At nearly every electric vehicle or smart grid conference, someone in the audience asks the question about just how clean electric vehicles are given that they are probably fueled by coal power plants.

There is a short answer and a long answer. The short answer is that if you are just asking about CO2, the electric vehicle, even from the dirtiest of U.S. coal plants, is still cleaner than a traditional internal-combustion vehicle.

However, toss hybrid vehicles, e-bikes, buses, renewable energy and particulates into the picture and it is not as clear-cut. A new study that examined electric vehicles, including e-bikes, in China found that EVs were not necessarily cleaner in terms of particulate emissions and their health effects, but rather the source of pollution was just offset from the urban centers to the areas where the power plants are located.

If you turn your head the other direction, however, increasing levels of renewables mean that an EV can become cleaner over time, while an internal combustion engine can increase efficiency but will still ultimately run on the same fuel source.

One of the most widely cited, and more recent studies, on the issue comes from Electric Power Research Institute and the Natural Resources Defense Council in 2007. That study found that if you compared a plug-in hybrid electric vehicle with a conventional vehicle, the PHEV would have 28 to 34 percent lower greenhouse gas emissions compared to a conventional vehicle, depending on the type of coal-fired power plant. However, when the comparison is made against a hybrid vehicle, the PHEV has a 1 to 11 percent higher GHG emission if it’s running on traditional coal-fired power plants.

The study found that the PHEV emitted about the same GHG emissions as a hybrid if it is running on electricity from pulverized coal, but once the electricity comes from natural gas, nuclear, renewables, biomass or any coal plant with carbon capture, the PHEV performs far better than an internal-combustion engine and better than a hybrid.

When the researchers looked out far into the future, the picture was rosier for PHEVs. Using a mix of electricity resources and a variation of ranges of PHEVs, the study came up with nine scenarios comparing conventional, hybrid and PHEVs. In every instance the electric vehicles had lower GHG emissions than both the hybrids and conventional cars.

But there are a lot of ifs between now and 2050, and CO2 is just one measurement. When researchers from the University of Tennessee, University of Minnesota and Tsinghua University in Beijing looked at electric vehicles and health impacts from fine particulates in China, it was not only a mixed bag, but EVs performed far worse.

EVs and Health Impacts in China

Automobile ownership has increased more than an order of magnitude in one decade in China. E-bikes in particular are skyrocketing, with an 86 percent annual growth rate during the past decade. The government has also been pushing for electric vehicle adoption. The move to EVs can curb smog in the country’s mega cities, but 85 percent of electricity in China is from fossil fuels, of which more than 90 percent is from coal. 

The e-bikes performed better than cars, motorcycles and buses on most emission metrics, but electric vehicles often fared worse when it came to comparing fine particulates emitted by the electricity source versus the tailpipe and refining facility for gasoline cars.

Fine particulate emissions per passenger-kilometer was 3.6 times greater for electric cars than for gasoline cars overall. About half of the fine particulates from electricity production that are inhaled by humans are inhaled by people near the power plants, rather than those in urban areas where most of the cars are.

The researchers found that electric vehicles fueled by coal-fired plants in China rate far worse in mortality due to fine particulates compared to gasoline cars, but it is hardly the final word.

The study acknowledged limitations, and like the EPRI study about CO2, any changes to the coal power plants would improve the position of EVs. Most importantly, it examined what would happen if people make substitutions between electrified and diesel or gas-powered transportation.  

E-bikes are the fastest growing segment, and the study found that most people would use diesel buses if e-bikes were no longer available. E-bikes, however, are far cleaner in every respect than diesel buses and even EVs, which had higher particulate emissions from power plants overall compared to diesel buses, did have similar health impacts from primary particulates.

The results of the studies on electric vehicles and emissions bolster some of the findings from Greentech Media’s recent report, Electric Vehicles 2011: Technology, Economics, and Market, which found that plug-in hybrids might make the most sense in the near-term. The report’s co-author, Travis Bradford, president and founder of the Prometheus Institute for Sustainable Development, noted that the real winner in terms of cost competitiveness, not to mention clean emissions, in the near future is not the fully electric vehicle but the plug-in hybrid.

“The real reason cost doesn’t get better is the internal combustion engine doesn’t stay the same,” said Bradford. He noted the increasing fuel economy standards coming to the U.S. as one example. “The electric vehicles will have to get better just to keep up,” he said.

Keeping Up with Gas Vehicles in the U.S.

In the U.S., EPRI looked at a scenario with an aggressive penetration of PHEVs by 2030 (about 40 percent on-road vehicles). It found that, “PHEVs result in small but significant improvements in ambient air quality and reduction in deposition of various pollutants such as acids, nutrients and mercury.” Unlike the study in China, EPRI looked at emissions rather than health outcomes.

Like China, a decrease in pollutants for some most of the population in the U.S. would also mean an increase for the 1 percent of the population that is near the power plants supplying the electricity for the new fleet of electric cars. Assuming the growth in electricity generation comes mostly from coal, primary emissions of fine particulates would increase by 10 percent, according to EPRI.

The EPA is putting in more stringent air quality standards on power plants in the U.S., but that also comes at time when miles per gallon standards are increasing substantially for light and heavy-duty vehicles.

NRDC, which was also involved in the study, held the position that cleaning up the air in one region should not come at the expense of another region. It also noted that “with sufficient emissions controls in place PHEVs have the potential to improve air quality and to substantially contribute to meeting our long term GHG reduction goals.”

The adoption of electric vehicles and plug-in hybrids are successful only if their advantages, and limitations, are taken into account along with the entire transportation picture and regulations on power plants.

Studies that take into account the lifecycle of EV batteries are also missing from the picture so far. Although accurate predictions of EV penetration are hard to come by, various models can help to inform policy makers about a balance of regulation and incentives to create a cleaner transportation fleet -- as long as that comes with a clear definition of what clean means.