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Heat Transfer Enhancement of Latent Thermal Energy Storage in Rectangular Components
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.ORCID iD: 0000-0002-8059-1546
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Latent Thermal Energy Storage (LTES) is an interesting choice to storethermal energy in a sustainable energy system. The primary benefit of LTESis the relatively high latent heat of fusion of the materials, known as PhaseChange Materials (PCM), used in such a system as the storage medium.However, as the thermal conductivity of PCMs is often very low, there is aneed to enhance the rate of heat transfer within the charging/dischargingprocess and to improve the thermal performance of the LTES systems.This thesis addresses the enhancing effect of extending heat transfer area inrectangular LTES enclosures. A key contribution of this thesis is acomprehensive visualization of the phase change processes for an organicPCM, including solidification and melting, constrained as well asunconstrained, known as Close-Contact Melting (CCM), in a cavity with andwithout fins. Observations have been carried out for fins of different lengthsand numbers with a varying angle of inclination, and a comprehensive analysisin terms of phase change time and thermal power is conducted.The observations show fins are more influential in solidification than inmelting, reducing the solidification time by 80% and increasing the meanpower by 395%, at a cost of 10% loss in the extracted energy. In contrast, inmelting, fins have a modest effect in enhancing the process. The relativeenhancement effect of fin is higher in constrained melting than inunconstrained melting. In a case with maximum enhancement, a reduction by52% in the constrained melting time and a relative enhancement in the meanpower by 90% is achieved at a cost of 9% loss in the stored energy. As thevolume fraction of fin increases, the discrepancies in melting time betweenthe constrained and unconstrained melting diminishes.A numerical model for solidification and constrained melting is validatedbased on the experiments, and a more inclusive sensitivity analysis of finparameters is performed. The enhancing effect of different parameters on thephase change time and the thermal power is analyzed and the relatively moreeffective measures are identified. Analyzing the simulation data withdimensionless parameters for a cavity oriented horizontally and enhancedwith vertical fins, overall dimensionless groups for solidification and constrained melting have been obtained. The dimensionless groupscontribute in general to achieving a better understanding of fins parametersand to facilitating the LTES designs.In addition, this thesis investigates a novel idea of extending the surface areavia incorporating mini-channels into LTES enclosures, used as passages forair as a low thermal conductive Heat Transfer Fluid (HTF). The mini-scaleinternal hydraulic diameter of the mini-channels and their high external areato-volume ratios make a potential for dual enhancement on both the PCMside and the HTF side. An existing design and a conceptual one with thepossibility of adding fins on the PCM side, capable of being manufactured viaproduction methods of extrusion and Additive Manufacturing (AM),respectively, have been simulated and studied.The two mini-channel types provide considerable enhancements in the rateof heat transfer for a PCM heat exchanger working with air. The degree ofenhancement increases as the air flow rate increases, at the cost of anincreasingly higher pressure drop. Regarding this, increasing the number ofchannels is identified as a more effective enhancing measure than adding finsto the PCM side. In addition, the conceptual design with a higher internalhydraulic diameter and considerably a higher aspect ratio has a lower pressuredrop than the existing design, charging/discharging the thermal energy at asimilar rate but with a lower fan power. More optimized designs withminimization of pressure drop, contribute to paving the way in facilitation ofthe utilization of the enhanced air-PCM heat exchanger in variousapplications.

Abstract [sv]

