Energy and exergy analysis of a combined system: cascade organic Rankine cycle and cascade refrigeration cycle

In this paper, a new combined system is proposed for recovering thermal energy at medium temperature using a cascade organic Rankine cycle to feed a cascade refrigeration cycle. Energy and exergy analysis is applied to the combined system to determine its performance using different working fluids under the same operating conditions taking into account the effect of some operating parameters and the selection of organic fluids on cycle performance. The pair of organic fluid (Toluene/R245fa) used for the cascade organic Rankine cycle and the pairs (R717/R744, R717/R23, R134a/R23) used for the cascade refrigeration cycles. The results show that the combined system function with the couple (R717/R23) for cascade refrigeration cycle gives better exergy efficiency 50.03% compared to other couples, 49.57% for the couple (R717/R744), and 48.01% for the couple (R134a/R23). The thermodynamic evaluation shows that the operating temperatures, such as the cascade organic Rankine cycle evaporation temperature and the cascade refrigeration cycle evaporation temperature influence the performance of the combined system. Cite this article as: Rabah T, Koten H, Fadhila B, Salma S, Mohamed H. Energy and exergy analysis of a combined system: cascade organic Rankine cycle and cascade refrigeration cycle. J Ther Eng 2021;7(5):1139–1149. Journal of Thermal Engineering Web page info: https://jten.yildiz.edu.tr DOI: 10.18186/thermal.977992


INTRODUCTION
Industrial processes are generally characterized by the loss of energy in a form of heat. This dissipated energy not only reduces the effectiveness of these processes but also contributes to the environmental impacts associated with the use of the fossil fuel. These low-temperature discharges cannot be used to generate electricity via a steam thermal plant (Conventional Rankine Cycle). Alternative technologies like the organic Rankine Cycle (ORC) can convert the waste heat to electricity at a relatively low temperature.
Hence, they contribute significantly to reduce fuel costs and pollution as well.
Organic Rankine cycles are cycles of mechanical power generated from waste heat. The organic Rankine Cycles are variants of water vapour cycles, but they use the heat sources to produce mechanical energy at low or medium temperatures [1,2]. Bellos and Tzivanidis optimized a system with Organic Rankine Cycle (ORC), an absorption chiller for cooling, heating, and electricity production. They found the maximum exergy efficiency to be 29.42% with toluene [3]. Furthermore, Bellos and Tzivanidis [4] examined a trigeneration system including a generator, an ejector, a condenser, a turbine, and an evaporator where the studied configuration forms a system able to produce heating, cooling, and electricity. The heat source generally comes from geothermal energy [5][6][7][8], biomass energy [9,10], or industrial waste streams [11,12], otherwise solar energy [13,14].
Another system for cold production using the absorption machine has been studied by [21][22][23][24][25]. Cooling technologies operating at low temperatures have attracted the attention of many researchers in different contexts.
A study was carried out by Lee et al. [26] on thermodynamic analysis of a cascade refrigeration system using carbon dioxide (CO 2 ) and ammonia (NH 3 ) as refrigerants. Regarding the same topic area, Getu et al. [27] conducted their study on the system (R744/R717) for optimizing the design and parameters of the studied system. The analysis here is based on sub-cooling at the condenser outlet at 40°C and overheating at the evaporator outlet with 50°C to develop mathematical expressions for the maximum COP. The results revealed that COP max was highest for ethanol followed by R717 and it was lower for R404 under the same conditions. Kilicarslan et al. [28] studied an irreversibility analysis of a cascade refrigeration system for various refrigerant couples, namely R152a/R23, R290/ R23, R507/R23, R234a/R23, R717/R23, and R404a/R23; the degrees of the condenser sub-cooling and evaporator superheat are 5°C and 7°C for all cases.
They found that COP of the cascade refrigeration system increases and the irreversibility decreases with rising evaporator temperature and polytrophic efficiency for all studied refrigerant couples and vice versa for the increasing of the condenser temperature. Dapazo et al. [29] have performed an analysis of CO 2 /NH 3 cascade cooling system design and its operating parameters and how they influence the COP and the exergy efficiency of that system; they concluded that the COP increases by 70% when the evaporation temperature of CO 2 varies from -55°C to -30°C and hence other parameters influence on the COP increases as well, but when the condenser temperature of NH 3 increases from 25°C to 50°C, the COP decreases. The authors tried also to use NH 3 and CO 2 as refrigerants to provide a cooling capacity at an evaporating temperature of -50°C [30]. This refrigeration capacity was showed in the results to be 9.45 kW. A thermodynamic analysis of theoretical performance for a cascade refrigeration system was conducted by Yst and Karakurt [31]; Different refrigerant couples are used in which the pair R23/R717 showed the best performances compared to the other pairs of refrigerants. The rate of total exergy destruction decreases with the increase of the temperature of the evaporator and increases with the increase of the temperature of the condenser. For the efficient recovery of waste heat in marine applications, a dual Organic Rankine Cycle system in parallel was proposed by Yun et al. [32]. The results showed that when the total annual output power in double ORC configuration ranges from 103% to 115%, the double ORC produces more energy than the single ORC. Molès et al. [33] studied a thermodynamic analysis of the performance of a combined system of organic Rankine cycle and vapour compression cycle driven by heat sources at low temperature and using low GWP fluids. The thermal COP of the ORC-VCC system varied between 0.30 and 1.10; it was concluded that the thermal COP increased with the ORC and VCC evaporating temperatures and decreased with the condensing temperature.
A new cascade cycle to recover energy from a medium-temperature waste stream was proposed by Sadreddini et al. [34] where both energy and exergy analyses are applied to the system. The results showed that the best performance is for a cascade cycle with Pentane as a working fluid for the ORC section. A combined system using a single ORC cycle stage to power a cascade refrigeration system has been studied by Lizarte et al. [35]. In the present study, a new configuration for a cascade refrigeration system is used; this configuration uses a two-stage cascaded ORC cycle instead of a single-stage, it is noted that each stage of the ORC cycle supply one stage of the cascade refrigeration cycle, knowing also that the upper stage turbine of the ORC cycle supply the compressor of the upper stage of the refrigeration cycle and the turbine of the lower stage of the ORC cycle supplies the compressor of the low stage of the refrigeration cycle. In addition, several pairs of organic fluids were used to conduct a comparative study in order to select the best pair that gives good performance to the combined system. The pair of the organics fluids used for each cycle are: (toluene / R245fa) chosen to supply the ORC cycle in cascade and the pairs (R717 / R744, R717 / R23, R134a / R23) chosen for the refrigeration cycle CRS Cascade.
The aim of this study is to improve the system performance. In addition, a comparative study of different couples of organic fluids was followed to select the best couple that can give good energy efficiency. The pairs used in this study are (Toluene/R245fa), (R717/R744, R717/R23, R134a/R23) for the cascade ORC and the cascade CRS cycles, respectively.

