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Preliminary experimental results from a linear reciprocating magnetic refrigerator prototype
Applied Thermal Engineering 52 (2013) 492e497
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Preliminary experimental results from a linear reciprocating magnetic refrigerator prototype Luca Antonio Tagliafico a, Federico Scarpa a, *, Federico Valsuani a, Giulio Tagliafico b a b
University of Genoa, DIME/TEC, Division of Thermal Energy and Environmental Conditioning, Via All’Opera Pia 15 A, 16145 Genoa, Italy University of Genoa, DCCI, Division of Chemistry and Industrial Chemistry, Via Dodecaneso 31, 16146 Genoa, Italy
h i g h l i g h t s < We give preliminary results from a linear reciprocating magnetic refrigerator prototype. < The design is intended to process visualization and investigation. < The prototype behavior gives us various suggestions to improve its general performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 August 2012 Accepted 19 December 2012 Available online 31 December 2012
A linear reciprocating magnetic refrigerator prototype was designed and built with the aid of an industrial partner. The refrigerator is based on the Active Magnetic Regenerative cycle, and exploits two regenerators working in parallel. The active material is Gadolinium in plates, 0.8 mm thick, for a total mass of 0.36 kg. The device is described and results about magnetic field and temperature span measurements are presented. The designed permanent magnet structure, based on an improved cross-type arrangement, generates a maximum magnetic field intensity of 1.55 T in air, over a gap of (13 50 100) mm3. The maximum temperature span achieved is 5.0 K, in a free run condition. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Magnetic refrigerator Prototype Regenerator Magnetocaloric effect Curie temperature
1. Introduction Magnetic refrigeration is an innovative way of cooling that is promising to be a breakthrough technology in the field of refrigeration at room temperature, and that is currently in a preindustrialization stage. Magnetic refrigeration is based on the magnetocaloric effect, that is to say the attitude of ferromagnetic solids to change their thermodynamic state if subjected to a magnetic field change. This effect, discovered by Warburg in 1881 [1], can be observed as a sudden, adiabatic, temperature change that occurs in the active material. It is highly reversible and with good approximation instantaneous, but of small intensity (about 2 K for a magnetic field change of 1 T in gadolinium). To overcome this limitation Brown [2] conceived a regenerative cycle, known as the Active Magnetic Regeneration (AMR), that reached temperature spans (temperature difference between the hot and the cold side) up to
* Corresponding author. Tel.: þ39 (0)103532861; fax: þ39 (0)10311870. E-mail address: [email protected] (F. Scarpa). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.12.022
50 K (under a superconducting magnetic field of 7 T), suitable for actual applications. If compared to traditional vapour compression technology, magnetic refrigeration avoids the two components with the major irreversibility, namely the gas compressor and the expansion valve. Furthermore a water-based liquid can be used as an energy carrier fluid instead of refrigerant fluids, eliminating all the restrictions related to containment and disposal. For the former reason, the energy conversion process could theoretically be more efficient with respect to traditional technologies, and for the latter reason a magnetic refrigerator device could be more environmentally friendly than a traditional one. The discovery of the Giant Magnetocaloric effect by Pecharsky and Gschneidner [3], and the performance (60% of Carnot efficiency) shown by the first experimental apparatus operating at room temperature [4] gave a strong impulse to the worldwide research on magnetic refrigeration. In the last six years more than 30 magnetic refrigerator prototypes have been presented to the scientific community, the most of them based on the process developed by Brown in order to amplify the small adiabatic temperature change provided
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by the magnetocaloric effect. Several prototypes and experimental studies were already published, showing that different configurations are feasible to perform room temperature magnetic refrigeration. Extended reviews on the existing prototypes can be found in [5e7]. Linear test sections were presented in [8e11] and a pre industrial linear prototype in [12]. Rotating prototypes were designed and described in [13e17]. A different solution, which exploits a nested concentric magnets configuration, is proposed by Tura and Rowe [18]. Among all the prototypes, we here recall some relevant examples. A Pre-industrial prototype has been presented by Balli et al. [12]. They obtained a temperature span of 22 K with a 1.45 T magnetic field and using gadolinium flat plates. The rotating prototype developed by Okamura et al. [13] is designed in a very compact form and exploits the concept of layered beds. The active material is arranged as a proper sequence of different gadolinium-based alloys (GdY and GdDy) in order to widen the magnetocaloric effect temperature range, and so the achievable temperature span. This device can reach a temperature span of 10 K, and a maximum cooling capacity of 60 W. Bahl et al. [14] and Tusek at al. [17] developed multipole magnet assemblies increasing the cycle frequency for a given rotational speed. Zimm et al. [15] designed a vertical axes rotary machine with a fixed external magnetic structure. A different solution is presented by Vasile and Muller in [16]. They developed an enhanced regenerative cycle named AM2R (Active magnetic double regenerative cycle). Tura and Rowe [18] developed a device with two AMRs operating in a variable magnetic field generated by two nested Halbach cylindrical magnets in reciprocal motion. Operating with gadolinium flakes the maximum temperature span achieved was 13 K, while with gadolinium spheres [19] a temperature span of 29 K and a maximum cooling power of 50 W was reached. In this paper a linear reciprocating magnetic refrigerator prototype is described, and the first preliminary results about magnetic field and temperature spans are presented. The prototype is intended as a proof of principle unit with no demand for performance, and the design is so intended to process visualization and investigation. 2. Prototype design The magnetic refrigerator is designed to be a demonstrative unit, and for this reason it is not subjected to compactness or performance constraints. Its design is oriented to simplicity, ease of use and accessibility, with most of the components available on the market. Nevertheless, the main guidelines followed in the design of the refrigerating machine are intended to realize good technological solutions. The device is based on AMR cycle, which is a four step sequences cycle here briefly reported: - solid magnetization: a magnetic field is applied to the solid, that warms up; - hot blow: with the magnetic field applied steadily, an intermediate fluid flows from one side of the regenerator to the other, cooling down the solid while warming up; - solid demagnetization: the magnetic field is removed, and the solid cools down; - cold blow: with no magnetic field applied, the intermediate fluid flows in the opposite direction with respect to the hot blow, warming up the solid while cooling down. The linear reciprocating device scheme is reported in Fig. 1. The two parallel regenerators, composed by parallel gadolinium plates, perform two AMR cycles, by entering and exiting from a high magnetic field region generated by a fixed permanent magnet
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Fig. 1. Linear magnetic refrigerator process scheme. One regenerator (bed A) is under applied magnetic field.
