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CHAPTER - 3

Design and Evaluation of a 3 TR VAR System

and Exergy Analysis of a Typical LiBr/H2O VAR

System

3.1 Introduction

The continuous increase in the cost and demand for energy has led to more

research and development to utilize available energy resources efficiently by

minimizing waste energy. It is important to note that system performance can be

enhanced by reducing the irreversible losses in the system by using the principles of

the second law of thermodynamics. A better understanding of the second law of

thermodynamics has revealed that entropy generation minimization is an important

technique in achieving optimal system configurations and/or better operating

conditions. Some researchers [1, 2] have used the principles of entropy generation

minimization to analyze different systems to improve the systems performance. The

absorption refrigeration system (ARS) is becoming more important because it can

produce higher cooling capacity than vapor compression systems, and it can be

powered by other sources of energy (like waste heat from gas and steam turbines,

sun, geothermal, biomass) other than electricity. Furthermore, an ARS does not

deplete the ozone layer and hence, it poses no danger to the environment. Theoretical

and experimental works on the performance characteristics and thermodynamic

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analysis of ARSs are available in the literature. The absorption cycle uses a heat-

driven concentration difference to move refrigerant vapors (ammonia) from the

evaporator to the condenser. The high concentration side of the cycle absorbs

refrigerant vapors (which, of course dilutes that material). Heat is then used to drive off

these refrigerant vapors thereby increasing the concentration again. Water is the most

common absorbent used in commercial cooling equipment, with ammonia used as the

refrigerant. Smaller absorption chillers sometimes use water as the absorbent and

ammonia as the refrigerant. Starting with the evaporator, ammonia is evaporating off

the chilled water tubes, thereby, bringing the temperature down being returned from

the air handlers to the required chilled ammonia supply temperature. This ammonia

vapor is absorbed by the concentrated water solution due to its hygroscopic

characteristics. The solution is then pumped to the concentrator at a higher pressure

where heat is applied to drive off the ammonia and thereby re-concentrate the water.

The ammonia driven off by the heat input step is then condensed, collected, and then

flashed to the required low temperature to complete the cycle. Since ammonia is

moving the heat from the evaporator to the condenser, it serves as the refrigerant in

this cycle.

In the recent years, the interest in absorption refrigeration system is growing

because these systems have environmental friendly refrigerant and absorbent pairs.

Due to this fact, wide spread efforts are currently underway to utilize available energy

resources efficiently by minimizing energy consumption, besides developing

alternative for refrigerants which are environmental friendly and do not harm the ozone

layer. The absorption refrigeration cycle is similar to that of a vapor compression cycle

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which employs a volatile refrigerant. Usually, ammonia or water which alternately

vaporizes under low pressure in the evaporator by absorbing latent heat from the

material being cooled and condenses under high pressure in the condenser by

surrendering the latent heat to the condensing medium. In the absorption cycle, vapor

compressor employed in the vapor compression cycle is replaced by an absorber and

generator. Also the energy input required by the vapor compression cycle is supplied

by the mechanical work of the compressor while the energy input in the absorption

cycle is in the form of heat supplied directly to the generator. The source of heat

supplied to the generator is usually low grade energy such as waste heat, Renewable

energy etc.

As the ammonia-water and LiBr-H2O mixtures are environmental friendly, are

the most commonly used for refrigeration purposes in absorption systems, and despite

of the new mixtures under investigation, the ammonia-water mixture is the only one

which has clear future [3]. The principle of absorption is providing the necessary

pressure difference between the vaporizing and condensing processes, which

alternately condenses under high pressure in the condenser by rejecting heat to the

environment and vaporizes under low pressure in the evaporator by absorbing heat

from the medium being cooled. Ammonia water absorption chillers have been widely

used for different occasions [4]. Amount of work associated with theoretical and

experimental analysis of the commercial absorption chillers, using ammonia water as

working fluid is available in the literature [5, 6]. However, most of the research is

carried out with commercially fashioned chillers that have been specially designed as

an air cooled system.

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To investigate the system characteristics being consistent with the designed

values, an experimental system is designed and fabricated, using mass and energy

balance equations. This study deals with the analysis of an AAR system with options

to operate with a low-temperature heat source. Application of absorption refrigeration

devices has a large potential for decreasing the consumption of primary energy

sources and for reducing environmental pollution. In the last few years, the quest for

new cycle models for these devices has become one of the important subjects in the

investigation of absorption refrigerators. Aqua-ammonia absorption refrigeration (AAR)

system is preferred to classical vapor compression when a heat source is available

cheaply.

