13

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

Absorption systems have been found in literature for more than 100

years. A detailed review of the literature on absorption systems has been

made by Liao (2004). Hence, the focus is made on the major and minor areas

of research such as absorption cycles and air-cooled systems respectively, in

this work.

Table 2.1 Research focus of absorption technology (Liao 2004)

Focus Area Contribution (%)

Absorption Additives 7 %

Absorption Cycles 46 %

Air-Cooled Systems 2 %

Alternative Working Fluids 6 %

Combined Heat and Power 4 %

Component Design 8 %

Economic Assessment 4 %

Heat and Mass Transfer 7 %

Modeling and Simulation 9 %

Properties 7 %

14

Table 2.1 shows the categories of research areas pertaining to absorption

technology.

VARS are classified into water-lithium bromide and ammonia-

water systems, based on the working fluids used as mentioned in the earlier

section. Water-lithium bromide systems are employed for air-conditioning

applications, and ammonia-water systems for industrial cooling/cold storage

(Ataer and Gö üs 1991) applications. The following section highlights the

literature review of the ammonia-water system, with regard to the

improvement in the system performance and also on the type of cooling

medium used, such as water, or air cooled systems.

The literature review that has been extensively done is classified into:

(a) Working fluid

(b) Theoretical studies and

(c) Experimental studies on the air-cooled GAX based vapour

absorption refrigeration systems.

2.2 WORKING FLUID

The performance of the absorption system depends on the

thermodynamic and chemical properties of the working fluids (Perez-Blanco

1984, Eisa and Holland 1987, Narodoslawsky et al 1988). Also, both the first

cost and the operating cost of an absorption system depends primarily on the

working fluid properties. Some of the desirable properties (Holmberg and

Berntsson 1990) of the working fluids are furnished below:

(a) The refrigerant should have high latent heat of vaporization.

(b) The difference in the boiling point between the pure

refrigerant and the mixture should be as large as possible.

15

(c) There should be high affinity between the refrigerant and the

absorbent.

(d) The mixture should be chemically stable, non-explosive and

non toxic.

(e) Both the refrigerant and the absorbent should be non-

corrosive, environment friendly and low cost.

(f) The transport properties such as viscosity, thermal

conductivity, etc that influence heat and mass transfer, should

be favourable.

A survey of different and alternative working fluids shows that the

most common working fluids are ammonia-water and water-lithium bromide

(Marcriss et al 1988). The reason why none of the alternative working fluids

have gained a market foothold is because the proposed alternatives may

address one or two drawbacks of the conventional working fluids, while

contributing to several of their drawbacks. Among the common working

fluids, ammonia-water is the most suitable one for the air-cooled system and

GAX operation, as discussed in the previous chapter.

2.2.1 Properties of ammonia and water

Ammonia is a colourless, alkaline gas at ambient temperature and

pressure, with a distinct pungent odour, (McKee and Wolf 1963) and is highly

soluble in water. Ammonia and Water are highly polar substances and have

the hydrogen bonding. Major properties of ammonia and water are given in

Table 2.2.

16

Table 2.2 Key properties of ammonia and water

Ammonia Water

Molecular weight (kg/kmol) 17 18

Boiling point at 1 bar (°C) - 33.2 100

Freezing point at 1 bar (°C) - 77.6 0

Critical pressure (bar) 113.5 221.2

Critical temperature (°C) 132.5 374.3

The required thermodynamic and transport properties of the

ammonia-water mixture that have been presented and discussed by various

researchers are explained as under:

Ziegler and Trepp (1984) presented a correlation used to calculate

the equilibrium properties of ammonia-water mixtures for a pressure and

temperature range of upto 50 bar and 500 K respectively. The equations of

state used are based on those of Schulz. The values of the specific volume,

vapor pressure, enthalpies and equilibrium constants for mixtures are

compared with the experimental data and the results are presented in the form

of vapor pressure and enthalpy concentration diagrams.

Renon et al (1986) formulated a cubic equation of state for the

ammonia-water vapour-liquid equilibria. The mixing parameters were

obtained from the data of Guillevic et al (1985), Pawlikowski et al (1982) and

Scatchard et al (1947). The parameters can be used to design a heat pump

easily, using the Aspen plus flow sheeting program.

17

Park and Sonntag (1990) calculated the thermodynamic properties

such as the bubble point temperature, dew point temperature, enthalpy and

entropy of ammonia-water mixtures, based on the generalized equation of

state. They compared the calculated thermodynamic properties with those of

the previous data in order to investigate the accuracy of the new approach.

The range of pressure and temperature is extended upto 20 MPa and 650 K

respectively, with an appropriate concentration of ammonia. For the

superheated region, the temperature range is easily extended to the limit of

chemical stability in principle.

Ibrahim and Klein (1993) formulated an equation of state for the

ammonia-water mixtures in the pressure range between 0.2 and 110 bar and

temperature range between 230 and 600 K. Different equations are used to

calculate the properties of the liquid and vapor phases. In the vapour phase,

the mixture is assumed to behave as an ideal solution, while in the liquid

phase, Gibbs excess energy is used to allow any departure from the ideal

solution behavior. The accuracy of the results showed a good agreement

between the computed properties and the experimental data.

Patek and Klomfar (1995) proposed a set of five equations for fast

calculations of the selected thermodynamic properties of ammonia-water. The

equations are suitable for the industrial design of the absorption refrigeration

system. The equations are constructed by fitting critically assessed

experimental data using simple functional forms, that cover the region in

which the absorption systems usually operate. The enthalpy of the gas phase

is calculated in the ideal mixture approximation and the results are presented

in the form of an enthalpy concentration diagram.

