John Voelcker’s article ”How Green Is My Plug-In?” focuses on the carbon impact of plug-in hybrid electric vehicles. Their overall effect will depend, of course, on how many are built in the coming years and just what their electric ranges are. IEEE Spectrum ’s David Schneider discussed this issue with Jeremy Michalek, an assistant professor of mechanical engineering and of engineering and public policy at Carnegie Mellon University. Michalek has an upcoming article on the sizing of batteries for such cars in the journal Energy Policy . According to his calculations, plug-in hybrids with big batteries, like the Chevy Volt, may never save consumers any money.
IEEE Spectrum: Tell me a little bit about your research.
Jeremy Michalek: I do work in three areas: The first is systems optimization. The second is design for market systems—that includes things like trying to measure consumer preferences and predict what people are likely to buy in the marketplace. The third area is green design and environmental policy, and that’s where the plug-in work primarily falls. You can have the greenest thing in the world, but if nobody buys it, it makes zero impact.
Spectrum: What sparked your recent study of plug-in hybrid vehicles?
JM: I was looking at other work going on that examined the economic and environmental implications of plug-ins, and I noticed that people made some assumptions I wasn’t sure were valid. One of those was that they would ignore the weight of all the batteries you would have to add to the vehicle.
I wanted to understand whether that added battery weight was important. If you add it all up—especially for a large-capacity plug-in like the Chevy Volt—you really bog down the vehicle. And that can affect efficiency. So even though you get to drive in electric mode for a larger portion of your miles, you get lower efficiency.
Our study showed that moving from a battery pack sized for 7 miles of electric travel to a heavier pack sized for 60 miles of electric travel would increase the amount of electricity needed to propel the vehicle in electric-only mode by about 10 percent and increase the gasoline needed in gas-only mode by about 8 percent. So there’s definitely an effect, but it is not the most significant factor. What turned out to matter much more is the charging pattern.
Most of the other studies of plug-in hybrids analyze what the vehicle does for the median driver. But we wanted to see how differences in driving patterns would influence the economic and environmental implications of such cars.
Spectrum: You found in your study that plug-in hybrids might not be good for everybody. But my understanding was that an electric range of 40 miles—the projected electric range of the Chevy Volt—encompasses the daily driving of a very large fraction of the driving population. Is that not true?
JM: That is true, but just because you drive fewer miles than the electric range of the vehicle doesn’t make the vehicle better, because you have to pay for all those batteries when you buy the car.
The base case in our study assumes batteries cost US $1000 per kilowatt-hour of capacity. We looked at cheaper batteries, too, but the base case might be a little bit cheap already. And even at that price, you cannot make up the cost of the batteries in fuel savings over the lifetime of the vehicle. So even though you get to operate it on electricity rather than gasoline more of the time, all the fuel savings in the future don’t add up to what you paid for a big battery up front.
Spectrum: But there are benefits that don’t have a price tag attached to them—the alleviation of smog in the inner city and the avoidance of imported petroleum, for example—isn’t that true?
JM: It is. The reduction of criteria pollutants could be very important in a place like Los Angeles, where even though the vehicle costs more, and even though I may not be reducing greenhouse gases as much as I would like, the car is still worth it because moving the pollution to the power plant, which is farther from the city, is important for human health.
That’s an aspect that’s not included in our assessment. And the benefit to public health could indeed be an additional reason for policymakers to push plug-ins regardless of other factors.
Still, the three main motivations for plug-in hybrids are that they are going to save consumers money, they are better for the environment in terms of the amount of carbon dioxide produced, and they reduce fuel consumption and thus reduce dependence on foreign oil. We examined all those things.
Spectrum: Wouldn’t a plug-in hybrid with, say, a 200-mile range reduce the fuel consumption considerably for virtually everyone?
JM: Yes, such a vehicle would reduce fuel consumption for everybody. The question is, at what cost? That’s why we did a sensitivity analysis—to see how much things would need to cost to make the plug-in look more attractive economically.
For example, what if a gallon of gas cost $6? We found that under that scenario, a small-capacity plug-in, one that has only enough batteries to travel 7 miles on electricity, could be cost-effective for most drivers. On the other hand, something like the Volt, with its 40 miles worth of batteries, wouldn’t be cost-effective for anybody, even with gasoline at $6 a gallon (assuming the consumer is paying for those batteries, not Uncle Sam).
We also looked at the effects of a $100-per-ton carbon tax, which is significantly higher than anybody in Washington is talking about—at least for the near term. If carbon were taxed at that rate, it would not change the picture very much in terms of pushing people to buy more plug-ins. That’s because the production of electricity generates carbon too; vehicular carbon emissions don’t just come from the production, distribution, and combustion of gasoline.
On average, using gasoline to propel a vehicle emits more carbon than does running a car on electricity. But if you were to pay $100 a ton for every extra ton of carbon released, it wouldn’t be enough to persuade many more drivers to purchase plug-in vehicles on financial grounds—although that changes if the infrastructure used to produce the electricity itself becomes more green.
Spectrum: Yet General Motors is designing its plug-in hybrid for a 40-mile electric range. Where did the folks at GM come up with that number?
JM: That’s a great question. I don’t want to speak for GM, but in the press they seem to focus on gasoline displacement for a large portion of U.S. drivers. Displacement is certainly good. The question is, what cost should we be willing to pay as a society to achieve it, and what is the most efficient route?
Spectrum: In light of your analysis, is it conceivable that GM might decide to offer the Volt with battery packs of several different sizes?
JM: I think that being able to customize the size of the battery for your driving style would make a big difference in terms of putting the right vehicles in the hands of the right drivers. You could buy a Volt with a 5-mile battery pack, for example, if you were typically able to charge every 5 miles.
But there are some technical issues that would make it difficult to deliver on that. Vehicles have to pass crash tests. And their dynamics will be altered when you remove lots of heavy batteries and change the center of mass. So the vehicle would have to be designed so that it would show acceptable performance and safety in every configuration. That’s a big challenge with these vehicles being so new. I’m not surprised that nobody has tried it.
Spectrum: So if you have to make a choice—big or small batteries for plug-in hybrids—which is best?
JM: From what we’ve found, if you have a higher-capacity plug-in, something like the Volt, it could lower greenhouse-gas emissions for some drivers, but that comes at a cost that wouldn’t be paid back by fuel savings. A $100-a-ton carbon tax doesn’t even do it.
On the other hand, a driver who is able to charge frequently would do well to buy a small-capacity plug-in. This person might not care at all about the environment or about the nation’s dependence on foreign oil, yet he or she would still benefit from buying such a vehicle.
Places where the economic, environmental, and national-security objectives are all well aligned—that’s where you’d want to break in a new technology. I would say to carmakers, go after those people. And to consumers: Buy small, charge often.
To Probe Further
A prepublication copy of Michalek’s upcoming Energy Policy article is available on Carnegie Mellon University’s Web site.