3.2 Aqua-ammonia absorption refrigeration cycle

The basic cooling cycle is the same for the absorption and electric chillers. Both

systems use a low-temperature liquid refrigerant that absorbs heat from the water to

be cooled and converts to a vapor phase (in the evaporator section). The refrigerant

vapors are then compressed to a higher pressure (by a compressor or a generator),

converted back into a liquid by rejecting heat to the external surroundings (in the

condenser section), and then expanded to a low-pressure mixture of liquid and vapor

(in the expander section) that goes back to the evaporator section and the cycle is

repeated.

The basic difference between the electric chillers and absorption chillers is that an

electric chiller uses an electric motor for operating a compressor used for raising the

pressure of refrigerant vapors and an absorption chiller uses heat for compressing

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refrigerant vapors to a high-pressure. The rejected heat from the power-generation

equipment (e.g. turbines, microturbines, and engines) may be used with an absorption

chiller to provide the cooling in a combined heat and power (CHP) system. The basic

absorption cycle employs two fluids, the refrigeratnt and the absorbent. The most

commonly fluids are ammonia as the refrigerant and water as the absorbent. These

fluids are separated and recombined in the absorption cycle. In the absorption cycle

the low-pressure refrigerant vapor is absorbed into the absorbent releasing a large

amount of heat. The liquid refrigerant/absorbent solution is pumped to a high-operating

pressure generator using significantly less electricity than that for compressing the

refrigerant for an electric chiller. Heat is added at the high-pressure generator from the

heating element. The added heat causes the refrigerant to leave the absorbent and

vaporize. The vapors flow to a condenser, where heat is rejected and condense to a

high-pressure liquid. The liquid is then throttled though an expansion valve to the lower

pressure in the evaporator where it evaporates by absorbing heat and provides useful

cooling. The remaining liquid absorbent, in the generator passes through a valve,

where its pressure is reduced, and then is recombined with the low-pressure

refrigerant vapors returning from the evaporator, so the cycle gets completed and can

be repeated.

A simple ammonia–water VAR system was designed and fabricated to study

the performance of this system. The cycle consists of four main components, namely

the condenser, evaporator, absorber and generator. A schematic diagram and a

photograph of the experimental apparatus are shown in Figs. (3.1-3.3).The mixture of

refrigerant-absorbent (NH3-H2O) is heated in the generator by an electric heater, which

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is equipped with single electric resistance of 1.5 kW capacity. The heater can be

controlled manually by on and off switch, the pressure gauge is installed on the

generator outlet and the discharge pressure under normal condition is 10.7bar. The

ammonia vapor along with fraction of water vapor enters an air cooled condenser

where it rejects heat and is condensed to a liquid before expanding in the expansion

device. The low pressure and low temperature refrigerant enters the evaporator.

During the cooling process, the liquid ammonia vaporizes and the transport fluid

(water) absorbs the vapor to form a strong ammonia solution in the absorber. After

leaving the evaporator, the refrigerant vapor enters the absorber where the

temperature rises due to mixing of refrigerant vapors with weak solution coming from

the generator through pressure regulating device. The mixture of vapor and weak

solution cools down by air or other cooling medium and converts it into liquid (strong

solution). The strong solution is pumped to the generator by solution pump and the

cycle is completed. Water and ammonia properties are obtained from standard

properties of pure substances table in the ASHRAE [7].

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Fig.-3.1 La

Fig.-3.

1-Condenser, 2-Evapo

71

out of a basic 3 TR Ammonia-water

: Front view of the vapor absorption

rator, 3-Pump, 4-Generator, 5-Expansio

AR system

ystem

Valve, 6-Absorber

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Fig.-3.3: Side view of vapor absorption system

6- Absorber, 7- Pressure reducing valve, 8- Fan.

3.3 Design and Experimental Analysis of a 3 TR VAR System

The thermodynamic analysis of the vapor absorption cycle is based on the

following three equations which can be applied to any part of the system:

Principle of Mass balance

∑ = 0m (3.1)

Principle of Material balance

∑ = 0mX (3.2)

Principle of Energy balance

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e The work input for the pump is negligible relative to the heat input in the generator.

Therefore, the pump work is neglected for the purpose of analysis.

3.3.2 Coefficient of Performance

The general refrigeration system can be considered as a perfectly reversible system,

the net refrigerating effect is the heat absorbed by the refrigerant in the evaporator.

