Environmental Engineering Reference
In-Depth Information
conversions producing electricity and liquid fuels indis-
pensable for decades to come. Any rapid substitutions
by low-power-density renewable flows are illusory with-
out dismantling existing urban societies. Escaping the
imperatives of scale built into the world's energy system
by more than a century of fossil fuel combustion and
electricity generation will not be easy. In fossil-fueled civ-
ilization we have been shifting downward, producing
fuels and thermal electricity with power densities 1-3
OM higher than the common power densities of energy
use in buildings, factories, and cities.
As a result, transportation and transmission rights-of-
way vastly surpass the land claims of fixed extraction and
conversion infrastructures. In a solar civilization inherit-
ing today's urban and industrial systems, we would at
best harness energies with the same power densities with
which they are used, and more often we would have to
concentrate diffuse flows, bridging power density gaps of
2-3 OM. This would not only increase fixed land
requirements but also require more extensive transmis-
sion rights-of-way. The inflexible location of electricity
generation based on renewable flows is another key con-
cern. But a solar society would face by far the greatest
land requirements if it were to supplant all crude oil-
derived liquid fuels used in modern transportation by
phytomass-derived ethanol.
During the first years of the twenty-first century global
consumption of gasoline and diesel fuel in land and ma-
rine transport, and kerosene in flying, was about 75 EJ.
Even if the most productive solar alternative (Brazilian
ethanol from sugarcane at 0.45 W/m
2
) could be repli-
cated throughout the tropics, the aggregate land require-
ments for producing transportation ethanol would reach
about 550 Mha, slightly more than one-third of the
world's cultivated land or nearly all agricultural land in
12.9
Comparison of global renewable energy flows and the
world's annual TPES.
imperatives of ultimate resource availability, infrastruc-
tural inertia, and contrasts between conversion and
consumption power densities limit our choices. Solar ra-
diation is the only truly unlimited renewable resource, 3
OM larger than global TPES at the beginning of the
twenty-first century. Other fluxes could supply only a
fraction of existing demand even if they were exploited
to the limit of technical capacities (fig. 12.9). At the
same time, enormous infrastructural investment in our
cities, industries, transportation, and energy networks
creates a highly inertial socioeconomic arrangement. In
the long run we may deurbanize; smaller, stabilized pop-
ulations with access to advanced communication techni-
ques may make large cities obsolete. But during the
twenty-first century we will still have to cater to scores
of megacities and conurbations, which are growing rap-
idly in most poor countries.
The further extension of these inherently energy-
intensive systems makes large-scale high-power-density
