Plug-in Hybrid - Greenhouse Gas Emissions

Greenhouse Gas Emissions

See also: Greenhouse gas emissions in plug-in electric vehicles

The effect of PHEVs on greenhouse emissions is complex. Plug-in hybrid vehicles operating on all-electric mode do not emit harmful tailpipe pollutants from the onboard source of power. The clean air benefit is usually local because depending on the source of the electricity used to recharge the batteries, air pollutant emissions are shifted to the location of the generation plants. In the same way, PHEVs do not emit greenhouse gases from the onboard source of power, but from the point of view of a well-to-wheel assessment, the extent of the benefit also depends on the fuel and technology used for electricity generation. From the perspective of a full life cycle analysis, the electricity used to recharge the batteries must be generated from renewable or clean sources such as wind, solar, hydroelectric, or nuclear power for PEVs to have almost none or zero well-to-wheel emissions. On the other hand, when PEVs are recharged from coal-fired plants, they usually produce slightly more greenhouse gas emissions than internal combustion engine vehicles. In the case of plug-in hybrid electric vehicle when operating in hybrid mode with assistance of the internal combustion engine, tailpipe and greenhouse emissions are lower in comparison to conventional cars because of their higher fuel economy.

There has been much debate over the potential GHG emissions reductions that can be achieved with PHEV. A study by the Electric Power Research Institute reports that a 338 TW·h or 5.8% increase in power generation needed as a result of PHEV. In the same report the EPRI also states that CO₂ emissions could increase by 430 million metric tons. The article concludes:

"In summary, the addition of PHEVs as a significant transportation option adds approximately 6% to the total national electricity demand in 2030 compared to the base case with no PHEVs. Due to the charging profile that results in most of this additional demand occurring during off-peak hours (late night/early morning) there is an increase in the need for baseload generation. The addition of coal-fired generation to meet this need for more baseload generation does not result in any significant differences in annual emissions of SO₂, NO_x and Hg because of the caps on those pollutants. Therefore, any reductions in emissions of SO₂, NO_x or Hg from non-electric generating sources would result in a net national decline in these emissions. However, it does result in an appreciable increase in CO₂ and PM emissions as this analysis has not assumed any limits on CO₂ or PM emissions."

A study by the American Council for an Energy Efficient Economy (ACEEE) predicts that, on average, a typical American driver is expected to achieve about a 15% reduction in net CO₂ emissions compared to the driver of a regular hybrid, based on the 2005 distribution of power sources feeding the US electrical grid. The ACEEE study also predicts that in areas where more than 80% of grid-power comes from coal-burning power plants, local net CO₂ emissions will increase, while for PHEVs recharged in areas where the grid is fed by power sources with lower CO₂ emissions than the current average, net CO₂ emissions associated with PHEVs will decrease correspondingly.

A 2007 joint study by the Electric Power Research Institute (EPRI) and the Natural Resources Defense Council (NRDC) similarly found that the introduction of PHEVs into America’s consumer vehicle fleet could achieve significant greenhouse gas emission reductions. The EPRI-NRDC report estimates that, between 2010 and 2050, a shift toward PHEV use could reduce GHG emissions by 3.4 to 10.4 billion metric tons. The magnitude of these reductions would ultimately depend on the level of PHEV market penetration and the carbon intensity of the US electricity sector. In general, PHEVs can be viewed as an element in the "Pacala and Socolow wedges" approach which shows a way to stabilize CO₂ emissions using a portfolio of existing techniques, including efficient vehicles.

A 2008 study at Duke University suggests that for PHEV's to reduce greenhouse gas emissions more than hybrids a carbon pricing signal that encourages the development of low carbon power is needed. RAND also in 2008 studied the questions of a carbon tax, carbon cap and trade systems, increasing gasoline tax, and providing renewable energy subsidies under various economic conditions and vehicle type availabilities. RAND found that subsidies were able to provide a smoother transition to new energy sources, especially in the face of energy source price volatility, because subsidies can be structured according to relative costs between renewables and fossil fuel, while taxes and carbon trading schemes alone do not take relative prices of energy into account.

The Minnesota Pollution Control Agency found that if Minnesota's fleet of vehicles making lengthy trips were replaced by plug-in hybrids, CO₂ emissions per vehicle would likely decrease. However, unless more than 40% of the electricity used to charge the vehicles were to come from non-polluting sources, replacing the vehicles with non-plug-in hybrids would engender a larger decrease in CO₂ emissions. Plug-in hybrids use less fuel in all cases, and produce much less carbon dioxide in short commuter trips, which is how most vehicles are used. The difference is such that overall carbon emissions would decrease if all internal combustion vehicles were converted to plug-ins.

