Research for more efficiency and climate protection
Development of an environment-friendly air conditioning unit with the refrigerant CO2 (carbon dioxide)
The effects of synthetic substances on the earth’s atmosphere are identified as incresingly problematic. These synthetic refrigerants will especially become a problem if they reach the environment due to accidents, maintenance or leakage of the air conditioning unit. The negative impacts on the greenhouse effect are 1300 times higher with the fluorocarbon “R134a” than with carbon dioxide.
The replacement of the previously used artificial refrigerant with the natural substance carbon dioxide provides a huge opportunity in the area of air conditioning and refrigeration. Carbon dioxide has already been used as a refrigerant at the end of the 19th century, however in slow operating, huge compressors with a subcritical operational mode.
Initially, today’s fast operating small vehicle compressors could not be used for this high pressure refrigerant. Here new gaskets, oils, and substances had to be found. The design of the air conditioning units for high pressures makes the use of the refrigerant CO2 even in the supercritical pressure range (higher 73 bar) possible. The so-called transcritical operational mode henceforth offers a sufficient capacity and efficiency when these air conditioning units are operated up to ambient temperatures of 50 degree Celsius. In subcritical operational mode, the capacity and efficiency decreases if outside temperatures of 25 degree Celsius are reached.
Due to the transcritical operational mode there is no liquid phase of the refrigerant in the high pressure part but only supercritical gas. This requires a different system architecture. The redesign of the refrigerant-carrying components leads to smaller dimensions, which makes the application in vehicles possible. On the other hand, the high operational pressures demand a great deal of the used valves, compressors and heat exchangers.
In contrast to conventional air conditioning units, with the transcritical CO2 unit it can be heated due to the circulation reversal (heat pump). Carbon dioxide has the highest heating capacity of all the refrigerants and is ready for use in a broad evaporation temperature range from +20 to -55 degree Celsius. No other current refrigerant can fulfil that task.
Model character
The developed air conditioning units and transport refrigeration units with carbon dioxide were presented on the IAA ’94 respectively IAA’98 as the first units world-wide with this new technology and a few years later they were tested in vehicles on the road.
Since summer 1996, a public-transport bus with a CO2 air conditioning unit has been in daily use. At the beginning of 1997 the second bus was put into operation. These buses are equipped with mobile measurement to conduct a comparison with an R134a unit in parallel. The operational experience of 7 years and the evaluation of the field test show the good applicability of CO2 as a refrigerant. Due to this experience, Konvekta plans on achieving the series-production readiness of CO2 units. Since August 2002 the first transport refrigeration unit with a CO2 compression circulation has been used by ALDI. In cooperation with the company Volkswagen, a CO2 air conditioning unit for a Midibus was developed and tested in detail.
Environment relief
When it was confirmed in the 80s that the commonly used refrigerant CFC destroys the ozone layer of the atmosphere, Konvekta consequently pushed the development of air conditioning units with a more compatible refrigerant. The result was that at the end of 1989 the first air conditioning units with the fluorocarbon free from chlorine were produced Europe-wide. Until 1992, all air conditioning units of the company had been converted to R134a. This refrigerant is still used world-wide today.
However, soon it was obvious that this refrigerant has a very high greenhouse potential. As a basis for the Global Warming Potential (GWP) carbon dioxide is assumed with the factor 1. The world-wide used refrigerant R134a has the factor 1300. Moreover it is a synthetically produced gas and harmful long-term effects on the environment cannot be excluded. Researches are known which describe the splitting up of this refrigerant into tri-fluorine acetic acid and hydrofluoric acid in the atmosphere. The caustic tri-fluorine acetic acid is persistent or not degradable at all, hydrofluoric acid is highly toxic and carcinogenic. The optimistic estimation of the greenhouse gas emissions by the DG-Environment of the European Commission assumes that there are approximately 30 megatons of CO2 equivalents because of the air conditioning units of the more than 200 million vehicles in the EU. Therefore, Konvekta was convinced that the currently used polluting refrigerant R134a had to be replaced.
Thereupon, a team of our development department systematically tested 25 possible refrigerants to detect an environment-friendly refrigerant. The result according to these criteria was carbon dioxide (CO2), a natural gas, which is found in the atmosphere and thus is completely harmless. In addition to this, carbon dioxide is a by-product in the production of technical gases and therefore available world-wide at a low price. Before the use of this refrigerant, comprehensive analysis had to be carried out. Then it was detected: The operational pressure of CO2 in an air conditioning or refrigeration unit is five to six times higher than the pressure of the present refrigerant. Therefore it is necessary that all of the components are constructed entirely new or are reconstructed. The development was conducted by two medium-sized manufacturers of components in the area of refrigeration engineering under the system leadership of Konvekta.
Miscellaneous (as for example: economic success, references, previous honours)
Within the scope of the “Förderpreis innovativer Mittelstand”, Konvekta received the exceptional price of the “Hessische Technologiestiftung” for this development in 1999. In the year 2000, Konvekta was awarded with the 2nd price for environment-friendly technologies by the Bundesverband der Deutschen Industrie e.V. (BDI) in the context of the European competition for a better environment. Konvekta received the commendation “The Technology Award for Sustainable Development” by the European Commission. In addition to this, this development was responsible for our presence as one of the world-wide projects of the EXPO 2000. Due to the obligation of the EU to reduce the greenhouse gas emissions by 8% CO2 as a refrigerant becomes more important today. When replacing R134a by CO2 after a possible legal regimentation greenhouse gas emissions can be reduced without threatening jobs in the affected industries.
On the basis of the development of a CO2 based air conditioning unit further innovations have been rendered. A deep-freezing unit for the transport of perishable goods with the refrigerant CO2 was developed. The development of a CO2 heat pump could replace conventional engine block heaters in the future. With the development of a climate module for accumulative means of transportation a further innovation was accomplished which leads to the reduction of pollutant emissions and product life cycle costs as well as an increase in the passenger’s comfort and an improvement of maintenance works.