Natural Gas for Cars

by Bill Chameides | April 17th, 2012
posted by Erica Rowell (Editor)

Permalink | 1 comment

Less polluting than its peers, natural gas is considered a good bridge between today’s fossil fuels and tomorrow’s renewables. But with respect to climate natural gas in cars (as in this Honda) may not be a winner after all. (Photo: Honda News/flickr)

Crossposted with National Geographic’s Great Energy Challenge blog.

Does it make climate sense to drive cars with natural gas?

Our nation appears to be rapidly moving to a natural gas-powered economy. Advances in hydraulic fracturing (fracking) and horizontal drilling have made huge deposits of natural gas in shale and tight sands commercially viable. (See “Hydrofracturing: An Energy Revolution.”) Suddenly the United States is awash with a cheap form of domestic energy. Cheap enough to compete successfully against coal for generating electricity and climate friendly enough to make it the Environmental Protection Agency’s electricity-generating choice as well.

But there are some troubling questions, some related to the environmental damage that fracking might cause to local communities and others related to the climate benefits of natural gas itself.

Because the major ingredient of natural gas is methane — a powerful greenhouse gas many times more effective a global warmer than CO2 — any leakage of natural gas during the extraction (production), transport or storage stage can offset or even overwhelm the climate benefits of burning natural gas instead of coal.

Where you end up coming down on the argument about climate benefits largely depends upon what you assume about the leakage rate for natural gas. Assuming a large leakage of as much as eight percent of the total production might lead you, as it did Cornell’s Robert Howarth et al, to conclude that sticking with “dirty” coal is more beneficial from a climate perspective. Assuming a more modest leakage of up to three percent, as EPA estimates, might result in the conclusion that switching to natural gas provides a climate benefit (although, because it is a fossil fuel, not a carbon-free one without carbon capture and storage).

Don’t count on that debate being resolved until the scientific community and gas industry team up to comprehensively determine just how much gas is actually leaking. 

But while the debate roils over the climate benefit of substituting natural gas for coal in electricity generation, there has not been a similar assessment of whether it makes climate sense to substitute natural gas (actually compressed natural gas or CNG) for petroleum to power our cars and trucks. That is, until now, with the publication of a new paper by Ramon Alvarez of the Environmental Defense Fund and co-authors (including yours truly) in the Proceedings of the National Academy of Sciences.

Why Natural Gas-Powered Cars and Trucks?

At current prices, switching to compressed natural gas as a fuel for your car or truck would seem a no-brainer — on an energy basis, natural gas costs about half as much as gasoline. And because natural gas is a domestic fuel, it makes great sense from a national security point of view. There are downsides:

Still it is an alternative, and one that some municipal transportation systems have opted for. So, does the switch make sense from a climate perspective?

An Estimate of Climate Impacts of Vehicles Powered by Compressed Natural Gas

When it comes to fossil fuels and carbon emissions, coal is the dirty fuel and natural gas the clean one with petroleum in the middle. Because coal is so “dirty,” it is relatively easy for natural gas to beat out coal in the climate-benefit contest for producing electricity. But the case is not that clear when it comes to natural gas and gasoline.

To assess the relative benefits, Alvarez et al used a metric we called the technology warming potential (TWP), which calculates the relative warming of a given fuel or technology choice over the lifetime of its use. The results were not encouraging for compressed natural gas-powered cars.

Even when adopting EPA’s modest leakage rates, we found that a full-scale U.S. move from gasoline to natural gas would initially lead to more warming than if we just stuck with gasoline.

Because methane is removed from the atmosphere more rapidly than CO2, the extra climate warming from natural gas leakage doesn’t last forever but eventually disappears. But that happens very slowly.

The break-even point between warming and cooling doesn’t happen in the Alvarez et al calculations until 80 years after the initial switch. And a 10 percent climate benefit compared to sticking with gasoline doesn’t accrue for 150 years. And that 10 percent benefit is approximately equivalent to improving a vehicle fleet that gets 30 miles per gallon by a mere three miles per gallon. Switching from diesel to compressed natural gas would take almost 300 years before seeing a climate benefit.

If we’re serious about cutting greenhouse gases in the coming decades, that, my friends, is simply not going to, as the phrase goes, cut it.*

Leakage the Key

Of course the wildcard in our study and all the related ones is the uncertainty surrounding the natural gas leakage rate. A colleague of mine commented on this subject saying, and I paraphrase, the more one learns about natural gas leakage, the more one realizes we don’t know much about it. It is conceivable that leakage rates are actually less than EPA’s estimate, in which case compressed natural gas might be a climate winner. And of course the opposite may also be the case.

And so the debate rages on, with claims and counterclaims (see here, here, and here). Of course one way to resolve the issue is for the scientific community and gas industry to team up and comprehensively determine just how much gas is actually leakin
g. (Here’s a study looking at methane leakage rates in Colorado.)

But there’s an easier way out that would make the whole debate over leakage moot: simply close up the leaks in the supply network and capture the natural gas before it escapes. In fact, in our paper we estimated that if leakage rates were cut to 1.6 percent (as opposed to the three percent estimate used in the paper) “CNG cars would result in climate benefits immediately and improve over time.”

You know what they say: a cubic foot of natural gas saved is a climate benefit earned.


End Note

* On the other hand, it should be noted that Alvarez et al did find an immediate climate benefit in switching from coal to natural gas for electricity generation when using EPA’s modest leakage rates.

filed under: climate change, coal, energy, faculty, fossil fuels, fracking, global warming, methane, natural gas, transportation
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1 Comment

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  1. Greg Neal
    May 2, 2012

    I used the web site to compare total greenhouse gases (GHG) of a 2012 Honda Civic CNG vs. a 2012 Toyota Prius. Assuming 15,000 miles per year, they calculate: (1) Civic: 3.7 tailpipe GHG + 2.0 “upstream” GHG (2) Prius: 2.9 tailpipe GHG + 0.7 “upstream” GHG This suggests that the highest MPG gasoline car currently available (Prius) is a better choice than the only natural gas car available (Civic). Now a Prius with switchable “dual-fuel” tanks, one of which being for higher density “Adsorbed Natural Gas,” (which would allow easy home CNG refills at normal pressures for in-town travel) and you might have a real low-GHG champ! Alas, technological feasibility might never be realized if affordability and consumer acceptance were lacking. Keep up the good work!

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