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CHAPTER 2
OVERVIEW OF LITERATURE
2. 1 INTRODUCTION
In the previous chapter, a detailed discussion of the need for the
effective utilization and conservation of energy, and various methods of
upgrading waste heat have been presented. The problem of water scarcity, the
requirement for water desalination and the combined system of vapour
absorption heat transformer and desalination system were also presented. In
this connection, a detailed survey of available literature on the absorption heat
transformer and desalination systems has been made. The review on the
literature has been focused with respect to
1. Working fluids for VAHT
2. First law analysis of VAHT
3. Second law analysis of VAHT
4. Theoretical studies on MED
5. Theoretical studies on absorption heat pump/ absorption heat
transformer based desalination systems
6. Experimental Studies on VAHT
7. Experimental studies on absorption heat pump/ absorption
heat transformer based desalination systems
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2.2 WORKING FLUIDS FOR VAHT
The cost, operating characteristics and performance of a VAHT are
mainly dependent on the properties of the refrigerant, absorbent and their
mixtures. The most important properties are: specific heat, heat of mixing,
heat of vaporization of the refrigerant, vapor pressure of refrigerant and
absorbent, solubility of the refrigerant, viscosity, density of the refrigerant and
solution, surface tension and thermal conductivity of the solution.
Tufano (1998) has discussed the simplified criteria for the
development of new absorption working pairs. A simplified mathematical
model that can be used to evaluate the effect of the working pair on both
dimensions, and the performance of the absorption heat pumps and heat
transformers has been developed. It has been shown that the solvent activity
coefficient determines the decrease of the solvent vapour pressure over the
solution and the solvent heat of vaporization affects mainly the dimension of
the apparatus. A lesser effect of the coefficient of performance can be
attributed to the excess enthalpy of the solution. The easy to use criteria
developed, are useful in optimizing the development of the new absorption
working pairs.
2.2.1 WATER BASED COMBINATIONS
Water is an excellent refrigerant, having high latent heat of
evaporation and low vapor pressure. It is also inexpensive, nontoxic, and
non-explosive. Lithium bromide and other similar salts are very good
absorbents, having high vapor pressures and low specific heat. The affinity
between water and lithium bromide is also very high and the mixture is safe,
nontoxic and environmentally friendly. Various absorbents with water as the
refrigerant are presented in Table 2.1.
20
Table 2.1 Water based working fluid combinations
Validity Range
Absorbent Data provided Source Temperature
(K)
Concentration
wt %
Lithium Bromide Thermodynamic properties Herold and Morjan (1987) 258 to 448 45 to 70
Lithium Bromide Vapor pressure and enthalpy
values
Kumar and Patwardhan
(1992)
283 to 453 45 to 70
Lithium Chloride Thermodynamic design data Grover and Devotta (1988)Generator 353
Absorber 373–
Lithium Chloride Vapour pressure data Uemura (1977) 268 to 363 10 to 42.5
Lithium Iodide Thermodynamic design data Patil et al (1991 a) 298 to 373 9.56 to 62
Lithium Iodide Thermodynamic design data Patil et al (1991 b) 298 to 373 9.56 to 62
Lithium Chloride + Lithium
Nitrate(2.8:1)
Vapour pressure data Iyoki et al (1993b) 278 to 443 10.3 to 77
Lithium Bromide +
Lithium Thiocyanate (1:1)
Thermodynamic design data Iyoki et al (1981) 301 to 362 0 to 70
Lithium Bromide + Lithium
Iodide (4:1)
Heat Capacities Iyoki et al (1990) 313 to 383 10 to 64
21
Table 2.1 (continued)
Validity Range
Absorbent Data provided Source Temperature
(K)
Concentration
(wt %)
Lithium Bromide + Lithium
Nitrate (4:1)
Physical and Thermal
Properties
Iyoki et al (1993a) 283 to 343 9.9 to 64
Lithium Bromide + Zinc
Bromide (2:1)
Vapour pressure data Adegoke and Gosney
(1991)
293 to 373 50.71 to 74
Lithium Chloride + Lithium
Nitrate(2.8:1)
Solubility Iyoki et al (1993c) 284.85 to 351.75 –––
Lithium Chloride + Calcium
Chloride+ Zinc Nitrate
(4.2:2.7:1)
Thermodynamic
properties
Pinchuk et al (1982) 298 to 573 5 to 60
Lithium Bromide + Lithium
Chloride + Zinc Chloride
(3:1:4)
Thermodynamic design
data
Iyoki (1993d) 302 to 383 32.6 to 72.6
Lithium Bromide + Lithium
Iodide +Ethylene glycol
(3:1:1)
Vapour pressure,
Specific heat and heat
of mixing
Iizuka and
Nagamatsuya (1990)
273 to 473 68 to 80
22
Table 2.1 (continued)
Validity Range
Absorbent Data provided Source Temperature
(K)
Concentration
(wt %)
Lithium Bromide + Zinc
Chloride + Calcium Bromide
(1:1.8:0.26)
Physical and thermal
properties
Iyoki and
Uemura
(1989 c)
283 to 343 10 to 74.4
Lithium Bromide + Zinc
Bromide + Lithium Chloride
(1:1.8:0.26)
Vapour pressure Iyoki and
Uemura
(1989 a)
303 to 392 5.2 to 77.7
Lithium Nitrate + Potassium
Nitrate + Sodium Nitrate
(5.3:2.8:1.9)
Enthalpy
concentration
temperature
correlations
Ally (1988) 305 to 723 70 to 94.1
P-T-X relation 273 to 473NaOH: KOH: CsOH
Mixture enthalpy
Herold et al.(1991) 273 to 453
50 to 75
23
Table 2.1 (continued)
Validity Range
Absorbent Data provided Source Temperature
(K)
Concentration
(wt %)
Lithium Bromide + Zinc
Bromide + Lithium Chloride
(1:1.8:0.26)
Heat capacity Iyoki and
Uemura
(1989 b)
283 to 343 5.1 to 77.1
Lithium Bromide + Lithium
Nitrate + + Lithium Iodide
+ Lithium Chloride (5:1:1:2)
Solubility, vapor
pressure and heat
capacity.
Koo et al
(1999)
Solubility
252 to 367
Vapor pressure 330 to 434
Heat capacity 298 to 328
Vapor pressure 50 to
70
Heat capacitiy 50 to
65
Lithium Bromide + Lithium
Nitrate + + Lithium Iodide
+ Lithium Chloride (5:1:1:2)
Thermodynamic design
data
Lee et al
(2000)
303 to 323 –––
24
2.2.2 ORGANIC BASED WORKING FLUID COMBINATIONS
The search for new working fluids has brought into focus the
possibility of using other working fluid pairs for use in absorption systems.
An overview of the works carried out on the properties of alternative working
fluids for absorption systems has been presented in Table 2.2.
Table 2.2 Organic based working fluid combinations
Working Fluids Data Source Temperature
range (K)
R21-DMF Solubility Zelhoefer andCopley (1937) 305
R22-DMF Solubilitycharacteristics
Agarwal andBapat (1985) 248 to 393
R22-DMF -Thermalconductivity
-Viscosity
-Density
-Surface tension
Dorairaj andAgarwal (1987)
270 to 360
TFE-CHI, TFE-
DTG, TFE-NMP,
TFE-PYR and
HFIP-NMP
P-T-X
Enthalpy-concentration
Bokelmann andSteimle (1986)
253 to 473
R22-NMP Enthalpyconcentration
Borde andJelinek (1987) 273 to 413
R134a - DMAC P-T-X
Excess enthalpy
Borde andJelinek (1991) 273 to 413
R134a -DMETEG
P-T-X
Excess enthalpy
Borde et al(1995) 273 to 413
25
Table 2.2 (continued)
Working Fluids Data Source Temperature
range (K)
n-butane-DMF P-T-X relation
Mixture enthalpyKumar et al.(1993)
250 to 380
TFE-PYR Thermo physicalproperties
Zhuo andMachielsen(1993)
253 to 373
R32 with DMAC P-T-X
Excess enthalpyBorde et al.(1995)
273 to 413
R124 withDMAC
P-T-X
Excess enthalpyBorde et al.(1997)
273 to 413
Propone with
N-hexadecane.
