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Magnetic Refrigeration for Near Room-Temperature Applications
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.ORCID iD: 0000-0001-9592-0202
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Refrigeration plays a crucial role in many different sectors and consumes about 17% of the electricity produced globally. This significant energy consumption implies large share of refrigeration in primary energy consumption and other environmental impacts. In addition to the environmental impacts associated with energy consumption, the vapor-compression systems contribute in global warming due to the release of their gaseous refrigerants into the atmosphere. As an alternative technology for near room-temperature applications, magnetic refrigeration is proposed by some researchers to eliminate the release of gaseous refrigerants into the atmosphere and to reduce the energy consumption. This thesis is a compilation of a number of studies done on magnetic refrigeration for room-temperature applications.

In the first study, the environmental impacts associated to magnetic refrigeration are looked at closely through a life cycle assessment. The life cycle assessment indicates that because of the environmental burdens related to the rare-earth materials used in magnetic refrigeration, the reduction in the environmental impacts is not guaranteed by switching to magnetic refrigeration technology. Accordingly to avoid the extra environmental impacts the magnetic refrigeration systems should use magnetic materials frugally, which requires an optimized design. In addition, operation with higher efficiency compared to vapor-compression systems is necessary to have environmental advantages, at least in some impact categories.

A practical method to optimize the design of magnetic refrigeration systems, e.g. to have a compact design or high efficiency, is utilizing a flexible software model, with which the effect of varying different parameters on the performance of the system can be simulated. Such a software model of the magnetic refrigeration system is developed and validated in this project. In developing the model one goal is to add to the precision of the simulated results by taking more details into consideration. This goal is achieved by an innovative way of modeling the parasitic heat transfer and including the effect of the presence of magnetocaloric materials on the strength of the field created by the magnet assembly. In addition, some efforts are made to modify or correct the existing correlations to include the effect of binding agents used in some active magnetic regenerators. Validation of the developed software model is done using the experimental results obtained from the prototype existing at the Department of Energy Technology, KTH Royal Institute of Technology.

One of the parameters that can be modified by the developed software model is the choice of the magnetocaloric materials for each layer in a layered active magnetic regenerator. Utilizing the software model for optimizing the choice of the materials for the layers reveals that materials with critical temperatures equal to the cyclic average temperature of the layers in which they are used do not necessarily result in the desired optimum performance. In addition, for maximizing different outputs of the models, such as energy efficiency or temperature lift sustained at the two ends of the regenerators, different choice of materials for the layers are needed. Therefore, in other studies seeking to improve one of the outputs of a system, the choice of the transition or critical temperatures of the materials for each layer is an additional parameter to be optimized.

The prototype existing at the Department of Energy Technology, KTH Royal Institute of Technology, was initially designed for replacing the vapor-compression system of a professional refrigerator. However, it could not fulfil the requirements for which it was initially designed. The aforementioned developed simulation model is used to see how much the choice of the materials, size of the particles, and number of layers can enhance the performance while the operation frequency and flow rate of the heat transfer fluid are at their optimum values. In other words, in that study the room for improvement in the performance without applying major changes in the system such as the geometry of the regenerator, which implies redesigning the whole magnet assembly, is investigated. In the redesign process the effect of binding agent and the limitations associated to different properties of it is also investigated theoretically. Nevertheless, the study did not show that with keeping the geometry of the regenerators and the currently existing magnetocaloric materials the initial goals of the prototype can be achieved.

In the next study more flexible choice of geometries and magnetocaloric materials are considered. In fact, in this study it is investigated how much the magnetocaloric materials need to be improved so that magnetic refrigeration systems can compete with vapor-compression ones in terms of performance. For the two investigated cases, the magnetic-field dependent properties of the currently existing materials are enough provided that some other issues such as low mechanical stability and inhomogeneity of the properties are solved. Nevertheless, for more demanding design criteria, such as delivering large cooling capacity over a considerable temperature span while the magnetic materials are used sparingly, the magnetic-field dependent properties need to be enhanced, as well.

A less explored area in room-temperature magnetic refrigeration is the subject of another study included in the thesis. In this study, solid-state magnetic refrigeration systems with Peltier elements as heat switches are modeled. Since the Peltier elements consume electricity to pump heat, the modeled systems can be considered hybrid magnetocaloric-Peltier cooling systems. For such systems the detailed transient behavior of the Peltier elements together with layers of magnetocaloric materials are modeled. The mathematical model is suitable for implementation in programing languages without the need for commercial modeling platforms. The parameters affecting the performance of the modeled system are numerous, and optimization of them requires a separate study. However, the preliminary attempts on optimizing the modeled system does not give promising results. Accordingly, focusing on passive heat switches can be more beneficial.

