Comparative Study of a Compression?Absorption Cascade System Operating with NH3-LiNO3, NH3-NaSCN, NH3-H2O, and R134a as Working Fluids

: This research presents a comprehensive bibliographic review from 2006 through 2020 about the state of the art of the compression?absorption cascade systems for refrigeration. In consequence of this review, this research identifies the significant development of systems that consider lithium bromide as a working fluid; however, the use of other working fluids has not been developed. This study is motivated toward the development of a parametric analysis of the cascade system using NH3-LiNO3, NH3-NaSCN and NH3-H2O in the absorption cycle and R134a in the compression cycle. In this study, the effect of the heat source temperature, condensation temperature in the compression cycle, the use of heat exchangers in the system (also known as economizers) and their contribution to the coefficient of performance is deepened numerically. The economizers evaluated are the following: an internal heat exchanger, a refrigerant heat exchanger, a solution refrigerant heat exchanger, and a solution heat exchanger. Mass and energy balance equations—appropriate equations to estimate the thermophysical properties of several refrigerant?absorbent pairs—were used to develop a thermodynamic model. The studied heat source temperature range was from 355 to 380 K, and the studied condensation temperature range in the compression cycle was from 281 to ?291 K; additionally, the importance of each economizer on the coefficient of performance was numerically estimated. In this way, NH3-NaSCN solution in the absorption cycle and R134a in the compression cycle provided promising numerical results with the highest COPs (coefficient of performance). Abstract: This research presents a comprehensive bibliographic review from 2006 through 2020 about the state of the art of the compression–absorption cascade systems for refrigeration. In consequence of this review, this research identifies the significant development of systems that consider lithium bromide as a working fluid; however, the use of other working fluids has not been developed. This study is motivated toward the development of a parametric analysis of the cascade system using NH 3 -LiNO 3 , NH 3 -NaSCN and NH 3 -H 2 O in the absorption cycle and R134a in the compression cycle. In this study, the effect of the heat source temperature, condensation temperature in the compression cycle, the use of heat exchangers in the system (also known as economizers) and their contribution to the coefficient of performance is deepened numerically. The economizers evaluated are the following: an internal heat exchanger, a refrigerant heat exchanger, a solution refrigerant heat exchanger, and a solution heat exchanger. Mass and energy balance equations—appropriate equations to estimate the thermophysical properties of several refrigerant–absorbent pairs—were used to develop a thermodynamic model. The studied heat source temperature range was from 355 to 380 K, and the studied condensation temperature range in the compression cycle was from 281 to –291 K; additionally, the importance of each economizer on the coefficient of performance was numerically estimated. In this way, NH 3 -NaSCN solution in the absorption cycle and R134a in the compression cycle provided promising numerical results with the highest COPs (coefficient of performance).


