Chemical Engineering March 2017 - 18

Thermo Fisher Scientific
Tenn.; www.ornl.gov) is developing
one such technology, leveraging a
thermal phenomenon known as the
magnetocaloric effect. The key to
magnetocaloric cooling is the precise
application of a magnetic field
to specialized powdered metallic
materials. The magnetocaloric materials
can expel and absorb heat
through a cycle of being magnetized
and de-magnetized. " Studies have
shown that these materials have
the potential to be 20-25% more
efficient than conventional vaporcompression
systems, " says Ayyoub
Momen, lead researcher for ORNL's
magnetocaloric refrigeration project,
which is working along with GE
Appliances toward commercializing
the first magnetocaloric refrigerator.
" When you put these materials inside
of a magnetic field, their temperature
suddenly goes up, " explains Momen,
" but when you remove the magnet,
their temperature goes down. " The
project aims at leveraging this cooling
effect in a refrigerator. The critical
property for magnetocaloric materials
is the temperature at which they
lose their magnetism, or the Curie
temperature. Researchers at ORNL
have fine-tuned cooling performance
by layering as many as 15-20 different
magnetocaloric materials based
on their Curie temperatures to expand
the temperature span of the
refrigerator, says Momen.
Beyond manipulating the magnetocaloric
materials to improve their
cooling behavior, a second facet of
the research is to design the refrigeration
machine itself (Figure 2). Inside
the refrigeration process, the
magnetocaloric materials are periodically
magnetized and de-magnetized
while a working fluid, such as
water or glycol or a mixture of the
two, moves into and out of the system.
As the fluid passes through the
particulate materials, on one side, a
cooling effect is generated, while the
other side generates a heating effect.
Analogous to a vapor-compression
refrigeration cycle, here, the cool side
acts as the evaporator and the hot
side acts as the condenser.
" One of the main challenges, from
a performance point of view, is the
pressure drop, " says Momen. Other
18
design challenges he mentions
are the complex valving system
required for the process and
optimizing the design to bring
down system costs. " It is iterative
research that we are performing
right now, " he explains.
The system's overall
safety
is
another advantage, since the
magnets are shielded, making
exposure to the magnetic field
unlikely. Also, the use of the innocuous
working fluids and solid
magnetocaloric " refrigerants "
decreases leakage concerns.
Additionally, the magnets and
the refrigerant materials can be
recycled when their service life
is up, adds Momen. " The target
is to make the system as robust
as a conventional refrigerator with a
lifetime of around ten to fifteen years, "
he explains. In addition to investigating
magnetocaloric cooling for refrigerator
applications, the ORNL team is
also looking into scaling the system
into a small air-conditioning unit.
Thermoelectric devices - those
that leverage the heat flux between
two materials of differing conductivity
and require no refrigerant chemicals
- are also being increasingly
considered for cooling applications.
A new refrigerated incubation system
from Thermo Fisher Scientific Inc.
(Waltham, Mass.; www.thermofisher.
com) is based on the thermoelectric
Peltier effect, providing both cooling
and heating in a single module.
The company developed the Peltierbased
Heratherm incubator (Figure
3) in order to overcome some of the
disadvantages of traditional compressor-based
cooling processes
used in similar products, explains
Konrad Knauss, global product manager
of Thermo Fisher's constant
temperature products. Compressorbased
systems are especially energy
consuming when systems must
regulate temperatures near ambient,
because both the compressor and
heating element run simultaneously
to stabilize temperature. In tests conducted
by Thermo Fisher, the Peltier
units consumed a fifth of the energy
required for running a compressorbased
cooling system.
Unlike compressor systems, the
Heat output
Functional principle of Peltier
module in cooling mode
Heat output
Cooling
External fan
Chamber fan
Environment
installation
location
Usable space
Cooling
Heat output
FIGURE 3. Based on Peltier thermoelectric technology, this
refrigerated incubator consumes much less energy than a
traditional compressor-based cooling system
Peltier system generates no vibrations.
Another advantage is that the Peltier
system generates very little heat exhaust,
so its operation will not impact
a laboratory's ambient conditions.
Within the Peltier element, the connection
between two metals with
different electric conductivities promotes
heating on one side and cooling
on the other side. " In the refrigerated
incubator, we switch the sides
when we need cooling or heating -
the cool side is on the inside, and the
hot side is on the outside, and vice
versa, " explains Knauss. While the
Peltier effect is extremely efficient in
the near-ambient range, there are efficiency
disadvantages when operating
at extremely high or low temperatures,
says Knauss.
Although Peltier cooling systems
are available commercially for smallscale
applications like household wine
coolers, Knauss says refrigerated incubation
is the first commercial application
of the technology in the industrial
science sector. Peltier Heratherm
incubators are currently available in a
178-L benchtop model and a 381-L
floor model, but with the addition of
more internal
Peltier elements, the
system could effectively be expanded
for higher capacities. According to
Knauss, the company plans to eventually
scale up the system for a larger
offering, but there are several challenges
to overcome with regard to
cost and ease of operation.
■
Mary Page Bailey
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
MARCH 2017
http://www.ornl.gov http://www.thermofisher http://WWW.CHEMENGONLINE.COM
Chemical Engineering March 2017

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