18

Sun (1997) presented the thermodynamic properties of water-lithium

bromide and ammonia-water mixtures. The results can be used to select the

operating conditions for absorption systems, and to realize automatic control

for the operation of these systems at optimum conditions.

Sun (1998) formulated the thermodynamic properties of ammonia-

water, ammonia-sodium thiocyanate and ammonia-lithium nitrate. The

thermodynamic properties were expressed in the form of polynomial

equations.

Tillner-Roth and Friend (1998) developed a thermodynamic model

based on the fundamental equation of state for the Helmholtz free energy of

the ammonia-water mixture. The model covered the entire two phase region

between the solid-liquid-vapour boundary and the critical locus. Experimental

data in the single phase region are restricted to sub critical temperatures, for

the liquid below 420 K and 40 MPa and for the vapour for pressures below 10

MPa. No experimental data are available for super critical temperatures. The

authors concluded that more experimental data are needed in order to verify

the reliability of the calculated thermodynamic properties, especially in the

single phase region.

Soleimani Alamdari (2007) presented a set of five simple and

explicit functions for the determination of the vapour-liquid equilibrium

properties of the ammonia-water mixture. The functions are constructed by

the least square method of curve fitting, using the valid available data in the

literature. The presented functions are valid upto a temperature and pressure

of 140°C and 100 bar respectively. A reasonable accuracy has been observed

when the obtained results are compared with those of other correlations in the

literature.

19

The required thermodynamic and transport properties of the

ammonia-water mixture for the theoretical analysis and the design of the

vapour absorption system have been obtained from the literature cited as

above.

2.3 THEORETICAL STUDIES

To analyze the performance of the vapour absorption refrigeration

system, several researchers have done simulation studies based on the

thermodynamic analysis. The influence of various operating parameters on

the system performance has been inferred, as these inferences are essential for

the design of the vapour absorption refrigeration system. A review of the

theoretical studies on the air-cooled and GAX based systems are furnished in

the subsequent sections.

2.3.1 Air-cooled systems

Oh et al (1994) simulated an air-cooled gas fired double effect

parallel flow water-lithium bromide absorption heat pump of capacity 7 kW.

The performance of the absorption heat pump in the cooling mode was

investigated by simulation to obtain the system characteristics, depending on

the temperature of the air at the inlet of the absorber, the working fluid

concentration, the ratio of the mass flow rate of the solution into the first

generator to the total mass flow rate of the solution from the absorber, and the

leaving temperature differences of the heat exchanging components. It was

observed that an increase in the inlet air temperature of above 37°C, decreases

the COP, and also causes a corrosion problem, due to the high temperature of

the first generator. A reasonable agreement was obtained when the predicted

results were compared with the measured data for the same design conditions.

It was suggested that for an optimum design, the leaving temperature

20

difference at the absorber, evaporator and condenser and the second generator

need to be maintained at -2°C, 2°C and less than 7°C respectively.

Kim et al (1999) analyzed the performance of the air-cooled system

for various operating conditions, using three different working fluids, namely,

LiBr + H2N(CH2)2OH + H2O, LiBr + HO (CH2)3OH + H2O, and LiBr +

(HOCH2CH2)2NH + H2O. All the three new working fluids were safe in

operation even at high sink temperatures, and served as alternatives to the

conventional water based absorption systems.

Lee et al (2000) investigated the performance of an air-cooled

double effect series flow absorption system, by using a new working fluid

H20+LiBr+LiI+LiNO3+LiCl. A cycle simulation was carried out for the

system and the thermodynamic design data were calculated at various

operating conditions. The performance of the new working fluid was found to

have a low crystallization temperature, reasonable COP, and was applicable

for air-cooled operation even at higher absorber temperature.

Salim (2001) performed simulation studies on an air-cooled water-

lithium bromide absorption system of capacity 7 kW using ABSIM software,

but no experimental validation was done.

Alva and Gonzalez (2002) investigated the technical feasibility of

air-cooled solar based absorption systems of capacity 10.5, 14 and 17.5 kW.

Simulations were conducted to evaluate the system's performance when

subjected to dynamic cooling loads. Within the computer model, heat and

mass balances were conducted on each component of the system, including

the solar collectors, thermal storage tank, the air-cooled condenser, and the

air-cooled absorber. The heat input to the absorption system generator was

supplied by an array of flat plate collectors that were coupled to a thermal

21

storage tank. The performance was compared with that of the water cooled

absorption system and no experimental validation was done.

Kim and Machielsen (2002) compared the performance and the

costs of different air-cooled solar vapour absorption refrigeration systems.

The single regenerative cycle showed a significant performance, but its

application is limited due to the high initial cost. Compared to the single

effect system of the same cooling capacity, the half effect system required

about 40% more heat exchange surface and 10 to 60% more collector area,

depending on the collector type. Ammonia-lithium nitrate systems showed

better results when different working fluids were used, and compared. The

authors suggested to develop low cost half effect absorption system to bring

down the initial cost. They concluded that the ammonia-sodium thiocyanate

system was not favourable due to an excessive pumping power requirement.

Izquierdo et al (2004) studied the performance of a double stage

air-cooled water-lithium bromide absorption cycle to produce cooling at 5°C,

operated by solar energy. When compared to that of the single stage cycle, the

double stage system allowed the use of condensation temperatures higher by

13°C than those of the former. The performance analysis showed that the

single effect system cannot be operated above 40°C of condensation

temperature, and crystallization does not occur in the double effect system

even if the condensation temperature is above 50°C.

