The use of CO2 as a refrigerant dates back more than a century, but it
fell out of favor in the air-conditioning and refrigeration industry with
the development of chlorofluorocarbons (CFCs) in the 1930s. Shortly thereafter,
hydrochlorofluorocarbons (HCFCs) such as HCFC-22 were developed, and HCFC-22
eventually became the primary refrigerant for stationary air-conditioning
systems. However, when concerns about the depletion of the stratospheric
ozone layer emerged in the 1970s, national and international agreements
were enacted to phase out CFCs and HCFCs.
At first, the phaseout of chlorine-containing refrigerants such as CFCs
and HCFCs led the industry toward another class of fluorocarbon refrigerants,
hydrofluorocarbons (HFCs) that did not contain chlorine and thus did not
harm the ozone layer. However, in the 1980s, scientists identified global
warming as a major environmental threat, and the global warming impact
of HFCs came under scrutiny, leading many researchers and manufacturers
to reconsider “natural” refrigerants such as CO2,
hydrocarbons, and ammonia, because these substances have negligible direct
impact and ozone-depletion potential. The signing and ratification by many
countries of the Kyoto Protocol has provided greater impetus to look for
alternatives to fluorocarbon refrigerants; several European countries have
already begun restricting their use and are planning for an eventual phaseout.
Carbon dioxide is non-flammable and non-toxic in contrast to other natural
refrigerants—hydrocarbons (flammable) and ammonia (flammable and
toxic). Furthermore, it is inexpensive, widely available worldwide from
numerous suppliers, and not subject to venting restrictions. The high operating
pressures of CO2 also provide a potential opportunity for system
size and weight reduction. The major challenge, however, is to design a
efficient, reliable system that accommodates the unique characteristics
of CO2, most significantly, five times the typical system operating
pressure and a low critical temperature that requires cooling a supercritical
rather than condensing a two-phase mixture.
The application areas attracting the most interest today for CO2 are
those where current system refrigerant leakage rates are high enough to
regulatory attention, as well as in high-temperature heat-pump applications
and in military cooling systems because of special logistics considerations.
Centralized refrigeration systems used in supermarkets are prone to leakage
due to the large number of refrigerant line joints, long runs of refrigerant
piping, and frequent thermal cycling. Carbon Dioxide can be used efficiently
in these systems, and some leakage can be tolerated. The same is true for
vehicular air-conditioning, where considerable engineering effort has been
expended by the major automobile manufacturers to develop prototype CO2-based
air-conditioners for cars and trucks. (Another potential advantage of the
CO2 cycle for vehicles is a heat-pump mode that delivers instant
heat in winter.) In Japan, CO2-based heat-pump water heaters
have been commercialized,
and design efforts are underway in the U.S. These heat pumps take advantage
of the high-temperature heat rejection from the transcritical CO2 cycle.
However, there is a downside to using CO2 as a refrigerant.
Many studies, both theoretical and experimental, have demonstrated that
efficiency of transcritical CO2 cycles is lower than that of
conventional fluorocarbon-based vapor compression systems, particularly
at high ambient
temperatures. This decrease in system efficiency could negate part or all
of the environmental advantage of the CO2 system by increasing
its indirect contribution to global warming due to the higher energy consumption.
it would likely be unacceptable from a marketing or regulatory standpoint
to introduce new air-conditioning and refrigeration systems with lower
efficiencies than existing units. Therefore, an approach to improving the
efficiency must be found in order to spur commercialization. Fortunately,
such an opportunity exists by recovering the losses that occur during the
expansion process as the refrigerant leaves the high-pressure gas cooler
and enters the evaporator.
In theory, recovery of energy lost during the expansion process in a vapor
compression cycle is of interest for any refrigerant. However, the relatively
large expansion losses attributable to the high operating pressures of
CO2 make a work-recovery device particularly important. Design
studies at my company have found that a reasonably efficient CO2 expander
on scroll technology can improve the efficiency of a CO2-based
system to parity with fluorocarbon-based equipment while achieving the
environmental benefits described.
With these potential benefits, why aren’t we seeing more research
or accelerated efforts by manufacturers to get CO2 on the market
sooner? The answer, of course, is that history has shown us that introducing
refrigerants is never easy. However, expect to see systems that accommodate
the unique characteristics of CO2 as a “green” refrigerant
in the years ahead.
Topping is director of Appliance Research at TIAX LLC (www.tiaxll.com).
He can be reached by phone at 617/498-6058, by fax at 617/498-7206,
or e-mail at Topping.R@tiaxllc.com. From the Top appears bimonthly
in APPLIANCE ENGINEER®.