June 25, 2018.
The electricity usage for refrigeration systems is accounted for more than 50 – 60 % of the country’s total electrical energy consumption. Based on 2017 data as shown in Figure 1, the total domestic electricity usage is as high as 185,370 million unit per year. With the cost of electricity production at 2 – 5 baht/unit, the cost of electricity usage for refrigeration systems in Thailand is higher than 200 billion baths per year
Figure 1. The Chart of electricity consumption of Thailand in 2017 (Source: Energy Policy and Planning Office, Ministry of Energy).
Therefore, the research and development of the refrigeration system with improved energy efficiency will greatly help reduce the electricity consumption and cost in Thailand. Current widely-used refrigeration system is a vapor compression refrigeration, which has the efficiency limitation lower than 10% of Carnot cycle resulting in high energy consumption. Moreover, the system requires the use of phase-changing refrigerant medium consisting of hydrofluorocarbons (HFC) and hydrochlorofluorocarbons (HCFC), which damage the ozone layer in the atmosphere. Therefore, it is necessary that a new and innovative refrigeration system, which exhibits high energy-consumption efficiency and environmental-friendly, is developed to replace the traditional vapor-compression system in the near future.
In 2017, Thailand Center of Excellence in Physics (ThEP Center) has initiated and conducted the inter-university research program on Innovative Physics for Magnetic Cooling Industry with the objectives to conduct R&D on magnetic materials for the productions of permanent magnets and magnetocaloric materials by applying innovative physics to develop magnetic refrigeration prototypes. The magnetic refrigeration uses a magnetocaloric material as a solid-state refrigerant without using any environmentally-harmful solution. The cooling/heating performance of the magnetocaloric effect (MCE) can potentially be as high as 60% of Carnot cycle. As a result, the magnetic refrigeration can potentially become an alternative refrigeration system for household and industrial sectors in the future to reduce the electricity cost of more than 100 billion baht per year.
The magnetic refrigeration is based on the principles of thermodynamics. The magnetocaloric material are heated up and cooled down, under magnetization and demagnetization, respectively. Therefore, the magnetic-field induced temperature change of the magnetocaloric material is exploited for refrigeration cycle by using the magnetocaloric material as a refrigerant.
The refrigeration cycles of the vapor compression system and the magnetic refrigeration system is schematically illustrated in Figure 2. For the magnetic refrigeration, the compressors, pressure-relief valve, and environmentally-hazard refrigerants (e.g. HFC, HCFC) are replaced with the solid-state magnetocaloric refrigerant and the magnetic field. The magnetocaloric material’s temperature increases under a presence of magnetic field (H field on) and decreases when the field is off. Unlike the refrigerant that is used in the vapor compression system, the temperature of the magnetocaloric material can be changed by the magnetic field application under normal atmosphere without the need for a compressor to pump up high pressure. An application of the magnetic field can be controlled by using a rotary permanent magnet with typically rotation speed lower than 10 rpm, which is much lower than the pumping frequency of a compressor. As a result, the magnetic field application potentially consumes much lower electrical energy than the pressurization process by a compressor. Moreover, the temperature change of the magnetocaloric material under a magnetic field change is a reversible process. Therefore, the efficiency of the magnetocaloric heating/cooling cycles can be close to that of Carnot cycle. As a result, the magnetic refrigeration shows high potentials to provide high energy-efficient and environmental-friendly refrigeration system.
Figure 2. Comparison of (left) refrigeration cycles of the vapor compression system with (right) refrigeration cycles of the
Based on the magnetic refrigeration principles, magnetocaloric materials and permanent magnets are key materials for the development of innovation magnetic refrigeration prototypes. Therefore, this research program is composed of 4 projects including:
Project 1:Magnetic and Physical Properties of Magnetocaloric Materials (LaMnO3) and Development of Magnetic-refrigeration-system Prototypes
Project 2:Fabrication of Magnetocaloric Materials for Magnetic Refrigeration
Project 3:Nanostructured Permanent Magnets for Magnetic Refrigeration
Project 4:Advanced Permanent Magnets for Magnetic Refrigeration
Projects 1 and 2 are aimed to study and develop magnetocaloric materials and processing for the magnetic refrigeration prototype fabrication. Projects 3 and 4 are aimed to study and develop magnetic materials to be used as permanent magnets to incorporate into the magnetic cooling prototypes. Below are details of activities and key results from each project in the past year.
