kai wang et. al. 201state-of-the-art review on crystallization control technologies for water/libr...
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a b s t r a c tThe key technical barrier to using water/lithium bromide (LiBr) as the working fluid in aircooledabsorption chillers and absorption heat-pump systems is the risk of crystallizationwhen the absorber temperature rises at fixed evaporating pressure. This article reviewsvarious crystallization control technologies available to resolve this problem: chemicalinhibitors, heat and mass transfer enhancement methods, thermodynamic cycle modifications,and absorption system-control strategies. Other approaches, such as boostingabsorber pressure and J-tube technology, are reviewed as well. This review can help guidefuture efforts to develop water/LiBr air-cooled absorption chillers and absorption heatpumpsystems.ª 2011 Elsevier Ltd and IIR. All rights reserved.Etat de l’art des technologies employe´es pour pre´venir lacristallisation lors de l’utilisation des pompes a` chaleur a`absorption au H2O/LiBrMots cle´s : Syste`me a` absorption ; Pompe a` chaleur ; Bromure de lithium ; Cristallisation ; Pre´vention1. IntroductionAccording to the 2009 Buildings Energy Data Book, spacecooling and heating, and water heating consume 49.8%and 25.2% of primary energy consumed in U.S. residentialand commercial buildings (U. S. Department of Energy, 2009).Energy-efficient, environmentally friendly heat pumpingtechnologies may be able to dramatically reduce this energyuse, and ultimately substantially reduce emissions. Absorptionheat pumps, first developed in the nineteenth century* Corresponding author.E-mail address: [email protected] (K. Wang).www.ii fiir.orgavailable at www.sciencedirect.comjournal homepage: www.elsevier.com/locate/ijrefrigi n t e r n a t i onal j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 3 2 5e1 3 3 70140-7007/$ e see front matter ª 2011 Elsevier Ltd and IIR. All rights reserved.doi:10.1016/j.ijrefrig.2011.04.006TRANSCRIPT
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 3 2 5e1 3 3 7
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Review
State-of-the-art review on crystallization control technologiesfor water/LiBr absorption heat pumps
Kai Wang*, Omar Abdelaziz, Padmaja Kisari, Edward A. Vineyard
Building Equipment Research Group, Energy & Transportation Science Division, Oak Ridge National Laboratory, One Bethel Valley Road,
P.O. Box 2008, MS-6067, Oak Ridge, TN 37831-6067, USA
a r t i c l e i n f o
Article history:
Received 3 December 2010
Received in revised form
1 April 2011
Accepted 18 April 2011
Available online 1 May 2011
Keywords:
Absorption system
Heat pump
Lithium bromide
Crystallization
Control
* Corresponding author.E-mail address: [email protected] (K. Wan
0140-7007/$ e see front matter ª 2011 Elsevdoi:10.1016/j.ijrefrig.2011.04.006
a b s t r a c t
The key technical barrier to using water/lithium bromide (LiBr) as the working fluid in air-
cooled absorption chillers and absorption heat-pump systems is the risk of crystallization
when the absorber temperature rises at fixed evaporating pressure. This article reviews
various crystallization control technologies available to resolve this problem: chemical
inhibitors, heat and mass transfer enhancement methods, thermodynamic cycle modifi-
cations, and absorption system-control strategies. Other approaches, such as boosting
absorber pressure and J-tube technology, are reviewed as well. This review can help guide
future efforts to develop water/LiBr air-cooled absorption chillers and absorption heat-
pump systems.
ª 2011 Elsevier Ltd and IIR. All rights reserved.
Etat de l’art des technologies employees pour prevenir lacristallisation lors de l’utilisation des pompes a chaleur aabsorption au H2O/LiBr
Mots cles : Systeme a absorption ; Pompe a chaleur ; Bromure de lithium ; Cristallisation ; Prevention
1. Introduction
According to the 2009 Buildings Energy Data Book, space
cooling and heating, and water heating consume 49.8%
and 25.2% of primary energy consumed in U.S. residential
g).ier Ltd and IIR. All rights
and commercial buildings (U. S. Department of Energy, 2009).
Energy-efficient, environmentally friendly heat pumping
technologies may be able to dramatically reduce this energy
use, and ultimately substantially reduce emissions. Absorp-
tion heat pumps, first developed in the nineteenth century
reserved.
Nomenclature
AAC Air-cooled absorption chillers
AHP Absorption heat pump
CHP Combined heat and power
CNT Carbon nanotube
COP Coefficient of performance
LiBr Lithium bromide
LPA Low-pressure absorber
MPA Medium-pressure absorber
VC Vapor compression system
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(Foley et al., 2000), have received growing attention in the
last few decades. The increasing cost of fossil fuels and
environmental concerns have made the particular features
of the thermally activated heat-pump cycle attractive for
residential, commercial, and industrial applications (Fiskum
et al., 1996).
The key market barrier to the application of water/LiBr
absorption chiller technology in combined heat and power
(CHP) systems is the need for a cooling tower to reject heat
from the condenser and absorber to the ambient air (Zogg
et al., 2005). The use of cooling towers in light-commercial
absorption chiller systems is unpopular because they
1) provide a breeding ground for bacteria, 2) increase initial
system costs, 3) require regular maintenance, and 4) require
extra space for their installation (Zogg andWestphalen, 2006).
The development of air-cooled water/LiBr absorption chiller
technology could effectively eliminate these disadvantages.
However, the key technology barrier in developing water/LiBr
air-cooled absorption chillers is solution crystallization at
high absorber temperatures for fixed evaporating pressure
(Foley et al., 2000; Kurosawa et al., 1988; Zogg et al., 2005).
