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158
MOTORINDIA
l
January 2012
In addition to the environmental
aspects, there is also a financial cri-
terion. A current survey showed that
the price for R134a has increased by
100 per cent in recent years. Whilst
R134a cost 13 Euros per kg in 2001,
it now costs 26 Euros.
The question now is what refrig-
erant should be used in the bus air-
conditioning units in the future?
Alternatives
A more environmentally compat-
ible alternative to the refrigerant
R134a is the chemical refrigerant
HFO 1234yf and the natural refrig-
erant R744 (CO2).
R744 is a natural component in
the air and only has a GWP of a fac-
tor of 1. As it is a waste product of
industrial processes and the envi-
ronmental effect has already been
“booked”, its further use is therefore
environmentally neutral (GWP = 0).
A system operated with R744
has a higher efficiency level than a
R134a system. Almost 25 per cent
less fuel is used for the same cooling
performance (UBA 2008, ADAC
2008, Hrnjak, SAE ARCRP1,
2007). This not only reduces the
exhaust gas emissions, it also cuts
down on operating costs.
Thanks to the simple and non-
hazardous handling of R744, the in-
spection times compared to R134a
are reduced by more than half. Due
to the fact that R744 is not a chemi-
cal refrigerant, it is also not subject
to the Chemicals & Climate Protec-
tion Act. The purchasing costs per
kg of refrigerant are over 90 per
cent lower which, in turn, equates
to an overall savings potential in
terms of the service costs of around
70 per cent.
Carbon dioxide (R744) is neither
toxic nor flammable (safety data
sheet). Measurements by the pro-
fessional association for traffic (BG
Verkehr) have confirmed the harm-
lessness of the use of R744 in buses.
The maximum occupational expo-
sure limit value (MAK) for R744 is
5000 ppm. This value was not ex-
ceeded in the tests.
Due to the high pressure, an air-
conditioning system that is operated
with carbon dioxide is also ideal for
reversing the cooling process. The
air-conditioning system can there-
fore be used as a heat pump for the
bus heating system. In contrast, a
heat pump with R134a would not
supply enough output at tempera-
tures below 0°C due to the lower
pressure levels.
The transcritical operation and the
high pressure means that the exist-
ing cooling system needs to be com-
pletely modified. The increased cost
caused by these changes will be am-
ortized within five-six years.
HFO-1234yf is a chemically
manufactured refrigerant. The cli-
mate protection goals defined by
RL 2006/40/EC are achieved with a
GWP of 4. The low GWP value of
4 is achieved because the substance
is quickly decomposed in the at-
mosphere. Moisture plays the major
role here. Additives are mixed in to
ensure that the refrigerant does not
decompose in the air-conditioning
unit. This prevents premature de-
composition of the R-1234yf in the
system. Also, much more care needs
to be taken during servicing com-
pared to R134a.
w
special article
In the future, environmentally-friendly air-conditioning
systems, in particular for all-electric buses and hybrid
busses, could be low-emission hermetical systems with
electrical compressors.