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CHAPTER 2

Literature Review

2.1 Introduction

A refrigeration system utilizes work supplied by an electric motor to transfer

heat from a space to be cooled to a high temperature sink (place to be heated). Low

temperature boiling fluids called refrigerants absorb thermal energy to get vaporized in

the evaporator causing a cooling effect in the region being cooled. While comparing

the advantages and disadvantages of various cooling systems, two most important

parameters i.e the operating temperature and the coefficient of performance are of

vital importance in these systems. These systems can be evaluated using energy and

exergy analyses which are based on first and second law of thermodynamics,

respectively and have been described in the previous chapter in detail.

An extensive review of the literature has been done on different refrigeration and

heat pump systems in present chapter. The main idea was to have possible future

direction of research. The literature review has been classified as under:

1. Vapor Absorption Refrigeration Systems.

2. Vapor Compression Refrigeration Systems.

3. Vapor Compression-Absorption Refrigeration Systems.

2.2 Vapor Absorption System

Vapor Absorption system is an attractive method for utilizing low grade energy directly

for cooling. This is an important advantage as against the conventional vapor

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compression system which operates on high grade energy. Another important feature

of these systems is that it does not use any moving component except a very small

liquid pump. Vapor absorption system consists of four basic components viz. an

evaporator, an absorber (located on low pressure side), a generator and a compressor

(located on high pressure side). A refrigerant flows from the condenser to the

evaporator, then via absorber to the generator and back to condenser, while the

absorbent passes from absorber to the generator and back to absorber. For maximum

efficiency, the pressure difference between the low pressure side and high pressure

side is maintained as small as possible. Although, the initial cost of these systems is at

present higher but their operating expenses are often appreciably lower, which can

further be reduced if efficient absorption and distillation can be achieved. Since, the

efficiency of these processes is determined largely by thermodynamic properties of the

refrigerant –absorbent combination, an extensive study of these properties is of utmost

importance in the development of an efficient absorption refrigeration cycle.

A large number of researchers have carried out research in the field of vapor

absorption refrigeration using different working pairs and the most common working

pairs are LiBr-H2O and NH3-H2O. Alizadeh et al [1] carried out theoretical study on

design and optimization of water – lithium bromide refrigeration cycle. They concluded

that for a given refrigerating capacity higher generator temperature causes high

cooling ratio with smaller heat exchange surface and low cost. There is a limiting factor

for water lithium bromide cycles because of the problem of crystallization. Anand and

Kumar [2] carried out availability analysis and calculation of irreversibility in system

components of single and double effect series flow water lithium bromide absorption

19

systems. The assumed parameters for computation of results were condenser and

absorber temperature equal to 87.8oC and 140.6oC for single effect and double effect

systems respectively.

Tyagi [3] carried out the detailed study on aqua-ammonia VAR system and

plotted the coefficient of performance, mass flow rates as a function of operating

parameters i.e. absorber, evaporator and generator temperatures. He showed that

COP and work done are the function of evaporator, absorber, and condenser and

generator temperature and also depends on the properties of binary solution. Ercan

and Gogus [4] showed the irreversibility’s in components of aqua-ammonia absorption

refrigeration system by second law analysis. They calculated the dimensionless

exergy loss of each component, exergetic coefficient of performance, coefficient of

performance and circulation ratio for different generator, absorber evaporator and

condenser temperature. They concluded that aqua-ammonia system needs a rectifier

for high ammonia concentrations but it will lead to additional exergy loss in the system.

They observed the highest exergy loss in evaporator followed by absorber. I was also

concluded that the dimensionalless total exergy loss depends on generator

temperature.

Oh et al [5] investigated a gas fired, air cooled LiBr/H20 double effect parallel

flow type absorption heat pump of 2TR being used as an air conditioner. They

investigated the performance of the absorption heat pump in the cooling mode through

cycle simulation. They obtained the system characteristics depending on the inlet

temperature of air to the absorber, the working solution concentration, the solution

distribution ratio of the mass of the solution into the first generator to he total mass of

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the solution from the absorber, and the leaving temperature differences of the heat

exchanging components. They concluded that there exists a critical value of the

solution distribution ratio that maximizes the cooling performance of the system.

Aphornratana and Eames [6] investigated single effect water lithium bromide system

using exergy analysis approach. It was shown that the irreversibility in generator was

highest followed by absorber and evaporator.

Bell et al [7] developed a LiBr-H20 experimental absorption cooling system

driven by heat generated by solar energy. The components of the system are housed

in evacuated glass cylinders to observe all the processes. They determined the

thermodynamic performance of the system by applying mass and energy balance for

all the components. Their work was based on the assumption that the working fluids

are in equilibrium and the temperature of the working fluid leaving the generator and

absorber is equal to the temperature of generator and absorber respectively. They

concluded that the COP of the system depends on generator temperature and there is

optimum value of generator temperature at which COP is maximum. They also

concluded that by operating the system at low condenser and absorber temperatures

a satisfactory COP is obtained at a generator temp. as low as 68oC. Horuz [8]

explained the fundamental vapour absorption refrigeration system and carried out

comparative study of such system based on ammonia-water and water lithium bromide

working pairs. The comparison of two systems is presented in respect of COP, cooling

capacity and maximum and minimum pressures. He concluded that VAR system

based on water-lithium bromide is better than ammonia-water. However, problem of

crystallization lies with water-lithium bromide system.

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Kumar et al [9] studied in detail the exergy variation in solar assisted absorption

system. They found that rise in first generator heat transfer, decreases the heat

transfer in second stage generator. The increase in generator-II temperature

decreases the exergy and energy transfer rates at the condenser. They concluded that

the availability at the devices varies with respect to quality of the device. Talbi and

Agnew [10] carried our exergy analysis on single effect absorption refrigeration cycle

with lithium bromide water as the working fluid pairs. They developed a computer

simulation model based on heat and mass balance, heat transfer equations and

thermodynamic properties. The cycle collects free energy from the exhaust of diesel

engine. They calculated the dimensionless total exergy loss and exergy loss of each

component. They found that the absorber has the highest exergy loss of 59.06%

followed by generator. They concluded that the absorption refrigeration cycle is

effective in demonstrating the advantages of exergy process which are other wise not

accounted in the heat balance method.