Water heating consumes 11% and 5.8% of primary energy
consumption in U.S. residential and commercial buildings,
respectively (U. S. Department of Energy, 2009). This reflects
a significant market for energy-efficient heat-pump technolo-
gies for domestic and commercial heating applications. Among
currentheat-pumptechnologies, absorptionheatpumps (AHPs)
are attractive because of their high primary energy efficiency
compared to other technologies and their use of environmen-
tally benign refrigerants. However, water/LiBr absorption heat-
pump systems are unable to operate at typical water heating
temperatures because of crystallization limits of the mixture
at high heat-rejection temperatures (Wang et al., 2011).
Fig. 1 e Schematic diagram of single-e
The single-effect water/LiBr absorption cycle, shown in
Fig. 1, is composed of a condenser, an evaporator, an absorber,
a solution heat exchanger, and a generator. The condenser
and evaporator are identical in function to the corresponding
components in a vapor compression (VC) refrigeration
system. Refrigerant (water) is boiled off and pressurized in the
generator (point 7 in Fig. 1), condensed to a liquid (point 8 in
Fig. 1) in the condenser, then expanded using an expansion
device and eventually evaporated in the evaporator to
produce the cooling effect (point 10 in Fig. 1). The generator,
absorber, and solution heat exchanger are used instead of
the compressor of a VC refrigeration system and sometimes
called a thermal compressor (Zaltash et al., 2007). High-
pressure refrigerant water vapor (steam) is “generated”
when heat is applied in the generator. The hot concentrated
LiBr solution (points 4, 5, and 6) flows through the solution
heat exchanger on its way to the absorber. The low-pressure
water vapor (point 10) from the evaporator is absorbed into
the concentrated LiBr solution in the absorber. As the vapor is
absorbed, the LiBr mass fraction in solution is reduced to the
level of the generator inlet and the low-concentration solu-
tion (points 1, 2, and 3) is pumped back to the generator,
passing through the solution heat exchanger. The solution
heat exchanger is used for internal heat recovery to preheat
the solution leaving the absorber with the hot concentrated
LiBr solution leaving the generator to improve system
efficiency.
The crystallization line for water/LiBr is usually very close
to the working concentrations needed for practical AAC and
AHP systems operation. LiBr is a salt and has a crystalline
structure in its solid state. There is a specific minimum
solution temperature for any given LiBr salt concentration
below which the salt begins to crystallize out of the solution.
ffect water/LiBr absorption cycle.
Fig. 2 e Duhring diagram of single-effect absorption
system (see Fig. 1 for definition of state points).
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For LiBr solution, LiBr begins to crystallize either when the
concentration ratio is increased or when the solution
temperature is reduced beyond the crystallization limit.
Crystallization results in interruption of machine operation
and possible damage to the unit. It is more prone to occur for
the strong solution entering the absorber, which is point 6 in
Fig. 1. Crystallization must be avoided because it may lead to
the formation of slush in the piping network, which may
result in complete flow blockage if the slush is solidified. If this
occurs, the concentrated solution temperature needs to be
raised significantly above its saturation point in order to
dissolve salt crystals within a reasonable time. Recovering
absorber operation after crystallization is a labor-intensive
and time consuming process.
Fig. 2 compares the pressureetemperatureeconcentration
(PeTeX) characteristics of a typical water-cooled water/LiBr
absorption chiller to those for AAC and AHP systems in the
Duhring diagram. The figure shows that the higher heat-
rejection temperatures associated with AAC or AHP systems
bring the cycle closer to the crystallization curve, increasing
the likelihood of crystallization. The following instances
may trigger crystallization, either independently or in
combination.
1.1. Presence of non-condensable gases, such as airand hydrogen (Liao and Radermacher, 2007)
Since the absorption system operates under vacuum, outside
air may leak into the system. The corrosion of metal in the
absorption system will generate non-condensable gas (such
as hydrogen), particularly at higher-temperature operation
such as double-effect cycles, especially with direct-fired
generators. The presence of non-condensable gases decreases
system capacity and COP, which causes the concentration of
the concentrated absorbent solution to increase, depending on
the amount of inert or non-condensable gases in the system.
As the solution becomes more concentrated it tends toward
saturation and may even become supersaturated to trigger
crystallization. The onset of non-condensable gases could be
controlled by designing the machine with routine purging
systems.
1.2. Excessively cold condenser water coupledwith a high load condition (Florides et al., 2003)
Sudden cooling of the condenser water to below normal
operating temperature results in lowering the temperature
of the dilute absorbent solution leaving the absorber. This in
turn lowers the temperature of the concentrated absorbent
solution in the heat exchanger to below the crystallization
point and will begin to block the heat exchanger.
1.3. Over-firing the generator (Florides et al., 2003)
Over-firing the generator, resulting in super-saturation of the
absorbent solution, may also cause blockage of the heat
exchanger passages by crystallization.
1.4. Electric power failure (Florides et al., 2003)
During normal shutdown, the machine undergoes a dilution
cycle, which lowers the concentration of the solution
throughout the system. In such cases, the machine may cool
to ambient temperature without crystallization occurring in
the solutions. Crystallization is most likely to occur when
the machine is stopped while operating at full load, when
highly concentrated solution is present in the solution heat
exchanger.
This article presents a state-of-art review of various crys-
tallization control technologies available to resolve the LiBr
crystallization problem using chemical inhibitors, heat and
mass transfer enhancement methods, thermodynamic cycle
modifications, and absorption system-control strategies.
Other approaches, such as boosting absorber pressure
and J-tube technology, are reviewed as well. This review of
relevant technologies can help the research and development
community gain a better understanding of the crystallization
issues, take corrective action, and pursue future efforts in
developing water/LiBr AAC and AHP systems.