DESCRIPTION OF THE COMBINED SYSTEM (ORC-CRS)
The combined system consists of two main cycles: a cascade Rankine organic cycle used for mechanical energy production and a cascade refrigeration cycle used for refrigeration. The mechanical power generated by the ORC cycle turbines is transmitted to the compressors to supply them with energy and so to complete the refrigeration process.

THERMODYNAMIC ANALYSIS
In this section, a thermodynamic modelling study is presented and the mass balance is defined as follows [36,37]; Where m, is the mass flow rate (kg/s), According to first law of thermodynamics, the energy balance equation is: Where; Q , Ẇ and h are the heat transfer rate, work rate and specific enthalpy. The entropy balance equality can be described as follows: Where s and S˙ are the specific entropy and entropy generation rate, respectively. Based on the second law of thermodynamic analysis, the exergy equity defined as given below; The exergy destruction denoted by Ėx D and the other terms in Eq. (4) can be given as: The flow exergy is inscribed as The energy and exergy analysis of the ORC-CRS combined cycle requires the application of the first and second law of thermodynamics. The general equations corresponding to this principle are presented below (Table 1):

Performance Evaluation
The performance of the ORC-CRS combined system is determined by the following equations: The net mechanical power is: Thermal efficiency is written as: The coefficient of performance is presented as: The efficiency of the ORC-CRS system is: The coefficient of performance of ORC-CRS system is written as:

RESULTS AND DISCUSSION
In this study, a novel design of the ORC-CRS combined system is based on a thermodynamic analysis. In this section, the effect of the operating parameters on the performance of the combined system will be detailed. In each case, a couple of working fluids (Toluene/R245fa) is used for the cascade ORC cycle with R717/R23 as working fluid for the cascaded CRS cycle taking into account the fact that the thermodynamic properties are determined using Solkane and Coolpack software and that Ẇ net ORC = Ẇ Com_CRS . Table 2 shows the input parameters values for the ORC-CRS combined system.
Before starting the parametric study on the ORC-CRS combined system, a comparative study based on the three couples organic fluids used for the CRS cascade refrigeration cycle is done. The finding are presented in Table 3 as follows: Following the results obtained by the comparative study of the different pairs of organic fluids used by the CRS refrigeration cycle. The couple (R717/R23) gives a good exergy efficiency with a value of 50.03%. The thermodynamic characteristics of each point constituting the combined cycle (Table 4) have been first determined followed by the energetic and exergy performances of this system ( Table 5).