Introduction
In the literature, there are several reviews about vapor compression and innovative thermodynamic cycles for refrigeration as absorption technologies. Srikhirin et al. in 2011 [1] presented a review of absorption refrigeration technologies wherein the authors defined the principle of absorption, i.e., the single-stage, double-stage, half-stage and GAX configurations based on the advances of the time. It is interesting to note the introduction of absorption-compression systems and the thermodynamic cycles which consider a compressor and an ejector within the cycle. Lithium-bromide water and water-ammonia were enlisted as the main working fluids used in these kind of systems. Xu and Wang [2] analyzed four configurations involving absorption cycles with the following configurations analyzed: the single effect cycle, external-circuit coupling cycles, internal-circuit coupling cycles and the cycle combined with an ejector/compressor. The ejector or compressor cycles are used in the combined cycles to boost the cooling efficiency or to lower the temperature. She et al. [3] reviewed seven energy efficient and economic technologies for vapor compression refrigeration systems, in which the objective is to increase thermal efficiency; elements such as: ground heat exchangers, solar panel, cooling towers, desiccant rotor and thermal storage devices are studied throughout the different case studies. Papadopoulos et al. [4] revise the organic working fluids mixtures and the absorption refrigeration cycles available with the aim of generating cooling from the point of view of cleaner and sustainable development. The discussion and recapitulation of the information is intense; it is discussed as a future perspective in which the economic and sustainable prospects for absorption refrigeration cycles must be extended, and additionally, it provides a more reliable estimation of the properties of the working fluids.
As far as the authors are aware, there has been a continuous analysis and modification of compression systems and the absorption cycles separately. This work presents a summary of literature of the main advances in compression-absorption systems for refrigeration, assuming the cascade configuration. A summary is shown in Table 1; the compressive review considers the period from 2006 through 2020 and the contributions of the articles [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. As can be seen, the research line on cascade configuration involving the absorption cycle and compression cycle has been growing over time, and mainly carried out in the theoretical and thermodynamic field.
The main trends that have been reported over time in the study of this class of systems are: • The refrigerant-absorbent pair commonly worked is LiBr-H 2 O and NH 3 -H 2 O. Fernández-Seara et al. [5] and Cimsit et al. [7] developed simulations with NH 3 -H 2 O, presenting advantages and disadvantages regarding to LiBr-H 2 O. • The refrigerant that has been considered most frequently in the compression cycle is R134a, and less the NH 3 . Recently, models and designs have been reported that involve R410a, as in the work of Boyaghchi et al. [12]. Additionally, it proposes the use of H 2 O/Cu oxide (CuO) nanofluid as a fluid that transports energy from a solar collector to the generator. • New configurations have been proposed, which involve double-effect absorption or the satisfaction of specific refrigeration applications as food conservation, cooling in naval ships, or solar energy coupled into the cascade system. These designs consider LiBr-H 2 O in the absorption cycle. An aspect that has caught our attention is the fact that the number of heat exchangers (also known as economizers) that the system must involve has not been identified. Additionally, the heat exchanger effectiveness changes from author to author, in ranges from 0.6 to 0.85.
The relevant differences and novelties studies by Cimsit et al. [7], Boyaghchi et al. [12], Jain et al. [17], Karamangil et al. [24] and the present work are: • Thermodynamic analysis and simulation were presented by Cimsit et al. [7], Jain et al. [17], and Boyaghchi et al. [12] considering LiBr-H 2 O as the working fluid in the absorption cycle in several conditions, and the design consideration of solution heat exchanger. Then, Cimsit et al. [7] simulated the coefficient of the system performance considering NH 3 -H 2 O; while this work proposes an extension of the knowledge of Cimsit et al. [7] adding the evaluation of NH 3 -LiNO 3 , NH 3 -NaSCN in the absorption cycle.

•
The works carried out by Karamangil et al. [24] can be compared with the present manuscript, developing a thermodynamic model of a visual software package to analyze the absorption single-stage refrigerant system. The solutions under study were H 2 O-LiBr, NH 3 -H 2 O, NH 3 -LiNO 3 , and acetone-ZnBr 2 , while this study presents a theoretical thermodynamic model to compute the energy efficiency parameters in the compression-absorption cascade system considering the influence of several economizers. The energy balance equation and the coefficient of performance were considered in both papers as comparison parameters.

•
To the extent of our knowledge, the refrigerant-absorbent pair selected in this manuscript were NH 3 -LiNO 3 and NH 3 -NaSCN for the first time for the compression-absorption system, the numerical results are compared with NH 3 -H 2 O performance; while R134a refrigerant remains in the compression cycle.
The authors proposed that, the COP for the compression-double absorption systems were higher than those obtained with compression-single absorption systems. The results suggest the use of advanced cycles in the absorption process for cascade systems. None Experimental results such as: coefficient of performance, power and the temperature in different points of the system were analyzed by the authors.
It is interesting to note that, the authors present enough experimental information to say that cascade chiller allows savings of about 50% with respect of the application of a vapor compression unit only.

