiencyfor heating is validated in the three new proposed integration layouts (Cases 2–4) against the condensercharging layout.

Well-integrated thermal energy storage units can enhance flexibility and profitability for a cogeneration system by enabling its decoupling of electricity and heat production. In the present study, novel latent heat thermal energy storage technologies are numerically investigated on their thermal and economic performance to evaluate their implementation at an existing combined cycle power plant. Three commercially available storage designs are analyzed: one shell-and-tube heat exchanger design based on planar spiral coils, and two types of advanced macro-encapsulated designs with capsules resembling ellipsoid and slab in shape, respectively. For the spiral coil design, three-dimensional flow velocity and temperature fields are simulated with finite volume method to predict the transient storage heat transfer process, including the effect of secondary flow induced by centrifugal forces. For the macro-encapsulated designs, effective heat transfer coefficients between heat transfer fluid (HTF) and phase change material (PCM) are inferred from scaled-down storage prototyping and testing. A onedimensional two-phase packed bed model was developed based on the apparent heat capacity-based enthalpy method to numerically study the heat transfer in macro-encapsulated PCM. With an operating temperature range of 46-72 degrees C and a HTF supplying flowrate range of 4.2-8.4 m3/h defined by the cogeneration strategy, thermal power and accumulated storage capacity are calculated and compared for the first three hours of charge and the first hour of discharge for the three designs. The effect from increasing the HTF flowrate to accelerate charging/ discharging processes is indicated by the simulation results. Performance comparison among the three designs shows that the slab capsule design exhibits the highest accumulated storage capacity (710 kWh) and state of charge (40%) after three hours of charge, though it has a lower theoretical total storage capacity (1760 kWh) than the spiral coil design (1830 kWh). The ellipsoid capsule design shows a slightly lower accumulated storage capacity (700 kWh) than the slab design for 3-hr charge and an equivalent accumulated storage capacity/depth of discharge (250 kWh/14%) as the latter. Furthermore, the storage power cost of the slab capsule design is the lowest, by 6-12% lower than the spiral coil design and by 2-3% lower than the ellipsoid capsule design. However, with the highest design flowrate of 8.4 m3/h, the low state of charge (below 40%) after three hours and the low depth of discharge (below 14%) after one hour indicate that redesigning the heat transfer boundary conditions and the configurations of the three units are necessary to meet desirable storage performance in cogeneration applications.

Latent heat thermal energy storage has been receiving increasing interests in residential heating applications. In this paper, a numerical heat transfer model was built with finite element method for a cylindrically encapsulated latent heat storage prototype and used for investigating its thermal performance optimization measures. The model was validated against four sets of experimental results for both charge and discharge, as the difference in accumulated storage capacity between simulation and experiment is less than 4%. Transient storage inlet boundary conditions were set in simulation for discharge considering the thermal output from the coupled radiators. The results of the optimization analyses show that: 1) reducing the capsule diameter from 69 mm to 15 mm shortens the completion time of charge and discharge by up to 70%, however, at the expense of 23% decrease in total storage capacity; 2) using parabolic or linear time-increasing heat transfer fluid flowrate profiles than a time-constant one extends around twofold the useful discharge timespan; 3) increasing the storage vessel diameter from 0.6 m to 0.7 m and to 0.8 m prolongs the useful discharge timespan from 2 hrs to the recommended 3 hrs, though the further enlargement to 0.8 m results in a lower state of charge after 3 hrs due to increase in unexploited storage capacity. From the numerical optimization study, we proposed a storage design adjustment of using 15 mm-diameter phase change material capsules in a 0.7 m-diameter cylindrical storage vessel, coupled with a parabolic flow strategy, to improve the storage on-peak discharging performance.