Wang et al (2007) carried out a thermodynamic analysis of a gas

fired air-cooled adiabatic absorption refrigeration system of capacity 16 kW

using water-lithium bromide as the working fluid. The system had two new

features, such as the waste heat recovery of condensed water and an adiabatic

absorber with an air cooler, which enhances the effective heat and mass

transfer. They found that the outdoor air temperature had a great influence on

22

the cooling capacity and the COP of the system. The results indicate that the

reduction in the solution distribution ratio helped the cycle to operate in

vacuum, and to obtain a higher COP when the outdoor air temperature was

higher than that in normal operating conditions. However, a low solution

distribution ratio also increases the risk of crystallization in the high

temperature heat exchanger, as the air temperature decreases. They suggested

that a proper control of the solution distribution ratio is crucial to have high

efficiency and ensure reliability in operation.

Kim and Ferreira (2009) investigated an air-cooled absorption

chiller operated by flat collectors. The cycle used diluted water-lithium

bromide as the working fluid so that the crystallization issue is less, even

when the ambient conditions are high. The direct and indirect air-cooled

chillers considered in the study delivered chilled water at 5.7°C and 7.8°C,

with a COP of 0.36 and 0.38 respectively, at 35°C ambient condition and for a

heat source temperature of 90°C.

2.3.2 GAX based systems

Kandlikar (1982) proposed an effective method of utilizing the heat

of absorption to improve the system performance. The performance of the

proposed system with a heat recovery absorber was found to be 10% higher

than that of the conventional ammonia-water absorption refrigeration system.

The improvement in the COP of the system also reduced the cost by the

effective utilization of solar energy which decreased the collector area. The

possibility of incorporating an air-cooled condenser and absorber has been

greatly improved. The author concluded that a detailed analysis has to be

done in order to arrive at optimum design conditions.

23

Kaushik and Rajesh Kumar (1985) showed that an absorber heat

recovery cycle yields a higher COP compared to the conventional cycle at

higher generator temperatures. Water-lithium bromide was used as the

working fluid pair. The analysis was carried out to produce evaporator

temperatures of 5°C and 10°C with similar condenser and absorber

temperatures of 30°C. They revealed that the addition of the absorber heat

exchanger does not increase the heat exchanger area, because it reduces the

size of the absorber and generator. Even at higher generator temperatures, a

higher COP has been obtained, unlike the conventional water-lithium bromide

and ammonia-water cycles. However, the system was restricted to a limited

range of operating conditions, due to the crystallization problem associated

with the working fluid pair.

Scharfe et al (1986) analyzed the advantages and the limitations of

a GAX cycle. An equation for the heat of desorption was derived and it

showed that at any temperature interval, the heat requirement of the generator

is higher than the amount of heat supplied by the absorber.

Herold et al (1991) proposed a branched GAX cycle which

addresses the main problem in the standard GAX cycle. It was found that the

heat available in the absorber at each temperature level was not sufficient to

meet the heat requirement of that temperature level in the generator. The

branched GAX cycle provided a better match between the hot and cold sides

of the GAX heat exchanger, by increasing the solution flow rate in the high

temperature end of the absorber. The authors analyzed and presented the

results of the performance evaluation in both the heating and cooling modes

over a range of typical ambient conditions. The performance of the branched

GAX cycle was higher than that of the simple GAX by 20%. However,

24

compared to the simple GAX cycle, the cost of the branched GAX cycle

increased due to the additional solution pump.

Mcgahey and Christensen (1993) investigated a natural gas fired

GAX absorption heat pump, by using a modular steady state simulation,

which was used for commercial applications. An enhanced version of the

simulation model developed by the Oak Ridge National Laboratory (ORNL)

was used to model the complete absorption system, including an indoor gas

fired generator and an outdoor air-to-hydronic heat exchanger. The model was

used to map the heat pump performance for outdoor air temperatures between

-18°C and 46°C. The simulation model was used to optimize the system,

based on the performance of the UA values of the heat exchangers.

Groll and Radermacher (1994) simulated an ammonia-water

desorber absorber heat exchange (DAHX) cycle, and reported that the internal

heat exchange between the desorber and the absorber resulted in a very low

pressure ratio of about 70% lesser, when compared to that of the conventional

ammonia refrigeration system, and up to 62% lesser than that of the R-22

system. The reduction in the pressure ratio leads to an energy savings in the

DAHX cycle because of a more efficient compression process. The cooling

COP of the DAHX cycle was 10% higher than that of the conventional

ammonia-water refrigeration system, and 26% higher than that of an R-22

system for the same operating conditions. The results showed that the COP

was highly dependent on the logarithmic mean temperature difference of the

internal desorber / absorber heat exchange.

Rane and Erickson (1995) presented a patented three pressure GAX

cycle, which addresses the problem that at a high temperature lift greater than

60°C, there is no GAX temperature overlap. At these higher temperature lifts,

the three pressure GAX cycle gives a better COP compared to that of the

25

conventional single effect system, by tapping into the lost availability in the

externally cooled absorber and rectifier. It was found that, for a temperature

lift of 85°C, the analyzed GAX cycle prevents 10% reduction in the COP due

to the rectification losses in a conventional cycle.

Grossman et al (1995) investigated the performance of a GAX heat

pump for both the heating and cooling modes by using Absorption Simulation

(ABSIM) software (Grossman et al 1991). The simulation was carried out

over a wide range of ambient conditions, and the performance of the system

was calculated along with the internal flows and concentrations of the

solution and the refrigerant. It was inferred that the rectifier could produce

distilled refrigerant vapor with 99% concentration over the entire range of the

heat rejection temperatures. By increasing the rectifier temperature, there was

a marginal increase in the COP of the system, but the water content in the

refrigerant increased, which led to a larger temperature glide in the

evaporator. The influence of some of the design parameters, such as the flow

rate in the GAX heat transfer loop, and the refrigerant flow control was also

investigated. A cooling COP of 1.0 and a heating COP of 2.0 were obtained.

