Cascade Heat Exchanger Research Articles

Thermodynamic analysis of new configurations of auto-cascade refrigeration cycles integrating the vortex tube

Auto-cascade refrigeration cycles have extensive application prospects for low-temperature refrigeration. To achieve low evaporating temperature, the cascade heat exchanger uses a significant portion of the cooling capacity, resulting in less cooling capacity entering the evaporator. A vortex tube with energy separation capability is integrated into the cycle. Then the refrigerant is further cooled, thereby reducing the cooling capacity of the cascade heat exchanger. Thus, more cooling capacity generated by the cycle is transferred to the evaporator. This paper proposes two novel auto-cascade refrigeration cycles integrating vortex tubes. The performance of novel configurations is evaluated through thermodynamic analysis. The results demonstrate that novel configurations outperform the baseline cycle in terms of energy and exergy efficiency. The coefficient of performance and exergy efficiency of cycles achieved maximum increases of 23.84 % and 11.62 %. The modified exergy analysis method distinguishes the destruction and indicates the direction of optimization. Furthermore, the economic and environmental analysis indicates that the novel cycles achieved a maximum reduction of 6.74 % in cost rate and 12.72 % in environmental impact under given conditions. Generally, the thermodynamic analysis reveal the potential of vortex tubes to enhance the performance of auto-cascade cycles.1

Effect of plate-fin heat exchanger structural parameters on the performance of a cascade refrigeration system

This study investigates the effect of various structural parameters in plate-fin heat exchanger on the performance of a cascade refrigeration system (CRS). Focusing on five key factors—fin thickness, fin type, fin height, porosity, and element axial length—we analyze their contributions to COP, exergy destruction, and entropy production within the CRS. The results reveal that the fin type barely influences the CRS’ performance, the element axial length has the greatest impact, followed by fin height, porosity, and fin thickness, respectively. The COP of the CRS drops by 26.25 % as the element axial length increases from 124 mm to 136 mm, 23.67 % (23.08 %) as the fin height of perforated (straight) fin increases from 6.1 mm to 6.7 mm, and 10.27 % (10.38 %) as the fin thickness of perforated (straight) fin increases from 0.1 mm to 1.3 mm, and the exergy destruction increases by 42.42 %, 37 % (37 %), and 13 % (10.38 %), respectively. The effect of porosity on the CRS’ performance is opposite to that of other parameters. When the element axial length is 124 mm, the fin height is 6.1 mm, and the fin thickness is 0.1 mm, the COP of the CRS reaches the maximum value (0.689). The conclusion can provide data support for the performance optimization of CRS from the structure of the cascade heat exchanger.

Optimal intermediate temperature of two-stage cascade heat pump with non-azeotropic refrigerants for simultaneous heating and cooling

This paper presents a model for finding the optimal intermediate temperature to achieve the highest coefficient of performance of the standard two-stage cascade heat pump with non-azeotropic refrigerants for simultaneous heating and cooling. The optimal intermediate temperature remains unaffected by the pinch point temperature at the cascade heat exchanger, though this temperature does impact heat pump performance. The optimal intermediate temperature rises with increasing high-side condensing and low-side evaporating temperatures and the value is deviating from the average temperature between the condensing and evaporating temperatures of the high and low-temperature sides of the cascade heat pump, respectively by a correction factor. The factor is found to be dependent on the critical temperature ratio of the working fluids in the low-temperature side and high-temperature side including the condenser temperature of the high-temperature side of the cascade heat pump cycle. A developed model consisting of the average temperature with the correction factor for estimating the optimal intermediate temperatures were examined with a set of non-azeotropic refrigerants with different compositions at different high-side condensing (65–95 °C) and low-side evaporating (5–15 °C) temperatures and pinch point (2–11 °C) temperatures. The results on the optimal intermediate temperatures and the maximum COPs agreed very well with those calculated from the enthalpy method within ±7 % and ±3 % deviations, respectively.