Latent termisk energilagring (LTES) är ett intressant val för att lagra termisk energi i ett hållbart energisystem. Den primära fördelen med LTES är den relativt höga smältvärmen av materialet, känt som fasändringsmaterial (PCM), som används som lagringsmedium i ett sådant system. I och med att värmekonduktiviteten hos PCM ofta är mycket låg finns det ett behov av att öka värmeöverföring-hastigheten för laddnings-/urladdningsprocessen samt att förbättra LTES-systemens termiska prestanda.  Denna avhandling tar upp den förstärkande effekten av att utöka värmeöverföringsytor i rektangulära LTES-behållare. Ett nyckelbidrag i denna avhandling är en omfattande visualisering av fasförändringsprocesserna i ett organiskt PCM, inklusive frysning/stelning och smältning, både som begränsad (fastnat) smältning och obegränsad (rörande) smältning, även känt som Close-Contact Melting (CCM), i en behållare med och utan flänsar. Observationer har gjorts för flänsar av olika längd samt antal, med varierande lutningsvinkel och en omfattande analys av processerna vad gäller fasändringstid och medeleffekt genomförs. Observationerna visar att flänsar är mer effektiva vid stelning än vid smältning, vilket minskar stelningstiden med 80% och ökar medeleffekten med 395%, dock minskar den utvunna energin med 10%. Däremot har flänsar vid smältning en måttlig effekt vad gäller att förbättra processen. Den relativa förstärkningseffekten av flänsarna är högre vid begränsad smältning än vid obegränsad smältning. I ett fall med maximal förbättring uppnås en minskning på 50% av den begränsade smälttiden och en relativ förbättring av medeleffekten på 90%, samtidigt som den lagrade energin minskar med 9%. När volymfraktionen av flänsarna ökar, minskar skillnaderna i smälttid mellan den begränsade och obegränsade smältningen. En numerisk modell för stelning och begränsad smältning valideras baserat på experimenten, och en mer utförlig känslighetsanalys av flänsparametrar utförs. Den förstärkande effekten av olika parametrar på fasändringstiden och den termiska effekten analyseras och de relativt effektivare åtgärderna identifieras. Genom att analysera simuleringsdata med dimensionslösa parametrar för en kavitet orienterad horisontellt och förstärkt med vertikala flänsar, har övergripande dimensionslösa grupper för stelning och begränsad smältning erhållits. De dimensionslösa grupperna bidrar generellt till att uppnå en bättre förståelse av flänsarnas parametrar och till att underlätta LTES-designerna. Denna avhandling undersöker dessutom en ny idé om att utöka ytan genom att introducera minikanaler i LTES-värmeväxlare, som används som passager för luft som en lågt värmeledande värmeöverföringsvätska (HTF). Minikanalernas interna hydrauliska diameter och deras höga externa area-tillvolym- förhållanden skapar en potential för dubbel förbättring på både PCMsidan och HTF-sidan. En befintlig design och en konceptuell med möjlighet att lägga till flänsar på PCM-sidan, som kan tillverkas via tillverkningsmetoder extrusion respektive Additive Manufacturing (AM), har simulerats och studerats. Den termiska prestandan hos de två minikanaltyperna ger en avsevärd förbättring av medeleffekten för en PCM-värmeväxlare som arbetar med luft. Förbättringen ökar när luftflödet ökar, till priset av ett allt högre tryckfall. Angående detta identifieras ökning av antalet kanaler som en mer effektiv förbättrande åtgärd än att lägga till flänsar på PCM-sidan. Dessutom har den konceptuella designen med en större inre hydraulisk diameter och större proportioner ett lägre tryckfall än den befintliga designen, samt genomför laddning/urladdning av termisk energi i samma takt men med lägre fläkteffekt. Mer optimerade konstruktioner med minimering av tryckfall bidrar till att underlätta användning av den förbättrade luft-PCM värmeväxlaren i olika applikationer.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2022. , p. 181
Series
TRITA-ITM-AVL ; 2022:7
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-309999ISBN: 978-91-8040-173-9 (print)OAI: oai:DiVA.org:kth-309999DiVA, id: diva2:1645302
Public defence
2022-04-13, F3 / https://kth-se.zoom.us/j/61582597458, Lindstedtsvägen 26, Stockholm, 09:00 (English)
Opponent
Supervisors
Funder
Swedish Energy AgencyAvailable from: 2022-03-17 Created: 2022-03-17 Last updated: 2022-09-19Bibliographically approved
List of papers
1. Experimental investigation of thermo-physical properties of n-octadecane and n-eicosane
Open this publication in new window or tab >>Experimental investigation of thermo-physical properties of n-octadecane and n-eicosane
Show others...
2020 (English)In: International Journal of Heat and Mass Transfer, ISSN 0017-9310, E-ISSN 1879-2189, Vol. 161, article id 120285Article in journal (Refereed) Published
Abstract [en]