The theoretical COP is given by:

g

e

QQ

COP= (3.4)

In the absorption refrigeration system, the total energy supplied to the system is the

total heat supplied in the generator and work done by the pump. The actual COP of

the ammonia-water absorption chiller is calculated from

P

e

W Qg

Q

COP +

=

(3.5)

The design of 3 TR basic vapor absorption refrigeration system is based on

parameters given in Table-3.1 and the temperature, pressure and specific enthalpy of

mixture at different state points are tabulated in Table–3.2. The calculated mass flow

rate of the refrigerant has been found to be 0.10341 kg/sec. Using mass balance

equations the mass flow rate of solution has been found to be 0.155 kg/sec. The

detailed calculations are shown in Appendix -I. Based on the cooling capacity of 3TR,

the area of the evaporator has been found to be 0.9423 m2. The heat rejected by the

condenser and the area of the condenser has been found to be 9.66 kW and 0.54 m2

respectively. The heat supplied to the generator and the collector area required to

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provide same quantity has been found to be 50.72 kW and 95.24 m2 respectively. This

is based on the insolation of 600 W/m2 and collector efficiency of 30%.

Table – 3.1: Design Parameters for Aqua Ammonia VAR system

S. No. Parameters Values

1 Capacity of system 3TR

2 Concentration of NH3 in refrigerant, Xr 0.98

3 Concentration of NH3 in solution, Xs 0.42

4 Concentration of NH3 in absorbent, Xw 0.38

5 Temperature of the Evaporator, TE 2oC

6 Generator or condenser pressure, PH 10.7 bar

7 Evaporator pressure, PL 4.7 bar

8 Temperature of the Condenser, TC 54

o

C

9 Temperature of the Absorber, TA 52oC

10 Temperature of the Generator, TG 120oC

The heat rejected from the absorber and the COP of the system has been found to be

49.5 kW is 0.2079 respectively. The detailed calculation has been shown in the

appendix. Based on the above parameters the system was fabricated and

experimental analysis on the actual system was carried out by actually measuring the

temperature at various state points and calculation of mass flow rate is done using

mass balance equations.

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Table – 3.2: Temperature, Pressure & Specific Enthalpy of mixture at different

state points

State

Points

Temperature

(oC)

Pressure

(bars)

Specific Enthalpy

(kJ/Kg)

1 54 10.7 1135

2 54 10.7 200

3 2 4.7 200

4 2 4.7 1220

5 52 4.7 0

6 52 10.7 0

7 120 10.7 255

8 120 4.7 255

3.4 Discussion of Results for the 3TR VAR System

The experiment is performed on the 3TR ammonia-water vapor absorption

refrigeration system and various sets of observations were recorded. These set of

observations include the temperature readings at the various locations of the

ammonia-water VAR system. The temperature readings include the generator

temperature, condenser temperature, evaporator temperature, temperature of working

fluid at pump inlet and outlet and absorber temperature. The temperatures were

measured and recorded using an infrared thermometer (non contact type) and are

shown in Table – 3.3.

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From Table–3.3, the average evaporator temperature is 9oC and the average

condenser temperature is 35oC. Corresponding to these values of Te and Tc, the

saturated pressures are Pg=13.5 bar and Pe = 6 bar. Based on the assumption that the

concentration of refrigerant solution (Xr) =0.98, concentration of weak solution (Xw)

=0.38, conc. of strong solution (Xs) =0.42, the COP for 3 TR ammonia water VAR

system = 0.599. Contrary to this, the design pressures were Pg =10.7 bar and Pe = 4.7

bar and the design values for temperature were Tc = 54˚C and Te = 2˚C based on the

design values the COP was found to be 0.2079.

In the present experimental systems, the heat source to the generator is

provided externally using an electric heating element of 1.5 kW. Table-3.4 shows the

pressure and specific enthalpy values obtained using enthalpy concentration

diagrams. Using the data in Tables 3.3 and 3.4 and applying the mass and energy

balance equations, the COP is found to be 0.599, which is greater than the designed

value and this is due to the fact that both the condenser and evaporator temperatures

and pressures are higher as compared to the designed values i.e Te = 9˚C, Tc = 35˚C,

Pe = 6 bar and Pg = 13.5 bar. The calculated cooling capacity increases almost linearly

with the generator heat input. When the heat input is lower, the system is not able to

produce any cooling capacity. When the heat input increases beyond the minimum

value, it can produce cooling capacity since there is enough liquid ammonia throttled

and entered the evaporator. Further increase in heat input produces a higher cooling

capacity as more pure refrigerant vapor is generated and this causes COP to increase.

When the heat input continues to increase, the cooling capacity also increases but the

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rate will depend on the temperature limitations and also on need of rectification which

is absent in this system.

