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  • 1.
    A Monfared, Behzad
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Magnetic Refrigeration for Near Room-Temperature Applications2018Doktoravhandling, med artikler (Annet vitenskapelig)
    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.

  • 2.
    Monfared, Behzad
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Design and optimization of regenerators of a rotary magnetic refrigeration device using a detailed simulation model2018Inngår i: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 88, s. 260-274Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 3.
    Monfared, Behzad
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Simulation of magnetic refrigeration systems with thermal diodes and axial conductive heat transfer2016Inngår i: Refrigeration Science and Technology Proceedings, 2016Konferansepaper (Fagfellevurdert)
    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.

  • 4.
    Monfared, Behzad
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Simulation of solid-state magnetocaloric refrigeration systems with Peltier elements as thermal diodes2017Inngår i: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, nr 74, s. 322-330Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 5.
    Monfared, Behzad
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Furberg, Richard
    Palm, Björn
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Magnetic vs. vapor-compression household refrigerators: A preliminary comparative life cycle assessment2014Inngår i: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 42, s. 69-76Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 6.
    Monfared, Behzad
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Palm, Björn
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    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))2017Inngår i: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 78Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 7.
    Monfared, Behzad
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Palm, Björn
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Design and Test of a Domestic Heat Pump with Ammonia as Refrigerant2011Inngår i: 4th IIR Conference: Ammonia Refrigeration Technology, France: International Institute of Refrigeration, 2011Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Among alternative refrigerants, ammonia with zero Ozone Depleting Potential (ODP) and Global Warming Potential (GWP), and favorable thermodynamic properties is a sensible choice as a replacement for the synthetic refrigerants, which are powerful greenhouse gases.

    In this paper, the results of experiments done on a new ammonia water-to-water heat pump prototype at different evaporation temperatures and compressor speeds are reported. The heat pump is designed to deliver 7 kW heat at evaporation temperature of -5°C and condensation temperature of 40°C. The hot discharge gas from compressor is utilized to provide sanitary hot water, and the rest of the heat is used for preheating the tap water and space heating of a single-family house. The compact design of the heat pump helps reducing the refrigerant charge. To reduce the charge further and to prevent oil accumulation at the bottom of evaporator, a minichannel aluminum heat exchanger is used as evaporator.

  • 8.
    Monfared, Behzad
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Palm, Björn
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Design and Test of a Small Ammonia Heat Pump2011Inngår i: 10th IEA Heat Pump Conference 2011, 2011, s. s3_p18-Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Since synthetic refrigerants may cause environmental damages, by depleting the ozone layer or by contribution in global warming, many researchers, in recent years, have focused on the use of natural refrigerants such as ammonia to replace the synthetic ones. Although ammonia has been used in large refrigeration systems, its application in small units, say a small heat pump, is quite rare.

    The work presented in this paper is design and test of a small water-to-water ammonia heat pump, providing about 7 kW heat, sufficient for space heating and tap water heating of a single-family house. To reduce the charge and to overcome the problem of accumulation of oil at the bottom of evaporator, a minichannel heat exchanger is used as evaporator. Oil miscible in ammonia is also used to facilitate the oil return to compressor. A permanent magnet motor together with an inverter is used to run the compressor. Supplying the 7 kW heat, the heat pump runs efficiently with heating capacity to compression work ratio of 5.1 at evaporation and condensation temperatures of -5 °C and 40 °C.

  • 9.
    Monfared, Behzad
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Palm, Björn
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik.
    Material requirements for magnetic refrigeration applications2018Inngår i: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 96, s. 25-37Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 10.
    Monfared, Behzad
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Palm, Björn
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik.
    New magnetic refrigeration prototype with application in household and professional refrigerators2016Inngår i: Refrigeration Science and Technology Proceedings, 2016Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The number of magnetic refrigeration prototypes with high cooling capacity and large temperature span islimited and there is ample room for new designs and improvements. In this paper a new prototype, designedand built, aiming at 200 W cooling capacity and about 40 K temperature span is presented. Such a unit issuitable for applications in household and professional refrigerators. In the current work, design of theprototype is described, practical issues solved to make the prototype running are explained, and preliminarytest results are presented.

  • 11.
    Monfared, Behzad
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Palm, Björn
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik.
    Optimization of layered regenerator of a magnetic refrigeration device2015Inngår i: International journal of refrigeration, ISSN 0140-7007, E-ISSN 1879-2081, Vol. 57, s. 103-111Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 12.
    Monfared, Behzad
    et al.
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik, Tillämpad termodynamik och kylteknik.
    Palm, Björn
    KTH, Skolan för industriell teknik och management (ITM), Energiteknik.
    Small Ammonia Heat Pump with Variable Speed Compressor2012Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Ammonia is a natural refrigerant which has been used continuously for more than 100 years. From almost any technical perspective, it is an attractive refrigerant, having good cycle efficiency, good heat transfer properties and low pressure drop compared to most other refrigerants. The volumetric refrigerating effect is also higher than for other fluids of the same vapor pressure. Applications are nowadays restricted almost exclusively to large industrial or commercial systems.

    We have previously demonstrated that ammonia is also a viable refrigerant for small systems (Palm 2008, Monfared and Palm 2011). In this paper we report on additional tests with a small capacity (7.2 kW) water to water heat pump for sanitary hot water production and space heating. Particularly, the losses in the compressor, electric motor and variable speed drive are investigated under full and part load conditions. New data for the general performance of the heat pump in terms of heat delivered at 60 °C for sanitary hot water production and at 40 °C for space heating, energy efficiency, etc. will also be reported.

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