Study of capillary tubes in a transcritical CO2 refrigeration system

Capillary tubes have been used in refrigeration systems for many years, but not with a transcritical CO₂ system. In this article, the effects of capillary tubes in a transcritical CO₂ refrigeration system have been investigated experimentally and theoretically. Different types of capillary tubes with different lengths (0.5–4 m) and diameters (1–2 mm) have been tested. The result of this work is a static model, which is used in the further work to make a simulation model (static) of a complete refrigeration system. The model is based on Friedel's and Colebrook's pressure drop correlations.

The behaviour of an adiabatic capillary tube in a refrigeration cycle has been investigated theoretically. The conclusion is that the COP of a system with capillary tubes generally is better than when a fixed high pressure is used, but not as good as when variable optimal high pressure is used. Capillary tubes are especially interesting in applications where the evaporation pressure is constant and the temperature out of the gas cooler varies no more than ±10 K from the design condition. The reduction in COP is more significant at low temperatures out of the gas cooler.     

Integration of a three-stage expander into a CO2 refrigeration system

The inclusion of an expander with work recovery provides two advantages for transcritical CO₂ refrigeration cycles: the COP is improved and the exhaust pressure of the main compressor is lowered. Several designs of expanders have been proposed for this application and some prototypes have been tested already. In our laboratory a three-stage expander has been developed, which replaces the throttle valve of the normal refrigeration cycle and expands into the two-phase region. For optimum integration into the overall system it is proposed to install a vapour-liquid separator between the second and third stage of expansion. The vapour is guided back to the third expander stage whereas the liquid is supplied to the cooling stations via thermostatic or electronic expansion valves.     

Numerical simulation and experimental validation of vapour compression refrigeration systems. Special emphasis on CO2 trans-critical cycles

A numerical and experimental comparative study of a carbon dioxide trans-critical refrigerating system and a conventional sub-critical refrigerating cycle is presented. Attention is focussed not only on the whole refrigeration cycle, but also on the behaviour of the hermetic reciprocating compressors used in these systems. The comparative cases presented have been specially designed for small cooling capacity units with an evaporation temperature around 0 °C. A detailed numerical simulation model for hermetic reciprocating compressors performance, widely validated under conventional fluid refrigerants, has been extended to numerically obtain the CO₂ compressor prototypes behaviour. Two CO₂ compressor prototypes working with CO₂ have been experimentally tested in a specific unit, specially designed and built to analyse high-pressure single stage vapour compression trans-critical refrigerating equipments. This set-up has allowed validating a detailed numerical simulation code for the thermal and fluid-dynamic behaviour of single stage vapour compression refrigeration system working with CO₂ as fluid refrigerant. The numerical results and the experimental data obtained to validate compressors, heat exchangers and whole cycle behaviour have shown a really good agreement. Finally, the numerical and experimental comparison between the carbon dioxide system and the sub-critical conventional cycle has shown the possibility of CO₂ as fluid refrigerant under the studied working conditions.     

Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part I: Modeling, simulation and optimization of two system solutions

Using CO₂ trans-critical system solutions in supermarket refrigeration is gaining interest with several installations already running in different European countries. Using a computer simulation model, this study investigates the performance of two main system solutions: centralized with accumulation tank at the medium temperature level and parallel with two separate circuits for low and medium temperature levels. Both system solutions are presented and the simulation model is described in details. Calculations have been performed to design the systems and optimize their performances where basic layout and size of each solution have been defined. For ambient temperature range of 10–40 °C, the reference centralized system solution shows higher COP of about 4–21% than the reference parallel solution. Using two-stage compression in the centralized system solution instead of single stage will result in total COP which is about 5–22% higher than that of the reference centralized system and 13–17% higher than that of the improved two-stage parallel system. The two-stage centralized system solution gives the highest COP for the selected ambient temperature range.     

Experimental validation of a prototype ejector designed to reduce throttling losses encountered in transcritical R744 system operation

This study presents experimental results obtained from a transcritical R744 system using a refrigerant ejector. The results were compared to that of a conventional system with an expansion valve. For the test conditions considered, the cooling capacity and COP simultaneously improved by up to 8% and 7%, respectively. Experiments were analyzed to quantitatively assess the effects on system performance as a result of changes in basic ejector dimensions such as motive nozzle and diffuser sizing. Small angles of 5° yielded best results for the static pressure recovery of the high-speed two-phase flow entering the diffuser. Experiments confirmed that like in a conventional transcritical R744 system with expansion valve, the high-side pressure control integrated into the ejector could be used to maximize the system performance. Numerical simulation results helped identifying this basic trend. Due to difficulties in the ejector throat pressure measurements, a more practical performance metric was introduced in order to quantify overall ejector efficiencies. According to this definition, the prototype ejector was able to recover up to 14.5% of the throttling losses.     

