Indirect Refrigeration Systems Research Articles

Experimental Investigation of R404A Indirect Refrigeration System Applied Internal Heat Exchanger: Part 2—Exergy Characteristics

Although the R404A indirect refrigeration system (IRS) with an internal heat exchanger (IHX) and R744 as the secondary fluid has potential applications in supermarkets and hypermarkets, the exergy characteristics of this IRS have not been extensively investigated. In this study, the factors affecting the R744 exergy characteristics (degree of subcooling (DSB) and degree of superheating (DSP) of the R404A cycle, DSP of the R744 cycle, condensation temperature (CT) and cascade evaporation temperature (CET), and IHX efficiency) were experimentally evaluated to obtain basic data for the design of R404A IRS with R744 as the optimal secondary fluid. The main results can be summarized as follows: (1) Under given conditions, the smallest change in the system exergy destruction rate (EDR) according to the change in each parameter is the DSP of the R744 cycle (0.3–1%), followed by the DSB of the R404A cycle (6.1–8.8%), the IHX efficiency of the R404A cycle (3.8–14.3%), the DSP of the R404A cycle (11.7–15.9%), the CET (29.4–41.9%), and the CT (35–47%). (2) Also, in terms of the exergy efficiency of system (EES), the largest value was obtained for the DSP of the R404A cycle (2.4–12.7%), followed by the IHX efficiency of the R404A cycle (3–10.2%), the CET (2.2–8.7%), the CT (4–6.9%), the DSB of the R404A cycle (2.7–6.2%), and the DSP of the R744 cycle (0.04–1.2%). (3) In order to lower the system EDR, DSP, DSB, and IHX efficiency of the R404A cycle, the CET must be increased to the maximum, and to lower the DSP of the R744 cycle, the CT must be reduced to the minimum.

Experimental Investigation of R404A Indirect Refrigeration System Applied Internal Heat Exchanger: Part 1—Coefficient of Performance Characteristics

In this study, the performance characteristics of an R404A indirect refrigeration system (IRS) applied to an internal heat exchanger (IHX) is evaluated for supermarkets and hypermarkets. In a direct expansion system, R404A is the primary refrigerant and R744, a brine, is the secondary fluid. While there are abundant studies analyzing the theoretical performance of IRS, experimental research on IRS is lacking, and there are no papers that address the results of changes in the IHX in detail. In this study, the results achieved by modifying various parameters are experimentally evaluated to provide fundamental data for designing the optimal IRS. In the main results, looking at the trend of the increase in IHX efficiency, the change is very minimal when the efficiency is above 50%, so it is ideal to apply an IHX efficiency of about 50% considering economics and COP, etc. Applying the results in this study enables the operation and maintenance of IRSs as an eco-friendly system by achieving energy efficiency through optimizing the system coefficient of performance and securing economic feasibility by minimizing the R404A charging amount of the refrigeration cycle. To serve supermarkets and hypermarkets, R744 as a secondary fluid may help to realize an ecologically friendly, compact IRS system with a high heat transfer coefficient that can operate at low temperatures (−35 to 5 °C).

Experimental investigation of thermophysical properties of propylene glycol based secondary fluids for ground source heat pumps and indirect refrigeration systems

More than 1.7 million ground source heat pumps are currently in operation in the European Union and a large majority of systems is using propylene glycol as secondary fluid. In this study the thermophysical properties of propylene glycol solutions with specific freezing points between −5 and −50 °C are investigated at low temperatures of interest for refrigeration and heat pump applications. The experimentally determined data are compared to a large number of data from the literature. It has been found that the freezing point results for all concentrations were in very good agreement with both Melinder (2010) and ASHRAE (2021) data. Moreover, the literature data and present study for concentrations above 40 wt-% have different density curve slopes compared to ASHRAE data. The viscosity results for concentrations below 40 wt-% were up to 21.5 % higher than ASHRAE data and the viscosity results for solutions with concentrations above 40 wt-% were lower by up to 9.7 %. The thermal conductivity results for concentrations above 40 wt-% were 10 % higher than ASHRAE (2021) data. The experimental specific heat capacity curves and other studies do not have the same linear trend as Melinder (2010) and ASHRAE (2021) data. This difference indicates that currently available ASHRAE data have been indirectly computed from thermal conductivity data or that they have not been fully experimentally validated. Finally, more studies are required to investigate concentrations below 25 wt-% to identify solutions with higher specific heat capacity than water that could be of interest for specific applications.