ferromagnetic structure. The intermediate fluid is distributed into each pipe by means of a collector connected to electric valves. The regenerators are constrained to a plastic structure that holds the inversion valves and connections, and that is mechanically linked to a pneumatic system for linear motion. The pneumatic drive is controlled by an electronic timer which allows to set the hot and cold blow durations independently. The hydraulic arrangement, in order to guarantee the proper synchronization between the two regenerators, is realized by means of four bi-stable electrically driven 3/2 valves. The circuit arrangement guarantees that in the circuit outside the regenerators the fluid motion is unidirectional. The flow distribution is conceived in order to minimize the dead volume in the regenerator geometry (the volume of fluid that changes its flow direction), given the geometrical constrains imposed by the magnetic structure. The global dead volume is included between the two three way valves on each side. The bi-stable valves are properly activated by an electronic device triggered by the signal of a photoelectric sensor. The sensor is constrained to the structure that holds the regenerators, and its position can be tuned to change the synchronization between the magnetic field variation and the fluid direction inversion. The electric valves are connected to the static part of the circuit by flexible pipes. The hot and cold part of the refrigerator are realized by means of two symmetric circuits, each of them equipped with a mass flow rate transducer, a variable speed volumetric pump and a fluid reservoir to give an extra inertia to the system. Different operating conditions can be easily realized, all centered around room temperature, as reported in [20]: “free run” with minimum inertia, that is with both the hot and cold reservoir bypassed; “free run” with symmetric inertias, that is with the reservoir connected to the circuit; asymmetric run with the hot or the cold reservoir bypassed. Fig. 2 shows the fully assembled prototype. The pneumatic drive, the variable speed pumps and the flow meter are visible. The piping is realized by means of plastic flexible pipes with a diameter of 4 mm, and the connections are realized with components for pneumatic applications. The two variable speed pumps are placed under the flow meter together with their by-pass and differential pressure transducers. The two heat reservoir are also clearly visible, together with the magnetic structure and with the support that holds the regenerator. Fig. 3 shows a picture of the regenerator geometry. The two regenerators are composed by gadolinium parallel plates positioned in a structure that allows a clear visualization of the regenerator inside. The sealing is guaranteed by standard strip sealing.
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(70 mm) in about 0.2 s. The maximum and minimum magnetic field mean intensity, measured in air, are respectively Bmax z 1.55 T and Bmin < 0.02 T. Following the proposal by Scarpa et al. [21], the 14 bit code for this system would be 1-0-1-0-1-0-0-0-1-1-0-0, which indicates double effect, permanent magnets with concentrators with magnetic field applied by immersion of the active material in the field, with a single layered bed with ordered geometry, liquid heat transfer fluid, bi-directional motion with a linear, discontinuous motion of the active material. The technical data of the refrigerator are given in Table 1. 3. Magnetic field and magnetocaloric effect
Fig. 2. Picture of the fully assembled linear magnetic refrigerator.
The thermodynamic process is a double AMR with the two regenerators acting in parallel with the result of doubling the active mass for the given operating frequency, or from another point of view, doubling the frequency with a given material mass. The linear drive and the three way valves are synchronized in order to realize the AMR cycles. The motion control system has been designed in order to enable the regenerators to enter the magnetic field, hold steadily inside of it and get out of it in a given time. The basic motion sequence is composed by four steps. An example of a 0.12 Hz cycle is reported: 1) Bed A enters the magnetic field while bed B exits from it (magnetization time about 0.2 s) 2) Bed A steady inside the field for a set time (4 s) 3) Bed A exits from the field while bed B enters it (0.2 s) 4) Bed B steady inside the field for a set time (4 s). The process is characterized by the time functions of the magnetic field applied to a single regenerator and by the fluid mass flow rate that crosses it. The fluid mass flow rate has no dwell period (except the valves commutation time), since the valves have only two positions. The pneumatic drive gives the complete displacement
Fig. 3. Picture of the regenerator. The housing is made by transparent plastic. The piping system for fluid distribution is visible.
Permanent magnet magnetic field sources are designed in order to obtain a magnetic field with the following characteristics: high intensity, uniformity and confinement of the magnetic field. In recent times, complex permanent magnet array topologies (Hallbach, see for instance [19]) have been proposed in order to generate strong magnetic flux densities. However, the required effort in term of magnetic material weight and production complexity suggests the use of simpler topologies still able to give high performance at lower cost. In this work we have chosen a topology aimed to achieve near Hallbach performance by using relatively cheap standard magnets. In particular, permanent magnets design and assembly to achieve relatively high magnetic fields in useful volumes (e.g. up to 1.5 T in a 15 mm field gap) is addressed with a cross-type magnetic structure based on ten magnets and two soft iron concentrators (Fig. 4). A comprehensive review on the permanent magnet structures for magnetic applications is available in Ref. [22], where a performance parameter (Lcool) is defined related to the magnetic field variation and the volumes of the magnetized region and of the magnets. Fig. 5 shows the measured magnetic field intensity in air in the longitudinal and cross median sections. On the left a comparison between the “as built” simulation (solid line), and the measured profile (dashed marked line) is presented. Simulated magnetic field profile was computed by a 3D FEM static simulation carried out with commercial software. The Table 1 Technical data of the magnetic refrigerator device. Process Thermodynamic cycle Magnet-active material relative motion Active material Intermediate fluid Fluid flow rate operating range Frequency operating range Utilization factor range
AMR Linear, reciprocating Commercial gadolinium plates watereethanol 50% 5 O 20 g s1 (0.3 O 1.2 l min1) 1/4 Hz 0.5 O 3
Regenerator Numbers of regenerator Regenerator size (each) Plates thickness Measured void fraction Gadolinium mass
2 50 8 100 mm3 0.8 mm 0.38 2 194 g
Magnet Type of structure Permanent magnets Magnetic circuit Magnet mass Magnetic structure mass Magnetic gap Maximum magnetic field Minimum magnetic field Max spatial magnetic field variation
Cross type NdFeB, N50 High saturation steel 5 kg 35 kg 13 mm 1.55 T (in air) <0.02 T 1.2 T cm1
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Fig. 6 shows the magnetocaloric effect measured directly on the regenerator by a thermocouple placed on a gadolinium plate, moved inside and outside the magnetic region in a configuration of dry regenerator. A time history example of the direct temperature measure is given. The values obtained are corrected with the aid of a first order dynamic model of the thermocouple, in order to take into account the effects of not perfect insulation and thermal inertia. A comparison of the data obtained with the data computed from specific heat measurements and reported in [23,24] shows a little reduction in maximum intensity (12%) and a shift of the transition temperature of about 1.5 K. The directly measured maximum intensity is obtained from the specific heat measurement if a magnetic field variation of 1.40 T is considered. 4. Preliminary results
Fig. 4. Picture of the magnetic assembly utilized in the experimental tests.
