Energy xxx (2010) 1e8
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Exergy analysis of an experimental heat transformer for water purification
W. Rivera a, *, A. Huicochea b, H. Martínez a, J. Siqueiros b, D. Juárez b, E. Cadenas c
a
Centro de Investigación en Energía, Universidad Nacional Autónoma de México A.P. 34, 62580 Temixco, Mor., Mexico
Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, 62209 Cuernavaca, Mor., Mexico
c
Facultad de Ingeniería Mecánica, Universidad Michoacana de San Nicolás de Hidalgo, Santiago Tapia No. 403, Centro, Mexico
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 March 2010
Received in revised form
14 October 2010
Accepted 16 October 2010
Available online xxx
First and second law of thermodynamics have been used to analyze the performance of an experimental
heat transformer used for water purification. The pure water is produced in the auxiliary condenser
delivering an amount of heat, which is recycled into the heat transformer increasing the heat source
temperatures and also the internal, external and exergy coefficients of performance. The theoretical and
experimental study was divided into two parts. In the first part, a second law analysis was carried out to
the experimental system showing that the absorber and the condenser are the components with the
highest irreversibilities. In the second part, with the results obtained from the second law analysis, new
test runs were carried out at similar conditions than the former but varying only one selected temperature at the time. Comparing the COP (coefficient of performance) between the old and new test runs, it
was shown that higher internal, external and exergy coefficients of performance were obtained in all the
new test runs. Also it was shown that the ECOP (exergy coefficient of performance) increases with an
increment of the amount of the purified water produced and with the decrease of the flow ratio.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Heat transformers
Water purification
Distillation
Exergy analysis
1. Introduction
Absorption heat transformers are some of the most interesting
devices for energy saving, since they can upgrade waste heat
temperature to a higher level to be reused in the industrial process,
consuming just a negligible amount of primary energy.
With regard to second law or exergy analysis of absorption heat
transformers, Ishida and Ji [1] analyzed the theoretical performance
of a single-stage heat transformer with a graphical exergy methodology based on energy-utilization diagrams. Rivero and Le Goff
[2] reported the performance criteria of sorption heat pumps and
heat transformers. The authors proposed new parameters for
exergy analysis including the improvement potential used in the
present work. Lee and Sherif [3] applied the second law of thermodynamics to theoretically analyze the performance of multistage water/lithium bromide absorption heat transformers. The
results provided theoretical basis for the optimal operation and
design of absorption heat transformers. Zhao et al. [4] theoretically
studied the performance of a double-absorption heat transformer
using TFE-E181 as working fluids. The results showed that the new
solution cycle has not only wider operating range of absorber
temperatures but also a higher COP (coefficient of performance)
than that of the water/lithium bromide mixture. Sozen [5] studied
* Corresponding author. Tel.: þ52 73 250044; fax: þ52 73 250018.
E-mail address: [email protected] (W. Rivera).
the irreversibilities in a single-stage heat transformer used to
increase a solar pond’s temperature. The results showed that the
absorber and the generator need to be improved thermally in order
to increase the efficiency of the system. Fartaj [6] compared the
energy, exergy and entropy balance methods for the analysis of
a double-stage absorption heat transformer cycle. The results
obtained show the influence of irreversibilities of individual
components on deterioration of the effectiveness and the COP of the
system. Sozen and Arcacklioglu [7] proposed the artificial neural
networks technique to determine the exergy losses for each one of
the main components of an ejector-absorption heat transformer.
The results showed good accuracy between the training data and
the output results. Martínez and Rivera [8] analyzed the theoretical
performance of a double-absorption heat transformer reporting
values of the ECOP (exergy coefficients of performance) and the
irreversibilities for the complete system and the main components.
