Constructed Climates

Figure 3.16: The tables compare trees and wood with personal energy use and the equivalents of various energy sources (after DeWald et al. 2005). Americans’ annual per capita energy use equals about four 22-inch diameter trees. The plot compares source emissions of fine particles and greenhouse gases for equivalent amounts yielding 10¹⁵ Btus of energy (after Houck et al. 1998). Wood wins in terms of greenhouse gases, but loses badly for fine particle emissions. Coal and electricity are generally worst, and natural gas has the lowest emissions. Newer EPA-certified wood-burning stoves (at the arrowhead) greatly reduce emissions.

The recent push toward biofuels takes us back to the future. Wood, a fine biofuel, provided most of our energy 100 years ago (Figure 3.1). The top table in Figure 3.16 relates tree size to energy content via the standard measure of wood, the cord, a stack of split wood 4 ft high by 4 ft wide by 8 ft long, with a mass of about 1,800 kg.[77] Over the year, the average North Carolinian uses about 1,100 kWh per month, or about 37 kWh per day, which equals about 10 kg of wood per day.[78] A wonderful University of Nebraska publication relates the energy from wood to other sources in the bottom left table.[79] My pickup truck’s gas tank holds about 10 gallons. Assuming a gallon of gasoline and a gallon of fuel oil have the same energy content, filling up my gas tank equals cutting down and burning up a 6.5-inch diameter tree. Do that once a week, and I know my yard will be quickly deforested.[80]

Wood provides heat. Space heating accounts for 8% of the United States’ present energy use, of which 86% comes from fossil fuels and 8% from wood.[81] How good is wood heat? On the one hand, in a wood-chipping operation the heating energy gained was almost 28 times the energy invested, with the greatest expenditure being transportation. The energy return for unchipped firewood ought to be higher.[82] On the other hand, wood is one of the worst sources of heat in terms of fine-particle emissions. In terms of greenhouse gases, CO₂ dominates the emissions for wood heating, with methane and NO_x being negligible. Remember, though, that trees contain “modern carbon,” not fossil carbon, meaning the release of trees’ CO₂ doesn’t increase present-day concentrations. Transporting wood to the fireplace, however, counts against its carbon budget.[83]

These tables show why we changed from wood to fossil fuels (Figure 3.1): Fossil fuels contain a lot of energy in smaller volumes. Though old steam-powered trains did it, nobody wants to pull a trailer full of cordwood behind a steam-powered automobile. Can we convert this old-fashioned, low-density plant material into a new-fashioned, high-density fuel like ethanol? Unfortunately, much of the recent discussions focus on energy and nitrogen-intensive biofuels farming, and I certainly don’t blame farmers for seeking greater market demand, given their economic realities (see Figure 1.7). But efficient fuel-farming requires rapid carbon sequestration by plants (in other words, they grow fast), with desired rates nearing 22 Mg/hectare/year, or 2.2 kg/m²/yr, of dry biomass. Seemingly, short-rotation woody crops, like silver maples, sweetgums, and poplars, have high potential with regard to biomass production.[84]

Diverted waste-stream biomass adds another fossil-carbon-free energy source,[85] an especially great idea if the biomass would otherwise go into landfills. These wastes might include corn stalks and other crop wastes, as well as paper products, but low-till, soil-enhancement practices might take a hit.

————————————-

[77]My conversions assume 14,000 Btu/kg and 4.1 kWh/kg, and for comparison, gasoline has about 115,000 Btu/gallon, or about 34 kWh/gallon.

[78]Pressing these wood–electricity connections further, a 100-Watt incandescent light bulb burning for 10 hours uses the energy in six-tenths of a pound of firewood.

[79]DeWald et al. (2005) wrote the University of Nebraska Extension Agency guide on wood heating. It also provides nice safety tips and instructions to follow when growing and felling trees for firewood.

[80]Put your energy use into the context of tree biomass. Look outside your window into your own yard and find a dozen 9-inch-diameter trees for each member of your household. Imagine cutting them all down. That’s your household’s electricity use for the year. Now think about next year’s electricity use. Don’t even think about your household’s gasoline use. Can we really sustain our present energy use with biofuel-based sources?

[81]The British Columbia Ministry of Water, Land and Air Protection (2004) conducted a 2003 survey of household wood use across the province and found that wood-burning households used from 1.6 to 4.9 cords of wood each year.

[82]Houck et al. (1998) discuss the use of various energy sources for heating.

[83]As part of a broader strategy to reduce fossil carbon emissions and carbon dioxide levels, Houck et al. (1998) points out that harvesting older trees for heating and letting younger trees replace them, it can be argued, shifts forests to younger trees and enhances short-term carbon sequestration, as implied in Figure 2.14.

[84]Graham et al. (1992) consider the sequestration rates in short-rotation woody crops (SRWC). Compare this cited sequestration number, 2.2 kg/m², to estimates of net carbon sequestration values in cities, calculated by Nowak and Crane (2002), ranging from 0.26 kgC/m²/year for Atlanta, Georgia, to 0.12 kgC/m²/year for New York City. Given the desire to yield 2.2 kg/m², it’s clear we won’t grow fuel crops in cities. Compare these numbers to plant growth rates in natural systems, shown in Figure 1.2. We’re talking high-growth rates needed for biofuels. Advocates of biomass energy hope to get to 35% efficiency. Corn-to-ethanol conversion has one big strike against it, primarily the fossil-fuel inputs to corn production, but SRWC have less fossil-fuel demand and thus some potential to offset fossil fuels.

[85]Fossil-carbon-free only if one counts any fossil-fuel production use against the biomass’s primary function, not its waste-stream use.