P-T-X
Excess enthalpyRogdakis et al.(1997)
275 to 375
TFE-DMI P-T-X
Heat of mixingIshikawa et al.(1999)
253 to 473
HFC refrigerants
and mixturesSurface tension
Heide
(1997)323 to 333
R125 with
organic
absorbents
p-t-x relation Excess
enthalpy
Jelenik and
Borde (1999)273 to 413
R125, R134a,
R143a and
R152a with
organic
absorbents
p-t-x relation Yokozeki (2005) 283 to 373
26
2. 3 FIRST LAW ANALYSIS OF VAHT
The performance of the vapour absorption heat transformer has
been predicted by many researchers, by developing models based on a
thermodynamic analysis. Observations on the effect of various operating
parameters form a part of the above mentioned studies, with the focus to find
the basic information for the design and fabrication of the vapour absorption
heat transformer. These studies are a reliable source of information for the
research, in which one should concentrate on the efforts to improve the
performance of the vapour absorption heat transformer.
The state of the art of heat transformation was presented by Trepp
(1983). The history and basic theory of heat transformation were observed in
detail. The heat transformation was primarily focused on house heating.
Single and multi stage plants were analyzed. The inverse heat pumps for
upgrading the waste heat, and single substance heat transformers were also
discussed.
Stephan and Seher (1984 a) reviewed the knowledge of heat
transformers. The principle of heat transformation, single stage absorption
and resorption heat transformers (RHT), and multi stage processes were
analyzed. Different types of two stage processes such as combining two single
stage AHTs, and combining the AHT with an absorption cooling machine
were presented. Examples of practical realizations and applications were
given. Developments in the field of heat transformers like the 50 kW useful
heat pilot plant, its operation and performance data, and a plant designed with
5 MW and more were described.
Eisa et al (1986 a) presented possible combinations of operating
temperatures and concentrations, including flow ratios, Carnot coefficients of
performance and enthalpy based coefficients of performance, for absorption
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heat transformers operating on water-lithium bromide. The temperature limits
were 50°C to 140°C for the absorber, 30°C to 90°C for the evaporator and
generator, and 10°C to 50°C for the condenser. The correlation between the
operating temperatures, together with the theoretical coefficients of
performance, and the flow ratio were presented.
Eisa et al (1986 b) proposed possible combinations of operating
temperatures and concentrations, including flow ratios, Carnot coefficients of
performance and enthalpy based coefficients of performance for absorption
heat transformers operating on water- calcium chloride. The temperature
limits were 50°C to 100°C for the absorber, 30°C to 80°C for the evaporator,
40°C to 80°C for the generator and 10°C to 50°C for the condenser. An
increase of the generator temperature above the evaporator temperature
improved the enthalpy based COP. The rate of increase was lower than when
the generator temperature was increased from a temperature level lower, than
the evaporator temperature.
Best et al (1987) presented possible combinations of operating
temperatures and concentrations, including flow ratios, Carnot coefficients of
performance and enthalpy based coefficients of performance for absorption
heat transformers operating on ammonia – water. The temperature limits were
40°C to 120°C for the absorber, 30°C to 70°C for the evaporator, 30°C to
90°C for the generator and 10°C to 50°C for the condenser.
Grossman (1991) studied single stage and double stage heat
transformers working with water-lithium bromide. Their performance under
various operating conditions for process heat generation, using the heat
extracted from the solar pond, has been discussed. The results showed that a
COP of 0.5 can be obtained up to a lift of 35 C, with the process heat
temperature of up to 120 C in the single stage mode. In the two stage mode a
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COP of 0.3 with a lift of 90 C can be obtained, with the process heat
temperature of up to 180 C.
A comprehensive energy mapping of an oleo chemical plant,
engaged in the production of technical fatty acids and refined glycerol, was
performed by Aly et al (1993). The vapour used in the plant had a heat
content of 314 kW and was condensed in a dump condenser and discharged.
The study showed that incorporating an absorption heat transformer system
would enable the recovery of almost half this energy, with a temperature lift
of 34°C. The heat transformer system delivers steam at 3 bar, which could be
fully reused. An economic analysis showed based on an annual operation time
of 7200 hours and a heat transformer efficiency of 0.45.a pay-off period of
less than 18 months.
Rivera et al (1994) carried out a thermodynamic analysis to study
the effect of the heat exchanger effectiveness on the performance of single
stage heat transformers. Moreover, an analysis of three different arrangements
of the two stage heat transformers was performed using a mathematical model
assuming water-lithium bromide as the working fluid. An increase in the
solution heat exchanger effectiveness greatly improved the performance of the
absorption heat transformers, when the absorber temperature was at least
40°C higher than the temperature of the heat supplied to the system. In the
two stage heat transformers, higher absorber temperatures were obtained by
coupling the absorber of the first stage to the evaporator of the second.
However, higher performance coefficients were obtained in general by
coupling the absorber of the first stage to the generator of the second.
Ismail (1995) studied the performance of the absorption heat
transformers for upgrading low level heat. A mathematical model was
developed utilizing the equation of state to calculate the properties of
29
ammonia-water mixture. The performance of the heat transformer was
defined by the COP and circulation ratio. The parameters that affect the
performance were the level of waste heat, condenser temperature, and the
effectiveness of heat exchangers. The COP, circulation ratio and duties of the
generator, evaporator and condenser were calculated for the required
upgraded heat rate at the absorber. The COP, circulation ratio and heat loads
on different components were computed for the generator temperature of
70oC, 80oC and 90oC. The absorber temperatures were varied from 70oC to
150oC, with a 10oC increment. It is reported that a COP value of up to 0.4 to
0.45 can be obtained with a temperature lift of 20oC to 40oC.
Chen (1995) investigated the optimal performance of an absorption
heat transformer using the cyclic model with continuous flow. The effect of
thermal resistances between the heat transformer and the heat reservoirs was
considered in the model. A general expression, related to the rate of heat-
pumping, the coefficient of performance, and the overall heat transfer area of
the heat transformer, was derived. The expression was used to optimize the
main performance parameters of the heat transformer. The maximum rate of
heat-pumping and the corresponding coefficient of performance were
calculated. For a given overall heat transfer area of the heat transformer, the
optimal relation of the heat transfer areas of the heat exchangers was obtained.
The problems concerning the optimal choices of the other performance
parameters were discussed. The results obtained provided some new
theoretical bases for the optimal design and operation of real absorption heat
transformers.
Zhuo and Machielsen (1996) investigated high-temperature
absorption heat transformers with alkitrate as the working pair. Alkitrate is a
mixture of alkali-metal nitrate salts and water. The performance parameters of
single-stage, double-lift and triple-lift cycles for 1 MW industrial scale
30
absorption heat transformers were calculated by a computer-simulation model,
based on the heat and mass balance of each cycle. A comparison of alkitrate
cycles and H2O-LiBr cycles is illustrated. It was concluded that alkitrate was
especially useful for operating at high temperatures; up to 260°C, the COPs of
alkitrate cycles were the same or better than those of H2O-LiBr cycles, under
the same operating conditions. However, attention should be paid to the
solubility problem of alkitrate at low temperatures; a condensing temperature
of the working fluid (i.e. water) below 50°C was not recommended.
Granfors et al (1997) developed a model for the dynamic
simulation of an absorption heat transformer incorporated into an evaporation
plant. The different components of the AHT were modeled with lumped
volumes. Volumes and areas are taken from the design data, whereas the
values of the heat transfer coefficients were tuned to match the corresponding
experimental values in the pilot AHT unit. Simulation was carried out to
investigate the dynamic response of the AHT.
The performance of a double absorption heat transformer was
studied by Barragan et al (1998), using a water-calcium chloride system as the
working fluid. To increase the efficiency and temperature lift, two stage and
double absorption systems are the possible arrangements. Double-absorption
heat transformers have a relatively simple design and smaller size compared
to two-stage heat transformers. The performance simulations of various
operating conditions had been carried out. The results showed that a
maximum lift of 40 C was possible with a COP of 0.3.
