im Journal of JET v°lume 11 (2°18) p.p. 59-69 Issue 3, November 2018 Type of article 1.04 Technology www.fe.um.si/en/jet.html THE EFFICIENCY OF MAGNETIC REFRIGERATION AND A COMPARISON WITH COMPRESSOR REFRIGERATION SYSTEMS UČINKOVITOST MAGNETNEGA HLAJENJA IN PRIMERJAVA S KOMPRESORSKIM HLADILNIM SISTEMOM Botoc DorinR1,2, Jurij Avsec1, Adrian Plesca2 Keywords: Magnetocaloric effect, magnetic refrigeration, compressor refrigeration, heat transfer. Abstract The objective of this paper is to study the magnetic refrigeration process that makes use of solid materials, such as gadolinium silicon compounds, as the refrigerant. This effect was observed many years ago and was used for cooling to low temperatures. Recently, materials are being developed with which sufficient temperature and entropy change are produced which makes them useful for a wide range of refrigeration applications. Magnetic refrigeration is an emerging technology that utilizes this magnetocaloric effect found in a solid state to produce a refrigeration effect. R1 Corresponding author: Botoc Dorin, E-mail address: dorinbotoc@yahoo.com 1 University of Maribor, Faculty of Energy Technology, Laboratory for Thermomechanics, Applied Thermal Energy Technologies and Nanotechnologies, Hočevarjev trg 1, SI-8270 Krško, Slovenia 2 Faculty of Electrical Engineering, Energetics and Applied Informatics, Gheorghe Asachi Technical University of lasi, Department of Power Engineering, Romania JET 59 Botoc Dorin, Jurij Avsec, Adrian Plesca JET Vol. 11 (2018) Issue 3 Povzetek Namen prispevka je preučiti magnetno hlajenje, v katerem se kot hladilno sredstvo uporabljajo trdni materiali, kot so na primer goldolinijeve silicijeve spojine. Ta učinek je bil opazen pred mnogimi leti in je bil uporabljen za hlajenje blizu absolutne ničle temperature. V zadnjem času se razvijajo materiali, v katerih se proizvaja zadostna temperaturna razlika in sprememba entropije, zaradi česar so uporabne za uporabo v širokem razponu temperature. Magnetno hlajenje je nastajajoča tehnologija, ki uporablja ta magnetno-kalorični učinek v trdnem stanju, da proizvede hladilni učinek. 1 INTRODUCTION Modern society mostly uses vapour compression cycles and vapour absorption cycle processes. Moreover, using refrigerants such as chlorofluorocarbons or hydrochlorofluorocarbons (CFCs and HCFCs) have adverse effects on our environment. Recently, the developments of new technologies such as magnetic refrigeration and thermoelectric refrigeration, have brought alternatives to the conventional gas compression technique. One of the most important researchers in this field was Prof. Emil Gabriel Warburg (1846-1931) [1], a German physicist and a professor of physics at the Universities of Strasbourg, Freiburg and Berlin. In 1881, he discovered the magnetocaloric effect in an iron sample. Magnetic refrigeration is a cooling technology based on this effect. This method can be used to attain low temperatures, as well as the ranges used in common refrigerators, depending on the design of the system. 2 WORKING PRINCIPLE OF MAGNETIC REFRIGERATION SYSTEM Magnetic refrigeration works on the principle of the magnetocaloric effect (MCE), which is a magneto-thermodynamic phenomenon in which a reversible change in temperature of a suitable material is caused by exposing the material to a changing magnetic field, [1-8]. 60 JET The efficiency of magnetic refrigeration and a comparison with compressor refrigeration systems Magnets Magnets N/-S3 .0 Vts to o o 0 C CT Ol Temperature Figure 1: Magneto-caloric offocC MCE is the behaviour of a magnetic solid when it is exposed to a changing magnetic field: its temperature may be appreciably increased or decreased, with both the sign and the extent of the temperature difference between the final and the initial states of the material being dependent on numerous intrinsic and extrinsic factors. The chemical composition, the crystal structure, and the magnetic state of a compound are among the most important material parameters that determine its MCE. Magnetic refrigeration is based on a fundamental thermodynamic property of magnetic materials: the so-called magnetic effect, which causes a temperature change if the material is subject to an applied magnetic field under adiabatic conditions. 