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25-01-2009, 08:15 AM
7. Activities (present and future)
8. Magnetic materials
10. Superconducting magnets
11. Active magnetic regenerators (AMR?s)
12. A rotary AMR liquefier
The compilation of this seminar and presentation would not have been possible without the support and guidance of the following people and organization. With my deep sense of gratitude, I thank my respected teachers for supporting this topic of my seminar and presentation. This seminar and presentation report provides me with an opportunity to put into knowledge of advanced technology. I thereby take the privilege opportunity to thank my guide and my friends whose help and guidance made this study a possibility.
I would like to express my gratitude with a word of thanks to all of those who are directly or indirectly with this report.
Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique can be used to attain extremely low temperatures (well below 1 kelvin), as well as the ranges used in common refrigerators, depending on the design of the system.
The effect was discovered in pure iron in 1881 by E. Warburg. Originally, the cooling effect varied between 0.5 to 2 K/T.
Major advances first appeared in the late 1920s when cooling via adiabatic demagnetization was independently proposed by two scientists: Debye (1926) and Giauque (1927).
The process was demonstrated a few years later when Giauque and MacDougall in 1933 used it to reach a temperature of 0.25 K. Between 1933 and 1997, a number of advances in utilization of the MCE for cooling occurred.
This cooling technology was first demonstrated experimentally by chemist Nobel Laureate William F. Giauque and his colleague Dr. D.P. MacDougall in 1933 for cryogenic purposes (they reached 0.25 K)
Between 1933 and 1997, a number of advances occurred which have been described in some reviews.
In 1997, the first near room temperature proof of concept magnetic refrigerator was demonstrated by Prof. Karl A. Gschneidner, Jr. by the Iowa State University at Ames Laboratory. This event attracted interest from scientists and companies worldwide who started developing new kinds of room temperature materials and magnetic refrigerator designs.
Refrigerators based on the magnetocaloric effect have been demonstrated in laboratories, using magnetic fields starting at 0.6 T up to 10 teslas. Magnetic fields above 2 T are difficult to produce with permanent magnets and are produced by a superconducting magnet (1 tesla is about 20,000 times the Earth's magnetic field).
MAGNETO CAROIC EFFECT
The Magneto caloric effect (MCE, from magnet and calorie) 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. This is also known as adiabatic demagnetization by low temperature physicists, due to the application of the process specifically to effect a temperature drop. In that part of the overall refrigeration process, a decrease in the strength of an externally applied magnetic field allows the magnetic domains of a chosen (magnetocaloric) material to become disoriented from the magnetic field by the agitating action of the thermal energy (phonons) present in the material. If the material is isolated so that no energy is allowed to (e)migrate into the material during this time (i.e. an adiabatic process), the temperature drops as the domains absorb the thermal energy to perform their reorientation. The randomization of the domains occurs in a similar fashion to the randomization at the curie temperature, except that magnetic dipoles overcome a decreasing external magnetic field while energy remains constant, instead of magnetic domains being disrupted from internal ferromagnetism as energy is added.
One of the most notable examples of the magnetocaloric effect is in the chemical element gadolinium and some of its alloys. Gadolinium's temperature is observed to increase when it enters certain magnetic fields. When it leaves the magnetic field, the temperature returns to normal.The effect is considerably stronger for the gadolinium alloy Gd5(Si2Ge2). Praseodymium alloyed with nickel (PrNi5) has such a strong magnetocaloric effect that it has allowed scientists to approach within one thousandth of a degree of absolute zero.
Magnetic Refrigeration is also called as Adiabatic Magnetization.
To develop more efficient and cost effective small scale H2 liquefiers as an alternative to vapor-compression cycles using magnetic refrigeration.
With the help of magnetic refrigeration our objective is to solve the problem of hydrogen storage as it ignites on a very low temperature. Hydrogen Research Institute (HRI) is studying it with the help of magnetic refrigeration. We provide the cooling for the hydrogen storage by liquefying it.
The hydrogen can be liquefied at a low temperature and the low temperature is achieved with the help of magnetic refrigeration.
Thus, the magnetic refrigeration also provides a method to store hydrogen by liquefying it. The term used for such a device is magnetic liquefier.
2. Hot Heat exchanger
3. Cold Heat Exchanger
5. Magneto caloric wheel
1. Magnets : - Magnets are the main functioning element of the magnetic refrigeration. Magnets provide the magnetic field to the material so that they can loose or gain the heat to the surrounding and from the space to be cooled respectively.
2. Hot Heat Exchanger : - The hot heat exchanger absorbs the heat from the material used and gives off to the surrounding. It makes the transfer of heat much effective.
3. Cold Heat Exchanger :-The cold heat exchanger absorbs the heat from the space to be cooled and gives it to the magnetic material. It helps to make the absorption of heat effective.
4. Drive : - Drive provides the right rotation to the heat to rightly handle it. Due to this heat flows in the right desired direction.
