Thursday, December 14, 2006

Energy Opportunity Cost of Agriculture

I was reading this interesting report on Life Cycle-Based Sustainability Indicators for Assessment of the U.S. Food System and the graph to the left caught my eye (click for larger version). Basically it shows that the US is creating 1.4 quads of food energy, but it takes an additional 10.2 quads of energy in the form of fossil fuels to produce, transport, process, package and cook the food. For every calorie of food we consume it takes 7.2 times as much energy in fossil fuels to put it on the table.

Looks like we are using a lot of energy in order to produce our food. But, what if we looked at it slightly differently. What if we looked at the energy opportunity cost of agriculture. What if we say, used the land to produce energy via solar panels, how much energy could we produce?

From the USDA, we find that there are 302 million acres of harvested cropland. From Google Calculator we find that this is equal to 1.2 × 10^12 square meters.

At Wikipedia we find that most parts of the US get 4.5kWh/m2/day of solar radiation a day or 1,642.5 kWh/yr.

Lets assume that the solar panels are 10% efficient (although scientists have just made 40% efficient ones).

If all harvested cropland had solar panels instead, they would capture:
1,642.5 kWh/yr * 10% efficient * 1.2 × 10^12 m^2 = 1.97 x 10^14 kWh.

Via Google calculator again we find that this equals 6.72 × 10^17 BTU or 672 quads of energy. That is 480 times as much as the 1.4 quads of energy in our food, and 66 times as much as the 10.4 quads of energy to produce, transport, process, package and cook the food.
Note: the calories in the food is based on what we eat rather than what we produce, so the 480 times ratio is probably on the high side as the US exports a lot of food. When I was comparing solar panels with sugar cane earlier I got a ratio of 50 to 1. Average caloric yield per acre in the US is much lower than that of sugar cane, but I don't know if it is just 1/10.

While it might appear that we are using lots of energy in order to produce our food, in fact the much bigger loss is the amount of energy that we could have produced had we put solar cells on the land.

It might seem that going organic would help to use less energy, as you won't use energy to create fertilizer. But, as we have seen here, organic farming tends to use more land than conventional farming (in this case by around 50%). Based on the numbers above, if you could switch from organic to conventional and decrease the amount of land you use by just 1.5%, you could put solar panels on that land and generate more energy than is used producing, transporting, processing, packaging and cooking the food (still assuming it takes 7 times as much energy as in the food). Free up 3% of the land and you have doubled the amount of energy. From an energy opportunity cost standpoint, it is not the fossil fuels that are expensive, it is the crops inability to capture much solar energy on the land that is expensive.

The one issue with this analysis is that in the short run (say 15 years) solar power will not economical and few would replace crop land with solar panels. Also, while fertilizer could be made from solar energy, currently it is not really possible to distribute the food using electric vehicles. So, in the short run does it make sense to buy organic now to try and reduce fossil fuel usage?

This is a tough choice. You are basically choosing between using more land or using more fossil fuels. By reducing the amount of land needed this protects forests from being turned into farmland or allows old farmland to become forest again. On the other hand by reducing fossil fuel usage it helps with global warming, air pollution and resource wars. I think I would go with reducing land usage, but I could understand those that went the other direction as well.


odograph said...

Well, note that the 10 quads goes all the way out to home preparation. It is a long ways from a simple "agriculture" number.

It's the calorie efficiency of a "meat lover's pizza" if you get my drift.


Fat Knowledge said...


Good point. Looks like 2.2 quads of energy use are directly attributable to agriculture production.

Anonymous said...

You would probably be interested in John Blackburn's "Solar Florida", Fat. Here is a direct quote: “The energy demands projected for Florida in the previous chapter could be met from about 2% of its land area at an average conversion efficiency of 10%”

He also points out that the land area does not have to be in agricultural production. Solar collectors, for example, could be installed over parking lots. Cars (and their drivers) would be much happier having the resulting shade.

Bill Mantis

Fat Knowledge said...

Thanks Bill, I will have to check that book out.

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