Table – 3.3: Actual Temperature readings at different state points

S. No. Tgen.

˚C

Tcond.in

˚C

Tcond.out

˚C

Tevap. ˚C

Tevap.to

abs. ˚C

Tweak soln.

to abs.

˚C

Tpump

in

˚C

Tpump out

˚C

1. 38.4 34.3 32.5 8.2 45.7 48.5 48.6 49.5

2. 38.6 34.4 34.1 8.6 52.0 55.4 56.7 57.2

3. 42.1 35.3 34.6 8.8 56.6 61.0 60.9 61.2

4. 45.6 35.0 34.2 8.9 63.7 66.0 65.1 66.2

5. 49.2 35.4 34.4 8.9 63.2 69.2 68.0 68.2

6. 49.9 35.4 34.6 10.0 66.4 70.7 70.5 70.8

7. 51.2 34.8 34.6 10.2 67.5 74.0 72.5 73.6

8. 53.5 34.8 34.7 9.2 71.0 76.0 75.0 75.1

9. 56.9 35.8 35.5 9.3 73.6 78.5 78.0 78.6

10. 56.8 36.2 35.9 9.5 75.0 76.2 78.8 79.2

The above system can be modified so that it can be used on solar collector based

heating sources to make it fully renewable with little electrical consumption of solution

pump. The increase in heat input produces a higher cooling capacity as more pure

refrigerant is generated this also enhances the COP. The rise in generator

temperature above the designed value is due to the fact that automatic temperature

controller is not used to maintain the temperature in the generator.

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Table – 3.4: Measure of properties of mixture at different state points

State

points

Pressure

(bar)

Specific enthalpy

(kJ/Kg)

1. 13.5 1407.00

2. 13.5 148.15

3. 6.0 148.15

4. 6.0 1277.8

5. 6.0 37.03

6. 6.0 74.07

3.5 Exergy Analysis of LiBr/H2O Absorption System

In recent years, there has been growing interest in the use of the principles of

2nd law of thermodynamics for analyzing and evaluating the thermodynamic

performance of thermal systems as well as their technologies [8]. 2nd law analysis is

based on the concept of exergy, which can be defined as a measure of work potential

or quality of different forms of energy relative to environmental conditions. A Large

number of researchers have used 2nd law analysis for thermodynamic optimization of

refrigeration plants based on the theoretical analysis given by Bejan et al. [9].

Szargut [10] presented energy and exergy balance of an NH3-H2O absorption

refrigerator. Kotas [11] has used exergy analysis method in the analysis of thermal

systems. Talbi and Agnew [12] carried out exergy analysis of an absorption LiBr-

H2O.refrigeration cycle. Numerical results for the cycle were tabulated. A design

procedure was applied to a lithium-bromide absorption cycle and an optimization

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procedure that consists of determining the enthalpy, entropy, temperature, mass flow

rate and heat rate in each component, and coefficient of performance was calculated.

Horuz and Callander [13] did an experimental investigation of the performance of a

commercially available Vapour Absorption System. The cooling capacity of the plant

was 10 kW and used aqua-ammonia solution as the refrigerant. The response of

system to variations in chilled water inlet temperature, chilled water level in evaporator

drum, chilled water flow rate and variable heat input were presented. Asdrubali and

Grignaffini [14] obtained an experimental evaluation of a plant aimed at stimulating and

verifying performances of single stage H2O-LiBr absorption machine. Antonio et al [15]

studied the employment of an alternative absorbent used in absorption refrigeration

cycles to replace the absorbent currently employed in this kind of engines i.e., lithium

bromide. The alternative system consists of absorbent (LiBr:CHO2K=2:1by mass ratio)

and refrigerant (H2O). Lee and Sherif [16] gave the 1st and 2nd law analysis of

absorption system for cooling and heating applications. Xu et al [17] presented an

advanced energy storage system using aqueous lithium bromide as working fluid. The

working principle and flow of the variable mass energy transformation and storage

system were introduced and the system dynamic models were developed.

