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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: wangk@ornl.gov (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

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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).

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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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