Theoretical and experimental studies on heat transformers have
been reported by several applications. Scott et al. [9] reported the
technical and economic feasibility of incorporating an absorption heat
transformer, to increase the efficiency of energy use of an evaporationecrystallization plant in a sugar mill; the simulation demonstrated that the total amount of live steam used in the evaporation
plant can be reduced by 11.8e16.4%. Chen et al., [10] reported the first
industrial scale heat transformer by recovering waste heat from
a synthetic rubber plant which was used to heat water from 95 to
110 C with heat flow of 5000 kW, obtaining a mean COP of 0.47.
0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.energy.2010.10.036
Please cite this article in press as: Rivera W, et al., Exergy analysis of an experimental heat transformer for water purification, Energy (2010),
doi:10.1016/j.energy.2010.10.036
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W. Rivera et al. / Energy xxx (2010) 1e8
Huicochea et al., [11] reported for the first time results about the
feasibility to obtain purified water by using a heat transformer. The
maximum COP and Gross temperature lift (GTL) were of 0.23 and
25.9 C respectively. Siqueiros and Romero [12] demonstrated through
thermodynamic models, that one way to increase the performance of
a heat transformer is by the integration of a water purification system.
The water purification system makes it possible to recycle a certain
quantity of heat into the heat source (generator and evaporator,
simultaneously). The new configuration makes it possible to reach
theoretical COP increases of up to 121% for some specific conditions.
Cortés and Rivera [13] presented the optimization of a pulp and
paper mill utilizing a single-stage heat transformer for heat
recovering. The optimization was realized with a methodology
which includes exergy, exergoeconomics, thermoeconomics and
pinch analysis. The proposed methodology was useful in determining not only the best plant operating conditions but also
establishing the components or subsystems with the highest irreversibilities. Costa et al. [14] reported a preliminary feasibility study
of the implementation of various absorption heat pump configurations in a Kraft pulping process. Three different cases were
considered: (i) integration of a double lift heat transformer into the
heat recovery circuit of the wood chips digesters to produce low
pressure steam (ii) a double effect chiller installed in the bleaching
chemicals making plant to chill cooling water and produce middle
pressure steam and (iii) a heat pump installed on the steam
extraction line of a turbine which, combined with the addition of
a condensing unit, increases substantially the power output. The
results showed that in all the cases the proposed equipment was
viable using the assumed cost and efficiency data in the study.
Based on bibliographic review, it is clear that there are not
experimental studies on exergy analysis applied to a single-stage
heat transformer used for water purification. In the present study,
an analysis based on the first and second law of thermodynamics is
carried out for a single-stage heat transformer for water purification, operating with the water/lithium bromide mixture. Internal,
external and exergy coefficients of performance are calculated for
the complete system, as well as the irreversibilities in the main
components.
2. Description of the system
2.1. Heat transformer cycle
A single-stage heat transformer consists of an evaporator,
a condenser, a generator, an absorber and an economiser. Fig. 1
shows a schematic diagram of an absorption heat transformer in
a plot of temperature against pressure. A quantity of waste heat QGE
is added at a relatively low temperature TGE to the generator to
vaporise part of the working fluid from the weak salt solution
containing a low concentration of absorbent. The vaporised
working fluid flows to the condenser delivering an amount of heat
QCO at a reduced temperature TCO. The liquid leaving the condenser
is pumped to the evaporator in the higher-pressure zone. The
working fluid is then evaporated by using a quantity of waste heat
QEV which is added to the evaporator at an intermediate temperature TEV. Next, the vaporised working fluid flows to the absorber
where it is absorbed in a strong salt solution containing a high
concentration of absorbent from the generator delivering heat QAB
at a high temperature TAB. Finally, the weak salt solution returns to
the generator to preheat the strong salt solution in the economiser
before repeating the cycle again.
2.2. The water purification system with the heat transformer
Fig. 2 shows a schematic diagram of a water purification system
integrated to a heat transformer. The water purification system
removes the useful heat obtained in the absorber of the heat
transformer. Under specific conditions of atmospheric pressure and
chemical composition, the impure water reaches the boiling point
and goes out in two phases (liquid and steam) to a phase separator.
