NATURAL REFRIGERANTS - SUMMARY

Under the Montreal protocol the application of CFC refrigerants in developed countries is forbidden since 1996, and in developing countries the phase-out will be implemented in 2010. HFC alternative refrigerants which are harmless to the ozone layer have appeared on the market, but they have a big potential for global warming, and they are put in groups covered by the Kyoto protocol. This was the reason for the use of natural refrigerants that were known before, such as ammonia, carbon-dioxide, hydro-carbons (isobutane, propane etc.), air and water, as well as new researches towards other alternative refrigeration technologies.

Ammonia use is becoming more and more widespread. In addition to large units in cold stores and industry, development has spread to small commercial units. Recent improvements include the minimization of ammonia charging for safety reasons, with production of high quality compact sealed systems which are factory assembled. With the indirect cooling concept the use of ammonia in air conditioning of buildings is increasing.

The use of CO2 has already started in cascade systems for temperatures down to -54°C, and in heat pumps for hot water. Improvement of the energy efficiency of the refrigerating cycles and development of commercial refrigerating system components are priorities, especially in vehicles air conditioning systems.

The use of hydro-carbons is increasing; isobutane is very widely used in domestic refrigerators and commercial units with charges up to 150 g (in some cases to 1 kg). Propane is an excellent substitute for R22, and can be used in air conditioning under safe conditions. Because of the safety reasons it is used mostly in systems with indirect cooling, and the possibility remains for its use in vehicle air conditioning units.

The main barriers to the use of ammonia are its toxicity and flammability in particular concentrations in the air. For hydro-carbons the concern is their high flammability. CO2 has low energy efficiency and very high working pressures in cycles with transcritical parameters. Systems with water require turbocompressors that can pass the large volume flows. Air systems have a very good energy efficiency only at large temperature lift making them suitable for heat pumps and very low temperature refrigeration applications. The safety barrier can be surpassed using technical standards and regulations, coupled with proper maintenance and training of personnel.

On the international scene, history repeats itself from the period of 1930 to 1940 when CFC refrigerants were struggling to gain acceptance because of the predominance of natural refrigerants. Now the battle is fought in the opposite direction. As sales volumes increase, systems with natural refrigerants will become more economic, therefore capital cost should not be seen as a barrier, but as a motivation to act. Most natural refrigerants are also considerably cheaper than their synthetic alternatives. The general conclusion is that there is not an ideal refrigerant today. Each refrigerating and air conditioning system type must be considered individually with regard to refrigerant choice. Natural refrigerant should be chosen wherever possible for the sake of environmental protection.