3 THERMODYNAMICS OF THE MAGNETIC REFRIGERATION SYSTEM The basic thermodynamic cycle of the magnetic refrigerator is the Ericsson cycle, which operates between two adiabatic and two isomagnetic field lines. The magnetic refrigeration system can be divided into four fundamental steps (Fig. 2, Fig. 3). The temperature of hot and cold heat exchangers has agreat influence on the refrigeration performance, [1-8]. JET 61 Botoc Dorin, Jurij Avsec, Adrian Plesca JET Vol. 11 (2018) Issue 3 Expelled heat keeps refrigerator cold Expelled heat v A r*— Electromagnetic field off Figure 2: Thermodynamic processes in magnetic refrigeration D C 1. Adiabatic magnetization: A magnetocaloric material, when placed in an insulated environment(Q=0) and the external magnetic field is increased (+H), causes the magnetic dipoles of the atoms to align, thereby decreasing the material's magnetic entropy and heat capacity. 2. Isomagnetic enthalpy transfer: The magnetic field is held constant during this process (H=0) and the heat added during the adiabatic magnetization is then removed (-Q) by a fluid or gaseous substance to prevent the dipoles from reabsorbing the heat. Figure 3: Thermodynamic processes in magnetic refrigeration 62 JET The efficiency of magnetic refrigeration and a comparison with compressor refrigeration systems 3. Adiabatic demagnetization: The substance is returned to another adiabatic process (Q=0), and hence the total entropy remains constant. However, this time the magnetic field is reduced, the thermal energy causes the magnetic moments to overcome the field, and thus the sample cools, i.e. an adiabatic temperature change. 4. Isomagnetic entropic transfer: The magnetic field is held constant to prevent the material from heating back up. The magnetic field sources that can be used in magnetocaloric devices are permanent magnets and electromagnets. From the theory, we could divide magnetic refrigeration systems into refrigeration systems with moving parts (rotary, linear) or without moving parts. This machine uses as magnetocaloric suspensions working fluid. Rotary valve Permanent m agnet Coolant flow Water pump Water tank Magnetic working materials Refrigeration section Figure 4: Components of a magnetic refrigeration system Fig. 4 presents a rotational magnetic refrigeration system. Heat is exchanged by means of a circulating coolant (water). Because the magnets rotate, the magnetic working material is repeatedly heated and cooled. The well-timed switching of the direction of coolant flow in coordination with the rotation of the magnets enables cooled coolant to be supplied to areas to be cooled, [17]. Generally speaking, a compressor refrigeration system comprises two exchangers, with one receiving heat from the environment (evaporator) and the other releasing heat to the heating water (condenser) by means of a compressor, an expansion valve, and connecting copper pipes (Figure 5). Naturally, a refrigerant is required for the operation of a compressor refrigeration system. Refrigerants are working fluids, transferring heat from a lower temperature level to a higher temperature level. Decades ago, chlorofluorohydrocarbons were widely used for refrigerants in cooling and heating systems. As a result of stricter environmental laws and measures and claims that these refrigerants damage the ozone layer, their application started to be phased out. The use of new pure refrigerants and mixtures of more environmentally friendly and degradable refrigerants has been increasingly adopted, in case they are unexpectedly released into the environment. JET 63 Botoc DorinJurijAvsec, Adrian Plesca JET Vol. 11 (2018) Issue 3 The coefficient of performance (COP) of a magnetic refrigerator is the measurement of the thermodynamic quality of the apparatus under consideration; it shows how much (electrical) power P is to be invested for cooling Qr, [16] COP = ^ (3.1) Vr \ QlAQ < 0 J AQ = Qbc Qda (3.3) In refrigeration units applying superconducting magnets, the efficiency would be further reduced due to the power demand of the magnetic field sources as well as of the supporting cryogen system. In this way, the compressor coefficient of performance (COP) in the cooling mode may be defined (Figure 5): cop = 0evaL= m(hl-hi) P P 4 ' 4 COMPARISON BETWEEN MAGNETIC AND COMPRESSOR REFRIGERATION SYSTEMS In this chapter, we will analyse magnetic and compressor refrigeration systems for a family house with the required cooling heat of 11 kW. As the refrigerant, we have used refrigerant R-134a in an ideal compression refrigeration cycle. For