Effect of saturation-temperature on the performance of

INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2006; 30:729–740 Published online 13 January 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/er.1174

SHORT COMMUNICATION Effect of saturation-temperature on the performance of a vapour-compression refrigeration-cycle working on different refrigerants using exergy method Mehmet Kopacn,y and Bilal Zemher Mechanical Engineering Department, Zonguldak Karaelmas University, 67100 Zonguldak, Turkey

SUMMARY In this study, the behaviour of a vapour-compression refrigeration cycle, for different refrigerants such as NH3, R-12, R-22 and HFC-134a was investigated using the exergy method. The cooling load of the plant and the saturation-temperature of the cold chamber were held constant, whereas the saturationtemperatures of the evaporator and the condenser were varied from 303 to 313 K and 258 to 248 K, respectively. The irreversibility rates (or exergy destruction rates) of sub-regions for the whole cycle, using energy and exergy analysis, were determined for each refrigerant. The effects of changes in the saturationtemperature in the condenser and evaporator on the irreversibility rate of the cycle were obtained for each refrigerant. The relations between the total irreversibility rate of the plant and the irreversibility rate of the condenser and the evaporator were determined for different values of saturation temperatures of the condenser and the evaporator. The COP of the cycle and the rational efficiency were determined for each of the refrigerants and compared with each other. Among the refrigerants used, R-12 was found to be the most economical refrigerant as compared with the others. Copyright # 2005 John Wiley & Sons, Ltd. KEY WORDS:

energy; exergy; exergy method; rational efficiency; refrigeration cycle

1. INTRODUCTION Due to the awareness of limited resources available in the world, the government of most of the countries are changing their energy policies in order to reduce/minimize the waste in various thermal energy conversion systems. It has also sparked interest in the scientific community to take a closer look at the energy conversion devices and to develop new techniques to better utilize the existing limited resources. The first law of thermodynamics deals with the quantity of energy and asserts that energy cannot be created or destroyed (Cengel and Boles, 1998). This n

y

Correspondence to: Mehmet Kopac, Mechanical Engineering Department, Zonguldak Karaelmas University, 67100 Zonguldak, Turkey. E-mail: [email protected], mehmet [email protected]

Contract/grant sponsor: Zonguldak Karaelmas University; contract/grant number: 2002-45-02-06

Copyright # 2005 John Wiley & Sons, Ltd.

Received 6 February 2005 Revised 26 June 2005 Accepted 1 August 2005

730

M. KOPAC AND B. ZEMHER

law merely serves as a necessary tool for the bookkeeping of energy during a process and offers no challenges to the engineer. The second law of thermodynamics (or exergy analysis), however, deals with the quality of energy. More specifically, it is concerned with the degradation of energy during a process, the entropy generation/irreversibility, and the lost opportunities to do work; and it offers plenty of space for improvement. The second law of thermodynamics has proved to be a very powerful tool in the optimization of complex thermodynamic systems (Bejan, 2002; Kopac and Zemher, 2004; Kopac and Kokturk, 2005). It is well known that the energy consumption in the refrigerating and air-conditioning units is as high as in the heating systems. Dincer et al. (1996) investigated the thermal performance of a solar powered absorption refrigeration system. Meunier et al. (1997) investigated the performance of adsorptive refrigeration cycles using second-law analysis. Nikolaidis and Probert (1998) investigated the behaviour of two-stage compound compression-cycle with flash intercooling for R-22 by the exergy method. The effects of temperature changes in the condenser and evaporator on the irreversibility rate of cycle were determined. The thermodynamic performance of a single-stage absorption/compression heat pump using the ternary working fluid trifluoroethanol–water–tetraethylenglycol dimethylether for upgrading waste heat was studied by Bourounis et al. (2000). In their paper, a simulation program was developed using a mathematical model based on the mass and energy balances in all components of the cycle considering the thermodynamic equilibrium. The effects of thermal resistances and internal irreversibilities on the performance of combined cycles for cryogenic refrigeration were investigated by Go¨ktun and Yavuz (2000) and improved equations for the coefficient of performance of the systems under condition were obtained. Smith and Few (2001) discussed the development of the combined heat and power plant concept and described the construction of an experimental plant. Exergy analysis is employed to examine the plant performance to indicate areas of improvement that could not be identified, only by the first law analysis. A computational model based on the exergy analysis was presented by Yumrutas¸ et al. (2002) for the investigation of the effects of the evaporating and condensing temperatures on the pressure losses, exergy losses, second law efficiency, and the coefficient of performance (COP) of a vapour compression refrigeration cycle. Kanogˇlu et al. (2004) developed a procedure for the energy and exergy analysis of open-cycle desiccant cooling systems and applied it to an experimental unit operating in ventilation mode with natural zeolite as the desiccant. Tyagi et al. (2004) presented the thermoeconomic optimization of an irreversible Stirling cryogenic refrigerator cycle. The predicted performance of a shell-and-tube (1-2) evaporator installed in a vapour compression liquid chiller was analysed by Torrella et al. (2004). A theoretical study of a triangular cycle with cooling and heating at variable temperatures was investigated by Rozhentsev et al. (2005). The aim of this study is to investigate the effects of evaporator and condenser saturation temperatures on the irrevesibilities of components, efficiency defects and the rational efficiencies for an existing refrigeration plant by using different refrigerants. In most of the studies given in literature, these types of studies were mainly analysed using one type of refrigerant, however in this study the analyses were extended for four different refrigerants (NH3, HFC-134a, R-12 and R-22) and the results were compared. 2. MATHEMATICAL MODELLING The whole system was considered as closed, but the units were considered separately as open systems. Energy and exergy balances were applied as follows for a steady state open system Copyright # 2005 John Wiley & Sons, Ltd.