Abstract [sv]

Kylning spelar en avgörande roll i många olika sektorer och förbrukar cirka 17 % av den elektricitet som produceras globalt. Kylprocessernas energiförbrukning utgör alltså en stor andel av primärenergiförbrukningen och innebär även annan miljöpåverkan. Förutom miljöpåverkan som är förknippad med energiförbrukningen bidrar ångkompressionssystemen till global uppvärmning på grund av utsläpp av köldmedier i atmosfären. Som en alternativ teknik för nära rumstemperaturapplikationer föreslås magnetisk kylning av vissa forskare, för att eliminera utsläpp av köldmedier i atmosfären och för att minska energiförbrukningen. Denna avhandling är en sammanställning av ett antal studier om magnetisk kylning för rumstemperaturapplikationer.

I den första studien undersöktes de miljöpåverkningar som är förknippade med magnetisk kylning noggrant genom en livscykelanalys. Livscykelanalysen indikerar att minskningen av miljöpåverkan inte garanteras genom att byta till den magnetiska kylprocessen på grund av de miljöbelastningar som är relaterade till de sällsynta jordartsmetaller som används i magnetisk kylning. För att undvika de extra miljöpåverkningarna bör de magnetiska kylsystemen använda så lite magnetiska material som möjligt, vilket kräver en optimerad design. Dessutom är energieffektivare drift jämfört med ångkompressionssystemen nödvändigt för att få miljöfördelar, åtminstone i vissa miljöpåverkanskategorier.

En praktisk metod för att optimera designen av magnetiska kylsystem, för att uppnå t.ex. en kompakt design eller hög effektivitet, är användning av en flexibel mjukvarumodell, som simulerar effekten av olika parametrar på systemets prestanda. En sådan mjukvarumodell av det magnetiska kylsystemet har utvecklats och validerats i detta projekt. Ett syfte med utvecklingen av modellen är att öka precisionen av de simulerade resultaten genom att ta hänsyn till mer detaljer än i tidigare modeller. Detta mål uppnås genom ett innovativt sätt att modellera den parasitära värmeöverföringen och inkludera effekten av närvaron av magnetokaloriska material på styrkan av fältet som skapas av magnetaggregatet. Dessutom görs vissa modifieringar eller korrigeringar i de befintliga korrelationerna för att inkludera effekten av bindemedel som används i vissa aktiva magnetiska regeneratorer. Validering av den utvecklade mjukvarumodellen görs med hjälp av experimentella resultat som erhållits från den prototyp som finns vid Institutionen för Energiteknik, Kungliga Tekniska Högskolan.

En av parametrarna som kan modifieras i den utvecklade mjukvarumodellen är valet av magnetokaloriska material för varje skikt i en skiktad aktiv magnetisk regenerator. Användning av mjukvarumodellen för att optimera valet av material för skikten visar att material med kritiska temperaturer som är lika med den cykliska genomsnittstemperaturen hos de skikt där de används inte nödvändigtvis resulterar i önskad optimal prestanda. Dessutom behövs olika materialval för skikten för att maximera modellernas resultat avseende energieffektivitet eller temperaturskillnaden som erhålls mellan de två ändarna av regeneratorerna. Därför är valet av kritiska temperaturer för skiktens material en ytterligare parameter som ska optimeras i studier med avsikt att förbättra ett systems prestanda.

Prototypen vid Institutionen för Energiteknik, Kungliga Tekniska Högskolan, var ursprungligen designad för att ersätta ångkompressionssystemet för ett restaurangkylskåp. Det kunde emellertid inte uppfylla de krav för vilka det ursprungligen utformats. Den ovan nämnda utvecklade simuleringsmodellen används för att undersöka hur mycket prestandan kan förbättras genom att förändra valet av material, partikelstorleken, antalet skikt, driftsfrekvensen och flödeshastigheten av värmeöverföringsvätskan. Med andra ord undersöks utrymmet för förbättring av prestandan utan att genomföra stora förändringar i systemet, såsom förändringar i regeneratorers geometri och i magnetaggregatet. Under processen undersöks effekten av bindemedel på prestanda och begränsningar som är förknippade med bindemedlets egenskaper. Trots detta kunde studien inte visa att de ursprungliga målen för prototypen kan uppnås utan att ändra regeneratorernas geometri och de befintliga magnetokaloriska materialen.