1800
1800
1600
1600
Magnetic induction [mT]
Magnetic induction [mT]
geometry was measured on the actual magnetic structure, and the magnets and iron properties were declared by the manufacturer. A very good agreement between experimental and simulated data is observed. The region of high magnetic field (in the range x ¼ 0O55 mm) is subjected to a mean field intensity of 1.52 T, with a peak of 1.55 T (0.01 T, 95% confidence). The main difference between the measured and simulated data is a greater leakage near the edges of the useful region, which results in a decrease of the mean intensity in the region of interest. The slope of the magnetic field variation is of the order of 1.1 T cm1. The low magnetic field region is found for x < 11 mm and x > 67 mm. The regenerator subjected to the minimum magnetic field is so subjected to a mean magnetic field lower than 0.02 T, with just one tube subjected to a magnetic field of about 0.1 T. On the right the comparison between the “as built” simulated data (solid line), and the measured magnetic induction profile (dashed marked line) is displayed. Again a very good agreement can be observed, with the experimental data that show a stronger effect of flux leakages. The high field region is in the range x ¼ 4 O 14 cm, and shows a mean field of about 1.5 T. The measured profile shows a little dissymmetry of the field in the longitudinal direction. The measurements were performed on the median cross and longitudinal sections with a digital gaussmeter, by sampling the magnetic field with a spatial resolution of about 3 mm in the cross measurements, and 10 mm in the longitudinal measurements. The performance parameter proposed by Bjork [22] gives Lcool ¼ 0.13 (the third value among the reviewed magnets).
Preliminary results obtained in free run conditions are reported in Fig. 7. Three set of results are shown. Two set are obtained with the system running in a “free run” with minimum inertia condition, with different working frequencies. The two different frequencies analyzed show similar trends, with the maximum temperature span decreasing with the increasing of the fluid and solid capacity rate ratio (utilization factor, V). Maximum achieved temperature span, DTSPAN, is very similar, around 5.0 K, but the degradation of performance with utilization is more severe at the higher frequency. This effect means that in this system the effects of friction and components dissipation are not negligible and limit the operating frequency also from the performance point of view. In particular, the electric power absorbed by the valves and transferred to the fluid is very relevant in higher frequency conditions. In a test with the valves operating with a continuous electrical activation (about 6 W each) the maximum temperature span achievable drops to 2.6 K. Another contribution comes from the heat transferred to the fluid by the variable speed pumps. All the temperature measurements have been obtained using type T thermocouples with a data acquisition system characterized by an uncertainty of 0.02 C (95% confidence). The third series is obtained with the thermal inertias connected to the system, but the steady state temperature span is about the same of the previous condition. The behavior observed by these preliminary results shows a modest maximum temperature span with a maximum regeneration ratio of 1.2 (the ratio between the obtained temperature span and the maximum adiabatic temperature change of the material under the applied magnetic field change), good repeatability of results obtained in different conditions, and evidenced the limitations of the system also in terms of operating frequency. Regenerator ratio is a dimensionless temperature span, and can be used as a quality index of the prototype performance [27]. The
1400 1200 1000 800 600 400 200 Predicted Measured
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Fig. 5. (Left) Magnetic field measurement in air on the median cross section: predicted profile (solid), measured profile (dashed marked). (Right) Magnetic field measurement in air on the median longitudinal section: predicted profile (solid), measured profile (dashed marked).
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T [°C]
28 DTMAG
26
DTDEMAG
24 22 700
720
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time [s]
Fig. 8. Temperature vs. time evolution at the ends of a regenerator. Cycle time: 7.9 s; no bypass; V ¼ 2.9. Marked dashed lines show the temperature measurements at the ends of the regenerator (near the valves); solid lines show the temperature measurements at one end the two heat exchangers.
5
DT AD [°C]
4 3 DTAD,MAG DTAD,DEM DTAD,MAG (cp meas)
2
DTAD,DEM (cp meas)
1 0
5
10
15
20
25
30
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T [°C] Fig. 6. (Top) Temperatureetime history of one direct measure of the effective temperature change in one gadolinium plate with the permanent magnet magnetic structure as field source. (Bottom) DTAD vs T from direct measurements compared to data evaluated by specific heat measurements.
value of 1.2 is small if compared to those obtained by other authors and suggest that a better exploitation of the material is possible (see again [27]). An example of the temperature histories is shown by Fig. 8. Working conditions are cycle time 7.9 s, no bypass and V ¼ 2.9. Temperature of the hot and cold side of a regenerator are reported, together with the environment temperature. In the time window showed the device is operating in steady state, with no appreciable mean temperature variations of the hot and cold sides. The dashed lines show the temperature variation at the ends of the regenerator (near the valves), and the environment temperature,
6
ΔTSPAN [°C]
5
which is steady in a 0.1 C range. Measurements at the ends of the regenerator are influenced by the temperature damping due to dead volumes and thermocouples positioning, so that the different amplitude of the hot and cold signals is not significant. With these temperatures, and given that the fluid flow rate is kept constant, the temperature oscillation at the hot side should be larger than the cold one, due to the lower value of the specific heat at higher temperature and applied magnetic field. The temperatures of the heat exchangers are steady, with a temperature span of 2.8 C. We compared experimental results with simulated ones coming from a properly adapted version of the model described in Ref. [20,23]. Only by assuming the presence of some bypass, low friction fluid paths in the regenerator we succeed in replicating such a degraded performance. Indeed, when the fluid flows through such paths the uniformity of the heat exchange with the solid matrix is compromised and the global performance is accordingly reduced. In comparison to other authors results the obtained temperature spans are modest. Some simple gadolinium based AMR test sections, under similar magnetic field changes, can reach temperature spans larger than 11 C [10,25], in no load conditions. Others more complex prototypes show temperature spans up to 25 C [12,19], in no load conditions. Literature results clearly show that a better exploitation of the material is possible, with a performance oriented design and accurate building solutions. Our results are, however, in agreement with the temperature span obtained in [26], equal to 4.45 K, and obtained with a configuration similar to ours (regenerator size (160 30 7) mm3, made of gadolinium parallel plates 0.85 mm thick, and maximum applied magnetic field 1.65 T). 5. Conclusion
4 3 2 bypass tc=7.9s bypass tc=3.6s no bypass tc=7.9s
1 0 0.0
0.5
1.0
1.5
2.0
2.5
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Φ Fig. 7. Temperature span vs. utilization for different working conditions. Filled circles: cycle time 7.9 s, system with bypass open; triangles: cycle time 3.6 s, system with bypass open; void circles: cycle time 7.9 s, system with bypass closed.