the modelling of the compressor refrigeration process, we have used the following boundary conditions: temperature in the condenser 400C temperature in the evaporator —70C. The presented analysis is used for the 11 kW required refrigeration heat for summer house cooling. h1 = 574, h2s = 600 — , h3 = h4 = 4Q0— (4.1) 1 ' 2s kg 3 4 kg With the isentropic efficiency of the compressor of 85% we obtain the enthalpy h2: h2 = h1+ -(h2 —h1) = 574 + -¿à(600 - 574) = 606 kj/kg (4.2) The performance of refrigerators and heat pumps is expressed in terms of the coefficient of performance (COP) defined as: _ Desired output _ Cooling effect ___ 1574-4801 _ _ _ . , . CUrR =-=-— I-^ — |-I = (4.3) Required input Workinput \hi-h2\ \574-606\ 64 JET The efficiency of magnetic refrigeration and a comparison with compressor refrigeration systems -.boiling liquid saturated vapour Figure 5: Ideal compressor refrigeration cycle Table 1: Processes in compressor refrigeration cycle Process Description 1-2 Isentropic compression 2-3 Constant pressure heat rejection in the condenser 3-4 Throttling in an expansion valve 4-1 Constant pressure heat rejection in the evaporator Compressor power can be calculated from the relation: PcomP =^ = ^1= 3.74kW Lump cop 294 The energy consumed by the compressor per day Wday = 3.74 ■ 24 = 89.67 kW per day (4.5) The energy consumed by the compressor per month Wmonth = 89.67 ■ 31 = 2782 kWhper month (4.6) The cost of energy per month C = 2782-0.13 = €361.6 (4.7) Cost per kWh -€0.13 (4.8) JET 65 Botoc Dorin, Jurij Avsec, Adrian Plesca JET Vol. 11 (2018) Issue 3 The magnetic refrigeration system The most important characteristic's of a magnetocaloric device is its efficiency. It is usually defined by the Coefficient Of Performance (COP). The COP presents a ratio between the refrigeration energy and the total electric energy input. According to the literature [9] the required electric power could be determined this way: Pel = = — = 1.21 kW el COP 9.09 (4.9) Where: Pel — the energy consumed by the electric motor The energy consumed by the motor per month Wmonth =1.21-24-31=900.2 (4.10) The cost of energy per month C = 900.2-0.13 = 117.0€ (4.11) Table 2: Compression Efficiency between classical heat pump and with magnetic heat pump Efficiency for heat pump Efficiency for magnetic heat pump 2.94 9.09 Consumed energy for compressor Consumed energy for compressor 2782 kWh/month 900.2 kWh/month The cost of energy/month The cost of energy/month €361.6 €117 Table 3: Economic analyses for, cost of installation, energy, maintenance and equipment for classical heat pump and magnetic heat pump [16] Initial cost Compressor refrigeration system Magnetic refrigeration system Cost of equipment € 10000 25000 Difference 15000 Cost of installation € 3000 1500 Difference 1500 Total initial cost € 13000 26500 Difference 13500 66 JET The efficiency of magnetic refrigeration and a comparison with compressor refrigeration systems Annual operating cost Heat pump Magnetic heat pump Energy consumption kWh 25176 9816 Difference 15360 Cost of energy € 3276 1276 Difference 2000 Cost of maintenance € 1800 1200 Difference 600 Total annual cost € 5073 2476 Difference 2597 From the analysis of Tables 2 and 3, it can be seen that the purchase of the magnetic cooling system is still significantly more expensive, while the cost of the heating is considerably lower, especially due to a better cooling number. The purchase of the magnetic cooling system pays for itself after around six years. Some benefits and disadvantages of the magnetic refrigeration system are as follows, [16]. Benefits: 1. Environmentally friendly - Refrigerant used is solid and non-volatile and hence has no greenhouse effect. Conventional refrigerators use refrigerants that contain CFC or HCFC, which have been linked to ozone depletion and global warming. Some refrigerants, such as ammonia, are toxic and flammable. 2. Low running and operating cost - There is no compressor, which is the most inefficient and costly part, in magnetic refrigerators. This leads to less energy consumption and hence low running costs. 3. Higher efficiency - Because it eliminates the need to expand and compress the liquid, a magnetic refrigerator consumes less energy and can operate at 60% efficiency. 4. Reliability - High energy density and more compact device, fewer moving parts in comparison to traditional systems and thus more reliable. 