I nästa studie övervägs mer flexibla val av regeneratorernas geometrier och magnetokaloriska material. I den här studien undersöks hur mycket de magnetokaloriska materialen måste förbättras för att magnetiska kylsystem ska kunna konkurrera med ångkompressionssystem vad gäller prestanda. För de två undersökta fallen är de magnetfältberoende egenskaperna hos befintliga materialen tillräckliga förutsatt att vissa andra problem, såsom låg mekanisk stabilitet och inhomogenitet hos egenskaperna, löses. Emellertid, för mer krävande designkriterier, såsom att ge stor kylkapacitet över en betydande temperaturdifferens samtidigt som de magnetiska materialen används sparsamt, behöver de magnetfältberoende egenskaperna också förbättras.

Ett mindre undersökt område för magnetisk kylning i rumstemperatur är föremål för en annan studie som ingår i avhandlingen. I denna studie modelleras fasta magnetiska kylsystem med Peltier-element som värmebrytare. Eftersom Peltier-elementen förbrukar elektricitet och pumpar värmen kan de modellerade systemen betraktas som hybrid magnetokalorisk-Peltier-kylsystem. För sådana system modelleras i detalj det transienta beteendet hos Peltier-elementen tillsammans med de magnetokaloriska materialskikten. Den matematiska modellen är lämplig för implementering i programmeringsspråk utan behov av kommersiella modelleringsplattformar. Parametrarna som påverkar det modellerade systemets prestanda är många, och optimering av dem kräver en separat studie. De preliminära optimeringsinsatserna ger emellertid inte lovande resultat. Följaktligen kan fokusering på passiva värmebrytare vara mer fördelaktiga.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2018. , p. 137
Series
TRITA-ITM-AVL ; 2018:18
Keywords [en]
Magnetic Refrigeration, Magnetic, Cooling, Magnetocaloric, Life Cycle Assessment, Modeling, Simulation, Optimization, Regeneration, Active Magnetic Regeneration, Bonded Regenerator, Heat Switch, Thermal Diode, Peltier, Solid-State, Prototype
Keywords [sv]
Magnetisk Kylteknik, Magnetisk, Kylning, Magnetokalorisk, Livscykela-nalys, Modellering, Simulering, Optimering, Regenerering, Aktiv Mag-netisk Regenerering, Bunden Regenerator, Värmebrytare, Termodiod, Peltier, Solid-State, Prototyp
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-227683ISBN: 978-91-7729-792-5 (print)OAI: oai:DiVA.org:kth-227683DiVA, id: diva2:1205142
Public defence
2018-06-05, D2, Lindstedsvägen 5, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20180514

Available from: 2018-05-14 Created: 2018-05-11 Last updated: 2022-06-26Bibliographically approved
List of papers
1. Magnetic vs. vapor-compression household refrigerators: A preliminary comparative life cycle assessment
Open this publication in new window or tab >>Magnetic vs. vapor-compression household refrigerators: A preliminary comparative life cycle assessment
2014 (English)In: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 42, p. 69-76Article in journal (Refereed) Published
Abstract [en]

This paper seeks to shed light on the question whether a magnetic household refrigerator with permanent magnets is more environmentally friendly than a conventional, vapor-compression refrigerator. Life cycle assessment has been used as a tool to investigate the environmental impacts associated with the life cycle of a magnetic refrigerator. The results of the assessment have been compared with those of a conventional, vapor-compression refrigerator with the same functionality. The comparison reveals that the magnetic refrigeration has higher environmental impacts mainly due to the use of rare-earth metals used in the magnet material. The possibility of compensating for this shortcoming through reuse of the magnetic materials or improving the design and efficiency of the magnetic refrigerator has been examined. In addition, the effect of the electricity mix consumed during the use phase, as one of the key factors determining the life cycle environmental impacts, has been investigated.

