It's a simple fact that electric cars get cleaner when the electricity grids used to power them do.

And, indeed, those grids are growing less dirty each year in the United States.

The amount of U.S. electricity generated from coal has dropped significantly over the past few years--and that fuel has largely been replaced by lower-emission natural gas.

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That, along with a drop in electricity demand during the years of the "Great Recession," is having a positive impact on U.S. grid carbon emissions, according to a new Breakthrough Institute report.

Natural gas is proving attractive to utilities, and could possibly wean them off coal as renewable sources continue to develop.

Electricity grid substation (Image: FirstEnergy Corp on Flickr, used under CC license)

Electricity grid substation (Image: FirstEnergy Corp on Flickr, used under CC license)

The study looked at electricity generation in eight regions of the North American Electric Reliability Council between 2007 and 2013.

Where there was significant growth in natural-gas generation, that growth almost always came at the expense of coal generation.

Coal's share of the electricity-generating load in these areas declined, while other established sources like hydro and nuclear remained mostly stable.

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Natural gas accounted for 45 percent of the decline in coal during the period analyzed, with declines in electricity demand relating to the sluggish economy accounting for the remainder, the study says.

While there were increases in electricity generation from renewable sources during the same period, researchers claim they did not significantly impact the use of coal.

Looking at wind generation, they found that areas that saw significant increases also saw declines in nuclear and hydro-electric generation.

Two BNSF locomotives hauling coal trains meet near Wichita Falls, Texas

Two BNSF locomotives hauling coal trains meet near Wichita Falls, Texas

Since this involves essentially trading one zero-carbon-emission source for another, wind power in these areas did not contribute to an overall decline in emissions.

For now, natural gas will likely be the green method of choice for electricity generation. It's already much cheaper than coal, and can take advantage of existing infrastructure.

About $65 billion was spent on interstate pipeline construction over the past 18 years, and this should be enough to handle increased demand, according to a recent Navigant Research blog post.

Natural-gas plants are also more broadly distributed than coal-fired plants, and aren't reliant on climate or weather conditions like solar or wind farms.

This ability to work within the current grid infrastructure while still cutting emissions makes natural gas a "bridge fuel" to low-carbon energy, according to Navigant.

Electric power plant outside Ithaca, New York

Electric power plant outside Ithaca, New York

The U.S. is already the largest global consumer of natural gas, and could still see a 33-percent increase in demand by 2040, according to the Energy Information Administration.

By then, however, utilities will likely have faced far more substantial competition from distributed generation through sources such as solar arrays, both large-scale and on individual properties.

Companies are already concerned about the effect home-generated electricity will have on their business model, even pressuring state governments to curb home solar through legislation.

However, continuing declines in the prices of photovoltaic cells and lithium-ion batteries (for energy storage) will only make distributed generation more economically viable.


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