Hanna et al (1995) analyzed the GAX system as shown in

Figure 2.1, by employing the pinch point analysis technique. This technique

has been commonly used in chemical process industries, where internal heat

recovery plays a major role from the process design point of view. The study

focused mainly on the processes in the cycle, and the advantage is the manner

in which one could view the details of the internal processes of the cycle. The

graphical method of the pinch point analysis showed the importance of

internal heat recovery for cycle efficiency. By knowing the closeness of the

state points of the heat recovery processes, an economic trade off of the cycle

components was achieved.

26

Figure 2.1 Schematic of the GAX heat pump (Hanna et al 1995)

Ozaki et al (1995) developed computer codes for cycle simulation,

to study the performance of the absorption heat pump. Several factors, which

influence the COP of the absorption cycle, such as the evaporating

temperature, condensing temperature, and efficiency of the heat recovery heat

exchangers were considered. It was found that 5K decrease of temperature

difference of the GAX heat exchanger increased the cooling by 5.5%, and

there was a maximum heating COP at 4K of temperature difference of the

GAX heat exchanger.

Staicovici (1995) introduced poly-branched regenerative GAX

advanced cycles, which combine the advantages of the GAX, branched GAX,

regenerative GAX and regenerative GAX with rectification heat recovery.

Figure 2.2 show the schematic of a multi branched GAX cooling system. The

27

results highlighted the use of high solubility combinations at elevated

temperatures, as well as increasing the boiling temperature and the number of

stages. Compared to a double effect cycle, a two stage poly branched

regenerative cycle with rectification heat recovery was simple in construction,

and its thermal performance was 40% better. A three stage poly branched

regenerative GAX cycle had a COP 1.3 to 1.9 times higher and 70 to 80%

Carnot cooling efficiency for lifts (temperature difference between the

condenser and the evaporator) varying between 68 and 47°C. The

polybranched regenerative cycles were capable of producing both cold and

hot water at 50-70°C.

Figure 2.2 Schematic of a multi branched GAX cooling system

(Staicovici 1995)

Kang et al (1996) established a theoretical model for the design of

rectifier in a GAX absorption heat pump and considered three different rectifier

configurations namely vertical fluted tube, confined cross flow with fluted tube

and coiled smooth tube, for the study. The study revealed that a minimum

temperature difference between the interface and the bulk regions and a high heat

transfer coefficient in the vapour region reduces the size of the rectifier.

28

Ozaki et al (1996) compared the performances of the ammonia-

water advanced cycles, a GAX cycle, a hybrid cycle (a combination of a basic

cycle and a mechanical compressor) and a GAX hybrid cycle. The GAX hybrid

cycle was found to be more efficient in the cooling mode, and the difference

between the condensing and evaporating temperature influenced the cooling

COP. In the heating mode no cycle had any advantage over the others.

Erickson and Anand (1996) developed a vapour exchange GAX

cycle (VXGAX) similar to the branched GAX cycle. It was a three pressure

cycle that incorporated the heat of absorption into both the high pressure and

intermediate pressure generators. The performance of the new cycle was

better than that of the conventional cycle. The economic analysis indicates

that the VX GAX cycle provides commercially viable industrial refrigeration

operated by prime fuel or waste heat.

Erickson and Tang (1996) investigated double lift waste heat GAX

cycles (semi GAX cycles) that utilize the internal heat exchange between the

intermediate pressure absorber and the high pressure generator. A computer

modeling was done in order to identify the key performance parameters. An

increase of 20% in the COP was obtained for the semi GAX cycle when

compared to the conventional double lift cycle, and it require less total heat

duty, implying lower first cost.

Garimella et al (1996) studied the performance of a GAX heat

pump in both the heating and cooling modes by using ABSIM software. The

variables that affect the system performance were systematically investigated

over a wide range of ambient temperatures. The system cooling COP at the

rating point was maximized by varying the UA values of the heat exchanger.

The decrease in the GAX overlap at low ambient temperature, and the

corresponding transformation into the absorber heat exchange cycle was also

modeled, and its performance were investigated. Also, the role of an

additional solution – solution heat exchanger at low ambient temperature in

29

enhancing the system COP was quantified. The results showed that the COP

of the cooling and heating modes was 0.925 and 1.51 respectively, for an

ambient of 35°C and 8°C. In the cold ambient heating mode, the liquid heat

exchanger introduced between the solution heated desorber and the solution

cooled desorber offered significant performance benefits.

Saghiruddin and Siddiqui (1996) analyzed the economic aspect and

the performance study of the absorber heat recovery cycle as shown in

Figure 2.3, using ammonia-water, ammonia-lithium nitrate and ammonia-

sodium thiocyanate. The performance of the system improved by about 20 to

30 % in the ammonia-water mixture and by 30 to 35% in the ammonia lithium

nitrate and ammonia-sodium thiocyanate mixtures. Also, there was a

considerable reduction in the energy costs also by about 10 to 25% in the

Figure 2.3 Schematic of an absorption refrigeration system with theheat recovery absorber (Saghiruddin and Siddiqui 1996)

ammonia-water system, and around 20 to 30% in the ammonia-lithium nitrate

and ammonia-sodium thiocyanate systems. The operating cost of the system was

the lowest when bio gas was used as the heat source compared to liquefied

petroleum gas.

