Mechanical Subcooling Research Articles

CO2 with Mechanical Subcooling vs. CO2 Cascade Cycles for Medium Temperature Commercial Refrigeration Applications Thermodynamic Analysis

A recent trend to spread the use of CO2 refrigeration cycles in warm regions of the world is to combine a CO2 cycle with another one using a high performance refrigerant. Two alternatives are being considered: cascade and mechanical subcooling systems. Both respond to a similar configuration of the refrigeration cycle, they being based on the use of two compressors and same number of heat exchangers. However, the compressor, heat exchanger sizes and energy performance differ a lot between them. This work, using experimental relations for CO2 and R1234yf semi-hermetic compressors analyzes in depth both alternatives under the warm climate of Spain. In general, it was concluded that the CO2 refrigeration solution with mechanical subcooling would cover all the conditions with high overall energy efficiency, thus it being recommended for further extension of the CO2 refrigeration applications.

Thermodynamic Performance Analysis of CO 2 Transcritical Refrigeration Cycle Assisted with Mechanical Subcooling

Abstract The thermal performance of CO 2 transcrtical refrigeration cycle can be improved by cooling the CO 2 fluid exiting the gas cooler with an assisted vapor compression refrigeration cycle (auxiliary cycle). Thus, a thermodynamic analysis is performed to study the operation characteristics of the subcooling CO 2 transcritical refrigeration cycle. The results indicate that a maximum COP is achieved at the corresponding optimum discharge pressure and the optimum subcooling temperature. The improvement in COP is more significant in the case of higher ambient temperature and lower evaporation temperature, and the discharge pressure and temperature can be obviously reduced. The auxiliary cycle refrigerant is screened and R717 performs with the highest COP. The CO 2 transcritical assisted with mechanical subcooling is recommended for the cases with higher ambient temperature and lower evaporation temperature.

Experimental evaluation of transcritical CO2 refrigeration with mechanical subcooling

Transcritical CO2 booster refrigeration systems have seen a growing interest in recent years as they can provide two-stage refrigeration with R744, which has a global warming potential of 1.0, as the working fluid. However, the efficiency of these systems, particularly in transcritical operation, is often not as good as the efficiency of comparable conventional equipment. One possible way to improve performance in hot climates is through the use of a mechanical subcooler. This study describes laboratory testing of a lab-scale transcritical booster system, with and without mechanical subcooling and in transcritical and subcritical operating modes. In addition, a new steady-state model for generalized vapor compression systems was developed and is presented here. The model is used to simulate the system tested in the current article. The testing showed a significant improvement in both capacity and efficiency at all operating conditions with the subcooler. The solver was able to match capacity, power, and efficiency to within 3% for all test conditions. The model was used to perform a parametric study of subcooler capacity on the cycle; the results show that added subcooling substantially improves overall coefficient of performance and reduces the need for bypass flow, but the overall coefficient of performance benefit diminishes as the efficiency of the main cycle increases.

Comparative Assessment of Low-GWP Based Refrigerating Plants Operating in Hot Climates

Abstract Carbon dioxide (CO 2 or R744) and HFO-1234ze(E) are two promising alternatives to the currently employed high-GWP working fluids for food retail applications. In this paper, two indirect refrigeration systems using respectively R1234ze(E) and CO 2 as the primary and the secondary fluid, a R744 booster configuration with parallel compression and a R744 booster solution with R290 dedicated mechanical sub-cooling are theoretically compared with a R404A multiplex direct expansion refrigerating system (baseline). The latter serves both the LT and the MT load. All the evaluated configurations are supposed to be for commercial refrigeration technologies. The results in terms of COP reveal that both the investigated “CO 2 only” configurations have the best performance at low outdoor temperatures. On the other hand, the indirect solutions can outperform all the selected refrigerating plants at the extreme climatic conditions. In comparison with the baseline, a maximum reduction in annual electricity consumption by 6.4% and 8.9% is observed for R1234ze(E) and CO 2 based systems, respectively.

Thermodynamic analysis of a vapour compression refrigeration system integrated with a subcooler cycle

In this paper numerical investigation of a vapour compression refrigeration system using mechanical subcooler loop has been presented. A mathematical model has been developed for the systems with and without subcooler based on energy and exergy equations. Refrigerant used in both the systems is R134a. The performance of the systems with and without subcooler has been analysed and compared. The predicted results show that cooling load of the system increases nearly by 11% when subcooler cycle is employed. COP and exergetic efficiency of the system increase and exergy destruction rate decreases when subcooler is incorporated into the system. The increase in COP is found to be 11.4% and 16% at 0°C and -10°C evaporator temperatures respectively. Exergy destruction rate of the system is found to decrease by 17.8% and 8.8% with the use of subcooler at evaporator temperatures of 0°C and -10°C respectively.

