A Unit of Perspective

Most people don’t seem to grasp the scale of the energy problems we face. In an idle moment I thought of an energy related unit that might bring it home for people. I call it the Biomass Acre Year, or BAY.

My thinking was prompted by a line from James Kunstler’s book, The Long Emergency. Almost as an aside, he mentions that a single squirt of charcoal lighter fluid is equivalent to the amount of sunlight that fell on a fern for seven years, some millions of years ago.

How does our energy use compare to the sunlight falling on an acre of forest in a year? I’m thinking in terms of the amount of usable biomass actually produced. It would depend on the amount of sunlight, which changes with location, and the efficiency with which the forest transforms that sunlight, along with carbon from the atmosphere, into woody biomass. I will also consider the practical problem of the energy required to extract the biomass from the forest, and the percentage of the total biomass that we can extract without damaging the forest.

The starting point is sunlight. In Vermont a square meter of earth receives about 1360 kilowatt-hours of solar energy in a year. I looked through various sources of information on photosynthetic conversion efficiency and came up with an average of 0.5%. This means that roughly half a percent of that solar energy ends up captured as wood. Our impressive 1360 kWh becomes 6.8 kWh. At 4,047 square meters per acre, we get 27,520 (I’m rounding off) kWh per acre per year. That converts to 93,902,000 BTU.

Of course, we can’t extract all of that – it would kill the forest. The rule of thumb for a well managed hardwood forest is that one can sustainably extract a cord of wood per year per acre. A cord of hardwood contains roughly 20 million BTUs, so that gives a ratio of 4.7 to one. I’ll be slightly conservative and round that up to 5:1. That reduces our output per acre/year to 18,780,400 BTU.

But wait, there’s more! It takes energy to get energy. That theoretical wood can’t be burned in place to get energy. It has to be felled, processed, and transported. I looked up numbers for the energy return on investment (EROI) of wood chip production, a mode of use popular for large scale biomass use. I found numbers between 24 and 28 to 1, meaning that 1 unit of energy invested gets us an average of 26 units of biomass energy at the woodchip burning facility. We net 25 units, in this case 18,058,076 BTU. I’ll round that down to 18 million BTU, and call it a VTBAY, or Vermont Biomass Acre Year.

How to apply this? Let’s say you have a car that gets 25 miles per gallon and you decide to drive from somewhere in Vermont to Boston and back, a round trip of 500 miles. You will have expended 20 gallons of gasoline at 115,000 BTU per gallon, or 2.3 million BTU. That is 0.127 VTBAY. One trip to Boston has sucked up the equivalent output of an acre of Vermont forest for about 47 days.

An average American drives 13,500 miles annually in a car that gets 21 mpg. That’s 73.9 million BTU, or 4.1 VTBAY. Just to bring it back down to basics, a gallon of gasoline is equivalent to 2.3 VT Biomass Acre Days (VTBAD) and a gallon of diesel or #2 fuel oil is 2.6.

The embodied energy numbers (the energy used in manufacturing) I find for automobiles vary between 138 and 255 million BTUs. That is 7.7 to 14 VTBAY.

An interesting article in Low Tech Magazine estimates the embodied energy of computers. The rough ratio the author found is 12:1, meaning 12 pounds (or kilos) of fossil fuel are needed to manufacture 1 pound (or kilo) of computer. A gallon of diesel weighs about 7.2 pounds, so every 9.6 ounces of computer has the equivalent of a gallon of diesel embodied in it. A six pound laptop computer is equivalent to 10 gallons of diesel or 26 VTBAD. Call it a month.

And the electricity to power that laptop? According to the Energy Information Agency, an average American household uses just over 11,000 kilowatt hours annually. That’s a hair over 2 VTBAY. Of course, if we were trying to convert that biomass equivalent to electricity by burning it in a wood chip power plant we only get 25% of that energy as electricity, increasing the impact to 8 VTBAY. That is, an average household would require 8 acres of well managed forest to keep it powered up with wood chip fueled electricity.

I could go on like this, but the acres are piling up. The point of this exercise is to give some idea of the relationship between our demands for energy and the amount produced by nature. So far, just for a car (lasting 7 years), a year’s gasoline supply, a laptop, and electricity, our average American needs 13-plus acres. We haven’t dealt with a house and its contents or the fuel to heat it, or workplace energy. In a previous essay I calculated that it would require all 25 million acres of our northeastern hardwood forest, perfectly managed for firewood, to replace the #2 heating oil now burned in the northeast. Obviously, that can’t happen, and it doesn’t account for the propane, natural gas, and electricity.

The answer is less. We need to do what we do with less material and less energy, and by a significant margin in order to be sustainable. Sure, there are renewable energy sources that are more spatially efficient than biomass, but they have their own costs. We will not be able to pour magic renewable pixie dust in our fuel tanks and just keep driving. This is the proverbial third rail of politics: asking Americans to exercise restraint. Luckily, I am not running for political office. I am saying: Reduce your “acreage” now, because nature will be crowding you in the future, like it or not.

Comments

[Concojones]

This is a gem of a post.

"So far, just for a car (lasting 7 years), a year’s gasoline supply, a laptop, and electricity, our average American needs 13-plus acres"
Following your calculation, I arrive at 24 acres for just a year of gasoline and electricity, both including a 25% energy efficiency. That's acres of hardwood forest. I also looked up that Vermont has about 10 acres of surface per person, but most of that would be required for food production, and what remains is not all hardwood forest.