30

Zaltash and Grossman (1996) demonstrated the potential of using

ternary fluid mixtures for the advanced cycle by comparing the performance

of the simple GAX and the branched GAX cycle using ammonia-water and

ammonia-water-lithium bromide mixture. ABSIM was used to investigate the

potential of combining the above advanced cycles with the ternary fluids. The

performance parameters of the cycles, including the COP and heat duties,

were investigated as functions of the different operating parameters in the

cooling mode for both the ammonia-water binary and the ammonia-water-

lithium bromide ternary mixtures. The high performance potential of GAX

and branched GAX cycles using the ammonia-water-lithium bromide ternary

fluid mixture was achieved, especially at a higher generator temperature of

around 200°C. The cooling COPs have been improved by 21% over the COP

achieved with the conventional ammonia-water binary mixture.

Engler et al (1997) performed the simulation of a gas fired

ammonia-water GAX system using an ABSIM modular program. They

analyzed different configurations, such as the conventional single effect cycle,

the simple GAX cycle, the branched GAX cycle and the absorber heat

exchange cycle. Each configuration was formed on the basis of the previous

one by adding one or two components at each stage resulting in increased

complexity, but in improved performance. The influence of the components

added at each stage on the cycle, and the effects of the important operating

parameters in the heating and cooling modes were investigated over a wide

range of conditions. The COP ranged from about 0.5 for the conventional

single effect cycle to about 1.1 for the branched GAX cycle.

Potnis et al (1997) simulated an ammonia-water GAX system for

simultaneous heat and mass transfer, for coexisting liquid film absorption and

flow boiling desorption. The simulated temperature profiles were found to be

31

close to the experimentally obtained profiles for different vapour and liquid

flow rates. Also, the simulated values of the absorption side vapour phase

flow rates were close to those of the experiment boundary condition. The

simulation of the coupled heat and mass transfer processes could predict the

pinch point, appropriate sizing of the equipment, the COP of the system as

well as the vapour and liquid flow rates. The flow-boiling-side heat transfer

coefficients were found to be an order of magnitude higher than the

absorption-side liquid-phase heat transfer coefficients and the absorption-side

vapour-phase heat transfer and mass transfer coefficients were found to be

about an order of magnitude lower than those of the liquid phase.

Kang et al (1999) developed an advanced GAX cycle, namely,

Type A, B and C, as shown in Figure 2.4, to utilize the waste heat, and

studied the parametric analysis of the effects of the waste heat source

temperature and the outlet temperature of the gas-fired generator. In the Type

A cycle, the solution heated desorber of the standard GAX is replaced by a

waste heat exchanger; an extra heat exchanger is added to the standard GAX

in type B, and in type C the solution heated desorber is placed between the

rectifier and the GAX desorber to transfer the extra heat of the strong solution

to the desorber column. It was found that the effect of the waste heat

temperature on the performance of the system was negligible for a given gas

fired generator outlet temperature. The corrosion problem in the standard

GAX cycle at temperatures higher than 200°C could be solved by employing

the GAX cycle utilizing the waste heat (WGAX). The GAX effect was

dominant for a temperature lower than 181°C, while the effect of exergy loss

was dominant for a temperature higher than 181°C. Type A was better from the

view point of the GAX effect, whereas, Type B was better from the point of

view of the exergy loss. A comparison of the type B and type C cycles showed

that the solution heated desorber should be placed below the GAX desorber to

improve the cycle performance. It was recommended that sub cooling is

necessary to improve the COP in WGAX systems.

32

Figure 2.4 Schematic of the WGAX cycle (Kang et al 1999)

Kang and Kashiwagi (2000) compared the performance of an

ammonia-water GAX system (PGAX) and a single effect cycle for panel heating

(PSE) applications. Due to the internal heat recovery in the GAX component, the

COP of the PGAX was higher than that of the PSE. It was reported that the

performance of a hydronically cooled absorber was more sensitive to the coolant

temperature than that of the solution cooled absorber, and therefore, the effect of

the overall conductance (UA) ratio on the total COP of the PGAX was higher

than that of the PSE. The panel heating COP was more significantly affected by

the UA variation of the absorber than the space heating COP. The parametric

study revealed that the UA ratio could be used to select absorbers for heating

capacities. The stream from the hydronically cooled absorber is split into the

rectifier and the solution heat exchanger based on the split ratio, and the optimum

UA value occurred for a split ratio of 0.87.

33

Anand and Erickson (2001) examined the characterization of the

absorption cycle performance in terms of cycle lift (difference between the

condenser and evaporator temperatures) and revealed that the absorption cycle

performance was dependent on the absorption compressor and absorber

temperature. Further, the variation in the absorber temperature affected the

overall system performance. An effective lift was defined for each cycle to

incorporate the influence of the absorber temperature. Variations of the

effective lift curve, such as those observed for the basic GAX and VXGAX

cycles at low lifts, indicate the limitations in the performance of the system.

The performance data of the actual system were presented for the VX GAX

cycle heat pump, and the concept of the effective lift was validated.

Velázquez and Best (2002) reported the thermodynamic analysis

of a 10.6 kW air-cooled GAX system operated with hybrid natural gas and

solar energy, as shown in Figure 2.5. The COP was found to be 0.86 for

cooling and 1.86 for heating together with total internal heat recovery of

16.9 kW, at the generator and evaporator temperatures of 200°C and 4°C

respectively. The efficiency of the system decreases as the temperature lift

(temperature difference between the condenser and evaporator) increases.

The mass flow of the analyzed cycle was compared with that of the basic cycle,

showing 73% and 62% less for the circulation ratio and flow ratio respectively.

The system was found to be an excellent option for air conditioning purposes,

where the temperature lift was small. The proposed methodology allowed to

find the best working condition for a particular design.