2018
Vaidhav Jain, Gulshan Sachdeva, SS Kachhwaha [17] R717 The authors propose a novel configuration of a compression-absorption cascade system; it includes the addition of intermediate compressor in the absorption cycle and a second solution heat exchanger.
Evaporators connected in series; the external flow connects to the two systems are considered in the configuration of the compression-absorption cycles.
Advanced exergy analysis of entire system was carried out; it means that energy and exergy analysis were formulated, and thermodynamic model computed to obtain the nature of irreversibility for each one of the pieces of equipment. The energy balance equation and the coefficient of performance were considered in both papers as comparison parameters. In the works previously presented, an aspect that has caught our attention is the fact that the number of heat exchangers (also known as economizers) that the system must involve has not been identified; also, the heat exchanger effectiveness changes from author to author, in ranges from 0.6 to 0.9. The goal is to show the influence of the economizer's effectiveness and present a performance evaluation of each exchanger within a compression-absorption cascade system.

System Description
The compression-absorption cascade system for refrigeration presented in this work consists of nine main components: a generator, a solution heat exchanger (SHE), an absorber, a solution refrigerant heat exchanger (SRHE), a condenser, a refrigerant heat exchanger (RHE), a cascade heat exchanger, an internal heat exchanger (IHE), a compressor and an evaporator showed in Figure 1. This kind of system is based on the union of two cycles in a cascade configuration, the absorption cycle needs two fluids: a substance capable of high absorption capacity and the refrigerant, which is capable of being evaporated and condensed with relative ease. Waste heat is used to separate the working fluid from the absorbent in the generator; a vapor stream is directed to the condenser at state 1, while the strong solution (with a high concentration of absorbent) goes to the absorber at state 12. Then, the refrigerant condenses at state 3 and its pressure is reduced to feed the cascade heat exchanger. On the one hand, isobaric evaporation is carried out in the cascade heat exchanger to produce a vapor stream at state 6; it is taken to produce absorption with the arrival of a strong solution to get a weak solution. The weak solution is pumped at state 9 to state 11 to finish the absorption cycle. On the other hand, for the cascade heat exchanger, the refrigerant at state 15 is isobarically condensed to state 16. The pressure of the refrigerant is reduced in the expansion valve to the evaporator pressure level. The evaporation of refrigerant from states 18 to 19 produces the main product of the entire system, the refrigeration effect. Then, the vapor stream is subject to mechanic compression to finish the refrigerant cycle. As can be seen, four heat exchangers were added in the configuration of the cascade system; they have the main objective of taking advantage of the temperature differential in the system and consequently to increase the coefficient of performance. The four economizers are described as following: 1.
Internal heat exchanger. Its objective is to reduce the work added by the compressor to the cycle, it exchanges the heat transfer from high pressure compressed liquid to saturated steam to low pressure; all previously in the compression cycle.

2.
Refrigerant heat exchanger. This heat exchanger was designed to increase the energy of the vapor stream at the cascade heat exchanger outlet, taking advantage of the condensed liquid obtained from the condenser.

3.
Solution refrigerant heat exchanger. This type of equipment is designed to operate between the two pressure levels of the absorption cycle, the strong LiBr solution and the refrigerant vapor are the working fluids that flow in the equipment. Solution heat exchanger. It is the heat exchanger commonly used in the literature for absorption cycles; its function is to preheat the solution from the absorber to the generator. Its use increases the use of the waste heat added in the generator.

Thermodynamic Modeling
Considering the characteristics of the cascade cycle, the following assumptions have been declared:  In the absorption cycle, the pump work necessary to circulate the solution is considered negligible [7].
 Pressure and temperature values could be worked with experimentally [9].
 Steady-state conditions are considered in this study.  Thermodynamic equilibrium at the inlet and outlet of the components is assumed  The heat losses from the equipment, the pressure drop in the piping, and the main components are considered negligible [20].  The expansion process in the valve is considered isenthalpic.  Solutions flowing out of the absorber and the generator are assumed to be saturated in equilibrium conditions at their respective temperatures and concentrations [24]. The principles of mass and energy conservation are developed for each component of the cascade system. The governing equations for a steady flow system are given according to the following equations.
The mass conservation:

Thermodynamic Modeling
Considering the characteristics of the cascade cycle, the following assumptions have been declared: • In the absorption cycle, the pump work necessary to circulate the solution is considered negligible [7]. • Pressure and temperature values could be worked with experimentally [9]. • Steady-state conditions are considered in this study. • Thermodynamic equilibrium at the inlet and outlet of the components is assumed • The heat losses from the equipment, the pressure drop in the piping, and the main components are considered negligible [20].