In its liquid phase, the impure water returns to the suction pump in
order to be pumped again to the absorber, meanwhile the steam
goes to an auxiliary condenser where the heat is transferred to the
heat source. The difference in temperature between the steam and
the heat source allows an increase of the heat source’s temperature,
and therefore, the generator and evaporator temperatures increase
proportionally. The water level to purify in the phase separator is
constant and the cumulative salts are periodically drained.
3. Mathematical model
As it is well known, analysis based on the first law of thermodynamics gives information about the amount of energy entering
and leaving from each one of the components as well as the entire
system; however, it does not give information about the energy
quality, neither the irreversibilities in the components and the
complete system. Therefore, an exergy analysis is important (based
on second law of thermodynamics) to determine a precise behaviour of the systems. The exergy is defined as the maximum possible
reversible work that can be produced by a stream or system in
bringing the state of the system with a reference environment.
Exergy is conserved in an ideal process and destroyed during a real
process. Neglecting nuclear, magnetic, electric and chemical effects,
the exergy for a specific state with reference to the environment
can be written as:
_ ¼ m½ðh
_
Ex
h0 Þ T0 ðs s0 Þ
(1)
Where h0 and s0 are evaluated at the reference environment
temperature T0 ¼ 293.15 K.
In steady state conditions and neglecting the kinetic and
potential energies by means of an exergy balance in an open
system, the exergy destruction or the irreversibility equation can be
written as:
I_ ¼
X
j
Fig. 1. Schematic diagram of an absorption heat transformer.
!
T0
1
$Q j þ
Tj
X
i
!
Exi
IN
X
i
!
W
Exi
(2)
OUT
From Eqs. (1) and (2) and realizing energy and exergy balances
for each one of the main components of the system with reference
to Fig. 2 the following equations can be obtained:
Please cite this article in press as: Rivera W, et al., Exergy analysis of an experimental heat transformer for water purification, Energy (2010),
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W. Rivera et al. / Energy xxx (2010) 1e8
3
Fig. 2. Schematic diagram of a heat transformer used for water purification.
Generator
Auxiliary condenser
_ 1 h1 þ m
_ 8 h8 m
_ 7 h7
Q_ GE;IN ¼ m
(3)
_ K ðhV hK Þ
Q_ CO;AUX;H ¼ m
(16)
_ GE;EXT ðhA hB Þ
Q_ GE;EXT ¼ m
(4)
_ I hJ hI
Q_ CO;AUX;C ¼ m
(17)
_
_
_
_
_
I_GE ¼ Ex
A þ Ex7 Ex1 Ex8 ExB
(5)
_ V þ Ex
_ I Ex
_ J Ex
_ K
I_CO;AUX ¼ Ex
(18)
Economiser
Condenser
_ 1 ðh1 h2 Þ
Q_ CO;IN ¼ m
(6)
_ 5 ðh5 h6 Þ
Q_ EC;H ¼ m
(19)
_ CO;EXT ðhD hC Þ
Q_ CO;EXT ¼ m
(7)
_ 10 ðh10 h9 Þ
Q_ EC;C ¼ m
(20)
_ þ Ex
_ Ex
_ Ex
_
I_CO ¼ Ex
D
1
C
2
(8)
_ þ Ex
_ 5 Ex
_
_
I_EC ¼ Ex
9
10 Ex6
(21)
Pumps
Evaporator
_ 1 ðh4 h3 Þ
Q_ EV;IN ¼ m
(9)
_ ¼ n
W
1
H2 O ðPAB PGE Þf
(22)
_ EV;EXT ðhE hF Þ
Q_ EV;EXT ¼ m
(10)
_ ¼ n
W
2
mixture ðPAB PGE Þf
(23)
_ þ Ex
_ E Ex
_ Ex
_ F
I_EV ¼ Ex
3
4
(11)
_ ¼ n
W
3
H2 O ðPATM PAB Þf
(24)
_ ¼ n
W
4
H2 O ðPATM PGE Þf
(25)
_ þW
_
WP;INT ¼ W
1
2
(26)
_ þW
_
WP;EXT ¼ W
3
4
(27)
_ þ Ex
_ þ Ex
_ L þ Ex
_ NþW
_ P;INT þ W
_ P;EXT
IW ¼ Ex
2
8
_ Ex
_ Ex
_ M Ex
_ I
Ex
(28)
Absorber
_ 4 h4 þ m
_ 10 h10 m
_ 5 h5
Q_ AB;IN ¼ m
(12)
_ AB;EXT ðhH hG Þ
Q_ AB;EXT ¼ m
(13)
_ L hL þ m
_ V hV
_ H hH ¼ m
m
(14)