Place, publisher, year, edition, pages
Elsevier, 2014
Keywords
Magnetic, Refrigeration, Life cycle assessment, Environment
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-147579 (URN)10.1016/j.ijrefrig.2014.02.013 (DOI)000338804200009 ()2-s2.0-84901675794 (Scopus ID)
Funder
Swedish Energy Agency, 33847-1
Note

QC 20140812

Available from: 2014-06-30 Created: 2014-06-30 Last updated: 2022-06-23Bibliographically approved
2. Design and optimization of regenerators of a rotary magnetic refrigeration device using a detailed simulation model
Open this publication in new window or tab >>Design and optimization of regenerators of a rotary magnetic refrigeration device using a detailed simulation model
2018 (English)In: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 88, p. 260-274Article in journal (Refereed) Published
Abstract [en]

In this work a comprehensive simulation of a magnetic refrigeration device is presented, validated, and used for redesigning the regenerators of an existing prototype. The redesigning process includes choosing the magnetocaloric materials and number of layers and optimizing for particle size, flow, and operation frequency. The simulation consists of the model of the magnetic field, parasitic heat transfer and active regeneration. The model of the magnetic field and parasitic heat transfer are embedded in the 1D model of the active regeneration cycle. The detailed model of the magnetic field, taking the effect of presence of the magnetocaloric materials into account, is described and validated separately against measured magnetic field. An innovative method for including the parasitic heat transfer in the active regeneration model without compromising the accuracy is used. The influence of the properties of the binding agent on the performance of the bonded beds as regenerators are also investigated.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Magnetic refrigeration, Modelling, Heat transfer, Regeneration, Bonded regenerator, Optimization
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-224224 (URN)10.1016/j.ijrefrig.2018.01.011 (DOI)000437748300027 ()2-s2.0-85044613952 (Scopus ID)
Note

QC 20180314

Available from: 2018-03-14 Created: 2018-03-14 Last updated: 2024-03-18Bibliographically approved
3. Optimization of layered regenerator of a magnetic refrigeration device
Open this publication in new window or tab >>Optimization of layered regenerator of a magnetic refrigeration device
2015 (English)In: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 57, p. 103-111Article in journal (Refereed) Published
Abstract [en]

Magnetic refrigeration, as an alternative to vapor-compression technology, has been the subject of many recent investigations. A technique to enhance the performance of magnetic refrigerators is using layers of different materials in the regenerator of such devices. In this study the choice of magnetocaloric materials in a multi-layered packed bed regenerator is investigated in order to optimize the performance. A numerical model has been developed to simulate the packed bed in this study. Optimized packed bed designs to get maximum temperature span or maximum efficiency are different. The results indicate that maximum temperature span can be achieved by choosing the materials with the highest magnetocaloric effect in the working temperature range, while maximum Carnot efficiency is achieved by choosing materials with Curie temperatures above the average layer temperature.

Place, publisher, year, edition, pages
Elsevier, 2015
Keywords
Magnetic refrigeration, Magnetocaloric, Layering, Optimization, Temperature span, Efficiency
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-175372 (URN)10.1016/j.ijrefrig.2015.04.019 (DOI)000361908200012 ()2-s2.0-84937408356 (Scopus ID)
Note

QC 20151015

Available from: 2015-10-13 Created: 2015-10-13 Last updated: 2022-06-23Bibliographically approved
4. Corrigendum to “Optimization of layered regenerator of a magnetic refrigeration device” (International Journal of Refrigeration (2015) 57 (103–111)(S0140700715001267)(10.1016/j.ijrefrig.2015.04.019))
Open this publication in new window or tab >>Corrigendum to “Optimization of layered regenerator of a magnetic refrigeration device” (International Journal of Refrigeration (2015) 57 (103–111)(S0140700715001267)(10.1016/j.ijrefrig.2015.04.019))
2017 (English)In: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 78Article in journal (Refereed) Published
Abstract [en]

The authors regret that, in three instances on page 105 the term “Maxwell equations” is used mistakenly instead of “thermodynamic relations”. However, this does not affect any results or conclusions and is just a correction in the terminology. The authors would like to apologise for any inconvenience caused.