Scott et al (1999 b) developed a mathematical model to describe
both the hydrodynamic and heat transfer characteristics of the multi-
compartment absorption heat transformer for different steam temperatures for
possible installation. The mathematical model was capable of predicting the
31
steady state, transient and dynamic behavior of the transformer. A number of
simulations were carried out to investigate the relationship between the useful
heat produced by the transformer, which was equivalent to the sum of the
high temperature steam generated in the absorber, as a function of the COP of
the cycle. The simulation results indicated that more useful heat can be
obtained at lower COP values. The transient and dynamic simulations showed
that the heat transformer attains steady-state conditions within 40 to 60 min.
Yin et al (2000) presented the performance analysis of an
absorption heat transformer with different working fluid combinations. A
comparative performance study was done for the absorption heat transformer
with H2O-LiBr, TFE-NMP, and TFE-E181and TFE-PYR. The results show
that the four working fluid combinations are all suitable for absorption heat
transformers. H2O-LiBr is suitable at lower operating temperatures, while
TFE-NMP, TFE-E181 and TFE-PYR are suitable at higher operating
temperatures. Considering these conclusions, a system might even include a
two-stage heat transformer with H2O/LiBr for the first stage and TFE-NMP,
TFE-E181 or TFE-PYR for the second stage.
A new ejection-absorption heat transformer was presented and
analyzed by Shi et al (2001). H2O-LiBr was chosen as the working fluid
combination. The ejection – absorption heat transformer is a simpler
configuration, than the double absorption and two-stage heat transformers.
Only an ejector is added to the single stage heat transformer. Besides the
ejector's very simple configuration, the systems combining ejectors and other
devices are also very simple, which makes applying the ejector simpler and
safer technologically, than applying mechanical devices which can increase
pressure. The delivered useful temperature in the ejection – absorption heat
transformer is higher than in a single stage heat transformer, and
simultaneously its system performance is raised.
32
Mathematical models of single stage and advanced absorption heat
transformers operating with water-lithium bromide and water/ Carrol mixtures
were developed by Rivera et al (2001). Carrol is a new mixture of lithium
bromide and ethylene glycol [(CH2OH)2] in the ratio 1 : 4.5 (ethylene glycol:
lithium bromide) by weight. The analysis was done to simulate the
performance of these systems coupled to a solar pond in order to increase the
temperature of the useful heat produced by solar ponds. The highest
coefficients of performance were obtained with the single stage heat
transformers. However, the gross temperature lift reached with these systems
was the lowest. Comparing the two stage and double absorption heat
transformers, it was observed that almost the same COPs and temperature
differences were obtained with both the systems under the same operating
conditions. The highest gross temperature lifts for the single stage heat
transformers were about 60oC and 105oC for the two-stage and double
absorption heat transformers.
Shiming et al (2001) proposed a self regenerated absorption heat
transformer, using TFE-NMP as working fluid. The proposal was for
upgrading the lower temperature level energy to higher temperature level
energy, and to recover more energy. As it was not possible to achieve both the
tasks with the working fluids of water-lithium bromide or ammonia -water, a
new organic working pair TFE- NMP was selected for the analysis. One of
the important features of this pair is that it has a wide working range, absence
of crystallization, etc. But it has some negative features like ammonia – water.
There is a low boiling temperature difference between TFE-NMP, a rectifier
is required. Thermal calculations under summer and winter conditions have
been worked out. The results showed that more energy in the waste heat can
be recovered.
33
Kurem and Horuz (2001) compared the use of ammonia - water and
water-lithium bromide solutions in absorption heat pumps and in absorption
heat transformers. The comparison of the two was presented with respect to
the COP, the flow ratio and the maximum system pressure. The AHT system
using the water-lithium bromide solution provided better performance than
the system using ammonia-water.
The thermodynamic performance of a new type of double
absorption heat transformer (DAHT) was studied by Zhao et al. (2003). The
water-lithium bromide solution was selected for the investigation. The
solution cycle in this new type of DAHT was different from the others, in
which the temperature of the absorbing evaporator is not an independent
variable and the degree of freedom of the system was less than that of the
DAHT with other solution cycles, by one. The results showed, that compared
with the other types of DAHT this new type of DAHT had a higher
coefficient of performance, especially when a larger temperature lift is needed.
The maximum coefficient of performance and the maximum gross
temperature were about 0.32 and 60 to 100°C respectively.
Qin et al (2004) established a generalized irreversible four heat
reservoir heat transformer cycle model, based on an endoreversible absorption
heat-transformer cycle. The heat resistances, heat leaks and irreversibilities
due to the internal dissipation of the working substance were taken in to
account. The fundamental optimal relations between the COP and the heating
load, the maximum coefficient of performance and the corresponding heating
load, the maximum heating load and the corresponding coefficient of
performance, as well as the optimal temperatures of the working substance
and the optimal heat-transfer surface areas of the four heat exchangers were
derived using finite-time thermodynamics. The effects of the cycle parameters
on the characteristics of the cycle were studied by numerical examples.
34
Zhao et al (2005) simulated the thermodynamic performance of the
double-effect absorption heat-transformer (DEAHT) using TFE-E181 as the
working fluid, based on the thermodynamic properties of the TFE/E181
solution. The results showed that, when the temperature in the high-pressure
generator exceeds 100°C and the gross temperature lift was 30°C, the COP of
the DEAHT was about 0.58, which was larger than the 0.48 of the single-
stage absorption heat transformer and the increase of the COP was about 20%.
But it was still less than 0.64 of the DEAHT using LiBr-H2O as the working
fluid. The COP of the DEAHT decreases more rapidly with an increase in the
absorption temperature than that of the heat transformer. The double-effect
absorption heat transformer is more suitable for applications in circumstances
having a higher temperature heat resource, and where a higher temperature lift
is not needed.
A theoretical model of an absorption heat transformer for a solar
pond was presented by Sencan et al (2007). The working fluid pair in the
absorption heat transformer was aqueous ternary hydroxide fluid, consisting
of sodium, potassium and cesium hydroxides in the proportion 40:36:24
(NaOH: KOH: CsOH). Different methods, such as linear regression , pace
regression , sequential minimal optimization , M5 model tree, M50 rules,
decision table and back propagation neural network (BPNN) were used for
modeling the absorption heat transformer. The best results were obtained by
the BPNN model. The BPNN procedure was more accurate, and required
significantly lesser computation time than the other methods.
Horuz and Kurt (2010) investigated an absorption heat transformer
with an industrial application. The AHT was analyzed with water-lithium
bromide as the working fluid. It was shown how the basic AHT system could
be modified to increase the COP and the heat transfer at the absorber. The
system performance data were presented in a tabular form, for different
35
system modifications of the base system, for comparison. It has been proved
that, by applying different modifications, the COP could be increased by
14.1%, the heat transfer at the absorber by 158.5% and the hot process water
produced by 3.59% compared to the basic AHT.
Colorado et al (2011 a) applied the error propagation with the
Monte Carlo method to the COP of a water purification system integrated into
an absorption heat transformer, predicted by the artificial neural network
(ANN). A new correlation for calculating the relative standard deviation of
the COP as a function of the experimental COP, and the percentage of the
relative standard deviation of the instrument, were obtained. The results of the
study showed that the percentage of the relative standard deviation of the
COP predicted by the ANN was decreased when the experimental COP was
increased.
2.4 SECOND LAW ANALYSIS OF VAHT
Exergy analysis or Second law analysis is more reasonable method
of analyzing thermal systems, as it involves with quality of energy rather than
the quantity. Many researchers have done second law analysis of absorption
heat pump systems, desalination systems and heat Transformer with water
purification system.
Thermodynamic analysis and optimization of a real single stage
absorption heat transformer were reported by Stephan and Seher (1984 b). A
mathematical model was applied for the analysis taking irreversible processes
into account. Ammonia – water was taken as the working mixture. The
analysis was done for a case where saturated humid air at 90°C was available
as the heat source, and cooling water at 15°C served as the heat sink for the
removal of waste heat. To get useful heat at 130°C an exergetic efficiency of
0.45, and heat ratio of 0.35 can be attained. The work required for operating
36
the pumps amounted for 5.9% of the useful heat. About one third of the
exergy received is dissipated, and a careful design and optimization of this
apparatus is indispensable.
