The Case for Using Carbon Dioxide as a Refrigerant

March 29, 2001
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The Montreal Protocol regulations on gases that deplete the earth’s ozone layer have led to the phaseout of chlorofluorocarbons (CFCs) as refrigerants in industrialized countries. Moreover, hydrochlorofluorocarbons (HCFCs) are only an interim solution in industrialized countries until the year 2020. In fact, certain national regulations prescribe an even earlier phaseout date (for instance, by the end of the year 1999 for R-22 in Germany).

Another environmental concern regarding these refrigerants is their behavior as greenhouse gases in the atmosphere. This also applies to the newly developed hydrofluorocarbons (HFCs). For this reason, these new HFC refrigerants are placed in a basket with five other gases covered by the Kyoto Protocol on greenhouse gases. This situation has led to increased use of the “old” refrigerants — ammonia and hydrocarbons. Although both are environmentally benign, they can exhibit a certain degree of local danger because of their flammability and/or toxicity. Therefore, carbon dioxide (CO2), an “old” refrigerant used in industrial and marine refrigeration, was proposed by the late Prof. Gustav Lorentzen in 1990 to be used as an alternative refrigerant, mainly because of its non-flammability.

Ozone Depletion Potential

As opposed to CFCs and HCFCs, ammonia, hydrocarbons, and CO2 all have an Ozone Depletion Potential (ODP) of zero and a negligible Global Warming Potential (GWP). As for HFCs, their ODP is zero. Their GWP ranges from a few hundred in the case of the flammable HFC-32 to several thousand in the case of the flammable HFC-143a and the non-flammable R-125. With respect to the safety of “old” refrigerants, only CO2 can compete with the non-flammable HFCs.

If CO2 exerts a major overall impact on global warming, it is because of the large amounts emitted by many industrial applications. However, contrary to HFCs, its GWP is negligible when applied as a refrigerant. Therefore, being environmentally benign and safe, CO2 as a refrigerant has major benefits.



The Carbon Dioxide Regerating Cycle

The following question remains: Is CO2 also well suited for application in refrigeration, air conditioning, and heat pump systems?

Here, because of its thermodynamic properties, CO2 differs from the other refrigerants mentioned. Its vapor pressure is much higher. Its critical temperature is around 31°C, because the heat discharge into the ambient atmosphere above this temperature is impossible through condensation, as happens in the normal vapor compression cycle.

CO2 can only be used in the classic and very efficient refrigeration cycle when heat discharge temperatures are lower than the critical temperature (e.g., when used in the lower stage of a cascade system, with another refrigerant being used in the higher stage).

For heat rejection at supercritical pressure, only gas cooling, not condensation, is possible. This leads to the cycle known as the trans-critical cycle. It was proposed by Lorentzen and his coworkers for automotive air conditioning and heat pump systems.

This trans-critical cycle is not new. It has been well known since the last century as the Linde-Hampson process for air liquefaction, based on the Joule-Thomson effect. In this context, it shows a certain lack of efficiency.

In classic refrigeration, air conditioning and heat pump applications, the principal energetic drawback of the trans-critical cycle with CO2 has to be taken into account. Therefore, it should only be applied where the environmental advantage is obvious and/or local safety is necessary, either of these measures compensating for the energetic drawback.



Applications of Carbon Dioxide

Refrigerant (direct effect) and CO2 emissions from energy supply to refrigerating systems (indirect effect) both contribute to greenhouse gas emissions expressed by using Total Equivalent Warming Impact (TEWI). Therefore, refrigeration systems with a high degree of emission are preferred application areas for CO2 as an alternative refrigerant, as long as the energy efficiency, defined as Coefficient of Performance (COP), can be kept at the same level.

In the 1991 Technical Options Report of UNEP, automotive air conditioning was identified as the application with the largest refrigerant consumption worldwide and the highest direct effect on TEWI, expressed as a percentage. Therefore, Lorentzen and his coworkers first drew attention to the application for CO2 as a refrigerant. Out of necessity, they employed the trans-critical cycle because of higher outside air and heat discharge temperatures when running mobile air conditioning systems. But the entire transport sector can be a main application for CO2 as a refrigerant.

Commercial refrigeration, including systems used in supermarkets, also has a rather large impact on TEWI due to the long refrigerant lines and the large refrigerant charges. Cascade systems with CO2 as the low-temperature refrigerant in a classic vapor compression cycle, or CO2 as secondary refrigerant, are possibilities enabling reduction of greenhouse gas emissions of refrigerants without the disadvantage of higher energy consumption.

