Transcritical CO2 Refrigeration System Research Articles

Comparative study of cycle modification strategies for trans-critical CO2 refrigeration cycle for warm climatic conditions

This paper presents a comparative study of performance of six prominent modifications on the basic trans-critical CO2 refrigeration system to investigate their suitability to high ambient temperature application (35–55°C). To explore the application in chiller, domestic refrigeration and air cooling the evaporator temperature chosen are −10°C, 0°C and 10°C respectively. In general the cycle modifications have a positive effect on the overall COP of the system. However, to comprehend practicability of these modifications for three application areas, a few other parameters which affect design and operation are also included in the study. These are compressor discharge pressure and temperature, mass flow rate, interstage pressure for multi-stage operation and exergy destruction. Effect of real time constraints like approach temperature, pressure drop in gas cooler, compressors efficieny, degree of superheat, expanders efficiency and effectivenesss of intermediate heat exchanger are also incorporated. Interrelation between these parameters are brought out from the study.

Field measurements of supermarket refrigeration systems. Part I: Analysis of CO2 trans-critical refrigeration systems

This study investigates the refrigeration performance of three CO2 trans-critical solutions based on field measurements. The measurements are carried out in five supermarkets in Sweden. Using the field measurements, low and medium temperature level cooling capacities and COP's are calculated for ten-minute intervals, filtered and averaged to monthly values. The results indicate that the systems using trans-critical booster system with gas removal from the intermediate vessel have relatively the highest total COP. The reasons are higher evaporation temperatures, lower internal and external superheat and higher total efficiency of booster compressors. Another important factor is gas removal from the intermediate vessel which leads to higher COP of low temperature level. Comparing the older and newer installed systems, a trend in energy efficiency improvement has been seen. The study shows this improvement originates from both changes in the system design (e.g. two stage expansion) and components efficiency improvement (e.g. higher total efficiency of compressors - lower internal superheat and higher evaporation temperatures of cabinets).

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.

Experimental Investigation of a Combined Power Refrigeration Transcritical CO2 Cycle

Optimization studies of the CO2 transcritical combined power refrigeration cycle are presented here. Steady state thermodynamic analysis including energy is carried out at various operating conditions employing a precision in-house property code CO2 PROP. CO2 transcritical combined power refrigeration cycle exhibits better performance than that of CO2 transcritical refrigeration cycle alone. However, optimum gas cooler pressure does not change with the type of the system; it remains same for both the system; refrigeration and combined cycle. However, system performance is significantly affected by the evaporator pressure and compressor isentropic efficiency unlike the CO2 transcritical refrigeration system Combined cycle exhibits significant improvement in COP at relatively higher evaporative temperature. However, at lower evaporator temperature cooling cycle shows better performance.

Optimal heat rejection pressure for transcritical CO2 refrigeration cycle with an expander

ABSTRACTHeat rejection pressure plays an important role in designing a transcritical CO2 refrigeration system, and it has an optimal value to maximize the system’s coefficient of performance (COP). With a thermodynamic simulation model, the optimal heat rejection pressure is studied in the paper for an expander cycle, as well as conventional throttle valve cycle. The effects of compressor efficiency, expander efficiency, gas cooler outlet temperature, and evaporation temperature on the optimal heat rejection pressure are analyzed. It is the first time for a transcritical CO2 expander cycle that the optimal heat rejection pressure is correlated with the gas cooler outlet temperature and the evaporation temperature at given compressor efficiency and expander efficiency. The average deviation from the correlation to simulation results is less than 1.0%. The correlation provides a guideline to system development and performance optimization of a transcritical CO2 expander cycle.