Kripalani et al (1984) presented a comparative performance study
of a single stage VAHT with water-lithium bromide, R21-Dimethyl
formamide (DMF), R22-DMF and R22- Dimethyl ether tetra ethylene glycol
(DMETEG) as working fluids. The heat source temperatures were considered
from 50 to 70°C, to represent the generating and evaporating temperatures,
and the heat sink temperatures were considered from 15 to 40°C to represent
the condensing temperature. Temperature boosts of up to 30°C can be
achieved with single stage systems at COPs around 0.55. Double staging with
water-lithium bromide in the first stage and R21-DMF in the second stage can
yield significantly higher temperature boosts of the order of 50-80°C. When
exergy efficiencies were considered, water-lithium bromide and R2I-DMF
systems did not experience much variation with the change in source
temperature, while R22-DMF and R22-DMETEG systems showed a
decreasing tendency with increasing source temperature.
Ciambelli and Tufano (1987) discussed the technical and economic
feasibility of a single stage water-sulphuric acid heat transformer. A
simplified mathematical model was used for the evaluation. Three different
criteria of optimality were considered. The first two respectively account for
the exergetic and enthalpy values of the useful heat. The third criterion was
based on an approximate evaluation of the fixed and operating costs,
considered proportional to the inverse of the heat storage capacity. This
apparatus is particularly suited for high temperature operation (i.e. for source
temperatures greater than about 100°C).
37
Tyagi (1987) investigated the theoretical performance
characteristics of single stage absorption and resorption heat transformers, and
double stage heat transformers, using ammonia/water as the binary working
mixture. The coefficient of performance, energy efficiencies, mass circulation
ratio and pump work have been discussed as a function of the heat delivery
temperature. The pump work and heat regenerator duty are less for the
resorption heat transformer than for the absorption heat transformer, for fixed
heat source and sink temperatures, when obtaining an equal amount of useful
heat at the same delivery temperatures. The COP of the AHT is higher than
that of the RHT. Higher delivery temperatures can be obtained by two-stage
processes. The COPs of the two stage heat transformers are low.
Cheng and Shih (1988) presented a detailed thermodynamic
analysis of absorption heat pumps and heat transformers. The system used
water-lithium bromide as the working fluid. Both first law and second law
methods were used to assess the behavior of the absorption heat pump,
absorption cooler and absorption heat transformer. The average COP value of
the heat pump was 1.7 and that of the cooler were 0.7. The COP value of the
heat transformer was lower than those of the heat pump and cooler. The
exergy effectiveness was the highest for the heat pump, moderate for the heat
transformer and the lowest for the cooler.
Theoretical performance characteristics of single stage heat
transformers using four binary working fluid combinations were studied by
Tyagi et al. (1989). Ammonia - 1, 4 butanediol, ammonia-2,3 butanediol,
ammonia- triethyleneglycol dimethylether (TEG-DME) and sulphur dioxide-
dimethyl acetaamide (DMA) were selected for the analysis. Heat source
temperatures of 50 to 60oC and heat sink temperatures of 25 to 30oC were
considered as the working range. A comparison of the working fluids has
been presented. The circulation ratio was the minimum for ammonia-2,3
38
butanediol. The heat delivery temperature is at a maximum for ammonia-
TEG-DME. The exergy efficiency and COP were the maximum for ammonia-
water mixtures. Ammonia-l, 4 butanediol and sulphur dioxide-dimethyl
formamide systems seem to be quite attractive.
Duarte and Bugarel (1989) studied the optimal working conditions
of an absorption heat transformer, based on a theoretical cycle using the H2O-
LiBr working pair. The efficiency parameters of the system were discussed
and a new parameter, namely, the exergetic index, directly related to the
exergetic efficiency but more significant for evaluating the performance of the
system, was introduced. A distinction was made between energetic
optimization, which involves maximal thermodynamic efficiency and
economic optimization, for which the knowledge of both the energy and
equipment costs is necessary. The existence of two kinds of decision variables
related to each type of optimization was discussed. Four criteria for energetic
optimization were defined from which simple prediction equations were
obtained. These equations enabled the prediction predict for a given pair, and
a range of energetically optimal working conditions, valid for a large number
of cold and waste energy source temperatures. Finally, the practical
applications of the proposed predictions were presented.
Jernqvist et al (1992) derived equations for four different
efficiencies, discussed and compared for absorption heat transformers. The
derived expressions provided an alternative to the commonly used COP.
Thermodynamic and exergetic efficiencies are the other expressions used in
the analysis. The working pair water-sodium hydroxide was used in the
simulation of two heat transformer systems. The thermodynamic efficiency
was shown to be a more logical measure of the heat transformer
efficiency, since it takes into account both heat losses, heat exchange and the
temperature lift. The authors concluded that exergetic index should be
39
considered as a more significant measure for evaluating the performance of
the AHT systems, since it properly takes into account the exergy losses which
inevitably occur in the system; it was however stressed, that the exergy
analysis should be used as a complement to the first law analysis.
Bisio (1998) presented the thermodynamic analysis of devices for
upgrading thermal energy. The upgrading of low-level energy can be achieved,
either by means of absorption and resorption cycles, or by heat transformers
or by their combinations. A classification of the various thermodynamic
systems on the basis of the Carnot factors of the inlet and outlet energies, the
relative entropy production and the exergy efficiency of some characteristic
energy upgrading devices were analyzed. With a general definition of
efficiency, the relative entropy production and the exergy efficiency of the
various upgrading techniques were considered. Also the possibilities and the
convenience of recompressing the vent steam were examined.
Wang et al (2002) investigated a two-stage heat transformer with
H2O-LiBr for the first stage and TFE-NMP for the second stage. Three kinds
of two - stage heat transformers were analyzed. The first method was to
connect the first stage absorber to the second stage evaporator. The second
method was to couple the first stage absorber to the second stage generator.
The third method was to split the heat delivered by the absorber between the
generator and the evaporator of the second stage. From the results, the two
stage heat–transformer arranged by coupling the first-stage absorber to the
second stage evaporator was found to be the best arrangement, with relatively
high delivered useful temperature, efficiency and delivered heat flow. It is
also technically simpler than the others.
Fartaj (2004) analyzed a double-stage H2O- LiBr absorption heat
transformer cycle, using the energy, exergy and entropy balance methods. A
comparison of the results by the second law exergy and entropy balances
40
indicated that they were consistent in identifying the location and relative
significance of the key non-idealities within the system. The results obtained
clearly showed the influence of the irreversibilities of the individual
components on the deterioration of the effectiveness and the coefficient of
performance of the system. The second law analysis offered an alternative
view of the cycle performance and provided an insight, which the first law
analysis could not. The differences between the first law analysis by energy
balance method and second law analysis by exergy and entropy balance
methods were illustrated quantitatively for the double-stage absorption heat
transformer cycle, and the limitations and advantages of these methods were
presented and discussed.
Sozen and Yucesu (2007) developed a mathematical model of an
absorption heat transformer, operating with ammonia - water. Simulation was
done to study the performance of the system coupled to a solar pond, in order
to increase the temperature of the useful heat produced by solar ponds, and
used a special ejector located at the absorber inlet. When compared to an
AHT with and without an ejector, the system’s COP and exergetic coefficient
of performance improved by 14% and 30%, respectively. The maximum
upgradation of the solar pond’s temperature by the AHT was obtained at
57.5°C and the gross temperature lift at 97.5°C with a COP of about 0.5.
Lee and Sherif (2000) conducted a performance analysis of the
single-stage, double-stage, and triple-stage absorption heat transformers,
using water-lithium bromide as the working medium. Under various operating
conditions simulations were done to evaluate the first law efficiency and the
entropy and exergy were calculated to evaluate the second law performance.
