تجزیه و تحلیل عملکرد از یخچال مغناطیسی فعال دوار
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Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Applied Energy, Volume 111, November 2013, Pages 669–680
Performance results for a novel rotary active magnetic regenerator (AMR) and detailed numerical model of it are presented. The experimental device consists of 24 regenerators packed with gadolinium (Gd) spheres rotating inside a four-pole permanent magnet with magnetic field of 1.24 T. A parametric study of the temperature span, cooling power, coefficient of performance (COP) and efficiency of the system was carried out over a range of different hot reservoir temperatures, volumetric flow rates and cooling powers. Detailed modeling of the AMR using a 1D model was performed and compared with the experimental results. An overall mapping of the thermal losses of the system was performed, and good agreement between the experimental and numerical results was found when parasitic heat losses were subtracted from the modeling results. The performance of the system was evaluated via the COP, the exergetic-equivalent cooling power (ExQ), and the overall second law efficiency, η2nd. Losses mapping indicated that friction and thermal leakage to the ambient are the most important contributors to the reduction of the system performance. Based on modeling results, improvements on the flow distributor design and reduction of the cold end thermal parasitic losses are expected to enhance the efficiency of the system. For an operating frequency of 1.5 Hz, a volumetric flow rate of 400 L/h, a hot reservoir temperature of 297.7 K, and thermal loads of 200 and 400 W, the obtained temperature spans, ΔTS, were 16.8 K and 7.1 K, which correspond to COPs of 0.69 and 1.51, respectively. The maximum overall second-law efficiency was 5.6% for a ΔTS of 12.9 K at 500 L/h and 400 W.
An active magnetic regenerator (AMR) refrigerator is an alternative cooling technology that uses a solid refrigerant that has no ozone depleting potential and no direct global warming potential. Instead the technology relies on the magnetocaloric effect, a coupling between the temperature and magnetic field of magnetic materials. Since the magnetization/demagnetization of a magnetocaloric material (MCM) can be reversible, it may be possible to build highly efficient cooling devices around the effect for a diverse number of applications including chip and sensor cooling  and low-cost magnetic energy conversion devices . A room temperature regenerative magnetic refrigerator was first demonstrated by Brown . In 1982 the active magnetic regenerator (AMR) concept was introduced, in which the regeneration was achieved by the active material itself . Since then, many new devices have been reported , with modern AMRs generally using permanent magnets and regenerators made of packed spheres  and , packed particles  or parallel plates  and  of MCM. A comprehensive overview of the material and the regenerator technology was recently given in Ref. . The AMR cycle uses a heat transfer fluid to transport the heat generated from magnetizing and demagnetizing the MCM to the hot and cold reservoirs. The AMR cycle has four basic processes: magnetization, the cold-to-hot blow, demagnetization, and the hot-to-cold blow. During magnetization, the temperature of the MCM increases, then fluid is pumped from the cold reservoir to the hot reservoir in order to reject the magnetic work to ambient. The regenerator is then demagnetized, causing a decrease in temperature and a cooling load is accepted from the cooled space by pumping fluid from the hot reservoir across the regenerator and into the cold reservoir. The system performance is mostly a function of the MCM, heat transfer characteristics in the regenerator, and cycle parameters such as frequency and fluid flow rate. Rotary AMRs, where the magnetic field in the regenerators is varied by rotating the regenerator relative to the magnet, have been shown to operate effectively at higher cycle frequencies. Devices with stationary magnets and rotating regenerators have been demonstrated by Ref.  and , while systems with stationary regenerators and rotating magnets have been demonstrated by Ref. , ,  and . Several AMR devices have been reported recently using Gd regenerators, which is the benchmark MCM for use in AMR devices. A magnetic refrigerator using stationary regenerator beds and a rotating permanent magnet was shown to produce a maximum cooling power of 844 W at zero temperature span and 400 W at a temperature span, ΔTS, of 8.1 K using a 0.89-kg Gd