The literature does not comprehensively distinguish between the merits of alternative energy sources and those of plug-in vehicles themselves. Not all of the benefits associated with plug-in vehicles are unique. This distinction can have important policy implications for regions that rely on non-petroleum sources of electricity, which is increasingly natural gas in much of the US. The natural gas available in a region could be utilized by the transportation sector in different ways: as CNG for conventional vehicles (CV) to provide the benefits of fuel switching from petroleum use; as CNG for hybrid electric vehicles (HEV) that also reduce life cycle energy use; as a source of electricity for plug-in battery electric vehicles (BEV) that can also shift CAC emissions from vehicles in urban areas to power plants in rural areas.

This study evaluates the incremental life cycle air emissions (GHG and CAC) impact benefits and life cycle ownership costs of non-plug-in (CV and HEV) and plug-in (BEV) vehicles using natural gas as a common primary energy source.

—luk et al.

The researchers determined the life cycle air emissions impacts and life cycle ownership costs of a set of light-duty passenger vehicle pathways: gasoline CV: vehicle fueled by gasoline with a conventional powertrain; CNG CV: vehicle fueled by compressed natural gas, with a conventional powertrain; CNG HEV: vehicle fueled by compressed natural gas, with a hybrid electric powertrain; NG-e BEV: vehicle powered by natural-gas-derived electricity, with a battery electric powertrain.

They used GREET 1 fuel-cycle and GREET 2 vehicle-cycle models to determine GHG (CO₂, CH₄, and N₂O) and criteria air contaminant (CAC) (PM_2.5, VOC, NO_x, and SO_x) emissions.

GREET assumes that a dedicated CNG CV and CNG HEV can achieve fuel economies that are 5% and 40% higher on an energy equivalent basis, respectively, than the reference vehicle. They then estimated the NPVs of climate change and human health impacts from the GHG and CAC emissions, respectively.

Base case life cycle (a) GHG climate change impacts, (b) CAC health impacts, and (c) ownership costs. GHG climate change (from the quantities of CO2, CH4, and N2O emissions) and CAC health impacts per vehicle lifetime (from the quantities and geographic distributions of NOx, SOx, PM2.5, and VOC emissions). Credit: ACS, Luk et al. Click to enlarge.

Among their findings from the pathway analysis were:

• The Gasoline CV has both the highest base case energy use (900 GJ per vehicle lifetime) and CO₂ emissions (60 t), while the CNG CV results are 10% and 20% lower, respectively.

• CNG HEV energy use and CO₂ emissions are both 30% lower than those of the CNG CV, because of fuel economy differences. These metrics are similar for both the CNG HEV and NG-e BEV because differences in vehicle operation and fuel production stages largely offset each other.

• Compared to CO₂ emissions, vehicle operation is a smaller contributor to the CAC emissions because gasoline and CNG both have low sulfur contents, and Tier 2 emissions standards require the use of emissions control equipment to reduce vehicle tailpipe and evaporative NO_x, PM_2.5, and VOC emissions.

• Vehicle operation stage is still a major contributor of VOC emissions because of windshield washer fluid (which contains methanol) use and PM_2.5 emissions, in part because of tire and brake wear.

• The Gasoline CV has both the highest base case GHG climate change ($3000) and CAC health ($700) impacts.

• The NG-e BEV has the lowest vehicle operation CAC health impacts because of the lack of tailpipe and fuel tank evaporative emissions, which often occur in populated areas, but life cycle CAC health impacts are approximately the same ($600) for all natural gas pathways.

• The NG-e BEV pathway has the highest cost of ownership, 30% higher than those of non-plug-in vehicle pathways, despite having the lowest operating expenses. This high cost is largely due to the $13,000 battery that provides a 125-km (80-mi) driving range.

They then performed Monte Carlo analysis for incremental costs and benefits for fuel switching (the CNG CV replacing the Gasoline CV); energy efficiency (the CNG HEV replacing the CNG CV); and emissions shifting (the NG-e BEV replacing the CNG HEV).

Life cycle incremental (a) benefit−cost Monte Carlo analysis, (b) benefit sensitivity analysis, and (c) cost sensitivity analysis results. Benefit refers to reduction in air emissions impact and cost refers to increase in ownership costs. Credit: ACS, Luk et al. Click to enlarge.

Among the findings of that incremental cost-benefit analysis were:

• Fuel switching in conventional vehicles (gasoline to CNG) can provide life cycle air emissions impact benefits without significantly changing life cycle ownership costs.

• Improving energy efficiency with hybrid electric vehicles can provide life cycle air emissions impact benefits and reduce life cycle ownership costs.

• Shifting emissions with plug-in vehicles can increase life cycle ownership costs without providing life cycle air emissions impact benefits. Uncertainty in BEV driving range and natural gas power plant efficiency (36−60%) overshadows the advantage of mitigating tailpipe emissions in urban areas. The emissions and ownership cost findings likely also apply to plug-in hybrid electric vehicles (PHEVs).

Targeting incentives at regions with poor air quality can limit unintended negative consequences of plug-in vehicles, which would be exacerbated if these vehicles become a mass market alternative across all geographical areas. This includes an increase in upstream emissions because of fuel and vehicle production. … policies should encourage the targeted adoption of plug-in vehicles in niche markets, particularly urban areas with poor air quality; because alternative fuel use in non- plug-in vehicles is likely more cost-effective at providing life cycle air emissions impact benefits.

—Luk et al.

The authors also observe that because the quantity of GHG emissions is highly sensitive to changes in the fuel cycle, while the majority of CAC emissions are from vehicle production, improvements in vehicle fuel economy can reduce climate change impacts without decreasing life cycle health impacts. Unlike GHG emissions, the impact of CAC emissions depends on the geographic location where they occur, which provides a local advantage for plug-in vehicles that is not captured when using climate change impacts (or GHG emissions) as metrics.

Climate change regulations and CAC emission policies should aim for synergies in reducing negative impacts. There can be trade-offs as illustrated by emissions control systems in non-plug-in vehicles; excess air (above stoichiometric air−fuel ratio) reduces GHG emissions by improving fuel economy but at the expense of higher NO_x emissions. Consequently, policies such as Tier 3 tailpipe CAC emissions standards are important to have alongside legislation designed to reduce GHG emissions to avoid unintentional increases in either health or climate change impacts.

—Luke et al.

The study was financially supported by the AUTO21 Network Centre of Excellence, Natural Sciences and Engineering Research Council, and General Motors.

Resources

• Jason M. Luk, Bradley A. Saville, and Heather L. MacLean (2015) “Life Cycle Air Emissions Impacts and Ownership Costs of Light-Duty Vehicles Using Natural Gas As a Primary Energy Source” Environmental Science & Technology doi: 10.1021/es5045387