5. Magneto caloric Wheel : - It forms the structure of the whole device. It joins both the two magnets to work properly.
The magnetic refrigeration is mainly based on magneto caloric effect according to which some materials change in temperature when they are magnetized and demagnetized.
Near the phase transition of the magnetic materials, the adiabatic application of a magnetic field reduces the magnetic entropy by ordering the magnetic moments. This results in a temperature increase of the magnetic material. This phenomenon is practically reversible for some magnetic materials; thus, adiabatic removal of the field reverts the magnetic entropy to its original state and cools the material accordingly. This reversibility combined with the ability to create devices with inherent work recovery, makes magnetic refrigeration a potentially more efficient process than gas compression and expansion. The efficiency of magnetic refrigeration can be as much as 50% greater than for conventional refrigerators.
The process is performed as a refrigeration cycle, analogous to the Carnot cycle, and can be described at a starting point whereby the chosen working substance is introduced into a magnetic field (i.e. the magnetic flux density is increased). The working material is the refrigerant, and starts in thermal equilibrium with the refrigerated environment.
? Adiabatic magnetization: The substance is placed in an insulated environment. The increasing external magnetic field (+H) causes the magnetic dipoles of the atoms to align, thereby decreasing the material's magnetic entropy and heat capacity. Since overall energy is not lost (yet) and therefore total entropy is not reduced (according to thermodynamic laws), the net result is that the item heats up (T + ?Tad).
? Isomagnetic enthalpic transfer: This added heat can then be removed by a fluid like water or helium for example (-Q). The magnetic field is held constant to prevent the dipoles from reabsorbing the heat. Once sufficiently cooled, the magnetocaloric material and the coolant are separated (H=0).
? Adiabatic demagnetization: The substance is returned to another adiabatic (insulated) condition so the total entropy remains constant. However, this time the magnetic field is decreased, the thermal energy causes the domains to overcome the field, and thus the sample cools (i.e. an adiabatic temperature change). Energy (and entropy) transfers from thermal entropy to magnetic entropy (disorder of the magnetic dipoles).
? Isomagnetic entropic transfer: The magnetic field is held constant to prevent the material from heating back up. The material is placed in thermal contact with the environment being refrigerated. Because the working material is cooler than the refrigerated environment (by design), heat energy migrates into the working material (+Q).
Once the refrigerant and refrigerated environment are in thermal equilibrium, the cycle begins a new
As shown in the figure, when the magnetic material is placed in the magnetic field, the thermometer attached to it shows a high temperature as the temperature of it increases.
But on the other side when the magnetic material is removed from the magnetic field, the thermometer shows low temperature as its temperature decreases.
The place we want to cool it, we will apply magnetic field to the material in that place and as its temperature increases, it will absorb heat from that place and by taking the magnetic material outside in the surroundings, we will remove the magnetic material from magnetic field and thus it will loose heat as its temperature decreases and hence the cycle repeats over and again to provide the cooling effect at the desired place.
1. HIGH EFFICIENCY : - As the magneto caloric effect is highly reversible, the thermo dynamic efficiency of the magnetic refrigerator is high.
It is some what 50% more than Vapor Compression cycle.
2. REDUCED COST : - As it eliminates the most in efficient part of today?s refrigerator i.e. comp. The cost reduces as a result.
3. COMPACTNESS : - It is possible to achieve high energy density compact device. It is due to the reason that in case of magnetic refrigeration the working substance is a social material (say gadolinium) and not a gas as in case of vapor compression cycles.
4. RELIABILITY : - Due to the absence of gas, it reduces concerns related to the emission into the atmosphere and hence is reliable one.
1. Competition in global market :-Research in this field will provide the opportunity so that new industries can be set up which may be capable of competing the global or international market.
2. Low capital cost :-The technique will reduce the cost as the most inefficient part comp. is not there and hence the initial low capital cost of the equipment.
3. Key factor to new technologies :-If the training and hard wares are developed in this field they will be the key factor for new emerging technologies in this world.
(present and future)
1. Development of optimized magnetic refrigerants (large magneto caloric effect):- These days we are trying to develop the more effective magnetic refrigerators with the help of some other refrigerants so that large magneto caloric effect can be produced. This research work is under consideration. We are trying to find the refrigerant element which can produce the optimum refrigeration effect.
2. Performance simulations of magnetic refrigerants:-Under the research we are studying the performance of various refrigerants and trying to simulate them. This will help us to develop the technology the most and at a faster rate.
3. Design of a magnetic liquefier:- The storage of hydrogen is also a big problem. The magnetic liquefier developed so far solves this problem. The magnetic liquefier is a device based on magnetic refrigeration which help us to store the hydrogen at a low temperature and after that it can be used for various purposes.
Only a limited number of magnetic materials possess a large enough magneto caloric effect to be used in practical refrigeration systems. The search for the "best" materials is focused on rare-earth metals, either in pure form or combined with other metals into alloys and compounds.
