Thermo project refrigeration

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Exergy analysis of refrigeration systems using an alternative refrigerant (hfo-1234yf ) to R-134a

Arif Emre Özgür *, Ahmet Kabul and Önder Kizilkan

Süleyman Demirel University, Faculty of Technology, Department of Energy Systems Engineering, 32260, Isparta, Turkey

*Corresponding author:

[email protected]

Abstract In this study, analysis studies of the first and second law of thermodynamics are carried out for vapor compressed refrigeration systems using an alternative refrigerant HFO-1234yf to HFC-134a. No important differences between cycle efficiencies were observed for both refrigerants. However, the exergy destruction rate of the compressor obtained with HFO-1234yf is lower than that calculated for R-134a. According to the exergy and energy analysis results obtained with this study, it can be evaluated that HFO-1234yf is a good alternative to R-134a. If the safety requirements ( flammability problem of the refrigerant) have been satisfied refrigeration systems charged with HFO-1234yf, this alternative refrigerant can be commonly used in the systems.

Keywords: exergy; refrigeration; R1234yf; R134a; thermodynamic analysis

Received 27 April 2012; revised 16 May 2012; accepted 17 May 2012

1 INTRODUCTION

All vapor compression refrigeration systems produced newly are charged with the refrigerants that have zero ozone deple- tion potential (ODP ¼ 0) today. However, almost all of these refrigerants have high global warming potentials (GWP). For example, the GWP of R134a (HFC-134a), which is one of the most popular refrigerants for the current refrigeration systems and air-conditioning systems, is �1300. The European Parliament has published a directive that bans refrigerants having a 100-year direct GWP exceeding 150 in mobile air conditioners [1]. As replacements for R134a, the manufac- turers in Europe are considering the use of R744 (CO2) and R1234yf (2,3,3,3 tetrafluoropropene, HFO-1234yf ) because these refrigerants have GWP of 1 and 4, respectively [2]. The R1234yf is a new refrigerant, and is also a flammable refriger- ant. However, this refrigerant has low toxicity property and can be directly charged into conventional refrigeration systems as the refrigerant instead of R134a. But R744 requires higher system operating pressures, and cannot be directly charged into conventional systems. Critical temperature and pressure of R744 is �31.18C and 7.377 MPa, respectively. Due to relatively low critical temperature value of R744, the transcritical cycle is established for a refrigeration system using R744 as the

refrigerant [3]. Then, the maximum pressure of the system using R744 is much higher than the systems’ using conven- tional refrigerants such as R134a.

Park and Jung studied for nucleate boiling heat transfer coefficients of R1234yf. They investigated these coefficients for plain and low fin surface conditions. They concluded that the nucleate boiling heat transfer coefficients of R1234yf are very similar to those of R134a. However, they also stated that con- ventional boiling correlations can be used for the design of evaporators and boilers with R1234yf [4]. Tanaka and Higashi measured thermodynamic properties of R1234yf at saturation condition. They stated that almost all thermodynamic proper- ties of R1234yf are lower than those of R134a [5]. SAE report published for R1234yf claims that R1234yf is the best replace- ment refrigerant for R134a [6]. Weissler [7] stated that if a comparison is made between R134a replacements, R1234yf and R744, in the view point of global warming, it was observed that R1234yf has the lowest contribution. Because of the rela- tively low lower-flammability limit and minimum ignition energy, some chemical companies proposed to use R1234yf to direct expansion refrigeration systems. However, safety class of R1234yf to be published is expected to be A2L [8].

In this study, energetic and exergetic analyses are studied under the same evaporation and condensation conditions for

International Journal of Low-Carbon Technologies 2014, 9, 56 – 62 # The Author 2012. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact [email protected] doi:10.1093/ijlct/cts054 Advance Access Publication 26 June 2012 56

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R134a and R1234yf. Additionally, adiabatic efficiency of the system for both refrigerants are taken to be 85%.

2 SYSTEM DESCRIPTION

The first law of thermodynamics is concerned with the conser- vation of energy. Unlike energy, exergy is not subject to a con- servation law for the real systems [9]. Exergy analysis is based on the second law of thermodynamics, and a powerful tool for the design, optimization and performance evaluation of energy systems [10].

A schematic drawing of a typical subcritical cycle for R1234yf and R134a and a typical transcritical carbon dioxide cycle is shown in Figure 1. In the subcritical cycle, while a con- denser is used for discharging the heat, a gas cooler is used for the same operation in the transcritical cycle. This is the main difference between these cycles. In Figure 2, the corresponding schematic ln P– h diagrams are illustrated for both cycles.

