Hybrid and plug-in cars may not be as green as perceived

WASHINGTON — People interested in environmental issues love a good game of this versus that. Which is better for the environment: Paper towels or hot-air hand dryers?

While it’s an interesting diversion, such cases are all cocktail party chatter compared with whether, how much and what you choose to drive. One-fifth of U.S. greenhouse gas emissions come from transportation, according to the advocacy group Environmental Defense Fund.

Environmental impact is just one of many considerations when buying a car. Quality and price — both of the car and, these days, of gasoline — come into play. Green cars are currently struggling to be price-competitive, leading GM recently to put a production hold on the highly publicized Volt.

But what if you wanted to base your choice on environmental merits alone? The biggest decision then is what sort of engine to buy.

There are hybrid vehicles such as the Toyota Prius, Honda Insight and Ford Fusion, which use gasoline to power the car but can capture the gas engine’s surplus energy as electricity to charge a battery.

There are also plug-in hybrids, which add the ability to charge the battery from an electrical socket while maintaining a gasoline backup. The Chevrolet Volt has grabbed headlines in this category and was the co-winner of the car of the year at this year’s Geneva Auto Show. Finally, there are such pure plug-in automobiles as the Nissan Leaf. No gas, just a rechargeable electric battery.

Other considerations include the energy required to extract the vehicle’s raw materials, the land taken up by the mines and factories, the energy used to transport the raw materials and the newly assembled car from place to place, the actual manufacturing of the product, the disposal of the car at the end of its useful life and the infrastructure required to support the vehicle (gas stations, charging stations, etc.).

You’re probably expecting an answer here: All things considered, are the new engines environmentally superior? Unfortunately, the answer is “It depends.”

But there are some factors to consider when making this decision.

Jeremy Michalek, a professor of mechanical engineering at Carnegie Mellon University, published a study on the relative impacts of these categories last year. He found that a variety of factors affect a car’s overall environmental impact. Some have to do with location, some with consumer behavior and some with manufacturing decisions beyond your control.

Consider, for example, what kind of fuel powers the car. There’s no question that 100-percent electric vehicles have lower tailpipe emissions, because no fossil fuels are combusted during use. Total greenhouse gas emissions, however, are a different story. The electricity that runs a plug-in car has to come from somewhere.

Coal is the most common source of electricity in the United States, and it emits 27 percent more carbon dioxide than oil, per unit of energy produced, by some calculations.

So, according to Michalek, your best environmental choice depends on your location: “Consumers in Seattle obtain much of their electricity from hydroelectric power, so a battery electric vehicle that uses no gasoline will be at its best. Consumers in West Virginia obtain much of their electricity from coal, so a regular gasoline-powered hybrid electric vehicle like the Toyota Prius is better.”

Disposal of the batteries is also resource-intensive, because they must be disassembled carefully rather than tossed into a landfill.