A linear reciprocating magnetic refrigerator has been designed and built by our institution in collaboration with an industrial partner. The prototype aim is to show the process in an open way, with components and processes clearly visible and accessible. Data from magnetic field measurements are in very good agreement with predicted performance. Direct magnetocaloric effect measurements obtained on the plates are presented and compared with data computed by heat capacity measurements of high purity gadolinium samples, showing a reduction in amplitude and a shift in the transition temperature. Preliminary results obtained in “free run” conditions are presented. The dependency of temperature span by utilization was observed for three different operating conditions, showing a good reproducibility of the experiments, and a trend which evidences the limit in terms of higher frequency operation of this design. A regeneration ratio of 1.2 was achieved, a result that can be improved with further
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modifications on the actual system which are foreseen in future work. Thanks to this experience important issues of regenerators behavior have been identified. Among these the most relevant are the presence of low friction paths inside the regenerator due to the deformation of the regenerator housing under pressure, which reduces strongly the regenerator effectiveness and can be avoided with a proper sealing configuration; the effect of auxiliary power inputs, such as electric dissipation of the valves, which reduces the maximum temperature span achievable; the effect of dead volumes, which can be reduced by a new piping design. On the basis of the knowledge gained with this work, future work foresees the improvement of this system, and the design of a new more performance oriented system. Acknowledgements The present work was developed in the framework of the Italian Ministry of Economic Development (MSE) project F.A.R n.6728/ 2002 “Refrigerazone magnetica”. The authors wish to thank the Technical Director of Zanotti S.p.A., Ing. Lorenzo Bulgarelli, for his collaboration and suggestions during the prototype development. References [1] E. Warburg, Investigation on magnetism, Ann. Phys. 13 (1881) 141e164 (in German, Magnetiche Untersuchungen). [2] G.V. Brown, Magnetic heat pumping near room temperature, J. Appl. Phys. 47 (1976) 3673e3680. [3] V.K. Pecharsky, K.A. Gschneidner Jr., Giant magnetocaloric effect in Gd5(Si2Ge2), Phys. Rev. Lett. 78 (2) (1997) 4494e4497. [4] C. Zimm, A. Jastrab, A. Sternberg, V.K. Pecharsky, K.A. Gschneidner Jr., M. Osborne, I. Anderson, Description and performance of a near room temperature magnetic refrigerator, Adv. Cryog. Eng. 43 (1998) 1759e1766. [5] K.A. Gschneidner Jr., V.K. Pecharsky, Thirty years of near room temperature magnetic cooling: where we are today and future prospects, Int. J. Refrig. 31 (6) (2008) 945e961. [6] B. Yu, M. Liu, P.W. Egolf, A. Kitanovski, A review of magnetic refrigerator and heat pump prototypes built before the year 2010, Int. J. Refrig. 33 (2010) 1029e1060. [7] L.A. Tagliafico, F. Scarpa, F. Canepa, G. Tagliafico, Room temperature magnetic refrigeration technology, in: Refrigeration: Theory, Technology and Applications, Nova Publishers, New York, USA, 2010, pp. 71e131. [8] D.S. Arnold, A. Tura, A. Rowe, Experimental analysis of a two material active magnetic regenerator, Int. J. Refrig. 34 (2011) 178e191.
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[9] K. Engelbrecht, C.R.H. Bahl, K.K. Nielsen, Experimental apparatus for a magnetic refrigerator using three different types of magnetocaloric material regenerators, Int. J. Refrig. 34 (2011) 1132e1140. [10] T. Kawanami, S. Hirano, K. Fumoto, S. Hirasawa, Evaluation of the fundamental performance on magnetocaloric cooling with active magnetic regenerator, Appl. Therm. Eng. 31 (2011) 1176e1183. [11] J.Y. Law, V. Franco, P. Keblinski, R.V. Ramanujan, Active transient cooling by magnetocaloric materials, Appl. Therm. Eng. 52 (1) (2013) 17e23. [12] M. Balli, O. Sari, C. Mahmed, C.H. Besson, P.H. Bonhote, D. Duc, J. Forchelet, A pre-industrial magnetic cooling system for room temperature application, Appl. Energ. 98 (2012) 556e561. [13] T. Okamura, K. Yamada, N. Hirano, S. Nagaya, Performance of a room temperature rotary magnetic refrigerator, Int. J. Refrig. 29 (2006) 1327e1331. [14] C.R.H. Bahl, K. Engelbrecht, R. Bjork, D. Eriksen, A. Smith, K.K. Nielsen, N. Pryds, Design concepts for a continuously rotating active magnetic regenerator, Int. J. Refrig. 34 (2011) 1792e1796. [15] C. Zimm, A. Boeder, J. Chell, A. Sternberg, A. Fujita, S. Fujieda, K. Fukamichi, Design and performance of permanent magnet rotary refrigerator, Int. J. Refrig. 29 (2006) 1302e1306. [16] C. Vasile, C. Muller, Innovative design of a magnetocaloric system, Int. J. Refrig. 29 (2006) 1318e1326. [17] J. Tusek, S. Zupan, A. Sarlah, I. Prebil, A. Poredos, Development of a rotary magnetic refrigerator, Int. J. Refrig. 33 (2010) 294e300. [18] A. Tura, A. Rowe, Design and testing of a permanent magnet magnetic refrigerator, in: Proc. Second IIF-IIR Conference on Magnetic Refrigeration at Room Temperature, IIF-IIR, 2007, pp. 365e370. [19] A. Tura, A. Rowe, Permanent magnet magnetic refrigerator design and experimental characterization, Int. J. Refrig. 34 (2011) 628e639. [20] G. Tagliafico, Numerical and Experimental Characterization of Components for Active Magnetic Regenerative Refrigerators, PhD thesis, University of Genova, Italy, Genoa, 2011. [21] F. Scarpa, G. Tagliafico, L.A. Tagliafico, Classification proposal for room temperature magnetic refrigerators, Int. J. Refrig. 35 (2) (2012) 453e458. [22] R. Bjork, C.R.H. Bahl, A. Smith, N. Pryds, Optimization and improvement of Halbach cylinder design, J. Appl. Phys. 104 (013910) (2008) 1e9. http:// dx.doi.org/10.1063/1.2952537. [23] G. Tagliafico, F. Scarpa, F. Canepa, A dynamic 1-D model for a reciprocating active magnetic regenerator; influence of the main working parameters, Int. J. Refrig. 33 (2) (2010) 286e293. [24] D. Vuarnoz, T. Kawanami, Numerical analysis of a reciprocating active magnetic regenerator made of gadolinium wires, Appl. Therm. Eng. 37 (2012) 388e395. [25] A. Tura, K.K. Nielsen, A. Rowe, Experimental and modeling results of a parallel plate-based active magnetic regenerator, Int. J. Refrig. 35 (2012) 1518e1527. [26] P.V. Trevizoli, J.R. Barbosa Jr., R.T.S. Ferreira, Experimental evaluation of a Gdbased linear reciprocating active magnetic regenerator test apparatus, Int. J. Refrig. 34 (2011) 1518e1526. [27] G. Tagliafico, F. Scarpa, L.A. Tagliafico, A dimensionless description of active magnetic regenerators to compare their performance and to simplify their optimization, Int. J. Refrig. (2012). http://dx.doi.org/10.1016/j.ijrefrig.2012.10.024. Available online 30 October 2012.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Preliminary experimental results from a linear reciprocating magnetic refrigerator prototype Luca Antonio Tagliafico a, Federico Scarpa a, *, Federico Valsuani a, Giulio Tagliafico b a b
University of Genoa, DIME/TEC, Division of Thermal Energy and Environmental Conditioning, Via All’Opera Pia 15 A, 16145 Genoa, Italy University of Genoa, DCCI, Division of Chemistry and Industrial Chemistry, Via Dodecaneso 31, 16146 Genoa, Italy
