Counting Electrons and Molecules

The electric vehicles are coming. Whether they are to be plug-in versions of hybrid cars, along the lines of the eagerly-awaited Chevrolet Volt or plug-in Prius, or the pure EVs recently promised by the CEO of Renault and Nissan, Carlos Ghosn, we will soon have significant numbers of cars on the road for which fuel economy metrics must track kilowatt-hours, in addition to or instead of liquid gallons. As I noted in my posting a few months ago on the usefulness of a possible "miles per dollar" metric, we haven't yet developed the vocabulary for comparing the efficiency of such vehicles to that of the traditional cars they are meant to replace.

If I told you that Car #1 gets 58 miles per gallon and Car #2 gets 4.5 miles per kWh, would you know which one was more efficient, and could you even assess that without including some factors that are external to the car, such as how its electricity was generated? Now complicate matters by considering Car #3, a plug-in hybrid touted by its manufacturer to achieve 100 mpg overall. But does that figure include or exclude the electricity it used when it wasn't using its gasoline engine, and if so, on what basis? The people running the Automotive X-Prize have made a very good start on a practical way to answer these questions, by creating a metric they call MPGe, for "miles per gallon equivalent ." I can't improve on their explanation for what this does:

"Basically we ask: how much energy was delivered to the vehicle, and how far did
it go? We convert the energy to the number of gallons of gasoline containing
equivalent energy, and we express the result as miles per gallon."

The value of such a formula is that it enables us to compare on a consistent basis the performance of any car running on any fuel, including gasoline, diesel, ethanol, biobutanol, natural gas, hydrogen, and/or electricity. And when the only fuel involved is gasoline, the formula collapses into the familiar equation for calculating plain vanilla mpg. Best of all, this results in a single, easily-comparable number for each car model.

Unfortunately, when I thought about this in the context of the wind power study I discussed in Wednesday's posting, I realized that the MPGe formula has a flaw. A competition aimed at creating the most efficient production car possible might properly convert electricity to BTUs at the standard energy-equivalent rate of 3,412 BTUs/kWh, without regard to the energy used in generating it. However, with wind turbines and photovoltaic arrays generating only a small fraction of the power we use today, most of the kWh's flowing through our power grids took a lot more than 3,412 BTUs to create. Because this factor occurs in the denominator, under-counting BTUs/kWh at their theoretical equivalent makes a transportation system based on electric vehicles appear more efficient than it really is, from the perspective of the overall economy. With national energy security a major driver of alternative energy, this is a problem.

Consider California, a key market for EVs and plug-in hybrids, because of its air quality regulations. Over 40% of the electricity generated there comes from natural gas, consuming anywhere from 7,000 to 10,000 BTUs per kWh. Using the X-Prize formula, the MPGe for an EV that gets 4 miles per kWh would drop from 136 to 66, if we substituted 7,000 BTU/kWh for the textbook conversion. That's not entirely fair, because it ignores the upstream energy losses associated with turning crude oil into gasoline, which are in the range of 10-20%. Nor are we including distribution losses for electricity, which can be significant, or the shifting generation mix at different times of the day or year. The proper basis of comparison involves a full "well to wheels" analysis, specific to each location and for every segment of the power dispatch curve. That's neither realistic nor very useful for future consumers.

Ultimately, I still prefer miles-per-dollar, or better yet, dollars per 100 miles, because it avoids these complexities and deals with the two factors that are of primary interest to drivers: cost and distance. However, I'm also realistic about the likelihood of replacing miles per gallon as the standard of comparison any time soon, particularly since that's the basis of the recent major revision to the Corporate Average Fuel Economy standard. MPGe represents the natural evolution of that metric, if we can agree on the appropriate way to compare electrons to molecules.