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PERFORMANCE OF A VAPOUR-COMPRESSION REFRIGERATION-CYCLE

731

following Cengel and Boles (1998) and Kotas (1995) when the kinetic and potential energies and exergies were negligible, as given below. General energy balance for steady-state open system: X X ’ in þ W ’ out þ W ’ in þ ’ out þ Q mh mh ð1Þ ’ ¼Q ’ out

in

’ out are the rate of heat input to the system and the rate of heat output from the ’ in and Q where Q ’ in and W ’ out are the rate of work input to the system and the rate of work system, respectively. W output from the system, respectively. 2.1. Energy analysis for components of plants ’ c ¼ mðh Evaporator ð4-1Þ : Q ’ 1 h4 Þ

ð2Þ

’ comp;s ¼ mðh Compressor ð1-2Þ : W ’ 2s h1 Þ

ð3Þ

’ comp;s is the isentropic power of compressor (Figure 1). Here W ’ comp;s W ’ comp ¼ W ’ comp;s =Za ; W ’ comp ¼ mðh Za ¼ ; W ’ 2 h1 Þ ’ comp W h2 ¼ h 1 þ

ðh2s h1 Þ Za

ð4Þ

ð5Þ

’ ’ el ¼ W comp W Zm Zel

ð6Þ

’ o ¼ mðh Condenser ð2-3Þ : Q ’ 2 h3 Þ

ð7Þ

Throttling valve ð3-4Þ : h3 ¼ h4

ð8Þ

Coefficient of Performance; COP : COP ¼

’c Q ’ el W

ð9Þ

General exergy rate balance for steady-state open system: I’ ¼

X

me ’

in

X

Q ’ me ’ E’ in E’ W out ¼ To S gen

ð10Þ

out

where I’ is the rate of irreversibility (or the rate of exergy destruction), me ’ is the exergy flow rate ’W is the rate of exergy of heat input, E is the rate of exergy of power of the working fluid, E’ Q out in output and S’ gen is the rate of entropy generation. The rate of heat exergy and the power exergy were defined as follows: To Q Q ’ ’ ’ 1 ð11Þ Rate of Heat Exergy; E in ðfor Tin 5To Þ : E in ¼ Qin Tin Copyright # 2005 John Wiley & Sons, Ltd.

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732


’W ’ Power Exergy; E’ W out : E out ¼ W

ð12Þ

Thermomechanical Exergy ðor flow exergyÞ; e : e ¼ h ho To ðs so Þ

ð13Þ

where ho and so are the specific enthalpy and the specific entropy at environmental state (Po, To), respectively. Exergy efficiency (or rational efficiency), c: I’ ’ el c ¼ 1 ; E’ i ¼ W ð14Þ E’ i 2.2. Exergy rate balance for each component of the plants *

For evaporator (4-1) (Figure 1) c I’ evap ¼ mðe ’ 4 e1 Þ E’ Q c c E’ Q c ¼

’c Q ; COPrev ; I’ evap

ð15Þ

Tcr ðfor Tcr to To Þ To Tcr To Tcr ’ ¼ mðe ’ 4 e1 Þ Q c Tcr COPrev ¼

’ el E’ Wel ¼ W

ð16Þ

ð17Þ

*

For compressor (1-2):I’ comp ¼ mðe ’ 1 e2 Þ þ E’ Wel ;

*

For condenser (2-3), I’ cond ¼ mðe ’ 2 e3 Þ

(19)

