Refrigerators, ice-boxes and freezers: daily objects which are not often thought about, except when standing in front of one, looking for something to snack on. The history behind this technology is much more interesting than might seem at first. Especially since it is being continuously further developed. Since the 1990s, it has become clear, that a new era of cooling technology needs to commence. Researchers had discovered, that a main component of the commonly used refrigerants was environmentally damaging: the chlorofluorocarbon (CFCs). This organic molecule reacts with the ozone layer in the stratosphere, which is split apart. Various new chemical bonds are formed which steadily reduce the amount of ozone. A CFC-ban was eventually introduced and alternative fluids, namely the fluorocarbons (FCs) were implemented as new refrigerants. These FCs are not damaging to the ozone but still contribute to the greenhouse gas potential. An EU-ordinance from 2006 (amended 2014) called for procedures for the reduction of damaging refrigerant-emissions from cooling systems. A worldwide policy was introduced with the Montreal Protocol 2016. 200 countries agreed to the step-wise but continuous reduction of FC implementation.
“The interest in environmentally friendly and efficient cooling technologies is currently very high,” reports Dr. Kilian Bartholomé from Fraunhofer-Institute for Physical Measurement Techniques (IPM). He joined forces with colleagues from the Department for Applied Mathematics of the University of Freiburg in order to tackle this complex subject. “We wanted to combine the benefits of magentocaloric materials with a new heat transfer concept based on heatpipes, in order to develop a new form of sustainable cooling.” With this goal in mind, the researchers from the University of Freiburg and Fraunhofer IPM collaborated in the pilot project “ActiPipe: Active heatpipes for sustainable cooling”, funded by the Sustainability Center Freiburg (LZN).
A cooling system based upon magnetocaloric (MK) materials would prove quite beneficial as compared to the conventional compressor-technology. First, environmentally damaging refrigerants are no longer required. On the other hand, this system allows for a large potential efficiency factor. The set-up also includes only a few parts susceptible to wear which leads to less maintenance expense as compared to common cool systems. A further not unimportant benefit of working with heatpipes is, that the system works significantly quieter.
MK-materials have been utilized for many years but a combination thereof with the so-called heatpipes has thus far never been examined. For the examination undertaken by the ActiPipe group, a LaFeSi alloy was chosen as a magentocaloric sample material. This alloy is well suited for implementation in a cooling system as it has a large magnetic entropy change and an adiabatic temperature change. The manufacturer thereof can also be completed on an industrial scale.
The principle behind cooling with MK-materials: the MK-material is placed into a magnetic field whereupon it becomes warm due to the created magnetic alignment. This created warmth is then removed and the Mk-material cools down once more; it cools down to its starting temperature. The MK-material is then removed from the magnetic field by which it cools even further down. In order to reach its starting temperature, it is attached to the cooling site and collects heat. In the current magnetocaloric systems, a fluid (generally water) is pumped back and forth in the so-called “bed” of the system. However, the cycle frequency is limited due to the restricted thermal transfer. At the same time, frictional loss occurs due to the high pressure which in turn also limits the system efficiency.
The researchers decided thereupon, to transfer the thermal transmission concept from heatpipes into their system. A heatpipe is nothing more than a gas-tight pipe (usually copper), from which all foreign gases have been removed and which contains a small amount of liquid (generally water). If one side of the pipe is heated, vaporizes the fluid on that side and then condenses on the cold side of the pipe. Thereby, very high rates of heat transfer can be reached.
In the ActiPipe project, magentocaloric materials along with pressure relief valves developed at IPM were integrated into a heatpipe. When the MK-material was magnetized and thereby warmed, the fluid evaporated with the result that the vapor pressure increased in that segment. The vapor then moved via the pressure relief valves into the neighboring element and condensed there, carrying with it thermal energy. If the magnet is moved to the next segment, the MK-material in the first segment cools back down and reaches its starting temperature. The pressure sinks, a vacuum is created. The result: the vapor and with it the warmth from the previous segment is collected. Such a system is also suitable as a heating mechanism.
“In our ActiPipe project, we created a complete experimental system based upon this theoretical concept,” explains Dr. Bartholomé. “Our university colleagues were busy with the development of mathematical descriptions of the model of a heatpipe. We at IPM meanwhile determined that our set-up worked very well and that we could create with two segments, a cooling cycle with a frequency of six hertz, which is an amazing result,” reports Bartholomé very proudly. “However, the heat pump power is not quite as high as we would like it.” Therefore, the set-up needs to be fine-tuned a bit more but the team is pleased with their first results. The models developed by the university team also provided important new observations. These include the exact modelling of how a drop of water condenses down an inner wall. With such simulations, it will be possible in the future to examine various parameters and how they influence the system performance.
Further projects handling the development of caloric cooling systems are being carried out with various other researchers, including Fraunhofer researchers. As part of the project “MagCon” (Magnetocalorics: Development of refrigerant-free, highly efficient heat pumps for heating and cooling), IPM researchers are working closely with colleagues from Fraunhofer IFAM to develop an efficient, magnetocaloric heat pump. ActiPipe is also embedded in a different project, namely “EC-Cool”. In this project, electrocaloric materials are being analyzed. The principle behind EC-Cool is quite similar to ActiPipe, just that the researchers are working with electrical fields instead of magnetic fields.
The first industrial partners have already voiced their interest in the first results of ActiPipe. Together with the Philipp Kirsch GmbH, the Vacuumschmelze GmbH and GSI Technology UG, the IPM is currently working on the development of a magentocaloric system for the medical technology sector. Goal of this collaboration is the development of an efficient medical refrigerator which must be able to reach very low temperatures. Dr. Bartholomé has high hopes for the future work being done: “Until we can have our first functioning magnetocaloric systems, we are going to continuously improve and fine-tune our system. But we are already well on our way to the end-product.”