h i g h l i g h t s < We give preliminary results from a linear reciprocating magnetic refrigerator prototype. < The design is intended to process visualization and investigation. < The prototype behavior gives us various suggestions to improve its general performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 August 2012 Accepted 19 December 2012 Available online 31 December 2012
A linear reciprocating magnetic refrigerator prototype was designed and built with the aid of an industrial partner. The refrigerator is based on the Active Magnetic Regenerative cycle, and exploits two regenerators working in parallel. The active material is Gadolinium in plates, 0.8 mm thick, for a total mass of 0.36 kg. The device is described and results about magnetic field and temperature span measurements are presented. The designed permanent magnet structure, based on an improved cross-type arrangement, generates a maximum magnetic field intensity of 1.55 T in air, over a gap of (13 50 100) mm3. The maximum temperature span achieved is 5.0 K, in a free run condition. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Magnetic refrigerator Prototype Regenerator Magnetocaloric effect Curie temperature
1. Introduction Magnetic refrigeration is an innovative way of cooling that is promising to be a breakthrough technology in the field of refrigeration at room temperature, and that is currently in a preindustrialization stage. Magnetic refrigeration is based on the magnetocaloric effect, that is to say the attitude of ferromagnetic solids to change their thermodynamic state if subjected to a magnetic field change. This effect, discovered by Warburg in 1881 [1], can be observed as a sudden, adiabatic, temperature change that occurs in the active material. It is highly reversible and with good approximation instantaneous, but of small intensity (about 2 K for a magnetic field change of 1 T in gadolinium). To overcome this limitation Brown [2] conceived a regenerative cycle, known as the Active Magnetic Regeneration (AMR), that reached temperature spans (temperature difference between the hot and the cold side) up to
* Corresponding author. Tel.: þ39 (0)103532861; fax: þ39 (0)10311870. E-mail address: [email protected] (F. Scarpa). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.12.022
50 K (under a superconducting magnetic field of 7 T), suitable for actual applications. If compared to traditional vapour compression technology, magnetic refrigeration avoids the two components with the major irreversibility, namely the gas compressor and the expansion valve. Furthermore a water-based liquid can be used as an energy carrier fluid instead of refrigerant fluids, eliminating all the restrictions related to containment and disposal. For the former reason, the energy conversion process could theoretically be more efficient with respect to traditional technologies, and for the latter reason a magnetic refrigerator device could be more environmentally friendly than a traditional one. The discovery of the Giant Magnetocaloric effect by Pecharsky and Gschneidner [3], and the performance (60% of Carnot efficiency) shown by the first experimental apparatus operating at room temperature [4] gave a strong impulse to the worldwide research on magnetic refrigeration. In the last six years more than 30 magnetic refrigerator prototypes have been presented to the scientific community, the most of them based on the process developed by Brown in order to amplify the small adiabatic temperature change provided
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by the magnetocaloric effect. Several prototypes and experimental studies were already published, showing that different configurations are feasible to perform room temperature magnetic refrigeration. Extended reviews on the existing prototypes can be found in [5e7]. Linear test sections were presented in [8e11] and a pre industrial linear prototype in [12]. Rotating prototypes were designed and described in [13e17]. A different solution, which exploits a nested concentric magnets configuration, is proposed by Tura and Rowe [18]. Among all the prototypes, we here recall some relevant examples. A Pre-industrial prototype has been presented by Balli et al. [12]. They obtained a temperature span of 22 K with a 1.45 T magnetic field and using gadolinium flat plates. The rotating prototype developed by Okamura et al. [13] is designed in a very compact form and exploits the concept of layered beds. The active material is arranged as a proper sequence of different gadolinium-based alloys (GdY and GdDy) in order to widen the magnetocaloric effect temperature range, and so the achievable temperature span. This device can reach a temperature span of 10 K, and a maximum cooling capacity of 60 W. Bahl et al. [14] and Tusek at al. [17] developed multipole magnet assemblies increasing the cycle frequency for a given rotational speed. Zimm et al. [15] designed a vertical axes rotary machine with a fixed external magnetic structure. A different solution is presented by Vasile and Muller in [16]. They developed an enhanced regenerative cycle named AM2R (Active magnetic double regenerative cycle). Tura and Rowe [18] developed a device with two AMRs operating in a variable magnetic field generated by two nested Halbach cylindrical magnets in reciprocal motion. Operating with gadolinium flakes the maximum temperature span achieved was 13 K, while with gadolinium spheres [19] a temperature span of 29 K and a maximum cooling power of 50 W was reached. In this paper a linear reciprocating magnetic refrigerator prototype is described, and the first preliminary results about magnetic field and temperature spans are presented. The prototype is intended as a proof of principle unit with no demand for performance, and the design is so intended to process visualization and investigation. 2. Prototype design The magnetic refrigerator is designed to be a demonstrative unit, and for this reason it is not subjected to compactness or performance constraints. Its design is oriented to simplicity, ease of use and accessibility, with most of the components available on the market. Nevertheless, the main guidelines followed in the design of the refrigerating machine are intended to realize good technological solutions. The device is based on AMR cycle, which is a four step sequences cycle here briefly reported: - solid magnetization: a magnetic field is applied to the solid, that warms up; - hot blow: with the magnetic field applied steadily, an intermediate fluid flows from one side of the regenerator to the other, cooling down the solid while warming up; - solid demagnetization: the magnetic field is removed, and the solid cools down; - cold blow: with no magnetic field applied, the intermediate fluid flows in the opposite direction with respect to the hot blow, warming up the solid while cooling down. The linear reciprocating device scheme is reported in Fig. 1. The two parallel regenerators, composed by parallel gadolinium plates, perform two AMR cycles, by entering and exiting from a high magnetic field region generated by a fixed permanent magnet
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Fig. 1. Linear magnetic refrigerator process scheme. One regenerator (bed A) is under applied magnetic field.