In 2009 researchers at Argonne National Laboratory adapted their GREET model to conduct a full well-to-wheels (WTW) analysis of energy use and greenhouse gas (GHG) emissions of plug-in hybrid electric vehicles for several scenarios, considering different on-board fuels and different sources of electricity generation for recharging the vehicle batteries. Three US regions were selected for the analysis, California, New York, and Illinois, as these regions include major metropolitan areas with significant variations in their energy generation mixes. The full cycle analysis results were also reported for the US generation mix and renewable electricity to examine cases of average and clean mixes, respectively This 2009 study showed a wide spread of petroleum use and GHG emissions among the different fuel production technologies and grid generation mixes. The following table summarizes the main results:

PHEV well-to-wheels Petroleum energy use and greenhouse gas emissions for an all-electric range between 10 and 40 miles (16 and 64 km) with different on-board fuels. (as a % relative to an internal combustion engine vehicle that uses fossil fuel gasoline)
Analysis	Reformulated gasoline and Ultra-low sulfur diesel	E85 fuel from corn and switchgrass	Fuel cell hydrogen
Petroleum energy use reduction	40–60%	70–90%	more than 90%
GHG emissions reduction	30–60%	40–80%	10–100%

The Argonne study found that PHEVs offered reductions in petroleum energy use as compared with regular hybrid electric vehicles. More petroleum energy savings and also more GHG emissions reductions were realized as the all-electric range increased, except when electricity used to recharged was dominated by coal or oil-fired power generation. As expected, electricity from renewable sources realized the largest reductions in petroleum energy use and GHG emissions for all PHEVs as the all-electric range increased. The study also concluded that plug-in vehicles that employ biomass-based fuels (biomass-E85 and -hydrogen) may not realize GHG emissions benefits over regular hybrids if power generation is dominated by fossil sources.

A 2008 study by researchers at Oak Ridge National Laboratory analyzed oil use and greenhouse gas (GHG) emissions of plug-in hybrids relative to hybrid electric vehicles under several scenarios for years 2020 and 2030. Each type of vehicle was assumed to run 20 miles (32 km) per day and the HEV was assumed to have a fuel economy of 40 miles per US gallon (5.9 L/100 km; 48 mpg). The study considered the mix of power sources for 13 U.S. regions, generally a combination of coal, natural gas and nuclear energy, and to a lesser extend renewable energy. A 2010 study conducted at Argonne National Laboratory reached similar findings, concluding that PHEVs will reduce oil consumption but could produce very different greenhouse gas emissions for each region depending on the energy mix used to generate the electricity to recharge the plug-in hybrids. The following table summarizes the main results of the Oak Ridge National Laboratory study for the 2020 scenario:

Comparison of carbon emissions and oil consumption by plug-in hybrids relative to hybrid electric vehicles (HEVs) by U.S. regional power generation sources on 2020
Region	Main electricity sources	Share total generation 2020	Carbon emissions relative to HEVs		Oil consumption relative to HEVs		States included in the region
			Plug-in hybrid	All-electric mode	Plug-in hybrid	All-electric mode
Northwest	Natural gas Nuclear	84.3% 15.7%	-20.0%	-37.2%	-47.0%	-99.6%	Includes ID, MT, NV, OR, UT, SD, WA, and WY.
California	Natural gas Renewable	99.0% 1.0%	-15.3%	-26.5%	-47.0%	-99.6%
Texas	Natural gas	100%	-15.0%	-25.7%	-47.0%	-99.6%
Florida	Natural gas Oil	96.1% 2.4%	-14.8%	-25.3%	-45.6%	-96.4%
New England	Natural gas Coal	70.3% 15.5%	-11.4%	-17.4%	-44.3%	-93.5%	Includes CT, MA, ME, NH, RI, and VT.
Lower Midwest	Natural gas Coal	88.6% 11.4%	-11.0%	-16.4%	-46.9%	-99.4%	Includes AR, KS, LA, NM, OK, and TX.
Southwest	Natural gas Coal	83.6% 16.1%	-9.40%	-12.8%	-46.9%	-99.4%	Includes AZ, CO, NM, NV, and TX.
Mid-Atlantic	Natural gas Coal	60.6% 37.0%	-1.2%	+6.1	-45.4%	-95.9%	Includes DC, DE, MD, ME, NJ, and PA.
Upper Midwest	Natural gas Coal	47.6% 46.0%	-0.8%	+7.2%	-46.7%	-99.0%	Includes IA, MN, MT, ND, NE, SD, and WI.
Southeast	Coal Natural gas	51.9% 44.9%	+2.4%	+14.4%	-46.7%	-98.9%	Includes AL, GA, LA, MS, NC, SC, and TN.
New York	Oil Natural gas	67.2% 29.4%	+4.3%	+19.0%	-8.6%	-10.9%
Greater Ohio	Coal Natural gas	65.7% 32.8%	+7.8%	+27.0%	-46.6%	-98.7%	Includes IN, KY, MI, OH, VA, and WV.
Greater Illinois	Coal Natural gas	75.4% 24.6%	+11.7%	+36.0%	-46.5%	-98.6%	Includes IA, IL, MI, MO, and WI.