Experimental investigation of thermophysical properties of ethylene glycol based secondary fluids

Aqueous solutions of ethylene glycol are commonly used as secondary fluids in different indirect refrigeration systems and heat pumps as well as nanofluids. A very extensive literature review has been done, including more than 90 references published from 1905 to 2023. Despite the wide application and importance, especially in the energy sector, ethylene glycol solutions seem to be less investigated in low temperature ranges and more research is required to improve the quality and quantity of available data. The novelty of this paper is to investigate the most important thermophysical properties of ethylene glycol solutions in low temperatures. In this study a different approach was made and solutions having a specific freezing point temperature (between -5 and -50 ºC) rather than specific concentration were investigates in temperature ranges applicable for different cooling applications. The concentrations giving a certain freezing point temperature seemed to deviate in some cases with 1–2 wt-% between different sources. Nevertheless, the density results were in rather good agreement with all reference data. The viscosity results were lower by up to ±10% compared to reference values. Additionally, the obtained experimental results for thermal conductivity were higher by up to 12% compared to ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) data. The specific heat capacity results were higher by up to 14.6% and 5.4% than current reference data. There is a high probability that the current ASHRAE data for specific heat are actually indirectly calculated values from thermal conductivity data and not validated using differential scanning calorimetry techniques.

Technical and economic evaluation of a novel liquid CO2 energy storage-based combined cooling, heating, and power system characterized by direct refrigeration with phase change

Energy storage systems combining cooling, heating, and power have higher flexibility and overall energy efficiency than standalone systems. However, achieving a large cooling-to-power ratio in direct-refrigeration systems without a phase change and in indirect refrigeration systems driven by heat is difficult, limiting the energy output of the system. Therefore, this study proposes a novel combined cooling, heating, and power system based on liquid CO2 energy storage. Using direct refrigeration with a phase change, the system has a large cooling capacity and can achieve a wide range of cooling-to-power ratios through the mass flow regulation of the refrigeration branch. Energy, exergy, and economic analyses were conducted based on models of the proposed system. Adjusting the split ratio of the system splitter S1 can realize a wide range of changes in the cooling- and heating-to-power ratios of the system and can flexibly meet the demand for cooling, heating, and power generation. The system with the best overall performance was obtained by optimizing the levelized cost of storage as the objective function, where the system’s power efficiency, exergy efficiency, energy efficiency, levelized cost of storage, and energy storage density were 0.56 %, 18.15 %, 319.76 %, 0.10 $/kW·h, and 19.17 kW·h/m3, respectively. The proposed system in the standalone power-storage mode has technical and economic performance advantages, as compared to other systems reported in the literature.

Comparative study regarding retrofitting with a low GWP refrigerant in an ice rink with energy recovery implementation

The authors make a comparative study between an existing R407C indirect refrigeration system which serves an ice rink refrigeration plant with a modern R744 indirect refrigeration system incorporating heat recovery, by using “Pack Calculation Pro” software. The proposed solution involves no alterations in the water-ethylene glycol circuit that ensures the rink’s cooling load. By considering the need for retrofitting to comply with current legislation, the study aims to identify the best solution in terms of environmental impact and the CO2-equivalent release into the atmosphere. The recovered energy can be further used for space heating, ventilation, and domestic hot water preparation. This study is targeted on an existing ice rink placed in Iasi, Romania, and the analysis demonstrates that the implementation of this solution reduces the environmental impact of CO2-equivalent reduction, and the global COP (Coefficient of Performance) value increases.

Rheological properties of CO2 hydrate slurries in presence of Dioctyl sodium sulfosuccinate (AOT) in a dynamic loop for refrigeration application

Recent studies have shown real potential for using hydrate slurries as a secondary fluid in indirect refrigeration systems. However, pipeline clogging problems can appear due to hydrate agglomeration. Additives are generally used to counteract these problems. In this study, rheological properties of CO2 hydrate slurries in the presence of an additive Dioctyl sodium sulfosuccinate (AOT) are investigated. The capillary viscometer method is used to determine the fluid behaviour and the AOT impact on the slurry rheology. A dynamic loop is set up to collect experimental pressure drop and volume flow rate data. Based on the Rabinowitch and Mooney approach and Herschel-Bulkley model to determine the rheological properties, a shear-thinning behaviour was observed. The importance of the behaviour index choice in the procedure is discussed. Finally, the results obtained with AOT are compared with pre-existing studies. The present study highlights the AOT potential to significantly improve the CO2 hydrate slurries properties.