Energy, modified exergy, exergo-economic and exergo-environmental analyses of a separation-enhanced auto-cascade refrigeration cycle

Auto-cascade refrigeration technology has broad application prospects in the field of low-temperature refrigeration. The separation efficiency of refrigerants is one of the main factors affecting system performance. This paper proposes a separation-enhanced auto-cascade refrigeration cycle. An additional cascade heat exchanger and an auxiliary phase separator are introduced before the original cascade heat exchanger. The configuration enhances cooling capacity and cycle performance. The energy, modified exergy, exergo-economic and exergo-environmental performance of the cycle are evaluated through simulation and compared with the basic cycle and previously modified cycle. The results demonstrate that the proposed cycle outperforms both the basic cycle and the modified cycle in both energy and exergy efficiency. The maximum coefficient of performance and exergy efficiency of the separation-enhanced cycle is 44.4% and 44.3% higher than those of the basic cycle, 17.1% and 24.8% higher than those of the modified cycle, respectively. Furthermore, the modified exergy analysis method identifies the exergy destruction caused by the components and zeotropic refrigerant. Actually, the exergy destruction of refrigerant contributes to 39.7% of the overall exergy destruction under typical conditions. Moreover, the exergo-economic analysis indicates a 7.6% reduction in the total cost rate for the proposed cycle compared to the basic cycle and a 2.35% reduction compared to the modified cycle. The exergo-environmental analysis highlights a decrease of 25.4% in overall environmental impact for proposed cycle compared to basic cycle and 12.1% reduction compared to modified cycle. In general, the comparative analysis reveals the improvement potential of the proposed cycle in terms of energy-saving, economic and environmental performance.

An ultralow-temperature cascade refrigeration unit with natural refrigerant pair R290-R170: Performance evaluation under different ambient and freezing temperatures

The global demand for ultralow-temperature (ULT) refrigeration units gets greatly promoted, for storage, transportation, and distribution of COVID-19 vaccines. In this work, a ULT freezer is developed with a cascade refrigeration system (CRS) utilizing environmentally friendly refrigerants R290 and R170. The performance of the ULT freezer is experimentally evaluated under different ambient temperatures Tamb and freezing temperatures Tfreezing. The result shows that once the refrigeration system starts, the freezer enters a pull-down period and then reaches stable or periodic on-off operation. The monitored temperatures present drastic variations in the low-temperature cycle (LTC) and show a relatively stable start-up process in the high-temperature cycle (HTC). The monitored temperature rises when Tamb increases from 16 °C to 32 °C. The increasing Tamb brings about a larger temperature drop crossing the pre-cooled condenser, cascade heat exchanger, and high-temperature condenser, and a smaller temperature reduction through the anti-condensation. As Tfreezing decreases from −60 °C to −86 °C, the suction/discharge gas temperatures increase in the low-temperature compressor, while the other monitored temperatures reduce. The largest temperature non-uniformity in the freezer is 9.19 °C, and the lowest wall temperature can reach −90.52 °C. With Tamb ranging from 16 °C to 32 °C, the power consumption of the freezer increases from 896 W to 912 W. When Tfreezing varies from −60 °C to −86 °C, the CRS’s consumed power reduces from 804 W to 904 W. The present freezer can easily obtain low temperatures e.g., −81 °C, and reach a lower temperature, such as −86 °C with proper improvements to reduce cold loss.

Experimental and theoretical investigation on the energetic and economic performance of low-temperature cascade air-source heat pump considering the effects of cascade heat exchanger

Air-source heat pump (ASHP) is wide used for its carbon neutrality. In cold climates, to ensure good efficiency and stability, cascade ASHP is a promising technology for high-temperature water application, such as space heating boilers retrofit. Cascade heat exchanger is a key component in cascade ASHP. However, there are few experimental studies on the effects of its area on system energetic, economic, and environmental performance under low ambient temperature and high water-supply temperature, which is essential for system improvement and application. Focusing on this research gap, this paper carries out experiment of cascade ASHP system performance and conducts analysis on system suitability based on recurrent Neural Network. To fairly compare system performance in different cold climate cities with different cascade heat exchanger areas, conversion factor γ is proposed and used in the analysis. The results show total energy consumption of compressors decreases the most in New York (21498 MJ) and least in Shenyang (9414 MJ) as cascade heat exchanger areas increases from 0.6 to 1.6 m2. Similarly, the system in New York shows the most significant reductions in carbon emissions (2701 kg/year) and total annual cost (225$), while that in Shenyang shows the smallest drop of 1277 kg/year and 118$, indicating it’s more meaningful to increase cascade heat exchanger area in a city with longer heating period. The results also show the COP varies significantly across cities, strongly correlated with average ambient temperature. The increase in COP slows down when cascade heat exchanger area reaches 1.6 m2, indicating even larger area is not recommended. These findings provide a foundation for improving and applying cascade ASHP in cold climates.

