Net emissions resulting from the use of plug-in hybrid electric vehicles (PHEVs) depend on the efficiency of the conventional vehicle fleet; PHEV CD (all-electric, charge-depleting mode) efficiency; charging strategy; battery pack capacity; driving patterns; and generator mix used for charging.
A team at Carnegie Mellon University has modeled the net emissions in two regional transmission operators (PJM and NYISO) from PHEVs under different scenarios for future power generation; different size battery packs; charging strategies (home, work and smart); and PHEV fleet percentages between 0.4 and 50%. Scott Peterson, J. F. Whitacre and Jay Apt found that compared to 2005 gasoline fleet efficiency levels, all charging strategies and CD mode efficiencies yield reduction of CO2 emissions.
If a 2020 conventional vehicle fleet efficiency target of 35 mpg (6.7 L/100km) is compared to the 2020 CD (charge depleting) efficiency, net CO2 emissions drop significantly in switching from gasoline to electricity in NYISO but less in PJM because of the differences in generation, unless power generation with carbon capture and storage (CCS) is used.
In a paper published in the ACS journal Environmental Science & Technology, they present their results for a 10% PHEV market share of the light-duty vehicle fleet. Other PHEV market shares are included in the paper’s Supporting Information, but the results are similar except for the lowest 0.45% level (with fewer PHEVs charging, emissions are more sensitive to the specific plant used to charge them).
We estimate that PHEVs are likely to have lower net emissions of NOx and CO2 than a conventional vehicle fleet, given current (10.7 L/100 km) efficiencies. When compared to 2020 CAFE standards (6.7 L/100 km), net CO2 emissions in New York are greatly reduced by switching from gasoline to electricity, but coal-heavy PJM shows lower benefits unless coal units are fitted with CCS or replaced with lower CO2 generation. NOx is reduced in both RTOs, but SO2 increases unless a cap binds...A $50/tonne CO2 price applied only to combustion emissions in the electric sector will have a negligible short-term effects on net CO2 emissions from PHEVs.—Peterson et al.
Among the results of their study:
Home charging occurs near peak system load, smart charging near minimum system load, and work charging occurs both near peak system load (at the same time as home charging) and earlier in the day when most vehicles are arriving at work. These differences in timing result in changes in generator mix and thus emissions. In PJM, home charging results in the greatest CO2 reductions with no CO2 price and relies more on natural gas generation. In NYISO, smart charging results in greater CO2 reductions because of the large number of natural gas generators predicted to be used to meet demand.
Battery size results in few qualitative changes. Large batteries increase the magnitude of emissions changes “but do not change the sign” except in the case of NOx emissions in NYISO with work or home charging. Large batteries are also more sensitive to charge rate.
In NYISO home charging does not decrease CO2 emissions as much as smart or work charging because it is displacing gasoline with plants near the peak, often using oil. Smart charging relies on 86% natural gas in NYISO, whereas home charging uses only 44% natural gas. In NYISO work and smart charging have similar CO2 emissions. PJM shows nearly the opposite result with smart charging having significantly lower reductions in CO2 emissions (relying on 98% coal). Home and work charging in PJM exhibit similar levels of CO2 emissions.
Although without a CO2 price there is no incentive to use a generator with lower CO2 emissions, adding a $50/tonne CO2 price does not significantly alter the plants used to meet a given load.
In the CCS scenario there is little change in NOx emissions.
Unlike the other pollutants, net SO2 emissions increase in most scenarios.
There are strong arguments in favor of electrification of the transportation sector in addition to net emissions. Combining numerous mobile emission sources into a far small number of stationary sources offers opportunities for cost-effective emissions reduction that may not otherwise be feasible in the transportation sector, and the location of emissions is likely to be moved farther from densely populated areas. If PHEV cars displace light trucks, SUVs, and vans from the fleet, emissions will be further reduced from the values reported here.
Enacting a CO2 price of $50/tonne will not be effective at reducing net CO2 emissions from a PHEV fleet. PHEVs are likely to place upward pressure on SO2 allowance prices if emission caps bind or to increase emissions if the caps do not bind. PHEVs will probably reduce net CO2 and NOx emissions but are unlikely to reduce net SO2 emissions.—Peterson et al.
Scott B. Peterson, J. F. Whitacre, Jay Apt (2011) Net Air Emissions from Electric Vehicles: The Effect of Carbon Price and Charging Strategies Environmental Science & Technology 45 (5), 1792-1797 doi: 10.1021/es102464y