Study evaluates the impacts of different hybrid-electric vehicles

With the introduction of vehicles such as the Chevrolet Volt, Nissan Leaf, and Toyota Prius, you now have a handful of choices of hybrid-electric vehicles. If the motivation for these vehicles is reducing our dependence on oil, how do they compare to one another? What about reducing greenhouse gas emissions? And on top of all of this, are the costs worth the benefits? Given the potentially harmful environmental impacts of continuing an oil- and gasoline-based automobile industry, a professor at Carnegie Mellon has been working to answer these questions.

Jeremy Michalek, a professor in the departments of engineering and public policy and mechanical engineering, has been working with colleagues to study the various effects of driving hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs) on the environment. They are interested in the benefits as well as the disadvantages, which may vary depending on numerous factors such as battery type, distance driven, and where the electricity used to charge the car battery comes from.

Something that the average consumer may not think about is that the emissions associated with driving these electric vehicles include more than what comes out of the tailpipe. Emissions are released many times during the life cycle of an electric vehicle: from smokestacks of factories that make the vehicles, as well as the power plants that make the electricity to charge the vehicle. Michalek explains that switching from a gasoline to a plug-in vehicle changes the types of emissions and where they are released.

“The nice thing about electricity is that you can make it in lots of different ways and in different locations. Gasoline comes from oil. Sure, you can make it synthetically, but it basically comes from oil. Electricity can be made in so many different ways, including renewable ways,” Michalek said.
Through the use of optimization models, he and his research group looked at which combination of battery properties — like type, capacity, and operating method — resulted in the best balance between greenhouse gas emissions and cost. They compared vehicles that were essentially identical in every way except for the powertrain options; thus, any effects that resulted from a change in battery properties could be assigned to that particular change.

The results of this study, which were recently published in the Journal of Mechanical Design, found that the size of the battery pack played an important role in greenhouse gas emissions and cost. Larger battery packs allow more miles to be driven on electricity, but those additional miles do not outweigh the amount of greenhouse gas emissions associated with the production of those batteries. Smaller battery packs, however, allow fewer miles to be driven on electricity, but are significantly cheaper, so there is a larger reduction in greenhouse gas emissions per dollar spent on the battery. Therefore, considering both economics and greenhouse gas emissions, a moderate- to small-sized battery pack was found to be optimal.

In an interview with the website Electric Vehicle World, Michalek explained, “It will be a good transition for people to start buying these HEVs and PHEVs with smaller battery packs because batteries will start being produced more frequently, and we can move quicker down the battery learning curve and hopefully make them cheaper.” As the technology on battery production improves, larger battery packs would therefore become more economical and have an even larger impact on greenhouse gas emissions.

Michalek has presented policy briefs to Congress members in Washington to make sure that people who are involved with energy policymaking are aware of the findings. Spreading this knowledge to make policymakers and the public more aware of the available technology is important in moving society toward a more sustainable way of living. The next step, Michalek said, will be in estimating the value of the benefits of electric vehicles. Quantifying all of the benefits will help us determine as a society whether or not it is worth the switch to a more electricity-based automobile market.