2. Chemical crystallization inhibitors
Macriss (1968) and Macriss and Rush (1970) explored the
performance of LiBreLiSCN (Lithium Thiocyanate)eH2O as
a working fluid in AACs, and its vapor pressure, crystalliza-
tion, viscosity, and density data as a function of temperature
and concentration. Compared with other working fluids such
as LiBreH2O, LiBrebutyrolactoneeH2O, and LiBreCsBreH2O,
the solution of LiBreLiSCN was found to have desirable
physical properties with good potential for air-cooled
systems. Hence, the correlation equations of vapor pressure,
heat of vaporization, and solution concentration were pre-
sented by Weil (1968), and were used for cycle analysis of
LiSCNeLiBreH2O combinations in which the LiSCN/LiBr mole
ratio is less than 4. A series of stability tests were conducted
on LiBreLiSCN solutions with and without inhibitor (Li2CrO4)
and with various construction materials (304 stainless steel,
carbon steel, aluminum, and copper). The results show that
304 steel and aluminum appeared to promote decomposition.
However, iron and copper, themost commonly usedmetals in
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 3 2 5e1 3 3 71328
absorption systems, have less effect on the stability of
LiBreLiSCN solution (Rush, 1968).
Biermann (1978) reviewed chemicals having the potential
to sustain an air-cooled, solar-powered absorption refrigera-
tion system. An aqueous chemical solution called “Carrol”
was developed based on the review results (Reimann, 1981).
It consists of LiBr, ethylene glycol, and 1-nonylamine (or
phenylmethylcarbinol) as an absorbent mixture and water as
the refrigerant. The ethylene glycol was used as a crystalliza-
tion inhibitor, and originally the 1-nonylamine was used as an
additive to enhance the heat and mass transfer but was
replaced by phenylmethylcarbinol because of its side effect
(when 1-nonylamine is heated in the presence of copper
oxide, it forms chemically refractory copper soaps) (Lof, 1993;
Zogg et al., 2005). The weight ratio of LiBr and ethylene glycol
in Carrol is 4.5:1. The thermophysical properties (pressuree
concentrationetemperature equilibrium properties, density,
film heat transfer coefficient, specific heat capacity, thermal
conductivity, and viscosity, as well as crystallization curve) of
Carrol had been documented by Reimann (1981). Fig. 3 shows
the equilibrium diagram for aqueous solutions of Carrol and
crystallization curves of LiBr and Carrol aqueous solution. The
comparison between Carrol and LiBr aqueous solution
crystallization curves shows that Carrol has a larger feasible
area of operation than LiBr aqueous solution. Carrol had been
tested extensively in solar-powered, water-cooled (Biermann
and Reimann, 1981b), and air-cooled (Biermann and
Reimann, 1981a) absorption applications both in the labora-
tory and in the field. Since a traceable amount of ethylene
glycol could possibly exist in the vaporized refrigerant in the
generator, Inoue (1993) and Park et al. (1997) suggested
utilizing a rectifier in absorption systems that use Carrol as
a working fluid.
Iyoki andUemura (1981) also studied the feasibility of using
LiBreethylene glycol aqueous solution as a working fluid in
solar-powered absorption refrigerating machines. The mole
ratio of water and ethylene glycol is 10:1 in the aqueous
solution. The specific gravity, solubility, vaporeliquid equi-
librium, vapor pressure, and heat of mixing of the watere
LiBreethylene glycol were measured experimentally. The
Fig. 3 e Equilibrium chart for aqueous solutions of Carrol,
reproduced according to Reimann (1981).
enthalpyeconcentration chart was constructed from these
results. The performance characteristics of single- and double-
effect solar-powered absorption refrigerating machines were
studied using this chart. Eisa et al. (1988) investigated the
operational characteristics of an experimental absorption
cooler using the same solution (water/ethylene glycol mole
ratio ¼ 10). Kim et al. (1995) measured and compared the
vapor pressure of LiBr þ ethylene glycol þ water (water/
ethylene glycol mole ratio ¼ 10) and LiBr þ LiCl þ ethylene
glycol þ water (LiCl/LiBr mass ratio ¼ 1, water/ethylene glycol
mole ratio ¼ 10).
The physical and thermal properties of watereLiBre
C4H6O2 (g-butyrolactone) solution were investigated to obtain
its absorption refrigeration performance characteristics (Iyoki
et al., 1984). The organic compound as a third component was
added to improve system COP. It was envisioned that this
additive might enlarge the difference in vapor pressure
between refrigerant and absorbent solution and thus allow for
an evaporating temperature near 0 �C. The performance
characteristics of this working fluid were proven to be better
than those of the watereLiBr solution.
A new absorbenterefrigerant pair using water as the
refrigerant and a 1:1 mixture of LiBr and zinc chloride (ZnCl2)
by weight as absorbent was developed by Manago et al. (1984)
and Ohuchi (1985). This new absorbent has been found to be
a promising candidate for a heat pump and air-cooled cooling
system to achieve superior performance. The simulation
results show that the new absorbent solution gave a heating
COP of 1.57 and a cooling COP of 1.00 for air-cooled, double-
effect absorption cycles, with a boiler efficiency of 80%.
Modahl (2002) invented a new absorbenterefrigerant pair for
the high-temperature loop of a dual loop triple effect
absorption chiller system. The absorbent is zinc bromide
(ZnBr2) and LiBr and the refrigerant is water. The mass ratio
of ZnBr2 to LiBr is 1.75. The absorbent also contains
0.003 g of lithium hydroxide per gram of contained salt, and
the concentration of solution is in the range of 80e91%
(by weight).
Park et al. (1997) carried out the experimental measure-
ment of four physical properties (solubility, vapor pressure,
density and viscosity) of LiBr þ 1,3-propanediol (b-propylene
glycol)þwater solution (LiBr/1,3-propanediolmass ratio¼ 3.5).