A review on thermodynamic optimization of irreversible refrigerator and verification with transcritical CO2 system

The transcritical vapor compression refrigeration cycle consists of isothermal heat addition process and isobaric non-isothermal heat rejection process with highly variable heat capacity unlike to the subcritical cycle. Hence, it is quite interesting whether the analysis and optimization results of irreversible Carnot-like refrigerator are applicable for this case. The present study consists of two parts: the detailed review on theoretical analysis and analytical optimizations of irreversible Carnot-like refrigerator with both infinite and finite capacity heat reservoirs, and verification of analytical results with the results obtained from a more elaborate numerical simulation of transcritical CO2 refrigeration system. Considered objective parameters are cooling load or COP and overall heat transfer surface area, and optimizing parameters are cold and hot side working fluid temperatures, ratio of residence times for heat addition and heat rejection, and heat transfer surface area ratio. Reasonably fair agreement has been obtained between analytical and numerical model predictions.

A review on thermodynamic optimization of irreversible refrigerator and verification with transcritical CO2 system

The transcritical vapor compression refrigeration cycle consists of isothermal heat addition process and isobaric non-isothermal heat rejection process with highly variable heat capacity unlike to the subcritical cycle. Hence, it is quite interesting whether the analysis and optimization results of irreversible Carnot-like refrigerator are applicable for this case. The present study consists of two parts: the detailed review on theoretical analysis and analytical optimizations of irreversible Carnot-like refrigerator with both infinite and finite capacity heat reservoirs, and verification of analytical results with the results obtained from a more elaborate numerical simulation of transcritical CO2 refrigeration system. Considered objective parameters are cooling load or COP and overall heat transfer surface area, and optimizing parameters are cold and hot side working fluid temperatures, ratio of residence times for heat addition and heat rejection, and heat transfer surface area ratio. Reasonably fair agreement has been obtained between analytical and numerical model predictions.

Experimental investigation on the influence of EEV opening on the performance of transcritical CO2 refrigeration system

In order to investigate the influence of electronic expansion valve (EEV) opening on the performance of the transcritical CO2 refrigeration system, an experimental test rig of the transcritical CO2 system was built up. The system operation parameters such as temperature, pressure were measured with different EEV opening, when the inlet temperatures of the gas-cooler water and the evaporator water were set to 30 °C and 15 °C, respectively. The effects of EEV opening on the pressure, temperature and the performance of the system were studied in detail. The results demonstrate that the EEV opening has great effects on the discharge pressure of the compressor, gas-cooler outlet pressure, and the discharge temperature of the compressor. The compressor input power decreases with the increasing of EEV opening. The cooling capacity and the coefficient of performance (COP) of the system peaks at the EEV opening of 40% and 60%, respectively.

Simulation and performance optimization of finned tube gas cooler for trans-critical CO2 refrigeration system in Indian context

This paper analyzes trans-critical CO2 refrigeration system in typical Indian conditions and the challenges associated with it. A mathematical model for air cooled finned tube gas cooler is developed, validated and used for the performance analysis at various design and operating conditions. A combination of input parameters is employed anticipating that it will enhance heat transfer, reduce the approach temperature and eventually improve the COP of refrigeration system. As the paper reiterates the success of CO2 as refrigerant, it emphasizes upon operating parameters that are based on local environmental conditions for best possible performance of overall system including fan power consumption. Thus the paper help to formulate design guidelines for optimized performance of gas cooler at different ambient temperatures.

Optimal analysis of gas cooler and intercooler for two-stage CO2 trans-critical refrigeration system

Energy-based 1st law and exergy-based 2nd law are both employed in the paper to assess the optimal design of gas cooler and intercooler for two-stage CO2 refrigeration system. A simplified mathematical model of the air-cooled coil is presented and validated against experimental data with good accuracy. The optimum circuit length under the influence of frontal air velocity and deep rows is investigated first. Thereafter, designed coil with optimum circuit length is further evaluated within the two-stage refrigeration system. It is found out the optimum point using 1st law does not coincide with the point using 2nd law in isolated component and the simulation results from isolated component by 2nd law are closer to system analysis. Results show optimum circuit length is much bigger for gas cooler than intercooler and the influence on the length from variation of frontal air velocity and deep rows may be neglected. There does exist optimum frontal air velocity which will decrease with more number of deep rows.

