“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, June 30, 2013

Low hanging fruit revisited

Photo credit: Lincolnloop.com
About three years ago, I posted an article on the "low hanging fruit" in fuel savings. In that article, I demonstrated that for a given increase in m.p.g., the lower the starting m.p.g. (before the increase) the more fuel will be saved by that increase. Thus, a driver who drives 12,000 miles per year and increases from 15 m.p.g. to 18 m.p.g., either by purchasing a new vehicle or changing driving habits, will save about 133 gallons of gasoline per year. Another 12,000 miles per year driver who increases from 25 m.p.g. to 28 m.p.g. will save only about 51 gallons per year. The best way to understand this is to think of gallons per mile or, more transparently, gallons per 100 miles, the inverse of m.p.g. This number is 100*1/(m.p.g.) Thus, the first driver uses 6.67 gallons per 100 miles before the change and 5.56 gallons per 100 miles after. This driver saves 1.11 gallons every 100 miles. The second uses 4 gallons per 100 miles before and 3.57 after. This driver saves 0.43 gallons every 100 miles.

I was reminded of that post by this article in Energy Trends Insider. The article discusses a construct from the Department of Energy (DOE) at a new website that discusses the so-called "eGallon." This number purports to give the quantity (or cost) of the electricity that it would take to move a "typical" electric vehicle (EV) as far as a gallon of gasoline takes an "average" conventional car. This is where the "low hanging fruit" concept comes in. Replacing a 20 m.p.g. vehicle with an EV saves MUCH more than replacing a 30 m.p.g. vehicle. The 30 m.p.g. vehicle goes half again as far on a gallon and thus the eGallon costs more for that driver.

I applaud the DOE for attempting to clarify the possible savings in fuel expenditure vs. electricity expenditure but, in some cases, it may be very misleading. The calculation is further complicated by the wide variance in how electricity is priced in various localities.

As to the "low hanging fruit," the plot below shows, for a driver who drives 12,000 miles per year, how many gallons of fuel are saved per year by moving from one m.p.g. driving regime to a higher one. It plots initial m.p.g. from 10 to 40 and final m.p.g. from whatever was the initial m.p.g. to 120 m.p.g.  The "front" axis is initial m.p.g., the axis that extends back and right is final m.p.g., and the vertical axis is gallons saved. As can be seen in the rightmost portion, the savings from a high starting point are not nearly as large. It's also easy to see, especially at the left end, that the big gains are in the initial improvements - the slope is dramatically steeper than at the higher final numbers.

No comments: