The future for mobile A/C systems

Prof. Dr.-Ing. Jürgen Köhler from the Institut für Thermodynamik, based at the Technical University Braunschweig, Germany, answers Christian Shelton’s questions about the issues surrounding the use of fluorinated greenhouse gases in mobile air-conditioning systems.

How do mobile air-conditioning (MAC) systems in buses and coaches cause greenhouse gas emissions?

There are two ways – direct and indirect. The direct way is when the refrigerant gas leaks out of the MAC system into the atmosphere. The indirect way is from the carbon dioxide (CO2) emissions from the engine – which needs to run in order to power the MAC. Running the MAC actually increases the fuel consumption – resulting in corresponding higher CO2 emissions of the engine.

How do the refrigerant gases escape from the MAC?

The gas can leak through the shaft seals and flexible hoses used in the system. The MAC systems use compressors, which have shaft seals. These shaft seals are not totally leak-proof. Flexible hoses are also used to compensate for the relative movement between the compressor, which is mounted to the engine, and the rest of the MAC system, which is mounted to the body of the bus. Again, these hoses are not entirely leak-proof.

What is the most commonly used refrigerant in MAC systems and how much damage does it do to the atmosphere?

The most commonly used refrigerant is HFC-134a. HFC-134a has a global warming potential (GWP) of 1,400. The GWP measurement is worked out in relation to CO2, which has a GWP value of 1. This means that 1kg of HFC-134a will have the equivalent damaging (or global warming) effect on the atmosphere as 1,400kg of CO2.

What does more actual harm to the atmosphere the direct or the indirect emissions?

Well no-one actually knows exactly how much refrigerant HFC-134a is released into the atmosphere due to leakage, service errors, accidents and so on. Experts all over the world are currently trying to determine what this figure could be and some institutions, such as the Ecole des Mines, in Paris, are working on developing measuring systems.

For buses, experts estimate the annual leakage rate of HFC-134a to be approximately 10% of the total charge of gas put into the system in the first place.

However, if you consider the high mileages that buses and coaches cover over a year – more indirect emissions are emitted. In fact we estimate the ratio of direct to indirect emissions to be in the region of 1:3 for a leakage rate of 10%. For tighter systems with a leakage rate of 5% it would be 1:6.

Having said this, if the bus and coach operators use bio-fuel then the direct emissions become comparable to or even greater than the indirect emissions and the ratio falls to approximately 1:1.

Is there any European Union (EU) legislation in place to control indirect greenhouse gas emissions caused by air conditioning systems from buses or coaches?

No, at the moment there is no legislation in place in the EU to control the indirect emissions of greenhouse gases. As a matter of fact, it is very difficult to control the indirect emissions because the amount of indirect emissions given off by buses and coaches are closely linked to the efficiency of the bus. And the efficiency of the bus depends upon its load. So it would be very hard to legislate against the efficiency of buses and coaches – as their load is variable.

However, it may be possible to support engine designs that have proven to be highly efficient with tax breaks.

Is there any EU legislation in place to control direct HFC-134a emissions?

Yes, legislation for cars and light trucks is about to be introduced. At the end of January the European Council and the European Parliament agreed on prohibiting the use of HFC-134a or any refrigerants with a GWP larger than 150 as refrigerants in MAC systems. Although this ban will only affect cars and light trucks, I fully expect this to be extended to include buses and coaches in the future.

Car and truck manufacturers will have to ensure that any newly designed type of car and light truck comply with this policy as of the 1st January, 2011. The legislation will extend to cover any new cars or trucks sold from 1st January, 2017.

So are there any alternative refrigerant gases that could be used in MAC systems to comply with the legislation?

The first alternative is to use CO2, which itself is a pretty effective refrigerant. Parallel to the car A/C developments, the German air-conditioning specialist Konvekta AG started its development of CO2 A/C systems for buses in the 1990’s and has already achieved encouraging results. There are actually a couple of buses equipped with Konvekta’s CO2 A/C units that have been running on the road for a significant number of years.

Dupont and Honeywell have both recently said that they have found new refrigerant blends that are safe to use and comply with the European GWP legislation. If these gases prove to be as effective as the manufacturers claim, then this might be an alternative too. At the moment, we do not know much about these refrigerants. The only thing we know is that the Honeywell blend needs a 40% higher mass flow rate compared to conventional systems when circulating within the air-conditioning system. This might cause significant higher pressure drops and corresponding lower efficiencies in bus A/C systems because they use very long suction line hoses.

The last option (which is not really an alternative for buses) could be to use either the refrigerant HFC-152a or a gas blend of propane and isobutane. Both are effective refrigerants and the propane/isobutane blend is currently used in refrigerators in Europe. However, these gases have the disadvantage that they are flammable. The average charge of refrigerant in a bus or coach MAC system is between 5–10kg, while 1kg of propane is enough to blow up a house! So for safety reasons, these gases are not used in vehicles.