The third largest quantity of refrigerant emission per system is shown by the unit air conditioning and heat pump systems. In the heat-pump application, unit systems and chillers offer good perspectives for CO2 as a refrigerant, thanks to use of the trans-critical cycle. The heat rejected on the high-temperature side is used for space heating or hot water production.

Since the trans-critical cycle also shows a temperature glide in the gas cooler, the temperature profiles of the refrigerant and the secondary fluid can be advantageously adapted in order to minimize heat-transfer loss and hence improve energy efficiency. Good results can be achieved only with similar and rather large temperature intervals on both sides, so the preferred application should be hot air or water production.



Advantages of Carbon Dioxide

In the trans-critical cycle, gas cooler pressure and temperature are not linked as in the sub-critical two-phase region.

Since the high-side pressure greatly affects — via the pressure ratio — compressor work and efficiency, high temperatures can be achieved with reasonable compressor power. Therefore, the application of CO2 in heat pumps (e.g., for hot water at 90°C) can be an excellent goal.

The high vapor pressure leads not only to a low-pressure ratio with the advantage of high compressor efficiency, but also to high heat transfer coefficients and low relative pressure losses. Thus, despite the lack of efficiency of the theoretical trans-critical cycle, the CO2 supercritical refrigeration cycle may still compete with the vapor compression cycle using other refrigerants.

A further advantage related to the use of CO2 is its higher volumetric capacity due to its high working pressures enabling small equipment components and small-diameter lines to be used. Also, the fact that one is not forced to recover, reclaim, or recycle the CO2 refrigerant means that CO2 appears to be very attractive in certain applications where the infrastructure is poor or too expensive, as in developing countries.



Drawbacks of Carbon Dioxide

The main drawback of carbon dioxide as a refrigerant is its inherent high working pressure. This pressure is much higher than that of the other natural and synthetic refrigerants mentioned.

On one hand, this means that for CO2 cycles, components must be redesigned. Since CO2 offers a much higher volumetric capacity, the problem of the higher working pressure can be overcome by optimal design involving smaller, stronger components.

Nevertheless, newly designed components have to be produced and can only be manufactured at reasonable prices if mass-produced in sufficient numbers. This can be a big hurdle to surmount before CO2 technology is introduced in refrigeration, air conditioning, and heat pump systems. If, for instance, the automotive and transport industries decided to move to this technology, other fields would benefit from mass-produced low-price components.



CONCLUSION

CO2 technology is very attractive as it is environmentally benign and safe, but needs a breakthrough enabling mass production of the necessary components in order to be cost-competitive compared with conventional refrigeration, air conditioning, and heat pump technology.

Extensive research and developmental work on CO2 technology has been performed internationally over the past nine years. This has especially been the case within two large European Community projects: the EC RACE Project for the development of CO2 automotive air conditioning systems and the EC COHEPS Project for CO2 heat pumps. These projects have shown that CO2 technology can compete with common technology in automotive and transport air conditioning. It works as well as in heat pumps providing high temperatures for hydronic systems and for the production of hot water at temperatures of up to 90°C. It also works well for industrial drying processes.

However, it should also be mentioned that there are some applications in which the nature of the less efficient trans-critical cycle cannot be compensated for by taking advantage of the particular operating conditions of the system. In conclusion, CO2 technology can meet the environmental and safety requirements of today’s challenges for refrigeration, air conditioning and heat pump systems, but only in suitable applications where the advantages outweigh the drawbacks of this technology.

Billiard is director of the International Institute of Refrigeration, located in Paris, France. For more information, visit www.iifiir.org (website).

Publication date: 04/02/2001

Sidebar: Out-of-Industry Source Puts Some CFCs in Pipeline

An Oklahoma company has found a way to keep CFCs in the pipeline by going well beyond traditional hvacr suppliers. Iceberg Refrigerant of Oklahoma City, OK, talked about the concept at the 15th Annual Mechanical Service Contractors of America (MSCA) Conference late last year.

Basically, booth officials said the company receives refrigerants from the pharmaceutical industry. The refrigerants were intended for such uses as part of a propellant for asthma inhalers, but were rejected for reasons unrelated to the quality of the refrigerant. (CFCs remain available to the pharmaceutical industry because of various regulatory exemptions.) Iceberg then “receives the mixed virgin refrigerants, separates them into their pure components and packages the pure components for use in the refrigeration industry,” booth personnel said. Refrigerants consist of CFC-11, -12, and –114, as well as HFC-134a.

R-11 comes packaged in 100-lb. containers and 55-gallon drums. R-11, R-114, and R-134a are packaged in 30-lb non-returnable cylinders.

For more information, contact Iceberg at 1300 W. Main St., Oklahoma City, OK 73106; 405-236-4255.

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