regenerator . This device operated continuously with a maximum reported operating frequency of 4.7 Hz, and an exergetic equivalent cooling power of 14 W was obtained. Efficiency was not reported. A rotary magnet device using a fluid displacer to distribute the fluid flow was reported to operate at a no-load temperature span of 29 K. This device produced a cooling power of 50 W at a ΔTS of 10 K, using a 0.11-kg Gd regenerator at a maximum operating frequency of 4 Hz . A maximum COP of 1.6, which included all motor inefficiencies and drive loss, was calculated for a cooling capacity of 50 W at a ΔTS of 2.5 K and a frequency of 1.4 Hz. The COP could be increased to 2.2 if the motor inefficiency were removed . The machine evaluated in this paper was described in detail in Refs. , ,  and  together with some experimental results. It consists of a novel concentric magnet assembly design  with a 24-bed regenerator rotating continuously in the magnet gap. Fluid flow is provided by a continuously operating gear pump and flow distributor system. The flow distributors are designed such that a minimum of eight regenerator beds are open to flow at any given time to minimize fluctuations in flow to the regenerator. This device demonstrated a maximum cooling power of 1010 W at a ΔT S of 0.3 K and a maximum no-load ΔT S of 25.4 K and that it absorbs a cooling power, View the MathML sourceQ̇C, of 100 W at a ΔTS of 21 K . A maximum operating frequency of 10 Hz was experimentally proved at a ΔTS of 12.1 K and 200 W . This paper presents experimental data, numerical results obtained using a 1D model of the AMR  and, for the first time, a detailed assessment of the system efficiency over a range of operating conditions. The power consumption associated with several components have been measured, and methods for improving the system performance and increasing its efficiency are suggested. A detailed thermal loss analysis of several system components was carried out, which was applied a posteriori to improve agreement between model and experiment. The performance of the system was evaluated in terms of COP, ExQ, and η2nd.
نتیجه گیری انگلیسی
Experimental results obtained with a novel rotary magnetic cooler prototype for a range of hot reservoir temperatures, cooling capacities and volumetric flow rates were presented. As a part of the analysis of the AMR performance, a detailed study of parasitic losses external to the regenerator beds was carried out. It was shown that flow distributor friction, heat transfer from the regenerator housing, heat losses from the connecting piping and heat losses from the cold end fluid filter contributed to reducing the performance of the AMR. When the AMR operates with a temperature span of 25 K, the losses to the ambient are estimated at approximately 74 W, which will directly reduce the power available for cooling. The efficiency of the AMR was studied for a range of operating conditions. For an operating frequency of 1.5 Hz, a volumetric flow rate of 400 L/h, and a hot reservoir temperature of 297.7 K, a cooling capacity of 200 W produced over a 16.8 K span with a COP of 0.69. For a 400-W cooling power, the temperature span was 7.1 K, with a COP of 1.51. The maximum experimentally measured COP was 1.62 when absorbing 400 W cooling power at a span of 1.5 K. The power loss associated with various components was quantified, which made clear that efficiency can be significantly increased by reducing friction in the fluid flow distributors. Efficiency can be further increased by reducing parasitic losses, which will increase the effective cooling power of the device. The experiments were compared to predictions from a 1D numerical AMR model for all experiments presented. When the calculated parasitic losses are included, good agreement between the model and experiment is achieved. This suggests that a simple 1D regenerator model can be used to predict a large scale AMR device with some accuracy. The paper also presented a systematic performance evaluation of the AMR device, which incorporated the data on mechanical losses in the various components and quantified the thermodynamic and mechanical efficiencies in a second-law framework. The maximum overall second-law efficiency was around 5%, which corresponded to approximately one-sixth of the maximum second-law efficiency calculated excluding the mechanical losses. This, together with the possibility of advanced magnetocaloric materials with multiple layers can certainly make magnetic refrigeration at room-temperature more competitive at certain niche applications, as suggested by .