Ames Lab and Astronautics Corporation of America, are making considerable progress towards a commercially viable refrigeration technology based on the magnetocaloric effect present in certain rare earth materials .
Magnetic refrigeration has been around for a long time, and was used principally to reach ultra low temperatures in cryogenics research. Developments on two fronts are mutually moving the technology towards room temperature and commercial application. One is the refrigeration cycle itself–new approaches have been developed, which are reaching performance at room temperature that is very competitive with vapor compression. A laboratory scale magnetic refrigerator built by Astronautics produces 600 watts of cooling power, achieves temperature span of 28 degrees K near room temperature with the lowest temperature being just above the freezing point of water, efficiencies up to 60% of Carnot, and a COPs of five to eight. It has been in continuous operation since December ’96. This work used traditional gadolinium spheres as the magnetic refrigerant.
The other key ingredient is the magnetic material. Ames Lab, a leader in the field of rare earth metals research, announced a breakthrough this summer of a giant magnetocaloric effect in new alloys of gadolinium, silicon and germanium. Magnetocaloric effect in these materials is 2 to 7 times larger than in other prototype refrigerant materials. Also, the operating temperature (the Curie point) can be tuned from -400 degrees F to 65 degrees F, by adjusting the ratio of silicon to germanium.
Magnetic refrigeration operates by magnetizing and demagnetizing the material, analogous to compression and expansion in a vapor cycle. However, magnetizing and demagnetizing losses are much less compared to friction losses during compression and expansion. Two ÒbedsÓ filled with magnetic material are pushed in and out of a magnetic field. As a bed enters high magnetic field space, it heats up (magnetocaloric effect) and the heat is picked up by a flow of heat transfer fluid (which is water in this laboratory scale magnetic refrigerator) and is dissipated into the surroundings. When a bed is pulled out of magnetic field, it cools down due to the reverse magnetocaloric effect, cooling the water.
The use of solid refrigerant material (gadolinium) and water as a heat transfer fluid offer another advantage compared to conventional vapor cycle refrigerators: it is the absence of harmful chemicals as liquid refrigerants that present serious environmental hazard .
Strong magnetic fields are needed, currently produced by superconducting magnets. However, the team is finding ways to lower the field required, while new developments in permanent magnets (materials, fabrication, and expiration of key patents) offer the possibility of simpler and less expensive systems. Also, high temperature superconductors are coming into their own, which likewise could change things dramatically.
The need for a strong field puts the economics in favor of larger systems, however smaller scale devices are also anticipated. The key differentiating features are:
1. Higher efficiency (which can be highly significant when power is limited–e.g. in an electric vehicle).
2. None of the environmental issues associated with
conventional liquid refrigerants.
3. Ability to cool continuously over a range of temperatures (e.g. in chilling a fluid stream) which is thermodynamically
4. Ability to scale down without significant losses of cooling efficiency, which is to the contrary of conventional vapor cycle refrigeration.
Initial applications will probably be in industrial and commercial (e.g. supermarkets) refrigeration, cooling and air conditioning. Other possibilities of interest to utilities are cooling of inlet air for combustion turbines, and district cooling.
The technology is at least five years from a practical commercial reality, however Ames and Astronautics are already fielding numerous inquiries from interested parties and potential partners. The developers are opened to the possibility of teaming with other companies who may do the manufacturing and marketing of actual products.
A number of technical and popular articles and other information are available from Ames.
Carl Zimm, Astronautics (principal investigator) Madison WI,
Karl Gschneidner, Ames Lab (principal investigator)
Vitalij Pecharsky, Ames Lab (principal investigator)
515-294-8220 , firstname.lastname@example.org
Alan Paau, Iowa State Univ. (intellectual property)
Todd Zdorkowski, Ames Lab (tech transfer)