2. Magnetic and Physical Properties of Magnetocaloric Materials (LaMnO3) and Development of Magnetic-refrigeration-system Prototypes Project
The objectives of this project is to develop a magnetic refrigeration prototype and to conduct the first-principle studies of the magnetocaloric phenomenon in materials. The understanding of the first principles of the magnetocaloric phenomenon will assist the design and development of the magnetic cooling innovation. The project is composed of the following studies:
The activities and key results of each sub-project are described as following:
2.1. Design and assembly of magnetic generating system
The primary process of the magnetic refrigeration prototype design is the study of magnetic refrigeration system including a regenerator, a magnet-generating structure, and a heat-exchange flow system. This project addresses the design and development of the magnet-generating assembly along with the measurement of the possible magnetic flux density using COMSOL Multiphysics program. Because of the limitation of the permanent magnet production in Thailand, the magnet has to be in a rectangular form. Consequently, the magnet designs are proposed in two structures, simple magnet and complex magnet assemblies. The simple rectangular magnet structure is a rotating magnet system consisted of an inner rotating permanent magnet centered inside the outer stationary iron yoke (Figure 3). The complex magnet structure consists of an inner rotating magnet of four rectangular permanent magnets connected with soft iron magnet and an outer yoke of a soft iron magnet (Figure 4). The average flux density is 0.5 and 0.65 T, respectively. The computed study using COMSOL Multiphysics provide the optimal magnet design before the real construction. Finally, the actual magnetic generating system will be produced and assembled for testing and comparing with the simulation and using in the prototype.
Figure 3. (a) A sketch of the simple rectangular magnet assembly. (b) The determined structure with magnetic flux density (T).
Figure 4. (a) A sketch of the complex rectangular magnet assembly. (b) The determined structure with magnetic flux density (T).
2.2 The design and production of automatic control system for the magnetic refrigeration prototype
This project designs a prototype of a magnetic refrigerator with the control of heat transfer fluid flow system, which is responsible for conveying heat energy from heat exchangers with magnetocaloric materials. There is a development of a flow system that directs the flow of hot and cold temperatures flowing in different directions using a solenoid valve to electrically control the flow directions. The control system also controls the rotating motor that governs the magnetic field generating system to induce magnetization and demagnetization states on the magnetocaloric materials. Three-phase motors control the speed of the magnetic rotation with the inverter and drive power through the pulleys and pulleys to the magnetic circuit. The rotation of the magnetic circuit must be in synced with the fluid flow direction, which is automatically controlled by the electrical system. Sensors for temperature, pressure and flow rate are embedded in the system to monitor these values real time for analyzing the performance of the magnetic refrigeration prototype. The system is shown in Figure 5.
Figure 5. Fluid flow and valve control system.
2.3The study and design of fluid flow and heat exchange in active magnetic regenerators
Magnetocaloric materials used for a magnetic refrigeration prototype in this project are gadolinium (Gd) and lanthanum compounds La(Fe,Mn,Si)13Hy. Lanthanum compounds are brittle material that can be damaged by high pressure while the system is running. Binding agent can increase the toughness of the lanthanum compounds packed-bed to prevent damage from the pressure (Figure 6). Binding agents are polymeric materials, which show low thermal conductivity resulting in poor heat exchange efficiency. Therefore, the use of suitable binding agents and the use of additives to increase the thermal conductivity of the binding agent must be studied.
Figure 6. Example of magnetocalric beds with epoxy binders.
This research studied the behavior of fluid flow past magnetocaloric material in a magnetic refrigeration. It commenced from the test to determine a relationship between pressure loss and velocity of water flow, passing through ferrous porous media at the Reynolds number of 3,000-7,000. This ferrous porous media was modelled to be porous material, having porosity as Gadolinium. The obtained information was used to organize a numerical model to predict the water flow past porous media. The example of simulation and experimental setups are shown in Figures 7 and 8, respectively. From the comparison of pressure loss, the results from the Computational Fluid Dynamics - CFD were in agreement with those measured from the experiment, as shown in Figure 9. This yielded model will be utilized for the design of heat transfer in magnetic refrigeration system using CFD.
Figure 7. Simulation model of the fluid flow through porous media.
Figure 8. Experimental setup for CFD study.
