Environmental Engineering Reference
In-Depth Information
Iowa cornfield on an overcast July day, although each
of these energies amounts to 30 MJ. A common denom-
inator does not differentiate between renewable and fossil
fuel energies, yet this distinction has many fundamental
implications for an energy system. Where animate labor
remains an important source of mechanical energy, there
is no easy way to compare such energy inputs with those
contributed by fuels and electricity. Should the animate
energy inputs be counted in terms of all the feed con-
sumed by draft animals (including those kept just for re-
production) or just in terms of the work they actually
accomplished? Total food intake should not be counted
as an energy input for working humans (they spend most
of it on basal metabolism and a part of it on nonlabor
tasks), but multiple assumptions are then needed to
quantify the labor-related energy input.
And how to account for mental work, the intellectual
input whose direct metabolic costs are negligible but
whose development (years of education) is clearly very
energy-intensive? This question becomes especially vex-
ing in a modern high-energy society, where human labor
is overwhelmingly invested in nonphysical tasks. Even
when the comparisons are limited to fossil fuels, there is
a considerable loss of information in conversion to a
common denominator. Some qualitative differences are
important even before these fuels are burned: they de-
termine the ease, costs, and impacts of extraction, trans-
portation, and distribution. And during combustion,
maximum flame temperatures obtainable from different
fuels set the limits on the availability of useful energy
and the flexibility of final conversions. Moreover, equal
amounts of available energy may be accompanied by
vastly different environmental effects. Odum (1996) tried
to take these important qualitative differences into ac-
count by calculating specific energy transformities based
on sequential upgrading of solar radiation, but these
largely arbitrary and often questionable constructs do lit-
tle to resolve the irresolvable (see section 12.1).
Differences of energy form, availability, density,
extractability, transportability, ease of conversion, pollu-
tion potential and convenience, and safety of use cannot
be subsumed by a single denominator. Equally intracta-
ble are comparisons of different electricity generation
modes. In the case of fossil-fueled generation, the energy
equivalent of electricity is clearly the heat content of the
consumed fuel. But what is the primary energy content
of nuclear electricity when the fission reactors use only a
small portion of uranium's thermal potential? And should
hydroelectricity be converted by using its thermal equiv-
alent (1 kWh
¼
3.6 MJ) or the prevailing rate of thermal
generation whereby 1 kWh
¼
9-15 MJ? A country that
generates only hydroelectricity will appear to have a rela-
tively small primary energy use if the first conversion is
used, and a highly inflated one if the second conversion
is applied, because it is most unlikely that a nation would
produce as much electricity if all of it were to be gener-
ated by burning fossil fuels. Both the United Nations
and BP employ a hybrid solution, using the thermal
equivalent for hydroelectricity and the prevailing effi-
ciency rate (about 33%) of fossil-fueled generation for
nuclear electricity.
Energy efficiency is captured by yet another set of
measures hiding considerable complexity (Lovins 2004).
There is no single or best yardstick to assess the perfor-
mance of energy transformations; the most commonly
used ratio is not necessarily the most revealing one; the
quest for the highest rate is not always the most desirable
goal; and inevitable preconversion energy losses may be
far greater than any conceivable conversion improve-
ments. The ratio of energy output or transfer of the