•
The expansion process in the valve is considered isenthalpic.

•
Solutions flowing out of the absorber and the generator are assumed to be saturated in equilibrium conditions at their respective temperatures and concentrations [24].
The principles of mass and energy conservation are developed for each component of the cascade system. The governing equations for a steady flow system are given according to the following equations.
The mass conservation: where m represents the mass flow rate in (kg/s) that goes in and out into control volume.
The conservation of species in the solution are depicted according to the following equation: where x is mass concentration of solution in the absorption cycle. The first law of the thermodynamics was applied according to the following equation: where h is the specific enthalpy in (kJ/kg) at each state point, Q the heat load that goes in and out into control volume in (kW), and W the work in (kW). The coefficient of performance was determined as the refrigeration capacity (Q evap ) per unit of heat and work added in the generator (Q gen ) plus compressor (W comp ). The following provides the calculation of coefficient of performance (COP): For the calculation of thermophysical properties of the solution and working fluids considering in this study, the following references were used:

•
The NH 3 -H 2 O thermophysical properties are obtained from the correlations provided by M. Conde Engineering [25].

•
The NH 3 -LiNO 3 thermophysical properties are evaluated by functions obtained by Libotean et al. [26,27] for vapor pressure, density and specific heat capacity, • Infante Ferreira's [28] correlation has been used to obtain the specific enthalpy of solution.

• For the NH 3 -NaSCN solution, vapor pressure and enthalpy correlation equation coefficients provided by D Cai et al. [29] and •
The correlation of density was provided by Chaudhari et al. [30] Furthermore, the calculation of the thermophysical properties of the NH 3 and R134a is achieved through a coupling with the "Coolprop" open-source thermodynamic properties database [31].
In accordance with previous hypotheses, the balances of mass, energy, and thermophysical properties calculation for each system, besides the simulation code and thermodynamic analysis are presented next.

Results and Discussion
This section presents the main numerical results in the following order: (1) comparison between the numerical results of this research and a model reported by other authors, (2) thermodynamic results in each state of the entire system for future design, (3) the effect of the heat source temperature, evaporation temperature in the compression cycle, the heat exchanger effectiveness, and the contribution of each economizer in the cascade system.