Note that the state points in Figure 2 are defined as the con- dition of the refrigerant characterized by its temperature, mass flow rate and quality. In Figure 2, evaporation, compression, condensation and expansion processes are shown between points 4 – 1, 1 – 2, 2 – 3 and 3 – 4, respectively. In this study, en- ergetic and exergetic efficiency comparisons are made at the same evaporating and condensing temperature conditions. The assumptions in the study are given as follows:

- steady-state operation, - negligible pressure drop, - negligible heat losses/gains for the pipe flows, - kinetic and potential energy terms are ignored, - negligible power consumption of the fans.

To obtain reliable and meaningful outcomes from the energy analysis studies, the second law of thermodynamics is used. To use this law, one has to use a term defined as the availability or exergy. For the pure substances, this term can be defined in Equation (1) with the assumptions made before.

c¼ðh�h0Þ�T0ðs � s0Þ

The availability loss and destruction terms of each system com- ponent shown in Figure 1 can be calculated by

Dc ¼ exergy supplied � exergy recovered

3 THERMODYNAMIC ANALYSES

3.1 The first law of thermodynamics The aim of the first law of thermodynamic analysis is to specify the variation of coefficient of performance (COP) with evaporator temperature, condenser temperature, compressor is- entropic efficiency and superheating temperature. For this reason, first law analysis was applied to each system compo- nent. The compressor capacity can be obtained from:

_W comp ¼ _mrðh2 � h1Þ ð1Þ where _W comp is the compressor capacity, _mr the mass flow rate of the refrigerant and h the enthalpy. Here, h2 is the actual en- thalpy at the compressor exit and defined as;

h2 ¼ h1 þ h2s � h1

his ð2Þ

where his is the isentropic compressor efficiency. The electric motor power can be found from the equation below:

_W el ¼ _W comp

hel hmech ð3Þ

Here hmech is the compressor mechanical efficiency and hel the electric motor efficiency. The condenser capacity of the system according to the first law of thermodynamics is defined as:

_QC ¼ _mrðh2 � h3Þ ð4Þ The refrigeration capacity of the system can be found from:

_QE ¼ _mrðh1 � h4Þ ð5Þ The overall energetic performance of the plant is determined by evaluating its COP, calculated as the ratio between the re- frigeration capacity and the electrical power supplied to the compressor [11]:

COP ¼ _QE

_W COMP ð6Þ

Figure 1. Schematic drawing of the refrigeration system.

Figure 2. Schematic drawing of the refrigeration cycle.

Exergy analysis of refrigeration systems

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3.2 The second law of thermodynamics The second law analysis is based on the concept of exergy. Exergy can be defined as a measure of work potential or quality of different forms of energy relative to environmental conditions. Exergy analysis that applied to a system describes all loses in the system components and the whole system. With the help of this improvement, potentials of these losses or irre- versibilities and their order of importance can be understood. With the use of irreversibility, which is a measure of process imperfection, it ’is more helpful on determining the optimum operating conditions. It is possible to say that exergy analysis can indicate the possibilities of thermodynamic improvement of the process under consideration [12].

The exergy method is a relatively new analysis technique in which the basis of evaluation of thermodynamic loses follows from the second law rather than the first law of thermody- namics. Thus, it belongs to that category of analyses known as second law analyses [13].

The general exergy balance can be expressed in the rate form as [14]:

_Ein � _Eout ¼ _Edest ð7Þ

where _Ein � _Eout is the rate of net exergy transferred by heat, work and mass and _Edest the rate of exergy destruction. The other form of Equation (10) is given as [15]:

_EQ � _EW þ _Emass; in � _Emass; out ¼ _Edest ð8Þ where subscripts Q and W denote heat and work, respectively. The exergy of heat, work and mass flow are defined as follows [14]:

_EQ ¼ _Q 1 � T0 T

� � ð9Þ

_EW ¼ _W ð10Þ

_Emass ¼ _mr1 ð11Þ

where Q is the heat transfer rate at temperature T, T0 the refer- ence temperature and 1 the specific ( flow) exergy.

The difference of the flow availability of the stream and that of the same stream at its restricted dead state is called flow exergy, 1, and given by ignoring the chemical exergy terms [13]:

1 ¼ðh � T0sÞþ 1

2 V 2 þ gz �ðh0 � T0s0Þ ð12Þ

where g is the gravitational acceleration, V the velocity and z the altitude of the reference level. Ignoring the potential and kinetic energy terms of Equation (12):

1 ¼ðh � T0sÞ�ðh0 � T0s0Þ ð13Þ

where h0 and s0 are the enthalpy and entropy values of the dead state of the refrigerant at pressure P0 and temperature T0. Rearranging Equations (9 – 12), the rate form of the general exergy balance becomes [15]:

_Edest ¼ X

_Q 1 � T0 T

� � � _W þ

X in

_mr1 � X out

_mr1 ð14Þ

In Equation (15), the _Edest term is also known as the irreversi- bility rate.