The COP of the three systems decreased slightly at first as the desirable
temperature boost increased, and then declined more when the temperature
boost increased further. On the other hand, the exergetic efficiency increased
41
slightly at first and then declined sharply as the temperature boost, increased.
The results also showed that there was a limitation of the achievable
temperature boost for absorption heat transformers, employing waste heat and
an external cooling water loop. It was also evident that increasing the heat
source temperature or decreasing the cooling water temperature, provided the
AHT systems a better COP and better exergetic efficiency in the higher
temperature boost range.
Sozen and Arcaklioglu (2007) proposed a technique with ANN to
find the exergy losses in an ejector-absorption heat transformer. As the
thermodynamic analysis is too complex, the ANN method was selected for
the analysis. The study can be considered to be helpful in predicting the
exergy losses of each component of an EAHT, prior to its being set up in an
environment.
Rivera et al (2010) analyzed the performance of a heat transformer,
operating with water-lithium bromide mixture used for water purification.
Plots of the irreversibilities for each one of the main components of the
system are reported against the main temperatures and the operating
parameters of the heat transformer. The highest irreversibilities occurred in
the absorber, which accounts for more than 30% of the irreversibilities of the
entire system, followed by the auxiliary condenser with about 25%. The
lowest irreversibilities were found in the pumps, which are almost negligible,
and in the economizer which was lower than 5%.
Martinez and Rivera (2009) developed a mathematical model for
analyzing a double lift vapour absorption heat transformer, based on the first
and second law. A simulation model was developed to estimate the coefficient
of performance, the exergy coefficient of performance, the total exergy
destruction in the system and the exergy destruction in each one of the main
42
components, as a function of the system temperatures, the efficiency of the
economizer, the GTL and flow ratio. The results showed that the generator
was the component with the highest irreversibilities or exergy destruction,
contributing to about 40% of the total exergy destruction in the whole system.
The optimum operating region of the analyzed system was also presented.
Colorado et al (2011 b) proposed a methodology to decrease the
total irreversibilities of a single stage heat transformer, using the artificial
neural network inverse. The proposed methodology could calculate the
optimal input parameters that should be used in order to operate the heat
transformer with lower irreversibilities.
2. 5 STUDIES ON MED
Hamed et al (1996) investigated the performance of a thermal vapor
compression (TVC) desalination system. Operational data of a four–effect,
low temperature thermal vapor compression desalination plant revealed that
performance ratios of 6.5 to 6.8 can be attained. Such ratios were almost twice
those of a conventional four–effect boiling desalination plant. The
performance ratios of the TVC system increase with the number of effects and
with the entrainment ratio of the thermo–compressor and decrease with the
top brine temperature. Exergy analysis revealed that the thermal vapor
compression desalination plant (TVC) was the most exergy–efficient when
compared with the mechanical vapor compression (MVC) and multi–effect
boiling ones. The subsystem most responsible for exergy destruction in all
three desalination systems investigated was the first effect, because of the
high temperature of its heat input. In the TVC system, this amounts to 39%,
with the second highest exergy defect being that of the thermo–compressor,
equal to 17%.
43
El–Dessouky et al (1998) modeled the multi effect evaporation
(MEE) desalination process to determine the effects of the important design
and operating variables on the parameters controlling the cost of producing
fresh water. The results showed that the heat transfer coefficients in the
evaporators and the pre heaters augmented with boiling temperature. The
plant thermal performance ratio was nearly independent of the top brine
temperature and strongly related to the number of effects. The specific heat
transfer area increases by reducing the top brine temperature and raising the
number of effects. The effect of top brine temperature on the specific heat
transfer area is more pronounced at high number of effects. The specific flow
rate of cooling water is nearly constant for different top temperature and
decreases rapidly as the number of effects is increased.
El–Dessouky and Ettouney (1999) analyzed several operating
configurations of multi effect evaporation, namely parallel flow, the
parallel/cross flow, and systems combined with thermal or mechanical vapor
compression. All models take into account dependence of the stream physical
properties on temperature and salinity, thermodynamic losses, temperature
depression in the vapor stream caused by pressure losses and the presence of
non–condensable gases, and presence of the flashing boxes. Analysis was
performed as a function of the number of effects, the heating steam
temperature, the temperature of the brine blow down, and the temperature
difference of the compressed vapor condensate and the brine blow down.
Results were presented as a function of parameters controlling the unit
product cost, which include the specific heat transfer area, the thermal
performance ratio, the specific power consumption, the conversion ratio, and
the specific flow rate of the cooling water.
Jernqvist et al (2001) developed a computer code for all types of
evaporation and flashing processes. Its advanced graphic capabilities enabled
44
the user to construct process flow sheets on the screen. The results can be
displayed either graphically, as text file, or in any other form chosen by the
user, and can be studied both for the whole plant such as concentration,
temperature, and pressure profiles, and for a particular piece of equipment.
The program also included a comprehensive database for the physical
properties of seawater as well as other liquors. There was a library containing
correlations for the heat transfer coefficient of different heat transfer surfaces
and flow regimes.
Spiegler and E1–Sayed (2001) analyzed the fundamentals of
energetic and economic of separation processes. The separation process of the
desalination technologies by membranes and by distillation is considered as
an example for the purpose of clarity and for the special importance to
desalination. Both the energetic and the economics of the separation process
are based on a quantitative formulation of the second law of thermodynamics
in terms of the concept of exergy and its destruction.
Aly and El–Fiqi (2003) developed a mathematical model to analyze
both the multi–stage and multi–effect desalination systems. For MSF the
model accounted for the geometry of the stages, the mechanism of heat
transfer, and the variation of the physical properties of seawater with
temperature and salinity. In addition, the model considered the role of fouling
and its effect on the plant performance ratio. Relationships among the
parameters controlling the product water cost to other operating and design
parameters were calculated. The parameters were plant performance ratio,
specific flow rate of re circulating brine, top brine temperature, and specific
heat transfer area. The study results indicated that the performance ratio was
completely dependent on the number of effects and slightly dependent on the
top brine temperature.
45
Alasfour et al (2005) presented thermal analysis of three different
configurations of a multi–effect thermal vapor compression desalting system.
Three configurations of ME – TVC system ME–TVC (without regenerative
feed heaters), ME–TVC with regenerative feed heaters and ME–TVC coupled
with a MEE system were considered. The impact of motive steam pressure,
temperature difference per effect, top brine temperature, feed seawater
temperature and motive steam flow rate on the system's performance for each
configuration were investigated. The exergy analysis showed that
irreversibilities in the steam ejector and evaporators were the main sources of
exergy destruction in the three configurations. The analysis showed that the
third configuration (ME–TVC+MEE) had two main features compared to
ME–TVC and ME–TVC. First it had a lower compression ratio, which makes
the motive steam capable of compressing larger amounts of the entrained
vapor; as a result, the amount of motive steam was reduced. Second, the
configuration can be used for large–scale production.
Mabrouk et al (2007) presented a thermo economic analysis of the
widely used and existing desalination processes. Thermo economic approach
was used to distribute the cost of the whole process on the internal streams
based on exergy not energy. The stream–cost equations were arranged in a
matrix form and solved to calculate the monetary cost of the process streams.
The cost associated with the rejected streams as well as the cost of the exergy
destruction were calculated for process units. This in turn enables to point out
the units which have the higher sum of investment and exergy destruction
costs. The most widely used desalination processes such as Multi stage flash
(MSF), multi effect evaporation (MEE), thermal vapor compression (MEE–
TVC), mechanical vapor compression (MEE–MVC), and reverse osmosis RO
were considered and compared.
46
Nafey et al (2008) designed a multi–effect evaporation mechanical
vapor compression (MEE–MVC) desalination process with thermo economic
approach. Exergy and thermo economic mathematical models of the
considered process units were developed. The energy analysis showed that the
thermal performance ratio of the considered system with external steam is 8%
less than that the system without external steam. Thermo economic analysis
showed the unit product cost was 29% higher in the system of external steam.
The unit product cost of the desalted water at the normal operation (without
external steam) was calculated by 1.7 $/m3. Design calculations showed that
increasing the required capacity of the considered system, the unit product
cost decrease.
