The Leaf: 99 is not as simple as it looks

Yesterday I wrote that 99 is a highly compelling number when applied to the miles per gallon one can obtain while driving an electric car, in this case the Nissan Leaf. Many more people will be attracted to take a look at a car with such a credential. But Vaclav Smil, a University of Manitoba professor, author of Energy Myths and Realities and a generally wise hand, responds by taking issue with the Environmental Protection Agency’s evaluation of the Leaf’s mileage. When one takes into account the use of coal or natural gas to produce the electricity to power the Leaf – as opposed to the more highly energy-dense gasoline in an internal combustion energy – the mileage plummets, Smil wrote me. Here is his beginning salvo:

33.7 kWh is indeed a thermal equivalent of a gallon of gasoline (America still prefers its medieval units). But that electricity has to be generated, and 80 percent of the United States’ electricity comes from coal and natural gas, which is about 35 percent efficient, so that 2.85 TIMES as much chemical energy in coal is needed to produce those 33.7 kWh, or, inversely, the actual ‘mileage’ would not be 99 but 34.7 mpg, MUCH worse than my Honda Civic. As always, little kindergarten science and basic numeracy goes a long way.

Thanks Professor Smil, I replied. What about natural gas? Isn’t it much more dense energy-wise than coal? Yes, but still nowhere near gasoline. Here is Smil’s breakdown:

A correction (which does not affect the outcome) first: I said 80 percent but it is actually 70 percent fossil-fueled (roughly 50 percent coal, 20 percent gas), 20 percent nuclear and 10 percent hydro.  If the electricity for electric cars would come ONLY from hydro or PV or wind then we could use the STRAIGHT thermal equivalent (1 Wh = 3600 J or 1 kWh = 3.6 MJ) and, obviously, no carbon burden is involved (except, of course, for fossil energy needed to MAKE the turbines, cells, steel for towers etc, all embedded energy).

Coal generation involves a mix of old and new plants, the former as low as 30-32 percent efficient, the best new ones about 40 percent. If we go with 35 percent as a good U.S. coal-generation mean, then we need 1/0.35 or 2.857 kWh of coal to generate a kWh of electricity.

Gas-fired generation is more efficient, 40 percent straight, and up to 60 percent in combined cycle with a steam turbine. So if we assume a liberal mean of 50 percent, we need 1/0.5 or 2 kWh of natural gas to generate a kWh of electricity. All in all, this involves two recurrent and a bit tricky problems of converting different energies to a common denominator and choosing the average efficiency rate (keeps changing!). One way to do it (particularly when doing international comparisons) is to assume that a kWh of hydro or wind or nuclear electricity equals its straight thermal value of 3.6 MJ; the other is to assume (for all, or at least for the nuclear) the prevailing mode equivalent, in the U.S. case the fossil fueled generation using coal and gas, and multiply the non-fossil kWhs roughly three times.

The EPA used a straight equivalent converting kWh to gasoline, but as most of those kWh will come from fossil-fueled combustion it should have used a weighted mean of the U.S. electricity generation, and hence at least roughly halving the 99 mpg claim. Of course, it would be nice if we could generate with 100% efficiency, but thermodynamics does not play nice.

The blogosphere is beginning to gripe about No. 99 as well. At Forbes, Warren Meyer gets hot under the collar, and sees an EPA conspiracy afoot. At the New York Times, Matthew Wald merely notes that two federal agencies – the EPA and the Federal Trade Commission – disagree on the Leaf’s range on a single charge. The former says it’s 73 miles, and the latter 96 to 110 miles.

The trouble here is the inherent imperfection of attempting to translate a numbering system intended for one thing (gasoline-fueled vehicles) to another (electric cars). Yet, since the entire energy edifice is changing before our eyes, with natural gas moving closer to center stage in terms of electricity production, and the electric-power grid becoming generally more efficient, the ball is not static. 

Update: The mileage numbers are just out for the Chevy Volt, which though advertised as an electric car actually is backed up by a gasoline-driven engine when the battery wears out. It clocks in at 60 mpg.