The Duhring chart was generated using correlation results
based on the experimental data and showed that the proposed
solution could have a high absorber temperature, which is
essential for the design of air-cooled absorption chillers.
Park and Lee (2002) investigated the heat and mass transfer
performance of water vapor absorption into the LiBr-based
working fluids (LiBr þ 1,3-propanediol þ water (LiBr/1,3-
propanediol mass ratio ¼ 3.5)), and LiBr þ LiI þ 1,3-
propanediol þ water solutions (LiBr/LiI ¼ 4 by mole ratio and
(LiBr þ LiI)/1,3-propanediol ¼ 4 by mass ratio). The experi-
mental results showed the heat and mass transfer charac-
teristics of LiBr þ LiI þ 1,3-propanediol þ water solution were
comparable with the LiBr water solution. Yoon and Kwon
(1999) carried out the cycle analysis of an air-cooled,
double-effect absorption chiller system using H2O/LiBr þ 1,3-
propanediol (HO(CH2)3OH) as a new working fluid. The simu-
lation results showed that the new working fluid might
provide an 8% higher crystallization limit than conventional
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water/LiBr solution. The authors noted that ambient air inlet
temperature plays an important role in system performance
and the corrosion problem.
Kim et al. (1996a,b, 1997) and Young et al. (1997) measured
the basic thermophysical properties for three solutions,
LiBr þ H2N(CH2)2OH þ H2O, LiBr þ HO(CH2)3OH þ H2O,
and LiBr þ (HOCH2CH2)2NH þ H2O (LiBr/H2N(CH2)2OH,
LiBr/HO(CH2)3OH, and LiBr/(HOCH2CH2)2NH mass ratios are
3.5). These were selected as possible working fluids for air-
cooled absorption chillers, and Kim et al. (1999) calculated
the theoretical COPs at various operating conditions. They
also checked the cooling capacity and crystallization problem
for air-cooled cycle operation. Among these three working
fluids, LiBr þ H2N(CH2)2OH þ H2O was found to have the
widest operating range. This advantage is mainly because of
the enhanced solubility of the working fluid. However, there
are several issues, such as corrosion, heat and mass transfer
performance deterioration, need for rectification, and higher
viscosity, that need to be explored before using it in air-cooled
absorption chillers.
Ally (1988) presented simulation results aimed at
comparing the potential performance of LiBr and ternary
nitrate aqueous mixtures in high-temperature absorption
heat-pump application. The ternary nitrate mixture is an
aqueous solution of LiNO3, KNO3, and NaNO3 in mass ratio
53:28:19. The simulation results indicated that the ternary
nitrate mixture may be operated at up to 260 �C boost
temperature, which is approximately 80 �C higher than what
has been demonstrated with LiBr. In lower-temperature lift
regimes, the ternary nitratemixtures are not competitive with
LiBr. In higher-temperature regimes, the nitrates show the
potential for 10% higher COPs and a marginally greater
absorber capacity than LiBr.
Herold et al. (1991) developed an aqueous ternary
hydroxide working fluid to replace LiBr aqueous solution.
This aqueous ternary hydroxide sorbent consists of sodium,
potassium, and cesium hydroxide in the proportions 40:36:24
(NaOH:KOH:CsOH). The crystallization characteristics of the
salt solution can be avoided by mixing the three hydroxides
with water over the full range of operating conditions
expected in water heating applications. However, there are
some corrosion problems in its application. Trace amounts of
nitrogen (from minor air leakage into the system) will react
with trace amounts of hydrogen (from the hydroxides) to
form ammonia, which can attack the copper tubing used in
water heaters (Zogg et al., 2005).
De Lucas et al. (2003) measured the density, viscosity and
vapor pressure of aqueous mixture of lithium bromide and
potassium formate (CHO2K). The mass ratio of LiBr/CHO2K
is 2. They (De Lucas et al., 2004) also theoretically investigated
the performance of absorption refrigeration cycle utilizing
this new absorbent. The results showed the efficiency of the
absorption cycle is improved. This new absorbent mixture
requires a lower-temperature level (55 �C) in the generator to
activate the absorption cycle due to its low boiling point. The
use of an aqueous mixture of LiBr and sodium formate
(CHO2Na) was recently investigated (De Lucas et al., 2007). The
new working fluid composition maintains a ratio of LiBr/
CHO2Na of 2 by weight. This working fluid was introduced as
a potential competitor to aqueous LiBr solution for absorption
systems due to its higher water vapor absorption rates and
lower required generation temperatures (De Lucas et al., 2004).
Wang et al. (2010) conducted a systematic study to explore the
crystallization temperature of LiBr/CHO2Na water solution
and compared it against aqueous LiBr solutions. The results
were then used to evaluate the feasibility of using this new
working fluid in heating applications. The aqueous solution of
LiBr þ CHO2Na showed poor crystallization performance,
which would limit its use in AAC and AHP system designs.
Physical and thermal properties and corrosion character-
istics of the H2O þ LiBr þ LiNO3 (Iyoki et al., 1993c),
H2O þ LiBr þ LiI and H2O þ LiBr þ LiNO3 (Iyoki et al., 1993a,b),
and H2O þ LiBr þ LiNO3 þ LiI þ LiCl (Koo et al., 1999) systems
have been reported in the open literature. LiNO3 behaves as
a crystallization inhibitor and corrosion inhibitor. LiI is also
selected as a crystallization inhibitor, and LiCl serves as
a vapor pressure suppression agent. A company in Japan
(Iizuka et al., 1992; Tongu et al., 1993) patented a water þLiBr þ LiCl þ LiI þ LiNO3 solution for an air-cooled, double-
effect absorption chillereheater which increases allowable
absorber and condenser operating temperatures to about
10 �C and 4 �C higher than for a water-cooled cycle. A new
corrosion inhibitor was also developed so that the high-
temperature generator could be operated at 175 �C in an
air-cooled absorption cycle. A comparative study based on
thermodynamic simulation, numerical simulation of the
absorption process in the vertical falling-film absorber, and
experimental tests of the absorption rate of working fluid in
a vertical tube absorber was performed by Bourouis et al.