Tozer and James [18] derived the thermodynamic absorption cycle performance and

temperature formulae. Ideal absorption cycle was demonstrated as the combination of

a Carnot driving cycle with a Reverse Carnot cooling cycle. Performance and

temperature relations of double, triple and multistage cycles were derived. Validation

of the fundamental thermodynamics of absorption cycles was presented by applying

an exergy analysis. Tozer et al [19] described the use of the T-S diagram of water

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extended with additional curves to represent real and ideal water/LiBr absorption

cycles. An explanation was provided on several methods available, including details of

the thermodynamic justification of the method used, to construct the extended

diagrams. Extended T-S diagram was provided with the representation of a real single-

effect water/LiBr absorption refrigeration cycle. Nikolaidis and Probert [20],

investigated the behavior of 2-stage compound compression cycle with flash inter-

cooling, using refrigerant R-22, by exergy method. The effects of temperature changes

in condenser and evaporator, on the plant’s irreversibility were determined. This paper

carries the exergy and energy analysis of 496 TR absorption cooling system using

LiBr-H2O as working fluids. Exergy analysis is done to look for losses with in the

systems. The coefficient of performance (COP) under different operating conditions for

cooling applications are determined and shown graphically.

3.6 System Description (Double Effect LiBr-H2O VAR System)

The system to be analyzed is a 496 TR vapour Absorption system and uses

saturated steam as heat source, water as refrigerant, lithium bromide as absorbent,

produces the chilled water under vacuum conditions for the purpose of air conditioning

and technology process. The chiller consists of following main parts: High pressure

generator (HP generator),Low pressure generator(LP generator),condenser

,evaporator, absorber, high temperature heat exchanger, low temperature heat

exchanger, and condensate heat exchanger, auxillary generator for high pressure

generator, and such auxillary parts such as purging unit, de-crystallisation piping and

hermetically sealed pumps(solution pump and refrigerant pump).

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A double-effect chiller is very similar to the single-effect chiller, except that it

contains an additional generator. In a single-effect absorption chiller, the heat released

during the chemical process of absorbing refrigerant vapor into the liquid stream, rich

in absorbent, is rejected to the cooling water. The main objective of a higher effect

cycle is to increase system performance when high temperature heat source is

available. As shown in Fig.-3.4, high temperature heat from an external source (steam)

is supplied to the first-effect generator.

Fig.-3.4 Block diagram of the Vapor Absorption Chiller System

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Where,

1 Strong water concentration Li-Br

solution leaving absorber


solution entering low temp. Heat

exchanger


solution leaving high temp. Heat

exchanger


solution entering Generator 1

5 Weak water concentration Li-Br

solution leaving Generator 1


solution entering Generator 2


solution leaving Generator 2


solution leaving low temp. Heat

exchanger


solution entering absorber

10 High pressure water vaporentering Generator 2

11 High pressure water vapor

entering condenser

12 High pressure water leaving

condenser

13 Low pressure water entering

evaporator

14 Low pressure water vapor

entering absorber

15 Steam in

16 Condensate out

17 Cooling water in

18 Cooling water out

19 Chilled water in

20 Chilled water out

21 Cooling water in

22 Cooling water out

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The chiller is purged from the non-condensable gases and kept under the

vacuum conditions. Weak solution from the absorber is pumped in to the HP generator

through LT, condensate and HT heat exchangers. It is heated by operating system

and concentrated in to intermediate solution, and high temperature refrigerant vapour

is produced. Intermediate solution enters LP generator through HT heat exchangers

in order to exchange heat with weak solution which is passed through the tubes, and

heated by refrigerant vapors from HP generator, concentrated to strong solution,

releasing refrigerant vapour at same time. The strong solution passes through the

outside tube space of LT heat exchanger, enters absorber, transmitting heat to weak

solution from absorber. In absorber the strong solution absorbs refrigerant vapour

again. Refrigerant vapour from HP generator is condensed in LP generator to form

condensate, which enters the condenser through throttle. Refrigerant vapour formed in

the LP generator flows to the condenser to form condensate also. These two parts of

refrigerant condensate flows in to the flash chamber through U-pipe. A part of

refrigerant vapour is flashed to form vapour, which flows in to the re- absorption

chamber at the bottom of absorber, while another part of refrigerant water is cooled,

and enters evaporator refrigerant pan. Refrigerant from evaporator refrigerant pan is

pumped over the evaporator tubes for the refrigeration effect, and evaporates to form

the vapour by absorbing heat of chilled water flowing through tubes. Produced

refrigerant vapour enters absorber, and absorbed by strong solution in the absorber.

Chilled water is cooled and return to the system of customer. Strong solution is diluted

by absorbing refrigerant vapour in absorber and absorbing flashed refrigerant vapour

in the re- absorption chamber, then it is transferred by solution pump to HP and LP

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generator for concentration. Heat generated is carried to atmosphere by cooling water.

This process is continued and refrigeration effect is repeated. The photographic

external view and side view of double-effect, steam-fired absorption chiller is given in

fig.-3.5(a) and fig.-3.5(b) respectively.