_I ¼ _Edest ð15Þ

If exergy analysis equations above were performed on each component of the vapor compression refrigeration system given in Figure 1, irreversibility rates of each component can be found.

Condenser exergy balance

_E2 ¼ _E3 þ _EQC þ _IC ð16Þ

Since, the thermal exergy associated with QC is zero; the exergy balance for the condenser region reduces [15]:

_IC ¼ _E2 � _E3 ð17Þ

Compressor exergy balance

_El þ Wel ¼ _E2 þ _IW ð18Þ

_IW ¼ _El � _E2 þ Wel ð19Þ

Expansion valve exergy balance

_E3 ¼ _E4 þ _IExp ð20Þ

_IExp ¼ _E3 � _E4 ð21Þ

Evaporator exergy balance

_E4 þ _EQE ¼ _E1 þ _IE ð22Þ

The thermal exergy associated with QE is defined as:

_EQE ¼ _QE 1 � T0 TE

� � ð23Þ

_IE ¼ _E4 � _El þ _QE 1 � T0 TE

� � ð24Þ

In the exergy balance equations above, the subscripts C, W, Exp and E denote condenser, compressor work, expansion valve and evaporator, respectively.

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The exergetic efficiency, j, of a vapor compression refriger- ation cycle is defined as [16]:

j ¼ _EQE Wel

ð25Þ

4 RESULTS AND DISCUSSION

In Figure 3a and b, COP variations and in Figure 4a and b, exergy efficiency variations of vapor compression systems using R134a and R1234yf are presented, respectively. For COP values and exergy efficiencies, negligible differences are observed between R134a and R1234yf at studied conditions. However, higher COP and exergetic efficiency values are obtained with R1234yf at higher evaporation temperatures and lower conden- sation temperatures. At this state, R1234yf can be assumed as better choice for mobile air-conditioning systems in the point of view of energetic performance.

The irreversibility variations with condensing temperature of the condenser, the compressor, the expansion valve and the evaporator are presented in Figures 5 – 8, respectively.

Figure 3. COP variations of vapor compression systems using (a) R134a and

(b) R1234yf. Figure 4. Exergy efficiency variations of vapor compression systems using (a)

R134a and (b) R1234yf.

Figure 5. Irreversibility variation of the condenser with the condensing

temperatures of R134a and R1234yf.

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Total irreversibility variation of the system with the con- densing temperatures of R134a and R1234yf are presented in Figure 9.

The irreversibility variations with evaporating temperature of the condenser, the compressor, the expansion valve and the evaporator are presented in Figures 10 – 13, respectively.

Total irreversibility variation of the system with the evapor- ating temperatures of R134a and R1234yf are presented in Figure 14.

The same result obtained with energetic and exergetic effi- ciency graphics can be seen in figures presented for irreversibil- ities. R1234yf has more advantages for lower condensing and higher evaporating temperature conditions than to R134a.

Figure 6. Irreversibility variation of the compressor with the condensing

temperatures of R134a and R1234yf.

Figure 7. Irreversibility variation of the expansion valve with the condensing

temperatures of R134a and R1234yf.

Figure 8. Irreversibility variation of the evaporator with the condensing

temperatures of R134a and R1234yf.

Figure 9. Total irreversibility variation of the system with the condensing

temperatures of R134a and R1234yf.

Figure 10. Irreversibility variation of the condenser with the evaporating

temperatures of R134a and R1234yf.

Figure 11. Irreversibility variation of the compressor with the evaporating

temperatures of R134a and R1234yf.

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In Figure 15, the Grassmann diagrams are plotted for both refrigerants at – 58C evaporation and 408C condensation tem- perature conditions. The maximum exergy destruction is obtained at the compressor for both refrigerants. However, R1234yf is assumed as more favorable refrigerant in the point of view of thermodynamics.

5 CONCLUSION

In this study, energetic and exergetic analysis of vapor compres- sion refrigeration cycles have been carried out for R134a and R1234yf refrigerants. No important differences between cycle efficiencies were observed for both refrigerants. So it can be said that R1234yf is a great alternative for R134a in thermo- dynamically point of view. However, the same sentences cannot be used in terms of safety requirements. Many research studies have been pursued to determine the safety class of R1234yf today. If safety requirements have been satisfied by systems using R1234yf, global warming problem originated R134a emissions will be solved by using R1234yf instead of R134a.

Figure 12. Irreversibility variation of the expansion valve with the

evaporating temperatures of R134a and R1234yf.

Figure 13. Irreversibility variation of the evaporator with the evaporating

temperatures of R134a and R1234yf.

Figure 14. Total irreversibility variation of the system with the evaporating

temperatures of R134a and R1234yf.

Figure 15. The Grassmann diagrams for a vapor compression refrigeration

system using (a) R134a and (b) R1234yf.

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