Krugman is correct in his analysis but wrong in his conclusions. This is what he said in today’s NYT column (behind a pay wall)

In fact, corn is such a poor source of ethanol that researchers at the University of Minnesota estimate that converting the entire U.S. corn crop — the sum of all our ears — into ethanol would replace only 12 percent of our gasoline consumption.

This analysis is flawed. If we are to consider ethanol as a primary fuel, and an even partial replacement for fossil fuels, then we must allow the ethanol production infrastructure to stand on its own spindly straw legs. We must demand self-sufficiency of the fuel, and assume in our calculations corn production and distillery are run on ethanol--not gasoline.

After all, we are looking to ethanol to replace some or all of our petroleum, so we must consider the entire corn-to-ethanol process is fueled by ethanol. Otherwise why use petroleum in a pointless inefficient conversion when the resulting ethanol is less energy dense and would require larger storage than gasoline? So some of the ethanol flowing out the back end of the ethanol plant must be rerouted around to the receiving dock to make more ethanol.

Hence the need for Net Energy calculations. Krugman’s UoM researchers stopped calculating at a very very inopportune moment. This is what they did.

The US consumes 9.5 million barrels gasoline per day
US consumer 145 billion gallons gasoline annually.(9.5 million gallons per day * 365 days in year * 42 gallons per barrel)
Annual US corn crop harvest is 10 billion bushels.
Ethanol yield is about 2.5 gallons per bushel.
10 billion bushels * 2.5 gallons ethanol per bushel corn = 25 billion gallons of ethanol.
Ethanol has less energy density so 25 billions/1.5=16.7 billion gallons of gasoline equivalent
16.7 billions gallons of ethanol / 145 billion gallons gasoline used annually = about 12%

This is the final calculation they neglected.

One gallon net production requires three gallons input into that production because corn ethanol has an EROEI of 1.34 to 1.

So 16.7 billions gallons/4 = 4.2 billion gallons.

Thus to restate Krugman's conclusions:

In fact, corn is such a poor source of ethanol that researchers at the University of Minnesota estimate that converting the entire U.S. corn crop — the sum of all our ears — into ethanol would replace only 3 percent of our gasoline consumption.

pete