Theoretical evaluation of trans-critical CO2 systems in supermarket refrigeration. Part II: System modifications and comparisons of different solutions

The performance of CO₂ refrigeration systems strongly depends on the operating conditions. The specific characteristics of low critical temperature and high operating pressure limit its applications and imply the implementation of different control strategies. This study compares the performance of different CO₂ system solutions for supermarket refrigeration with R404A system. Some possible modifications and improvements on the CO₂ system have been investigated. The COP of the investigated CO₂ system solution can be improved by about 3–7% along the ambient temperature range of 10–40 °C. The annual energy consumption calculations in three different climates; cold, moderate and hot, show that the centralized trans-critical CO₂ system is good solution for cold climates whereas the NH₃–CO₂ cascade system has the lowest energy consumption in hot climates. Both systems proved to be good alternatives to R404A DX system for supermarket refrigeration.     

The optimum high pressure for CO2 transcritical refrigeration systems with internal heat exchangers

The system performance of a CO₂ refrigeration system is greatly affected by the compressor discharge pressure. An internal heat exchanger (IHX) with high effectiveness is an important factor to achieve high system performance. The expression traditionally used to describe the heat exchange effectiveness is not suitable for CO₂ systems. As a result a practical effectiveness expression for IHX, based on enthalpy difference, has been derived and is reported in this paper. Detailed analysis on the relationship between the optimum high pressure P_k,opt and other systematic parameters was performed. Evaporating temperature has little influence on P_k,opt; and IHX can minimize the sensitivity of the system to the refrigerant quality x at the evaporator outlet. Based on simulation data, a correlation of P_k,opt was developed that predicts the simulation values with a deviation of less than 3.6% in the whole range and 0.94% when the evaporating temperature t₁=5.3 °C. The results reported in this paper can be used in optimum control and performance evaluation of the whole system.     

Simulated performance and cofluid dependence of a CO2-cofluid refrigeration cycle with wet compression

Recent experiments demonstrate the viability of a low-pressure CO₂-cofluid compression refrigeration cycle in which CO₂ and a non-volatile cofluid are circulated in tandem and co-compressed in a compliant scroll compressor. This work explores the theoretical performance limitations of such a cycle operating under environmental conditions representative of automotive air conditioning and studies the dependence of this performance on the properties of the CO₂-cofluid mixture. The vapor–liquid equilibrium and thermodynamic properties of the mixture are described using a previously reported activity-coefficient model. A coupled system of physically based equations that allows for consideration of both ideal and real hardware components is used to represent the system hardware and its interaction with the environment. The system efficiency is analyzed in terms of entropy generation rates in the various hardware components; entropy generation in the internal heat exchanger—a component required to achieve sufficiently low cooling temperatures—strongly influences overall system efficiency. The vapor pressure of the CO₂-cofluid mixture and the heat of solution of CO₂ in cofluid have large and somewhat independent contributions to the system performance: lower saturation pressure lowers the optimal operating pressures at fixed CO₂ loading, while increasingly negative heat of solution contributes to higher specific refrigeration capacity and efficiency.

Résumé

Recent experiments demonstrate the viability of a low-pressure CO₂-cofluid compression refrigeration cycle in which CO₂ and a non-volatile cofluid are circulated in tandem and co-compressed in a compliant scroll compressor. This work explores the theoretical performance limitations of such a cycle operating under environmental conditions representative of automotive air conditioning and studies the dependence of this performance on the properties of the CO₂-cofluid mixture. The vapor–liquid equilibrium and thermodynamic properties of the mixture are described using a previously reported activity-coefficient model. A coupled system of physically based equations that allows for consideration of both ideal and real hardware components is used to represent the system hardware and its interaction with the environment. The system efficiency is analyzed in terms of entropy generation rates in the various hardware components; entropy generation in the internal heat exchanger—a component required to achieve sufficiently low cooling temperatures—strongly influences overall system efficiency. The vapor pressure of the CO₂-cofluid mixture and the heat of solution of CO₂ in cofluid have large and somewhat independent contributions to the system performance: lower saturation pressure lowers the optimal operating pressures at fixed CO₂ loading, while increasingly negative heat of solution contributes to higher specific refrigeration capacity and efficiency.     