_ þ Ex
_
_
_
_
I_AB ¼ Ex
4
10 þ ExG Ex5 ExH
(15)
3
9
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doi:10.1016/j.energy.2010.10.036
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The most important parameters to analyze the performance of
a heat transformer from the first and second law of thermodynamics are the flow ratio of the water purification production, the
COP, the irreversibilities in the components and the whole system
and the ECOP.
The flow ratio (FRWP) is an important parameter since it is directly
related to the water purification production, size and cost of the
absorber, the auxiliary condenser and the pump. It is defined as the
_ H Þ to
ratio of the mass flow rate of the water leaving the absorber ðm
_ K Þ.
the mass flow rate of the water purification productionðm
FRWP ¼
_H
m
_K
m
(29)
The internal coefficient of performance represents the efficiency
of an absorption heat transformer. It is defined as the heat delivered
in the absorber per unit of heat load supplied to the generator and
the evaporator plus the work done by the pumps.
COP ¼
Q_ AB
_
_
_
Q GE þ Q EV þ W
P;INT
(30)
The COPExT (external coefficient of performance) is:
COPEXT
Q_ AB;EXT
¼
_
_ P;EXT
Q GE;EXT þ Q_ EV;EXT þ W
(31)
The exergy loss or irreversibility for the entire cycle ICYCLE is
given by
I_CYCLE ¼ I_GE þ I_CO þ I_EV þ I_AB þ I_CO;AUX þ I_S þ I_EC þ I_W
(32)
The ECOP or the exergy effectiveness is defined as the maximum
exergy obtained from the systems to the exergy supplied in the
generator and the evaporator plus the exergy of the pumps work.
ECOP ¼
_
_
Ex
H ExG
_
_
_
_
_
ðEx
A ExB Þ þ ðExE ExF Þ þ WP;EXT
(33)
The exergy effectiveness is defined as a measurement of the
capacity of the system to produce the desired effect [15]. The exergy
effectiveness is calculated with the following expression:
I_
P_
Ex
np
3 ¼ 1 PCYCLE
¼ P
_ ns
_ ns
Ex
Ex
(34)
_ np and Ex
_ ns are the net exergy produced and net exergy
Where Ex
supplied
With the previous equations for each one of the main components of the system it is possible to determine the flow ratio of the
purified water production and the COP.
The physical and thermodynamic properties for the water/
lithium bromide mixture were taken from McNeely [16]. The
entropy values of the waterelithium bromide mixture were
obtained from the work of Kaita [17]. The physical and thermodynamic properties for water were taken from the data published by
Irvine and Liley [18].
4. Experimental methodology
4.1. Experimental equipment description
Fig. 3 shows the experimental water purification system integrated to a heat transformer of 700 W designed and built at the
Centro de Investigación en Ingeniería y Ciencias Aplicadas of the
Universidad Autónoma del Estado de Morelos. Its size is 0.80 1.20 1.60 m. The main components of the heat transformer are
Fig. 3. Experimental water purification system integrated to a heat transformer.
made of 316L stainless steel. The water purification circuit is made
of copper and stainless steel. The condenser and evaporator are
heat exchangers made of double concentric coil-shaped tubes. The
generator and absorber are heat exchangers made of shell and
tubes. The generator worked as a boiling pool and the absorber as
a falling film with horizontal tubes. A thermal insulator made of an
expanded elastomer with a thermal conductivity of 0.040 W/m K
was used.