Lee and sheriff [11] carried out the second law analysis of a single effect water

lithium bromide absorption refrigeration system. The effect of heat source temperature

on COP and exergetic efficiency was evaluated. However, they did not analyzed effect

of variation in absorber and condenser temperatures and also the effectiveness of

solution heat exchanger was also not specified. Lee and sheriff [12] carried out the

second law analysis of single effect and various double effect lithium bromide water

absorption chillers for chilled water temperature of 7.22oC and cooling water

temperatures 29.4oC and 35oC and computed COP and exergetic efficiency. The

effect of heat source temperature on COP and exergetic efficiency was investigated. In

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this study, the effectiveness values of solution heat exchangers considered for

analysis has not been specified and their results are only valid for water cooled

systems.

Sozen [13] studied the effect of heat exchangers on the system performance in

an ammonia water absorption refrigeration system. Thermodynamic performance of

the system is analyzed and the irreversibility’s in the system components have been

determined for three different cases. The COP, ECOP, circulation ratio, and non

dimensional exergy loss of each component of the system is calculated. They

concluded that the evaporator, absorber, generator, mixture heat exchanger and

condenser show high non-dimensional exergy losses. They also concluded that using

refrigerant exchanger in addition to mixture heat exchanger does not increase the

system performance. Fernandez-Seara and Vazquez [14] studied the optimal

generator temperature in single stage ammonia – water absorption refrigeration

system. They studied the behavior of this temperature on thermal operating conditions

and system design parameters. They carried out study based on parametric analysis

by developing a computer program and based on the results designed a control

system. The control system developed maintains a constant temperature for the space

to be refrigerated and also control the optimal temperature in the system generator.

De Francisco et al [15] developed and tested the prototype of a 2kW capacity

water ammonia absorption system operating on solar energy for rural applications.

The system also suffered from leakages in different components and need further

improvements. They concluded that the efficiency of the system is very low. The new

and improved prototype has to be developed. Horuz and Callander [16] described the

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experimental investigation of the performance of a commercially available absorption

refrigeration system. The system is natural gas fired with a capacity of 10 kW. They

studied the response of the refrigeration system to variations in chilled water

temperature, chilled water level in evaporator drum; chilled water level flow rate and

variable heat input are presented. They concluded that lower the energy input, lower

will be the cooling effect.

De Lucas et al [17] studied the use of alternative absorbent used in absorption

refrigeration cycles to replace the existing absorbent. New absorbent used is a mixture

of lithium bromide and potassium formate in a 2:1 w/w. The performance of the system

is compared by developing a program. They concluded that less energy is required in

the generator and due to this; waste heat with a temperature of 328.15K is required.

The efficiency of the system is increased and the new absorbent is less corrosive and

less expensive to manufacture. Sencan et al [18] carried out the exergy analysis of a

single effect water lithium bromide absorption refrigeration system and calculated the

exergy losses in the system components. The effect of heat source temperature on

COP and exergetic efficiency was computed. They did not analyze the effect of

variation in absorber and condenser temperature. They concluded that the cooling and

heating COP of the system increases slightly when increasing the heat source

temperature but the exergetic efficiency of the system decreases when increasing the

heat source temperature for both cooling and heating applications.

Kilic and Kaynakli [19] investigated single effect and series flow double effect

vapour absorption systems using energy analysis approach. The effect of different

parameters such as generator temperature, absorber temperature, condenser

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temperature, solution circulation ratio and solution concentration etc had been

investigated by these researchers on COP. Their results revealed that COP of double

effect absorption refrigeration system is higher in comparison to single effect system.

Kilic and Kaynakli [20] used the first and second law of thermodynamics to analyze the

performance of a single stage water lithium bromide absorption refrigeration system by

varying some working parameters. They introduced a mathematical model based on

exergy method. They found that the performance of the ARS increases with increasing

generator and evaporator temperatures but decreases with increasing condenser and

absorber temperatures. They concluded that the highest exergy loss occurs in

generator regardless of operating conditions and therefore it is most important

component of the system.

Gong et al [21] presented the method of product exergy cost for scheme

selection optimization of cooling and heating source system of air conditioning system.

They developed the optimization algorithm which adopts an integrative; multiple

objective decision method with the analysis of the product exergy cost and concluded

that the method is scientific and reliable. Kaynakli and Yamankaradeniz [22] studied

the single effect VA system on the basis of entropy generation method. Kaynakli and

Yamankaradeniz [22] performed calculations for a 10kW cooling load system. The

evaporator and condenser temperature was taken as 4oC and 38oC respectively. The

generator temperature was taken as 90oC. Effectiveness of solution heat exchanger

was assumed as 0.5 and efficiency of pump was assumed equal to 0.9. They

concluded that entropy generation of the generator is an important fraction of the total

entropy generation in the system basically due to the temperature difference between

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the heat source and the working fluid and in order to decrease the total entropy

generation of the system, the generator should be developed

Morosuk and Tsatsaronis [23] used an absorption refrigeration machine to

represent splitting the exergy destruction into endogenous/exogenous and

unavoidable/avoidable parts and this is new development in the exergy analysis of

energy conversion systems. They concluded that advanced exergetic evaluation of an

ARM supplies useful additional information which is not provided by exergy analysis.

The avoidable exergy destruction identifies the potential for improving each system

component. Gomri and Hakimi [24] carried out exergy analysis of double effect lithium

bromide/water absorption refrigeration system. They showed that the performance of

the system increases with increasing LP generator temperature, but decreases with

increasing HP generator temperature. They concluded that the highest exergy loss

occurs in the absorber and in the HP generator and therefore the absorber and HP

generator is the most important component of the double effect refrigeration system.

Gomri [25] carried out the comparative study between single effect and double

effect absorption refrigeration systems. They developed the computer program based

on energy balances, thermodynamic properties to carry out thermodynamic analysis.