ferromagnetic structure. The intermediate fluid is distributed into each pipe by means of a collector connected to electric valves. The regenerators are constrained to a plastic structure that holds the inversion valves and connections, and that is mechanically linked to a pneumatic system for linear motion. The pneumatic drive is controlled by an electronic timer which allows to set the hot and cold blow durations independently. The hydraulic arrangement, in order to guarantee the proper synchronization between the two regenerators, is realized by means of four bi-stable electrically driven 3/2 valves. The circuit arrangement guarantees that in the circuit outside the regenerators the fluid motion is unidirectional. The flow distribution is conceived in order to minimize the dead volume in the regenerator geometry (the volume of fluid that changes its flow direction), given the geometrical constrains imposed by the magnetic structure. The global dead volume is included between the two three way valves on each side. The bi-stable valves are properly activated by an electronic device triggered by the signal of a photoelectric sensor. The sensor is constrained to the structure that holds the regenerators, and its position can be tuned to change the synchronization between the magnetic field variation and the fluid direction inversion. The electric valves are connected to the static part of the circuit by flexible pipes. The hot and cold part of the refrigerator are realized by means of two symmetric circuits, each of them equipped with a mass flow rate transducer, a variable speed volumetric pump and a fluid reservoir to give an extra inertia to the system. Different operating conditions can be easily realized, all centered around room temperature, as reported in [20]: “free run” with minimum inertia, that is with both the hot and cold reservoir bypassed; “free run” with symmetric inertias, that is with the reservoir connected to the circuit; asymmetric run with the hot or the cold reservoir bypassed. Fig. 2 shows the fully assembled prototype. The pneumatic drive, the variable speed pumps and the flow meter are visible. The piping is realized by means of plastic flexible pipes with a diameter of 4 mm, and the connections are realized with components for pneumatic applications. The two variable speed pumps are placed under the flow meter together with their by-pass and differential pressure transducers. The two heat reservoir are also clearly visible, together with the magnetic structure and with the support that holds the regenerator. Fig. 3 shows a picture of the regenerator geometry. The two regenerators are composed by gadolinium parallel plates positioned in a structure that allows a clear visualization of the regenerator inside. The sealing is guaranteed by standard strip sealing.
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(70 mm) in about 0.2 s. The maximum and minimum magnetic field mean intensity, measured in air, are respectively Bmax z 1.55 T and Bmin < 0.02 T. Following the proposal by Scarpa et al. [21], the 14 bit code for this system would be 1-0-1-0-1-0-0-0-1-1-0-0, which indicates double effect, permanent magnets with concentrators with magnetic field applied by immersion of the active material in the field, with a single layered bed with ordered geometry, liquid heat transfer fluid, bi-directional motion with a linear, discontinuous motion of the active material. The technical data of the refrigerator are given in Table 1. 3. Magnetic field and magnetocaloric effect
Fig. 2. Picture of the fully assembled linear magnetic refrigerator.
The thermodynamic process is a double AMR with the two regenerators acting in parallel with the result of doubling the active mass for the given operating frequency, or from another point of view, doubling the frequency with a given material mass. The linear drive and the three way valves are synchronized in order to realize the AMR cycles. The motion control system has been designed in order to enable the regenerators to enter the magnetic field, hold steadily inside of it and get out of it in a given time. The basic motion sequence is composed by four steps. An example of a 0.12 Hz cycle is reported: 1) Bed A enters the magnetic field while bed B exits from it (magnetization time about 0.2 s) 2) Bed A steady inside the field for a set time (4 s) 3) Bed A exits from the field while bed B enters it (0.2 s) 4) Bed B steady inside the field for a set time (4 s). The process is characterized by the time functions of the magnetic field applied to a single regenerator and by the fluid mass flow rate that crosses it. The fluid mass flow rate has no dwell period (except the valves commutation time), since the valves have only two positions. The pneumatic drive gives the complete displacement
Fig. 3. Picture of the regenerator. The housing is made by transparent plastic. The piping system for fluid distribution is visible.
Permanent magnet magnetic field sources are designed in order to obtain a magnetic field with the following characteristics: high intensity, uniformity and confinement of the magnetic field. In recent times, complex permanent magnet array topologies (Hallbach, see for instance [19]) have been proposed in order to generate strong magnetic flux densities. However, the required effort in term of magnetic material weight and production complexity suggests the use of simpler topologies still able to give high performance at lower cost. In this work we have chosen a topology aimed to achieve near Hallbach performance by using relatively cheap standard magnets. In particular, permanent magnets design and assembly to achieve relatively high magnetic fields in useful volumes (e.g. up to 1.5 T in a 15 mm field gap) is addressed with a cross-type magnetic structure based on ten magnets and two soft iron concentrators (Fig. 4). A comprehensive review on the permanent magnet structures for magnetic applications is available in Ref. [22], where a performance parameter (Lcool) is defined related to the magnetic field variation and the volumes of the magnetized region and of the magnets. Fig. 5 shows the measured magnetic field intensity in air in the longitudinal and cross median sections. On the left a comparison between the “as built” simulation (solid line), and the measured profile (dashed marked line) is presented. Simulated magnetic field profile was computed by a 3D FEM static simulation carried out with commercial software. The Table 1 Technical data of the magnetic refrigerator device. Process Thermodynamic cycle Magnet-active material relative motion Active material Intermediate fluid Fluid flow rate operating range Frequency operating range Utilization factor range
AMR Linear, reciprocating Commercial gadolinium plates watereethanol 50% 5 O 20 g s1 (0.3 O 1.2 l min1) 1/4 Hz 0.5 O 3
Regenerator Numbers of regenerator Regenerator size (each) Plates thickness Measured void fraction Gadolinium mass
2 50 8 100 mm3 0.8 mm 0.38 2 194 g
Magnet Type of structure Permanent magnets Magnetic circuit Magnet mass Magnetic structure mass Magnetic gap Maximum magnetic field Minimum magnetic field Max spatial magnetic field variation
Cross type NdFeB, N50 High saturation steel 5 kg 35 kg 13 mm 1.55 T (in air) <0.02 T 1.2 T cm1
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Fig. 6 shows the magnetocaloric effect measured directly on the regenerator by a thermocouple placed on a gadolinium plate, moved inside and outside the magnetic region in a configuration of dry regenerator. A time history example of the direct temperature measure is given. The values obtained are corrected with the aid of a first order dynamic model of the thermocouple, in order to take into account the effects of not perfect insulation and thermal inertia. A comparison of the data obtained with the data computed from specific heat measurements and reported in [23,24] shows a little reduction in maximum intensity (12%) and a shift of the transition temperature of about 1.5 K. The directly measured maximum intensity is obtained from the specific heat measurement if a magnetic field variation of 1.40 T is considered. 4. Preliminary results
Fig. 4. Picture of the magnetic assembly utilized in the experimental tests.