(2005) to evaluate the performance of air-cooled absorption
air-conditioning systems working with a solution of water þ(LiBr þ LiCl þ LiI þ LiNO3) (5:2:1:1 molar ratio). The safety
margin against crystallization of strong solution leaving
solution heat exchanger is higher for water þ LiBr þ LiCl þLiI þ LiNO3 solution than for LiBr aqueous solution. At the
minimum generator temperature, the single-effect cycle with
water þ (LiBr þ LiCl þ LiI þ LiNO3) solution shows a feasible
temperature range in the absorber about 7 �C wider than with
LiBr aqueous solution.
Ring et al. (2001) and Dirksen et al. (2001) tested the
crystallization temperature of 27 crystallization inhibitors
(at concentrations of 250e1500 ppm) within industrial LiBr
solutions cooled at a rate of 20 �C h�1. Some of these additives
(such as Methylene diphosphonic acid, Pyrophosphoric acid,
Amino tri(methylene phosphonic acid), Diethylenetriamine
pentamethylene phosphonic acid and 1-Hydroxyethylidene-1,1-
diphosphonic acid) further decreased the crystallization
temperature by up to 13 �C below the experimental crystalli-
zation temperature and up to 22 �C below the equilibrium
solubility of the same LiBr solution without additive.
Nemoto et al. (2010) used H2OeLiBre1,4-dioxane as a new
working fluid for solar-powered absorption refrigeration
system. The 1,4-dioxane is an organic substance whose
azeotropic relationship with water lowers the boiling point of
water. The pressureetemperatureeconcentration character-
istics, evaporation characteristics, and solubility of this new
working fluid were experimentally examined. The results
concluded that the absorption refrigerator can be expected to
produce the chilled fluid at �5 �C with the hot-water
temperature at 85 �C from a solar collector.
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3. Heat and mass transfer enhancement
The enhancement of heat and mass transfer performance
of working fluids and an absorber heat exchanger
decreases the overall temperature lift, and lowers the risk
of crystallization. The additional benefits of this approach
are reducing absorber size and initial system costs.
Vertical falling-film heat exchangers were used as
absorbers (as shown in Fig. 4) by Reimann (1981) and Tongu
et al. (1993). The solution and the refrigerant vapor flow
down inside the tube while the outside of the tube is cooled by
air. The authors have further used vertical in-tube conden-
sation where condensed refrigerant flows down the tube
forming a new continuous condensation surface to increase
the heat transfer coefficient. Kiyota et al. (2003) carried out
experimental and simulation studies for absorption inside air-
cooled vertical pipes. The results indicated that the heat
transfer area of air-cooled absorber is about three times larger
than that of a water-cooled absorber utilizing a vertical falling-
film configuration. Tsuda and Perez-Blanco (2001) employed
a vertical vibrating screen to enhance the heat and mass
transfer of working fluid in the absorber. The experimental
results covered a range of solution Reynolds numbers from
20 to 300, with mixing scales from 0.2 to 1 mm, and mixing
frequencies from 20 to 100 Hz for solution flow over a vertical
flat plate. The analysis showed that this technology could
significantly increase the absorption rate. A comprehensive
review of the mathematical model of the coupled heat and
mass transfer phenomena that occur during falling-film
absorption is given in Killion and Garimella (2001). Miller and
Keyhani (2001b) carried out mathematical and experimental
analysis on the effect of roll waves (wavy-laminar flow) on the
hydrodynamics of falling films in vertical absorbers. Regres-
sion analysis showed that the Reynolds number (Re) and the
Kapitza number (Ka) could describe the data trends in roll
waves. The correlation could explain 96% of the total variation
in the data and the deviation between experimental data and
simulated data was within �4%. Miller and Keyhani (2001a)
provided correlation equations for the coupled heat and
mass transfer of aqueous LiBr solution in vertical column
absorber without heat and mass transfer additive. Compared
against experimental results (internally cooled smooth tube of
Fig. 4 e Air-cooled vertical falling-film absorber (Zogg et al.,
2005).
19.05 mm outside diameter and of 1.53 m length), the average
absolute error in the Nusselt (Nu) correlation was �3.5%, and
the average absolute error in the Sherwood (Sh) correlation
was �5%.
Warnakulasuriya (1999) and Warnakulasuriya and Worek
(2006) investigated a spray absorber (as shown in Fig. 5),
which utilizes pressure atomization of working fluid to
increase the heat and mass transfer rates due to increasing
the area exposed to the brine solution compared to the falling-
film technique. A spray absorber was evaluated experimen-
tally to investigate the effects of differential pressure across
the nozzle and liquid absorbent flow rate on the absorption
rate. A mathematical model was also developed to predict the
mass transfer rate. Since the absorption process in this
concept is adiabatic, the concentration of absorbent fluid
doesn’t change significantly. Hence, solution circulation
rates through the absorber should be relatively high. This will
increase the power consumption and decrease the efficiency
of the absorption system. In this design, absorbent solution is
cooled by a separate process. Therefore, a rather large solution
heat exchanger is necessary to accommodate the high
solution circulation rates. Ryan et al. (1995) experimentally
studied the absorption rate of water vapor into a spray of
LiBr aqueous solution at sub-ambient pressure. Four kinds of
sprays generated by four different nozzles were tested and
compared. The test results showed that absorption rates are
significantly affected by any lamella produced by the spray
before breaking up into drops. Orian et al. (2006) studied the
performance of a special nozzle device and the influence of
nozzle geometry on spray characteristics, such as mean
droplet diameter, size distribution and velocity, and spray
cone angle and core length. The number of wings and spinner
pitch angle were the dominant parameters affecting spray
characteristics. Venegas et al. (2005) investigated the mass
transfer coefficient of ammonia refrigerant vapor absorbed by
lithium nitrateeammonia solutions in a spray absorber. The
experimental analysis showed that the mass transferred
reached a maximum (about 60% of the total) during the drop
deceleration period. A time-average mass transfer coefficient
equal to k¼ 1.86� 10�5m s�1 could be attained using the spray
absorption method.