34

Figure 2.5 Schematic of the solar GAX cycle using ammonia-watersolution pair (Velazquez and Best 2002)

Kang et al (2004) developed four different advanced hybrid

GAX cycles and carried out a parametric analysis. The development of the

four cycles was: Type A for performance improvement, Type B for low

temperature applications, Type C for the reduction of the generator

temperature and Type D for hot water temperature applications. The

schematic of the hybrid GAX cycle is shown in Figure 2.6. A compressor has

been placed between the evaporator and the absorber in Type A and Type B,

and between the desorber and the condenser in Type C and Type D. The

improvement in the COP of Type A was 24% higher than that of the simple

GAX for the same operating conditions. In Type B, it was observed that at an

evaporation temperature of -80°C the COP was 0.3. The maximum generator

temperature that could be reduced was 164 °C, and eventually this eradicates

the corrosion problem, that occurs at temperatures above 200°C in the simple

GAX. In Type D, the highest hot water temperature that could be obtained

was 106°C, which can be subsequently used for space heating and panel or

floor heating applications.

35

Figure 2.6 Schematic of the HGAX cycle (Kang et al 2004)

Sabir et al (2004) analyzed the performance of a novel GAX -

Resorption heat driven refrigeration cycle. The novel system was as simple as

that of a single effect cycle and the performance was found to be sensitive to

the inlet temperature of the cooling / chilled water. The COP of the system

was better than that of the conventional single effect vapour absorption and

resorption cycles, but less than that of the GAX cycles. However, it was

anticipated that, a wide range of water temperatures, and mass and heat

transfer effectiveness would result in a better performance than that of a

simple GAX system.

Ramesh Kumar and Udayakumar (2007) simulated a GAX

absorption compression cycle operated with the ammonia-water working fluid

pair. The degassing range (difference between the mass fraction of the weak

and strong solutions) of the cycle was optimized for the maximum COP and

the effect of the absorber pressure on the component heat duties was

investigated. It was found that the maximum COP occurs at an optimum

degassing range of about 0.4 kg of ammonia per kg of strong solution. The

hybrid GAX cycle showed an increase of 30% COP compared to that of the

36

simple GAX cycle. It was reported that the required COP of the hybrid cycle

could be attained in the lower degassing ranges, and it can be operated by

utilizing low temperature energy sources.

Zheng et al (2007) simulated a single stage ammonia absorption

system and a GAX cycle, and reported that the COP and the exergy efficiency

of the latter were 31% and 78% respectively higher than those of the former,

for the heat source temperatures of tH = 120°C, tM = 25°C and tL = 5°C. Based

on the concept of exergy coupling, the absorption cycle was divided into the

heat pump and heat engine sub-cycles. By means of the energy grade factor-

enthalpy diagram, the thermodynamic analyses of the two frameworks were

studied , which showed that the exergy demand of the heat pump sub-cycle in

the GAX cycle was the same as that of a single stage cycle. Also, the energy

grade factor-enthalpy diagram, clearly illustrated the level of the energy

quality at each state of the cycle and the energy load caused in the process. A

reduction in the external heat loss, external exergy loss and the internal

exergy loss of the heat engine sub-cycle, reduced the energy consumption,

and an increased benefit was obtained from the overall cycle.

Park et al (2008) developed an ammonia GAX absorption cycle that

could supply both chilled and hot water, using a single hardware. The effect of

the outlet temperature of the hot water, and the split ratio (the ratio of the

solution flow rate into a hydronically cooled absorber to the total flow rate of

the weak solution from the GAX section of the absorber) of the solution on the

cooling and heating COP were investigated. It was inferred that when the

system was operated at a full cooling load and the temperature of the hot water

was 55°C, the cooling COP of the three modes was 60%, 42% and 87%

respectively. Mode 1 gave a better result from the hot water supply point of

view. However, during summer, when the cooling mode is the primary purpose

rather than the hot water supply, case 3 was the most desirable. It was

recommended that the optimum UA values of the solution cooled absorber and

hydronically cooled absorber for mode 3 should be less than those of mode 1.

37

Ramesh Kumar and Udayakumar (2008) studied the effect of the

compressor pressure ratio on a 3.5 kW ammonia-water GAX absorption

compression cooler. The effects of the generator, sink and evaporator

temperatures on the performance of the cycle as a function of the pressure

ratio were studied. The COP increased with an increase in the low side

pressure ratio and generator temperature, and decreased with an increase in

the absorber and condenser temperatures. The low side pressure ratio of the

cycle was optimized for the optimum COP. The optimum COP corresponding

to the optimum pressure ratio was found to be independent of the sink and

evaporator temperatures, for a given value of the generator and approach

temperatures. The performance of the analyzed cycle was nearly 25% higher

than that of the standard GAX cycle.

Ramesh Kumar and Udayakumar (2008a) carried out simulation studies

based on approach temperature on a 3.5 kW GAX absorption compression cooler

and compared its performance with that of a simple GAX cycle. Three working

fluids namely ammonia-water, ammonia-sodium thiocycnate and ammonia-

lithium nitrate were used. The GAX absorption compression cycle showed a

better performance than that of a simple GAX cycle.

Ramesh Kumar et al (2009) reported the heat transfer modeling of an

11.5 kW GAX absorption compression cooler as shown in Figure 2.7. The effect

of the UA value of each component of the cycle on the COP and the cooling

capacity was analyzed, and it was found that the UA value of the absorber and

generator had a significant impact on the cycle performance. At an operating

condition of the minimum UA value of all the heat exchanging components, the

maximum COP of the system was found to be around 1.2. The variation in the

temperature of the cooling medium in the absorber was found to have a greater

effect on the COP and capacity of the system, than a variation in that of the

condenser. The effects of the mass flow rate and the inlet temperatures of the hot

fluid, chilled water and cooling water were also investigated.