Miles Per Dollar

Have you ever encountered an idea so blindingly simple and obvious that you slapped your forehead in frustration that it didn't occur to you first? I had one of those moments the other day, reading an article that popped up on my personalized MSN portal, concerning fuel economy comparisons. The link appears broken, but the gist of author's argument was that if we focused on how many miles our vehicles travel on a dollar's worth of fuel, rather than per gallon, we might make fewer unnecessary trips and choose more efficient vehicles to start with. I agree with that logic, though from my perspective "mp$" could be even more useful as we enter a world in which the gallons we're using aren't directly comparable, and as electricity enters the transportation mainstream, resisting easy conversion to gallons without heroic assumptions and creating potentially over-optimistic assessments of the overall efficiency of plug-in hybrid cars.

As fuel diversity increases, the utility of measuring vehicle energy efficiency in terms of miles per gallon (mpg) diminishes, unfortunately coinciding with a much greater emphasis on mpg thanks to last year's Energy Bill that raised the required new car fleet standard to 35 mpg. This is more than a technicality, when you consider that carmakers get to count "flexible fuel vehicles" (FFVs) that can run on E-85 or gasoline as though they achieved higher mileage on ethanol, rather than about a quarter less. The Energy Bill, which included provisions strongly promoting E-85 and FFVs, at least limited the contribution of this factor to 1.2 mpg of a carmaker's average through 2014, phasing out to zero in 2020.

I could not find any cars that were available in all possible energy permutations, but the 2008 Chevrolet Tahoe large SUV came close. It's available in gasoline, FFV and hybrid versions. With retail gasoline averaging $2.96/gal. this week and E-85 at $2.48/gal. (both varying widely by state,) and using the EPA's fuel economy estimates for this vehicle on both fuels, the "mp$" comparison is interesting:

• Tahoe V8 on gasoline: 5.4 mp$
• Tahoe V8 on E-85: 4.8 mp$
• Tahoe Hybrid on gasoline: 7.1 mp$

While I'm sure there are locations where E-85 would have an advantage over regular gasoline, that requires it to be priced in a way that fully reflects its 25% lower energy content.

The next comparison is between gasoline and diesel, which has been significantly more expensive than gasoline this winter. There are a few manufacturers with comparable cars available in both fuels, including Mercedes and Volkswagen. Since I couldn't find 2008 diesel results for VW, I picked the former's E-series sedan to compare. Since the gasoline E350 requires premium fuel, I added $0.25/gal. to the US average price.

• E350 6-cyl. on premium gasoline: 5.9 mp$
• E320 Bluetec turbodiesel: 7.9 mp$

Finally, let's compare a Prius-style hybrid with the likely result for a plug-in hybrid, such as the Chevrolet Volt. In the absence of actual efficiency data for the Volt, I assume it would be comparable to the Prius on gasoline. Electric efficiency should be around 4 miles per kilowatt-hour. The average residential electricity price last year was 10.7 cents/kWh, with some markets considerably above that and others offering time-of-day pricing that would allow for overnight recharging at a lower price, so the following is a rough estimate:

• Prius or Volt on regular gasoline: 15.5 mp$
• Volt on residential electricity: 37.4 mp$
• Volt in 50/50 driving mix: 22.0 mp$ (very impressive, but not quite the 100 mpg equivalent often touted)

Miles per dollar has much to recommend it, particularly for its simplicity and alignment with the priority consumers put on value. However, it also has two key disadvantages. Unlike mpg, it changes every time fuel prices do, so any comparisons based on mp$ are only snapshots at a point in time. Nor does it address the emissions associated with that dollar's worth of energy, though mpg doesn't do that, either. A carbon tax or cap-and-trade system would help align fuel prices with their environmental consequences and make the resulting mp$ comparisons reflect both price and emissions. In that case, mp$ would be a significant improvement over mpg, particularly in helping consumers cut through an increasingly complex set of different fuel and power-train options. And while I don't expect Congress to rewrite the new CAFE standard in mp$ terms, or carmakers to embrace a metric that calls some of their marketing into question, how hard would it be for consumer-oriented car websites to display mp$ alongside mpg? More information might just lead to better decisions.