Place, publisher, year, edition, pages
Elsevier Ltd, 2017
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-216479 (URN)10.1016/j.ijrefrig.2017.03.011 (DOI)000403029800018 ()2-s2.0-85018629928 (Scopus ID)
Note

QC 20171201

Available from: 2017-12-01 Created: 2017-12-01 Last updated: 2022-06-26Bibliographically approved
5. Material requirements for magnetic refrigeration applications
Open this publication in new window or tab >>Material requirements for magnetic refrigeration applications
2018 (English)In: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 96, p. 25-37Article in journal (Refereed) Published
Abstract [en]

A primary motivation underlying the research on room-temperature magnetic refrigeration is reaching energy efficiency levels beyond what is achievable with vapor-compression technology. However, the goal of building commercially viable magnetic refrigeration systems with high performance and competitive price has not been achieved yet. One of the obstacles to reach this goal is the inadequate properties of the currently existing magnetocaloric materials. In this article, the needed improvements in the properties of the magnetocaloric materials are investigated. Two existing vapor-compression refrigerators are used as reference for the required performance, and magnetic refrigerators are simulated using a numerical model. Apart from the requirements such as uniformity of transition temperature for each layer, small increment in transition temperature in adjacent layers, and mechanical strength of the materials, the study shows that for the investigated cases materials with adiabatic entropy change 2.35 times larger than the existing materials are needed to outperform vapor-compression systems.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Magnetocaloric, Material, Magnetic, Refrigeration, Cooling
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-227681 (URN)10.1016/j.ijrefrig.2018.08.012 (DOI)000452347000005 ()2-s2.0-85054296445 (Scopus ID)
Note

QC 20181008

Available from: 2018-05-10 Created: 2018-05-10 Last updated: 2024-03-15Bibliographically approved
6. Simulation of solid-state magnetocaloric refrigeration systems with Peltier elements as thermal diodes
Open this publication in new window or tab >>Simulation of solid-state magnetocaloric refrigeration systems with Peltier elements as thermal diodes
2017 (English)In: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, no 74, p. 322-330Article in journal (Refereed) Published
Abstract [en]

Magnetic refrigeration as an alternative for vapor-compression technology has been the subject of many recent studies. Most of the studies focus on systems with limited cycle frequency in which a fluid transfers heat to and from the magnetocaloric material. A suggested solution for increasing the frequency is use of solid-state magnetic refrigeration in which thermal diodes guide the heat from the cold end to the warm end. In this work a solid-state refrigeration system with Peltier elements as thermal diodes is modeled in details unprecedented. The performance of Peltier elements and magnetocaloric materials under their transient working conditions after reaching cyclic steady state are simulated by two separate computer models using finite element method and finite volume method. The models, in parts and as a whole, are verified. The verified finite element model is used for a parametric study and the results are analyzed.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
magnetic refrigeration, solid-state, Peltier element, thermal diode, heat gate, hybrid
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-196515 (URN)10.1016/j.ijrefrig.2016.11.007 (DOI)000394399000028 ()2-s2.0-85002863275 (Scopus ID)
Note

QC 20161117

Available from: 2016-11-15 Created: 2016-11-15 Last updated: 2024-03-18Bibliographically approved
7. Simulation of magnetic refrigeration systems with thermal diodes and axial conductive heat transfer
Open this publication in new window or tab >>Simulation of magnetic refrigeration systems with thermal diodes and axial conductive heat transfer
2016 (English)In: Refrigeration Science and Technology Proceedings, 2016Conference paper, Published paper (Refereed)
Abstract [en]

In conventional magnetic refrigeration cycles with heat transfer fluid, the achievable cycle frequency, andtherefore, specific cooling capacity is limited. Furthermore, ineffective use of magnet in low frequencydevices makes them expensive. In this work, as an alternative technique, utilizing conductive heat transfercontrolled by two different types of thermal diodes, gas-liquid and contact-break diodes, is investigated. Forthat purpose two software models are made to simulate the performance of a magnetic refrigerator with eachof the diodes. The results of simulations are presented and comparison is made between these results andthe results of older studies which used ideal properties. According to the results, due to the limited thermalconductivity of the magnetocaloric materials, the increase in the capacity becomes small with too highfrequencies. Among the thermal diodes and materials studied, the liquid metal Galinstan as the conductingfluid in an active diode gave the best results.

Keywords
magnetocaloric, refrigeration, solid-state, thermal diode, heat gate, heat switch
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-192595 (URN)10.18462/iir.thermag.2016.0143 (DOI)000402542400037 ()2-s2.0-85017613159 (Scopus ID)978-2-36215-016-6 (ISBN)
Conference
7th International Conference on Magnetic Refrigeration at Room Temperature
Note

QC 20160921

Available from: 2016-09-15 Created: 2016-09-15 Last updated: 2022-06-22Bibliographically approved

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