Two studies released by the Electric Power Research Institute (EPRI) and the Natural Resources Defense Council (NRDC) show that widespread use of plug-in hybrid electric vehicles (PHEVs) in the United States could significantly reduce greenhouse gas (GHG) emissions and has the potential to provide small but significant improvements in ambient air quality in most areas of the US.
Widespread adoption of PHEVs could reduce GHG emissions from vehicles by more than 450 million metric tons annually in 2050—equivalent to removing 82.5 million passenger cars from the road. Cumulative GHG emissions reductions from 2010 to 2050 could reach 10.3 billion metric tons under the most aggressive scenarios for the development of a lower-carbon electrical infrastructure and PHEV penetration.
The analysis is the first to combine models of the US electric system and transportation sector with atmospheric air quality models to account for the future evolution of both sectors in technological advances, electricity load growth and capacity expansion.
PHEV Impact on Nationwide Greenhouse Gas Emissions. The GHG research measures the impact of increasing numbers of PHEVs between 2010—the year the researchers assumed PHEVs would become available on the US market—and 2050. The “well-to-wheels” analysis accounted for emissions from the generation of electricity to charge PHEV batteries and from the production, distribution and consumption of gasoline and diesel motor fuels.
The researchers combined three scenarios for the development of the electric sector— high-, medium-, and low-levels of both CO2 and total GHG—and three scenarios for the rate of adoption of PHEVs—low, medium and high.
The different scenarios of CO2 and GHG intensity for the electric sector are:
High. There is limited availability of higher efficiency and non-emitting generation technologies and a low cost associated with allowances to emit CO2 and other GHGs in this scenario. Total annual electric sector GHG emissions increase by 25% from 2010 to 2050.
Medium. Advanced renewable and non-emitting generation technologies, such as biomass and IGCC with carbon capture and storage, are available in this scenario. There is a moderate cost associated with allowances to emit CO2 and other GHGs. Total annual electric sector emissions decline by 4% between 2010 and 2050.
Low. Carbon capture and storage retrofit technology for existing coal plants are available in this scenario. In addition, there is significantly slower load growth indicative of a nationwide adoption of energy efficiency, or other demand reduction, and a high cost to emit CO2 and other GHGs. Total electric sector emissions decline by 85% in this scenario from 2010 to 2050.
The three different PHEV adoption scenarios each presume PHEVs enter the market in 2010 and achieve maximum new vehicle market share in 2050. PHEVs reach a maximum of 20% new vehicle market share in the Low PHEV scenario, 62% in the Medium PHEV scenario, and 80% in the High PHEV scenario.
|2050 New Vehicle Market Share by Scenario||Vehicle Type|
|PHEV Fleet Penetration Scenario||Low||56%||24%||20%|
Total system emissions from a given level of PHEV use will be determined by a combination of the vehicle type (PHEV with a 0, 20 or 40 miles of electric range), annual vehicle miles traveled by vehicle type, and the types of generating resources that are built and dispatched to serve the electrical load from grid-connected PHEVs.
The researchers found that annual GHG emissions reductions were significant in every scenario combination of the study, reaching a maximum reduction of 612 million metric tons in 2050 (High PHEV fleet penetration, Low-electric sector CO2 intensity case).
|2050 Annual GHG Reduction (million metric tons)||Electric Sector CO2 Intensity|
|PHEV Fleet Penetration Scenario||Low||163||177||193|
Air Quality Analysis. The air quality study evaluated two scenarios for the year 2030:
- A base case without any penetration of PHEVs in the US vehicle fleet; and
A PHEV case with PHEVs having reached 50% of new vehicle sales and constituting 40% of on-road vehicles by 2030. In the PHEV case, the overall fraction of vehicle miles traveled by the US vehicle fleet using electricity stored in PHEV batteries is 20%.
The study used a high electric-sector emission case where nearly all additional electricity demand needed to power an aggressive market penetration of PHEVs was assumed to be met by an increase in the use of present-day coal-fired generation technology with only currently required environmental controls.
The study consisted of four steps:
Transportation Sector Modeling. For both the base case and the PHEV case, the transportation sector and its emissions were modeled out to 2030. Emissions offset due to vehicle miles traveled using electricity (and reductions in upstream emissions) are calculated by the transportation models. In addition, the incremental electricity demand due to PHEVs was calculated for the PHEV case. The incremental load takes into account losses during transmission and battery charging. This incremental load is also attributed to different hours of the day assuming an overall charging profile for the fleet.
Electric Sector Modeling. For both the base case and PHEV case, the US electric sector was modeled from 2006 to 2030. New generation capacity and electricity dispatch is simulated by the models to account for increased load due to population and economic growth. Emissions associated with electricity generation is also calculated and constrained by environmental regulations as explained earlier. In the PHEV case, the incremental electrical load due to PHEVs is added for all intermediate years in which PHEVs are present as well as 2030.
Emissions Processing. For each scenario, emissions from the transportation sector and electric sector were merged with emissions from all other sectors into an emissions inventory. Natural emissions from vegetation and soil are also added into the emissions inventory. The emissions inventory is then transformed into a format suitable for use in a three-dimensional model of air quality for the entire continental United States.
Air Quality Modeling. The US Environmental Protection Agency’s Community Multiscale Air Quality (CMAQ) model was used to simulate US air quality in 2030 in each scenario. The key air quality indicators investigated in the air quality modeling were: ozone mixing ratios; daily and annual particulate matter concentrations (for both PM10 and PM2.5); deposition of sulfate, nitrate, total nitrogen (sum of oxidized and reduced nitrogen) and mercury; and visibility at Class I areas (e.g. national parks). In addition, population-weighted exposure indicators were also calculated for ozone and particulate matter.
The study found that:
In most regions of the United States, PHEVs result in small but significant improvements in ambient air quality and reduction in deposition of various pollutants such as acids, nutrients and mercury.
On a population weighted basis, the improvements in ambient air quality are small but numerically significant for most of the country.
The emissions of gaseous criteria pollutants (NOx and SO2) are constrained nationally by regulatory caps. As a result, changes in total emissions of these pollutants due to PHEVs reflect slight differences in allowance banking during the study’s time horizon.
Considering the electric and transportation sector together, total emissions of VOC, NOx and SO2 from the electric sector and transportation sector decrease due to PHEVs. Ozone levels decreased for most regions, but increased in some local areas. When assuming a minimum detection limit of 0.25 parts per billion, modeling estimates that 61% of the population would see decreased ozone levels and 1% of the population would see increased ozone levels.
Mercury emissions increase by 2.4% with increased generation needs to meet PHEV charging loads. The study assumes that mercury is constrained by a cap-and-trade program, with the option for using banked allowances, proposed by EPA during the execution of the study. The electric sector modeling indicates that utilities take advantage of the banking provision to realize early reductions in mercury that result in greater mercury emissions at the end of the study timeframe (2030).
Primary emissions of particulate matter (PM) increase by 10% with the use of PHEVs due primarily to the large growth in coal generation assumed in the study. In most regions, particulate matter concentrations decrease due to significant reductions in VOC and NOx emissions from the transportation sector leading to less secondary PM.
Environmental Assessment of Plug-In Hybrid Electric Vehicles (EPRI reports)