Experimental evaluation of a CO2 transcritical refrigeration plant with dedicated mechanical subcooling

CO2 transcritical refrigeration cycles require optimization to reach the performance of conventional solutions at high ambient temperatures. Theoretical studies demonstrated that the combination of a transcritical cycle with a mechanical subcooling cycle improves its performance; however, any experimentation with CO2 has been found. This work presents the energy improvements of the use of a mechanical subcooling cycle in combination with a CO2 transcritical refrigeration plant, experimentally. It tested the combination of a R1234yf single-stage refrigeration cycle with a semihermetic compressor for the mechanical subcooling cycle, with a single-stage CO2 transcritical refrigeration plant with a semihermetic compressor. The combination is evaluated at two evaporating levels of the CO2 cycle (0 and −10 °C) and three heat rejection temperatures (24, 30 and 40 °C). The optimum operating conditions and capacity and COP improvements are analysed with maximum increments on capacity of 55.7% and 30.3% on COP.

Energy and environmental performance assessment of R744 booster supermarket refrigeration systems operating in warm climates

This paper presents a theoretical comparison among different commercial refrigeration systems in terms of annual energy consumption and environmental impact. Eight configurations were studied: a R744/R134a cascade refrigeration system (baseline), a conventional and an improved R744 booster system, two R744 booster solutions with dedicated mechanical subcooling, a R744 booster with parallel compression and two solutions which combined the parallel compression and the mechanical subcooling. The evaluation was based on the weather data in Valencia (Spain) and in Athens (Greece), as well as on the running modes of a conventional European supermarket. A transition zone, which occurred between subcritical and transcritical operations, was adopted.The results showed that all the enhanced configurations may achieve a comparable energy saving to the one of the baseline in both the selected locations. Furthermore, they allow reducing the Total Equivalent Warming Impact (TEWI) by at least 9.6% beside the cascade solution.

Energy improvements of CO2 transcritical refrigeration cycles using dedicated mechanical subcooling

In this work the possibilities of enhancing the energy performance of CO2 transcritical refrigeration systems using a dedicated mechanical subcooling cycle are analysed theoretically. Using simplified models of the cycles, the modification of the optimum operating conditions of the CO2 transcritical cycle by the use of the mechanical subcooling are analysed and discussed. Next, for the optimum conditions, the possibilities of improving the energy performance of the transcritical cycle with the mechanical subcooling are evaluated for three evaporating levels (5, −5 and −30 °C) for environment temperatures from 20 to 35 °C using propane as refrigerant for the subcooling cycle. It has been observed that the cycle combination will allow increasing the COP up to a maximum of 20% and the cooling capacity up to a maximum of 28.8%, being both increments higher at high evaporating levels. Furthermore, the results indicate that this cycle is more convenient for environment temperatures above 25 °C. Finally, the results using different refrigerants for the mechanical subcooling cycle are presented, where no important differences are observed.

Performance evaluation of an ejector subcooled vapor-compression refrigeration cycle

In this study, a novel vapor-compression refrigeration cycle with mechanical subcooling using an ejector is proposed to improve the performance of a conventional single-stage vapor-compression refrigeration cycle. In the theoretical study, a mathematical model is developed to predict the performance of the cycle by using R404A and R290, and then compared with that of the conventional refrigeration cycle. The simulation results show that the performance of the ejector subcooled cycle is better than that of the conventional cycle. When the evaporator temperature ranges from −40 to −10°C and the condenser temperature is 45°C, the novel cycle displays volumetric refrigeration capacity improvements of 11.7% with R404A and 7.2% with R290. And the novel cycle achieves COP improvements of 9.5% with R404A and 7.0% with R290. In addition, the improvement of the COP and cooling capacity of this novel cycle largely depends on the operation pressures of the ejector. The potential practical advantages offered by the cycle may be worth further attention in future studies.

Design of a mechanical subcooling system device for increasing a low temperature refrigeration system’s capacity

El subenfriamiento mecánico se emplea para incrementar el coeficiente de desempeño de cualquier ciclo de refrigeración,especialmente en las aplicaciones de baja temperatura. Cuando es necesario reconfigurar un cicloexistente, un dispositivo de subenfriamiento mecánico permite una gran flexibilidad para encontrar un puntoespecífico de operación, ya sea un sistema combinado con un mejor coeficiente de desempeño (COP) y una mayorcapacidad de refrigeración o un sistema combinado de la misma capacidad que trabaja por menos tiempo y unmejor COP. Las simulaciones corridas en Aspen HYSYS® permiten estudiar tantos casos posibles como puedenpresentarse para lograr un diseño óptimo en el sistema reconfigurado, teniendo en cuenta la restricciones querepresentan cada uno de los componentes existentes en el sistema principal.