Sayyaadi and Saffari (2010) performed thermo economic
optimization of a MED desalination system with thermo vapor compressor.
They presented a model based on energy and exergy analysis. With the Total
Revenue Requirement method they developed an economic model of the
system. The objective functions based on the thermodynamic and thermo
economic analysis were developed. The optimization was considered for the
proposed multi effect distillation system including six decision variables. A
deterministic optimization approach genetic algorithm was utilized as an
optimization method. This approach was applied to minimize the cost of the
system product water.
2.6 THEORETICAL INVESTIGATIONS ON HEAT PUMP / HEAT
TRANSFORMER BASED WATER DESALINATION SYSTEMS
Application of absorption heat pumps and heat transformers into
the field of water purification is an attractive option and is gaining interest of
researchers in recent years. Many investigators have reported their theoretical
47
findings. The following are the various works carried out in this most
promising area.
Elshamarka (1991) described a solar air-conditioning system
including an absorption heat pump for potable water production, while
performing its air-conditioning duty in a solar house. An integrated system
composed of a solar-heating loop, an absorption heat pump, and an air-
cooling coil was proposed for arid areas. This process of water desalination
produced twice as much water as that produced by a solar still. The proposed
modification of the basic absorption solar air-conditioning system would pay-
back in about six years.
Riffat (1995) analyzed various arrangements of the absorption
system, using the H2O-LiBr pair for water desalination. An in-house computer
program was used to determine the performance of the system arrangements.
The performance of the conventional absorption system for water desalination
was examined, and then a thermodynamic analysis was carried out for the
hybrid system, and it was concluded, that experimental work is required to
assess the performance of the absorption system, using different
refrigerant/absorbent combinations.
Jacques and Larger (1999) analyzed the concept of cogeneration,
relating a gas turbine to a multi effect desalination unit + lithium bromide
absorption heat pump. Cogeneration was applied to very high efficiency
thermal seawater desalination plants, to reduce the cost of desalinated water.
A 9 MW gas turbine related to a 9600 m3/d desalination unit. The specific
heat consumption of the desalination unit was 35 kWh/m3 and the estimated
cost of distilled water at the outlet of the plant was less than US $0.5/m3,
under reasonable economic conditions.
48
Rodriguez and Camacho (1999) analyzed thermo-economically
some aspects of a solar desalination comprising of a MED system coupled to
a one-axis tracking collector field and to a double-effect absorption heat
pump. They evaluated the influence on the product cost of some parameters,
thermal energy cost, number of effects, plant capacity and daily operation
hours. Water costs of this solar MED plant were compared with a
conventional energy source plant. Moreover, the effect on the competitiveness
of the solar desalination system of the financial and fiscal politics parameters
were studied as well as the effect of the fuel and equipment cost evolution.
Mandani et al (2000) investigated a new configuration of
combining a single effect evaporation process with a water-lithium bromide
heat pump. The results showed that the performance ratio varied over a range
of 2.4 to 2.8. The performance at higher operating temperatures was
attractive.
Bourouis et al (2004) simulated numerically a water purification
system integrated with a single stage heat transformer with the LiBr + LiI +
LiNO3 + LiCl working fluid combination. An absorber temperature at 100°C,
waste heat supplied to the generator and the evaporator operating at
temperatures ranged from 60 to 80°C, heat sink cooling the condenser
operating at a temperature between 10 and 40°C were considered for the
analysis. The results showed that, the wider range of solubility of the multi-
component salt solution made possible the operation of the heat transformer
cycle at higher concentrations of the strong solution.
49
Generator Absorber
Condenser Evaporator
Auxiliarycondenser
Phase separator
Impure waterHeat source
Pure water
Generator Absorber
Condenser Evaporator
Auxiliarycondenser
Phase separator
Impure waterHeat source
Pure water
Figure 2.1 Schematic of absorption heat transformer for water
purification (Siqueiros and Romero 2007)
Siqueiros and Romero (2007) proposed a system to increase the
COP of a heat transformer used in a water purification system, shown in
Figure 2.1. This was done by increasing the original heat source temperature
when recycling the steam latent heat from the purification process. Absorber
temperatures from 104 to 115°C for atmospheric pressure water purification
were obtained from the heat transformer. The results showed that the
proposed system was capable of increasing the original value of the COP to
more than 120%. The proposed system was also practical for any other
distillation system integrated into a heat transformer, and was independent of
the working fluid-absorbent pair.
Romero et al (2007) presented a method of increasing the COP of a
heat transformer for water purification systems, without increasing the source
heat temperature. A new COP called the water purification coefficient of
performance was proposed for the system, which considered a fraction of the
50
heat recycled. Simulation with proven software compared the performance of
the modeling of an absorption heat transformer for water purification
operating with water-lithium bromide, as the working fluid-absorbent pair.
Plots of enthalpy based COP and water purification COP were shown against
the absorber temperature under several thermodynamic operating conditions.
It was shown that the proposed system was capable of increasing the original
value of the enthalpy based COP upto 1.6 times its original value, by
recycling energy from a water purification system.
Auxiliary condenser
Distilled water
Waste heat Impure water
Evaporator Absorber
Condenser Generator
PumpPump
Exp. Value
Auxiliary condenser
Distilled water
Waste heat Impure water
Evaporator Absorber
Condenser Generator
PumpPump
Exp. Value
Figure 2.2 Integration of the water purification process to an
absorption heat transformer with energy recycling.
(Hernandez et al 2008)
Hernandez et al (2008) proposed a model for predicting the
COP of a water purification process integrated in an absorption heat
transformer, using the artificial neural network. The configuration is shown in
Figure 2.2. The neural network model was successfully trained with an
51
experimental database and validated with a fresh database. This neural
network model showed that the input and output concentrations in the
absorber and generator, and the pressures (evaporator- absorber and
condenser-generator) have a strong bearing on the performance of the cycle,
represented by the COP. The results from the neuronal model show an
improvement in the performance of the water purification process integrated
to an absorption heat transformer, over that of other configurations of energy
recycling.
Romero and Martinez (2008) presented a proposal for rational
energy saving with waste heat. A thermodynamic-mathematical model was
presented for heat transformer operation for water purification using low
grade waste heat. The proposed system was theoretically evaluated with low
grade energy, with inlet temperatures of 65 to 80°C, which could be
industrial waste heat. Surroundings for condensate the working fluid remain
for the calculations between 25 and 30°C, for absorber temperature higher
than 100°C water purification. Absorber temperature for the system was the
power for simple distillation of impure water, with values from 105 to 115°C
which can be able for purification of brackish water. The enthalpy based
coefficient of performance rose from 0.3 to 0.43.
Escobar et al (2008) developed different algorithms for on-line
estimation of the COP of a waste energy recovery heat transformer by the
water purification process. A thermodynamic model was used to predict the
COP on-line under steady state conditions. The applied thermodynamic model
was satisfactory to predict the COP, with the assumptions considered. The
developed algorithms were validated with the experimental system.
Hernandez et al (2009 a) compared the thermodynamic and neural
network models to estimate on-line the COP in an absorption heat transformer
integrated with a water purification process. The neural network model
52
computed 16 input variables of the inlet and outlet temperatures of four major
components, pressures and concentrations. A thermodynamic model was used
to estimate the COP under steady state with average temperatures, whereas
the neural network model predicted the COP under steady and unsteady
conditions.
Hernandez et al (2009 b) evaluated the optimal operating conditions
using the artificial neural network inverse for a water purification process
integrated in to an absorption heat transformer with energy recycling. This
inverse methodology in ANN considered the Nelder-Mead simplex method to
evaluate the optimal operating conditions. A neural network model was
developed using 16 neurons of normalized form in the input layer, three
neurons in the hidden layer and one neuron in the output layer. The output
layer in the ANN model is the prediction for the behavior of the COP in a
water purification process integrated in to an absorption heat transformer.
Gomri (2009) studied a combination of flat plate solar collectors, a
single effect heat transformer, and distillation process with a capacity of 500
litres per day used for a beach house. To simulate the performance of this
combination, mathematical models were developed. In the overall
desalination plant, the highest exergy loss was in the flat plate collectors. The
exergetic efficiency of the heat transformer increased slightly, with an
increase during the daytime.