Kang et al. (2000) carried out a parametric analysis to
comparatively study two different absorption modes in
ammoniaewater absorption heat-pump systems, namely,
falling-filmmode and bubblemode (as shown in Fig. 6). A plate
heat exchanger with an offset strip fin in the coolant side was
used to simulate the falling-film absorber and the bubble
absorber. The results indicated that the local absorption rate
of the bubble mode is always higher than that of the falling-
film mode. The difference is due to the larger mass transfer
area, better mixing, and higher heat transfer coefficients
experienced in bubble mode. Furthermore, the size of bubble
absorber heat exchanger is 48.7% less than that of falling-film
absorber. Merrill and Perez-Blanco (1997), Merrill (2000), and
Castro et al. (2009) achieved similar conclusions. However,
bubbling a vapor through a liquid solution will require higher
energy compared to a falling-film absorber (Garimella, 1999);
consequently it will limit the efficiency of this system. For this
reason, a bubble absorber may not be appropriate for water/
LiBr systems (Palacios et al., 2009).
Fig. 5 e Conventional absorber (above) and spray absorber with solution sub-cooler (below) (Warnakulasuriya and Worek,
2006).
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Zaltash et al. (2007) carried out comprehensive experi-
mental tests to evaluate the system performance of a 4.5 kW
air-cooled water/LiBr hot-water-driven absorption chiller unit
in a climate-controlled environment with rotating heat
exchangers as absorbers and generators. The COP and cooling
capacity of system were approximately 0.58 and 3.7 kW,
respectively, at 35 �C ambient temperature with 40 �C cooling
water temperature and 16.7 �C chilled water temperature.
Izquierdo et al. (2008) conducted the trial tests using the same
type of absorption chiller in Madrid, Spain, in August 2005.
The average COP for the test period as a whole was 0.49.When
the electric power consumed by auxiliary equipment was
counted into the calculation of COP, the COP decreased to 0.37.
The heat exchangers of this unit are all enclosed in a hermeti-
cally sealed drum as shown in Fig. 7. The rotating speed of
drum is about 300 revolutions per minute. This unit utilized
rotational force to form thin films in order to improve heat and
mass transfer in the absorber and generator (Aoune and
Ramshaw, 1999; Pravda, 1994; Ramshaw and Winnington,
1991; Winnington and Green, 2001; Winnington et al., 2001;
Zogg et al., 2005).
Theoretical investigation of an air-cooled micro-channel
absorber performance in an absorption-based miniature
electronics cooling system (having cooling capacity of 100 W)
was reported by Kim et al. (2008). As shown in Fig. 8, thewater/
LiBr pair is used as the working fluid and refrigerant vapor
(water) flows counter-current against the LiBr aqueous solu-
tion. The heat released from the absorption process is rejected
to the coolant in a liquid-cooled absorber or via the offset-
strip-fin array in an air-cooled absorber. The simulation
results showed that the air-cooled micro-channel absorber
could have comparable performance to a liquid-cooled micro-
channel absorber.
TeGrotenhuis et al. (2005) prototyped an ammoniaewater
absorption heat pump, which utilized a micro-channel heat
exchanger with thin-wick materials for absorber and used
branching fractal structures for a generator. Fig. 9 is a sche-
matic diagram of a thin-wick absorber. A planar wick (thick-
ness varying from 0.1 to 0.5 mm) was put into a small channel
with a plenum adjacent to the wick. High concentration
ammonia solution flows through this wick from one end to
the other, and the water vapor is absorbed by the fluid in the
wicks. Whenever the pressure of the ammonia solution in the
wicks is lower than the water vapor plenum pressure,
the capillarity will guide fluid through the wicks, and vapor
will flow through the adjacent plenum. The advantages of this
Fig. 6 e Schematic diagram of plate heat exchanger with falling film and bubble absorption modes (Kang et al., 2000).
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concept are the ability to recover from process upsets as well
as orientation-independence of the working fluid. The design
cooling capacity of this prototype machine is 250 W when the
evaporating temperature and condensing temperature are
10 �C and 49 �C, respectively. The experimental results showed
that the cooling capacity and COP of this system are 190 W
and 0.39, respectively, at the aforementioned operating
conditions.
A chemical, 1-nonylamine, was used as an additive to
enhance the heat and mass transfer of Carrol by Reimann
(1981), but it was replaced by phenylmethylcarbinol due
to a side effect of 1-nonylamine (Lof, 1993). The film heat
transfer coefficients of Carrol solution (70 wt%) with/without
1-nonylamine were measured as a function of absorber
loading. Results showed that the addition of 1-nonylamine in
Carrol would increase the film heat transfer coefficient by at
least 100% (Reimann, 1981).
Kang et al. (2008) experimentally investigated the effect of
nanoparticles of Fe and carbon nanotubes (CNT) on heat and
mass transfer enhancement of the water/LiBr absorption
process. It was found that mass transfer enhancement is
much more significant than heat transfer enhancement.