38

Figure 2.7 Schematic of the GAX absorption compression cooler

(Ramesh Kumar et al 2009)

Velázquez et al (2010) presented a numerical simulation of the

solar GAX cycle of cooling capacity 10.6 kW, as shown in Figure 2.8. A

linear fresnel reflector concentrator (LFRC) was used as an ammonia vapour

generator. A mathematical modeling, considering the geometrical, optical,

thermal and fluid dynamic aspects of the LFRC was carried out. The COP of

the solar GAX cycle and the efficiency of the LFRC were about 0.85 and 0.60

respectively. The numerical result also revealed that the availability of the

solar beam radiation had a negligible effect on the COP of the system but a

significant effect on the capacity of the LFRC and the refrigeration cycle. The

study also revealed the technical feasibility of using an LFRC as a generator

in the solar GAX cycle.

39

Figure 2.8 Schematic of the ammonia-water solar GAX refrigeration

system with LFRC (Velázquez et al 2010)

Yari et al (2011) compared the energy and exergy analyses of GAX

and GAX hybrid absorption refrigeration cycles. It is inferred from the

parametric analyses that the generator temperature has more influence on the

second law efficiency that that on the COP of the cycles. The generator,

absorber and the expansion valve contributes to the highest and the lowest

exergy destruction respectively, in both the cycles.

2.4 EXPERIMENTAL STUDIES

The experimental studies on the air-cooled and GAX based systems

carried out by various researchers are explained in detail in the next section.

40

2.4.1 Air-cooled systems

Kurosawa and Fujimaki (1989) conducted experimental studies on

an air-cooled double effect gas fired 70 kW absorption chiller, using water-

lithium bromide as the working fluid. Even though the ambient temperature

reached 34°C, the system operated normally. When the ambient temperature

reached its peak, the temperatures of the solution at the absorber inlet and

outlet were 56°C and 30°C respectively. The temperatures of the cooling

medium at the inlet and outlet were 34.1°C and 44°C respectively. Since the

outlet temperature of the absorber was found to be lower than the outlet

temperature of the cooling medium, it indicates that the counter flow heat

exchange was conducted smoothly.

Castro et al (2002) developed a 3 kW water-lithium bromide

vapour absorption refrigeration system and compared the performance of the

system theoretically and experimentally, for hot water temperatures ranging

from 80 to 95°C. The initial results indicate discrepancies between the cooling

capacity and the COP, due to the poor performance of the absorber.

Incomplete wetness had been observed in the horizontal tube heat exchangers

(generator and evaporator) with the consequent reduction of the useful heat

exchange area.

Castro et al (2008) presented the recent developments in the design

of air-cooled hot water driven water-lithium bromide absorption chiller. From

the view point of methodology, the main novelty was the systematic

application of mathematical models for the design and prediction of the

thermal behavior of the new prototype. Due to the cycle configuration (single

effect), the temperature lifts between the evaporator and condenser are

41

limited, if the hot water temperature is restricted to 95°C in order to avoid

crystallization. The results showed that an evaporator temperature of 5 to 6°C

could be reached. A maximum COP of 0.65 has been obtained, and an

acceptable agreement between the experimental COP and the theoretical one,

especially with the primary fluid data, and under conditions far from the

minimum driving temperature at the generator was observed.

2.4.2 GAX based systems

Erickson et al (1996) reported the results of a gas-fired branched

GAX cycle heat pump as shown in Figure 2.9. A novel thermosypon cooled

absorber that was used, eliminated the need for the outdoor hydronic loop and

reduced the cost by 10%. The inference from the results highlighted that a

Figure 2.9 Schematic of the branched GAX prototype (Erickson et al1996)

42

cooling COP of 1.06 was obtained at a temperature effective lift (the

difference between the evaporator temperature and the average of the

condenser and absorber temperatures) of 38.9 °C and 14.7 kW cooling

capacity. For the same lift, the cycle cooling COP in the branched GAX was

1.04, and at an ambient condition of 35° C, a cooling load of 15.7 kW was

achieved at a cooling COP of 0.95. The performance of the branched GAX

was marginally lower than that of the GAX cycle due to the sub-cooling of

the absorbent liquid at the top and bottom of the GAX component.

Zhou and Radermacher (1997) experimentally investigated the

vapor compression cycle with a solution circuit and desorber/absorber heat

exchanger. The working fluid employed was an ammonia-water mixture. For

a temperature lift between 60 °C and 80 °C, a COP in the range of 1.2 to 1.8

was obtained and the cooling capacity range was found to be between 7 and

12 kW. The results were compared with those of a single stage and two stage

cycle which revealed, that the two stage cycle had the highest temperature lift

and lowest cooling COP. However, the single stage cycle had the highest

cooling COP but the lowest temperature lift. The experimental results showed

that when a bypass was introduced between the outlet of absorber I and the

inlet of generator II, the sub-cooling decreased and the variation in the

cooling COP was between 1% and 3%, and the temperature lift increased by a

maximum of 6 °C.

Ng et al (1998) examined a 7 kW gas fired ammonia-water

absorption chiller with a generator heat exchanger, an absorber heat exchanger

and a regenerative GAX configuration. The COP of the system was about 0.8 at

an operating condition of a generator temperature of 200°C, condenser

temperature of 44°C, absorber temperature of 41°C and evaporator temperature

of 5°C. The study revealed the significance of the process average temperature

in the analysis of the chiller. From the thermodynamic considerations, a proper

43

process average temperature has been derived for the non-isothermal processes

occurring in the absorption chiller, and quantified the inaccuracies that derived

from an incorrect process average temperature, in predicting chiller

performance and in estimating optimal operating conditions.