Research and Improvement of Heat Transfer Performance of Plate and Shell Evaporator

Evaporator plays a key role in indirect refrigeration system, so improving the efficiency of evaporator is of great significance to improve the overall performance of refrigeration system. For the plate and shell evaporator, the reliability of the model is proved by numerical simulation of the existing evaporator and comparison with the experiment. An improved scheme is proposed for the existing evaporator, and the improved model is simulated and analyzed. The result shows that compared with the previous model, the improved model improves the heat exchange efficiency, reduces the pressure drop, and improves the overall efficiency of the evaporator.

Increasing the energy efficiency of an indirect refrigeration system through automation

In this publication, the authors explore in detail ways to improve the energy efficiency of the refrigeration system with an intermediate coolant. After all, cold systems are systems that are used to cool and preserve food in factories, shops and homes. Their efficiency directly affects the safety of products, as well as saving energy and resources. Automation is one of the methods to increase the energy efficiency of a cooling system. This technology can help to manage the processes of the system in automatic mode, which reduces the human factor and increases the accuracy of control. Automation can help identify possible problems and malfunctions in the refrigeration system, which reduces equipment downtime. Automation is not the only way to increase the energy efficiency of a refrigeration system. Optimizing heat transfer can also greatly improve system performance. This method aims to reduce heat loss and improve heat exchange, which helps to reduce the energy consumption of the system. Also, the installation of energy-saving devices and the use of environmentally friendly refrigerants can greatly improve the efficiency of the system. The authors draw attention to the importance of developing individual solutions for each cooling system, which take into account its specifics and features. Such solutions may include not only automation, but also other methods for optimizing the operation of the system. Based on the foregoing, we can conclude that in order to improve the energy efficiency of the refrigeration system, it is necessary to use an integrated approach, which includes automation, heat exchange optimization, installation of energy-saving devices, the use of environmentally friendly refrigerants, as well as the development of individual solutions for each particular refrigeration system.

Assessment of Efficiency of Heat Transportation in Indirect Propane Refrigeration System Equipped with Carbon Dioxide Circulation Loop

Recent research on indirect cooling systems using natural refrigerants has become increasingly common. One such solution is the gravity-induced circulation loop. The paper provides model considerations of the configuration of an indirect propane refrigeration system equipped with a circulation loop using carbon dioxide as a heat transfer fluid. Close attention has been paid to the analytical modelling of the carbon dioxide circulation loop operation. The model was formulated to determine the optimum height of the liquid downcomer based on the determination of flow resistance and heat transfer rate in evaporation and condensation processes. A validation of the proposed analytical model against the available literature on two-phase flow structure predictions and thermal performance predictions was performed. The effect of the change in the refrigeration capacity of the system on the coefficient of performance COP of the entire indirect system was analysed for the first time. The analysis was performed for three different carbon dioxide evaporation temperatures for the system’s refrigeration capacity, ranging from 0.5 to 10 kW. It has been proven that the system efficiency increases by up to 23% with an increase in the refrigeration capacity of the system. An increase in evaporation temperature in the circulation loop from −20 °C to 0 °C improves the COP of the entire indirect refrigeration system by approximately 50%. The above findings indicate that indirect cooling systems using naturally circulated CO2 as a heat transfer fluid should be designed for operation at maximum refrigeration capacity.

Investigation of Heat Transfer and Pressure Drop for R744 in a Horizontal Smooth Tube of R744/R404A Hybrid Cascade Refrigeration System—Part 1: Intermediate Temperature Region