Figure 9. Pressure loss vs Flow velocity.
2.4 The study of magnetocaloric properties based on the thermodynamics principles and numerical simulation
Physics of the cooling system based on magnetocaloric materials relate to several phenomena including heat transfer, electromagnetic induction, viscosity of fluid, among others. A numerical modeling designed for the analysis of influences of various parameters to the cooling efficiency is essential in the design and development of a magnetocaloric-based cooling system. In this work, a one-dimensional numerical model representing the working of the cooling system is introduced and systematically solved. The model straightforwardly gives the COP as a function of important variables such as volume flow rate (as shown in Figure 10). Moreover, the model is capable of computing the COP of a cooling system based on multi-layered magnetocaloric materials (MMCM) (as shown in Figure 11). This particular arrangement of MMCM in a cooling system may lead to a temperature reduction of approximately 40 oC.
Figure 10. Relation between Coefficient of Performance (COP) and volume flow rate.
Figure 11. Relation between Coefficient of Performance (COP) and number of layers.
3. Fabrication of Magnetocaloric Materials for Magnetic Refrigeration Project
Refrigeration technology is very important to our lives. However, refrigeration in use today relies almost entirely on vapor compression cycle that requires the CFC or HFC to function as a heat exchanger. It is well known that CFC and HFC may cause of global warming. Thus the environmentally friendly materials for refrigeration are absolutely necessary.
MCE was discovered by E. Warburg in 1881. By subjecting pure iron rod under magnetic fields at low temperature, it is found that pure iron rods release and absorb heat when magnetic fields change. MCE is a result of the change in entropy of magnetic material due to the arrangement of the magnetic moment with the external magnetic field as shown in Figure 12. MCE materials have been shown to have potential to be developed for use in magnetic refrigeration systems . Moreover, the theoretical study found that the system can be developed to outperform the vapor compression cycle system. However, the applications of MCE materials for refrigeration also have the drawback that the production cost is high and it requires a strong magnetic field (electromagnet or superconduction magnet). Therefore, the invention of materials that can exhibit MCE effect at a lower field will lead to great changes in the cooling technology.
Figure 12. Temperature variation of the MCE material due to entropy changing with the external magnetic field.
The application of MCE material for refrigeration has three main drawbacks: expensive precursors (rare earth), narrow temperature span, and require a high-intensity magnetic field. With such obstacles, the application of MCE material for refrigeration is not as successful as it should be.
The NiMnCrIn alloys exhibiting MCE effect near room temperature was synthesized by Sharma et al. . The discovery of rare earth free MCE materials sparks an application of MCE material for refrigeration. However, more research is still needed to increase the temperature span of the MCE materials.
This research project focuses on the development of low cost and low field MCE materials for use in refrigeration applications. The MCE materials will be studied and developed to adjust for the temperature span. In order to be able to apply MCE materials to refrigeration using electromagnetic field or permanent magnets, the selected materials will be coated with a soft magnetic material as shown in Figure 13.
Figure13. Soft magnetic-coated MCE materials.
Preliminary research results indicate that magnetic phase transition of FeNiCr alloy can be tuned by variation of Cr content. When the Cr content increases, the phase transition occurs at a lower temperature as shown in Figure 14.
Figure 14. Magnetic phase transition inFeNiCr alloy.
The element composition investigation indicates that composition of NiMnIn sintering at 900 °C is inhomogeneous. The variation of element composition results in the broadening of MCE effect as shown in Figure 15. However, the entropy change of the sample is lower than that previously reported .
Figure 15. Isothermal M-H curve and entropy change of NiMnIn alloy.
4. Nanostructured Permanent Magnets for Magnetic Refrigeration Project
Permanent magnets are vital components in newly proposed magnetic refrigerators and other instruments but the shortage of raw materials for Nd-Fe-B and Sm-Co magnets is impending. The research and development on rare-earth free permanent magnets is in good agreement with the national strategy to promote new industry based on nanotechnology in Thailand.
The research on new nanostructured magnets has been focused on manganese bismuth (MnBi) as an alternative to rare-earth magnets. Its notable low temperature phase (LTP) has a high magnetocrystalline anisotropy and the positive temperature coefficient of coercivity above room temperature indicating its performance at elevated temperature.