Reliable knowledge of phase change materials (PCM) thermo-physical properties is essential to model and design latent thermal energy storage (LTES) systems. This study aims to conduct a methodological measurement of thermo-physical properties, including latent enthalpy, isobaric specific heat, thermal conductivity and dynamic viscosity, of two n-alkanes, n-octadecane and n-eicosane. The enthalpy and isobaric specific heat of the materials are measured via differential scanning calorimetry (DSC) technique, using a pDSC evo7 from Setaram Instrumentation with a sample mass of 628.4 mg. The influence of the scanning rates, varying from 0.5 K/min to 0.025 K/min, in dynamic continuous mode within temperature range of 10-65 degrees C is investigated. The thermal conductivity and the dynamic viscosity are measured via Hot Disk TPS-2500S instrument and Brookfield rotational viscometer, respectively, up to 70 degrees C. The thermal analysis results via the pDSC show that the isothermal condition can be approached at a very low scanning rate, however at the cost of a higher noise level. A trade-off is observed for n-octadecane, achieving the lowest deviation of 0.7% in latent heat measurement at 0.05 K/min, as compared to the American Petroleum Table values. For n-eicosane, the lowest deviation of 1.2% is seen at the lowest scanning rate of 0.025 K/min. The thermal conductivity measured values show good agreements with a number of documented literature studies in the solid phase, within deviations of 2%. Larger deviations of 5-16% are found for the measurement in the liquid phase. The viscosity values also show a good agreement with the literature values with maximum deviations of 2.9% and 6.3%, with respect to the values of American Petroleum Tables, for n-octadecane and n-eicosane, respectively. The good agreements achieved in measurements establish the reliable thermo-physical properties contributing to the future simulations and designs. 

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2020
Keywords
PCM, Thermo-physical property, n-octadecane, n-eicosane
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-283867 (URN)10.1016/j.ijheatmasstransfer.2020.120285 (DOI)000571813900023 ()2-s2.0-85089414623 (Scopus ID)
Note

QC 20201126

Available from: 2020-11-26 Created: 2020-11-26 Last updated: 2022-12-07Bibliographically approved
2. Experimental investigation of solidification and melting in a vertically finned cavity
Open this publication in new window or tab >>Experimental investigation of solidification and melting in a vertically finned cavity
2021 (English)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 198, article id 117459Article in journal (Refereed) Published
Abstract [en]

Extending the heat transfer area is a simple technique to improve the thermal performance of phase change materials with low thermal conductivity. However, as the governing mechanisms differ in solidification and melting, fins can affect the processes in different ways. This demands assessment of fin enhancement in a combined analysis on both solidification and melting, often neglected in literature. This paper presents visual-izations of solidification and melting of n-eicosane in a rectangular cavity and experimentally investigates the enhancing effect of vertical fins with varying number and length. Experiments were conducted at water inlet temperature ranges of 15-25 degrees C and 50-60 degrees C for the solidification and melting processes, respectively. The results show that the vertical fins can be more influential in solidification rather than in melting with similar losses in the storage capacity. In the solidification process, as natural convection is absent, the mean power is enhanced by a maximum of 395% with a 10% loss in the storage capacity, as compared to the benchmark. In the melting case, the mean power is increased by a maximum of 90% with a 9% loss in the storage capacity. Although increasing the surface area with vertical fins contributes to development of convective structures, it makes a modest enhancement. In overall, increasing the fin volume fraction, in exchange for the loss in the storage capacity, enhances the solidification significantly while it has relatively low enhancement effect in melting. At the end, the performed experiments could be helpful for validation of future simulation tools with complex features, particularly solidification models lacking in literature.

Place, publisher, year, edition, pages
Elsevier BV, 2021
Keywords
PCM, Cavity, Vertical fin, Solidification, Melting
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-303757 (URN)10.1016/j.applthermaleng.2021.117459 (DOI)000701603600001 ()2-s2.0-85113634102 (Scopus ID)
Note

QC 20211028

Available from: 2021-10-28 Created: 2021-10-28 Last updated: 2022-06-25Bibliographically approved
3. Numerical investigation of melting in a cavity with vertically oriented fins
Open this publication in new window or tab >>Numerical investigation of melting in a cavity with vertically oriented fins
2019 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 235, p. 1027-1040Article in journal (Refereed) Published
Abstract [en]