The working fluid used in the above system is LiBr-H2O where refrigerant water is

handled from the refrigerant pan of evaporator, and sprayed over the tubes in the

evaporator. System water to be chilled in the evaporator gives heat to the refrigerant,

and decreases temperature. In the mean time, the refrigerant water gains heat, and

evaporates. As absorbent for the chiller, a lithium bromide solution is used. It can be

taken as the carrier of the refrigerant water, and functions as to absorb the refrigerant

the refrigerant vapour, produced in the evaporator by removing the heat of the chilled

water, and carries refrigerant in to HP and LP generators. Weak solution is divided in

to water and strong solution under the heat of supplied steam. Then the strong

solution returned in to absorber to absorb water vapour, produced in the evaporator.

Refrigerant vapor enters condenser to be condensed by dissipating heat into the

atmosphere through cooling water. Refrigerant condensate returns in to evaporator to

produce cooling effect.

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Table-3.5: Energy balance equations of various components in the absorption

System

S. No. Component Energy balance equation

1 Evaporator ( )201919

hhmQ e −=

2 Condenser ( )171817

hhmQ c −=

3 Generator-I ( )161515

hhmQ I g −=−

4 Generator-II ( )766

hhmQ II g −=−

5 Absorber ( )212221

hhmQ a −=

6 Pump

ρ

ag

p

PPmW

−

=1

The refrigeration system can be considered as a perfectly reversible system and the

net refrigerating effect is the heat absorbed by the refrigerant in the evaporator and

therefore the theoretical COP is given by

g

e

Q

QCOP= (3.9)

Whereas in case of absorption refrigeration system, the total energy supplied to the

system is the total of the heat supplied in the generator and work consumed by the

pump. The actual COP of absorption chiller is calculated from the equation below

( )P I g

e

W Q

QCOP

+

=

− (3.10)

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3.7.2 Second law analysis (Exergy Analysis)

Second law analysis is a relatively new concept, which has been used for

understanding the irreversible nature of real thermal processes and defining the

maximum available energy. The second law analysis is based on the concept of

exergy, which can be defined as a measure of work potential or quality of different

forms of energy relative to the environmental conditions. In other words, exergy can be

defined as the maximum theoretical work, derivable by the interaction of an energy

resource with the environment. Exergy analysis applied to a system describes all

losses both in the various components of the system and in the whole system. With

the help of this analysis, the magnitude of these losses or irreversibilities and their

order of importance can be understood. With the use of irreversibility, which is a

measure of process imperfection, the optimum operating conditions can be easily

determined. The advantage of exergy analysis based on thermo-economic

optimization is that the different elements of the system could be optimized

independently. It is possible to say that exergy analysis can indicate the possibilities of

thermodynamic improvement of the process under consideration. The physical exergy

component is associated with work obtainable in bringing a stream of matter from

initial state to a state that is in thermal and mechanical equilibrium with the

environment. Mathematically, physical exergy is expressed as [13]:

( ) ( )[ ]ooo x ssT hhm E −−−= (3.11)

Where, Ex is the exergy of the fluid at temperature T. The terms h and s are the

enthalpy and entropy of the fluid, whereas, h o and s o are the enthalpy and entropy of

the fluid at environmental temperature To .

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Table-3.6: Exergy losses for absorber, evaporator, generator-I,condenser and exergy efficiency

S. No Exergy Loss

(absorber)

(kW)

Exergy Loss

(evaporator)

(kW)

Exergy Loss

(generator-I)

(kW)

Exergy Loss

(condenser)

(kW)