Refrigerant charge in refrigerating systems and strategies of charge reduction

For some years, European regulation concerning refrigerating systems has become more and more severe. Because of their design and technology, refrigeration units cannot avoid refrigerant leaks, and can thus have a harmful impact on the environment. Many studies aimed at minimizing the charge in a refrigerating machine were thus developed. On a global level, reduction of refrigerant charges must not affect energy aspects while respecting environmental constraints [Montreal 1987, Kyoto 1997]. The architecture and refrigerant charge distribution in the systems are of particular importance; their analyses permit the determination of the target elements and thus define those in which the refrigerant charge could be reduced (heat exchangers, liquid pipes, receivers, …). This paper presents a review of the refrigerant charge studies in a refrigerating plant (methodologies of charge measurement, void fraction correlations and characteristic values for various technologies). It also evaluates the influence of the refrigerant charge on the coefficient of performance (COP) and on the cooling capacity. Finally, it inventories the different means that allow the charge in a refrigerating system to be minimized.     

Thermodynamic optimization of ejector actuated refrigerating cycles

A cold generation system featuring a Rankine cycle powered refrigeration cycle actuated by a supersonic ejector was theoretically investigated in view of the thermo-fluid-dynamic optimization of the working fluid characteristics.

The ejector model was validated against well established performance charts relating to water. A reference system was considered in which a Rankine cycle at moderate top temperature delivers its expansion power by means of an ideal turbine to an ideal compressor of a refrigeration cycle. Two main optimizing variables were ascertained: the fluid critical temperature and the complexity of the fluid molecule. The best performance of such reference cycle is around 80% of that of an ideal fully reversible, Carnot cycle based, system (COP of 2.0 for t_E,PC = 150 °C, t_E,RC = 5 °C, and t_C = 35 °C). As easily predictable the ejector compression introduces severe losses mainly due to the normal shock and the mixing of the motive and of the driven fluid. Overall COP for the above quoted temperatures decreases from 2.0 (reference cycle) to 0.4–0.7. The optimization of the working fluid showed that comparatively low critical temperatures are favoured and that a fluid complexity similar to that of CH₅N or CH₂Cl₂ gives the best performance. A detailed losses analysis explains this behaviour. In particular at low reduced temperatures the theoretical gain related to the better shape in the T–S plane of both the power and the refrigeration cycle is more than offset by the higher ejector losses due to the stronger normal shock needed to cope with an increased pressure ratio.

Notwithstanding an extensive fluid screening we did not succeed in finding a fluid that could be considered optimum from all points of view including ambient and safety issues. However, a number of traditional (non-zero ODP) chloro-fluoro-carbons and of new (zero ODP) refrigerants were found that yield, on the whole, a satisfactory performance.

Provided calculated COP will be confirmed by experimental testing, ejector powered refrigerators could compete with absorption systems in many applications.     

Experimental investigation on the performance of NH3/CO2 cascade refrigeration system with twin-screw compressor

This paper describes the experiment carried out to analyze the performance of a refrigeration system in cascade with ammonia and carbon dioxide as working fluids. The effect of operation parameters, such as the evaporating temperature of the low temperature cycle, the condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and superheat degree, on the system performance was investigated. Performance of the cascade system with NH₃/CO₂ was compared with that of two-stage NH₃ system and single-stage NH₃ system with or without economizer. It was found that the COP of the cascade system is the best among all the systems, when the evaporating temperature is below −40 °C. Also, the cascade system performance is greatly affected by evaporating temperature, condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and is only slightly sensitive to superheat degree. All the experimental results indicate that the NH₃/CO₂ cascade system is very competitive in low temperature applications.     