Technical-economic-environmental analysis of high temperature cascade heat pump with R718 (high stage) and six different low global warming potential refrigerants (low stage)

High temperature cascade heat pump with two separate refrigerants is a crucial system for decarbonizing industrial heating through electrification. The selection of optimal working refrigerants and temperature lifts relies on not only the thermodynamic analysis outputs such as coefficient of performance but also economic and environmental analyses. In this study, a technical–economic-environmental investigation was conducted for the proposed high-temperature cascade heat pump employing water as the high-stage refrigerant and six low global warming potential refrigerants in the low stage. To facilitate a detailed economic-environmental analysis, precise equipment sizing was undertaken. A comprehensive heat transfer model was developed to size the falling film shell-and-tube cascade heat exchanger, and empirical correlations were employed to size the internal heat exchanger at various cascade temperature lifts. The cascade heat exchanger model is validated against experimental data with less than 5 % error. The distribution of refrigerant charged mass was illustrated using instantaneous density and temperature data. To consolidate all thermal, economic, and environmental sub-indicators into a single parameter, the coefficient of performance, volumetric heating capacity, total annual cost, and total equivalent warming impact were combined as (COP × VHC)/(TAC × TEWI). This proposed comprehensive indicator can be applied to high-temperature cascade heat pumps to identify the optimum working pair and conditions. The results highlighted that R718 + R1234ze(Z) and R718 + R600 were the best working refrigerant pairs, considering all energy, economic, and environmental criteria.

Thermal Performance Evaluation of a Novel Ejector-Injection Cascade Refrigeration System

In this article, a novel cascade refrigeration system is proposed for moderately low temperature applications. An ejector expansion vapor compression cycle (EEVCC) and a vapor injection cycle with the flash tank (VICFT) are coupled together through a cascade heat exchanger to develop the novel ejector injection cascade refrigeration system. The goal of this study is to investigate the potential improvement scope of a conventional cascade refrigeration system by integrating EEVCC and VICFT. A detailed energy and exergy balance analysis are conducted on the novel proposed system by considering the following three design parameters: (1) evaporator temperature; (2) pressure drop at the ejector; and (3) lower circuit condensation temperature. The results reveal that the performance of the proposed system is sensitive to pressure drop at the ejector and lower circuit condensation temperature for each evaporator temperature. Therefore, it is necessary to determine the optimal values of these factors in order to conduct an accurate performance analysis. Furthermore, a comparative study between the proposed system and the conventional system is conducted based on the first and second laws of thermodynamics. The results show that at −60 ℃ of evaporator temperature, the proposed system has 8.571 % COP improvement and 7.241 % exergy efficiency improvement over conventional cascade refrigeration system. This study also explores the suitable pair of refrigerants for this proposed system to minimize environmental impact. Among seven pairs of refrigerants investigated, R170 and R601 at LTC and HTC respectively shows the best performance. The proposed system was found to be more sensitive to change of LTC refrigerant than HTC.