*

For throttling valve (3-4), I’ tvalve ¼ mðe ’ 3 e4 Þ

(20)

(18)

So, the total irreversibility: I’ total ¼ I’ evap þ I’ comp þ I’ cond þ I’ tvalve

ð21Þ

2.3. Efficiency defects and rational efficiency These are the fractions of the input which are lost through irreversibilities, i.e. I’ i di ¼ ’ el W The rational efficiency of the cycle, c c¼1

X

di

ð22Þ

ð23Þ

i

3. CALCULATIONS The refrigeration cycle used for the calculations was shown in Figure 1. In this study, the behaviour of a vapour-compression refrigeration cycle, using different refrigerants such as NH3, R-12, R-22 and HFC-134a was investigated by the exergy method. The cooling capacity and the temperature of the cooling room were held constant for the present analysis using different Copyright # 2005 John Wiley & Sons, Ltd.

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Environment

To Qo

Condenser

3

2

Compressor

T

Throttling Valve

2 2s

4

1

Electrical Engine

Evaporator

3

0

To

Qc

Tcr 1

4

Qc Cooling room

s

Figure 1. Schematic and T–s diagrams of a vapour compression refrigeration cycle.

refrigerants as mentioned above. Two cases were considered in the analysis. In the first case, the saturation temperatures of the evaporator were varied from 258 to 248 K. The inlet temperatures of the compressor were considered 5 K higher than that of the evaporator temperature for each case. The saturation and the outlet condenser temperatures were held constant at 303 and 298 K, respectively. The environmental temperature and the pressure were assumed to be 293 K, 101 kPa, respectively. In the second case, the saturation temperatures of condenser were varied between 303 and 313 K. The outlet condenser temperatures were assumed to be 5 K lower than that of the condenser saturation temperature for each case. The evaporator saturation temperature was held constant as 258 K. The values of operating parameters of refrigeration cycle were given in Table I. Thermodynamic properties of the refrigerants were taken from Wylen et al. (1994).

4. RESULTS AND DISCUSSION 4.1. Effect of the saturation-temperature of evaporator Using the conditions stated by the first case, the rate of irreversibilities of each components of the plant and the rate of total irreversibilities and the input electrical powers of the cycle were obtained for different refrigerants. The percentage ratios of the rate of irreversibilities to inlet Copyright # 2005 John Wiley & Sons, Ltd.

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Table I. Operating parameters of the refrigeration plants. Parameter ’ c (kW) Cooling capacity, Q Ambient temperature, To (K) Temperature of cooling room, Tcr (K) Saturation temperature of the evaporator, Ts,evap (K) Compressor inlet temperature, T1 (K) Saturation temperature of the condenser, Ts,cond (K) Condenser outlet temperature, T3 (K) Mechanical efficiency of the compressor, Zm Efficiency of the electrical engine, Zel Adiabatic efficiency of the compressor, Za

Value 93 293 272 258 263 303 298 0.83 0.90 0.90

Principal assumptions: 1. Heat loses and heat leaks are negligible. 2. Negligible kinetic and potential components of exergy.

electrical powers (or efficiency defects), the rational efficiencies of the cycle and the COP were determined. It was found out that the efficiency defects of the compressor were higher than those of the efficiency defects of the other components of the cycle. In general, when the saturation temperature of the evaporator was decreased from 258 to 248 K, the efficiency defects of the evaporator and the throttling valve were increased, whereas the efficiency defects of the compressor and the condenser were decreased. As the saturation temperature of the evaporator decreased, the total efficiency defect of the plant was increased. This increase in the irreversibility of the evaporator affected the whole plant. On the other hand, as the saturation temperature of the evaporator was decreased, the rational efficiency of the plant was also decreased. For NH3, the rational efficiency value of the plant was decreased from 24.94 to 18.76% (from 258 to 248 K) which implied that a 10 K decrease in the evaporator saturation temperature caused a 6.18% decrease in the rational efficiency (for NH3). The efficiency defect values were decreased from 31.04 to 30.62% in the compressor, 20.62 to 19.79% in the condenser and increased from 4.70 to 5.69% and 18.70 to 25.14% in the throttling valve and the evaporator, respectively. For a 10 K decrease in the evaporator saturation temperature, the total efficiency defect was increased from 75.06 to 81.24% (for NH3). For HFC-134a, the efficiency defect values were almost the same for the compressor, namely 32.15–32.12%, whereas decreased from 13.97 to 11.39% for the condenser, for the throttling valve increased from 10.09 to 12.41% and for the evaporator increased from 18.66 to 25.20%. The rational efficiency was decreased from 25.13 to 18.88%. A 10 K temperature decrease caused a 6.25% decrease in the rational efficiency (for HFC-134a). The rational efficiency decreased from 25.42 to 19.18% for R-12 and decreased from 25.02 to 18.90% for R-22 for a 10 K temperature decrease. The relations between the total irreversibility rate of the plant and the irreversibility rate of evaporator were obtained for the stated values of the saturation temperature of the evaporator (Ts,evap), and for different refrigerants. The values of the slopes are 2.1706, 2.1802, 2.2312 and 2.2361 for R-12, R-22, HFC-134a and NH3, respectively. The value of the minimum slope was found as 2.1706 for R-12 and the value of the maximum slope was found for NH3 as 2.2361. Plots of the total irreversibility rate of the plant versus the irreversibility rate of the evaporator for different values of Ts,evap were shown for different refrigerants in Figure 2. For an irreversibility rate value in the evaporator, the total irreversibility rate of the plant for R-12 was Copyright # 2005 John Wiley & Sons, Ltd.