Exergy

Efficiency

1 7.31 55.29 278.24 7.31 19.87

2 25.48 58.97 282.16 25.48 20.90

3 10.89 52.68 293.50 10.89 17.94

4 43.92 54.87 287.95 43.92 19.05

5 44.35 58.17 294.96 44.35 19.72

6 30.91 59.14 286.55 30.91 20.647 38.80 52.92 298.52 38.80 17.73

8 14.84 95.86 300.83 14.84 31.86

9 32.56 65.61 282.04 32.56 23.26

10 44.35 68.46 258.75 44.35 26.45

11 46.53 56.00 261.51 46.536 21.41

12 59.64 63.93 298.36 59.64 21.42

13 31.97 59.61 266.79 31.97 22.34

14 9.88 57.45 291.66 9.88 19.69

15 55.94 58.06 273.63 55.94 21.2116 17.10 61.66 276.68 17.10 22.28

17 56.58 56.23 281.87 56.58 19.95

18 49 61.05 276.71 49 22.06

19 7.56 60.58 285.08 7.56 21.25

20 47.76 60.58 283.57 47.76 21.36

21 24.86 52.35 293.01 24.86 17.86

22 6.72 57.29 272.78 6.72 21.00

23 8.4 58.00 289.89 8.4 20.00

24 9.24 62.49 292.73 9.24 21.34

25 75.76 52.70 293.45 75.76 17.95

26 6.80 48.34 299.15 6.80 16.16

27 17.86 50.56 304.50 17.86 16.60

28 56.86 61.66 269.58 56.86 22.87

29 22.87 51.92 307.33 22.87 16.89

30 57.88 63.20 263.91 57.88 23.95

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Table-3.7: Energy analysis of Li-Br/H2O Absorption System

S.No

Qe

(kW)Qg-I

(kW)Qa

(kW)Qc

(kW)WP

(kW)COP

1 816.64 1178.75 560 560 2.25 0.6912 774.84 1181.17 504 504 2.29 0.6543 735.95 1183.51 616 616 2.30 0.6204 774.84 1181.46 560 560 2.34 0.6545 816.64 1183.51 504 504 2.38 0.6886 815.67 1181.88 616 616 2.17 0.6887 776.78 1183.97 532 532 2.21 0.654

8 734.98 1187.45 476 476 2.17 0.6179 857.48 1181.25 532 532 2.28 0.72410 735.95 1174.46 504 504 2.17 0.62511 774.84 1172.99 504 504 2.17 0.65912 775.81 1184.68 504 504 2.17 0.65313 816.64 1175.57 616 616 2.17 0.69314 776.78 1182.76 532 532 2.17 0.65515 857.48 1176.91 504 504 2.17 0.72616 817.62 1178.33 476 476 2.17 0.69217 774.84 1179.14 504 504 2.17 0.655

18 816.64 1178.85 476 476 2.19 0.69119 815.67 1181.88 504 504 2.25 0.68820 815.67 1180.75 532 532 2.17 0.68921 734.98 1184.1 588 588 2.17 0.61922 735.95 1176.78 504 504 2.34 0.62323 775.81 1182.84 504 504 2.19 0.65424 816.64 1184.18 504 504 2.17 0.68825 776.78 1183.3 504 504 2.25 0.65426 773.87 1184.22 476 476 2.28 0.65127 774.84 1186.78 476 476 2.21 0.651

28 817.62 1174.31 504 504 2.19 0.69429 774.84 1188.12 504 504 2.21 0.65030 735.95 1175.2 532 532 2.14 0.624

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Figure 3.6 shows the variation of the COP of the absorption system with the

evaporator temperature. As can be seen from the figure, the COP of the system

increases slightly as the evaporator temperature increases and this is due to the fact

that with change in heat extracted by the evaporator the enthalpy difference between

the chilled water inlet and outlet of evaporator also varies and finally shows the

variation in COP of the system. Unlike COP, the exergy efficiency of the system

decreases with increase in evaporator temperature and the same is shown in Fig.

3.10.

Fig.-3.6: Variation of COP with evaporator temperature

Fig.-3.7 shows the variation of COP with generator-I temperature. As can be seen

from the figure, COP of the system increases with increase in generator-I temperature

upto a certain level and further increase will have adverse effects because of

increased rate of heat input which leads to higher heat transfer losses at higher

temperatures and this is why the performance of the absorption system largely

depends on the operating parameters of the system. However, the COP shows a

0.56

0.58

0.6

0.62

0.64

0.66

0.68

0.7

0.72

0.74

7 .

5

7 .

5

7 .

7

7 .

9

8 .

4

8 .

6

8 .

6 9 9

9 .

5

9 .

7

9 .

7

9 .

8

9 .

8

9 .

9

9 .

9

1 0

. 2

1 0

. 3

1 0

. 5

1 0

. 6

1 0

. 6

1 0

. 6

1 0

. 7

1 0

. 9

1 1

1 1

. 4

1 1

. 6

1 1

. 7

1 1

. 7

1 2

. 2

C O P

EVAPORATOR TEMPERATURE/o C

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downward trend at increased generator temperatures because of larger heat transfer

losses at higher temperature. Figure 3.11 shows the effect of generator-I temperature

on exergy efficiency and it is found that it does not show the same trend as that for

COP of the system with generator temperature.