Gomri (2010) carried out a comparative study of the single and
double effect absorption transformer for seawater desalination. To compare
the influence of the absorber temperature and the intermediate heat source
temperature on the energy efficiency, exergy efficiency, and fresh water
production of the two systems, simulation was done. The results showed that
the energy efficiency and the exergy efficiency of the double effect absorption
53
heat transformer were higher, than those of the single effect absorption heat
transformer.
Wang and Lior (2011) presented the thermal and economic
performance analysis of low temperature multi effect evaporation (LTMEE)
water desalination system coupled with a LiBr-H2O absorption heat pump
(ABHP). A thermodynamic sensitivity analysis of the ABHP-MEE has been
performed. The thermal performance of the ABHP-MEE with an integrated
ejector heat pump (EHP) system has been compared. The ABHP has a more
favorable thermal performance than the EHP only in certain parameters
ranges. The unit steam cost is an important factor in determining whether the
ABHP-MEE or the EHP-MEE is economically favorable. A general
procedure for economic comparison between the ABHP-MEE and EHP-MEE
has been outlined.
Huicochea and Siqueiros (2010) studied the improvement of the
efficiency of a heat transformer with a water purification system. The
behavior of the enthalpy coefficient of performance is presented as a function
of the absorber temperature, under different performance conditions in the
generator and evaporator. The coefficients of performance are compared to
various increases in the temperature of the heat source, by applying heat
recycling to only one component and both components. Under identical
performance conditions, the best scenario for increasing the efficiency of
energy use in a heat transformer using a water purification system, is applying
the heat recycled to the generator.
2.7 EXPERIMENTAL INVESTIGATIONS ON VAPOUR
ABSORPTION HEAT TRANSFORMERS
Several researchers have conducted experiments on vapour
absorption heat transformers with different configurations and with various
54
working fluids. The experiments have been aimed at commercializing these
systems in an economical way. Their findings and successes are summarized
below.
Bokelmann and Steimle (1986) proposed the following working
fluids, trifluoroethanol (TFE)-quinoline, TFE-tetraethylene glycol dimethyl
ether, TFE-ethylpyrrolidone (EP), TFE-isoquinoline, TFE-methylpyrrolidone
(MP), TFE-N methylpyrrolidone (NMP), TFE-pyrrolidone (PYR), TFE-
tetraethylene glycol (TEG), hexafluoroisopropanol-N methylpyrrolidone
(HFIP-NMP) and pentafluoropropionic acid (PFPA)- NMP based on the
experiments conducted on a pilot plant with water –lithium bromide.
Solubility data were tabulated and equilibrium (p-T-X) charts were
constructed for some working pairs. It was concluded, that heat transformers
comprising single stage water-lithium bromide units were safe and profitable.
Jansen et al (1987) conducted experiments on a single stage heat
transformer with water-lithium bromide. The plant absorbing heat released
during condensation of 22 tons of water vapor at 100°C was tested. 11 tons of
steam at 150°C was produced. The overall COP was estimated at 0.45 for an
initial period of 6 months. Due to the corrosion problem in the solution heat
exchanger, the COP was decreased afterwards.
Eriksson and Jernqvist (1989) reported the experimental results of a
heat transformer with self circulation, working with water-sodium hydroxide.
The results obtained in a pilot plant were presented. The COP obtained ranged
from 0.26 to 0.39. A temperature lift of around 20°C was obtained.
Yumikura et al (1989) tested the characteristics of a two stage heat
transformer using a water–lithium bromide solution. Experiments were
conducted on a plant of 75 kW capacity. It was observed that a temperature
55
lift of 60°C can be obtained, with the heat delivered at a temperature of more
than 150°C from the waste heat available at 90°C with a COP of 0.32. The
experimental results in the steady state were compared with the results
calculated by the computer system simulation. It was found that the
experimental results were within the calculated results.
George and Srinivasa Murthy (1993) experimentally investigated a
heat transformer of 3 kW heating capacity working with R21-DMF. The
experimental performance under different operating conditions has been
reported. The waste heat temperature was varied from 50°C to 75°C, and the
condenser temperatures varied from 20°C to 40°C. With the maximum
temperature lift of 20°C, the upgraded heat temperatures were up to 85°C.
The exergetic efficiency obtained was in the range of 0.2 to 0.35.
Abrahamsson et al (1995) tested a heat transformer with self
circulation. Stabilizing the self-circulation was the primary objective of the
work. The heat transformer plant was of 100 kW capacity, and was designed
to be installed at a major pulp and paper mill. The operation data of the plant
operated under real conditions were presented. The heat transformer used
steam at 100°C in both the generator and evaporator, and the absorber
produced steam at 123°C.
Barragan et al (1995) described the experimental performance of a
heat transformer working with water-lithium chloride. Experimental
investigation has revealed that a gross temperature lift of up to 30 C was
possible with this pair for an absorber temperature of 110 C. A COP of 0.45
was possible with the circulation ratio of less than 10.
Barragan et al (1996) conducted experiments on a heat transformer
using water–calcium chloride. The experimental results qualitatively agreed
56
with the results from the theoretical modeling of the water/calcium chloride
system in a heat transformer. It was observed that both the actual and the
enthalpy coefficients of performance decrease as the flow ratio increases.
Experimental investigation has shown that a maximum of 19 C lift could be
achieved with the maximum possible COP of 0.45 and the absorber
temperature of up to 84 C.
The experimental performance of a heat transformer with water-
magnesium chloride as working fluid was presented by Barragan et al. (1997).
Experiments were conducted at two different ranges of absorber temperature.
In the first case it was varied from 81 to 90 C, and in the second case from 91
to 101 C. The corresponding gross temperature lifts obtained were between
7.8 and 10.2 C in the case 1, and between 15 and 18.4 C in the case 2.
Barragan et al (1998) conducted experiments on a heat transformer
using water based ternary solutions, water– calcium chloride-zinc chloride
and water-lithium chloride-zinc chloride. The results showed that the gross
temperature lift for ternary fluids is higher than that of binary pairs. Also it
has been observed that water-lithium chloride-zinc chloride combination
gives a better performance than the other one. The highest gross temperature
lift of 35 C was realized.
Scott et al (1999 a) developed a novel multi-compartment
absorption heat transformer for different steam temperatures. A number of
experiments were performed, using three different working fluid pairs. The
heat transfer experiments showed that the flow rate of the steam fed to the
absorber is the most important process variable which affects the U value, and
consequently dominates the design process of the heat transfer area of the
absorber. As the steam flow rate increased, both the degree of mixing within
the solution bulk outside the lamella, and the two-phase region inside the
57
lamella increased. This resulted in increasing both the local heat transfer
coefficients in the compartments and in the lamella, due to increased surface
wetting
Scott et al (1999 c) studied a multi-compartment absorption heat
transformer for different steam temperatures, installed in a sugar mill, for
getting steam at four different temperatures. An economic analysis has also
been carried out. The variation in the payback period with energy cost for
various operating hours is also obtained. It ranges from 2 to 10 years for
different conditions.
Rivera et al (1999) evaluated experimentally the behavior of a heat
transformer using water and Carrol. Flow ratios, gross temperature lifts,
useful heat, and coefficients of performance were plotted for the heat
transformer for different temperatures and solution concentrations. The COPs
were in the range of 0.1–0.2 for the mixture. The highest gross temperature
lift (GTL) was 52°C, which was greater than the values reported in the
literature for water-lithium bromide mixtures. Because the water and Carrol
mixture has a higher solubility than water-lithium bromide, high experimental
values were obtained for the GTL.
Rivera and Romero (2000) operated a heat transformer with water-
lithium bromide to demonstrate the feasibility of the use of these systems, to
increase the temperature of the heat obtained from solar ponds. The COP was
in the range 0.10 to 0.16. The highest gross temperature lift was 44°C. The
maximum temperature of the useful heat produced by the heat transformer
was 124°C.
