Energy exergy and sustainability analysis of two-stage vapour compression refrigeration system

Abstract

Keywords: COP; Irreversibility; Exergetic efficiency

Introduction

Refrigeration technology based on the principle of rejection of heat to the surrounding at higher temperature and absorption of heat at low temperature evaporator [1], expansion valve, condenser and compressor are the main four components of single stage vapour compression system. Vapour compression refrigeration systems consume large amount of electricity. This difficulty can be removed by improve the performance parameters of system. Coefficient of performance and exergetic 440

replace R134a with these hydrocarbon mixtures. They observed that mixture of propane 50%, butane 40%, and isobutane 10% was best hydrocarbon mixture to replace R134a. Jung et al. [19], Arcaklioglu [20], and Arcaklioglu et al. [21] suggested to use of pure hydrocarbon instead of their mixtures due variation in condenser and evaporator temperature during phase changing at constant pressure. These changes in condenser and evaporator temperature cause for problem in vapour compression refrigeration cycle. Liedenfrost et al. [22] investigated freon as refrigerant on the performance of a refrigeration cycle Through above literature, it was found that energy, exergy and sustainable analysis of single stage vapour compression refrigeration systems have been done. But no literature contributed for energy and exergy analysis of two-stage vapour compression refrigeration system. Present works analyze the system in terms of energy and exergy efficiencies and explain the effect of exergy losses on two-stage vapour compression refrigeration system with hydrocarbons and R134a.

energy technologies. Utilization of green energy can be increased by this method [2-4]. In past decades, refrigerants such as R12, R02, R22 etc. used in vapour compression refrigeration system responsible for increasing of global warming and ozone depletion potential. An international society named Montreal protocol discussed and signed on the refrigerants having higher global warming and ozone depletion potential values for all countries. In order to control the emission of greenhouse gases one more committee was formed named as Kyto protocol [5]. After 90’s a program was ran to phase out the higher GWP and ODP refrigerants (CFC and HCFC) for the purpose of environmental problems. To replace “old” refrigerants with “new” refrigerants lots of researches has been carried out [6-11]. Selladurai and Saravanakumar [12] evaluated performance parameters such as COP and exergetic efficiency with R290/R600 hydrocarbon mixture on a domestic refrigerator designed to work with R134a and observed that performance of same system is higher with R290/R600a hydrocarbon mixture compared to R134a. In their analysis condenser, expansion valve and evaporator showing lower exergy destruction compared to compressor. Reddy et al. [13] presented theoretical analysis of R134a, R143a, R152a, R404A, R410A, R502 and R507A in vapour compression refrigeration system and effect on coefficient of performance and second law efficiency with variation of superheating of evaporator outlet, evaporator temperature and degree of subcooling at condenser outlet, vapour liquid heat exchanger effectiveness and degree of condenser temperature was discussed. They reported that COP and exergetic efficiency significantly affected with change of evaporator and condenser temperatures and also observed that R134a and R407C show highest and lowest performance in all respect. Kumar et al. [14] carried out energy and exergy analysis of single stage vapour compression refrigeration system using R11 and R12 as working fluids. Evaluation in terms of COP, exergetic efficiency and exergy losses in different components (compressor, evaporator, expansion valve and condenser) was done. Cornelissen[15] proposed that non-renewable energy sources are useful for minimizing the irreversibility of the system for sustainable development of systems. He also observed that emissions of gases put adverse effect on environmental conditions. In Nikolaidis and Probert [16]’ study, effect of condenser and evaporator temperatures on two-stage vapour compression refrigeration system using R22 was studied and suggested that there is requirement to optimize the condenser and evaporator conditions. Many researchers carried out researches on different proportion of hydrocarbons as working fluid in vapour compression refrigeration systems. Fatouh and Kafafy [17] suggested to replace R134a with hydrocarbon mixtures such as propane, propane/isobutane/n-butane mixtures, butane, and various propane mass fractions in domestic refrigerator. Pure butane showed high operating pressures and low coefficient of performance among considered refrigerants. Wongwises et al. [18] did experimental investigation on automotive airconditioners with isobutene, propane, butane and suggested to

Compression Refrigeration System

Some mathematical calculations are required to analyze the two-stage vapour compression refrigeration system based on energy and exergy method. Two stage vapour compression refrigeration system consist of low and high pressure compressor, condenser, evaporator, expansion valves, waterintercooler and flash chamber. Energy and exergy efficiencies are different for different refrigerants for same system. Following assumptions are taken for thermodynamic analysis of the system:

Temperature and pressure losses are not considered. All components are running under steady state conditions. Energy and exergy losses due to potential and kinetic energy are neglected. Mechanical efficiencies of low and high pressure compressors are assumed to be 80%.