They concluded that for each condenser and evaporator temperature, there is an

optimum generator temperature where change in exergy of single effect and double

effect absorption refrigeration system is minimum. Their study showed that the COP of

double effect system is approximately twice the COP of single effect system but there

is marginal difference between the exergetic efficiency of the system. Kaushik and

Arora [26] presented the energy and exergy analysis of single effect and series flow

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double effect water–lithium bromide absorption system. They developed the

computational model for parametric investigation. Their analysis involves the effect of

generator, absorber and evaporator temperatures on the energetic and exergetic

performance. They concluded that the irreversibility is highest in the absorber in both

systems as compared to other systems. Zhu and Gu [27] used the first and second law

of thermodynamic to analyze the performance of ammonia–sodium thiocyanate

absorption system for cooling and heating applications. A mathematical model based

on exergy analysis was developed. The performance of the system is analyzed using

different operating conditions. They concluded that the cooling and heating COP

increases with increasing generator and evaporator temperatures but it decreases with

increasing condenser and absorber temperatures. Garousi Farshi et al [28] developed

a computational model to study and compare the effects of operating parameters on

crystallization phenomena in three classes of double effect lithium bromide–water

absorption refrigeration systems (series, parallel and reverse parallel) with identical

refrigeration capacities. They concluded that the range of operating conditions without

crystallization risks in the parallel and the reverse parallel configurations is wider than

those of the series flow system. Behrooz and Ziapour [29] carried out thermodynamic

analysis of a diffusion absorption refrigeration heat pipe (DARHP) cycle. A computer

code was developed for an ammonia–water DARHP cycle with helium as the auxiliary

inert gas using EES software. The second law efficiency was examined parametrically

by the computer simulation. They validated the model by comparison with previously

published experimental data for DARHP system. The cycle performance results under

different conditions indicated that the best performance was obtained for the

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concentration rich solution of 0.35 ammonia mass fraction and the concentration of

weak solution about 0.1. They concluded that the exergy losses in the evaporator,

condenser and dephlegmator were small. Also the second law efficiency increases

with increasing evaporator temperature; and decreases with increasing thermosyphon

temperature.

Khaliq et al [30] carried out first and second law investigation of waste heat

based combined power and ejector-absorption refrigeration cycle using R141b as a

working fluid. Estimates for irreversibilities of individual components of the cycle lead

to possible measures for performance improvement. Results show that around 53.6%

of the total input exergy is destroyed due to irreversibilities in the components, 22.7%

is available as a useful exergy output, and 23.7% is exhaust exergy lost to the

environment, whereas energy distribution shows 44% is exhaust energy and 19.7% is

useful energy output. They concluded that proposed cogeneration cycle yields much

better thermal and exergy efficiencies than the previously investigated cycles and the

current investigation clearly show that the second law analysis is quantitatively

visualizes losses within a cycle and gives clear trends for optimization.

2.3 Vapour Compression System

In vapor compression system there are four major components: evaporator,

compressor, condenser and expansion device. Power is supplied to the compressor

and heat is added to the system in the evaporator, whereas in the condenser heat

rejection occurs. Heat rejection and heat addition are dissimilar to different

refrigerants. A standard vapor compression cycle consists of four processes viz. a

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reversible adiabatic compression from the saturated vapor to the compressor pressure

followed by a reversible heat rejection at constant pressure causing de-superheating

and condensation. This is further extended to an irreversible expansion at constant

enthalpy from saturated liquid to evaporator pressure and there after a reversible heat

addition at constant pressure causing evaporation to saturated vapor.

Keshwani and Rastogi [31] determined the optimum interstage pressure in a

two stage VCR system for refrigerant CFC12. They concentrated their research on

minimization of overall compressor work. Arora and Dhar [32] used the discrete

maximum principle, discusses by Katz (1962), to solve the problem of optimum

interstage pressure allocation in multistage compression systems for R-12, with and

without intercooling between the stages. They concluded that the optimum interstage

pressure approximately equals the geometric means of the condensation and

evaporation pressure but when the flash inter cooler was incorporated, they found a

considerable difference between the geometric means and the optimal pressure

values. Prasad [33] determined the optimum interstage pressure in a two stage vapour

compression refrigeration system for the refrigerant R-12 with a view to maximize the

COP. They concluded that the inter-stage temperature of a two-stage refrigeration

cycle is given by the geometric mean of the condensation and evaporation

temperatures.

Kumar et al [34] explained a method of carrying out exergy analysis on a

vapour compression refrigeration system using R-11 and R-12 as refrigerants. They

presented the exergy-enthalpy diagrams to facilitate the analysis. They explained the

procedure to calculate various losses in different components of the system.

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McGovern and Harte [35] presented an exergy method for compressor analysis. This

is used to find and quantify defects in the use of compressor shaft power and will lead

to the improvement in design of the compressor. The exergy destruction and its

location are identified. They analyzed the refrigerant compressor using R-12 as

refrigerant. They presented the graphs of the instantaneous rates of exergy

destruction. They concluded that it is particularly suitable for applications in computer

simulation of compressors and provides a sound basis for design optimization.

Zubair and Khan [36] showed that the optimum interstage pressure for a two

stage refrigeration system can be approximated by the saturation pressure

corresponding to the arithmetic mean of the condensation and evaporation

temperatures. Zubair et al [37] found that optimum interstage pressure for refrigerant

R-134a for maximum COP of the system was close to the saturation pressure

corresponding to the arithmetic mean of the refrigerant condensation and evaporation

temperatures. They showed that the system irreversible losses are lowest at an

intermediate saturation temperature near to arithmetic mean of the condensation and

evaporation temperatures. Aprea et al [38] reported that vapour compression

refrigeration systems are widely used for cold storage and super market refrigeration.

The suitable working fluid for these applications is the refrigerant R-502 which is an

azeotropic mixture of refrigerants R-22 and R-115.

Aprea et al [39] experimentally evaluated the general characteristics and

system performances of substitutes for R-502 in a refrigeration plant. They examined

different refrigerants such as R-402A, R-402B, R-403B, R-408A, R-404A, R-407A and

FX-40. All the refrigerants showed performances very close to those of R-502 except

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R-403B whose COP was found to be about 8% lower than that of R-502. They

concluded that the above refrigerants can be used as substitutes to R-502. Doring et

al [40] carried out an experimental study of R-507 to measure thermodynamic data.