Distribution of Exergy Destroyed in the ORC-CRS Combined System
The distribution of the destroyed exergy in the ORC-CRS combined system using the best organic fluid pair (Toluene/R245fa to cascade ORC cycle and R717/R23 to cascade CRS cycle) is studied. The results of Figure 2 shows Evaporator ORC Compressor 1 ṁ 9 h 9 + Ẇ C1 = ṁ 10 h 10 ṁ 9 ex 9 + Ẇ C1 = ṁ 10 ex 10 + Ė xD,C1 Heat Exchanger CRS ṁ 10 h 10 + ṁ 16   the largest exergy destroyed is in the ORC condenser by 62% and at the ORC exchanger by 13%. Following the comparative study, a parametric study on the best organic fluid R717/R23 has been done. Latter is based on the effect of the operating temperatures; in particular, the evaporation temperature T Evap_ORC and the evaporation temperature T Evap_CRS on the exergy efficiency of the ORC-CRS combined cycle was carried out. In this part, the effect of ORC evaporation temperature on the performance of the CRS-ORC combined cycle was studied. For this, some operating parameters are fixed such as the evaporation temperature and the condensation temperature of the ORC and CRS cycle: T Evap_CRS = -40°C, T cond_CRS = 40°C, and . In addition, the evaporation temperature of the ORC varies between 200°C and 310°C. Figures 3-5 shows the influence of ORC evaporation temperature on the coefficient of performance, the energy efficiency, and exergy efficiency of the ORC-CRS combined system, respectively. The other parameters are kept constant, which are presented in Table 2. Figure 3 shows the effect of the ORC evaporation temperature on the coefficient of performance of the combined system ORC-CRS. The result show that the coefficient of performance of system decreases considerably with the increase of the ORC evaporation temperature. The coefficient of performance decreased from 0.71 at a temperature of 200°C to a minimum value of 0.64 at the temperature of 310°C. Figure 4 shows the effect of the ORC evaporation temperature on the energy efficiency system of the ORC-CRS combined system. The results show that the energy efficiency of the combined system decreases considerably   On the other hand, the results of Figure 5 show that the exergy efficiency of the combined system decreases significantly with the increase of the ORC evaporation temperature from the maximum value of 69.55% to 50.03% over the temperature range of 200°C at 310°C.
The effect of the CRS evaporation temperature on the performances of the cascade refrigeration cycle and on the ORC-CRS combined cycle has been also studied in which the best selected organic fluid is used. To do this, some parameters are set; the evaporation temperature and the condensation temperature of the ORC cycle, as well as the condensation temperature of the CRS cycle, which respectively carry these values. T Evap_ORC = 310°C, T Cond_ORC = 40°C and T Cond_CRS = 40°C, while the evaporation temperature varies over a temperature range from -60°C à -30°C. Figures  6-8 show the influence of CRS evaporation temperature on the coefficient of performance, the energy efficiency, and exergy efficiency of the ORC-CRS combined system, respectively while keeping the other parameters constant are presented in Table 2. The results of Figure 6 show that the coefficient of performance of the combined system increases considerably over the temperature range considered and reaches a maximum value of 1.05 at -30°C and the minimum value is 0.32 at -60°C. The results of Figure  7 show that the energy efficiency of the combined system increases considerably over the temperature range considered and reaches a maximum value of 75.15% at -30°C and the minimum value is 32.36 % at -60°C of temperature.
The results of Figure 8 show that the exergy efficiency of the combined system increases significantly with the increase of the CRS refrigeration temperature, the maximum value being 63.62% at -30°C and the minimum value of 38.11% at -60°C.

CONCLUSION
In order to improve the performance of a cascade refrigeration system (CRS), a new combined ORC / CRS system was used in this study where the energy produced by the ORC cycle has been used to supply mechanical compressors of the CRS cycle. From the results found, it was concluded that: • The pair of R717/R23 gives better performances in terms of exergy efficiency for the CRS cycle with the Toluene / R245fa pair for the ORC cycle in cascade with 50% of , and the most great exergy destroyed was in the ORC condenser with 62%. • The coefficient of performance of the system decreases considerably with the increase of the ORC evaporation temperature. • The exergetic efficiency of the combined system increases significantly with the increase of CRS refrigeration temperature. • The energy efficiency of the combined system increases considerably over the temperature range considered and reaches a maximum value of 75.15% at -30°C and the minimum value is 32.36% at -60°C of temperature. • The exergy efficiency of the combined system decreases significantly with the increase of the ORC evaporation temperature.

ACKNOWLEDGEMENT
The authors thank the University Djilali Bounaama Khemis Miliana (Algeria) for funding this study.

DATA AVAILABILITY STATEMENT
No new data were created in this study. The published publication includes all graphics collected or developed during the study.

CONFLICT OF INTEREST
The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

ETHICS
There are no ethical issues with the publication of this manuscript.