The magnetocaloric effect is an intrinsic property of a magnetic solid. This thermal response of a solid to the application or removal of magnetic fields is maximized when the solid is near its magnetic ordering temperature.
The magnitudes of the magnetic entropy and the adiabatic temperature changes are strongly dependent upon the magnetic order process: the magnitude is generally small in antiferromagnets, ferrimagnets and spin glass systems; it can be substantial for normal ferromagnets which undergo a second order magnetic transition; and it is generally the largest for a ferromagnet which undergoes a first order magnetic transition.
Also, crystalline electric fields and pressure can have a substantial influence on magnetic entropy and adiabatic temperature changes.
Currently, alloys of gadolinium producing 3 to 4 K per tesla of change in a magnetic field can be used for magnetic refrigeration or power generation purposes.
Recent research on materials that exhibit a giant entropy change showed that Gd5(SixGe1 ? x)4, La(FexSi1 ? x)13Hx and MnFeP1 ? xAsx alloys, for example, are some of the most promising substitutes for Gadolinium and its alloys (GdDy, GdTy, etc...). These materials are called giant magnetocaloric effect materials (GMCE).
Gadolinium and its alloys are the best material available today for magnetic refrigeration near room temperature since they undergo second-order phase transitions which have no magnetic or thermal hysteresis involved.
Magnetic refrigeration requires excellent heat transfer to and from the solid magnetic material. Efficient heat transfer requires the large surface areas offered by porous materials. When these porous solids are used in refrigerators, they are referred to as "regenerators?. Typical regenerator geometries include:
(b) Perforated plates
© Wire screens
(d) Particle beds
Super Conducting Magnets
Most practical magnetic refrigerators are based on superconducting magnets operating at cryogenic temperatures (i.e., at -269 C or 4 K).These devices are electromagnets that conduct electricity with essentially no resistive losses. The superconducting wire most commonly used is made of a Niobium-Titanium alloy.
Only superconducting magnets can provide sufficiently strong magnetic fields for most refrigeration applications.
A typical field strength is 8 Tesla (approximately 150,000 times the Earth's magnetic field).An 8 Tesla field can produce a magneto caloric temperature change of up to 15 C in some rare-earth materials.
Active Magnetic Regenerators (AMR's)
A regenerator that undergoes cyclic heat transfer operations and the magneto caloric effect is called an Active Magnetic Regenerator (AMR).An AMR should be designed to possess the following attributes:
These requirements are often contradictory, making AMR's difficult to design and fabricate.
1. High heat transfer rate
2. Low pressure drop of the heat transfer fluid
3. High magneto caloric effect
4. Sufficient structural integrity
5. Low thermal conduction in the direction of fluid flow
6. Low porosity
7. Affordable materials
8. Ease of manufacture
A Rotary AMR Liquefier
The Cryofuel Systems Group at UVic is developing an AMR refrigerator for the purpose of liquefying natural gas. A rotary configuration is used to move magnetic material into and out of a superconducting magnet.
This technology can also be extended to the liquefaction of hydrogen.
The magneto caloric effect can be utilized in a thermodynamic cycle to produce refrigeration. Such a cycle is analogous to conventional gas-compression refrigeration:
The added advantages of MR over Gas Compression Refrigerator are compactness, and higher reliability due to Solid working materials instead of a gas, and fewer and much slower moving parts our work in this field is geared toward the development of magnetic alloys with MCEs, and phase transitions temperatures suitable for hydrogen liquefaction from Room temperature down to 20 K.
We are also collaborating with The University of Victoria (British Columbia, Canada), on the development of an experimental system to prove the technology.
ADVANTAGES OVER VAPOUR COMPRESSION CYCLES:-
Magnetic refrigeration performs essentially the same task as traditional compression-cycle gas refrigeration technology. Heat and cold are not different qualities; cold is merely the relative absence of heat. In both technologies, cooling is the subtraction of heat from one place (the interior of a home refrigerator is one commonplace example) and the dumping of that heat another place (a home refrigerator releases its heat into the surrounding air). As more and more heat is subtracted from this target, cooling occurs. Traditional refrigeration systems - whether air-conditioning, freezers or other forms - use gases that are alternately expanded and compressed to perform the transfer of heat. Magnetic refrigeration systems do the same job, but with metallic compounds, not gases. Compounds of the element gadolinium are most commonly used in magnetic refrigeration, although other compounds can also be used
Magnetic refrigeration is seen as an environmentally friendly alternative to conventional vapor-cycle refrigeration. And as it eliminates the need for the most inefficient part of today's refrigerators, the compressor, it should save costs. New materials described in this issue may bring practical magneto caloric cooling a step closer. A large magnetic entropy change has been found to occur in MnFeP0.45As0.55 at room temperature, making it an attractive candidate for commercial applications in magnetic refrigeration.
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