Environmental Engineering Reference
In-Depth Information
the tropics. Thus global transportation fuel demand
cannot be filled by even the most productive alcohol pro-
duction. In the United States corn-derived ethanol
provided an equivalent of less than 1.5% of gasoline on
an energy basis in 2005, and its output should double
by 2012 (Farrell et al. 2006). Ethanol's power density of
0.22 W/m
2
means that about 390 Mha (slightly more
than twice the country's entire cultivated area) would be
needed to satisfy the U.S. demand for liquid transporta-
tion fuel. The power densities of a fully solar operation
(fueling the machinery with ethanol, distilling with heat
derived by the combustion of crop residues) would
drop, even with the highest claimed EROI, to about
0.07 W/m
2
.
The United States would then require 1.2 Gha, more
than six times its entire arable area and about 75% of the
world's cultivated land, planted to corn destined for fer-
mentation. The prospect does not change radically by us-
ing crop residues to produce cellulosic ethanol because
only a part of these residues should be removed from
fields in order to maintain key ecosystemic services of
recycling organic mater and nitrogen, retaining moisture,
and preventing soil erosion (Smil 1999a). Moreover,
even large efficiency improvements in alcohol fermen-
tation or car performance will not make up for the
inherently low power densities of cropping. The U.S.
transportation sector, three times more efficient than it
was in 2000, would still claim some 75% of the country's
farmland if it ran solely on ethanol produced at rates pre-
vailing in 2005.
Nor would it be easy to supplant the most important
metallurgic use of fossil fuels in iron and steel production.
A return to charcoal would be the only practical choice.
Using the best Brazilian smelting practices (0.725 t of
charcoal/t of pig iron) and yields of 10 t/a for tropical
eucalyptus (Ferreira 2000) would require (for nearly
600 Mt of pig iron smelted annually in the early 2000s)
about 250 Mha of tree plantations. Half of Brazil's total
forested area in 2000 would have to be devoted to grow-
ing wood for the world's metallurgical charcoal, a most
unlikely proposition. And it would be even more difficult
to solarize the production of nitrogenous fertilizer.
Haber-Bosch synthesis uses mostly natural gas as a
source of hydrogen and as a fuel (oil and coal are more
cumbersome choices), and no large-scale nonfossil alter-
natives to this technique are commercially available (Smil
2001).
Solarization will be challenging even in the case of the
best match of conversion and final utilization densities. A
nearly perfect power density overlap between small-scale
solar conversions and household energy needs means
that mature, reliable flat plate and photovoltaic tech-
niques could cover significant portions of domestic
energy needs in warmer, sunny regions without any fun-
damental changes of current land use. And on a sunny
January day, enough radiation could be captured even at
50
N to heat a superinsulated single-story house by
rooftop collectors. But heating all houses through the
entire winter in temperate latitudes would be very diffi-
cult without interseasonal energy storage and major
modifications of both neighborhoods and energy needs.
Photovoltaic cells, even on ideally oriented (SW expo-
sure) roofs, would not suffice to cover peak daily needs
or heating loads during cold but overcast days. Two-
and three-story houses would be especially disad-
vantaged, as would many old houses with insufficient
insulation. For such dwellings any retrofits of expensive
energy storage would be prohibitive. In reality, unsuit-
ably oriented roofs are common, as is partial or total
shading of houses by surrounding trees and buildings.
