Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportations Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
Also, the addition of a secondary loop can make MAC systems more efficient. While
conventional, or primary loop, MAC systems use refrigerant to directly chill the in-cabin
heat exchanger, secondary loop systems employ a second fluid to chill the vehicle’s
interior. This secondary fluid would likely be a water/antifreeze solution. There are three
primary benefits to a secondary loop system. In the case of a system with a toxic
refrigerant, secondary loop systems isolate passengers from the refrigerant, for example in
the event of a rapid leak resulting from a collision. Additionally, these systems can be
used to reduce power requirements by allowing the compressor to run primarily during
deceleration or at efficient engine operating conditions. The antifreeze itself also could be
used as a thermal sink to maintain low temperatures in the time between these operational
modes. Another benefit is that these systems allow a reduction in the size of the primary
refrigerant circuit and, as a result, a reduction in the required size of the refrigerant
charge. It is estimated that the charge size can be reduced by about 50 percent for typical
vehicles. Designs and prototypes for these systems exist, but the increased cost and
complexity has kept them out of the market so far (U.S. EPA, 2007d).
Even without a secondary loop, changes can be made to component designs and
compressor cycling to reduce engine loads. Improved compressor cycling and control,
similar to that proposed for secondary loop systems, also could be employed to take
advantage of efficiency gains possible during vehicle deceleration. It also is possible to
allow the system to run at higher temperatures while in defrost mode. These changes,
coupled with more efficient heat exchangers, would allow MAC systems to operate with
reduced energy requirements. Such enhancements also would help hybrid vehicles
maintain their efficiency advantages over conventional vehicles during MAC system
operation: the continuous compressor load dramatically reduces the amount of engine-off
time that hybrids can utilize, so reducing MAC power requirements can help hybrid
vehicles operate even more efficiently.
Magnitude and Timing of GHG Reductions
EPA estimates that in 2007, mobile air conditioner (MAC) emissions accounted for 3.5
percent of total transportation greenhouse gas emissions (U.S. EPA, 2009d). Because the
Calilfornia Air Resources Board (CARB) has begun evaluating possible methods of
reducing “do-it-yourself” (DIY) MAC servicing in California, much of the available data
concerning DIY methods come from that State. DIY refrigerant emissions in California
represent about 0.39 percent of GHG emissions from the California transportation sector.
It is estimated that a ban of DIY refill cans would reduce refrigerant emissions by
66 percent of this amount (CARB, 2008e). As an alternative to a can ban, adopting a
deposit program to recycle can heels is estimated to reduce DIY emissions by about
31 percent. The effect of environmental fees would be dependent on the amount of the
fee, with an upper limit near that of the can ban. Because CARB anticipates that the above
reductions could be achieved in two years, methods of DIY reductions are considered
near-term options (CARB, no date).
The impact of a change in MAC refrigerant type depends primarily on the GWP of the
refrigerant and the mechanical efficiency of the system. HFC-1234yf would reduce CO2-
3-120