Environmental Engineering Reference
In-Depth Information
factories, and along busy transportation corridors; me-
dium to low power density use in suburban residential
areas; and extremely low power density use in parks and
other green zones. Typical urban power densities thus
range from less than 10 W/m 2
is radiated by living organisms and heat rejected by mod-
ern energy conversions. Heat rejection by autotrophs and
by ectothermic heterotrophs is merely a matter of slightly
delayed reradiation of absorbed solar flux. There is no
additional thermal burden for the biosphere, no climatic
consequences. Similarly, combustion of phytomass is
only a slightly delayed (and often highly concentrated)
return of solar energy that was converted into new chem-
ical bonds months (for crop residues) to tens of years (for
woody tissues) before its harvest and use. Thermoregula-
tion is a taxing matter for many heterotrophs (see section
4.2), but heat released by endothermic bodies is only a
delayed return of solar radiation resulting from the me-
tabolism of digested food.
Heat generated by combustion of fossil fuels is in a dif-
ferent class of delays because up to 10 8 years may have
elapsed before the conversion of solar radiation into
chemical bonds and their severance and reconstitution
during combustion. As shown in figure 11.4, the power
densities of these anthropogenic heat releases range over
10 OM (from 10 3 W/m 2 for heat flux prorated over
national territories to 10 7 W/m 2 of gas turbine nozzles),
with power ratings of individual phenomena spanning 8
OM and their areas extending over 16 OM. Some of
these power densities are high enough to disrupt local
heat balances, and even regional effects may be discern-
ible. Worldwide combustion of fossil fuels remains a neg-
ligible fraction of the Earth's thermal balance, and it
causes a flux exactly 2 OM smaller than do anthropo-
genic greenhouse gases.
On the planetary level, primary commercial energy
consumption in the year 2000 (380 EJ) prorated to
a mere 0.025 W/m 2 of the Earth's surface. This heat
is overwhelmingly dissipated over the continents, so a
more meaningful terrestrial average was 0.081 W/m 2 ,
in uncrowded,
low-
income cities to around 100 W/m 2
in affluent, crowded
megacities.
Elvidge (2004) conducted a study of U.S. impervious
surface area (ISA), the first of its kind. (He used a 1-km
grid for the coterminous states and information from
night-time light radiance and LANDSAT-derived land
cover values). The study found that the total U.S. ISA,
including all highways, city streets, parking lots, build-
ings, and other solid structures, amounted to nearly
113,000 km 2 , or slightly less than area of Ohio. Perma-
nent (roofed or paved) energy infrastructures are, of
course, a part of this total, but they accounted for a very
small fraction of it (less than 5%). More than 90% of the
country's energy use takes place within the ISA; only
trains, airplanes, ships, agricultural machinery, and off-
road vehicles operate outside of it. This means that in
the early 2000s, for every 5 m 2 of ISA, about 1 m 2 of
land was taken up elsewhere by extraction, conversion,
transportation, and transmission facilities of the U.S.
fossil-fueled energy system. Taking into account much
lower per capita energy use in other high-income coun-
tries (except Canada and Australia), one could thus gen-
eralize that affluent high-energy nations in temperate
zones need land equivalent to 10%-20% of their ISAs
for
the infrastructures of
their
fossil-fueled energy
systems.
11.2 Energy Conversions and Heat Rejection
Heat is the only inevitable product of any metabolism,
but there are fundamental differences between heat that
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