Model Validation
For validation of the present work, the simulation results are compared with research work reported by Cimsit and Ozturk [7], which is presented in Table 2. The relative error was calculated for each item available; the following equation was applied: The evaluated energy flow rates in all parts of the system are according to [7]. A maximum relative error was calculated to 0.0287. Agreement in values of COP, heat flow, and work in the components prove that the model is appropriate. Tables 3-5 show the thermodynamic data for the compression-absorption system with four economizers operating with NH 3 -LiNO 3 , NH 3 -NaSCN and NH 3 -H 2 O solutions in the absorption cycles and R134a in the compression cycle, all under the conditions of T gen = 363 K, T evap1 = 263 K, T abs = T con1 = 313 K, ε HX's = 0.8, 50 kW of refrigeration load. Table 3. Thermodynamic data of the compression-absorption cascade system using NH 3 -LiNO 3 solution and R134a as working fluid.   Table 6 shows a comparison between the heat capacities and work of the components of the proposed cascade cycles, and the traditional vapor compression refrigeration cycle. As can be seen, for the same cooling load, there is a decrease in the compressor work of 51.46%, it is the main reason why the COP increases.  Figure 2 shows the coefficient of performance for the compression-absorption systems as a function of the heat source temperature; for simulation T 16 = 291 K and T 6 = 283 K remains constant. For the numerical results, three evaporation temperatures in the absorption cycle were selected: 253 K, 263 K, and 273 K. As can be seen, for all working fluids, if the evaporation temperature increases the COP increases. The performance of the cycles that consider NH 3 -H 2 O and NH 3 -LiNO 3 are very similar, and even the same for heat source temperature higher than 365 K. The COPs are approximately 13.3% higher for of compression-absorption system considering NH 3 -NaSCN than those obtained with NH 3 -H 2 O.  Figure 3 shows the coefficient of performance against the T16 from 281 K to 291 K for all systems described above. The simulation considers, simultaneously, a change of T6 from 273 to 283 K, which allows us to maintain a temperature differential of 8 K between the condensation temperature in the compression cycle and evaporation temperature in the absorption cycle. It can be observed that the COPs increase with an increase of T16 keeping the consideration described for the cascade heat exchanger effectiveness constant.  Figure 3 shows the coefficient of performance against the T 16 from 281 K to 291 K for all systems described above. The simulation considers, simultaneously, a change of T 6 from 273 to 283 K, which allows us to maintain a temperature differential of 8 K between the condensation temperature in the compression cycle and evaporation temperature in the absorption cycle. It can be observed that the COPs increase with an increase of T 16 keeping the consideration described for the cascade heat exchanger effectiveness constant. Figure 3 shows the coefficient of performance against the T16 from 281 K to 291 K for all systems described above. The simulation considers, simultaneously, a change of T6 from 273 to 283 K, which allows us to maintain a temperature differential of 8 K between the condensation temperature in the compression cycle and evaporation temperature in the absorption cycle. It can be observed that the COPs increase with an increase of T16 keeping the consideration described for the cascade heat exchanger effectiveness constant. The results in Figure 4 can explain the increase in COP. The COP in the compression cycle is reduced, due to the increase in the pressure difference and the work of the compressor. This behavior is the same for all the systems under study, because R134a is used in the compression cycle. Figure 4 illustrates the COP in the absorption cycle, it increases due to the increase of T6 in the simulation. As The results in Figure 4 can explain the increase in COP. The COP in the compression cycle is reduced, due to the increase in the pressure difference and the work of the compressor. This behavior is the same for all the systems under study, because R134a is used in the compression cycle. Figure 4 illustrates the COP in the absorption cycle, it increases due to the increase of T 6 in the simulation. As can be seen, in the range of T16 from 281 to 285 K the system with the highest COP was NH 3 -LiNO 3 , while in the range of T16 from 285 to 291 K the system with the highest COP was NH 3 -NaSCN. This is evidence that the combination of NH3-NaSCN would show the best performance when the compressor consumption increases and COP in the compression cycle decreases. can be seen, in the range of T16 from 281 to 285 K the system with the highest COP was NH3-LiNO3, while in the range of T16 from 285 to 291 K the system with the highest COP was NH3-NaSCN. This is evidence that the combination of NH3-NaSCN would show the best performance when the compressor consumption increases and COP in the compression cycle decreases.  Figure 5 depicts the comparison of all compression-absorption systems considered in this study, the plot considers a heat exchanger with known effectiveness from 0 to 1, and its corresponding coefficient of performance is calculated. As it can be seen, the system with the highest coefficient of performance is the system with the NH3-NaSCN solution in the absorption cycle and R134a in the compression cycle; a similar trend has systems that consider NH3-H2O and NH3-LiNO3 in the absorption cycle; in the range under study, their discrepancy is less than 6.5%. As shown, the much higher improvement in COP was calculated considering the SHE compared to RHE, SHRE and IHE;  Figure 5 depicts the comparison of all compression-absorption systems considered in this study, the plot considers a heat exchanger with known effectiveness from 0 to 1, and its corresponding coefficient of performance is calculated. As it can be seen, the system with the highest coefficient of performance is the system with the NH 3 -NaSCN solution in the absorption cycle and R134a in the compression cycle; a similar trend has systems that consider NH 3 -H 2 O and NH 3 -LiNO 3 in the absorption cycle; in the range under study, their discrepancy is less than 6.5%. As shown, the much higher improvement in COP was calculated considering the SHE compared to RHE, SHRE and IHE; for all simulations, the use of IHE could be considered negligible when deciding to build the compression-absorption cascade system. Special attention should be paid to SHE, as it is the heat exchanger that showed a significant contribution in the COP. The results of the coefficient of performance for the compression-absorption cascade system using NH 3 -LiNO 3 solution and R134a refrigerant as working fluids are described. The COP increases rapidly when the solution heat exchanger is used, an increase of ≈42% is calculated when the solution heat exchanger increases from 0.6 to 0.85. If the refrigerant heat exchanger is only considered, the COP increases ≈2.9% when the heat exchanger's effectiveness was increased from 0.6 to 0.85. The use of the refrigerant solution heat exchanger and internal heat exchanger does not represent a significant contribution to the increase in COP. The behavior of the coefficient of performance of the compression-absorption cascade system considering each one of the economizers using the NH 3 -NaSCN solution and the R134a refrigerant as working fluids are analyzed. The heat exchanger effectiveness is changed in the range of 0 to 1, as depicted in Figure 5. If the heat exchanger effectiveness is fixed to 0.8, a common value for clean and new heat exchangers, the greatest contribution to the coefficient of performance was made by the solution heat exchanger, followed by the refrigerant heat exchanger and solution refrigerant heat exchanger. The contribution of the internal heat exchanger was insignificant. The maximum point of the coefficient of performance was 0.479 considering the heat exchanger effectiveness equal to 0.8. As can be seen, the coefficient of performance in the compression-absorption cascade system considering the NH 3 -H2O solution and R134a as working fluids increased linearly when the refrigerant heat exchanger, solution refrigerant heat exchanger and internal heat exchanger were simulated. According to what is illustrated in Figure 5c, the influence of the solution heat exchanger of the entire coefficient of performance produces an increase from 0.319 to 0.45 when the heat exchanger effectiveness was increased from 0 to 0.8. simulated. According to what is illustrated in Figure 5c, the influence of the solution heat exchanger of the entire coefficient of performance produces an increase from 0.319 to 0.45 when the heat exchanger effectiveness was increased from 0 to 0.8.

