Establishment of Leakage Rates of Mobile Air Conditioners in Trucks

Januar 2007
Prepared for the European Commissions (DG Environment) ENV.C.1/SER/2005/0091r
Winfried Schwarz

On behalf of the EU Commission, this study establishes empirically the annual leakage rate of mobile air conditioners (MACs) in trucks for the use-phase of the vehicles. The approach consists of measurements of the bigger part of use-phase emissions, called "regular leakage". Regular leakage takes place gradually from intact MACs. This is quite different to "irregular leakage", attributable to system failures caused by internal or external reasons, often by accidents. 

From July to November 2006, 271 measurements were carried out in Germany on trucks of the seven EU relevant brands, from model year 2000 onwards. The measuring sample should come close to the EU wide MAC-equipped truck fleet by models and makes, age, usage pattern, mileage, refrigerant charges, etc. The age of the MACs to be measured should not exceed seven years.

The measurements were made at 12 vehicle pools. For every MAC it was determined how much of the initial (norm) refrigerant charge was still in the system, and the deficit was related to the time elapsed since the vehicle's first registration. The assessment of the refrigerant deficiency in each MAC followed a dedicated Measurement Protocol which was in principle identical to that which was developed and used for establishing the leakage rate of MACs in passenger cars, in 2002/2003. 

The most important results of the analysis are:

1. Based on the measurements, the EU wide average leakage rate of MACs in trucks is estimated to be 87.8 grams per year, with an error band of ± 8.9 grams at a confidence level of 95 percent. Expressed as percentage of the original refrigerant charge this equals 8.3 percent/year, with an error band of ± 0.8 percent. 

2. The annual leakage rate (grams/year) is not constant over time. There is indication that newer systems of up to 35 months in use show higher leakage rates per year than older ones (36th to 88th months in use). However, the statement that leakage rates generally diminish with increasing age is too risky from a statistical point of view given the very small number of cases in the upper age categories. 

3. Although the absolute leakage rates differ widely between the seven makes from 70.2 to 153.5 in grams/yr., these differences are not significant statistically. The uniformity of leakage rates of MACs from different truck makers is confirmed when the leakage rates are expressed as a percentage of the initial charges. These figures show a limited spread ranging from 7.3 to 11.5 percent, annually. 

4. MACs with different charges lose an equal percentage of their refrigerant per year. As the norm fills vary widely from 700 to over 1,450 grams, smaller charged MACs (700-750 grams) were found to have a significant lower leakage rate in grams per year than bigger charged systems. Average loss per year is 58 grams from smaller charged MACs, and 87-123 grams from larger systems. From this it follows that a MAC design for trucks that reduces the refrigerant charge would contribute substantially, to reducing HFC-134a emissions from this source. The scope for reduction in overall regular emissions through charge reduction is estimated at 30% - if every MAC had a charge of only 750 grams. 

5. The annual mileage of the trucks tested varies widely from 89,810 km/year to 196,382 km/year, if the quarter of cases with the lowest mileage is compared to the quarter with the highest values. The difference in annual leakage rate is much smaller, ranging from 72.5 grams (1st quarter) to 101.9 grams (4th quarter), and is statistically not significant. In conclusion, the influence of annual mileage on leakage rates of MACs in trucks is quite small - and it is even smaller on percentage leakage rates. 

The results of this study need to be compared with other HFC-134a emissions that occur during the lifetime of the air conditioner. 

In addition to the regular emissions established in this study, "irregular" emissions have to be considered also in the use-phase of a truck. The author of this study estimates "irregular" emissions at roughly 30 grams. Adding "regular" and "irregular" emissions and assuming that the expected lifetime of a truck is ten years, the expected average greenhouse gas emission from a truck mobile air conditioner in the EU is about 1.5 tonnes of CO2 equivalent over the use-phase. 

These emissions need to be complemented by HFC-134a emissions before the vehicle has been taken into use, the service emissions and the end-of-life emissions, as well as CO2 emissions due to the increased fuel consumption as a consequence of operating the air conditioner. In conclusion, while "regular" HFC-134a emissions are likely to be the single most important source of greenhouse gas emissions from mobile air conditioners in trucks, it is also important to estimate the amount of other HFC-134a and CO2 emissions to understand the full climatic impact of these mobile air conditioners.