The heat supplied to the generator and evaporator was provided
by means of a thermal bath with a variable electric coil using
a commercial centrifuge pump of 0.5 hp and 3450 rpm. Heat was
extracted from the condenser with an external circuit, by using
a commercial centrifuge pump of 1 hp and 3450 rpm. The gained
heat in the absorber was removed by the water purification system,
by using a magnetic pump with variable speed from 0 to 5600 rpm.
The water/lithium bromide mixture was used because of its
good thermodynamic properties. This pair has some advantages
such as non toxic and non inflammable, high affinity and high
latent heat. The main disadvantage is its high corrosive at high
temperatures, which can be reduced with inhibitors [19,20].
Experimental tests were carried out using the water/lithium
bromide mixture without inhibitors in order to use conventional
thermodynamic correlations.
Solution flows were measured with two analogical flowmeters
with a reading accuracy of 2%. Lithium bromideewater concentrations in the generator and absorber were measured using
a refractometer with an accuracy of 0.0002 and a correlation of
the refraction index obtained in a previous work [21]. Two
analogical flowmeters with a full scale accuracy of 3% were used
to measure water flows of the generator and evaporator; impure
water flow was measured with an analogical flowmeter with a full
scale accuracy of 2%; while an analogical flowmeter with a full
scale accuracy of 3% was used to measure de condenser flow.
Magnetic pumps were used to move LiBrewater solutions and
working fluid, with variable speed from 0 to 5600 rpm.
Type J thermocouples and an Agilent data acquisition unit of 6½
digits with a Bench Link Data Logger software for temperatures
measurement were used. All temperatures were adjusted to
a temperature standard, with a maximum error of 0.3 C in the
interval from 0.0 C to 105.0 C. Two pressure transducers with
design exactitude of 0.25% in the total range were used to
measure low and high pressure. The relation between direct
voltage and pressure was obtained using an analogical pressure
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Table 1
Operating ranges of temperatures, pressures and concentrations for the heat
transformed.
Variables
Units
Major or equal
Minor or equal
T2
T4
T5
T9
TA
TC
TE
PHIGH
PLOW
XGE
XAB
22.6
77.1
97
78.9
85.1
19.2
84.8
31.4
9.6
50.9
49.5
31.3
82.7
101.3
83.1
89.9
29.7
89.4
39.5
12.5
55.7
54.2
C
C
C
C
C
C
C
kPa
kPa
%
%
standard with a full scale accuracy of 0.25% and with a resolution
of 666.6 Pa.
4.2. Experimental test runs
Romero and Siqueiros [12] proposed theoretically the use of
heat transformers to produce purified water recycling the heat
delivered during the water condensation. In order to corroborate
the established by the authors, the experimental heat transformer
was operated at the proposed conditions.
Several experimental test runs were carried out, however, just
a few tests satisfied all the requirements of the heat transformer
used for water purification. In every test, the relative standard
deviation was used to determine the steady state conditions [22].
The variations permitted were less than 0.3% in process variables
for a period of time of at least 30 min.
The theoretical and experimental study was divided into two
parts. In the first part, a second law analysis was carried out to the
data already obtained of five experimental test runs. Then, with the
results of second law analysis, new experimental test runs were
carried out trying to reduce the system and components irreversibilities. The new experimental test runs were realized at very
similar operating conditions than the old test runs, varying exclusively one parameter selected from the second law analysis. Finally,
a comparison was made between the old and new test runs.
In order to analyze the performance of the experimental heat
transformer used for water purification the operating ranges of
temperatures, pressures and concentrations are shown in Table 1.