Conclusions
A compressive and synthesized literature review and thermodynamic modeling of the compression-absorption cascade system considering several economizers were carried out for refrigeration purposes using NH3-LiNO3, NH3-NaSCN, NH3-H2O, and R134a as working fluids. The main results of the present paper are the following:

Conclusions
A compressive and synthesized literature review and thermodynamic modeling of the compression-absorption cascade system considering several economizers were carried out for refrigeration purposes using NH 3 -LiNO 3 , NH 3 -NaSCN, NH 3 -H 2 O, and R134a as working fluids. The main results of the present paper are the following: • For the refrigerant-absorbent pairs considered in this analysis, NH 3 -NaSCN in the absorption cycle and R134a in the compression cycle showed the highest coefficient of performances considering the heat source temperature and evaporator temperature conditions described in this work.

•
Approximately 50% of the work consumed in the compressor can be reduced in a cascade system using NH 3 -NaSCN and R134a as working fluids, compared to a traditional vapor compression refrigeration system. • The solution heat exchanger was the economizer that significantly benefits the coefficient of performance of the compression-absorption cascade system for all working fluids under study. The refrigerant heat exchanger is the economizer that secondly contributes a benefit to the performance of the entire system. The correct selection, cleaning, and maintenance of these economizers is suggested, to guarantee a greater heat exchanger effectiveness than 0.6.

•
The contribution of a solution refrigerant and an internal heat exchanger was not theoretically significant in the coefficient of performance for all working fluids under study.
Future research could consider the theoretical study of the compression cycle and double-effect-absorption cycles in the cascade configuration using NH 3 -LiNO 3 , NH 3 -NaSCN, NH 3 -H 2 O and eco-friendly refrigerants in the compression cycle.