1800
1800
1600
1600
Magnetic induction [mT]
Magnetic induction [mT]
geometry was measured on the actual magnetic structure, and the magnets and iron properties were declared by the manufacturer. A very good agreement between experimental and simulated data is observed. The region of high magnetic field (in the range x ¼ 0O55 mm) is subjected to a mean field intensity of 1.52 T, with a peak of 1.55 T (0.01 T, 95% confidence). The main difference between the measured and simulated data is a greater leakage near the edges of the useful region, which results in a decrease of the mean intensity in the region of interest. The slope of the magnetic field variation is of the order of 1.1 T cm1. The low magnetic field region is found for x < 11 mm and x > 67 mm. The regenerator subjected to the minimum magnetic field is so subjected to a mean magnetic field lower than 0.02 T, with just one tube subjected to a magnetic field of about 0.1 T. On the right the comparison between the “as built” simulated data (solid line), and the measured magnetic induction profile (dashed marked line) is displayed. Again a very good agreement can be observed, with the experimental data that show a stronger effect of flux leakages. The high field region is in the range x ¼ 4 O 14 cm, and shows a mean field of about 1.5 T. The measured profile shows a little dissymmetry of the field in the longitudinal direction. The measurements were performed on the median cross and longitudinal sections with a digital gaussmeter, by sampling the magnetic field with a spatial resolution of about 3 mm in the cross measurements, and 10 mm in the longitudinal measurements. The performance parameter proposed by Bjork [22] gives Lcool ¼ 0.13 (the third value among the reviewed magnets).
Preliminary results obtained in free run conditions are reported in Fig. 7. Three set of results are shown. Two set are obtained with the system running in a “free run” with minimum inertia condition, with different working frequencies. The two different frequencies analyzed show similar trends, with the maximum temperature span decreasing with the increasing of the fluid and solid capacity rate ratio (utilization factor, V). Maximum achieved temperature span, DTSPAN, is very similar, around 5.0 K, but the degradation of performance with utilization is more severe at the higher frequency. This effect means that in this system the effects of friction and components dissipation are not negligible and limit the operating frequency also from the performance point of view. In particular, the electric power absorbed by the valves and transferred to the fluid is very relevant in higher frequency conditions. In a test with the valves operating with a continuous electrical activation (about 6 W each) the maximum temperature span achievable drops to 2.6 K. Another contribution comes from the heat transferred to the fluid by the variable speed pumps. All the temperature measurements have been obtained using type T thermocouples with a data acquisition system characterized by an uncertainty of 0.02 C (95% confidence). The third series is obtained with the thermal inertias connected to the system, but the steady state temperature span is about the same of the previous condition. The behavior observed by these preliminary results shows a modest maximum temperature span with a maximum regeneration ratio of 1.2 (the ratio between the obtained temperature span and the maximum adiabatic temperature change of the material under the applied magnetic field change), good repeatability of results obtained in different conditions, and evidenced the limitations of the system also in terms of operating frequency. Regenerator ratio is a dimensionless temperature span, and can be used as a quality index of the prototype performance [27]. The
1400 1200 1000 800 600 400 200 Predicted Measured
0
1400 1200 1000 800 600 400 200
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0
20
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80
100
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Fig. 5. (Left) Magnetic field measurement in air on the median cross section: predicted profile (solid), measured profile (dashed marked). (Right) Magnetic field measurement in air on the median longitudinal section: predicted profile (solid), measured profile (dashed marked).
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T [°C]
28 DTMAG
26
DTDEMAG
24 22 700
720
740
760
780
time [s]
Fig. 8. Temperature vs. time evolution at the ends of a regenerator. Cycle time: 7.9 s; no bypass; V ¼ 2.9. Marked dashed lines show the temperature measurements at the ends of the regenerator (near the valves); solid lines show the temperature measurements at one end the two heat exchangers.
5
DT AD [°C]
4 3 DTAD,MAG DTAD,DEM DTAD,MAG (cp meas)
2
DTAD,DEM (cp meas)
1 0
5
10
15
20
25
30
35
T [°C] Fig. 6. (Top) Temperatureetime history of one direct measure of the effective temperature change in one gadolinium plate with the permanent magnet magnetic structure as field source. (Bottom) DTAD vs T from direct measurements compared to data evaluated by specific heat measurements.
value of 1.2 is small if compared to those obtained by other authors and suggest that a better exploitation of the material is possible (see again [27]). An example of the temperature histories is shown by Fig. 8. Working conditions are cycle time 7.9 s, no bypass and V ¼ 2.9. Temperature of the hot and cold side of a regenerator are reported, together with the environment temperature. In the time window showed the device is operating in steady state, with no appreciable mean temperature variations of the hot and cold sides. The dashed lines show the temperature variation at the ends of the regenerator (near the valves), and the environment temperature,
6
ΔTSPAN [°C]
5
which is steady in a 0.1 C range. Measurements at the ends of the regenerator are influenced by the temperature damping due to dead volumes and thermocouples positioning, so that the different amplitude of the hot and cold signals is not significant. With these temperatures, and given that the fluid flow rate is kept constant, the temperature oscillation at the hot side should be larger than the cold one, due to the lower value of the specific heat at higher temperature and applied magnetic field. The temperatures of the heat exchangers are steady, with a temperature span of 2.8 C. We compared experimental results with simulated ones coming from a properly adapted version of the model described in Ref. [20,23]. Only by assuming the presence of some bypass, low friction fluid paths in the regenerator we succeed in replicating such a degraded performance. Indeed, when the fluid flows through such paths the uniformity of the heat exchange with the solid matrix is compromised and the global performance is accordingly reduced. In comparison to other authors results the obtained temperature spans are modest. Some simple gadolinium based AMR test sections, under similar magnetic field changes, can reach temperature spans larger than 11 C [10,25], in no load conditions. Others more complex prototypes show temperature spans up to 25 C [12,19], in no load conditions. Literature results clearly show that a better exploitation of the material is possible, with a performance oriented design and accurate building solutions. Our results are, however, in agreement with the temperature span obtained in [26], equal to 4.45 K, and obtained with a configuration similar to ours (regenerator size (160 30 7) mm3, made of gadolinium parallel plates 0.85 mm thick, and maximum applied magnetic field 1.65 T). 5. Conclusion
4 3 2 bypass tc=7.9s bypass tc=3.6s no bypass tc=7.9s
1 0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Φ Fig. 7. Temperature span vs. utilization for different working conditions. Filled circles: cycle time 7.9 s, system with bypass open; triangles: cycle time 3.6 s, system with bypass open; void circles: cycle time 7.9 s, system with bypass closed.