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Total Irriversibility Rate of the Plant, (kW)

32 NH3 HF C-134a R-12 R-22

30

248 K

250 K

28 252 K 26 254 K 24 256 K 22 258 K 20 5

6

7 8 Irriversibilty Rate of Evaporator, (kW)

9

10

Figure 2. Plots for determining the CSB of the evaporator for different refrigerants with I’ evap as a variable Ts,evap and Ts,cond=303 K.

Rational Efficiency of the Plant, (%)

26 NH3 HFC-134a R-12 R-22

24

22

20

18

16 260

258

256

254

252

250

248

246

Saturation Temperature of Evaporator, (K)

Figure 3. Rational efficiency of the plant versus the evaporator saturation temperature for different refrigerants.

lower than the other refrigerants. For different refrigerants, the change of the rational efficiency with respect to the evaporator saturation temperature was given in Figure 3. As the saturation temperature of the evaporator decreased, the rational efficiency decreased. The rational efficiency values changed very close to each other for NH3, HFC-134a and R-22. Copyright # 2005 John Wiley & Sons, Ltd.

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3.4 NH3 HFC-134a

3.2

R-12 R-22

COP

3.0 2.8 2.6 2.4 2.2 260

258

256

254

252

250

248

246

Saturation Temperature of Evaporator, (K)

Figure 4. COP of the plant versus the saturation temperature of evaporator for different refrigerants.

But for R-12 the rational efficiency values were approximately 0.5% higher than the others (Figure 3). Plots of the values of the COP of the plant versus Ts,evap are shown in Figure 4. Changes in Figure 4 were similar to the changes in Figure 3. The maximum COP value was for R-12 again.

4.2. Effect of the saturation-temperature of condenser Using the conditions for case 2, the rate of irreversibilities of each component of the plant and the rate of total irreversibilities and the input electrical powers of the plant were obtained for different refrigerants. The % of the ratio of the rate of irreversibilities to input electrical powers (or efficiency defects), the rational efficiencies of the cycle and the COP were determined. The efficiency defects of the compressor were higher than the efficiency defects of the other components of the plant. In general, when the saturation temperature of the condenser was increased from 303 to 313 K, the efficiency defects of the condenser and the throttling valve increased, whereas the efficiency defects of the compressor and the evaporator decreased. As the saturation temperature of the condenser increased, the total efficiency defect of the plant increased also. This increase in the efficiency defect of the condenser affected the whole cycle. Also as the saturation temperature of the condenser was increased, the rational efficiency of the cycle decreased. When the saturation temperature of the condenser was increased by 10 K (or 108C) (from 303 to 313 K), the rational efficiency of the cycle decreased from 24.94 to 19.58% for NH3, from 25.13 to 19.26% for HFC-134a, from 25.44 to 19.69% for R-12 and finally from 25.02 to 19.33% for R-22. A 10 K increase in the condenser saturation temperature caused a 5.36, 5.87, 5.75 and 5.69% decrease in the rational efficiency of the cycle, respectively, for NH3, HFC-134a, R-12 and R-22. Copyright # 2005 John Wiley & Sons, Ltd.

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Total Irriversibility Rate of the Plant, (kW)

32 NH3 30

HF C-134a

R-12

R-22

313 K

28

311 K 309 K

26 307 K

24

305 K 22

303 K

20 3

4

5

6

7

8

9

10

11

12

Irriversibility Rate of Condenser, (kW)

Figure 5. Plots for determining the CSB of the condenser for different refrigerants with I’ cond as a variable Ts,cond and Ts,evap=258 K.