4.3. Uncertainty analysis
5
Table 3
Operating parameters used of the heat transformer for water purification of the five
initial experimental tests.
Parameters
Tests
T1 ( C)
T2 ( C)
T3 ( C)
T4 ( C)
T5 ( C)
T6 ( C)
T7 ( C)
T8 ( C)
T9 ( C)
T10 ( C)
T11 ( C)
T12 ( C)
TA ( C)
TB ( C)
TC ( C)
TD ( C)
TE ( C)
TF ( C)
TG ( C)
PHIGH (kPa)
PLOW (kPa)
XGE ( C)
XAB ( C)
1
2
3
4
5
61.1
24.7
32.0
80.1
100.4
87.6
72.8
86.3
79.5
72.1
82.1
81.0
86.6
85.2
21.4
23.4
86.4
81.9
88.9
33.4
9.6
53.8
52.2
67.4
23.2
30.3
79.3
97.6
90.7
77.7
91.2
80.9
74.2
84.2
83.5
86.1
84.5
19.6
22.1
86.1
82.4
90.7
35.6
12.1
53.5
51.9
61.7
30.6
33.7
77.1
98.7
88.1
73.1
86.6
79.3
72.8
82.4
81.6
85.1
83.3
29.7
32.0
84.8
79.4
87.2
31.4
9.6
50.9
49.5
69.7
25.2
48.8
82.7
100.8
88.9
74.3
87.8
83.1
74.5
84.2
82.8
89.9
88.1
21.0
22.9
89.4
85.2
91.0
34.2
10.8
55.7
54.2
67.7
23.4
30.4
79.4
97.6
90.7
77.7
91.2
80.9
74.1
84.2
83.5
86.2
84.5
20.0
22.5
86.2
82.4
90.7
34.2
10.8
53.2
52.2
the region 40 < X (%) <70, and 15.55 < T ( C) < 160 compared with
the data of McNeely [16] is less than 1.5%. The correlation of
concentration vs. refraction index has an uncertainty less than 1.2%
compared with the experimental data at 40 C of Zaltash and Ally
[24]. The maximum relative measurement error is <10% in the
mass flow of the evaporator’s external stream, caused by the small
flow.
The combined uncertainty was obtained through the Taylor
Series Method [25]. When y is given by a model, y ¼ f(x1, x2,., xN),
the combined uncertainty uc(y) is given as:
u2c ðyÞ ¼
N X
vf 2
i¼1
vxi
u2 ðxi Þ
(35)
The partial derivatives were evaluated numerically. The results
of uncertainty relative to the cycle reversibility are presented in
Table 2. The maximum uncertainty due to propagation of this
measurement error is less than 37% caused by the irreversibility in
the evaporator and in the absorber. In the auxiliary condenser this
value is less than 10%; in the other components the uncertainty is
To validate the model, it was tested at different operating
conditions and compared with the experimental data. Redundant
measurements were eliminated. The uncertainty in the correlation
of properties for water, compared with NIST database is less than
1% [23]. The uncertainty in the correlations for LiBr properties for
Table 2
Uncertainly relative lo cycle irreversibility in the evaluations of the components of
the heal transformer.
Component
Uncertainty relative to cycle
irreversibility uc 100/ICYCLE
Generator
Condenser
Evaporator
Absorber
Economiser
Pumps
Auxiliary condenser
Cycle
11.60
4.10
36.26
35.13
0.62
0.22
9.35
13.60
Fig. 4. COP, COPExT and ECOP for the five experimental test runs.
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Fig. 5. Exergy effectiveness of the main components of the system for the five
experimental test runs.
less than 5%. The cycle uncertainty is less than 14% in the calculation of the irreversibility (Table 3).
Fig. 7. Comparison of the exergy effectivenes for each one of the main components for
tests 1 and 4.
Figs. 4e6 show the internal COP, the COPExT, the ECOP and the
system and components irreversibilities for the first five test runs.