Fig.-3.7: Variation of COP with generator – I temperature

Fig.-3.8: Variation of COP with generator –II temperature

0.56

0.58

0.6

0.62

0.64

0.66

0.68

0.7

0.72

0.74

1 1 9

1 1 9

1 2 0

1 2 2

1 2 2

1 2 3

1 2 4

1 2 5

1 2 5

1 2 7

1 2 8

1 2 9

1 2 9

1 3 0

1 3 0

1 3 0

1 3 0

1 3 1

1 3 3

1 3 3

1 3 4

1 3 4

1 3 4

1 3 5

1 3 5

1 3 6

1 3 7

1 3 7

1 3 8

1 3 9

C O P

GENERATOR-I TEMPERATURE/o C

0.56

0.58

0.6

0.62

0.64

0.66

0.68

0.7

0.72

0.74

8 5

8 5

8 5

8 5

. 1

8 5

. 1

8 5

. 1

8 5

. 2

8 5

. 2

8 5

. 2

8 5

. 2

8 5

. 2

8 5

. 2

8 5

. 2

8 5

. 4

8 5

. 4

8 5

. 4

8 5

. 5

8 5

. 5

8 5

. 6

8 5

. 7

8 5

. 7

8 5

. 7

8 5

. 7

8 5

. 7

8 5

. 8

8 5

. 8

8 6

8 6

. 1

8 6

. 1

8 6

. 1

C O P

GENERATOR -II TEMPERATURE/o C

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Fig.-3.9: Variation of COP with condenser temperature

Fig.-3.10: Variation of exergy efficiency with evaporator temperature

Also, Fig.-3.11 shows the variation of exergy efficiency of the system with generator

temperature and it is evident that the exergy efficiency decreases with the increasing

0.56

0.58

0.6

0.62

0.64

0.66

0.68

0.7

0.72

0.74

2 8

. 8

2 9

. 2

2 9

. 6

2 9

. 8

3 0

. 4

3 0

. 6

3 0

. 8

3 1

3 1

3 1

. 6

3 1

. 8

3 1

. 8

3 2

. 1

3 2

. 2

3 2

. 2

3 2

. 6

3 2

. 6

3 2

. 8

3 2

. 8

3 3

. 6

3 3

. 7

3 3

. 8

3 3

. 8

3 4

3 4

. 3

3 4

. 3

3 4

. 3

3 4

. 6

3 4

. 6

3 4

. 8

C O P

CONDENSER TEMPERATURE/oC

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

7 .

5

7 .

5

7 .

7

7 .

9

8 .

4

8 .

6

8 .

6 9 9

9 .

5

9 .

7

9 .

7

9 .

8

9 .

8

9 .

9

9 .

9

1

0 .

2

1

0 .

3

1

0 .

5

1

0 .

6

1

0 .

6

1

0 .

6

1

0 .

7

1

0 .

9 1 1

1

1 .

4

1

1 .

6

1

1 .

7

1

1 .

7

1

2 .

2

E X E R G Y E F F I C I E N C Y

EVAPORATOR TEMPERATURE/o C

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generator temperature. This is due to the fact that the system operating on higher

temperature can improve the COP of the system but the more input exergy is supplied

to the system and more exergy losses occur in the generator during the heat transfer

process. Fig.-3.12 shows the comparative exergy loss in each component.

Fig.-3.11: Variation of exergy efficiency with generator – I temperature

Fig.-3.12: Comparative exergy Loss of different components of Li-Br-H2Oabsorption system

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1 1 9

1 1 9

1 2 0

1 2 2

1 2 2

1 2 3

1 2 4

1 2 5

1 2 5

1 2 7

1 2 8

1 2 9

1 2 9

1 3 0

1 3 0

1 3 0

1 3 0

1 3 1

1 3 3

1 3 3

1 3 4

1 3 4

1 3 4

1 3 5

1 3 5

1 3 6

1 3 7

1 3 7

1 3 8

1 3 9 E

X E R G Y E

F F I C I E N C Y

GENERATOR-I TEMPERATURE/o C

0

50

100

150

200

250

300

E X E R G Y

L O S S / k W

GENERATOR

EVAPORATOR

CONDENSER ABSORBER

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Results show that as expected the COP of the system increases minutely as

the generator temperature is increased but the exergy efficiency of the system drops

with the increase in generator temperature. It is also found that the COP of the system

increases with increase in evaporator temperature this largely depends on the

enthalpy difference between the chilled water at inlet and outlet of evaporator.

However, it is reverse in case of exergy efficiency.

The results with respect to exergy losses in each component and exergy

efficiency are very important for the optimization of absorption system. These results

are helpful for designers to bring changes in the actual system for optimum

performance and less wastage of energy.

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References

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