They presented the data in the form of mathematical correlations. Their theoretical

results show that the compressor discharge temperature for R-507 was approximately

8K below in comparison to R-402. Camporese et al [41] experimentally investigated

the mixtures such as HC290/HFC134a, HFC125/HC290/HFC134a,

FC125/HFC143a/HC290, HFC125/HFC143a/HCC270 and HFC32/HFC125/HFC143a

for their influence on the solubility of various lubricant oils by measuring critical

solubility temperatures. The experiments were conducted to compare refrigerating

capacity, COP, discharge temperature and mass flow rates. The mixtures selected for

new units were the mixture of HFC143a/HC290/R-22 showed the best performance

and its COP and cooling capacity were found to be higher in comparison to R-502

Nikolaidis and Probert [42] used exergy analysis to investigate the behaviour of

two stage compound compression cycle with flash intercooling using R-22 as

refrigerant by varying the condenser saturation temperature and evaporator saturation

temperature from 298 to 308 K and 238 to 228 K respectively. They determined the

effect of temperature change in condenser and evaporator on plants irreversibility rate.

They concluded that the changes in the temperatures of condenser and evaporator

significantly effect the plants overall irreversibility and therefore the system needs

optimization. Sami and Desjardins [43] carried out performance evaluation of R-407B,

R-507, R-408A and R-404A as substitutes to R-502. Their results revealed that R-

408A blend has a superior performance than R-502 but it is characterized by high

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discharge pressure. Ratts and Brown [44] used entropy generation minimization

method to compute the optimum reduced intermediate temperature for R-134a in a

two stage VCR system. They concluded that this method gives better results than

geometric means method for evaluation of interstage temperature charge pressure

compared to R-502. Rakehesh et al [45] carried out experimental study on a heat

pump with different refrigerant R-22, R-407C and R-407A. They concluded that R-

407C heat pump chiller systems offered higher exergy efficiency than those operating

with R-22 and in the case of R-407A systems; the exergy efficiency was higher than

that of HCFC at condensing temperatures less than 50oC.

Aprea et al [46] studied the performance of a VCR experimental plant both as

water chiller and as a heat pump using R-22 and its substitute R-417A. The use of

R-417A does not require lubricant change and equipment redesign. The results

showed that the COP and exergetic efficiency of the plant is higher for R-22 than

R-417A. Aprea and Renno [47] experimentally investigated the energetic and

exergetic performance of a VCR plant for cold storage application using both R-22 and

its substitute R-417A. The results showed that COP was 15% greater than R-417A

where as exergy destruction for R-417 was greater than R-22. Areaklioglu et al [48]

numerically calculated the rational efficiency and components based irreversibility

ratios of a cooling system based on the second law of thermodynamics using HFC and

HC based pure refrigerants such as HFC32, HFC125. HFC134a, HFC143a, HFC152a,

HC290, HC600a and there binary and turnery mixtures, along with CFC12, R-22 and

R-502. The effect of temperature glide, occurring at the condenser and evaporator, on

the rational efficiency of the cooling system was evaluated. The irreversibility in

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condenser was found to varying between 40-55% of the total irreversibility. The results

also suggest that for both binary and ternary mixtures the rational efficiency increases

against temperature glide.

Xuan and Chen [49] carried out an experimental study of a ternary near

azeotropic mixture of HFC161 as an alternative to R-502. Without any modification to

system components, experimental tests were performed on a vapour compression

refrigeration plant with a reciprocating compressor which was originally designed to

use R-404A, a major substitute for R-502. The experimental results under two different

rated working conditions indicated that the pressure ratios of this new refrigerant were

nearly equal to those of R-404A. Under lower evaporative temperature, its COP was

almost equal to that of R-404A and its discharge temperature was found to be slightly

higher than that of R-404A, while under higher evaporative temperature, its COP was

found was found to greater than that of R-404A and its discharge temperature was

lower than that of the latter. This new refrigerant achieved a high level of COP and

hence was considered as a promising retrofit refrigerant to R-502.

Park and Jung [50] studied two pure hydrocarbon refrigerants, R-1270

(Propylene) and R-290(Propane) and three binary mixturescompared to R-1270, R-

290 anf R-152a were tested in a refrigerating bench tester with a scroll compressor I

an attempt to substitute R-502. The results showed that all refrigerants tested had 9.6

to 18.7% higher capacity and 17.1 to 27.3% higher COP than R-502. There was

problem with mineral oil. They concluded that these alternative refrigerants offer better

system performance and reliability than R-502. They studied the thermodynamic

performance of two pure hydrocarbons and seven mixtures composed of propylene

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(R-1270), Propane (R-290), HFC152a and dimethyl ether (RE170,DME) in an attempt

to substitute R-22 in residential air conditioners. The mixtures are all near azeotropic

having the gliding temperature difference of less than 0.6oC. Test results revealed that

the COP of these mixtures was up to 5.7% higher than that of R-22. The compressor

discharge temperatures were reduced by 11-17oC with these fluids. There was no

problem found with mineral oil since the mixtures were mainly composed of

hydrocarbons. These fluids provide good thermodynamic performance with reasonable

energy saving.

Arora et al [51] carried out parametric investigation of actual vapour

compression refrigeration cycle in terms of COP, exergy destruction and exergy

efficiency for R-22, R-407C and R-410A by developing a computational model. The

results showed that COP and exergy efficiency for R-22 are higher in comparison to

R-407C and R-410A. It was concluded that R-410A is better alternate as compared to

R-407C with high coefficient of performance and low exergy destruction ratio when

considering for refrigeration applications. For air conditioning application R-407A is

better alternative than R-410A. Park et al [52] experimentally tested the

thermodynamic performance of R-433A, R-432A for possible replacement to R-22 in a

heat pump bench tester under air conditioning and heat pumping conditions. The test

results showed that the COP of R-433A was 4.9-7.6% higher than that of R-22 while

the capacity of R-433A was found to be 1.0-5.5% lower than that of R-22 under both

test conditions. The COP of R-432A was found to be 8.5-8.7% higher than HCFC and

its refrigerating capacity was 1.9-6.4% higher than that of R-22 under both test

conditions. The compressor discharge temperatures of R-432A and R-433A were

34

lower than that of R-22. The amount of charge required for both of these refrigerants

were 50-57% lower than that of R-22 due to their low density. They concluded that

these refrigerants are good long term environmental friendly alternatives to replace

R-22 in residential air conditioners and heat pumps due to their excellent

thermodynamic and environmental properties with minor adjustments. However, they

did not comment on the compatibility of these refrigerants with lubricating oil.

Palm [53] reported tat vapor pressure curves of the propane and propene are

quite similar to those of R-22 and ammonia, indicating that the application areas would

be same. Recently, air conditioning provided by ammonia refrigeration systems has

found application on college campuses and office parks, small scale building such as

convenience stores, and larger office buildings. These applications have been

achieved by using water chillers, ice thermal storage units and district cooling systems.

