One
of the difficulties with conventional vapour-compression refrigeration cycles
is that most of the better refrigerants are ozone depleting substances
consisting of chlorinated fluorocarbons (HCFCs) like freon gas.
'Freon' is a trade name for a family of haloalkane refrigerant manufactured by DoPont and other companies. These
refrigerants are commonly used due to their superior stability and safety
properties: they are not flammable nor obviously toxic as are the fluids they
replaced, such as Sulphur dioxide. Unfortunately, these
chlorine-bearing refrigerants reach the upper atmosphere when they escape. In
the stratosphere, CFCs break up due to UV-radiation, releasing their chlorine
atoms. These chlorine atoms act as catalyst in the breakdown of ozone, thus causing severe damage to the ozone layer that shields the Earth's surface from the Sun's strong UV radiation. The
chlorine will remain active as a catalyst until and unless it binds with
another particle, forming a stable molecule. So the major risk involved with
this refrigerator is that the manufacturers have to be careful with not to let
the harmful freon gas leak out. Newer refrigerants, currently being the subject
of research, have reduced ozone depletion effect that include HCFCs (R-22, used in most homes today) and HFCs
(R-134a, used in most cars) and have replaced most CFC use. HCFCs in turn are being
phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs), such as R-410A, which lack chlorine.
However, CFCs, HCFCs, and HFCs all have large global warming potential.
Supercritical carbon dioxide, known as R-744 have similar efficiencies compared to existing CFC and HFC based
compounds, and have many orders of magnitude lower global
warming potential .
Although modern refrigerators have replaced freon with a less harmful liquid,
other environmental cooling techniques are being actively explored. One novel
possibility is to use magnets to extract heat away, where rather than going
into the expansion of a gas—as in conventional refrigerators—the thermal energy
goes into disordering the aligned spins of a magnet. Magnetic refrigeration has
three prominent advantages compared with compressor-based refrigeration. First,
there are no harmful gases involved; second, it may be built more compactly as
the working material is a solid; and third, magnetic refrigerators generate
much less noise.
Magnetic
refrigeration utilizes the magnetocaloric effect (MCE). This effect causes a temperature
change when a certain metal is exposed to a magnetic field. All transition
metals and lanthanide series elements obey this effect. These metals, known as
ferromagnets, tend to heat up as a magnetic field is applied. As the magnetic
field is applied, the magnetic moments of the atom align. When the field is
removed, the ferromagnets cool down as the magnetic moments become randomly
oriented. A
magnetic field can easily align the spins on the manganese sites so that if the
magnetized material is allowed to come into thermal contact with a ‘hot’
object, then heat can depolarize the spins as per the scheme suggested in the
flow chart.
Soft ferromagnets are the most efficient and have very low heat loss due to
heating and cooling processes. Gadolinium, a rare-earth metal, exhibits one of the
largest known magnetocaloric effects. Also one can employ arc-melted alloys of
gadolinium, silicon, and germanium that provide greater temperature ranges at
room temperatures in the design of most modern magnetic refrigeration system.
Keeping this
principle in mind, hypothetically one can design a magnetic refrigerator as
follows: The heat transfer fluid for the magnetic refrigeration system may be a
liquid alcohol-water mixture having a given freezing point, assuring the
mixture does not freeze at the set operating temperatures. This heat transfer
fluid shall be cheaper than traditional refrigerants and also eliminates the
environmental damage produced from these refrigerants. During the operation the
heat transfer fluid gets cooled to desired lower level of temperature by the
non-magnetized cold set of beds that contain the small spheres of
magnetocaloric material. This cooled fluid is then sent to the cold heat
exchanger where it absorbs the excess heat from the freezer. This fluid leaves
the freezer at 0°F. The warm fluid then flows through the opposite magnetized
set of beds, where it is heated up to the desired higher level. This hot stream
is then cooled by room temperature air in the hot heat exchanger. The cycle
then repeats itself after the beds have switched positions back and forth to
the field while still keeping them in contact with the heat transfer plates.
Thus there will have an eco-friendly, non-compressor, noise free and highly
compatible refrigeration system for house hold and automobile cooling
applications. In this perspective, our group is engaged in developing such an
eco-friendly solid state system based on superpapramagnetic nanoferrites
especially for applications at near room temperature magnetic refrigeration.