Energy comparison based on experimental results of a cascade refrigeration system pairing R744 with R134a, R1234ze(E) and the natural refrigerants R290, R1270, R600a

Refrigeration architectures for low-temperature applications are gaining interest in the Industrial, Commercial and Health fields. One of the most classical architectures is cascade systems, where two mechanical single vapor compression cycles are thermally paired by the cascade heat exchanger (HXcc). The HXcc, acts as condenser for the low temperature cycle and as evaporator for the high temperature cycle. Each cycle is charged with a different refrigerant adapted to the operating conditions. New environmental rules make necessary to find proper refrigerant pairs to get refrigeration plants with high energy efficiency together with environmentally friendly fluids. This experimental work compares the energy performance of a cascade refrigeration system working with the pair R134a/R744 against four alternative refrigerant pairs: R290/R744, R1270/R744, R600a/R744CO2 and R1234ze(E). Laboratory tests were conducted at three heat rejection temperatures: 20, 30 and 40°C, maintaining a fixed low-temperature heat source level (−20°C). The energy analysis shows that R290/R744 and R1270/R744 improve the cooling capacity at low heat sink temperatures, and the pair R290/R744 improves the energy performance at low temperatures. The environmental analysis shows that all the refrigerant pairs achieve a reduction in CO2eq emissions.

MULTIOBJECTIVE OPTIMIZATION OF CASCADE REFRIGERATION SYSTEM USING NH3-CO2 AND NH3-N2O REFRIGERANT PAIRS

In this paper a comparative analysis based on multiobjective optimization of a cascade refrigeration system using NH<sub>3</sub>-CO<sub>2</sub> and NH<sub>3</sub>-N<sub>2</sub>O refrigerant pairs has been done. Exergetic effciciency and overall cost rate have been taken as the two conflicting objective functions, and evaporator temperature, condenser temperature, LTC condenser temperature, and cascade heat exchanger temperature difference are the four design variables. The optimization work has been carried out using genetic algorithm, which results in a nondominated optimal solution represented in the form of pareto-optimal curve, and the technique for order performance by similarity to ideal solution (TOPSIS) method is used to select a unique solution. The results show that NH<sub>3</sub>-N<sub>2</sub>O is a better alternative as compared to NH<sub>3</sub>-CO<sub>2</sub> refrigerant pair from a multiobjective optimization point of view. The corresponding optimal operating conditions have also been suggested for both the refrigerant pairs.

Comparison study of conventional and advanced exergy analysis on cascade high temperature heat pump system based on experiment

Cascade high temperature heat pump (CHTHP) has shown remarkable application potential for industrial waste heat recovery owing to its superiority of large temperature lift. Great efforts have been made to optimize the performance of CHTHP by means of conventional exergy analysis (CEA), which was not qualified to uncover the thermodynamic interactions among components. Thus, advanced exergy analysis (AEA) has been adopted to comparatively evaluate the relationships of reversibility among the CHTHP components. Herein, both CEA and AEA were based on the experiment results from a prototype which could realize maximum temperature lift of 100 °C.The CEA results show that total exergy efficiency is 43.23%, while AEA results indicate that endogenous part accounts for 98.27% of the total exergy destruction and 67.69% of it is avoidable. The AEA reveals that the highest improvement priority belongs to low temperature compressor (LTCOM), followed by cascade heat exchanger (CHE) and high temperature compressor (HTCOM) and that low temperature expansion valve (LTEV) and high temperature expansion valve (HTEV) has no improvement potential which is different from the results of CEA theory. Conclusively, AEA can give more detailed guidance about the potential for improvement.

Exergoeconomic analysis and optimization of a novel integrated two power/cooling cogeneration system using zeotropic mixtures

This work deals with an exergoeconomic analysis for a new integrated power/cooling cogeneration system using zeotropic mixtures as working fluids with two levels of cooling temperature. The proposed system is comprised of two combined ejector refrigeration and power (CERP) cycles with different configurations using geothermal energy as the heat source. One subsystem is a conventional CERP system called basic subsystem and the other one is a CERP system employing a vapor-liquid separator called the bottoming subsystem. These subsystems are connected via a cascade heat exchanger in a parallel mode. The results of the parametric study show that geofluid mass fraction (that indicates the amounts of the mass flow rate of geofluid served thermal energy for basic and bottoming subsystems) has the biggest effect on the proposed system in comparison to other operating parameters. Furthermore, a multi-objective optimization with energy and exergy efficiencies and overall unit cost of products as objective functions is carried out for both basic and integrated CERP systems. The results of the comparison study for optimized systems show a significant performance improvement for the proposed system in comparison to the basic system. In this case, for the integrated CERP system, energy and exergy efficiencies increase by 3.88% and 4.76%, respectively, while the overall unit cost of products increases by 0.96 $ GJ−1, in comparison to the basic CERP system.

