Biodiesel can be produced from crops, such as soybeans. The reported EROI for biodiesel from soybeans is 3.2(2). Note that this is over double the EROI for ethanol, and that doesn’t even account for the higher efficiency of the diesel engine. Soybeans yield about 40 bushels per acre, which translates into around 60 gallons of biodiesel per acre. This is far short of the 350 gallons or more of ethanol that can be produced from an acre of corn, but we have to take into account the net energy produced. Given that the real energy return of grain ethanol is around 1.3, it took the energy equivalent of around 350/1.3, or 269 gallons of ethanol to make the 350. We netted out 81 gallons. For the soybeans, it took 60/3.2, or 19 gallons of biodiesel equivalent to produce the biodiesel, for a net of 41. But recall that 1 gallon of biodiesel is worth 2.25 gallons of ethanol when both are used in their respective engines, so the biodiesel yield is "worth" 2.25*41, or 92 gallons of ethanol. (Please note that these calculations are approximate. If I were going to try to publish this somewhere, I would convert everything into BTUs to calculate the net yields.)

However, I do not wish to make the argument that we should be making biodiesel from crops, unless we are doing so from by-products left over from food production. Production of biodiesel (or ethanol) from crops can’t make a significant dent in our current usage of motor fuels. Fortunately, there may be a better way. A couple of years ago, I ran across an article that really caught my attention. It was my Reference 1  (http://www.unh.edu/p2/biodiesel/article_alge.html), a report by Michael Briggs at The University of New Hampshire. Briggs explained that biodiesel can be produced from algae, at yields as high as 15,000 gallons per acre! Briggs did a number of calculations of the feasibility and cost of replacing the entire motor fuel supply of the U.S. with biodiesel. I checked his calculations and read his references, and his analysis - based on experiments conducted by NREL - appeared to me to be spot on. In his own words, regarding the acreage that would be required:

In the previous section, we found that to replace all transportation fuels in the US, we would need 140.8 billion gallons of biodiesel, or roughly 19 quads (one quad is roughly 7.5 billion gallons of biodiesel). To produce that amount would require a land mass of almost 15,000 square miles. To put that in perspective, consider that the Sonora desert in the southwestern US comprises 120,000 square miles. Enough biodiesel to replace all petroleum transportation fuels could be grown in 15,000 square miles, or roughly 12.5 percent of the area of the Sonora desert (note for clarification - I am not advocating putting 15,000 square miles of algae ponds in the Sonora desert. This hypothetical example is used strictly for the purpose of showing the scale of land required). That 15,000 square miles works out to roughly 9.5 million acres - far less than the 450 million acres currently used for crop farming in the US, and the over 500 million acres used as grazing land for farm animals.

It would be preferable to spread the algae production around the country, to lessen the cost and energy used in transporting the feedstocks. Algae farms could also be constructed to use waste streams (either human waste or animal waste from animal farms) as a food source, which would provide a beautiful way of spreading algae production around the country. Nutrients can also be extracted from the algae for the production of a fertilizer high in nitrogen and phosphorous. By using waste streams (agricultural, farm animal waste, and human sewage) as the nutrient source, these farms essentially also provide a means of recycling nutrients from fertilizer to food to waste and back to fertilizer.

Regarding the costs, he writes:

In "The Controlled Eutrophication process: Using Microalgae for CO2 Utilization and Agircultural Fertilizer Recycling", the authors estimated a cost per hectare of $40,000 for algal ponds. In their model, the algal ponds would be built around the Salton Sea (in the Sonora desert) feeding off of the agircultural waste streams that normally pollute the Salton Sea with over 10,000 tons of nitrogen and phosphate fertilizers each year. The estimate is based on fairly large ponds, 8 hectares in size each. To be conservative (since their estimate is fairly optimistic), we'll arbitrarily increase the cost per hectare by 100% as a margin of safety. That brings the cost per hectare to $80,000. Ponds equivalent to their design could be built around the country, using wastewater streams (human, animal, and agricultural) as feed sources. We found that at NREL's yield rates, 15,000 square miles (3.85 million hectares) of algae ponds would be needed to replace all petroleum transportation fuels with biodiesel. At the cost of $80,000 per hectare, that would work out to roughly $308 billion to build the farms.

The operating costs (including power consumption, labor, chemicals, and fixed capital costs (taxes, maintenance, insurance, depreciation, and return on investment) worked out to $12,000 per hectare. That would equate to $46.2 billion per year for all the algae farms, to yield all the oil feedstock necessary for the entire country. Compare that to the $100-150 billion the US spends each year just on purchasing crude oil from foreign countries, with all of that money leaving the US economy.

I spent a lot of time reading through his references (some are very long reports), and I could not understand why we weren’t massively funding this research. It turns out that NREL stopped funding the program in 1996. The reason remains unclear to me, but this concept had given me hope that there might be a viable alternative out there after all that didn’t require us to turn all our forests into farmland. I spent a lot of time wondering just how I could involve myself in this area and contribute. I did e-mail Michael Briggs and we had a nice discussion, and I came away convinced that he knew what he was talking about. So why on earth weren’t we all over this? Frankly, I still don’t know the answer to that.