Jones Beene wrote:
The efficiency of the system is irrelevant. If every single leaf, grass,
branch and food crop that grows in North America were converted into
ethanol with 100% efficiency it would not be anywhere near enough.
Whoa !!! This is so outrageously false it defies the imagination !!!!
Nope. It is correct, as I shall show below.
Don't you have a calculator?
As a matter of fact, I do. Let's go over three methods of estimating this.
Method 1. You estimated 40 tons of wood per acre. Assuming those are metric
tons, that's 40,000 kg per 4047 square meters, or 10 kg per meter. Ordinary
wood produces 15 MJ per kg, so that's 150 MJ/year. There are 31,557,600
seconds per year, so that comes to 5 watts per m^2. I find that a little
implausible but perhaps in an area where the growing season lasts all year,
for trees which shade the ground considerably, it is possible. It is within
an order of magnitude of my estimate.
Method 2. Take the entire US land area and divide by population. You get
about 2 ha per person, but unfortunately half that land is not arable. It
is desert, steep rocky mountain, asphalt or Wall Mart floorspace. Arable
land in the US is estimated at about a hectare per person. In Florida or
the tropics with the Sun directly overhead is produces about 1020 kW per
square meter (http://en.wikipedia.org/wiki/Solar_power), but in the US
lower 48 it's about 900 Watts I think. Photosynthesis is roughly 0.1%
effective, so that comes to 0.9 watts/m^2. However, the sun only shines 12
hours per day, and in the US the growing season is six months per year, so
that comes to about 0.23 W/m^2. In Florida, maybe 0.5 W/m^2.
As I said, the average American has about 1 ha of productive land per
person (10,000 m^2), so this comes to 2.3 kW continuous power. That is
impressive. It is enough to run your house and your car, but unfortunately
Americans use a lot more energy than that. See the Annual Energy Review,
Table 1.5. The total is 338 million BTU per capita, or 356,609 MJ. Divide
jules by seconds per year (31,557,600) and you get 11.3 kW continuous
consumption, almost all of it fossil fuel. In other words, my
back-of-the-envelope estimate is that we consume roughly four times more
than all photosynthesis creates, whereas Pimentel, using a much more
sophisticated and careful method, came up with two times more than all
photosynthesis. (Yesterday I said that farmland produces only about one
third as much biomass as the fossil fuel we consume. The numbers I am
estimating here is for all land including national parks and other
non-farmland, which Pimentel estimates at 50% of fossil fuel.)
Very roughly, Pimentel is estimating that the average U.S. land per capita
produces 5 kW/ha continuously, or 0.5 watts. That is somewhat more
optimistic than me, but still an order of magnitude below your optimum tree
growing area. That's quite plausible. Even on the land I own, some areas
are far more productive than others. Are you sure this does not include
fertilizer? That would be an external energy input.
Method 2. Take the old rule of thumb that in hard times an acre garden can
support a family of four. This was true in the UK and the US, in the
preindustrial era without massive amounts of fertilizer. (You can ignore
animal or tractor input energy because that does not increase yields much.
Mainly it reduces human labor.) Assume the family follows the European
tradition, and disposes of human waste elsewhere, but it brings in a
roughly an equal amount of animal manure. Plus the family mulches kitchen
scraps, such as pea pods and corn silk. The RDA diet is about 2000 kcal per
day, although I doubt that a premodern family of four scraping by with an
acre garden ate 8000 kcal per day, let's say they did. To put it another
way, a person with a modern quarter acre garden can grow just about all the
food he eats, assuming he trades some of it to avoid dietary monotony. My
mother-in-law lives on this amount of land. (She has plenty of other land
for cash crops.) Actually, US land is not as productive as land in places
like Japan, which has a longer growing season and much more rain. Third
world farmers barely survive on 0.1 ha per person (0.25 acres).
Okay, so 4,000 kcal = 16.7 MJ, and there are 86,400 seconds per day, so
that comes to 193 W, which is close to the estimate for basal metabolism
plus the amount of work you do in a day. 0.25 acres = 1012 m^2, so that's
0.2 W/m^2, over the course of a year. That is about in line with what I
estimated before. Remember this is for land without modern agriculture;
i.e., no energy input. Of course, a person does not get to eat every
calorie of food grown in a garden. Insects eat a significant amount, there
is spoilage, bacteria, stalks, pea pods and other left-overs which are
plowed back into the ground. Still, people and other animals are remarkably
good at exploiting food resources, so it is a reasonable estimate that we
eat about 2/5ths of the food calories. (We excrete some of them, but I have
accounted for that already.) That brings us back to Pimentel's estimate.
Any questions?
- Jed