Fig. 7 e Rotating absorption chill
The mass transfer enhancement factors of CNT are 2.16 for
0.01 wt% and 2.48 for 0.1 wt% solutions. Comparatively, the
mass transfer enhancement factors of Fe nanoparticles are
1.71 for 0.01 wt% and 1.90 for 0.1 wt%. These results show that
CNT has superior performance to Fe nanoparticles and hence
is a better candidate for LiBr aqueous solution mass transfer
enhancement. Kim et al. (2007a) found that the size of the
ammonia absorber can be reduced greatly by adding surfac-
tants or nanoparticles, which is beneficial to the miniaturi-
zation of absorption equipment. Kim et al. (2007b) further
studied the use of Cu, CuO, and Al2O3 nanoparticles as addi-
tives to NH3/H2O solutions. They also investigated the use of
2-ethyl-1-hexanol, n-octanol, and 2-octanol as surfactants to
improve heat and mass transfer coefficients. The results
showed that the addition of surfactants and nanoparticles
improved the absorption performance up to 5.32 times.
4. Thermodynamic cycle modification
Kim and Infante Ferreira (2005) proposed half-effect, air-
cooled water/LiBr absorption cycles driven by hot water from
er unit (Zaltash et al., 2007).
Fig. 8 e Conceptual diagrams of (a) liquid-cooled and (b) air-cooled micro-channel absorber (Kim et al., 2008).
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 3 2 5e1 3 3 7 1333
a solar collector. The half-effect absorption cycles can be
categorized as single pump half-effect cycle and double pump
half-effect cycle. Fig. 10 shows the Duhring chart of the single
pump half-effect absorption cycle. Compared to the single-
effect absorption cycle, the half-effect cycle has one extra
medium-pressure absorber (MPA). The heat released from the
low-pressure absorber (LPA) is cooled by the evaporator. Hence
the LPA could operate at lower temperature in a half-effect
cycle than in a single-effect cycle, which decreases the risk of
crystallization. A half-effect absorption prototypemachinewas
built and tested in the laboratory by the same authors. The
cooling capacity and COP of this unit are 4.36 kW and 0.25,
which is lower than the design goals (10 kW and 0.38, respec-
tively) due to system leakage and cavitation in the refrigerant
gear pump. An air-cooled, heat-coupled, half-effect, parallel-
flow, water/LiBr absorption cycle has been theoretically inves-
tigated for solar air conditioning in extremely hot climates
(Kim and Infante Ferreira, 2009). Modeling results indicate that
the chillers could produce chilled water at 7 �C with a COP of
0.37 when the hot-water temperature and ambient air
temperature are 90 �C and 35 �C, respectively. The cooling
power would decrease by 64% when the ambient temperature
Fig. 9 e Schematic of a microwick absorber (TeGrotenhuis
et al., 2005).
increases from 35� to 50�. A dilute LiBr aqueous solution was
utilized as theworking fluid (concentrations varied from44.5 to
57.4 wt%) so that the risk of crystallization is less than with
other water-cooled absorption chiller systems.
A comparative study of single-stage and double-stage
absorption cycles has been performed to examine the crys-
tallization limitation of air-cooled water/LiBr absorption
systems using low-grade heat (Izquierdo et al., 2004). Simu-
lation results show that the single-stage cycles could not
operate due to crystallization when the condensing temper-
atures are higher than 40 �C using the heat from solar panels.
However, double-stage absorption cycles could avoid crystal-
lization until condensing temperatures reach 53 �C.
5. Absorption system-control strategies
A self-decrystallization technique has been proposed and
experimentally tested in a double-effect absorption air
conditioner/heater prototype unit (De Vuono et al., 1992).
Fig. 10 e Duhring chart of half-effect absorption cycle d
single pump cycle (Kim and Infante Ferreira, 2005).
Fig. 11 e Boosting absorber pressure to avoid
crystallization: (a) mechanical compression approach;
(b) Duhring diagram of absorption cycle with boosted
absorber pressure (Zogg et al., 2005).
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The flue gas from the burner is bypassed to heat the solution
heat exchanger after the event of crystallization. The authors
thought the solution heat exchanger was the most likely
component prone to crystallization. Test results showed that
it might require approximately 1.4 h to allow crystallized salt
to dissolve and drain from the solution heat exchanger.
Martini et al. (1998, 2004) developed and patented an over-
concentration control system to prevent the crystallization of
LiBr solution. They used an analog-type level switch that can
respond to the changing refrigerant level in the evaporator,
which is a direct indication of weak concentration in the
solution leaving the absorber. Solution concentration and
system temperatures were then used to theoretically evaluate
the absorption cycle. The fluid state closer to the crystalliza-
tion curve was then monitored closely and compared to
current operating conditions. Once the operating conditions
approach crystallization concentrations, corrective action is
taken to reduce the LiBr concentration and protect the chiller.
Through the use of a microprocessor, the chiller can operate
in a proactive way to prevent crystallization (Kalogirou, 2008).
Liao and Radermacher (2007) proposed a newmethodology
to prevent crystallization by raising the chilled water
temperature setting or reducing the exhaust inlet temperature
according to the control strategy maps developed in their
research. Wang et al. (2011) studied the impact of process
water flow configuration on the performance of a single-effect
AHP. Two flow configurations (the process water flowing
either from condenser to absorber or vice versa) have been
simulated and investigated at typical single-effect AHP oper-
ating conditions using ABSIM (Grossman et al., 1987). It was
shown that allowing the process water to flow first through
the absorber enables wider operating conditions. However,
this flow configuration resulted in AHP performance degra-
dation of 3.3% in efficiency (COP) and 4% in heating capacity
at design conditions due to higher condensing temperatures.
