Global warming is the matter of grave concern and has accompanied one more is that the aspiration regarding the quality of life. Eventually, this quality now depends on the cooling appliances and so the energy requirements for cooling processes. This is still fine to desire the cooling systems but the problem is that most coolants cause environmental and health damage.
Considering this concern, a novel technology has emerged as refrigeration using magnetic materials in magnetic fields. Researchers at the Technische Universität (TU) Darmstadt and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have developed an idea, which is materialized in six steps, a cooling cycle based on the ‘magnetic memory’ of special alloys.
Already known concept is the magnetic properties of metals can be changed when cooled or heated.
Oliver Gutfleisch, Professor of Functional Materials at the TU Darmstadt, said, “Iron, for example, is only ferromagnetic below 768 degrees Celsius; nickel’s transformation temperature is 360 degrees Celsius.”
“Conversely, some alloys become ferromagnetic when they warm up. This phase transition is connected with the so-called magnetocaloric effect: when these shape-memory alloys are placed in an external magnetic field just below their transformation temperature, they spontaneously jump to their magnetic order and simultaneously cool down.”
He explained, “The stronger the magnetic field, the more they cool.”
The scientists also have discovered that if pressure applied externally can reverse the magnetization process, causing the alloy to heat up.
How did they conclude the results?
Six steps cycle
Both Prof. Antoni Planes and Prof. Lluís Mañosa from the University of Barcelona, the scientists succeeded in providing experimental proof. Dr. Tino Gottschall, who is now researching at the HZDR’s Dresden High Magnetic Field Laboratory (HLD), explained, “We used an alloy of nickel, manganese, and indium for our experiments because its conversion can be triggered at room temperature.”
The researchers generated the magnetic field utilizing the strongest permanent magnets in the first step; the rare-earth metal neodymium in addition to iron and boron. They can generate magnetic fields up to a flux density of 2 tesla – that is 40,000 times stronger than the Earth’s magnetic field.
Gottschall explained, “Under such conditions, our alloy cools down by several degrees.” “Measurements we have made at the HLD have shown that a millisecond in the magnetic field is already enough for permanent transformation.”
In the second step, the researchers removed the cooling element from the magnetic field, which retained its magnetization.
In step three, the heat sink comes into contact with goods to be cooled down and absorbs its heat. The alloy even remains magnetic if the material returns to its original temperature. This can be remedied by mechanical pressure in step four.
A roller compresses the shape memory alloy in step four. Under pressure, it switches to its denser, non-magnetic form and heats up in the process.
When the pressure is removed in step five, the material retains its state and remains demagnetized.
In the final step, the alloy releases heat into the environment until it has returned to its initial temperature and the cooling cycle can recommence.
Expensive raw materials used
Gutfleisch said, “Just a few years ago, alloys with a magnetic memory were regarded as unusable because they can only be cooled in the magnetic field once.” “Global research, therefore, focused on materials that have no memory effect. However, refrigerators produced according to this principle come at a price.”
“In the case of reversible magnetization, the cooling effect only lasts as long as the cooling element is exposed to the magnetic field. Even in the best-case scenario, half of the coolant must be placed between the magnets. This means that you need four times as much permanent magnet as cooling medium.”
The most effective Neodymium magnets are also the most expensive on the market. Similarly, the rare-earth metal is a critical raw material, and considerable amounts of it are required.
Electromagnets cannot be used for magnetic cooling. For physical reasons, the level of efficiency would be lower than with vapor compression, which is used in billions of refrigerators and air conditioners. However, the researchers are convinced that this cooling technology no longer has a future.
Gottschall says, “The coolants commonly used today are highly effective as heat-transfer media, but their greenhouse impact is a thousand times greater than that of carbon dioxide. The production licenses for most of them in Europe will expire in the near future. Propane or butane are effective coolants but form highly explosive mixtures in contact with the air. Ammonia is toxic and corrosive. Carbon dioxide is not especially effective as a coolant.”
Why would use rare earth sparingly?
Oliver Gutfleisch is convinced that the future belongs to solid coolants. The functional materials expert says, “We have been able to show that shape-memory alloys are highly suitable for cooling cycles.” “We need far fewer neodymium magnets but can nevertheless generate stronger fields and a correspondingly greater cooling effect.”