03-08-2013, 12:58 PM
Review on research of room temperature magnetic refrigeration
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Abstract
Room temperature magnetic refrigeration is a new highly efficient and environmentally protective technology.
Although it has not been maturely developed, it shows great applicable prosperity and seems to be a substitute for the
traditional vapor compression technology. In this paper, the concept of magnetocaloric effect is explained. The devel-
opment of the magnetic material, magnetic refrigeration cycles, magnetic field and the regenerator of room temperature
magnetic refrigeration is introduced. Finally some typical room temperature magnetic refrigeration prototypes are
reviewed.
Introduction
Although Montreal Protocol has been restricting the
harm to environment of ODS (Ozone Depletion Sub-
stance) to a great extent, the greenhouse effect problem
is not be solved completely yet. So, in addition to fur-
ther developing the vapor compression technology, sci-
entists and engineers have begun to explore new
refrigeration technology such as thermoelectric refrig-
eration, thermoacoustic refrigeration, absorption/
adsorption refrigeration, and magnetic refrigeration.
The study of magnetic refrigeration was started with
the discovery of magnetocaloric effect (MCE) 120 years
ago [1]. Then it has been used in cryogenic refrigeration
since 1930s. It is maturely used in liquefaction of
hydrogen and helium. In 1976, at Lewis Research Cen-
ter of American National Aeronautics and Space
Administration, Brown first applied the magnetic
refrigeration in a room-temperature range [2]. By
employing rare-earth metal gadolinium (Gd) as the
magnetic refrigeration working substance, he attained a
47 K no-load temperature difference in a 7 T magnetic
field.
Magnetic material
Selection of room temperature magnetic material
From the previous entropy analysis of magnetic
materials, only magnetic entropy SM is changeable with
the magnetic field change. In the range of room tem-
perature, the influence of lattice entropy SL is too
remarkable to neglect. Therefore, part of the cooling
capacity of the magnetic system is consumed for cooling
lattice system for the entropy flow from the lattice sys-
tem, though the temperature decreases to some extent
during adiabatic demagnetization. Thus the gross cool-
ing capacity is less than that of the condition of
(SL+SE)%0 [20].
Perovskite and perovskite-like compounds
Large magnetic entropy change has been found in the
perovskite manganese oxides in recent years, so that
these materials attract more and more attention. The
main advantages of this series of compounds over Gd
and GdSiGe alloys are low cost, non-active chemical
property (no oxidation), little coercive force as well as
high electric resistance. Many studies on these com-
pounds are led mainly in China, Spain and United
States [49–65]. From Table 1, it is clear that their Curie
temperature also can be easily tuned to the needed range
by introducing some kinds of metal additions. However,
ÁSM will decrease much in the meantime, lowering their
practicability.
Magnetic refrigeration cycle
Magnetic refrigerator completes cooling/refrigeration
by magnetic material through magnetic refrigeration
cycle. In general a magnetic refrigeration cycle consists
of magnetization and demagnetization in which heat is
expelled and absorbed respectively, and two other
benign middle processes.
The basic cycles for magnetic refrigeration are mag-
netic Carnot cycle, magnetic Stirling cycle, magnetic
Ericsson cycle and magnetic Brayton cycle, among
which the magnetic Ericsson and Brayton cycles are
applicable for room temperature magnetic refrigeration
for the Ericsson and Brayton cycles employ a regenerator
to achieve a large temperature span and are easy to
operate. Fig. 2 shows the Ericsson and Brayton cycles.
Structure and principle of AMR
A single-stage AMR is generally a porous bed of
magnetic refrigerant material, which acts as both the
refrigerant (coolant) that produces refrigeration and the
regenerator for the heat transfer fluid. Flowing through
the active magnetic regenerator the fluid carries heat to
and from the external heat exchangers.
The working principle of AMR is presented in Fig. 4
[106,110]. Assume that the bed is at a steady state con-
dition with the hot heat exchanger at TH ($ 24 C) and
the cold heat exchanger at TC ($ 5 C). The AMR cycle
experiences four processes: (a) adiabatic magnetization
process. Each particle in the bed warms up; (b) isofield
cooling process. In a high field, the fluid is blown from
the cold end to the hot end, absorbs heat from the bed
and expels heat at a temperature higher than TH in the
hot heat exchanger; © adiabatic demagnetization pro-
cess. Each particle in the bed cools again (after the for-
mer process); (d) isofield heating process.
Conclusion
Progresses of room temperature magnetic refrigera-
tion have been made worldwide. However, the develop-
ment of room temperature magnetic refrigeration is not
in mature status yet. Room temperature magnetic
refrigeration will be a new refrigeration method with
extreme potential on account of high efficiency and
environment-safe.