Fig. 4 shows the internal, external and exergy coefficients of
performance for the five experimental test runs. As it was expected,
in this figure it can be seen that the internal COP for the heat
transformer are higher (varying from 0.23 to 0.33) than the COPExT
since the former do not consider the heat losses of the components
with the environment. On other hand, it can be seen that the COPExT
and the ECOP have similar values varying from 0.15 to 0.24 and
from 0.16 to 0.25, respectively.
Fig. 5 shows the exergy effectiveness for each one of the main
components and the entire cycle for the five experimental test runs.
In this figure it can be seen that the components with the highest
exergy effectiveness are the evaporator and the generator followed
by the economiser, meanwhile the component with the lowest
exergy effectiveness are the condenser (varying from 0.08 to 0.44)
and the absorber. The exergy effectiveness for the cycle varied from
0.17 to 0.22 which can be considered relatively as a good value for
a small capacity heat transformer. The high exergy effectiveness of
the evaporator and generator is due to the low heat losses since the
components were well designed and operated at moderated
temperatures.
In Fig. 6 it can be seen the irreversibilities for each one of the main
components, as well as the irreversibility of the entire cycle. It can be
observed that the components with the highest irreversibilities, in
average, are the condenser and the absorber which is in concordance
with the results of the exergy effectiveness presented in Fig. 5. The
high irreversibility values in the absorber were expected because of
the high irreversibilities of the involved processes, such as the
exothermic water vapor absorption in the solution and the high
gradient temperatures among the streams entering and leaving the
component, and also because this component operates at the
highest system temperature, meanwhile, the high irreversibilities in
the condenser are due to the low heat transfer between the water
vapor and the cooling water circulated inside the coil. Also it can be
seen that the cycle irreversibility varies from 0.21 to 0.33 kW.
In order to reduce the system irreversibilities basically it is
necessary to improve the components design or to modify the
operating conditions. Because of to design and build new components requires a considerable amount of money, it was decided just
to modify the operating conditions in such way that the irreversibilities may be reduced and the diverse COP may be increased. In
order to do that and with the previous results of the second law
analysis made to the components and the entire cycle, for the five
experimental test runs, new test runs were carried out. Each one of
the new five test runs were similar than the previous kipping all the
main system temperatures almost equal (with deviations no higher
than 0.2 C), with exception of one temperature which was varied
trying to improve the system efficiency. The analysis was divided
into two cases. In the first case, for tests 1 and 4, the absorber
temperatures were decreased 2 C in order to reduce the heat
losses to the environment and to decrease the high gradient
Fig. 6. Irreversibility of the main components and the complete system for the five
experimental test runs.
Fig. 8. Comparison of the COP, COPExT and ECOP for tests 1 and 4.
5. Results
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7
Fig. 9. Comparison of the exergy effectivenes for each one of the main components for
tests 2, 3 and 5.
temperatures among the streams entering and leaving the
component. In the second case, for tests 2, 3 and 5, the evaporator
temperatures were increased 2 C. It is important to mention that
the reduction of the condenser temperature was also considered as
alternative in order to decrease the heat losses and irreversibilities
in this component; however, this could not be done because of the
restriction of the environment temperature.
5.1. Case I
Figs. 7 and 8 show the components exergy effectiveness and the
COP respectively, for the old and new tests 1 and 4. In the new tests
the absorber temperatures were decreased 2 C with respect the
old test runs. In this figures it can be seen that in spite all the
components exergy effectiveness change because of new internal
equilibrium, the absorber exergy effectiveness increases for the two
test runs (see Fig. 7) producing a net effect of the increase of the
COP (see in Fig. 8). Comparing the results between the old and new
experimental test runs it can be seen in Fig. 8 that for the new test 1,
the internal COP, the COPExT and the ECOP increased 7.1%, 8.9% and
7.4% respectively, meanwhile, for the new test 4 increased 13.8%,
26.1% and 25.0%, respectively.