A proposed subcooling method for vapor compression refrigeration cycle based on expansion power recovery

This study proposes a new subcooling method for vapor compression refrigeration cycle based on expansion power recovery. In a main refrigeration cycle, expander output power is employed to drive a compressor of the auxiliary subcooling cycle, and refrigerant at the outlet of condenser is subcooled by the evaporative cooler, which makes the hybrid system get much higher COP. Various refrigerants, including R12, R134a, R22, R32, R404A, R41, R507A, R717, and R744, are considered. Thermodynamic analysis is made to discuss the effects of operation parameters (expander efficiency and inlet temperature of cooling water) on the system performance. Results show that the proposed hybrid vapor compression refrigeration system achieves much higher COP than the conventional vapor compression refrigeration system, conventional mechanical subcooling system and conventional expansion power recovery system, with maximum COP increments 67.76%, 19.27% and 17.73%, respectively when R744 works as the refrigerant in the main refrigeration cycle. It is most beneficial for R12 and R717 in the auxiliary subcooling cycle and R744, R404A and R507A in the main refrigeration cycle.

Cost optimization of heat exchanger inventory for mechanical subcooling refrigeration cycles

In this paper, thermoeconomic considerations are given to heat exchanger inventory allocation in vapor compression cycles with mechanical subcooling. Investigation is made with respect to constant work rate, heat rejection and cooling rates as well as heat transfer in the subcooler. It was found that no minima exist for any of the cost functions with respect to the absolute temperature ratio (ξ) and the average subcooling absolute temperature ratio (θ3). The derivatives for the integrated subcooling cycle can be generated from the derivatives of the dedicated subcooling cycle. It was concluded that the cost optimization of the integrated mechanical subcooling system is qualitatively the same as the dedicated subcooling system.

Experimental energetic analysis of a vapor compression refrigeration system with dedicated mechanical sub-cooling

In this work, an experimental investigation into the effects, in terms of energy, of employing a dedicated mechanical subcooling cycle with a residential 1.5ton simple vapor compression refrigeration system is presented. A comparative analysis of the experimental cycle performance is conducted with and without the dedicated subcooler cycle when the room temperature is kept between 18 and 22°C. This is done in order to ascertain the percentage increase in efficiency due to use of a dedicated subcooling loop. R22 is employed as the refrigerant in the main cycle whereas R12 is flowing in the dedicated subcooling cycle. The experimental outcomes indicate that the load carrying capacity of the evaporator increased by approximately 0.5kW when R22 was subcooled, in the main cycle, by 5–8°C. It was also noted that, by using the subcooling, the second-law efficiency of the cycle increased by an average of 21%. Furthermore, the general trend indicated that this percentage increase is inversely proportional to the ambient temperature variation. The experimental work proves that dedicated subcooling can be used for increasing cooling capacity and efficiency.

The effect of refrigerant combinations on performance of a vapor compression refrigeration system with dedicated mechanical sub-cooling

Performance characteristics due to use of different refrigerant combinations in vapor compression cycles with dedicated mechanical sub-cooling are investigated. For scratch designs, R134a used in both cycles produced the best results in terms of COP, COP gain and relative compressor sizing. In retrofit cases, considering the high sensitivity of COP to the relative size of heat exchangers in the sub-cooler cycle and the low gain in COP obtained due to installation of a dedicated sub-cooling cycle when R717 is the main cycle refrigerant, it seems that dedicated mechanical sub-cooling may be more suited to cycles using R134a as the main cycle refrigerant rather than R717. With R134a as the main cycle refrigerant, no major difference was noted, by changing the sub-cooler cycle refrigerant, in the degradation of the performance parameters such as COP and cooling capacity, due to equal fouling of the heat exchangers.

The impact of fouling on performance of a vapor compression refrigeration system with integrated mechanical sub-cooling system

Performance characteristics due to fouling in a vapor compression cycle with integrated mechanical sub-cooling are investigated for various applications. Considering the first set of refrigerants i.e. R134a, R410A and R407C, from a first law standpoint, the COP indicates that R134a always performs better unless only the evaporator is being fouled. From a second law standpoint, the second-law efficiency indicates that R134a performs the best in all cases. Considering the second set of refrigerants i.e. R717, R404A and R290, the COP indicates that R717 always performs better unless only the evaporator is being fouled; however, the second-law efficiency indicates that R717 performs the best in all cases. Furthermore, the performance degradation due to fouling of the evaporator always has a larger effect on cooling load capacity while the performance degradation of the condenser always has an overall larger effect on the sub-cooler compressor power requirement as well as the COP of the system. In light of the data generated, a new performance degradation law, presented earlier, is shown to be applicable for this system as well that can reduce the amount of experimentation and help predict relevant quantities of the refrigeration system.

