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 didThe 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.
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."
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.
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