In this study, the evaporation heat transfer characteristics of an intermediate temperature of R744 in a smooth horizontal tube, when operating as an indirect refrigeration system (IRS) among hybrid cascade refrigeration systems (HCRSs), are evaluated. Studies on the characteristics of intermediate-temperature evaporation heat transfer under the operating conditions of evaporators used in actual refrigeration systems, such as IRS, cascade refrigeration systems, and HCRS, used in supermarkets are lacking. Thus, this study provides basic data on the characteristics of evaporation heat transfer of R744 in the evaporators of refrigerators used in supermarkets. The tube employed to the evaporation experiment in this study was a horizontal smooth copper tube with a length of 8000 mm and an inner diameter of 11.46 mm. The experimental variables were measured over a wide range of mass flux of 200–500 kg/(m2·s), heat flux of 10–40 kW/m2, and saturation temperature of −40–0 °C. The main results are summarized as follows: (1) The application of Kandlikar’s correlation formula at an evaporation temperature of −20 °C in an IRS helps in a good prediction of the R744 evaporation heat transfer coefficient. (2) The pressure drop according to the heat and mass flux showed the same heat transfer coefficient trend, but the pressure drop at saturation temperature was different from the trend of heat transfer coefficient.

Influence of secondary fluid on the performance of indirect refrigeration system

• Performance of three new secondary liquids was compared with that of ethylene glycol. • Secondary liquid with best performance and its best working parameters were obtained. • Considering cost and safety, LM-4 secondary fluid can be used to directly replace ethylene glycol. This paper proposes solutions to address the issues of traditional refrigerant substitution and the compatibility between secondary fluid and indirect refrigeration system. By using heat balance test method, the performance of three new different secondary fluids, LM-1, LM-4, LM-8, is compared with ethylene glycol at different flow rates. The secondary fluid with the best performance and its optimal working parameters are obtained. Furthermore, the Reynolds and Prandtl numbers corresponding to the optimal working parameters are derived, which provides theoretical basis for the design of intermediate heat exchanger in indirect refrigeration systems. The test results show that in the cold storage temperature of −18 °C, compared to the aqueous solution of ethylene glycol, the coefficient of performance ( COP c ) of LM-8, LM-1, and LM-4 is higher by 12.3%, 3.9%, and 1.7%, respectively. When the cold storage temperature of is 0 °C, compared to that of ethylene glycol solution, the COP c of LM-1 and LM-8 increases by 9.3% and 6.1%, respectively, while the COP c of LM-4 decreases by 0.9%. Therefore, these three secondary fluids can be used as potential alternatives for ethylene glycol, but LM-1 has a low flash point and certain risk, so it is not recommended to use. LM-8 exhibits the best performance, but the price is more expensive. The LM-4 is less expensive and has the closest performance to ethylene glycol. In the existing systems, considering cost and safety, LM-4 secondary fluid can be used to directly replace ethylene glycol.

Exergy characteristics of R404A indirect refrigeration system using CO2 as a secondary refrigerant

In this paper, R404 refrigeration system is evaluated to understand the exergy characteristics of an indirect refrigeration system for R404A that adopted CO2 as a secondary refrigerant. This provides basic data for the optimal design of indirect refrigeration system for the R404A. The results are summarized as follows. It was confirmed that the system’s exergy loss increased as the R404A condensation temperature, the CO2 cooler temperature difference, and the CO2 mass flow rate increased under a fixed experimental condition. However, the system’s exergy loss decreased as the CO2 evaporation temperature increased. It was also found that the exergy loss of the R404A refrigeration system was higher than that of the CO2 secondary refrigerant system. It was also confirmed that total exergy loss of the R404A refrigeration system and the indirect refrigeration system tended to be inversely related to the coefficient of performance. Regardless of any experimental conditions, the exergy loss rate of components was the largest in the compressor, followed by the R404A condenser, the expansion valve, the internal heat exchanger, the CO2 cooler, the CO2 evaporator and the pump. Therefore, in conclusion, it is necessary to reduce exergy loss in the compressor and the R404A condenser to improve the overall performance of this system.

Conversion of a Direct to an Indirect Refrigeration System at Medium Temperature Using R-134a and R-507A: An Energy Impact Analysis

This work presents the experimental evaluation of energy consumption and refrigerant charge reduction when a commercial direct expansion refrigeration system is converted into an indirect system. The evaluation (with R-134a and R-507A) used a commercial cabinet with doors for medium temperature and a single-stage refrigeration cycle using a semi-hermetic compressor and electronic expansion valve; 24-h energy consumption tests were performed at laboratory conditions for each refrigerant and configuration at three heat rejection levels (23.3, 32.8 and 43.6 °C), maintaining an average product temperature inside the cabinet of 2 °C. The work analyses the impact of the conversion on temperature and pressure indicators, as well as, in the energy performance of each element. For R-134a the refrigerant charge was reduced in a 42.9%, but the energy consumption rose by 22.0%–22.8%; for R-507A the charge reduction was of 32.8% with an increase in energy consumption of between 27.7% and 38.7%.

