In northern California, in a region where forests are abundant, one cord of wood sold in 2008 for about $150. Although one cord is a stack of 4 by 4 by
Table 1.3 Conversion Coefficients
To convert from
British thermal unit (Int. Steam Table) British thermal unit (mean) British thermal unit (thermochemical) British thermal unit (39 F) British thermal unit (60 F)
Calorie (International Steam Table)
Calorie (l5 C)
Calorie (20 c)
Cubic foot (methane, STP)
Electron volt ERG Foot LBF Foot poundal kWh
Ton of TNT Power
Foot LBF/second Foot LBF/minute Foot LBF/hour
Horsepower (550 foot LBF/sec) Horsepower (electric) Horsepower (metric)
LBF stands for pounds (force).
to multiply by
joule 1.60206 x 10~19
joule 4.2140 x 10~2
joule 3.6 x 106
BTU 1.0 x 1015
joule 4.2 x 109
watt 2.2597 x 10~2
watt 3.7662 x 10~4
pascal 1.0133 x 105
kg 1.660531 x 10-27
8 ft (128 cubic feet), the actual volume of wood is only 90 cubic feet—the rest is empty space between the logs. Thus, one cord contains 2.5 m3 of wood, or about 2200 kg. The heat of combustion of wood varies between 14 and 19 MJ/kg. If one assumes a mean of 16 MJ per kilogram of wood burned, one cord delivers 35 GJ. Therefore, the cost of energy from wood was $4.3/GJ in northern California.
Still in 2008, the price of gasoline was about $3 per gallon ($1.2 per kg) although if fluctuated wildly. Since the heat of combustion of gasoline is 49 MJ/kg, gasoline energy used to cost $24/GJ, over five times the cost from burning wood.
In California, the domestic consumer of electricity paid $0.12 per kWh, or $33/GJ.
From these statistics, it is clear that when we buy energy, we are willing to pay a premium for energy that is in a more convenient form—that is, for energy that has a higher utility. Utility is, of course, relative. To stoke a fireplace in a living room, wood has higher utility than gasoline and, to drive a car, gasoline has higher utility than electricity, at least for the time being. For small vehicles, liquid fuels have higher utility than gaseous ones. For fixed installations, the opposite is true.
The relative cost of energy is not determined by utility alone. One barrel contains 159 liters, or 127-kg of oil. With a heat of combustion of 47 MJ/kg, this corresponds to 6 GJ of energy. In mid-1990, at a price of $12/barrel or $2/GJ, oil cost less than wood (then at $3.2/GJ) notwithstanding oil being, in general, more useful. However, oil prices are highly unstable depending on global political circumstances. The 2008 price of oil (that peaked well above $100/barrel, or $17/GJ) is now, as one might expect, substantially higher than that of wood and is one of the driving forces toward the greening of energy sources. Perhaps more importantly, there is the dangerous dependence of developed nations on oil from countries whose interests clashes with those of the West.
Government regulations tend to depress prices below their free market value. During the Carter era, natural gas was sold in interstate commerce at the regulated price of $1.75 per 1000 cubic feet. This amount of gas yields 1 GJ when burned. Thus, natural gas was cheaper than oil or wood.
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