“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Sunday, September 25, 2011

CO2 and the Nissan Leaf

Image Credit: Biomass Technology Group
In an earlier post I gave a few of the details of the Lexus CT 200h I've been driving for a few weeks now. Then, in my immediate predecessor post I discussed the fact that a much larger portion than I'd have thought of the electricity supplied by Anaheim Public Utilities, my electrical provider, comes from the burning of coal. Finally, in a much earlier post, I discussed the Nissan Leaf. The question to be addressed here: given my driver profile, which of the two vehicles would have the smaller carbon footprint with Anaheim Public Utilities supplying my electricity?

Starting with the CT 200h, let's calculate the CO2 emissions per mile (for my driving style). I'm averaging 51.16 m.p.g., and from this EPA site I find that running a gallon of gasoline through the internal combustion engine in a car (see the site for the assumptions which seem very reasonable) results in the emission of 8.8 kilograms of CO2. So dividing 8.8/51.16 yields 0.172 kilograms (172 grams) of CO2 emitted per mile.

I'm not sure of the accuracy, this site says a Prius, at 51 m.p.g. city and 48 m.p.g. highway, emits 127 grams/mile. I achieve mileage this good, so I should have a similar number. On the other hand, here at Grist we read that the Prius emits 238 grams per mile. These are large discrepancies. Since the EPA site lists its assumptions and since these seem reasonable, and since the number calculated from them is intermediate between the other two, that's what I'm using.

The Leaf would have gotten most of its energy from my home and thus, as best I can determine, 65% from the burning of coal, 20% from the burning of natural gas, and 15% from renewable sources (hydroelectric, geothermal, wind, and biomass in that order - I'm assuming no emissions from these). Here we find that the Leaf uses 34 kilowatt hours per 100 miles or 0.34 kilowatt hours/mile.

I'd need to supply this electricity, and I'll assume that the charging system is 85% efficient and that the transmission from the power plant is 80% efficient. That means that the output of the sources of my electricity need to supply 0.34/(.8*.85) or 0.50 kilowatt hours to propel my hypothetical Nissan Leaf for a mile. 65% of this half a kilowatt hour, or 0.325 kilowatt hours would be supplied by burning coal. From the same site I used in the previous post, I find that a typical 500 megawatt coal power plant will emit 3.7*10^6 (short) tons of CO2 to produce 3.5*10^9 kilowatt hours. Thus, the production of a kilowatt hour entails the emission of 1.06*10^(-3) tons of CO2. This is 0.962 kilograms, so the burning of coal to charge the Leaf will result in 962*0.325 or 313 grams emitted per mile.

For the 20% of my electrical energy supplied by natural gas,  this site shows (after some calculations, from the details of which I will spare my patient readers) that the 0.2*0.5=0.1 kilowatt hours that will derive from that source will produce 59.6 grams of CO2.

Thus, driving a mile in the Leaf will entail the emission of 313+60 or 373 grams of CO2. This is higher even than the high Grist site estimate for a gasoline powered Prius and over twice the estimate I derived for my CT 200h from the EPA site. I'd, without a doubt, spend less money on electricity in the Leaf at $0.14 per kilowatt hour vs. $3.899 (today) per gallon of gasoline in the CT 200h but my CO2 footprint would be over twice as high. And this is in the allegedly green state of California.

Finally, I'm now driving about 21,000 miles per year, thereby emitting 3.6 tonnes or 4.0 short tons (US tons of 2000 pounds) of carbon dioxide.

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