Pearson [54] specified that ammonia is widely used in industrial systems for food

refrigeration, cold storage, distribution ware housing and process cooling. It has more

recently been proposed for use in applications such as water chilling for air

conditioning systems. Bhattacharyya et al [55] carried out analysis of an

endoreversible two-stage cascade cycle and obtained an optimum intermediate

temperature for maximum exergy and refrigeration effect. They developed a

comprehensive numerical model of a transcritical CO2-C3H8 cascade system. The

cycle was optimized with the operating temperatures and the results obtained were in

line with the simulation results.

Arora and Kaushik [56] presented a detailed exergy analysis of an actual

vapour compression refrigeration cycle. They developed a computational model for

35

computing coefficient of performance, exergy destruction, exergetic efficiency and

efficiency defect for different refrigerants. They concluded that R-507A is a better

substitute to R-502 than R-404A and the efficiency defect in condenser is highest and

lowest in liquid vapour heat exchanger for the refrigerant considered. Mafi et al [57]

carried out exergy analysis for multistage cascade low temperature refrigeration

systems used in olefin plants. They developed the equations of exergy destruction and

exergetic efficiency for heat exchanger, compressors and expansion valves. The

relations for total exergy destruction in the system and the system overall exergetic

efficiency are obtained. They also developed the expression for minimum work

requirement for cascade low temperature refrigeration used in olefins plants. They

determined the overall exergetic efficiency to be 30.88%

Dopazo et al [58] reported the analysis of the parameters in design and

operation of a CO2/NH3 cascade cooling system and their effect on system COP and

exergetic efficiency. They carried out the analysis based on general mathematical

model which was validated using experimental results. They concluded that for

specific installation, the isentropic efficiency for each compressor in cascade system

should be determined as accurately as possible from the manufacturer or experimental

data in order to obtain reliable values for the optimum CO2 condensing temperature

and maximum COP. Miguel Padilla et al [59] did exergy analysis of the impact of direct

replacement of R-12 with zeotropic mixture R-413A on the performance of a domestic

vapour compression refrigeration system originally designed to work with R-12 using a

simulated analysis model. They concluded that the overall energy and exergy

performance of system working with R-413A is better than R-12.

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Qureshi and Zubair [60] investigated the performance degradation due to fouling in a

vapor compression cycle for various applications. The investigation was carried out

using refrigerants R-134a, R-410A and R-407C. The first law analysis indicates that

R-134a always performs better unless only the evaporator is being fouled. However,

the second law shows that R-134a performs the best in all cases. While considering

the second set of refrigerants i.e. R-717, R-404A and R-290. The first law shows that

R-717 always performs better unless only the evaporator is being fouled. In contrast to

this, from a second-law standpoint, the second-law efficiency indicates that R-717

performs the best in all cases. Volumetric efficiency of R-410A and R-717 remained

the highest under the respective conditions studied. Furthermore, performance

degradation of the evaporator often has a larger effect on compressor power

requirement while that of the condenser has an overall larger effect on the COP.

Sayyaadi and Nejatolahi [61] considered a cooling tower assisted vapor

compression refrigeration machine has for optimization with multiple criteria. Two

objective functions, the total exergy destruction of the system and the total product

cost of the system are considered as thermodynamic and economic criterion

respectively, have been considered simultaneously. They developed the

thermodynamic model based on energy and exergy analyses and an economic model

according to the Total Revenue Requirement (TRR) method. The exergetic and

economic results obtained for three optimized systems have been compared and

discussed. They concluded that the multi-objective design more acceptably satisfies

generalized engineering criteria than other two single-objective optimized designs.

37

Zhu and Jiang [62] developed a refrigeration cycle which combines a basic

vapor compression refrigeration cycle with an ejector cooling cycle. The ejector cooling

cycle is driven by the waste heat from the condenser in the vapor compression

refrigeration cycle. The additional cooling capacity from the ejector cycle is directly

input into the evaporator of the vapor compression refrigeration cycle. The governing

equations are derived based on energy and mass conservation in each component

including the compressor, ejector, generator, booster and heat exchangers. They

concluded that the COP is improved by 9.1% for R-22 system.

Qureshi and Zubair [63] investigated the performance characteristics due to use

of different refrigerant combinations in vapor compression cycles with dedicated

mechanical sub-cooling. For basic designs, R-134a used in both cycles produced the

best results in terms of COP, COP gain and relative compressor sizing. In retrofit

cases, considering the high sensitivity of COP to the relative size of heat exchangers

in the sub-cooler cycle and the low gain in COP obtained due to installation of a

dedicated sub-cooling cycle when R-717 is the main cycle refrigerant, it seems that

dedicated mechanical sub-cooling may be more suited to cycles using R-134a as the

main cycle refrigerant rather than R-717. With R-134a as the main cycle refrigerant, no

major difference was noted, by changing the sub-cooler cycle refrigerant, in the

degradation of the performance parameters such as COP and cooling capacity, due to

equal fouling of the heat exchangers.

A cascade system consists of two independently operated single-stage refrigeration

systems: a lower system that maintains a lower evaporating temperature and

produces a refrigeration effect and a higher system that operates at a higher

38

evaporating temperature. These two separate systems are connected by a cascade

condenser in which the heat released by the condenser in the lower system is

extracted by the evaporator in the higher system.

Wang et al. [64] examined the potential of a double-stage coupled heat pumps heating

system, whereby an air source heat pump was coupled to a water source heat pump.

Comparatively, they found that such a coupling process improved energy efficiency

ratio by 20% compared to a purely air source heat pump.

Satoru Okamoto [65] carried out the analysis of a heat pump system with a latent heat

storage utilizing sea water installed in an aquarium. In this study a heat pump installed

in an aquarium with latent heat storage potential using sea water is analyzed. This

installation is quite helpful in maintaining the indoor conditions at constant temperature

and humidity. In this study the comparison of the actual operating characteristics and

efficiency of sea water source heat pump is carried out with two assumed conventional

systems that are, an air source heat pump without ice storage and an oil-fired

absorption refrigeration system. The results indicate that cost of generation of heat

energy with sea water heat pump is significantly lower than that of air-source heat

pump and the oil-fired heat pump and. The actual operating costs of sea water heat

pump is 42% lesser than the air fired and oil fired heat pumps and the energy

consumption for the generation of heat is also 19% lesser for the sea water heat

pump. Also the emission of the harmful gases like CO2 is also lesser for sea water

heat pump when compared to air fired and oil fired heat pumps.

