My Dirty, Coal-Powered Plug-in Prius?

By Bill Moore

Posted: 22 Oct 2009

The National Research Council just issued a report that claims plug-in vehicles like our plug-in Prius are worse for the environment than gasoline-powered vehicles. The media has quickly jumped all over this -- as did a local physician friend of mine -- casting electric cars and PHEVs in a very negative light.

I fully admit that here in Nebraska, our PICC-converted 2009 Prius is being powered by a significant fraction of Wyoming coal, hopper car loads of it every hour. My local power company generates nearly 58% of our electricity from this dirty, CO2-laden fossil fuel. Another 27% comes from natural gas and oil. A nuclear plants located within 25 miles of our home here in Papillion, a suburb of Omaha, provide another 15%. Pathetically, only 0.2 percent comes from wind and landfill gas.

Concerned about how much CO2 we're indirectly contributing by plugging in our car every night and recharging its battery, I did some calculations that I'll share with you, though bear in mind, my math skills leave a lot to be desired.

First off, how much CO2 would our Prius emit if we simply kept it in its stock hybrid mode (actually we can easily switch it back through a simple computer display)? My wife, who drives the car daily, commutes a total of 11 miles round trip. Assuming she gets 45 mpg, she will burn about a quarter gallon of gasoline a day. Depending on whose numbers you use, a gallon of gasoline will produce between 20-24 pounds of CO2. So, in hybrid mode, her commute produces 5.28 lbs of CO2 if we use 22 lbs per mile as our average. In European teams that about 135 g/km.

To drive her 11 miles at 45 mpg (assuming colder weather), in stock hybrid mode, the Prius will produce just over 5 pounds of carbon dioxide and consumes around a quart (0.9 L) of petroleum (60% from overseas).

Now let's switch the car back to PHEV mode and what happens?

I've been tracking the amp hour usage of the car on a daily basis and my wife is using about 30% +/- of the 26.6 amp hour capacity of the Plug-In Conversions Corporation battery pack over the 11 miles. At a finished voltage of 220 volts, that translates into about 1.8 kWh of electric power consumed. Some bit of gasoline is also burned to warm up the catalytic converter, but its difficult to measure exactly how much at the moment because of the '09 Prius' bladder-type tank. I am guessing it's way under a quart, probably measured in fractions of a cup, though as it gets colder, it will go up, as it does with any IC engine car in winter.

Generating a kilowatt hour of electric power produces an estimated 2.095 lbs of CO2. So, recharging the Prius' battery back to its starting 89% SOC results in about 3.2 lbs (1710 gram) of fossil fuel-produced CO2.

Although a nuclear power plant may not produce CO2 on site, carbon dioxide is produced during the fuel creation process. Estimates vary, but one figure places it at around 40 grams per kilowatt hour.

So, to the 1710 grams add another 72 or 1782 grams for the 17.69 km of her commute. That calculates out to be equivalent to 100.73 grams/kilometer in European terms and 0.356 lbs of CO2 per mile in American nomenclature.

Finally, let's add 0.25 liters of gasoline that's burned starting the car to warm the catalytic converter and defrost the windshield on cold damp days like today. At the moment, this is just a guess; I'll have a better handle on this once we upgrade the Prius display to more accurately display fuel burned each day. So, for the sake of argument, let's add roughly another 37 g/km.

Running the car on gasoline alone in standard hybrid mode for my wife's 11 mile commute generates something around 135 g/km of CO2, while in PHEV mode, it is right around 137 g/km, not as good as I'd like, obviously, but one that is likely to improve over time as more of our local power comes from wind and other less-carbon intense energy sources.

Even in a state so heavily reliant on fossil fuels, our PHEV Prius generates about the same amount of CO2 as a stock 2009 Prius in our particular driving situation. I would, of course, prefer that instead of being so dependent on coal, natural gas and oil, we were more like Quebec, Norway or Brazil where nearly all of their power comes from renewable hydroelectricity. Then it would be no contest.

And unspoken in all this brouhaha is the fact most of the power propelling my wife's Prius comes from the American power grid, where coal is becoming less important and renewable energy is growing at double digit rates. In the my book, both our car and the grid is heading in the right direction. You can't say that about your average car on the road.

ADDENDUM: 5 November 2009

My local power company responded to my request to determine the percentage of coal vs. nuclear power-produced electricity I use overnight when I recharge the Prius. It turns out that on a yearly average it's a 50/50 split, which means that the amount of CO2 I am directly responsible for is just over 2 pounds for my wife's commute of 10 miles. In grams of CO2 per kilometer it works out to be 54.7g/km.

Here's how I arrive at that number. I've been collecting data twice a day from the car for several weeks now and it appears that the commute consumes about 2.4kWh of energy from the battery. Of this, I now know that half comes from coal or 1.2kWh. Assuming 1.8 lbs of CO2 per kWh means that we'd be directly responsible for just over 2 lbs of CO2 for my wife's commute to and from work, at least when the temperature is in the high 50's to low 60's, and the IC engine remains off. If the car were operating as a conventional Prius, the same trip would create 5.3 lbs at 45 mpg. A conventional gasoline car getting 22 mpg in city driving would generate 11 lbs of CO2, over five times what we're producing as a PHEV, even using coal. Total electricity used to make the commute each day? Less than 17¢ a day.

But in actuality, the car is carbon-free. We monthly contribute $15 to our utility's green energy fund. The utility tells me that equates to 500 kWh of "renewable, carbon-free" energy per month the utility purchases in our behalf (though it doesn't actually come off my monthly utility bill). Assuming we consume 3 kWh of energy a day recharging the car, that's only 90 kWh a month, less than 20% of the 500 kWh we're voluntarily paying for. I'll assume the rest offsets my computer and other electronics I use to produce EV World.

So, when I tell someone from now on that I drive a plug-in Prius, I don't want to hear any more "Oh, you're plugging into a lump of coal." Actually, no I am not, thank you very much. By-the-way, what's your car running on? Unless it's sunshine or wind, don't even bring the issue up.

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