THERMODYNAMIC ANALYSIS AND EXPERIMENTAL RESEARCH OF TRANSCRITICAL CO2 CYCLE WITH INTERNAL HEAT EXCHANGER AND DUAL EXPANSION

This work presents the experimental evaluation of the energy performance of transcritical CO2 refrigeration and heat pump systems. The optimal gas cooler pressures and the optimal COP have been analyzed from a thermodynamic point of view. The systems used a new dual expansion valve and a balance CO2 liquid receiver adjustment device, which can control high and low side pressure effectively. Moreover, we demonstrate the influence of the internal heat exchanger (IHX) on the systems' performances, on the basis of the analysis of the relative COP index RCOPI, the compressor power index RPCI and other parameters which can confirm the truth of. The experimental evaluation covers five evaporating levels (-10 to 10°C) and in a wide range of gas cooler pressures (75 to 120 bar). It is concluded that with the IHX system, compressor power is relatively low when the high side pressure is over 100 bar, and the evaporation temperature is below 0°C. The COP of the system without the IHX is slightly higher than the system with the IHX; it is increasing about 3% to 5%, when the evaporation temperature is over 5°C. Relative to the single expansion process, the dual expansion cycle can decrease the influence of pressure fluctuations of CO2 supercritical fluid and liquid mixture on the systems.

Simulation and Performance Evaluation of Finned Tube Gas Cooler for Trans-Critical CO<sub>2</sub> Refrigeration System

Carbon dioxide is one of the rediscovered sustainable options, as CFCs and HFCs are now in the list of regulated substances, to be phased out in time bound way. There are however various challenges to the successful use of CO2 as refrigerant. Cost effective and widely acceptable technology is demand of the time. Performance of trans-critical CO2 refrigeration systems is found to be sensitive to the gas cooler design. In the present work a detailed mathematical model is developed for a finned tube air cooled gas cooler and the same is validated using published literature. Subsequently, to achieve the lowest possible approach temperature, the performance of the gas cooler is analyzed for various air velocity and alternate pipe arrangements. The effect of increase in size of gas cooler to its performance is also studied.

Thermodynamic Analysis and Experimental Investigation of Internal Heat Exchanger Influence on CO<sub>2</sub> Trans-Critical Cycle Performance

This work present the experimental evaluation of the energy performance of trans-critical CO2refrigeration and heat pump systems. The optimal gas cooler pressure and the optimal COP have been analysed from a thermodynamic point of view. The experimental evaluation covers five evaporating levels (-10 to 10°C) and in a wide range of gas cooler outlet pressures (75 to 120bar). It is concluded that: With the the internal heat exchanger (IHX) system compressor power is relatively low, when the high side pressure is over 100bar, and the evaporation temperature is below 0°C. The COP of the system without IHX is slightly higher than the system with the IHX, it is increase about 3% to 5%, when the evaporation temperature is over 5°C.

Theoretical evaluation on effect of internal heat exchanger in ejector expansion transcritical CO2 refrigeration cycle

The performance of the transcritical CO2 refrigeration system requires further improvement in order to save energy. In this paper, the effect of the internal heat exchanger (IHE) on the performance of the ejector expansion transcritical CO2 refrigeration system is analyzed theoretically based on the first law of thermodynamics. The possible parameters affecting system efficiency are investigated. The variation of ejector entrainment ratio, pressure recovery, ejector efficiency and the coefficient of performance (COP) is obtained for the ejector expansion transcritical CO2 refrigeration cycles with and without IHE. It is found that the addition of IHE in the CO2 ejector refrigeration cycle increases the ejector entrainment ratio and the ejector efficiency, and decreases pressure recovery under the same gas cooler pressures. Unlike in a conventional throttle valve cycle, an IHE addition does not always improve the system performance in the ejector expansion cycle. Whether the energy efficiency of the ejector cycle by IHE can be improved depends on the isentropic efficiency level of the ejector. The utilization of IHE is only applicable in the cases of lower ejector isentropic efficiencies or higher gas cooler exit/evaporator temperatures for the ejector expansion system from the view of energy efficiency.

