http://www.motherjones.com/environment/2013/09/green-tesla-electrical-grid

This story first appeared on the Slate website and is reproduced here as part of the Climate Desk collaboration.
The knock on electric cars has always been the same: They’re great for the environment, but they’re pokey and impractical, and nobody wants to buy one. The stunning success story of the Tesla Model S has, improbably, flipped that equation. It’s blazingly fast, surprisingly practical, and everyone wants to buy one. But now some critics are asking: How green is it, really?

The quick answer: If current trends hold, it could be pretty darn green in the long run. But as of today, the calculation isn’t as straightforward as you might think. Depending on whom you ask, what assumptions you make, and how you quantify environmental impact, the answer could range from "greener than a Prius" to "as dirty as an SUV." And where the Tesla falls on that spectrum depends to a surprising extent on where you live and how much you drive it.

Charts: Tesla Motors is thriving—thanks the government.

Electric cars are squeaky clean, of course, in the sense that they don’t burn gas. With no engine, no gas tank, and no exhaust, they’re considered to be zero-emissions vehicles. But there’s more to a vehicle’s environmental impact than what comes out of the tailpipe. The Tesla doesn’t run on air. It runs on electricity, which in turn is generated from a range of different sources, from nuclear fission to natural gas to the darkest, dirtiest fossil fuel of them all: coal.

So if you’re going to stack a Tesla’s per-mile emissions against those of a gas-powered vehicle, you’ll need to start by looking at the composition of the electrical grid. Nationally, the grid is roughly 40 percent coal, 25 percent natural gas, 20 percent nuclear power, and about 10 percent renewable sources, led by hydroelectricity. So it’s fair to say that your average Tesla is powered in large part by burning fossil fuels.

Tesla acknowledges this, and insists that its cars are still far cleaner than their internal-combustion competitors. That’s because battery-powered cars are more efficient at converting their stored energy into forward progress. A Model S can travel upwards of 265 miles on a single charge of its 85 kilowatt-hour battery, which equates to less than 3 gallons of gas. Its official EPA miles-per-gallon equivalent is 89, far greater than a standard Toyota Prius.

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