58
Genssle and Stephan (2000) conducted experiments on a heat
transformer with compact heat exchangers and the mixture TFE-E181. New
and less expensive plate heat exchangers were used in the system. The results
were presented and compared with the results of a computer simulation model.
The COP of the heat transformation process with the mixtures, water-lithium
bromide and ammonia–water, were compared. A maximum COP of 0.42 was
reached for an internal temperature lift of 348.2 K to 366.2 K.
Alonso et al (2003) experimentally studied a heat transformer with
partially miscible working mixtures. The building and operation of an
experimental pilot plant using n-heptane-DMF demonstrated the practical
feasibility of a heat transformer cycle using a liquid phase separation step.
The results of the operation of a laboratory scale pilot unit were presented. A
temperature lift of 8°C has been reported. A thermal efficiency of 30 % to 40
% has also been reported. A new COP which is the ratio between the absorber
heat output and pump work has been defined.
Ma et al (2003) reported the test results of the first industrial scale
absorption heat transformer to recover the waste heat released from the
mixture of steam and organic vapor at 98°C in a synthetic rubber plant. The
recovered heat was used to heat hot water from 95 to 110°C, feeding back to
the coagulator as the supplementary heating source. The AHT system was
operated with a H2O-LiBr solution with a heat flow of 5000 kW. The COP,
the thermal efficiency and the temperature lift of the AHT system were
presented. The results showed that the mean COP was 0.47, the gross
temperature lift 25°C could be realized and the payback period was 2 years.
The summary of the experimental works carried out by various
researchers are presented in the Table 2.3.
59
Table 2.3 Summary of experimental results
Source Working Pair COP GTLMaximum Absorber
Temperature
Bokelmann and Steimle (1986) H2O–LiBr 0.45 45°C 145°C
Jansen et al (1987) H2O–LiBr 0.45 50°C 150°C
Eriksson and Jernqvist (1989) H2O – NaOH 0.26 to 0.39 20°C 125°C
George and Srinivasa Murthy (1993) R21 –DMF 0.2–0.35 20°C 85°C
Abrahamsson et al (1995) H20 – NaOH 0.45 23°C 123°C
Barragan et al (1995) H2O–LiCl 0.45 30 C 110 C
Barragan et al (1996) H2O–CaCl2 0.45 19 C 84 C
Barragan et al (1997) H2O – MgCl2 0.27 7 8 to 10.2°C
15 to 18.4°C
81 to 89°C
91 to 101°C
60
Table 2.3 (continued)
Source Working Pair COP GTL Maximum Absorber
Temperature
Barragan et al (1998) H2O – LiCl + ZnCl2
H2O – CaCl2 + ZnCl2
0.28 37.5°C 96°C
Scott et al (1999 a) H2O – CaCl2 0.504 65°C 130 C
Rivera and Romero (2000) H2O – LiBr 0.16 44 C 124 C
Genssle and Stephan (2000) TFE – E181 0.42 18°C 93°C
Alonso et al. (2003) n–heptane / DMF 0.3 to 0.4 8°C 103°C
Ma et al. (2003) H2O – LiBr 0.47 25°C 110°C
61
2.8 EXPERIMENTAL INVESTIGATIONS ON HEAT PUMP /
HEAT TRANSFORMER BASED WATER DESALINATION
SYSTEMS
The various experimental studies carried out by researchers on heat
pump / heat transformer based water desalination systems are presented
below.
Siqueiros and Holland (2000) reported the experimental work on
water purification assisted by heat pumps. Vapour compression heat pumps
were first used. Then, heat driven vapour absorption heat pumps replaced the
system to operate on low grade energy. These thermally driven units could be
designed for small scale mobile units, or for very large plants designed to
produce potable water for towns and even cities. It was concluded that the
heat driven heat transformer or temperature amplifier assisted desalination
and water purification units, were worthy of consideration alongside other
systems, in view of their potential for cost reduction.
Huicochea et al (2004) presented the experimental results of a
portable water purification system integrated into a heat transformer. The
waste heat for the heat transformer was simulated, and the working fluid used
in it was water-lithium bromide. The system worked with waste heat
temperatures of 68-78°C. At higher absorber concentrations the absorber heat
output and COP were higher. The water production rate increased when the
COP increased. The results of the water analysis showed that the distilled
water quality was similar to such obtained from a laboratory's electrical
distiller.
Ziqian et al (2009) designed and tested a regeneration absorption solar
desalination unit. The performance ratio and the flow rate of the freshwater of
the unit at different operating temperatures and pressures were studied; the
62
performance ratio of the unit is high, because most of latent heat of the vapor
and part of the sensible heat of the brine are utilized many times; the solar
heating system of the unit was studied by simulation under conditions of the
least cost of fresh water. The optimal parameters, which are solar collector
areas, storage volume, start-up and break temperatures, are given.
Condenser Generator
Economizer
AbsorberEvaporator
Heat
source
Auxiliary
condenser
Pure
waterImpure
water
Condenser Generator
Economizer
AbsorberEvaporator
Heat
source
Auxiliary
condenser
Pure
waterImpure
water
Figure 2.3 Experimental schematic diagram of a heat transformer used
for water purification. (Rivera et al. 2011)
Rivera et al (2011) analyzed the performance of an experimental
heat transformer used for the purification of water, as shown in Figure 2.3.
The heat delivered in the auxiliary condenser by condensing the water vapour,
has been recycled in to the heat transformer to increase the source heat
temperature and performance. The second law analysis was carried out on the
experimental system to find the irreversibilities. With the results obtained
from the second law analysis, new test runs were carried out under similar
conditions, varying only one selected temperature at a time. Comparing the
COP of the old and the new test runs, it was shown that higher internal,
external and exergy coefficients of performance were obtained in all the new
63
test runs. The results showed that decreasing the absorber temperature and
increasing the evaporator temperature, produced not only a higher COP but
also low flow ratios and a higher amount of purified water. The summary of
experimental results is presented in Table 2.4.
Table 2.4 Summary of experimental studies on heat pump / heat
transformer based desalination systems
2.9 CONCLUSIONS
In this chapter the works carried out by various researchers in the
fields of water based working fluids for heat transformers, theoretical and
Source Heat Pump /
Heat
Transformer
Working fluid Performance Distilled water
flow rate
Siqueiros
and Holland
(2000)
Heat Pump Water / lithium
bromide
Water / Carrol
-- 4.5 liters per h
Gutiérrez et
al (1998)
Heat Pump Water / lithium
bromide
1.1 to 1.4 0.5 and 4.3 kg
h 1
Gutiérrez et
al (2000)
Heat Pump water/Carrol 1.35 to 1.55. 1.2 ansd 4 kg/h
Padilla et al
(2007)
Heat Pump Water / lithium
bromide
2.2 3 m3/h
Rodriguez et
al (1999)
Heat Pump Water / lithium
bromide
2.2 3 m3/h
Huicochea
et al (2004)
Heat
Transformer
Water / lithium
bromide
0.3 0.5 to 1.4
liters per h
Rivera et al
(2011)
Heat
Transformer
Water / lithium
bromide
0.3 Less than 1
liters per h
64
experimental studies on heat transformers, theoretical studies on MED,
theoretical and experimental studies on heat transformer coupled with water
purification system have been presented in detail. The following conclusions
are arrived from the literature review:
1. VAHT is the efficient tool for the objective of process heat up
gradation.
2. Water based working fluid combinations are most suitable for
absorption heat transformers from view point of COP and
temperature lift.
3. To meet the growing water demand, desalination of seawater
is an important alternative, since the only inexhaustible source
of water is the ocean.
4. Application of absorption heat pumps and heat transformers
into the field of water purification is an attractive option and is
gaining interest of researchers in recent years.
5. Several investigators have studied on theoretical basis the
application of absorption heat transformer for purifying
seawater.
Due to the increasing energy demand, environmental issues related
to the use of fossil fuels and water scarcity, there are urgency for new and
sustainable sources and the associated technologies for practical and
economical solution to produce potable water. The present work is intended to
the theoretical and experimental study of the performance of vapour
absorption heat transformer coupled with multi effect distillation system.
