Exergy saving and exergy production in municipal wastewater ...

Exergy saving and exergy production in municipal wastewater treatment
last when the microorganism decomposes the matter the remaining part
of the exergy which cannot be used by the microorganism will be stored
in peat. Peat in the matter of time turns into oil or coal.
Part of the input exergy (sun light with short wave length which is high
quality energy) to the system (planet earth) will be reflected out as long
wavelength with low quality energy. The difference between the visible
sunlight and invisible sunlight reflected from the planet is the motive
force of all the processes happening on earth (Wall, 1977).
All the resources on the planet are divided into energy sources and other
material. The quality index is a factor which specifies the exergy amount
of an specific type of energy and in this way the highest quality index
(100) belongs to electrical energy, potential energy (stored in water
resources situated in a high level position) and kinetic energy stored in
waterfall and nuclear fuel. The chemical energy stored inside the oil, gas
and coal have the quality index of about 95 and the sunlight has 93. The
heat radiation reflected from the earth thus has zero amount of the
exergy (Wall, 1977).
The quality index also has another definition which shows the amount of
the elements in ordered form. In this case the quality index is high for an
energy type containing high exergy content or an element in high
ordered form. The Einstein’s relation between energy and matter here
can be used to relate these two concepts so to say exergy in matter and
exergy in energy together:
(7)
Equation 7 shows that exergy in the form of extra high quality energy
can be converted into exergy in a shape of high ordered form of
matter (Wall, 1977).
As another example, exergy is consumed to turn the mineral form of an
ore into pure form of it. A power plant is turning the exergy inside the
coal or oil into electricity but this process of course happens with losses.
These losses are around 60% and even higher for nuclear power plant
which is around 70% (Wall, 1977).
1.12. Definition in thermodynamic
Exergy is the part of the energy which is convertible to all other form of
the energies (a definition by H.D Baehr). In an environment which
contains a system, the exergy of the system is the amount of the
maximum mechanical work that can be extracted from the system.
In an ordinary battery, the reaction happens inside, changes the chemical
material into another material and electricity (one form of pure exergy);
this means the chemical material inside the battery has a higher exergy
comparing to the dead battery.
Another definition for exergy is the distance of the system from the
equilibrium state of its environment:
(8)
U = energy content of the system
U 0 = energy content of the system after transfer of entropy to the
environment
∆U = increase of the free energy or entropy in the environment
E = exergy
U = internal energy of the system
(9)
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