Latent heat thermal energy storage (LHTES) has been receiving increasing attention from researchers and engineers. A practical LHTES installation requires a deep understanding of phase change material’s (PCM’s) thermal behavior under thermal property testing and realistic operating conditions. To enrich this understanding, an experimental study on a commercial sodium acetate trihydrate-based PCM (Climsel C58) is presented in this article. C58 was characterized with two test methods: T-history tests and full-scale LHTES tests. The results are presented and discussed to exhibit the thermal behavior of C58 with these two test methods and the variations between them. With T-history tests, the thermal properties of C58 such as melting/solidification temperature range (57–61 °C/55–50 °C) and latent heat of fusion (216 kJ/kg) were determined. In full-scale LHTES tests, a parametric study was conducted to investigate the effects of heat transfer fluid flowrate and operating temperature range on the thermal performance of a 0.38 m3 storage prototype containing cylindrically macro-encapsulated C58. Moreover, longitudinal and radial PCM temperature distributions in full-scale tests were analyzed, suggesting the presence of phase separation. In general, C58 behaved differently between the two test methods regarding phase separation (negligible in T-history tests), supercooling effects (within 3 K in full-scale but up to 10 K in T-history tests), and thermal energy storage capacity (10% lower in full-scale tests). When using C58 or other salt hydrate-based PCMs for large-scale heat storage, these thermal behavior differences between the property-measurement and the application-oriented environments should be properly addressed in the design stage to ensure performance.

In the framework of this research work, the principle focus is to assess the applicability & reliability of the Phase change Effective Convectivity Model (PECM) as a numerical analysis tool to investigate natural convective heat transfer in single and two-fluid density PCM molten pools. The model is applied in ANSYS FLUENT as User Defined Function (UDF) to predict convective melt pool thermal hydraulics in a volumetrically heated PCM (Phase Change Material) melt pool. As a part of this work, PECM is tested first by a benchmark case against CFD to gain confidence in its applicability as an analysis tool. Two commercial PCMs: RT50 and C58, are introduced in a 3D semicircular vessel slice with their thermo-physical properties as input for modelling. The sidewalls made of quartz glass are used for direct visualization of convective heat transfer phenomena. It is ensured that the conditions of nearly constant density of power deposition over the entire volume of the PCM melt pool throughout the series of simulation cases. The values of characteristic numbers ranged within the following limits with different pool height corresponding modified Rayleigh number Ra=1012-1013 and for Prandtl number Pr=5-7. The selected modelling approach is validated against SIGMA experiment with respect to the angular distribution of heat flux that qualify our model to run in the proceeding calculation using PECM. Following benchmark test results of PECM compared with that of conventional enthalpy porosity method embedded in ANSYS FLUENT, PECM is applied in 1-layer and 2-layer PCM configuration to study in details of the influence of different boundary conditions, internal heat sources (QV) and heat transfer fluid (HTF) cooling condition to quantify the thermal loads. Finally, the comparison is made between two PCM configurations in terms of the quantification of the thermal load to justify PECM as an efficient numerical analysis tool for investigating convective heat transfer phenomena during PCM melting. 

Short-term latent heat thermal energy storage (LHTES) can be integrated in building heating systems for performing load shifting from on- to off-peak hours. In this study, the coupling effects of LHTES integration in heat pump heating systems are investigated for a multi-family building in Stockholm. As the reference heat pump provides heat in two separate heat exchangers, a de-superheater for domestic hot water and a condenser for space heating, two configurations for LHTES integration with the two respective heat exchangers are compared using a quasi-stationary mathematic system model. The performances of the two integrating configurations are also compared with the basic heat pump heating scenario (no heat storage) regarding the total electricity consumption, the seasonal performance factor (SPF), and the electric bills for end-users over a selected heating-season week. The simulation results reveal that, while using the condenser for charging an LHTES unit containing phase change materials melted at 58 oC inevitably increases the total electricity consumption in the heating system relative to the basic scenario, using the desuperheater instead for LHTES charging can reduce the total electricity consumptions and bills of 24 kWh and 173 SEK (17€), respectively, during that specific week.