Zhen et al. [66] carried out a study on the use of the ocean energy or sea water as

heat source as well as sink for district cooling and heating in Dalin city of China. They

39

suggested that coastal areas are the best suitable location for the use of sea water

source heat pump technology for both cooling as well as heating. The government

helped in the commissioning of the plants both for heating as well as cooling with the

capacity of 68 MW and 76 MW respectively for this study. In this study the economic,

energy and environment impacts of the sea water source heat pump technology are

analyzed. In this study, the system is compared with coal fired heating system and the

conventional air-conditioning system in terms of the economic, energy and

environment impacts. The economic impacts include the analysis of money through

series of sensitivity cases which needs to be invested in different forms like annual

cost, net present value used for the calculation of coal price, electricity price and

interest rate of loans. The energy effects include the analysis of change in the sea

water temperature which has been done with the simulation study of sea water

temperature field with a two dimensional convection-diffusion equation. The results of

the study indicates that the sea water source heat pump can be used for both heating

as well cooling applications and has a great potential if used in other locations

depending on the geographical conditions and local environment.

Shu Haiwen et al. [67] carried out the study of the energy saving judgment of electric-

driven sea water source heat pump district heating system over boiler house district

heating system. They concluded that for renewable energy utilization system, the

electricity driven heat pumps are gaining popularity, but the energy saving condition is

still not clear. In this study, an expression of the critical COP of the heat pump system

for energy saving is derived through the comparison of the system and conventional

boiler house district heating system in the energy consumption aspects. Also, the

40

actual COP values of the heat pump unit are calculated by the experimental data

regression model based on the details from the supplier of the heat pump. The

comparison of the values of both COP’s critical as well as actual brings out a judgment

on the energy saving aspect of an electric driven sea water heat source pump for

district heating. The results also indicate that both the heating radius and the natural

conditions of sea water are the most important factors to determine the energy

efficiency of the system. The results indicate that selection of sea water area should

be done in a way that it should have the largest sea water temperature difference that

could be utilized thereby decreasing the water head of the sea water pump. The type

of fuel used in the boiler greatly influences the critical COP value of the heat pump.

2.4 Vapor Compression-Absorption System

Vapor Compression-absorption heat pump/refrigeration cycle represents a

cycle in which vapor is mechanically compressed, absorbed and then desorbed using

a liquid solution cycle. These systems may be considered as hybrid systems between

conventional vapor compression and vapor absorption systems. The hybrid vapor

compression/absorption heat pump cycle combines two well known heat pump

concepts, the compression heat pump and the absorption heat pump. It uses a mixture

of refrigerants as the working fluid, one as the absorbent and the other as the

desorbent. A key advantage of the hybrid heat pump is the extended range of

temperatures available for a mixture compared to pure refrigerants. This is the effect of

the reduced vapor pressure obtained for a refrigerant in a mixture with less volatile

component. Another advantage is the gliding temperature obtained in the absorber

41

and desorber. It reduces irreversibility during heat exchange process between working

fluids and results in improved system performance.

Pourreza-Djourshari and Radermacher [68] presented the performance

calculation of two vapour compression heat pump cycles, one with single stage

solution circuit and the other with two stage solution circuit. The working fluid chosen

was R-22-DEGDME. They found that both cycles show a significant increase in COP

as compared to R-22. The results indicate that there is potential of control capacity by

a ratio of 7:1, energy saving up to 50% and significant reduction in pressure ratio

compared to conventional R-22 cycle. Radermacher [69] examined the performance of

vapour compression heat pump cycle with desorber/absorber heat exchange working

on R-22-R-113 mixture using successive substitution method. The results showed an

improvement in the cooling COP by 57% and a reduction in pressure ratio by 69%

compared to a conventional R-22 cycle. Stroker and Trepp [70] presented the first

simulation model which includes the calculation of the overall heat transfer resistance.

The heat transfer resistance has been calculated as a function of the mass flow rate

for the working pair NH3-H2O from the experimental data. They presented design and

experimental results of a compression heat pump with solution circuit. The test plant

heats water from 40 to 70oC and cools water from 40 to 15oC. A COP of 4.3 was

measured and an energy saving of 23% was achieved.

George et al [71] studied the performance of compression-absorption heat

pump working on R-22-Dimethyl formamide (DMF) through thermodynamic analysis.

The heating COP, concentration difference and the circulation ratio are calculated by

varying compression ratio and operating temperatures at the absorber and desorber.

42

The assumptions taken are that the absorbent does not evaporate in the considered

temperature range to necessitate rectification; equilibrium conditions exist at the exit of

absorber and desorber; effectiveness of heat exchanger is 100%; isentropic

compression in compressor; isenthalpic expansion in pressure reducing valve and no

heat losses and pressure drops in various components etc. They concluded that at

certain operating conditions COP as high as 6 and temperature lift as high as 60oC

can be achieved. Amrane et al [72] developed two simulation models, one for vapour

compression cycle with single stage solution circuit and another for vapour

compression cycle with two stage solution circuit utilizing NH3-H2O mixture. The

analysis of heat exchangers has been carried out by using UA values as input to

program.

Herold et al [73] analyzed a hybrid refrigeration cycle which combines a

mechanical compressor and an absorption cycle using single evaporator. The analysis

involves using the output from internal combustion engine efficiently. LiBr-H2O is used

as working fluid and the cycle has been analyzed assuming oil free compressor.

Although, these types of compressors are available, they are rarely used due to their

high capital cost and low isentropic efficiency. High initial cost and low performance

results in poor economics for the hybrid cycles. Some other assumptions taken in the

analysis are no pressure drops in heat exchangers and pipes; all phases are in

thermodynamic equilibrium. In this analysis also, only internal behavior of the cycle

has been studied and external temperatures are not taken into account.