A linear reciprocating magnetic refrigerator has been designed and built by our institution in collaboration with an industrial partner. The prototype aim is to show the process in an open way, with components and processes clearly visible and accessible. Data from magnetic field measurements are in very good agreement with predicted performance. Direct magnetocaloric effect measurements obtained on the plates are presented and compared with data computed by heat capacity measurements of high purity gadolinium samples, showing a reduction in amplitude and a shift in the transition temperature. Preliminary results obtained in “free run” conditions are presented. The dependency of temperature span by utilization was observed for three different operating conditions, showing a good reproducibility of the experiments, and a trend which evidences the limit in terms of higher frequency operation of this design. A regeneration ratio of 1.2 was achieved, a result that can be improved with further
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modifications on the actual system which are foreseen in future work. Thanks to this experience important issues of regenerators behavior have been identified. Among these the most relevant are the presence of low friction paths inside the regenerator due to the deformation of the regenerator housing under pressure, which reduces strongly the regenerator effectiveness and can be avoided with a proper sealing configuration; the effect of auxiliary power inputs, such as electric dissipation of the valves, which reduces the maximum temperature span achievable; the effect of dead volumes, which can be reduced by a new piping design. On the basis of the knowledge gained with this work, future work foresees the improvement of this system, and the design of a new more performance oriented system. Acknowledgements The present work was developed in the framework of the Italian Ministry of Economic Development (MSE) project F.A.R n.6728/ 2002 “Refrigerazone magnetica”. The authors wish to thank the Technical Director of Zanotti S.p.A., Ing. Lorenzo Bulgarelli, for his collaboration and suggestions during the prototype development. References [1] E. Warburg, Investigation on magnetism, Ann. Phys. 13 (1881) 141e164 (in German, Magnetiche Untersuchungen). [2] G.V. Brown, Magnetic heat pumping near room temperature, J. Appl. Phys. 47 (1976) 3673e3680. [3] V.K. Pecharsky, K.A. Gschneidner Jr., Giant magnetocaloric effect in Gd5(Si2Ge2), Phys. Rev. Lett. 78 (2) (1997) 4494e4497. [4] C. Zimm, A. Jastrab, A. Sternberg, V.K. Pecharsky, K.A. Gschneidner Jr., M. Osborne, I. Anderson, Description and performance of a near room temperature magnetic refrigerator, Adv. Cryog. Eng. 43 (1998) 1759e1766. [5] K.A. Gschneidner Jr., V.K. Pecharsky, Thirty years of near room temperature magnetic cooling: where we are today and future prospects, Int. J. Refrig. 31 (6) (2008) 945e961. [6] B. Yu, M. Liu, P.W. Egolf, A. Kitanovski, A review of magnetic refrigerator and heat pump prototypes built before the year 2010, Int. J. Refrig. 33 (2010) 1029e1060. [7] L.A. Tagliafico, F. Scarpa, F. Canepa, G. Tagliafico, Room temperature magnetic refrigeration technology, in: Refrigeration: Theory, Technology and Applications, Nova Publishers, New York, USA, 2010, pp. 71e131. [8] D.S. Arnold, A. Tura, A. Rowe, Experimental analysis of a two material active magnetic regenerator, Int. J. Refrig. 34 (2011) 178e191.
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[9] K. Engelbrecht, C.R.H. Bahl, K.K. Nielsen, Experimental apparatus for a magnetic refrigerator using three different types of magnetocaloric material regenerators, Int. J. Refrig. 34 (2011) 1132e1140. [10] T. Kawanami, S. Hirano, K. Fumoto, S. Hirasawa, Evaluation of the fundamental performance on magnetocaloric cooling with active magnetic regenerator, Appl. Therm. Eng. 31 (2011) 1176e1183. [11] J.Y. Law, V. Franco, P. Keblinski, R.V. Ramanujan, Active transient cooling by magnetocaloric materials, Appl. Therm. Eng. 52 (1) (2013) 17e23. [12] M. Balli, O. Sari, C. Mahmed, C.H. Besson, P.H. Bonhote, D. Duc, J. Forchelet, A pre-industrial magnetic cooling system for room temperature application, Appl. Energ. 98 (2012) 556e561. [13] T. Okamura, K. Yamada, N. Hirano, S. Nagaya, Performance of a room temperature rotary magnetic refrigerator, Int. J. Refrig. 29 (2006) 1327e1331. [14] C.R.H. Bahl, K. Engelbrecht, R. Bjork, D. Eriksen, A. Smith, K.K. Nielsen, N. Pryds, Design concepts for a continuously rotating active magnetic regenerator, Int. J. Refrig. 34 (2011) 1792e1796. [15] C. Zimm, A. Boeder, J. Chell, A. Sternberg, A. Fujita, S. Fujieda, K. Fukamichi, Design and performance of permanent magnet rotary refrigerator, Int. J. Refrig. 29 (2006) 1302e1306. [16] C. Vasile, C. Muller, Innovative design of a magnetocaloric system, Int. J. Refrig. 29 (2006) 1318e1326. [17] J. Tusek, S. Zupan, A. Sarlah, I. Prebil, A. Poredos, Development of a rotary magnetic refrigerator, Int. J. Refrig. 33 (2010) 294e300. [18] A. Tura, A. Rowe, Design and testing of a permanent magnet magnetic refrigerator, in: Proc. Second IIF-IIR Conference on Magnetic Refrigeration at Room Temperature, IIF-IIR, 2007, pp. 365e370. [19] A. Tura, A. Rowe, Permanent magnet magnetic refrigerator design and experimental characterization, Int. J. Refrig. 34 (2011) 628e639. [20] G. Tagliafico, Numerical and Experimental Characterization of Components for Active Magnetic Regenerative Refrigerators, PhD thesis, University of Genova, Italy, Genoa, 2011. [21] F. Scarpa, G. Tagliafico, L.A. Tagliafico, Classification proposal for room temperature magnetic refrigerators, Int. J. Refrig. 35 (2) (2012) 453e458. [22] R. Bjork, C.R.H. Bahl, A. Smith, N. Pryds, Optimization and improvement of Halbach cylinder design, J. Appl. Phys. 104 (013910) (2008) 1e9. http:// dx.doi.org/10.1063/1.2952537. [23] G. Tagliafico, F. Scarpa, F. Canepa, A dynamic 1-D model for a reciprocating active magnetic regenerator; influence of the main working parameters, Int. J. Refrig. 33 (2) (2010) 286e293. [24] D. Vuarnoz, T. Kawanami, Numerical analysis of a reciprocating active magnetic regenerator made of gadolinium wires, Appl. Therm. Eng. 37 (2012) 388e395. [25] A. Tura, K.K. Nielsen, A. Rowe, Experimental and modeling results of a parallel plate-based active magnetic regenerator, Int. J. Refrig. 35 (2012) 1518e1527. [26] P.V. Trevizoli, J.R. Barbosa Jr., R.T.S. Ferreira, Experimental evaluation of a Gdbased linear reciprocating active magnetic regenerator test apparatus, Int. J. Refrig. 34 (2011) 1518e1526. [27] G. Tagliafico, F. Scarpa, L.A. Tagliafico, A dimensionless description of active magnetic regenerators to compare their performance and to simplify their optimization, Int. J. Refrig. (2012). http://dx.doi.org/10.1016/j.ijrefrig.2012.10.024. Available online 30 October 2012.