The effect of operating conditions on the performance of a supersonic ejector for refrigeration

A previously developed one-dimensional model, based on a forward marching solution technique of the conservation equations has been used to study ejector operation and performance in a large range of refrigeration working conditions. Several important features of ejector operation characteristics were simulated. Global parameter values, their local distributions along the ejector including the temperature, the pressure and the Mach number were calculated for design and off design conditions. Operation parameters such as the entrainment ratio ω, compression ratios P_exit/P_ev, P_g/P_exit and the geometric ratio (D/D_c)² were found to significantly affect performance. The impact of the generator, the evaporator, the condenser and related thermodynamic parameters, which have been assessed in this study, are summarized as:

Fluid mixing conditions dictated by the fluid type, the mixing chamber geometry, the inlet and outlet constraints, may lead to off design operation with related stability and performance deterioration

Internal superheat generation, due to inefficient mixing and normal shock waves is very important in off design operation

Some degree of inlet superheat (around 5 °C) is necessary to prevent internal condensation but excess superheat is detrimental to the condenser efficiency at exit

Generator pressure conditions and the evaporator temperature significantly affect ejector performance.

Rotary system for the continuous production of cold by solid–gas sorption: modeling and analysis of energy performanceSystème rotatif de production continue de froid par sorption solide–gaz : modélisation et analyse des performances énergétiques

The rotary process presented here is designed for continuous operation and to use the concept of a heat regeneration cycle developed for solid sorption cold production systems. Based on the analysis of the thermodynamic cycle followed by the reagent, the system is modeled in the form of counter-flow heat exchangers in series. This allows an estimate of the energy performance of the process in terms of coefficient of performance (COP) and cold production capacity. It appears that for a given set of thermodynamic operating conditions, the number of transfer units (NTU) of the heat exchangers is the parameter, which conditions the value of the COP. A comparison between the rotary system by adsorption and by chemical reaction helps to select the ideal reagent according to the temperature level for cold production.     

Commercial refrigeration system using CO2 as the refrigerant

Various field-test systems using carbon dioxide as the only refrigerant have been installed since December 2001. In this paper we will analyse an ‘all-CO₂’ supermarket, which has been operating in the North of Italy since January 2003.The seasonal COP is calculated, based on prior laboratory measurements, and a comparison is made with a conventional direct expansion system using R404A.The total annual energy consumption of the installed CO₂ system is estimated to be about 10% higher than the direct expansion R404A solution. It is still possible to further improve efficiency and approach the efficiency of present R404A systems. These improvements are identified.The cost of the CO₂ installation is compared to the cost of an equivalent direct expansion R404A installation, the most economic among the various present types of commercial refrigeration systems. Because of the lack of suitable mass-produced components, the CO₂ installation is estimated to be, today, about 20% more expensive.     

Influence of the objective function on the optimisation of a steam ejector cycle

Optimisation is a design method to search the best value of the defined system's goal that can be expressed by an objective function to be minimised or maximised. A set of unknowns subject to constraints controls the values that the objective function can assume. A multi-variable approach enables one to address the optimisation of a thermodynamic system: the best working conditions of the system are different from those corresponding to component optimisation. Then we have to model the complete system to find an optimumty.

Different objective functions are presented as optimisation criteria of the design data of a steam ejector cycle, keeping the same boundary conditions and convergence limits and using a numerical optimisation of the cycle published by the authors. The comparison between the results obtained with different objective functions is presented to show the influence of the function chosen on the system design.

The comparison also shows that the choice of the objective function decisively influences the robustness of the numerical code results and the convergence performances of the code.     

Performances d'une machine tritherme à éjecteur utilisant des mélanges de fluides frigorigènesPerformance analysis of a jet cooling system using refrigerant mixtures

The optimisation of a jet cooling system using refrigerant mixtures as substitutes of pure refrigerants has been investigated. A steady-state simulation program, for given temperatures of the sources, integrating simple models of each component has been developed. A Peng-Robinson equation of state assuming equality of the fugacities of the two phases was used to model the thermodynamic properties of the vapour and liquid-vapour equilibrium. The refrigerants investigated in this study are: the pure refrigerants R142b, R152a, RC318, R124, R134a, R22 and the binary refrigerants R22/RC318, R22/R142b, R22/R124, R22/R152a, R22/R134a, R134a/R142b, R152a/R142b and R134a/R152a. Results show that the use of a binary mixture does not always increase the performance of system. Generally, when the mixture is strongly zeotropic (e.g.: R22/RC318), the cooling efficiency of the system decreases. However, when the mixture is mildly zeotropic (e.g. R134a/R142b) or almost azeotropic (e.g. R134a/R152a), efficiency and energetic efficiency increase.     