Analytical study on core flow in a novel scheme of cascade heat exchanger intensified with open foam partially filled in an annulus

Due to the importance of energy-saving and efficiency in the industry, a novel scheme for waste heat recovery was utilized in the cascade refrigeration cycle. This system is composed of a cascade heat exchanger combined with a recuperator intensified with open foam. The exhaust air from the heat exchanger is used for the air handling unit. Also, core flow in an annular channel is proposed as an innovation to enhance heat transfer. An analytical study was performed to evaluate the impact of metal foam properties on the performance of the partially filled annular heat exchanger with asymmetric isothermal boundary conditions. The energy equations were decoupled by normalizing, linear combination, and the variable change method. Then by forming the Bessel differential equations, temperature solutions were achieved in terms of the modified Bessel functions of the first and second kinds. In the energy and momentum equations of the porous region, the local thermal non-equilibrium (LTNE) and the Darcy-Brinkman models were used, respectively. The analytical solutions of velocity, pressure drop, mass flow ratio, temperature, and Nusselt numbers were obtained. A hydraulically critical point exists at the nondimensional interfacial radius of 2.86 for Da=10−4. It revealed that the Darcy number and the thermal conductivity ratio are two crucial parameters for improving performance. Based on the Performance evaluation criterion (PEC) definition, the optimal porosity value of 0.91 was determined. Also, a design triangle in the range of metal foams was proposed to select the local Nusselt and Biot numbers. At a particular Biot number, the PEC value increases 3.76 times in the design range of metal foams. Results demonstrated that the core flow in an annular heat exchanger leads to high performance, several times greater than non-core flow.

Experimental Analysis of the R744/R404A Cascade Refrigeration System with Internal Heat Exchanger. Part 2: Exergy Characteristics

This paper examines the exergy efficiency and exergy destruction rate of the R744/R404A cascade refrigeration system (CRS) using an internal heat exchanger in supermarkets according to various conditions affecting the system. A refrigerant of a low-temperature cycle uses R744 and a refrigerant of a high-temperature cycle in the CRS uses R404A. Experiments were conducted by changing various conditions on the high- and low-temperature side, and exergy analysis was performed accordingly. The main results are summarized as follows: (1) the lower the total exergy destruction rate of the CRS, the higher the exergy efficiency of the system, and accordingly the coefficient of performance (COP) of the system is also improved. (2) In the CRS, since the optimum cascade evaporation temperature exists (about −16 °C), it can be said that the limit point, that is, the cascade evaporation temperature with the maximum COP of the system, is the optimum point at about −16 °C. Therefore, at this optimum point, the exergy destruction rate of the cascade heat exchanger becomes the minimum. In other words, it should be noted that when the cascade evaporation temperature is the optimum point, the exergy destruction rate of the R744 compressor and the cascade heat exchanger is minimal. The purpose of this study is to provide basic design data by analyzing the exergy characteristics according to various conditions on the high- and low-temperature side for optimal design of a CRS to which R744 is applied.

Comparative analysis of a cooling systems working on an environmentally friendly refrigerants

The paper treats with refrigerants which affect on excessive heating of atmosphere. They are in group offluorinated greenhouse gases which are regulated by European and polish law. The main aim is to lower their amount in industry leading to overall removal from use. We present possible way to design refrigeration system lowering their adverse effect on natural environment. So three options of cooling systems are analyzed: one-stage refrigeration system working with R449A, cascade refrigeration system with R744 (carbon dioxide)/ R134a and R717 (ammonia) refrigeration plant. Due to the nowadays raising use of the cascade systems, an analysis of the operating parameters of such installation was carried out with considering the cascade heat exchanger as a condenser/evaporator. The installations concerned are three alternative offers for meat processing manufactory. A thermal balance was prepared for chambers located in building and the operating parameters of the installation were assumed. The equipment corresponding to the required cooling capacities were selected.