At Walailak University, MnBi magnets were produced by mixing and arc-melting the raw materials. Subsequently, they were annealed (below 300 °C) to enhance the formation of LTP MnBi and then ball-milled to enhance the coercivity with reducing grain sizes. However, the obtained hysteresis loops are narrow signifying the soft magnetic behaviors.
In order to improve a control over the phase formation, equipment is currently developed for the oxygen-free production. The lab-made microwave furnace under argon atmosphere will be used in the initial melting step (Figure 16). The following annealing and pressing steps are combined in a single system (Figure 17).
Figure 16. Microwave furnace (working temp. > 1,300 °C, argon atmosphere).
Figure 17. Pressing-annealing machine (working temp. < 450 °C, pressure < 40 N/cm2, working time < 200 h, argon atmosphere).
5.Development of Rare-earth-free High-performance Magnetic Materials for Magnetic Refrigerators Project
Over the past decade, demand for high performance permanent magnets is on the rise since a permanent magnet is an essential component in constructing a device that transforms between mechanical and electrical energies, and is crucial to many energy-harvesting technologies such as those found in wind turbines and hybrid vehicles. Current high-performance permanent magnets in use today are Nd-Fe-B and Sm-Co, which contain a large percentage of rare-earth elements. Extracting these rare-earth elements with high purity and in large quantity is difficult and costly. The demand for these elements, especially with the emergence of cooling technology such as magnetic refrigeration, would inevitably further drive up their price. Search for new high-performance, rare-earth-free magnetic materials is essential to avoid the shortage of the conventional magnetic elements.
A team at the Suranaree University of Technology has been researching for the new rare-earth-free permanent magnetic materials. The research initially focused on synthesizing the α" phase iron nitride (Fe16N2). Based on their experimental findings and literature reviews, it was found that such a synthesis can be obtained by thermal nitridation process, i.e. reaction of ammonium gas with metallic iron at low temperatures (< 200 oC). The experiments involved the use of ammonia gas, and thus must be performed with strict safety regulations since the laboratory was shared by other parties. Another important issue concerns the stability of Fe16N2 structure. At the temperature above 200°C, the material undergoes a structural change and loses its magnetic property. This makes it impractical to be used as a permanent magnet. The research was latter shifted towards the study of Manganese Bismuth (MnBi). Low temperature phase (LTP) MnBi exhibits high magneto-crystalline anisotropy with an increase in magnetic coercivity with increasing temperature.
The research team has set up a simple system for synthesizing MnBi via Bi diffusion into Mn powder at the BL3.2a station of the Synchrotron Light Research Institute. The complication in preparing MnBi originates from the difference in the melting point between manganese (1246 oC) and bismuth (271 oC) and the oxidation of MnBi. The base pressure of the developed system is 10-8 mbar. At that pressure, the remaining oxygen gas is extremely low which helps avoid the oxidation issue. The system is set up so that one may study the effect of oxygen to the magnetic properties of MnBi. Moreover it is equipped with a temperature controller to maintain the temperature at a required value with an error of ± 1°C. The system allows the research team to refine the preparation conditions that may lead to a successful preparation of high performance Mn-based magnetic materials.
Figure 18. The system for preparing Mn-based magnetic materials based on thermal diffusion.
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 V. K. Sharma, M. K. Chattopadhyay and S. B. Roy (2010). Largemagnetocaloric effect in Ni50Mn33.66Cr0.34In16 alloy. J Phys D: Appl Phys 43: 225001.
 X. Moya, L. Manosa and A. Planes (2007). Cooling and heating by adiabatic magnetization in the Ni50Mn34In16 magnetic shape-memory alloy. Phys Rev B 75: 184412.
Assoc. Prof. Dr. Kittiwit Matan 1), Asst. Prof. Dr. Ratchatee Techapiesancharoenkit 2), Asst. Prof. Dr. Pongsakorn Jantaratana3), Assoc. Prof. Dr. Chitnarong Sirisathitkul 4), and Assoc. Prof. Dr. Prayoon Songsiriritthigul 5)
1)Department of Physics, Faculty of Science, Mahidol University, 2) Materials Innovation Center, Faculty of Engineering, Kasetsart University, 3) Department of Physics, Faculty of Science, Kasetsart University, 4) Department of Physics, School of Science, Walailak University, 5) Shool of Physics, Institute of Science, Suranaree University of Technology