Environmental Engineering Reference
In-Depth Information
overall entropy growth for such a reversible cycle is zero and takes into account only
entropies of exchanged heat at the respective heat source temperatures
T
I
and
T
II
:
0
Q
I
/T
I
. The efficiency
η
C
of the considered Carnot cycle is the ratio of
work
W
to the cycle input
Q
I
;
η
C
=
Q
II
/T
II
−
=
W/Q
I
, which is:
T
II
T
I
η
C
=
1
−
(2.3.16)
The commonly called Carnot efficiency is in fact the efficiency of the Carnot cycle
and is the most important efficiency in thermodynamics. All other defined efficiencies
are less general, mostly arbitrary or specifically adjusted to the objects or situations.
One of the most significant properties of the Carnot efficiency is that it is valid inde-
pendently of the nature of the working fluid and can be applied to any material or field
matter used as the working fluid.
The Carnot efficiency can be used as a reference value for calculation of exergy
efficiency of a thermal engine. The energetic and exergetic efficiency of an engine
are respectively
η
E
,
eng
=
W/B
QI
. Based on formulae (2.3.16) and
(2.2.4) the ratio of energetic and Carnot efficiencies is:
W/Q
I
and
η
B
,
eng
=
η
E
,
eng
η
C
η
B
,
eng
=
(2.3.17)
The exergy efficiency
η
B
,
eng
of engine demonstrates how much the real energy
efficiency departs from the ideal efficiency represented by the Carnot efficiency. In the
ideal case (
η
E
,
eng
=
η
C
) the exergy efficiency approach 100%.
2.3.4.2 Perfection degree of process
Practically, process efficiency can be defined in different ways. For example, energy or
exergy can be used for expressing the numerator and denominator of the efficiency.
However, the best method for reviewing the process seems to be the application of
the degree of perfection recommended by Szargut et al. (1988) for measuring the
thermodynamic perfection of process.
The energy and exergy degrees of perfection are defined analogously for convenient
comparison. To determine the degree of perfection, all terms of the energy (or exergy)
balance equation are categorized either as useful product, or process feeding, or loss.
The perfection degree is then defined as the ratio of useful product to the process
feeding. The loss is not disclosed in the perfection degree formula because it is a
compensation of the perfection degree to 100%.
As was discussed in paragraph 2.2.2, the exergy losses can be internal and external.
The energy balance can disclose only energy external loss, whereas the exergy balance
can contain the terms of the external and internal exergy losses. Internal exergy loss
is calculated from the Guoy-Stodola law. External loss is equal to the energetic or
exergetic value of the unavoidably released waste heat or matter which however, can
be somehow utilized beyond the considered system in an additional process of “waste
recovery''. In multi-processes systems, in contrary to external losses, only the internal
exergy losses can be summed.
Search WWH ::
Custom Search