Roughly a third of the US' carbon dioxide emissions come from burning petroleum products in vehicles, a habit that also has significant implications for foreign policy and public health. The government and automakers have agreed to significant increases in fuel economy, a change that some of them intend to meet in part through the development of plug-in hybrid electric vehicles (PHEVs), which can complete short trips without burning any gasoline. But a report from the National Academies of Science suggests that getting these vehicles to market in significant numbers faces a series of challenges, and that it may take decades before PHEVs have a significant impact if fuel prices stay low.

The report was prepared by a committee that was initially assembled to evaluate hydrogen fuel cell vehicles; once that was complete, the Department of Energy asked them to also consider PHEVs, since much of the data on fuel efficiency and consumption of existing vehicles is relevant to both problems.

The authors consider two types of PHEV. One is a short-range PHEV-10, which would be similar to a plugin Prius that can go 10 miles on battery alone before switching to hybrid mode. The second is a longer-range vehicle that's similar to the Chevy Volt. That can go about 40 miles before its engine kicks in, and said engine never drives the wheels directly; instead, it simply powers an on-board generator to recharge the batteries.

In either case, both suffer from the same problem, namely that we shouldn't expect any major breakthroughs in battery technology. As far as the committee could determine, any PHEVs likely to reach the market in the next few decades will be using some form of lithium battery, which (thanks to its heavy use in personal electronics) is already mature technology. Any changes, in terms of cost, weight, charge density, etc., are likely to be incremental. The same goes for just about any other key component. We're already pretty good at building generators and electric motors, so we can't expect any breakthroughs there.

But it's really the batteries that will be the biggest barrier to adoption for a simple reason: price. The PHEV-10's battery currently adds $3,300 to the price of the car, and the battery of a PHEV-40 adds a whopping $18,000. Since change will be incremental, the report estimates that it will still be costing $11,000 in 2030 under likely estimates of battery progress; even in its optimistic scenario, the battery will still be pushing up the price tag by $9,000. Ouch.

(The costs don't end there, either. For a reasonable rate of charge, people will have to install 220V, 40A outlets in their garages.)

Because of the price barrier and the slow turnover of the total vehicle fleet, the most optimistic estimates are that only 13 percent of the US' total vehicles will be PHEV by 2030, and a more realistic estimate is for less than half that value. Worse still, unless the price of gasoline stabilizes at over $4.00 a gallon, PHEV-40s won't be cost effective until sometime in the 2040s (PHEV-10s will get there before 2030). The only way to drive their adoption in the interim will be some sort of incentive; the report estimates that up to $300 billion will be needed to push any sort of accelerated adoption. Although it's not discussed by the report, an obvious issue is whether the expected technical developments will actually occur if there aren't sufficient expectations of future sales.

Because of the slow pace of adoption, PHEVs are expected to have a negligible impact on oil use prior to 2030. And, once adoption rates are higher, they won't necessarily have a large impact on CO 2 emissions unless the electric grid is substantially decarbonized in the meantime, either through nuclear and/or renewables, or by sequestering some of the emissions from the coal plants that currently produce half the US' electricity.

Getting the electricity to the vehicles will also require some changes, as many areas of the country are already at grid capacity during peak usage hours. Unless there's some pricing incentive that encourages most people to charge their vehicles at night, PHEVs will put an uncomfortable strain on the grid.

So, given this grim picture, it might be tempting to question whether it's worth going this route at all. But the committee points out that the US has very little in the way of options for reducing its reliance on oil imports, so it really has to work on having all of its bases covered if it's serious about doing so. Although the individual effects are much smaller, combining PHEVs with biofuels and increased efficiency in conventional vehicles would be sufficient to drop US gasoline use to less than five percent of what it would be otherwise by 2050.

Unfortunately, it will be decades before significant differences in oil use become apparent, and require a significant up-front investment in technology and incentives. A limited oil supply that pushes prices well above $4.00 a gallon seems far more likely.