 |
 esearchers are making progress in perfecting automotive
and portable air-conditioning systems that use environmentally
friendly carbon dioxide as a refrigerant instead of conventional,
synthetic global-warming and ozone-depleting chemicals.
 It was
the refrigerant of choice during the early 20th century but was
later replaced with man-made chemicals. Now, carbon dioxide may
be on the verge of a comeback, thanks to technological advances
that include the manufacture of extremely thin yet strong aluminum
tubing.
Engineers
discussed their most recent findings last month during the Gustav
Lo-rentzen Conference on Natural Working Fluids, one of three
international air-conditioning and refrigeration conferences
held concurrently at Purdue University. Unlike the two other
conferences, the biannual Gustav Lorentzen Conference, which
is being held for the first time in the United States, focuses
on natural refrigerants that are thought to be less harmful to
the environment than synthetic chemical compounds.
"The
Gustav Lorentzen Conference focuses on substances like carbon
dioxide, ammonia, hydrocarbons, air and water, which are all
naturally occurring in the biosphere," says James Braun,
an associate professor of mechanical engineering at Purdue who
heads the organizing committee for all three conferences. "Most
of the existing refrigerants are man-made."
Purdue engineers
presented several papers detailing new findings about carbon
dioxide as a refrigerant, including:
- Creation of the first computer model that
accurately simulates the performance of carbon-dioxide-based
air conditioners. The model could be used by engineers to design
air conditioners that use carbon dioxide as a refrigerant.
- The design of a portable carbon-dioxide-based
air conditioner that works as well as conventional military "environmental
control units." Thousands of the units, which now use environmentally
harmful refrigerants, are currently in operation. The carbon
dioxide unit was designed using the new computer model. A prototype
has been built by Purdue engineers and is being tested.
- The development of a mathematical "correlation,"
a tool that will enable engineers to design heat exchangers -
the radiator-like devices that release heat to the environment
after it has been absorbed during cooling or future carbon dioxide-based
systems. The mathematical correlation developed at Pur-due, which
will be published in a popular engineering handbook, enables
engineers to determine how large a heat exchanger needs to be
to provide cooling for a given area.
- The development of a new method enabling
engineers to predict the effects of lubricating oils on the changing
pressure inside carbon dioxide-based air conditioners. Understanding
the drop in pressure caused by the oil, which mixes with the
refrigerant and lubricates the compressor, is vital to predicting
how well an air conditioner will perform.
Although
carbon dioxide is a global-warming gas, conventional refrigerants
called hydrofluorocarbons cause about 1,400 times more global
warming than the same quantity of carbon dioxide. Meanwhile,
the tiny quantities of carbon dioxide that would be released
from air conditioners would be insignificant, compared to the
huge amounts produced from burning fossil fuels for energy and
transportation, says Eckhard Groll, an associate professor of
mechanical engineering at Purdue.
Carbon dioxide
is promising for systems that must be small and lightweight,
such as automotive or portable air conditioners. Various factors,
including the high operating pressure required for carbon-dioxide
systems, enable the refrigerant to flow through small-diameter
tubing, which allows engineers to design more compact air conditioners.
More stringent
environmental regulations now require that refrigerants removed
during the maintenance and repair of air conditioners be captured
with special equipment, instead of being released into the atmosphere
as they have been in the past. The new "recovery" equipment
is expensive and will require more training to operate, important
considerations for the US Army and Air Force, which together
use about 40,000 portable field air conditioners. The units,
which could be likened to large residential window-unit air conditioners,
are hauled into the field for a variety of purposes, such as
cooling troops and electronic equipment.
"For
every unit they buy, they will need to buy a recovery unit,"
Groll says. "That's a significant cost because the recovery
unit is almost as expensive as the original unit. Another problem
is training. It can be done, but it's much more difficult than
using carbon dioxide, where you could just open a valve and release
it to the atmosphere."
The recovery
requirement would not apply to refrigerants made from natural
gases, such as carbon dioxide, because they are environmentally
benign, says Groll, who estimates that carbon dioxide systems
probably will take another five to 10 years to perfect.
Carbon dioxide
was the refrigerant of choice a century ago, but it was later
replaced by synthetic chemicals.
"It was
actually very heavily used as a refrigerant in human-occupied
spaces, such as theaters and restaurants, and it did a great
job," says Groll, who is chair of the Gustav Lorentzen Conference.
But one drawback
to carbon dioxide systems is that they must be operated at high
pressures, up to five times as high as commonly seen in current
technology. The need to operate at high pressure posed certain
engineering challenges and required the use of heavy steel tubing.
During the
1930s, carbon dioxide was replaced by synthetic refrigerants,
called chlorofluorocarbons, or CFCs, which worked well in low-pressure
systems. But scientists later discovered that those refrigerants
were damaging the Earth's stratospheric ozone layer, which filters
dangerous ultraviolet radiation. CFCs have since been replaced
by hydrofluorocarbons, which are not hazardous to the ozone layer
but still cause global warming.
However, recent
advances in manufacturing and other technologies are making carbon
dioxide practical again. Extremely thin yet strong aluminum tubing
can now be manufactured, replacing the heavy steel tubing.
Carbon dioxide
offers no advantages for large air conditioners, which do not
have space restrictions and can use wide-diameter tubes capable
of carrying enough of the conventional refrigerants to provide
proper cooling capacity. But another natural refrigerant, ammonia,
is being considered for commercial refrigeration applications,
such as grocery store display cases, Groll says.
Engineering
those systems is complicated by the fact that ammonia is toxic,
requiring a more elaborate design in which the ammonia refrigerant
is isolated from human-occupied spaces. The first ammonia systems
are currently being tested in Europe, and results will be presented
during the Gustav Lorentzen Conference, Groll says.
Groll's work
is funded by the US Army, Air Force and the American Society
of Heating, Refrigerating and Air-Conditioning Engineers, as
well as the Air Conditioning and Refrigeration Technology Institute.
Purdue
University News Service, 1132 Engineering Administration Building,
West Lafayette, IN 47907-1132; 765-494-2096; fax 765-494-0401.
Related
Web sites:
- The conferences:
- www.ecn.purdue.edu/Herrick/Events/Conference/2000conf/index.whtml.
- American Society of Heating, Refrigerating
and Air-Conditioning Engineers:
- www.ashrae.org/
- James Braun's Web page:
- ME.www.ecn.purdue.edu/ME/Fac_Staff/braun.whtml
- Eckhard Groll's Web page:
- ME.www.ecn.purdue.edu/ME/Fac_Staff/groll.whtml
|