Thermodynamic analysis of optimal condensing temperature of cascade-condenser in CO2/NH3 cascade refrigeration systems

This study thermodynamically analyzed a cascade refrigeration system that uses carbon dioxide and ammonia as refrigerants, to determine the optimal condensing temperature of the cascade-condenser given various design parameters, to maximize the COP and minimize the exergy destruction of the system. The design parameters include: the evaporating temperature, the condensing temperature and the temperature difference in the cascade-condenser. The results agreed closely with the reported experimental data. The optimal condensing temperature of the cascade-condenser increases with T_C, T_E and ΔT. The maximum COP increases with T_E, but decreases as T_C or ΔT increases. Two useful correlations that yield the optimal condensing temperature of the cascade-condenser and the corresponding maximum COP are presented.     

Experiment and simulation on the performance of an autocascade refrigeration system using carbon dioxide as a refrigerant

The main purpose of this study is to investigate the performance of an autocascade refrigeration system using zeotropic refrigerant mixtures of R744/134a and R744/290. One of the advantages of this system is the possibility of keeping the highest pressure of the system within a limit by selecting the composition of a refrigerant mixture as compared to that in the vapor compression system using pure carbon dioxide. Performance test and simulation have been carried out for an autocascade refrigeration system by varying secondary fluid temperatures at evaporator and condenser inlets. Variations of mass flow rate of refrigerant, compressor power, refrigeration capacity, and coefficient of performance (COP) with respect to the mass fraction of R744 in R744/134a and R744/290 mixtures are presented at different operating conditions. Experimental results show similar trends with those from the simulation. As the composition of R744 in the refrigerant mixture increases, cooling capacity is enhanced, but COP tends to decrease while the system pressure rises.

Résumé

The main purpose of this study is to investigate the performance of an autocascade refrigeration system using zeotropic refrigerant mixtures of R744/134a and R744/290. One of the advantages of this system is the possibility in keeping the highest pressure of the system within a limit by selecting the composition of a refrigerant mixture as compared to that in the vapor compression system using pure carbon dioxide. Performance test and simulation have been carried out for an autocascade refrigeration system by varying secondary fluid temperatures at evaporator and condenser inlets. Variations of mass flow rate of refrigerant, compressor power, refrigeration capacity, and coefficient of performance (COP) with respect to the mass fraction of R744 in R744/134a and R744/290 mixtures are presented at different operating conditions. Experimental results show similar trends with those from the simulation. As the composition of R744 in the refrigerant mixture increases, cooling capacity is enhanced, but COP tends to decrease while the system pressure rises.     

Efficiency of the indicated process of CO2-compressors

The compressor of a refrigerant compression process is the component with the major influence on the efficiency and reliability of the entire system. Due to the fluid properties of carbon dioxide (CO₂), the pressure ratio of the refrigeration process with CO₂ as the working fluid is, in relation to common refrigeration processes, rather low while the pressure difference is extremely high. From experimental and theoretical considerations it becomes obvious that at these conditions a high volumetric and energetic efficiency of the compressor may be achieved if its design is appropriate. In this paper, the effects on the efficiency of the indicated compression process of a CO₂-compressor are discussed and evaluated and a promising design concept for an efficient CO₂-compressor is derived.     

Optimized transcritical CO2 heat pumps: Performance comparison of capillary tubes against expansion valves

A capillary tube based CO₂ heat pump is unique because of the transcritical nature of the system. The transcritical cycle has two independent parameters, pressure and temperature, unlike the subcritical cycle. In the present study, a steady state simulation model has been developed to evaluate the performance of a capillary tube based transcritical CO₂ heat pump system for simultaneous heating and cooling at 73 °C and 4 °C, respectively against optimized expansion valve systems. Capillary tubes of various configurations having diameters of 1.4, 1.5 and 1.6 mm along with internal surface roughness of 0.001–0.003 mm have been tested to obtain the optimum design and operating conditions. Subcritical and supercritical thermodynamic and transport properties of CO₂ are calculated employing a precision in-house property code.

It is observed that the capillary tube system is quite flexible in response to changes in ambient temperature, almost behaving to offer an optimal pressure control. System performance is marginally better with a capillary tube at higher gas cooler exit temperature. Capillary tube length turns out to be the critical parameter that influences system optimum conditions. A novel nomogram has been developed that can be employed as a guideline to select the optimum capillary tube.