requirements (Hi-Z 1996). A 12-W unit could provide approximately 8 Ah, but it would not
totally eliminate the need for battery power.
The results of a literature review of APUs suggest that about 23,000
40,000 Btu/h of
primary energy is required for heating and cooling, which is considerably less energy than the
128,500 Btu/h used when the truck idles (ATC undated a; ATC undated b; Greer 1999).
If electrical power supplies the energy for cab/sleeper heating and engine block heating,
about 4.3 kW is required to supply the assumed 14,750 Btu/h requirement. The primary energy
consumed at the power plant is 45,378 Btu/h, assuming a conversion efficiency of 32.5%. Note
that very little of the primary energy used to generate electricity is in the form of petroleum, so
electrification minimizes oil use.
4.2 Emissions Reductions
Using more efficient alternatives to diesel engine idling also reduces emissions. Estimated
emissions for current practice and alternatives are listed in Table 4. No data are available for
storage options, which produce no emissions during their operation, but they are expected to
cause a small increase in truck operating emissions because of increased load on the heating,
ventilation, and air-conditioning (HVAC) system to heat or cool the thermal storage medium.
The data on equipment were provided by the suppliers and should be considered preliminary.
Emissions of carbon dioxide were calculated on the basis of the fuel input. Emissions from
electricity generation are based on the current average U.S. generation mix. Emissions would
differ somewhat by region or time of day (truck drivers sleeping at truck stops at night would be
using off-peak power, which removes gas and oil from the generating mix). Further, more
detailed work could examine the implications of changing the generating mix. With the caveat
that additional, more reliable data are needed, we can make some tentative observations.
Truck engine idling produces significantly higher greenhouse gas (CO
) emissions than any
of the other options considered (because of its higher energy use) and, in almost all cases, emits
more of all pollutants. The direct-fired heater and auxiliary power units generate significantly
than electricity use, assuming heating loads in each case are equal. Compared with
idling, electricity use significantly reduces all of the emissions tabulated (except possibly SO
see Appendix B) for the current U.S. generation mix. Electricity use reduces particulate
emissions compared with all of the other options for which data were available. These findings
However, conclusions about any of the other emissions from onboard units are highly
speculative because of the uncertainty in the data reported. The direct-fired heater and APU may
produce more hydrocarbons than electrification because emissions are relatively high during
start-up and as the heater warms up. Although reported emissions from APUs were significantly
higher for THC (total hydrocarbons), CO (carbon monoxide), NO
(nitrogen oxides), and
particulates than those from the direct-fired heater, both sets of data are suspect. Therefore, these
numbers should not be used to support one type of equipment over another.