Energy and exergy analysis of absorption- compression cascade refrigeration system

Abstract

Keywords: Absorption; compression; COP; exergy destruction; exergetic efficiency

Introduction

There are various applications which require temperature in the range of - 30°C to - 100°C. It includes food preservation, rapid freezing, ice production, storage of medical products, drugs and so on. It is not economical to obtain temperature below -40°C, using single stage vapour compression refrigeration (VCR) system or single stage vapour absorption refrigeration (VAR) system [1]. Therefore, in order to obtain such low temperatures cascade refrigeration system is the best choice. The conventionally employed two stage cascade compression refrigeration systems consume a lot of electricity. About 56% of total electricity generation in India is

cascade refrigeration system is limited. In this study attention is focussed on the components which are the major sites of exergy destruction. It is also endeavoured to find out the effects of various operating and design parameters on exergy destruction in different components, COP and exergetic efficiency.

Fernández-Seara et al. [9] carried out a study on compression-absorption cascade system. Ammonia-water was the working substance in absorption refrigeration cycle whereas carbon dioxide was used as a refrigerant in compression cycle. The COP of 0.253 was reported by them. Kairouani and Nehdi [10] proposed a geothermal energy driven absorptioncompression cascade refrigeration system and reported the COP of 5.4-6.2 (excluding pump work and generator heat load). Garimella et al. [11] developed a computational model of a waste heat driven single effect LiBr/H2O absorption-subcritical CO2 compression cycle for megawatt scale low temperature (-

System Description

40 ) cooling for high heat flux electronic application. They reported that this novel cascade cooling system consumed 31% less electricity than the equivalent VCR system. Cimsit and Ozturk [12] used different refrigerants and working pairs for the analysis of compression-absorption cascade systems. Ammonia, R134a, R-410A were used in compression section while NH3/H2O and LiBr/H2O were used as working substances in absorption section of cascade cycle. It was concluded that electricity consumption in cascade systems is 48-51% lower than conventional VCR systems. They also reported that LiBr/H2O based cascade system outperformed NH3/H2O based cascade refrigeration system by registering 33% higher COP. Wang et al. [13] studied the solar assisted R134a compressionLiBr/H2O absorption cascade refrigeration system. Electric power consumption was reported to be lower by 50% in comparison with VCR system. Jain et al. [2] performed the first law and second law based thermodynamic analysis of cascaded vapour compression-absorption system (CVCAS) which consists of single effect VAR system coupled with VCR system. The electric power consumption in CVCAS was 61% lower than that in VCR system for same operating conditions. Colorado and Velazquez [14] carried out exergy based thermodynamic analysis of compression-absorption refrigeration cycle using NH3, CO2 and R134a in VCR section and H2O-LiBr in VAR section so as to find out best working substance and suitable operating parameters. It was shown that highest irreversibility occurs in cascade condenser, accounting for around 19.96%, 19.31% and 13.28% of the total irreversibilities using NH3, CO2 and R134a respectively. A thermodynamic analysis of compression-absorption cascade refrigeration system using modified Gouy-Stodola equation was carried out by Jain et al. [15]. In their study they obtained the optimum temperature of cascade condenser which corresponds to minimum irreversibility and maximum COP of the system. Further, a comparative study of compression-absorption cascade refrigeration system and two stage vapour compression refrigeration system (TSVCS) reveals that primary energy consumption of compression absorption cascade system is 60.6% less and electrical COP is 153.6% more than that of TSVCS. It is obvious from the literature review that though a lot of work on energy based analysis is reported, yet the exergy based thermodynamic analysis of absorption-compression