This paper investigates the effect of vertical fins, as an enhancement technique, on the heat transfer rate and energy density of a latent heat thermal energy storage system. This contributes with knowledge on the interaction of heat transfer surface with the storage material for optimizing storage capacity (energy) and power (heat transfer rate). For the assessment, numerical modeling is employed to study the melting process in a two-dimensional rectangular cavity. The cavity is considered heated isothermally from the bottom with surface temperatures of 55 degrees C, 60 degrees C or 70 degrees C, while the other surfaces are insulated from the surrounding. Aluminum and lauric acid are considered as fin/enclosure material and phase change material, respectively. Vertical fins attached to the bottom surface are employed to enhance the charging rate, and a parametric study is carried out by varying the fin length and number of fins. Thus, a broad range of data is provided to analyze the influence of fin configurations on contributing natural convection patterns, as well as the effects on melting time, enhanced heat transfer rate and accumulated energy. The results show that in addition to increasing the heat transfer surface area, the installation of vertically oriented fins does not suppress the natural convection mechanism. This is as opposed to horizontal fins which in previous studies have shown tendencies to reduce the impact of natural convection. This paper also highlights how using longer fins offers a higher rate of heat transfer and a better overall heat transfer coefficient rather than increasing the number of fins. Also, fins do not only enhance the heat transfer performance in the corresponding melting time, but also maintain similar total amount of stored energy as compared to the no-fin case. This paper discusses how this is the result of the enhanced heat transfer allowing a larger portion of sensible heat to be recovered. For example, in the case with long fins, the relative mean power enhancement is about 200% with merely 6% capacity reduction, even though the amount of PCM in the cavity has been reduced by 12% as compared to the no-fin case. Although the basis for these results stems from the principles of thermodynamics, this paper is bringing it forward with design consideration. This is because despite its importance for making appropriate comparisons among heat transfer enhancement techniques in latent heat thermal energy storage, it has not been previously discussed in the literature. In the end, the aim is to accomplish robust storage systems in terms of power and energy density.

Place, publisher, year, edition, pages
ELSEVIER SCI LTD, 2019
Keywords
PCM, Melting, Cavity, Fin, Conduction, Convection
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-246275 (URN)10.1016/j.apenergy.2018.11.025 (DOI)000458942800083 ()2-s2.0-85056639432 (Scopus ID)
Conference
14th International Conference on Energy Storage (EnerSTOCK), APR 25-28, 2018, Cukurova Univ, Adana, TURKEY
Note

QC 20190325

Available from: 2019-03-25 Created: 2019-03-25 Last updated: 2022-06-26Bibliographically approved
4. Numerical Investigation of Latent Thermal Storage in a Compact Heat Exchanger Using Mini-Channels
Open this publication in new window or tab >>Numerical Investigation of Latent Thermal Storage in a Compact Heat Exchanger Using Mini-Channels
2021 (English)In: Applied Sciences, E-ISSN 2076-3417, Vol. 11, no 13, p. 5985-, article id 5985Article in journal (Refereed) Published
Abstract [en]

This paper aims to numerically investigate the thermal enhancement of a latent thermal energy storage component with mini-channels as air passages. The investigated channels in two sizes of internal air passages (channel-1 with d(h) = 1.6 mm and channel-2 with d(h) = 2.3 mm) are oriented vertically in a cuboid of 0.15 x 0.15 x 0.1 m(3) with RT22 as the PCM located in the shell. The phase change is simulated with a fixed inlet temperature of air, using ANSYS Fluent 19.5, with a varying number of channels and a ranging air flow rate entering the component. The results show that the phase change power of the LTES improves with by increasing the number of channels at the cost of a decrease in the storage capacity. Given a constant air flow rate, the increase in the heat transfer surface area of the increased number of channels dominates the heat transfer coefficient, thus increasing the mean heat transfer rate (UA). A comparison of the channels shows that the thermal performance depends largely on the area to volume ratio of the channels. The channel type two (channel-2) with a slightly higher area to volume ratio has a slightly higher charging/discharging power, as compared to channel type one (channel-1), at a similar PCM packing factor. Adding fins to channel-2, doubling the surface area, improves the mean UA values by 15-31% for the studied cases. The variation in the total air flow rate from 7 to 24 L/s is found to have a considerable influence, reducing the melting time by 41-53% and increasing the mean UA values within melting by 19-52% for a packing factor range of 77.4-86.8%. With the increase in the air flow rate, channel type two is found to have considerably lower pressure drops than channel type one, which can be attributed to its higher internal hydraulic diameter, making it superior in terms of achieving a relatively similar charging/discharging power in exchange for significantly lower fan power. Such designs can further be optimized in terms of pressure drop in future work, which should also include an experimental evaluation.

Place, publisher, year, edition, pages
MDPI AG, 2021
Keywords
PCM, mini-channels, air, melting, solidification
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-298952 (URN)10.3390/app11135985 (DOI)000672298700001 ()
Note

QC 20210726

Available from: 2021-07-26 Created: 2021-07-26 Last updated: 2022-06-25Bibliographically approved

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