Ahlby et al [74] carried out optimization study on the compression-absorption

cycle operating on NH3-H2O mixture. The improvement in cycle performance which is

43

gained by optimizing the temperature gradient in the absorber is considerable

particularly for situations with small external temperature gradient. The assumptions

taken in the analysis are: saturated conditions at the desorber and absorber outlets;

adiabatic absorption and desorption in the first part of the absorber and desorber,

respectively, until equilibrium is reached; constant UA values for heat exchanger; no

pressure drops and heat losses. The optimum point of operation is found by studying

the changes in the compressor and pump and the heat loss obtained in the solution

heat exchanger with the working conditions. They concluded that for each external

situation, an optimum working condition can be found. The improvement in cycle

performance gained by optimizing the temperature gradient in the absorber is

considerable. A comparison of performance with the vapour compression cycle shows

that compression-absorption cycle is better or equally good.

Ahlby et al [75] studied the performance of compression-absorption heat pump

with the ternary working fluid NH3-H2O-LiBr. At 60% by mass salt concentration,

ternary mixture showed 10% better cycle performance than binary fluid NH3-H2O. The

calculations are uncertain since the properties of such mixtures are estimated from

properties for NH3-H2O and NH3-H2O-60%LiBr and have not been experimentally

validated. Results indicate that the best mixture would be a solution with a salt content

of about 40-50% by mass. Riffat and Shankland [76] described the integration of

different types of absorption systems and vapour compression system. They analyzed

the performance of such systems using various refrigerant/absorber pairs. Their study

is concerned with the intermittent absorption system, intermittent absorption/vapour

compression system and combined intermittent absorption/vapour compression

44

system. They concluded that integrated compression absorption systems could

provide higher COP than individual systems

Rane et al [77] compared the performances of four versions of two stage

vapour compression heat pump with solution circuits. This represents a cascade

system. They developed a computer simulation model based on heat and mass

balances of each component. For heat exchangers, calculations, UA values have been

taken as input parameter. Performance of the cycles has been compared and it has

been found that the cycle with bleed line and desuperheater has 40-50% higher COP

than the cycle with rectifier. Various parameters, viz., cooling COP, solution heat

exchanger effectiveness, pressure ratio, temperature glides in the absorber and

desorber, and low temperature desorber load have been studied as a function of weak

solution concentration. The results indicate that the above system can work at

temperature above 100oC and achieve a temperature lift of more than 100K.

Groll and Radernacher [78] presented the simulation model for the vapour

compression cycle with single stage solution circuit and cycle with desorber/absorber

heat exchange. The working pair used was CO2-acetone and R-23-DEGDME. It has

been found that the mixtures CO2-acetone and R-23-DEGDME are not suitable for

higher absorber temperatures in heat pump applications due to low COP and high

absolute pressures compared to that for NH3-H2O mixture. Contrary to this, high COP

and low absolute pressures were obtained with CO2-acetone and R-23-DEGDME

compared to NH3-H2O mixture for low desorber temperature in refrigeration

applications. It has also been found that for temperature lifts below 70K, the vapour

compression cycle with single solution circuit is better compared to the cycle with

45

desorber/absorber heat exchange because the former gives more COP and capacity

at lower pressure ratio but for temperature lifts above 70K, the cycle with

desorber/absorber heat exchange was found better than the cycle with single stage

solution circuit.

Itard and Machielsen [79] surveyed the problems encountered when modeling

compression/resorption heat pumps. Their design showed that LMTD method cannot

be used for modeling of heat exchanger and for COP calculations when working with

large temperature glides. They concluded that a mixture can be more advantageous

than a pure refrigerant. They concluded that for certain external conditions, an

optimum overall concentration exists which is the determining factor for the COP of the

system. Ayala et al [80] carried out simulation study of ammonia/lithium nitrate

absorption/compression refrigeration cycle. They modeled a cycle over a range of

proportions from 0 to 100% mechanical vapour compression. They considered

different power generation and distribution efficiencies in deriving the primary energy

ratio. The main consideration for the hybrid model was the assumption that the heat

losses were zero, except in the generator, the pump work was not considered. They

concluded that it is possible to achieve up to a 10% increase in overall efficiency using

combined absorption/compression refrigeration systems.

Groll [81] presented the simulation results of vapour compression cycles with

solution circuit for the working pair carbon dioxide and acetone. The two cycles

investigated are vapour compression cycles with single stage solution circuit and

vapour compression cycles with desorber/absorber heat exchange and parameters

studied are circulation ratio, mass concentration, temperature level in desorber and

46

temperature lift between heat source and sink. Itard [82] carried out experimental and

simulation work on wet compression cycle and found that wet compression cycle is

better cycle because it gives better COP that solution recirculation cycle. Tarique and

Siddiqui [83] compared the performances and economic analysis of the combined

absorption/compression cycle using NH3-NaSCN solution and pure ammonia in the

compression cycle under various operating conditions. They concluded that the capital

and running costs are highly reduced while working with NH3-NaSCN as compared to

the cycle with pure ammonia.

Arun et al [84] carried out analysis of a single stage compression-absorption

heat pump using R-134a-dimethyl acetamide. Effect of variation in suction and

discharge pressures and generator and absorber temperatures on circulation ratio,

discharge temperature and heating COP have been studied. Results of the analysis

show that at low pressure ratios and high temperature lifts, compression-absorption

heat pump exhibits better performance than compression heat pump. For compression

absorption system the discharge temperature remains constant with the increase in

heat delivery temperature for a given pressure ratio and solution concentration

whereas for vapour compression system discharge temperature varies almost linearly

and increases sharply as the great delivery temperature reaches the critical

temperature of the refrigerant.

Swinney et al [85] investigated away of manipulating the composition change of

a refrigerant mixture using two components of similar volatility in order to reduce the

compression ratio. They examined the use of composition change with a mixed

refrigerant to achieve a temperature lift. They also examined the possibility of

47

integrating the column into a closed cycle along with implication of this in energy

requirements. They concluded that the performance is shown to be comparable to

conventional absorption refrigeration units. The cycle is able to use heat sources

below 100oC as input to the distillation column. Satapathy et al [86] carried out

thermodynamic analysis of a compression-absorption heat pump working on R-22-

DMETEG for simultaneous heating and cooling. It is found that by operating the

system at slightly higher discharge pressures than normal, excellent performance is

achieved despite the required large temperature lift.

