Replacement of CFCs
A. The first generation replacement of CFCs as refrigerants (HCFC)
HCFCs used as substitutes for CFCs are listed in Table 2.
HCFCs are compounds containing carbon, hydrogen, chlorine and fluorine. The
HCFCs have shorter atmospheric lifetimes than CFCs and deliver less reactive
chlorine to the stratosphere where the "ozone layer" is found. Consequently,
it is expected that these chemicals will contribute much less to stratospheric
ozone depletion than CFCs. Because they still contain chlorine and have the
potential to destroy stratospheric ozone, they are viewed only as temporary
replacements for the CFCs.
HCFCs are less stable than CFCs because HCFC molecules contain carbon-hydrogen bonds. Hydrogen is attacked by the hydroxyl radical in the lower part of the atmosphere known as the troposphere. When HCFCs are oxidized in the troposphere, the chlorine released typically combines with other chemicals to form compounds that dissolve in water and ice and are removed from the atmosphere by precipitation. When HCFCs become destroyed in this way their chlorine does not reach the stratosphere and contribute to ozone destruction.
A certain portion of HCFC molecules released to the atmosphere will reach the stratosphere and be destroyed there by photolysis (light-initiated decomposition). The chlorine released in the stratosphere can then participate in ozone depleting reactions as does chlorine liberated from the photolysis of CFCs. Because HCFCs are degraded significantly by two mechanisms in the atmosphere (as opposed to the CFCs which are destroyed almost exclusively by photolysis in the stratosphere), and because photolysis rates of HCFCs are generally slower than those for CFCs, proportionately less chlorine is released from HCFCs in the lower stratosphere when compared to CFCs. These properties explain why HCFCs are expected to deplete much less stratospheric ozone than equivalent amounts of CFCs. HCFCs phase-out dates are shown in Figure 3.
Figure 3: HCFC refrigerant phase-out dates (13)
B. The second generation replacement of CFCs as refrigerants (HFCs)
Table 3: HFCs as alternative for CFCs
|HFC-23 (CHF3)||HFC-143a (CF3CH3 )|
|HFC-32(CH2F2 )||HFC-152a (CH3CHF2)|
|HFC-43-10mee (CF3CHF2CHFCH2FCF3)||HFC-227ea (CF3CHFCF3)|
|HFC-125(CHF2CF3 )||HFC-236fa (CF3CH2CF3 )|
|HFC-134a (CH2FCF3 )||HFC-245fa (CF3CH2CHF2)|
Hydrofluorocarbons (HFCs) are compounds containing carbon, hydrogen, and fluorine.
Certain chemicals within this class of compounds are viewed by industry and
the scientific community as acceptable alternatives to CFCs and HCFCs on a long-term
basis. Because the HFCs contain no chlorine they do not directly affect stratospheric
ozone. Furthermore, mechanisms for ozone destruction involving fragments produced
as HFCs are decomposed within the atmosphere (CF3 radicals) have been shown
to be insignificant.All HFCs have an ozone depletion potential of 0.
Like HCFCs, the HFCs contain hydrogen that is susceptible to attack by the hydroxyl radical. Oxidation of HFCs by the hydroxyl radical is believed to be the major destruction pathway for HFCs in the atmosphere. Atmospheric lifetimes of the most commonly used HFCs (HFC-134a and HFC-152a) are limited to <12 years because of this reaction.
Although it is believed HFCs will not deplete ozone within the stratosphere, this class of compounds has other adverse environmental effects. It has been postulated that extensive use of these chemicals in the future could contribute significantly to enhanced radiative atmospheric heating. Also, a number of the HFCs, for example HFC-134a, are expected to decompose in the atmosphere and produce a long-lived chemical called trifluoroacetic acid (or TFA) that is known to have adverse effects on certain biota. Concern over these effects may make it necessary to regulate production and use of these compounds at some point in the future. (14)
C. The third generation replacement of CFCs
The third generation substitutes for CFCs are listed in Table 4.
D. The future replacement of CFCs as refrigerants
Looking further down the road, carbon dioxide may someday replace today’s refrigerants. Though carbon dioxide is a "greenhouse gas" that may contribute to global warming, it is non-toxic, non-flammable, cheap and abundant. But to work as a refrigerant, carbon dioxide must be run at extremely high pressures - up to several thousand psi! As long as the gas is safely contained at high pressure, it works pretty well as a refrigerant. But such high pressures pose a potential danger to technicians who must ultimately work on such systems.
Someday a perfect air conditioner may use no refrigerant whatsoever. Some time back, the Rovac Corporation in Rockledge, FL, announced it had developed a revolutionary A/C system that required no refrigerant at all, and used air itself as the working medium. The Rovac system used a "circulator" that was essentially an expander rather than a compressor. By expanding the volume of air, the drop in pressure produced a corresponding drop in temperature. The system supposedly required 35 to 40% less power than a refrigerant-based A/C system and provided equivalent cooling. Mitsubishi has licensed rights to the technology and was investigating it further. That was 20 years ago. We’re still waiting.
Ten years ago, the Naval Postgraduate School in Monterey, CA, claimed it had developed a refrigeration process using an acoustic generator. Sound waves were used to create pressure changes that had a chilling effect. No word as to what ever happened to this technology.
Although there are a lot of substitutes for CFCs as refrigerants, scientists continue to research new substitutes, which are less expensive, less destructive for ozone layer and more practical for industry. (16)