5. Quiet operation - This refrigerator unit is substantially quieter than traditional refrigeration systems. 6. Compactness: - It is possible to achieve a high energy density compact device. This is because in case of magnetic refrigeration the working substance is a solid material (e.g. gadolinium) and not a gas as in the case of vapour compression cycles Disadvantages: 1. The initial investment is very high in comparison to conventional refrigeration. 2. The magnetocaloric materials are rare earth materials hence their availability also becomes a disadvantage. These materials need to be developed to allow larger frequencies of rectilinear and rotary magnetic refrigerators. 3. Protection of electronic components from magnetic fields. However, it must be noted that they are static, of short range and may be shielded 4. Permanent magnets have limited field strength, while electromagnets and superconducting magnets are very expensive. 5. Temperature changes are limited. Multi-stage machines lose efficiency through the heat transfer between the stages. 6. Moving machines need high precision to avoid magnetic field reduction due to gaps between the magnets and the magnetocaloric material. JET 67 Botoc Dorin, Jurij Avsec, Adrian Plesca JET Vol. 11 (2018) Issue 3 5 CONCLUSION This article presents thermodynamic and economic analysis between magnetic refrigeration system and a conventional refrigeration system. From the analysis, it can be seen that the purchase of the magnetic cooling system is still significantly more expensive, while the cost of heating is considerably lower. The return period for the purchase of the magnetic cooling system is approximately six years. References [1] Y. Kulkarni: A review on Magnetic Refrigeration at Room Temperature, International of Innovative Research in Science, Engineering and Technology, Vol. 4, Iss. 12, 2015 [2] Q. Mary: Novel Thermodynamic Cycles involving Ferrofluids displaying Temporary Magnetic Remanence, School of Engineering and Material Science, p.p.69-73, 2013 [3] E. Gedic, M. Kaufeci, A. Kecebas, H. Kurt: Magnetic refrigeration technology applications on near-room temperature, International Advanced Technologies Symposium Karabuk Turkey, 2009 [4] U. Lucia: Exergy analysis of magnetic refrigeration, Spalto Marengo Alessandria, 2010 [5] P. W. Egolf, A. Kitanovski, D. Vuarnoz, M. Diebold, C. Besson: An Introduction to magnetic refrigeration, Institute of Thermal Sciences, Switzerland CH 1401 [6] Dr. Terry, J.Hendricks, Valerie H.Johnson, Matthew A.Keyser: Heat- Generated Cooling Opportunities, Center for Transportation Technologies and System, Colorado [7] V. K. Pecharsky, K. A.Gscheidner: Magnetocaloric Materials, Iowa State University, USA [8] A. Kitanovski, V. Plaznic, J. Tusek, A. Poredoš: New thermodynamic cycles for magnetic refrigeration, International Journal of refrigeration 37, ELSEVIER, 2014 [9] N. Biswas, N. K.Hanna, P. Datta, P.S. Mahapatra: Analisys of heat transfer, Journal Power Technology, ELSEVIER, 2018 [10] S. Labrosse: Thermal evolution of the core with a high thermal conductivity, Journal of Physics of the Earth and Planetary Interiors, 2012 [11] J. Tusek, S. Zupan, I. Prebil, A. Poredos: Magnetic Cooling-Development of Magnetic Refrigeration, Journal of Mechanical Engineering, Slovenia, 2009 [12] Z.G.Zheng, Y.Yu, X.C.Zhong, D.C.Zeng, Z.W.Liu: Design and performance study of the active magnetic refrigerator for room-temperature application, International Journal of refrigeration 32, 2009 [13] A. Rowe, A. Tura, J. Dikeos and R. Chahineb: Near Room Temperature Magnetic Refrigeration, Proceedings of the International Green Energy Conference, Waterloo, Ontario, Canada, 2005 [14] J. Avsec, U. Novosel: Control of air conditioning, 1st ed. Maribor: University of Maribor Press, 2017 68 JET The efficiency of magnetic refrigeration and a comparison with compressor refrigeration systems [15] M. Arnemann: Energy efficiency rf Refoigeoatirr 5yotem,Karlsru he UAS Institute of Refrigeration, Air Conditioning and Environmental Engieerieg (IKKU) 76133, Karlsrache Germany [16] Bj0rk, Rasmus; Smith, Anders; Bahl, Christian; Pryds, Nini: Determining the; minimum maoo and arot rf a magnetic oefoigeoatro, Technical Universy of Denmark 2011 [17] System developed by Railway Institute for evaluation of performance of magnetic working materials Nomenclature t time V specific volume COP coefficient of performance Q heat released JET 69