Environmental Engineering Reference
In-Depth Information
changing of ice into water at 0
o
C. Heating of water also
accounts for most of the difference between gross (or
higher) heat of combustion and net (lower) heating
value. The first rate is the total amount of energy released
by a unit of fuel during combustion with all water con-
densed to liquid (and hence the heat of vaporization is
recovered); the second rate subtracts energy required to
evaporate the water formed during combustion. The dif-
ference between the two rates is negligible for charcoal
or anthracite (both being virtually pure carbon) but
huge for fresh (green) wood.
Equivalence of energies makes it theoretically very easy
to add up disparate flows, and various common denomi-
nators have been used to offer aggregate supply or utili-
zation accounts in terms of SI units (J, Wh) or other
measures commonly used in energy studies, such as hard
coal or crude oil equivalents or barrels of oil. But these
quantitatively impeccable reductions to a common de-
nominator mislead in several ways. On the most general
level, the common denominators ignore the fundamental
distinction between low-entropy stores or flows and
high-entropy states. Yet the practical difference between
equivalent totals of available energy (in fuels, steam, or
electricity) and dissipated energy (in heat) is obvious.
The concept of exergy (the maximum work possible in
an ideal process) has been introduced to quantify this
loss of quality: unlike energy, exergy is not conserved
but is destroyed in every real process because of the in-
crease of entropy (Ahern 1980; Sciubba 2004; Sciubba
and Ulgiati 2005; Hermann 2006).
Energy units also do not capture qualitative differences
that determine actual modes of use. Three kilograms of
freshly cut wood are not qualitatively equivalent to 1 kg
of bituminous coal, or to a day's intake of food of three
adults, or to solar radiation absorbed in 1 s by 1 ha of
ability to transform a system, a process that can involve
any kind of energy (fig. 1.6), is thus much more helpful.
The standard physical derivation of the basic energy
unit via Newton's second law of motion—1 joule is the
force of 1 newton (mass of 1 kg accelerated by 1 m/s
2
)
acting over a distance of 1 m—certainly provides an im-
peccable definition, but it pertains only to kinetic energy
and, simple as it is, it hardly fosters an intuitive under-
standing of the elusive entity. The classical derivation of
calorie, a common non-SI unit of thermal energy, as the
amount of heat needed to raise the temperature of 1 g of
water from 14.5
C to 15.5
C, describes a process that is
easy to imagine, but it is not easily related to other ener-
gies. As already explained, Mayer's and Joule's experi-
ments made it possible to express heat in dynamic units:
1 cal
¼
4.1855 J.
Heat content (enthalpy, H) of a thermal energy source
equals the sum of the internal energy (E, the measure of
the molecular activity in the absence of motion, external
action, and elastic tension) of a system plus the product
of the pressure-volume work done on the system:
H
¼
E
þ
pV
:
Heat of combustion (or specific energy, J/kg) is the dif-
ference between the bonds in initial reactants and the
bonds in a newly formed compound. Heat can be trans-
ferred in three distinct ways: by conduction (direct
molecular contact, most commonly in solids), by convec-
tion (moving liquids or gases), and by radiation (emis-
sion of electromagnetic waves).
Latent heat is the amount of energy needed to effect a
physical change with no temperature change: changing
water to steam (latent heat of vaporization) at 100
C
requires exactly 6.75 times more energy than does the