6. Other approaches
Zogg et al. (2005) discussed boosting the absorber pressure to
avoid crystallization for air-cooled absorption chillers. Fig. 11
shows the schematic diagram of this concept accomplished
through mechanical compression. As shown in Fig. 11 (b), the
absorber would work further away from the crystallization
curve if the pressure of absorber could be elevated. Mechan-
ical compression devices such as axial-flow fans could be
utilized to lift the pressure of the absorber. A preliminary
analysis performed by the authors concluded that a pressure
lift equating to an 8.34 �C increase in absorber saturation
temperature was desired. Several issues should be considered
for this approach such as designing a practical and cost-
effective method to integrate mechanical compression
devices to separate the absorber and evaporator and optimize
the boosted pressure, and system cost to achieve the
maximum benefit for system operation.
J-tube technology is considered to be another method to
prevent LiBr solution crystallization in a solution heat
exchanger. J-tube technology was developed and has been
widely deployed in absorption refrigeration industry (Johnson
Controls, 1997; Wang and Chua, 2009). As shown in Fig. 1,
crystallization is more prone to occur in a strong solution
entering the absorber. When crystallization occurs, strong
solution would be forced back up to the generator. When the
salt solution level reaches a certain value in the generator, the
hot, highly concentrated solution will bypass or overflow into
the absorber through the J-tube to immediately increase the
temperature of the low-concentration solution. The heated
low-concentration solution will warm the crystallized salt
solution in the solution heat exchanger. This transfer of heat
will increase the solubility of the salt solution and dissolve
LiBr crystals into solution, consequently allowing the system
to continue operation.
Abu-Zour and Riffat (2010) proposed to exploit the exhaust
air from buildings to cool down the absorber and condenser in
the air-cooled absorption system. The exhaust air is cooler than
the outdoor ambient air, and it will be mixed with the outdoor
air then induced into the inlet passage of the absorber and
condenser units to lower inlet air temperature in hot climates.
7. Conclusions
In this review, we highlighted previous efforts to reduce or
avoid crystallization problems in LiBr-based absorption
systems. Most of these efforts were focused on air-cooled
absorption chiller technology. Water/LiBr absorption heat-
pump systems also need to conquer the technical hurdle of
limiting crystallization. Absorption heat-pump systems reach
higher absorption temperatures than air-cooled absorption
chillers, making these systems more vulnerable to crystalli-
zation problems. The article presented a detailed review of
several crystallization control strategies such as chemical
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 1 3 2 5e1 3 3 7 1335
inhibitor additives, heat and mass transfer enhancements,
thermodynamic cycle modifications, system-control strate-
gies, techniques to boost absorber pressure, and J-tube tech-
nology. This review of the relevant technologies can help
guide future efforts to develop water/LiBr air-cooled absorp-
tion chillers and absorption heat-pump systems.
One of the most effective crystallization control tech-
nologies is using crystallization inhibitors. The criteria for
choosing a chemical crystallization inhibitor are 1) effective-
ness in inhibiting the crystallization, 2) stability under
system’s highest temperature, 3) material compatibility, 4)
heat and mass transfer performance, and 5) personal and
environmental safety. Themost successful chemical inhibitor
used to date is ethylene glycol. The trade name Carrol
(LiBr þ ethylene glycol þ phenylmethylcarbinol þ water)
developed by a company in United States has been extensively
tested both experimentally and in field settings for solar-
driven, air-cooled absorption chiller application. The major
drawback of this fluid is the toxicity of ethylene glycol. Efforts
are still being pursued to develop an ideal additive for LiBr-
based absorption systems.
Another promising technique is to enhance heat and mass
transfer using chemical additives (such as 2-ethyl-1-hexanol
with chromate or molybdate, and phenylmethylcarbinol
with 1-nonylamine) and nanoparticles (such as iron nano-
particles and carbon nanotubes). Heat andmass transfer could
also be improved using different heat exchanger designs such
as the spray absorber, bubble absorber, rotating absorber, and
micro-channel absorber. These heat exchanger designs have
been shown to significantly enhance system performance and
have the potential to greatly reduce the size of absorption
systems as well.
Half-effect absorption cycles have been proposed as
a viable option to avoid solution crystallization. In these
cycles, part of the refrigerant is used to cool down the
absorber and the rest of the refrigerant is used for cooling
effect. Hence, the absorber operates at a lower temperature
than in a single-effect cycle. This shifts the absorber opera-
tion away from the crystallization curve. The main drawback
of such cycles is the lower COP and the design complexity.
Finally, several systems-control strategies have been
recommended in the open literature, such as self-
decrystallization, over-concentration control, evaporator
pressure control, and process water flow direction. The self-
decrystallization control strategy only works after the event
of crystallization, while the patented over-concentration
control is a preventive technique and minimizes the inci-
dence of crystallization. Evaporator pressure control, another
preventive technique to control crystallization, might limit
the system’s ability to meet cooling load requirements.
Process water flow direction control provides wider operating
conditions to the absorption heat-pump cycle and allows the
system to meet various load conditions.
In conclusion, different crystallization control strategies
need to be coordinated to reach optimum performance and
allow air-cooled water/LiBr absorption chillers and absorption
heat-pump systems to operate without onset of crystalliza-
tion. This would allow engineers to use renewable and waste
heat sources to cool buildings in arid regions and provide
more efficient heating using absorption heat pumps.
Acknowledgment
The authors would like to acknowledge Dr. Abdolreza Zaltash
and Dr. Moonis R. Ally of Oak Ridge National Laboratory
for their support, enlightening discussions and insights.
This work was performed with funding from the U.S. DOE
Office of Energy Efficiency and Renewable Energy, Building
Technologies Program.
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