Page 297 - Encyclopedia Of World History

energy 647

energy sources and by varieties and efficiencies of prime
movers that they rely on—but these constraints cannot
explain such critical cultural factors as creative brilliance or
religious fervor, and they offer little predictive guidance
regarding a society’s form and efficiency of governance or
The windmill is the major mechanical means
its dedication to the welfare of its citizens.The best expla-
used to harnass wind energy on the ground.
nation of energy’s role in history thus calls for a difficult
This diagram shows the primary mechanical
task of balancing these two realities, of striving for expla-
parts of the upper portion of a windmill.
nations that take account of these opposites.
Periodization based on the dominant uses of primary
energy cleaves world history into just two highly asym-
metrical spans: the renewable fuel era and the nonre- Biomass Fuels
newable fuel era. All premodern societies relied Biomass fuels had two inherent disadvantages: low
exclusively, or overwhelmingly, on solar, that is, perpetu- power density (expressed in watts per square meter—
2
ally (when measured on civilizational time scales) renew- W/m ) and low energy density (expressed in joules per
able energies. They derived their heat and light from kilogram—J/kg). Even in rich forests biomass was har-
2
biomass (the amount of living matter) that is produced by vested with densities not surpassing 1 W/m , but most
photosynthetic conversion of sunlight and harvested people did not have tools to cut mature tree trunks and
mostly as wood and crop residues, above all straws and had to rely on smaller trees, branches, and leaves gath-
stalks; plant and animal fats were also used in lighting. ered with much lower density. Similarly, the collection of
Their kinetic energy came from human and animal crop residues, needed also as feed and as a raw material,
metabolism (energized, obviously, by eating the bio- rarely yielded more than 0.1 W/m . Consequently, exten-
2
mass) and, to a much lesser extent, from wind and flow- sive forested areas were needed in order to supply the
ing water, the two forms of converted solar radiation energy needs of larger settlements. A large preindustrial
(after it is absorbed by the earth’s biosphere) that power city in a temperate climate would have required at least
the global water cycle and atmospheric circulation. 20 to 30 W per square meter of its built-up area for heat-
Fossil fuels, too, had their origin in photosynthesis, but ing, cooking, and manufacturing, and, depending on the
the constituent biomass was subsequently transformed kind fuel it used, it would have needed a nearby area of
over a period of between 1 million and 100 millionyears up to three hundred times its size to supply its fuel. The
by high temperatures and pressures in the uppermost lay- constraint is clear: No temperate-climate megacities of 10
ers of the earth’s crust into qualitatively new materials. million people or more could have existed during the era
Consequently, fossil fuels—ranging, in the ascending when wood was the main source of energy.
order of quality, from peats through various coals (lignites These power density limitations became even more
to anthracites) to hydrocarbons (crude oils and natural acute after charcoal became used on a relatively large
gases)—are not renewable on historic time scales. This scale. Conversion from wood to charcoal was done to
means that premodern, solar societies had an energy increase wood’s low energy density: In its air-dried form
basis whose potential longevity coincided with the (about 20 percent moisture) the fuel had about 18 MJ/kg,
remaining duration of the biosphere (the part of the whereas charcoal rates about 60 percent higher at 29 MJ/
world in which life can exist) itself (i.e., still hundreds of kg. The obvious advantages of the better fuel include
millions of years to go). On the other hand, modern soci- smaller mass to be transported and stored, smaller fur-
eties will have to change their energy base if they are to naces (or braziers), less frequent stoking, and less air pol-
survive for more than a few hundred years. lution. However, traditional charcoaling was inefficient,