Fernandez-Seara et al [87] investigated a compression absorption cascade

refrigeration system. The results were computed for refrigerants carbon-dioxide and

ammonia in the compression stage and ammonia water in absorption stage. It is

shown that the intermediate temperature level is an important design parameter that

causes an opposite effect on the COP of the compression and absorption systems.

Infante Ferreira et al [88] investigated the use of twin screw oil free compressor

operating under wet compression conditions in an ammonia–water compression

absorption heat pump cycle. The compressor performance is assessed with respect to

the influence of the location of liquid intake, injection angle and mass flow rate of the

injected liquid on compression performance. Labyrinth seals are used to separate the

oil free side from the lubricated side. They concluded that there is significant impact of

the liquid injection location and due to this the isentropic efficiency increases from 5 to

50% in the model and 10 to 35% in the experiment. The labyrinth leakage flow is

substantial and has a very large impact on the compression performance.

48

Kairouani and Nahdi [89] developed a novel combined refrigeration system and

discuss the thermodynamic analysis of the cycle. The possibility of using geothermal

energy for hybrid system is studied. They selected three working fluids R-717, R-22

and R-134a for the conventional and ammonia–water Pair for the absorption system.

The geothermal temperature source is in the range of 343-349K and the results show

that the COP of a combined system is significantly higher than that of a single stage

refrigeration system. The system presents an opportunity to reduce the continuously

increasing electrical energy consumption. Satapathy et al [90] carried out a

comparative thermodynamic investigation on R-22-E181 and R-134a-E181 working

pairs for vapour compression-absorption system for cooling and heating applications.

Results show that R-134a-181 working pair gives better performance at lower solution

concentration and lower system capacity. At higher solution concentration and higher

system capacity R-22-E181 is slightly better than R-134a-E181. They concluded that

since R-22 can be used for some more years, it may be considered as it gives higher

volumetric capacities. They also concluded that the actual performance is poor due to

use of non-optimized components.

Garimella et al [91] analyzed a novel cascaded absorption/vapor-compression cycle

with a high temperature lift for a naval ship application. A single-effect LiBr–H2O

absorption cycle and a subcritical CO2 vapor-compression cycle were coupled together

to provide low-temperature refrigerant −40°C for high heat flux electronics applications,

medium-temperature refrigerant 5°C for space conditioning and other low heat flux

applications, and as an auxiliary benefit, provide medium-temperature heat rejection

∼48°C for water heating applications. They developed a thermodynamic model to

49

analyze the performance of the cascaded system, and parametric analyses were

conducted to estimate the performance of the system over a range of operating

conditions. The performance of the cascaded system was also compared with an

equivalent two-stage vapor-compression cycle. This cycle was found to exhibit very

high COPs over a wide range of operating conditions and when compared to an

equivalent vapor-compression system, was found to avoid up to 31% electricity

demand. Yari et al [92] studied and compared the GAX and GAX hybrid absorption

refrigeration cycles from the viewpoint of both first and second law of thermodynamics.

They performed the exergy analyses in order to calculate the total exergy destruction

rate within the cycles and also reveal the contribution of different components to the

destructions. They concluded that in both cycles the generator temperature (Tgen) has

more influence on the second law efficiency whereas, the coefficient of performance

(COP) of the cycles are comparatively less affected by this temperature. An increase

of about 75% in the second law efficiency of the GAX cycle was found as the

generator temperature was varied from 400 to 440 K. With this variation of the

generator temperature, the increase in the corresponding COP was around 5%. In

addition, compared to that in the GAX cycle, the maximum value of exergetic efficiency

in the GAX hybrid cycle occurs at a slightly higher value of Tgen.

Zheng and Meng [93] studied the thermodynamic mechanism of the hybrid

refrigeration cycle. They proposed the two fundamental concepts, which are the

ultimate refrigerating temperature (or the ultimate temperature lift) and the behavior

turning. They investigated the impact of compressor pressure increasing on the cycle

performance. The key-parameters include the concentration difference, the circulation

50

ratio of working fluid, etc. They showed that the refrigeration cycle performance varies

with the change of compressor outlet pressure and depends on which one achieves

dominance in the hybrid refrigeration cycle, the absorption sub-system or the

compression sub-system.

2.5 Conclusions of Literature Review

A comprehensive review of the literature on Vapour Absorption Systems,

Compression-Absorption System and Vapour Compression System has been carried

out on various aspects of energy analysis, the type of cycles analyzed, working pairs

used and exergy analysis. With regards to vapour absorption cycles, it is found that

mostly the studies are carried out on large capacity systems and the investigation had

been carried out with in a limited range of system design parameters. The literature on

small vapour absorption systems is scant and very few studies have been done on

smaller systems. The above studies are simulation studies.

Regarding compression-absorption systems studies have been carried out by

many researchers mostly analytically and experimentally. The investigations have

been done on wet compression cycles which eliminated the need of solution pump.

The literature provides details with regard to the applications of this cycle. However the

literature on exergy analysis of such systems is scant. In CA systems, refrigerant –

absorbent mixtures are used as working fluids which provide temperature gradient

profiles in the absorber and desorber. Literature reveals that NH3-H2O is the most

suitable working fluid due to its high latent heat and excellent heat and mass transfer

properties.

51

Literature review revealed that thermodynamic optimization on compressor-

absorption system was carried out to find optimum working condition for a given

external condition. The temperature gradient in the absorber is optimized. The

literature reveals that cost optimization of the system is essential to minimize the cost

as this system is more capital intensive than the conventional VC and VA system. With

regard to vapour compression systems, literature review revealed that natural

refrigerants such as ammonia, propane, propylene are halogen free and are safe for

the environment. Many researchers have carried out theoretical and experimental

investigations on alternative refrigerants. Much is talked about the replacement of

R-22 but proper substitutes are still to be found out. The literature on exergy analysis

of vapour compression refrigeration system is available but the exergy analysis of

such system with variable refrigerant charge is not reported.

In view of the increase in the cost of our existing resources, the advantage of

minimizing losses in the use of this energy is very important and essential. Exergy

analysis is a prime area for effective improvement of the systems. In the present work

energy and exergy analysis of the refrigeration and heat pump systems is done in

order to improve the system thermodynamically. The main aim of the study is to locate

for components in the system for maximum irreversibility and to find ways to improve

the system. The overall objective is to accomplish the thermodynamic analysis of the

refrigeration and heat pump systems and study their thermodynamic viability.

52

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