Qe Figure 1. Absorption-compression cascade refrigeration system A cascade refrigeration system in general comprises of a low temperature circuit (LTC) and a high temperature circuit (HTC). Actual cooling load is supplied to LTC while its heat of condensation acts as a cooling load to HTC which ultimately rejects heat to the surroundings. Thus LTC and HTC are coupled through a common heat exchanger referred to as cascade condenser, which acts as condenser for LTC refrigeration cycle and evaporator for HTC refrigeration cycle. In case of absorption-compression cascade refrigeration system, VAR cycle is utilized in HTC and VCR cycle is employed in LTC. In this study single effect LiBr-H2O vapor absorption refrigeration cycle is used in HTC and R134a, CO2 and NH3 have been considered as refrigerants in the compression stage as shown in Fig.1. The compression system comprises of the evaporator, compressor, condenser and an expansion device. The major components of single effect VAR system are the absorber, generator, condenser, evaporator, solution heat 996

exchanger, pump, solution throttle valve and a refrigerant throttle valve. The single effect absorption cycle is separately described by authors [16].

Exergy Balance By the application of second law of thermodynamics, exergy destruction in each component of the absorptioncompression cascade refrigeration system is obtained and furnished below:

 10 h10  T0 s10   m  6 h6  T0 s6   m  1 h1  T0 s1  E Dabs  m

(c) Heat losses and pressure drops in connecting lines and various components are neglected.

E Dgen  m 3 h3  T0 s3   m 4 h4  T0 s 4   m 7 h7  T0 s7 

(d) The subcooling and superheating in discharge and suction lines are neglected.

 T   Q gen 1  0  T gen    7 h7  T0 s7   m  8 h8  T0 s8  E Dcond  m

Thermodynamic Analysis

E Dcc  m r (h9  h10 )  T0 ( s9  s10 )   m r ,ltc (h12  h13 )  T0 ( s12  s13 ) 

(e) The expansion process is isenthalpic. (f) The solutions at the exit of generator and absorber are saturated in equilibrium at their respective concentrations and temperatures. (g) Reference environmental temperature and pressure are 25°C and 101.3 kPa respectively. The thermodynamic analysis of compression absorption system involves the principles of mass conservation, energy conservation and exergy balance. Mass Balance The mass flow rate through each component of low  r ,ltc . It is calculated using eqn. (1). temperature circuit is m

E Dtotal  E Dabs  E Dgen  E Dcond  E Dcc  E Dshe  E Drtv,htc  E Dstv  E Dcomp  E Devap  E Drtv,ltc

 10h10  m  6 h6  m  1h1 Q abs  m  h m  h m  h Q  m

 r is mass flow rate of refrigerant through condenser and Here, m evaporator.

E Dshe  m s (h2  h3 )  T0 ( s2  s3 )   m w (h4  h5 )  T0 ( s4  s5 ) 

Model validation A simple steady state simulation model based on sequential modular approach has been developed and implemented in a computer program using EES software [18]. The model equations are formulated from species, mass, energy and exergy balances. The thermodynamic model of absorptioncompression cascade refrigeration system developed in this work is validated by the numerical data of Cimsit and Ozturk [12]. R134a and LiBr-H2O are considered as working substances in VCR and VAR cycles for validation.

Table 2 clearly indicates that there exist good agreements between the present data and those provided by Cimsit and Ozturk [12]. The maximum error is , which may be attributed to the usage of different correlations for the calculation of thermophysical properties of LiBr-H2O.

Exergy destruction ratio In order to identify and compare the sites of thermodynamic inefficiencies from the point of view of exergy analysis, the exergy destruction ratio Yd , k for each

Table 2. Comparison of performance data of present model with that of Cimsit and Ozturk [12]

component is calculated. It is defined as the ratio of exergy destruction rate in a component to the total exergy destruction rate of the system [17] and it is expressed as:

Initial/operating parameters The parameters assumed for computation of results are mentioned in Table 1.

Approach in cascade condenser (OT) Difference between evaporator and space temperature (DT)

Results And Discussion

For the base case the operating parameters considered are as follows: generator, condenser, evaporator, cascade condenser temperatures are 85°C, 40°C, -45°C and 7°C. The absorber temperature is same as the condenser temperature. The approach in cascade condenser is 7°C, Isentropic efficiency of compressor is 0.8 and effectiveness of solution heat exchanger is 0.7. In order to perform the parametric analysis one parameter is varied within the given limits while others are kept constant.