5.2. Case II
Figs. 9 and 10 show the exergy effectiveness of the components
and COP for tests 2, 3 and 5 respectively, in which for the new
experimental test runs the evaporator temperatures were
increased 2 C with respect the old test runs. In Fig. 9 it can be seen
that for the new three tests runs the exergy effectiveness increase
Fig. 10. Comparison of the COP, COPExT and ECOP for tests 2, 3 and 5.
Fig. 11. Comparison of the exergy coefficient of performance against the flow ratio for
the old and new test runs.
with respect of the old test runs. As it was mentioned in Fig. 7 all the
components exergy effectiveness change because of the new
internal operating conditions in the system but newly the net effect
over the COP is positive as can be seen in Fig. 10. In this case, the
internal COP, the COPExT and the ECOP increased for test 2; 5.7%,
5.2%, 5%, for test 4; 4.3%, 25%, 23.8% and for test 5; 25%, 10.5% and
10%, respectively.
Figs. 11 and 12 compare the ECOP against the flow ratio and the
amount of water purified produced respectively for the old and
new test runs.
In Fig. 11 it can be seen that in general the lowest values of the
ECOP are obtained for the old test runs, meanwhile the highest are
obtained for the new test runs. This indicate that the changes made
in the temperatures of the new test runs not only increases the
system efficiency but also reduce the flow ratio, decreasing with
this the amount of the solution circulating into the system.
Fig. 12 shows the ECOP against the purified water production.
In this figure it can be seen that the ECOP increases with an
increment of the water production. Based on Fig. 2, it is clear that
when the amount of water production increases, the heat recycled to the generator and evaporator is higher, reducing the
amount of exergy supplied to these components increasing the
ECOP (see Eq. 33). On other hand, it can be observed that in
general the amount of water purified increased for the new test
runs, which is also other advantage.
Fig. 12. Comparison of the exergy coefficient of performance against the water purification for the old and new test runs.
Please cite this article in press as: Rivera W, et al., Exergy analysis of an experimental heat transformer for water purification, Energy (2010),
doi:10.1016/j.energy.2010.10.036
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W. Rivera et al. / Energy xxx (2010) 1e8
6. Conclusions
The results showed that the exergy analysis was useful to
determine not only the internal, external and exergy coefficients of
performance of the system, but also the irreversibilities in the main
components of the heat transformer used for water purification.
With the analysis carried out to the first five test runs, new operating conditions were proposed which lead to higher COP which
increased from 4.3%to 26.1%. The results showed that decreasing
the absorber temperature and increasing the evaporator temperature not only higher COP can be obtained but also low flow ratios
and higher amount of purified water. It is important to mention
that there are some limits to decrease the absorber temperature
and to increase the evaporator temperature, in the first case; the
limit is related with the boiling temperature of the water, meanwhile in the second case, the limit is related with the availability of
the heat source temperature.
Acknowledgements
The authors would like to thanks to PAPIIT-UNAM project
IN103409 to partially sponsor the present study.
Nomenclature
COP
ECOP
Ex
E181
h
I
_
m
N
P
Q
s
T
TFE
u
W
X
x
y
coefficient of performance (dimensionless)
exergy coefficient of performance (dimensionless)
exergy (W)
tetraethylenglycol dimethylether
specific enthalpy (kJ/kg)
irreversibility (W)
mass flow rate (kg/s)
number of measured variables
pressure (kPa)
heat load (kJ/s)
entropy (kJ/kg C)
temperature ( C)
trifluoroethanol
uncertainty
pump work (kJ/s)
solution concentration (% wt)
measured variable
dependent variable
Subscripts
AB
absorber
AC
auxiliary condenser
CO
condenser
C
cold
c
combined
EC
economiser
EV
evaporator
EXT
external
GE
generator
H
heat
i
variable number
IN
INT
OUT
S
WP
input
internal
output
separator
water purification production
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