Energy, exergoeconomic and environmental optimization of a cascade refrigeration system using different low GWP refrigerants

Due to the environmental concerns, refrigerants with low global warming potential (GWP) are good alternatives to use in the most refrigeration system. In this paper, an energy-exergoeconomic-environmental model is presented for a cascade refrigeration system (CRS) at high and low temperature cycles operating with six pairs of refrigerants with low GWP including R41-R161, R41-R1234yf, R41-R1234ze, R744-R161, R744-R1234yf, R744-R1234ze. The optimal performance of the system is studied under the effects of temperature variations in condenser, evaporator and cascade heat exchanger. To do this, the coefficient of performance (COP), exergy efficiency and total cost rate are optimized for each pair of refrigerants to evaluate the optimal operational conditions of CRS using Pareto front curve. Results show that the maximum COP of 2.09 and maximum exergy efficiency of 35.32% are obtained at condenser temperature of 40 °C and evaporator temperature of − 30 °C. In addition, the minimum total rate cost is equal to 13587 $/year when the condenser temperature is 40 °C and evaporator temperature is 32.5 °C. Finally, it is concluded that R41-R161 and R41-R1234ze are optimal refrigerant pairs with the highest COP/exergy efficiency and lowest total cost rate.

Comparative study of air source and ground source heat pumps in 10 coldest Russian cities based on energy-exergy-economic-environmental analysis

As one of the environmentally friendly technologies, heat pumps have been widely utilized in residential and commercial buildings all over the world. The use of renewable energy sources and environmentally friendly refrigerants are two major challenges associated with heat pumps applications. Due to the advantages of cascade heat pumps over single-stage heat pumps and the higher energy demand met by cascade heat pumps, investigation of the effects of geographic conditions (weather and soil conditions) as well as the simultaneous use of two refrigerants for high and low temperature cycles on performance of cascade heat pumps would provide critical information for practical applications. This paper compares the performance of air source and ground source cascade heat pumps using refrigerants by low global warming potential and zero ozone depletion potential in top 10 coldest cities of Russia. The effects of variations in temperature of evaporator and cascade heat exchanger on optimal operational conditions are investigated based on energy, exergy, economic and environmental analysis using Pareto front curve. Optimization results show that there is an optimal point for evaporator temperature and cascade heat exchanger temperature where the maximum COP, maximum exergy efficiency and minimum total cost rate for ASHP and GSHP systems are obtained. It is found that at low air temperature, since the underground temperature is higher than ambient air, the efficiency of GSHP system is better than efficiency of ASHP system. The maximum COP of 2.357 and 2.44 are respectively obtained for ASHP and GSHP systems in Saint Petersburg. In addition, Yakutsk city has the maximum exergy efficiency of 40.7% and 43.2% and maximum total cost rate of 14714 $ year−1 and 13693 $ year−1 for ASHP and GSHP systems, respectively. Finally, it is concluded that Saint Petersburg is the best city to employ ASHP system while GSHP system is suggested for Khabarovsk city.

Indirect mechanical heat pump assisted humidification‐dehumidification desalination systems