Applying mechanical subcooling to ejector refrigeration cycle for improving the coefficient of performance

This paper describes a new ejector refrigeration system with mechanical subcooling which uses an auxiliary liquid–gas ejector to enhance subcooling for the refrigerant from condenser. The new system can have larger subcooling degree when circulating pump consumes a little more power compared with conventional ejector refrigeration system. Based on the built mathematical model, the performance of the new ejector refrigeration system was discussed and compared with that of a conventional ejector refrigeration system for refrigerant R142b. Theoretical analyzing results show that the new system can efficiently improve the coefficient of performance (COP) of ejector refrigeration.

Modelling and experimental validation of advanced refrigeration systems in supermarkets

The effective use of energy and the replacement of CFC and HCFC refrigerants are two factors that have influenced the design and operation of refrigeration systems in supermarkets during recent years. The potential for increasing energy efficiency in refrigeration systems, indoor climate and refrigerated cabinets is large. Since the energy systems of a supermarket are relatively complex, improvements in one subsystem affect other systems, thus making analysis of potential improvements non-additive. A computer model, CyberMart, that predicts building heating and cooling loads, HVAC (heating, ventilation, and air conditioning), and refrigeration system performances of a supermarket, has been developed. The focus of the model is on energy use, environmental impact (TEWI), and life cycle cost (LCC) of the refngeration system. The refrigeration system solutions included in the model are: direct system, completely indirect system, partially indirect system, cascade system, parallel system with mechanical sub-cooling (where the refrigerant in the low-temperature system is sub-cooled with the brine of the intermediate temperature level), and district cooling (that cools the condenser of the refrigeration machines). Measurements of different parameters such as outdoor and indoor temperatures, relative humidity, and compressor power have been carried out in several stores to validate the model. A theoretical description of the model and results from the model and measurements are presented in this paper.

Thermodynamic optimization of finite time vapor compression refrigeration systems

The optimization of refrigeration systems involves finding the optimum operating point at which the coefficient of performance of the system is maximum. For most vapor compression refrigeration systems, optimum performance is obtained by proper distribution of the total heat exchanger area between the evaporator and condenser of the system. In this paper, finite time thermodynamic models of a simple vapor compression refrigeration system, a two stage vapor compression refrigeration system (TSS), a dedicated mechanical subcooling vapor compression refrigeration system (DMSS) and an integrated mechanical subcooling vapor compression refrigeration system (IMSS) are presented to study the systems with respect to the heat exchanger parameters. In addition, the model of the TSS is used for investigating the optimum performance with respect to interstage temperature, while the models of the DMSS and IMSS are used for investigating the optimum performance with respect to the subcooler saturation temperature.

Design and rating of an integrated mechanical-subcooling vapor-compression refrigeration system

Subcooling of the refrigerant at the exit of the condenser in a simple vapor-compression refrigeration system allows the refrigerant to enter the main cycle evaporator with low quality. Thus, allowing the refrigerant to absorb more heat in the evaporator; thereby improving the coefficient of performance (COP) of the system. In an integrated mechanical-subcooling vapor-compression refrigeration system, the subcooling is performed by utilizing a small integrated vapor-compression refrigeration cycle, known as the subcooler cycle. This subcooler cycle is coupled to the main cycle at the exit of the condenser and it utilizes the main cycle condenser for rejecting the heat. In this paper, thermodynamic models of an integrated mechanical subcooling system are developed to simulate the actual performance of the subcooling system, particularly with respect to the subcooler saturation temperature in addition to heat exchanger areas. It is demonstrated that the performance of the overall cycle is improved over the corresponding simple cycle. This improvement is found to be related to the refrigerant saturation temperature of the subcooler. The model is also used for predicting an optimum distribution of the total heat exchanger area between the evaporator and condenser.

5620046 Heat exchanger, particularly a refrigerant evaporator

Using mechanical sub-cooling systems to increase COP of vapor compression cycles is a known method in literature to save energy and increase efficiency. Recently, much progress has been made with respect to investigation into its different aspects that can help to put it into practice. Numerical and experimental works are considered for the purpose of highlighting this progress. These can be categorized as: a) simulation of performance characteristics resulting from different refrigerant combinations in dedicated mechanical sub-cooling systems, b) variation in performance characteristics for a vapor compression cycle using integrated mechanical sub-cooling because of fouling, c) experimental study about consequences of employing a dedicated mechanical subcooling cycle with a simple vapor compression system, and d) experimental investigation about consequences of employing a subcooler in a two-stage refrigeration cycle. Some important results are discussed. Finally, some suggestions are made to provide direction into future research in this area to help put it into practice.