First law analyses Figures 2(a), (b) and (c) respectively show the variation of COP, generator heat load and absorber heat load with the generator temperature. It is clear from Fig. 2(a) that absorption-compression cascade refrigeration system achieves maximum COP at a particular generator temperature. The maximum values of COP for NH3, CO2 and R134a are 0.4307,

0.3874 and 0.4316 respectively and the corresponding generator temperature is 98°C. The COP of the system operating with R134a is approximately 10% higher than that obtained with CO2. The COP of NH3 system is marginally lower than that of R134a based system. The variation of COP with generator temperature depends inversely on the variation of generator heat load for constant cooling capacity system. It is obvious from Fig 2(b) that there exists a generator temperature corresponding to which generator heat load is minimum. The value of generator temperature corresponding to minimum generator heat load is equal to that corresponding to maximum COP. Considering the absorber heat load, shown in Fig. 2(c), it can be stated that highest amount is delivered in case of CO2 whereas lowest in case of R134a. Fig. 3(a) shows the COP of absorption-compression cascade refrigeration system as a function of evaporator temperature. As evaporator temperature is increased from -55°C to -35°C, increase in COP is registered.

(c) Figure 2. (a) COP, (b) generator heat load and (c) absorber heat load against generator temperature

circulation ratio (=Xw/ (Xw-Xs)) decreases. The reduction in solution circulation ratio decreases the heat required in the generator as shown in Fig. 4(c). Thus, COP of cascade refrigeration system may increase or decrease depending upon the increase in compressor power requirement and reduction in generator heat duty. It is clear from Figs. 4(a), (b) and (c) that maximum values of COP occur at different cascade condenser temperature for different refrigerants. In case of NH3, maximum COP is 0.4249 and it occurs at 12.47°C whereas for R134a and CO2 the maximum COP values are 0.4237 and 0.3743, occurring at 11.45°C and 8.64°C respectively. For all values of cascade condenser temperature, the performance of CO2 based system is inferior to the performance of the other two systems. However, the NH3 based system and R134a based system perform differently at different cascade condenser temperature, though the difference is marginal. It can be observed that NH3 based system outperforms R134a based system at higher cascade condenser temperature, particularly above 10°C for the conditions considered in this study.

(c) Figure 3. (a) COP, (b) generator load and (c) work of compression against evaporator temperature

The COP of R134a based system increases from 0.3695 to 0.4605 whereas the COP of CO2 based system increases from 0.3261 to 0.4214. The COP of NH3 based system is lower but closer to R134a based system. In Fig. 3(b) it can be seen that as evaporator temperature decreases from 35°C to -55°C, heat required in generator increases from 92 to 108 kW, 98 to 119 kW and 102 to 109 kW respectively for R134a, CO2 and NH3 based systems. The decrease in compressor work of absorption-compression cascade refrigeration system with increase in evaporator temperature is shown in Fig. 3(c). As evaporator temperature increases from 55°C to -35°C, compressor work decreases by almost 40% for all the refrigerants. It means rise in evaporator temperature by 1°C can reduce the electricity consumption by 2%. Carbon dioxide based system requires highest electrical input (20-35 kW) in the form of compressor work while R134a based system needs minimum electrical energy (16-27 kW). The cascade condenser temperature is one of the most important design parameters as it has intense effect on the performance of the absorption-compression cascade refrigeration system. Its lowest value is restricted to be above 0°C when LiBr-H2O is the working substance in VAR subsystem because it depends on the freezing point of water. From Fig. 4(a) it can be seen that with the increase in cascade condenser temperature from 2 to 16°C, COP first increases, achieves a maximum and then reduces. The increase in cascade condenser temperature causes increase in absorber pressure, increase in pressure ratio across the compressor and increase in mass flow rate in VCR subsystem. The first two factors account for increase in compressor power as shown in Fig. 4(b) while the last factor i.e. increase in absorber pressure accounts for decrease in strong solution concentration (Xs). The weak solution concentration (Xw) remains constant. Hence, solution