In this study, four new indirect heat pump assisted mechanical vapor compression humidification-dehumidification (HDH) systems are proposed and their superiorities over the reference system are demonstrated from thermodynamics and thermoeconomics viewpoints. The proposed models are configured based on an HDH unit and a simple cascade heat pump, an HDH unit and a heat pump with an ejector, an HDH unit and a cascade heat pump with an ejector, and an HDH unit and a vapor injection heat pump. Although employing a heat pump with cascade and ejector configurations improved gain-output-ratio (GOR) and specific power consumption (SPC) values in comparison with the base system, the performance metrics of such layouts are still inferior to those of the HDH unit coupled with the vapor injection heat pump. It is found that the desalination unit with vapor injection mechanism has the highest GOR and the lowest SPC values of 9.22 and 71.29 kWh/m3, respectively, under constant electrical power of 31.26 kW. Overall, proposal of the cascade heat pump, heat pump with an ejector, cascade heat pump with an ejector, and a vapor injection heat pump instead of the conventional heat pump improved GOR by 4.83%, 10.03%, 11.52%, and 14.25%, respectively. Meantime, the SPC of the HDH unit coupled with a simple heat pump decreased by 4.6%, 9.1%, 10.26%, and 12.46% when a cascade heat pump, a heat pump with an ejector, a cascade heat pump with an ejector, and a vapor injection heat pump were used instead of the conventional heat pump, respectively. Also, it is found that a maximum improvement of 14.13% can be achieved in exergy efficiency with a cost penalty of only 1.59 $/m3. Also, it was found that with the use of a simple cascade heat pump instead of the simple heat pump, the exergy efficiency can be enhanced by 4.9%, while the unit cost of the distilled water (UCDW) was degraded by 51.32%. Likewise, the HDH unit coupled with the cascade heat pump with an ejector was resulted in high UCDW compared to the base system, while its exergy efficiency was not enhanced so significantly. Variation of exergy efficiency versus the intermediate temperature had a maximum value of 1.93% (for the HDH unit coupled with a simple cascade heat pump) and 2.05% (for the HDH unit coupled with a cascade heat pump with an ejector) at Tint = 315 K. Moreover, it was found that the UCDW can be decreased with the rise of the intermediate temperature and cascade heat exchanger temperature difference as the fresh water cost rate decreases as a result, while the exergy efficiency can be increased with the rise of the cascade heat exchanger temperature difference.

Energy and Exergy Analysis of a Subcritical Cascade Refrigeration System With Internal Heat Exchangers Using Environmentally Friendly Refrigerants

AbstractThis study focuses on a thermodynamic performance analysis of a subcritical cascade refrigeration system (CRS) with internal heat exchangers (IHXs) using R41/R601, R41/R602A, and R41/cyclopentane as refrigerant pairs. The effect of evaporator temperature (Tev), condenser temperature (Tcond), and temperature difference in the cascade heat exchanger (ΔTCHX) on examined performance parameters are investigated. Each performance parameter is scrutinized by an optimum low-temperature circuit (LTC) condenser temperature. The operating parameters have some implications on the overall thermodynamic performance of the system. A change of 10 °C in the Tev and Tcond affects the performance of the system by approximately +26% and −8%, respectively. Moreover, a variation of 1 °C in the ΔTCHX reduces the performance of the system by about 2%. The effect of IHXs on the system has some interesting results. The coefficient of performance (COP) and exergy efficiency values of the system using R41/cyclopentane tend to constantly decrease by nearly 4.05%. Although not as much as R41/cyclopentane, there is also a slight drop in the performance of other refrigerant pairs. The discharge temperature in LTC and high-temperature circuit (HTC) compressors exceeds 120 °C for low-temperature refrigeration requirements, which is highly undesirable. Furthermore, the top priority components for the system improvement are HTC condenser, HTC compressor, and CHX. The refrigerant pairs with the thermodynamic performance from best to worst are R41/R601, R41/cyclopentane, and R41/R602A, respectively. Finally, the COP and exergy efficiency values of the modeled system are 10.40% higher and 3.06% lower, respectively, compared with current models in the literature.

Thermodynamic Analysis of a Cascade Heat Pump Incorporated in High-Temperature Heating System

In this article is presented a thermodynamic analysis of a cascade heat pump system designed for using in high-temperature heating systems. The own thermodynamic model was built by using properties of working fluids from the CoolProp base. The cascade heat pump was designed to use ambient air as heat source with temperature t_amb= –20 °C and for heating water in the high-temperature heating system up to 70 °C. The projected heating capacity of the cascade heat pump was 100 kW. The coefficient of performance (COP) of the cascade heat pump system due to use of different working mediums combinations in cycles of the cascade heat pump was investigated. For the best combination of working fluids (mediums) sub-cooling, super-heating, pressure loss in compressor’s suction line, as well as exergy efficiency of the heat pump were analysed as a function of the mean temperature of the cascade heat exchanger.