Priedeman and Christensen (1999) presented a general ammonia-

water absorption heat pump cycle of capacities 10.5, 11.5 and 17.5 kW that

was modeled and tested. The experimental results were used to calibrate both

the cycle simulation and the component simulations, yielding computer

design routines that could accurately predict the component and cycle

performance. The modeling incorporated a heat loss from the gas-fired

generator and pressure drops in both the evaporator and absorber. The

experimental findings of the 17.5 kW capacity chiller were found to be in

close agreement with the simulation results.

Priedeman et al (2001) tested a gas operated, 17.5 kW ammonia-

water GAX system under steady state operation at 35°C ambient conditions.

The COP of the system was found to be 0.68 at a full load condition.

Simulation was also carried out, and the results of the experimentation were

found to be in close proximity with the simulation results. Lower burner

generator efficiency, a pressure drop between the evaporator and pump inlet,

and less heat recovery in the GAX resulted in a lower performance of the

system, and improvements in these conditions could result in achieving the

target COP of 0.70 and delivering chilled water at the required temperature of

7°C.

Gomez et al (2008) evaluated an indirect thermal oil fired, 10.5 kW

GAX cycle. A simulation model was developed, calibrated and validated with

experimental findings in order to predict the performance of the system

outside the design parameters. The COP of the system was found to be 0.58

with a generation temperature of 192 °C. An internal heat recovery of

44

approximately 55% of the total heat supplied to the generator was obtained. A

maximum cooling load of 7.1 kW was obtained. The experimental results

were lower than the design values due to a lower range of the operating

temperature of the heating oil and the low performance of the absorber. The

performance of the GAX absorption system integrated to a micro gas turbine

as a cogeneration system, was also simulated, and it was found that the

overall efficiency of the cogeneration system varied between 29% and 49%

for cooling loads of 5 kW and 20 kW respectively.

Saravanan et al (2008) evaluated the performance of a biomass

heated GAX vapour absorption refrigeration system of 140 kW cooling

capacity, used for milk chilling operation. The evaporator was operated

between -2°C and 0°C. The GAX component recovers about 28 kW of

internal heat to attain a cooling capacity of 130 kW. An actual and real COP

of 0.58 and 0.52 were obtained for a generator temperature of 120°C and a

sink temperature of 30°C. Compared to the single effect absorption system,

the COP of the GAX system was found to be 30% higher. A saving of about

70% of electrical energy was obtained, compared to that of a vapour

compression system of the same capacity. The reduction in the emission was

about 586 tonnes of CO2 per year.

García-Arellano et al (2010) presented the dynamic analysis of a

shell and tube evaporator of an air-cooled absorption refrigeration GAX

system, to reach an operational stabilization. The dynamic analysis of the

evaporator was carried out using mathematical tools. The experimental results

showed that the evaporator can be simulated by means of a linear model. The

comparison between the theoretical and experimental results was found to be

in close agreement. The results obtained between the heat load and the

refrigerant mass flow rate could be used to design a control algorithm in order

to obtain a complete autonomy system. Also, it will maintain the COP of the

system closer to the design values.

45

2.5 CONCLUSION

In this chapter, the works carried out by various researchers on

working fluids, theoretical studies and experimental investigations on the air-

cooled GAX based vapour absorption refrigeration systems are reviewed in

detail. The following conclusions are drawn from the literature review.

1. The performance of the heat recovery absorber cycle is 10%

higher than that of a conventional single effect ammonia-

water system.

2. The GAX cycle is an elegant way of improving the

performance of a conventional absorption system. The COP of

the GAX cycle can be enhanced upto 40% than that of the

conventional single effect system for the same operating

conditions.

3. The GAX cycle has the ability to operate over a wide

concentration difference with respect to the ammonia-water

mixture, which cannot be realized in the water-lithium

bromide system.

4. In addition to ammonia-water, a few other absorbents for

ammonia have been studied. Studies on the GAX cycles with

respect to the different absorbents for ammonia need further

investigations to exactly predict the best one.

5. A majority of the results that are available in the literature for

the air-cooled GAX absorption systems are theoretical

simulations, and the experimental findings are limited.

It is inferred from the literature review that a significant amount of

work has been done on cycle modifications, to improve the performance of

the system. In the present work, the conventional ammonia-water cycle is

modified by incorporating GAX arrangements both on the low and high

46

pressure side, condensate pre-cooler, solution cooler and an additional

solution heat exchanger to recover the energy from the exhaust gas. The GAX

arrangement at the high pressure side, namely, the high pressure GAX

(HPGAX), heats the weak solution by the refrigerant vapour from the

generator and thereby the refrigerant vapour is purified. This eliminates the

necessity of a separate reflux cooler / rectifier as in the case of a conventional

NH3-H2O system. The GAX arrangement at the low pressure side, namely the

low pressure GAX (LPGAX), reduces the absorber heat load by absorbing a

partial amount of refrigerant vapour, by splitting the refrigerant vapour from

the evaporator by a factor, termed as split factor (Z), which is defined as the

ratio of the mass flow rate of the refrigerant to the absorber to the total mass

flow rate of the refrigerant in the cycle. These two new concepts are

employed in the air-cooled GAX system and hence, it requires indepth

analysis.

It is also inferred from Table 2.1, that the research work on absorption

systems with the air-cooled concept is meagre. Due to the increasing energy

costs (Sieres et al 2008), a concern for the environment and the unavailability

of commercial products with the air-cooled concept, the present research work

is aimed at developing an air-cooled GAX based vapour absorption

refrigeration system, using the conventional working fluid ammonia-water,

suitable for cold storage and other industrial applications.