Performance of nanofluid-cooled shell and tube gas cooler in transcritical CO2 refrigeration systems

Abstract The theoretical performance analyses of the nanofluid cooled shell-and-tube gas cooler in transcritical CO 2 refrigeration cycle have been done to study the performance improvement of gas cooler as well as CO 2 cycle using Al 2 O 3 , TiO 2 , CuO and Cu nanofluids and effects of various design and operating parameters. Use of nanofluid as coolant in shell-and-tube gas cooler of CO 2 cycle improves the gas cooler effectiveness, cooling capacity and COP without penalty of pumping power. The CO 2 cycle yields best performance using Al 2 O 3 –H 2 O as coolant in shell-and-tube gas cooler followed by TiO 2 –H 2 O, CuO–H 2 O and Cu–H 2 O. The maximum cooling COP improvement of transcritical CO 2 cycle for Al 2 O 3 –H 2 O is 26.0%, whereas that for TiO 2 –H 2 O is 24.4%, for CuO–H 2 O is 20.7% and for Cu–H 2 O is 16.5% for the given ranges of study. Study shows that the nanofluid may effectively use as coolant in shell-and-tube gas cooler to improve the performance of transcritical CO 2 refrigeration cycle.

EXPERIMENTAL ANALYSIS OF TWO-PHASE EJECTOR SYSTEM WITH VARYING MIXING CROSS-SECTIONAL AREA USING NATURAL REFRIGERANT CO2

The use of two-phase ejector in improving the performance of transcritical CO2 refrigeration system needs further experimental verification particularly the effects of its geometrical design. In this study, experimental data were gathered for ejectors with different mixing cross-sectional areas at different operating temperature and pressure. The results have shown that a smaller mixing area yields higher efficiency due to its higher pressure recovery and entrainment ratio, but its advantages are limited to lower ejector inlet pressure Pc. A larger mixing area is required for higher cooling capacity which can be achieved at higher ejector inlet pressure or lower ejector inlet temperature but excessive increase in this area considerably decreases the efficiency of the system. In this study, the ejector with the largest mixing area was the most inefficient and reduced the COP up to 10% compared to most efficient type. It demonstrates the significant effect of ejector's geometrical features, particularly the mixing cross-sectional area and its related geometrical ratio, in the performance of CO2 ejector system. The effect of motive nozzle inlet condition on pressure recovery profile has been more evident for ejector with smaller mixing area while the evaporator temperature has the least effect in the performance of the system. In the conditions used in this study, using ejector yielded a COP improvement of up to 35% compared to the conventional system.

Performance characteristics of multi-evaporator transcritical CO2 refrigeration cycles with hybrid compression/ejection

The theoretical analyses and optimization studies of two novel multi-evaporator transcritical CO2 refrigeration systems using constant pressure mixing ejector are presented in this article. Comparisons of novel hybrid compression—ejection CO2 cycles, based on coefficient of performance (COP) improvement over conventional cycle and other optimum parameters, are also done for various operating conditions. Study shows that the cycle performance is dependent on allocation of evaporator cooling capacities for both cycles. The optimum discharge pressure for both cycle layouts compared to corresponding basic cycle reduces by the use of ejection—compression. For both cycle layouts, the effect of gas cooler exit temperature on COP improvement is negligible, whereas the COP improvement at optimum condition increases with the increase in temperature difference between evaporators. Results reveal that the CYC1 is superior due to lower discharge pressure and significantly higher COP improvement (maximum of 38.1 per cent for CYC1 whereas 21.4 per cent for CYC2 at optimum condition for the studied ranges).