Figure 5. Exergetic efficiency and total exergy destruction rate against generator temperature

Figure 4. (a) COP, (b) generator load and (c) work of compression against cascade condenser temperature

Figs. 6(a), (b) and (c) respectively show the variation of exergy destruction ratio of the main components of NH3LiBr/H2O, CO2- LiBr/H2O and R134a- LiBr/H2O absorptioncompression cascade refrigeration systems with generator temperature. At the generator temperature corresponding to maximum exergetic efficiency, the sites of major irreversibilities are different for different working substances. The absorber, the cascade condenser and the condenser are the major contributors of exergy destruction for NH3-LiBr/H2O system with exergy destruction ratio of 24.57%, 22.92% and 15.61% respectively. In case of CO2- LiBr/H2O system the

Second law analyses Fig. 5 shows the exergetic efficiency and exergy destruction rates of R134a, ammonia and carbon dioxide based systems versus generator temperature at design conditions. It indicates a maximum value of exergetic efficiency (

and a minimum value of exergy destruction rate as increases from 80°C to 110°C.The variation of exergetic efficiency with generator temperature can be explained in . However, similar way as variation of COP with temperature of the heat source is additional factor which govern the exergetic efficiency, resulting in higher slope of exergetic efficiency as compared to corresponding COP. The maximum values of exergetic efficiency for NH3, CO2 and R134a based absorption-compression cascade refrigeration system are 35.43%, 29.99% and 35.56% respectively, occurring at same generator temperature of 85°C. The corresponding minimum values of exergy destruction rates are 24.43 kW, 31.29 kW and

Conclusions

The following conclusions are drawn from the present study: (a) The maximum value of COP and exergetic efficiency occur corresponding to different generator temperatures. The maximum exergetic efficiency occurs at a temperature lower than the generator temperature corresponding to maximum COP. (b) The generator temperature has greater impact on COP while exergetic efficiency is comparatively less affected by it. An increase of around 28% in COP is found with the increase in generator temperature whereas the corresponding increase in exergetic efficiency is about 11%. (c) Maximum COP and maximum exergetic efficiency occur at different cascade condenser temperature for different refrigerants in VCR subsystem. Maximum COP occurs at higher cascade condenser temperature than at which maximum exergetic efficiency is obtained. (d) The sites of highest irreversibilities are different for different refrigerants. For NH3-LiBr/H2O system it is cascade condenser, for CO2-LiBr/H2O system it is refrigerant throttle valve of VCR subsystem and in case of R134a-LiBr/H2O system it is compressor. (e) At design point, R134a-LiBr/H2O absorptioncompression cascade refrigeration system is the best performer from the view point of both first law and second law of thermodynamics.

Nomenclature

Specific enthalpy (kJ.kg-1) High temperature circuit Low temperature circuit Mass flow rate (kg.s-1) Heat transfer rate (kW)

Temperature ( or K) Vapour absorption refrigeration Vapour compression refrigeration Exergy destruction ratio work transfer rate (kW)

Greek letters Effectiveness of heat exchanger efficiency Exergetic efficiency

Figure 12. Exergy destruction ratio of main components against compressor isentropic efficiency for (a) Ammonia (b) Carbon dioxide and (c) R134a refrigerants

Cascade condenser Compressor Evaporator Generator High temperature circuit Low temperature circuit pump Refrigerant, room Refrigerant throttle valve Strong solution Solution heat exchanger Solution throttle valve Weak solution

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Acknowledgments

The support of Ministry of New and Renewable Energy (MNRE), government of India is duly acknowledged.

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