Natural refrigerant air-conditioning in trains and buses

By team, Sep 21, 2011, 11:39 6 minute reading

The 12th Karlsruher Fahrzeugklima Symposium took place on 15 September 2011. Around 100 experts assembled to discuss new approaches to mobile air-conditioning systems in trains and buses. The event was organised by the TWK GmbH, test and formation centre for heat pumps and refrigeration technologies. Dr. Sonnekalb from Konvekta elaborated on advantages and disadvantages of using natural refrigerants in mobile air-conditioning and CO2 MAC buses in Germany.

Out of a total of 26 buses equipped with CO2 mobile air-conditioning systems (MAC), 22 buses are still in operation today, 21 of which in Germany, 1 in Luxembourg. The currently oldest one of these buses belongs to Saar-Pfalz-Bus (SPB, the public transport company of the regions Saarland and West Palatinate) and uses a CO2 MAC since 2004. The good experiences have encouraged the regional transport company to acquire in total 5 buses with CO2 air-conditioning in 2011, while the public transport company of Berlin (BVG) integrated 7 buses with CO2 MAC into their fleet in 2010.

In total, the fleet of CO2 buses in Germany has so far covered over 3.3 million km, accumulating over 16,500 hours of compressor operating time, creating a profound basis of reference and practical experience with CO2 MAC in buses.

Konvekta has equipped the first urban bus with their CO2 systems as early as 1996. This bus with the CO2 MAC was in regular operation until 2005. Currently, Konvekta is working with the bus manufacturers Evobus (8 CO2 buses), MAN (10 CO2 buses) and Solaris (4 CO2 buses). One of the Solaris buses, the H18, is a hybrid articulated bus. The CO2 air-conditioning system of this bus consists of 2 gas coolers and 5 evaporators. The system contains 9 kg of CO2.

The newest two buses with CO2 air-conditioning systems have been introduced in August 2011 by the public transport company of Jena.

Reversible CO2 heat pump for heating and cooling in buses

In 2010, the regional transport company of Kurhessen (RKH) introduced in Kassel for the first time a bus equipped with a reversible CO2 MAC, which does not only cool but also heat the bus. In the period from November 2010 to June 2011, over 5 million data sets have been recorded and evaluated. In these 8 months, 1,400 motor operating hours and 500 compressor operating hours were recorded. The maximum registered outdoor temperature for the bus in operation was +33°C, while the minimum was -12°C.

The data showed that at ambient temperatures around 10-15°C the compressor was rarely used. At these temperatures, additional cooling with help of the compressors was only necessary when the cooling with external air was insufficient due to high passenger numbers and direct insolation. In general, the air-conditioning system was mostly used at ambient temperatures between 20-25°C, while the heat pump was mainly put to work at temperatures between 0-5°C.

The average coefficient of performance (COP) of the reversible CO2 system, calculated from the ratio of cooling/heating performance to motor output, was 3.54 (3.1 in A/C mode, 3.8 in heat pump mode). The average motor output was 5.13 kW. A particularly interesting application for this reversible system could be in electric buses, as much of the energy for heating can be sourced from ambient air.

Safety aspects of CO2 MAC in buses: latent leaks and accidents

The German occupational insurance association (BGV) has tested the CO2 bus of RKH in Kassel to see what would happen in case of a latent leak by loosening the largest screw connection of the system in the bus air duct. In such a case, the CO2 is released with a loud noise. Partial formation of dry icy was observed. During the test, the air-conditioning system was on and the doors of the bus were kept closed. The CO2 concentration was measured on different points in the bus. The results were far below 30,000 ppm, which is the limit value defined by the US Environment Protection Agency, and below 5,000 ppm, the maximum allowed concentration value at the workplace. The conclusion of BG Verkehr was therefore that no additional safety measures have to be taken for buses using CO2 mobile air-conditioning systems.

Rather unintentionally, the Saar-Pfalz-Bus (SPB) has also tested the safety of CO2 MAC in case of an abrupt leak. One of their buses equipped with CO2 MAC was involved in a frontal collision with a passenger car. The lead to the front evaporator was damaged, but no harm was caused to the passengers.

The conclusion is that if existing standards and regulations (e.g. EN378) are respected, CO2 is as safe to use in MAC as R134a.

Technical aspects of CO2 in MAC

CO2 has good thermodynamic properties but a low critical temperature, explained Dr. Sonnekalb. At temperatures above 31°C, CO2 becomes supercritical and necessitates in air-conditioning a transcritical system. As the highly compressed gas cannot condense anymore, the evaporator becomes in this case a gas cooler. CO2 is neither toxic nor flammable and can therefore be deployed in direct systems. However, the high working pressure of CO2 has to be considered. The suction pressure is about 10 times higher than with R134a and the high pressure is about 5 times higher.

Other natural refrigerants might be less fit for MAC

Water has outstanding thermodynamic characteristics. However, due to the low vapour pressure the air-conditioning system would have to work with negative pressure, which creates the risk of external air entering and disrupting the system. The low vapour pressure and density also necessitate the circulation of massive volumes of air, which needs turbo compressors and big suction tubes to minimise the loss of pressure between evaporator and compressor. Another technical challenge is the high triple point of water: at temperatures below 0°C the system can easily ice up.

Ammonia has the second best evaporation enthalpy and is popular in industrial applications. The pressure in an ammonia system is comparable to that of current synthetic refrigerants, however the compatibility of materials is problematic. Also due to safety reasons, ammonia can only be deployed in indirect systems, resulting in a certain efficiency loss due to the necessary additional heat exchange and pumps.

Hydrocarbons in mobile air-conditioning face similar safety concerns. Nonetheless, they are increasingly popular in passenger cars in Australia and some Asian countries as alternatives to synthetic refrigerants.

Motivation to change: laws and environmental image

The entering into force of the European Directive 2006/40/EU on emissions from air-conditioning systems in motor vehicles in January 2011 has revitalised the discussion about alternative refrigerants in mobile air-conditioning (MAC). The Directive prescribes the use of low GWP refrigerants in cars' MAC and forced the automotive industry to look for alternatives. While buses and trains are not subject to this directive, public transport companies feel the need to follow suite and find an acceptable solution in terms of potential future regulations but even more in terms of environmentally friendly image.

While the automotive industry has for now opted for a next generation synthetic refrigerant, representatives from German train and public transport companies have clearly stated that they cannot envisage the use of flammable refrigerants in mass-transport. In this context, CO2 is a viable option for air-conditioning in buses, while the Deutsche Bahn is for over 10 years now collecting experiences with air as refrigerant. This does not exclude the use of CO2 in German trains as proven by the introduction of CO2 MAC in the new Desiro trains to be deployed on the Westfrankenbahn.


By team (@r744)

Sep 21, 2011, 11:39

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