Environmental Engineering Reference
In-Depth Information
accepted, binding definitions. Perhaps the best example
is the treatment of power density in Encyclopedia of En-
ergy (2004). In this comprehensive, authoritative treat-
ment of energy matters, four authors define this rate
in four different ways: as ''power per unit of volume''
(W/m 3 ) in the first volume (Thackeray 2004); as
''power per unit of weight'' (W/kg) in the third volume
(German 2004); as ''power per unit of land area''
(W/m 2 ) also in the third volume (Smil 2004b); and as
''energy harnessed, transformed or used per unit area''
(J/m 2 ) in the sixth volume (Gr¨bler 2004).
All of these rates are used in practice by different disci-
plines. Nuclear engineers use volume power density in
order to express the energy release in reactor cores: it
ranges, expressed usually in kW/dm 3 , from just 1-5 for
MAGNOX and advanced gas reactors to 70-110 for the
most widespread pressurized water reactors (Zebroski
and Levenson 1976). Both volume (W/m 3 ) and weight
(W/kg) power density are commonly used to quantify
the performance of batteries. Radio engineers and
builders of wind turbines calculate power density from
both isotropic and directional antenna with area in the
denominator; for an isotropic antenna the density in
W/m 2 is simply a quotient of the transmitted power
and the surface area of a sphere at a given distance,
P t /4pr 2 . I should also add that the official list of SI-
derived units with special names calls W/m 2 ''heat flux
density'' or ''irradiance,'' has no special name reserved
for W/m 3 , and calls J/m 3
manner. My leading choices for universal measures are
applicable to all segments of energetics. No matter if the
processes are animate or inanimate, natural or anthropo-
genic, they can be profitably studied in terms of energy
and power densities and intensities, the rates that relate
energy and power to space and mass (or volume), mak-
ing it possible to express key physical realities in a few
simple yet revealing measures.
Energy density (J/m 3 ) and specific energy (J/kg) con-
vey critical information about fuels and foodstuffs, telling
us how concentrated sources of energy they are. This at-
tribute is decisive for all portable applications or where
space and weight are at premium: airplanes cannot cross
oceans powered by natural gas, a fuel whose volume den-
sity is only 1/1,000 that of aviation kerosene, and Hima-
layan climbers do not subsist on carrots, whose specific
energy is 1.7 MJ/kg, one-tenth that of power bars. Spe-
cific energy is also used to sum up the amount of fuels
and electricity that have been invested in the extraction
of minerals, the production of food and manufactures,
the distribution of goods, and the provision of services.
This quantity, also called energy cost or energy intensity
(MJ/kg or GJ/t), reveals relative energy needs (alumi-
num needs 1 OM more energy than steel) and informs
about low-energy-intensity substitutions.
Energy intensity of energy (J/J), expressed simply as a
fraction of gross energy content, appraises the net energy
gain of commonly used fuels and electricity. It is very
high for natural gas from giant Persian Gulf fields but
actually negative for early versions of corn-based ethanol
fermentation. Energy intensity of an economy (J/unit of
currency, J/$ for international comparisons, and J/$ in
constant monies for long-term series) is a valuable indica-
tor (albeit one that needs careful interpretation taking
into account national peculiarities) of both the stage of
''energy density'' (BIPM
2006).
Consequently, readers should be forewarned that the
definitions of key rates used in this topic cannot coincide
with every definition found elsewhere, but (in the ab-
sence of universal agreement) all measures are unambig-
uously defined and used in a logical and consistent
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