The relations between the total irreversibility rate of the cycle and the irreversibility rate of the condenser were obtained for the stated values of Ts,cond, and for different refrigerants. The values of slopes are 2.0384, 1.9957, 2.173 and 1.6523, respectively, for R-12, R-22, HFC-134a and NH3. The value of the minimum slope is 1.6523 for NH3 and the value of the maximum slope is 2.173 for HFC-134a. Plots of the total irreversibility rate of the plant versus the irreversibility rate of the condenser for different values of the saturation temperature of the condenser (Ts,cond) were shown for different refrigerants in Figure 5. Although the value of the slope was the smallest for NH3, the maximum irreversibility rate for this refrigerant at the condenser was determined for the same condenser saturation temperature. The rational efficiencies and the values of COP of the plant versus the saturation temperature of the condenser are shown for different refrigerants in Figures 6 and 7, respectively, at Ts,evap=258 K. In this temperature range investigated, the values of the rational efficiency of R-12 were found higher (in Figure 6). This was also true for Figure 7.

5. CONCLUSIONS A computational study based on the exergy analysis were presented for the investigation of the effects of the saturated temperatures of the condenser and the evaporator on the efficiency defects (or the ratio of the rates of the irreversibility to the rate of electrical exergy input) in each of the components of the plant, the total efficiency defect of the plant, the rational efficiencies (or the second law efficiencies) and the values of COP of a vapour compression refrigeration plant for NH3, HFC-134a, R-12 and R-22. It was found that the saturation temperatures of the condenser and the evaporator had strong effects on the efficiency defects of the condenser and Copyright # 2005 John Wiley & Sons, Ltd.

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Rational Efficiency of the Plant, (%)

26 NH3 HFC-134a R-12 R-22

24

22

20

18

16 301

303

305

307

309

311

313

315

Saturation Temperature of Condenser, (K)

Figure 6. Rational efficiency of the cycle versus the saturation temperature of condenser for different refrigerants, Ts,evap=258 K.

3.4 NH3 HFC-134a R-12 R-22

3.2

COP

3.0 2.8 2.6 2.4 2.2 301

303

305

307

309

311

313

315

Saturation Temperature of Condenser, (K)

Figure 7. COP of the cycle versus the saturation temperature of condenser for different refrigerants, Ts,evap=258 K.

the evaporator, and on the total efficiency defects, and the rational efficiency of the plant and the COP but little effects on the efficiency defects of the other components for the whole refrigerant. The total irreversibility rates of the plant and the irreversibility rates of the evaporator increased, and the rational efficiencies of the plant and the COP decreased with decreasing the Copyright # 2005 John Wiley & Sons, Ltd.

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saturation temperature of the evaporator. But the total irreversibility rates of the plant and the irreversibility rates of the condenser increased, and the rational efficiencies of the plant and the COP decreased with increasing the saturation temperature of the condenser. Finally, it was found out that the R-12 was the most economical refrigerant as compared with the others. The procedures given in this paper for the exergy analysis of vapour compression refrigeration plant could be applied to the whole refrigeration plant for optimization. NOMENCLATURE COP CSB e E’ Q E’ W h I’ m ’ P ’ Q ’c Q s S’ gen T ’ W ’ comp;s W ’ el W

=coefficient of performance =coefficient of structural bond =flow exergy (kJ kg1) =rate of heat exergy (kW) =power (or rate of work) exergy (kW) =specific enthalpy (kJ kg1) =irreversibility rate (kW) =mass flow rate (kg s1) =pressure (kPa) =heat rate (kW) =cooling heat rate (kW) =specific entropy (kJ kg1 K1) =rate of entropy generation (kW K1) =temperature (K) =power (kW) =isentropic power of compressor =electrical power (kW)

Greek letters d Zel Zm Za c

=efficiency defect (dimensionless) =efficiency of electrical engine (dimensionless) =mechanical efficiency (dimensionless) =adiabatic efficiency of compressor (dimensionless) =rational efficiency (dimensionless)

Subscripts c cr cond e el evap i in

=cooling =cooling room =condenser =exit =electrical =evaporator =number of component =inlet


Int. J. Energy Res. 2006; 30:729–740

740 o out rev s tvalve


=environmental state =outlet =reversible =saturation, isentropic =throttling valve

ACKNOWLEDGEMENTS

This work was funded by Research Fund AFP 2002-45-02-06 of Zonguldak Karaelmas University. The authors would like to acknowledge Professor Dr Turkan Kopac of Zonguldak Karaelmas University, Department of Chemistry for useful suggestions and helpful comments in preparing this manuscript.

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