Two stage vapour compression refrigeration system and its P-H plot shown in Figure 1 and Figure 2 respectively. Exergy, energy and sustainability analysis can be done as follow:

Results And Discussion

In this discussion effect of change of evaporator and condenser temperature on performance parameters like coefficient of performance, exergy loss, exergetic efficiency and sustainability index was studied for considered refrigerants Change of coefficient of performance with change in evaporating temperature for considered refrigerants As cleared from Table.1 that coefficient of performance of R134a is 4.2-8.9% and 3.6-5.3% lower than R152a and R600 respectively or in other words R134a consumes more electricity than R152a and R600.Ambient condition play an important role in electricity consumption of vapour compression refrigeration systems because higher the temperature difference between system and surrounding higher will be compressor work that’s why COP of vapour compression refrigeration system increase with increase in evaporator temperature and decrease with decrease in evaporator temperature.

Evaporating Temperature For Different Refrigerants

    
     !   T   !  For evaporator 
 %     T %       )

For condenser 
 !  2   T !  2   &/01 '1     )*+, )3

For expansion valves    "4
 2  5   T 2  5    6  %   T 6  %  (6) For water-intercooler  (7)   78
   9   T   9  For flash chamber    :/
 9     T 9      5  (8)  6   T  5   6  Exergetic efficiency ;<

COP of considered system with R152a, R600 ,R134a ,R600a ,R290 ,R410a,R1234yf and R404a varied between 1.454.88,1.40-4.86,1.33-4.69,1.33-4.72,1.30-4.57,1.31-4.43,1.304.47 and 1.12-4.15 respectively between -50 oC to 5 oC evaporator temperature.

irreversible losses occurred in the system. He also found that with increase in difference between evaporator and condenser temperatures exergy losses increases with R12, R134a, R22, and R502 used as refrigerants. 12

Change of exergy loss with change in evaporating temperature for considered refrigerants As shown in Fig. 3 exergy destructions or exergy losses decreases with increase of evaporator temperature. This is because that if evaporating temperature decreases the heat exchange between working fluid entered into the evaporator tubes and space being cooled also decreases, which finally decrease the cooling effect and therefore exergy destruction increases. Among selected refrigerants R404a (6.02-30.83 KW) and R152a (4.94-22.06 KW) shows higher and lower exergy loss for selected evaporator temperature range respectively. It was also observed that flash chamber, compressor, condenser, expansion valve, water-intercooler and evaporator are in increasing order of exergy loss for different refrigerants.

Evaporating Temperatures For R152a

Change of exergy loss with change in condensing temperature for considered refrigerants It is observed from Fig. 5 that for all considered refrigerants exergy destructions increased with increase of condensing temperature. This is due to increase of temperature difference between condenser and surrounding.

Evaporating Temperature For Different Refrigerants

Condensing TemperatureTCond (oC)… Change of exergy loss with change in evaporating temperature for R152a as working fluid

Condensing Temperatures For Different Refrigerants

Fig. 4 shows the variation of exergy loss for individual component with change in evaporating temperature with R152a used as working fluid. Behaviors of exergy destruction in different components of two stage vapour compression refrigeration system for rest of refrigerants are also observed similar. Flash chamber responsible for highest and evaporator shows lowest exergy destruction compared to other components. The exergy destruction in the components increase with the decrease of evaporating temperature.Yumrutas et al. [23] observed the effect on exergy loss with change of evaporation and condenser temperature. Khan [24] studied that due to the low expansion process and compressor efficiency most of the

Change of exergetic efficiency with change in evaporating temperature for considered refrigerants It is found that exergy losses decreases with increase of evaporating temperature for considered refrigerants. Fig. 6 shows that R152a gives highest exergetic efficiency among selected refrigerants. The purpose of condenser to take out the heat produced by compressor in discharge line and carried by refrigerant during cooling effect in evaporator. This heat in 443

refrigerant removed by transferring heat to the wall of condenser tubes due to convection and then transfer of heat due to conduction from tubes wall to surrounding.

However the performance of R152a and R600 is higher than R134a but hydrocarbons are flammable in nature and can be used in limited applications. Therefore R134a recommended for all kind of applications.

Nomenclature

CFC chlorofluorocarbon Variation of sustainability index with change in evaporating temperature for considered refrigerants As shown in Fig. 7 with increase in evaporator temperature sustainability index increases for selected refrigerants. R152a shows higher sustainability index than R134a for selected evaporating temperature range. It is also found that R152a and R600 have higher sustainability index and low impact on surrounding compared to R134a.

Conclusions And Recommendation

Energetic and exergetic analysis of two stage refrigeration system was carried out with different refrigerants and following conclusion and recommendation are presented below:

5. Sustainability index of the R152a and R600 are higher than

that of R134a at every evaporator temperature. It indicates also less environmental impact for hydrocarbons.

ambient conditions water intercooler high temperatur e low temperatur e low pressure expansion valve generation

hydrochlorofluorocarbo n rate of heat transfer (kW) work (kW)

ozone depletion potential global warming potential mass flow rate (kg/s)

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