Environmental Engineering Reference
In-Depth Information
concluded that corn production and fermentation claim
nearly 30% more energy than is contained in ethanol, or
EROI of 0.77. Pimentel's studies contained some exag-
gerated estimates (notably the energy cost of fertilizer
N), but his main arguments remain intact even if energy
return were unambiguously positive. Because of the at-
tendant environmental degradation (soil erosion, N
losses, mining of groundwater), U.S. corn production is
not renewable, and the large-scale output needed to re-
duce U.S. dependence on crude oil imports would claim
very large shares of U.S. farmland. Similar options based
on corn or other grain crops are untenable in any land-
scarce populous nation.
The controversy about corn ethanol's net gain is a per-
fect illustration of the inherent limitations of energy anal-
ysis. No such doubts attach to calculating the energy
costs of fuel ethanol production from Brazilian sugar-
cane. High yields of this tropical grass (commonly in ex-
cess of 60 t/ha or more than 7 t/ha of sugar) and no
need either for nitrogen fertilizer (thanks to the plant's
endophytic N-fixing bacteria) or for external fuel to ener-
gize the fermentation (thanks to the combustion of bag-
asse, fibrous residue that is available after expressing the
juice from cane stalks) combine to make the cultivation
of sugarcane and ethanol production highly rewarding.
A detailed account by Macedo, Leal, and da Silva (2004)
shows that typical practices in the state of S˜o Paulo have
EROI 8.3, and the best operations can have EROI as
high as 10.2.
LCAs for roof-top photovaltaic systems have shown
EROI ranging from barely positive for some early sys-
tems to values as high as 10 for modern setups in sunny
subtropical locations. Blakers and Weber (2000) calcu-
lated the overall embodied energy cost of 3.8 GJ/m
2
for a roof-top PV panel that will produce 5.5 GJ of elec-
tricity over its 10-year lifespan, implying EROI of 1.45.
In contrast, Meier (2002) put the total life cycle cost of
an 8-kW building-integrated PV system (1.08 GJ/m
2
)
at about 205 GJ and its total electrical output during
30 years at 1.165 TJ, implying EROI of 5.7. Clearly,
assumptions about the system's longevity make the key
difference.
10.2 Basic Materials: From Concrete to Fertilizers
Process analysis can produce fairly reliable and revealing
values for energy costs of basic raw materials, metals,
and synthetic compounds. Moreover, in some instances,
there is an extensive historical record that allows for
some fascinating secular comparisons. Given the multi-
tude of materials for which we have either detailed pro-
cess analyses or basic calculations of energy embodied in
their production, I decided to focus here on energy costs
of three products whose ubiquity and importance define
modern civilization in material terms: concrete, a leading
building material in terms of overall mass; steel, the dom-
inant metal; and ammonia, the compound that is synthe-
sized in greater abundance than any other industrial
chemical.
Reinforced concrete is the dominant building material
of modern civilization, and it is used everywhere in build-
ings, bridges, highways, runways, and dams. Concrete is
an old Roman invention, but modern cement became
available only in 1824, when Joseph Aspdin began firing
limestone and clay at temperatures high enough to vitrify
the alumina and silica materials and to produce a glassy
clinker (Shaeffer 1992). Its grinding produced a stronger
Portland cement named after limestone, whose appear-
ance it resembled when set. Concrete, a mixture of
cement, gravel, and water, is made by hydration, a reac-
tion between cement and water that produces tight