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.

Retrofit of lower GWP alternative R449A into an existing R404A indirect supermarket refrigeration system

R404A is going to be phased out from most of the commercial refrigeration systems due to its high GWP value of 3943. R449A (GWP of 1282) has been proposed to replace R404A with only minor system modifications in supermarkets. This paper presents the measurements of a light retrofit replacement of R404A using R449A in a medium temperature indirect refrigeration system (secondary fluid temperature at the evaporator outlet between −9 and −4 °C). It has been demonstrated that with a slight expansion device adjustment and 4% increase of refrigerant charge, R449A can be used in this refrigeration system designed for R404A because of its suitable thermodynamic properties and acceptable maximum discharge temperature. At a secondary fluid temperature at condenser inlet of 30 °C, the COP of R449A nearly matches that of R404A (both were between 1.9 and 2.2), despite having approximately 13% lower cooling capacity. As a conclusion, attending to the GWP reduction and similar energy performance, it was demonstrated using the TEWI methodology that the use of the recently developed refrigerant R449A in these applications can reduce the total CO2 equivalent emissions of an indirect supermarket refrigeration system designed for R404A refrigerant.

Theoretical evaluation of supermarket refrigeration systems using R1234ze(E) as an alternative to high-global warming potential refrigerants

Hydro-fluoro-olefin-1234ze(E) features an insignificant global warming potential and it could be successfully used in supermarket applications in an attempt to foster solutions which are more eco-friendly than the currently employed ones. In this article, four indirect refrigeration systems using R1234ze(E) as the primary working fluid were theoretically compared with both a R404A multi-plex solution and a R744/R134a cascade configuration. The annual electricity consumption and the total equivalent warming impact were assessed in three different American locations and in two European cities. The outcomes obtained demonstrate that R1234ze(E) can suitably replace R404A in commercial refrigerating plants, especially in warm climates. In fact, R1234ze(E)-based solutions exhibits similar energy saving and at least a decrement by 26.7% in total equivalent warming impact beside the configurations employing R404A.

2차 냉매로 천연냉매 R744를 사용하는 냉동시스템의 성능 특성

In this paper, the performance characteristics of R404 indirect refrigeration system using R744 as a secondary refrigerant were investigated experimentally to obtain a optimum design data for this system. First, for the constant experimental conditions, the COP of R404A indirect refrigeration system using R744 as secondary refrigerants decreases with respect to the increases in R404A condensation temperature and temperature difference in R744 cooler. And, the COP of indirect refrigeration system using R744 as secondary refrigerants decreases slightly with decreasing the mass flowrate of R744.

2차 냉매로서 CO2를 사용하는 간접 냉동시스템의 열전달과 펌프 소비동력

In this paper, the heat transfer coefficient and pump consumption power of indirect refrigeration system using CO2 as a secondary refrigerant were investigated experimentally. First, from the comparison of pump consumption powers of existing brines(EG, PG, EA etc.) and CO2 as secondary refrigerants at the same experimental conditions, PG and CO2 show the highest and lowest power, respectively. Second, the heat transfer coefficient of CaCl2 is the highest, but PG is the lowest among other secondary refrigerants. From the above results, it is confirmed that CO2 as the secondary refrigerant has excellent characteristics when comparing to existing brines. Thus, it is concluded that CO2 is applicable as the secondary refrigerant of indirect refrigeration system.

Thermo-hydraulic behavior of ice slurry in an offset strip-fin plate heat exchanger

Ice slurry is a promising alternative to conventional single-phase coolants in indirect refrigeration systems. In this paper, an experimental analysis of an offset strip-fin heat exchanger operating with ice slurry as working fluid is presented. The pressure drop and thermal performance have been determined. In order to obtain the partial thermal resistance in the ice slurry side an empirical correlation for the secondary fluid side was determined by applying the Wilson plot method in a set of tests performed previously. An empirical correlation in terms of the Colburn j-factor to describe the thermal behavior of the heat exchanger with ice slurry was obtained. On the other hand, the direct pressure drop measurements operating with different flow rates and ice fractions are shown and compared with values obtained with single-phase fluids. Pressure drop instabilities have been observed for flow rates lower than the nominal value provided by the manufacturer.