Transcritical CO2 Refrigeration Cycle Research Articles

Thermodynamic analysis and multi-objective optimization of a waste heat recovery system with a combined supercritical/transcritical CO2 cycle

This study firstly develops a novel combined cycle system consisting of a supercritical CO2 recompression Brayton cycle and a transcritical CO2 refrigeration cycle to recover waste heat from a marine turbine for both power generation and refrigeration. The pressure drop of the total heat exchanger in the whole cycle is considered in the thermodynamic and economic modeling process. Then, the influence of crucial system parameters on the system performance and economics is investigated through parametric sensitivity analysis. Finally, the system is optimized parametrically by multi-objective optimization. The results show that the higher inlet pressure of the high-pressure compressor helps to improve the thermal efficiency but reduces the exergy efficiency; the low-temperature heat exchanger plays a decisive role in the overall system exergy destruction; the HRHE accounts for a more significant proportion of the system cost, and the pressure drop has the most significant impact on the network of the system. The optimal solution is COP = 3.059, LCOE = 18.348$/(kW/h), and ηWhr = 0.651 when waste heat recovery efficiency, COP, and LCOE are considered optimization objectives.

Evaluation of the ejector two-stage compression refrigeration cycle with work performance from energy, conventional exergy and advanced exergy perspectives

Low temperature cold chain is the top priority of logistics development at this stage. CO2 (carbon dioxide) refrigeration system is widely used. However, the low efficiency of CO2 refrigeration system is the biggest obstacle to its development. In this research work, EES (Engineering equation solver) is used to simulate the system, which is widely used in thermodynamic modeling and calculation, a novel trans-critical CO2 refrigeration cycle with ejector for low-temperature storage is proposed. The energy and conventional exergy model of the system are established, and the advanced exergy model is established based on the conventional exergy model, and the actual performance of the system is analyzed, and the optimization potential of the system components is analyzed through the commonness of the two exergy analysis methods. The results show that the exergy destruction of each component of the ejector is the largest in the system, and the optimization potential is the highest in both conventional and advanced exergy models. After studying the influence of main parameters in the system cycle on the system performance and performance, it is found that there is an optimum intermediate pressure and gas cooler pressure in the system to maximize the system performance and efficiency. According to the influence of the change of system parameters on the system, it is found that the outlet temperature of gas cooler has the greatest influence on the optimization of ejector.

An investigation into the thermodynamic improvement potential of a transcritical automotive CO2 refrigeration cycle

To identify the underlying reasons for the poor efficiency of a transcritical automotive CO2 refrigeration cycle in hot climates clearly, a thermodynamic analysis of ideal and real cycles was performed. The analysis considered energy and exergy (conventional and advanced) viewpoints, determined the ideal cycle efficiency, and identified the improvement potential of each component. Energy analysis results revealed that the ideal efficiency reaches 25.3 at 35 °C outdoor condition, which is far better than currently available real efficiencies. This indicated excellent potential for efficiency improvement. The transition from a pseudo-optimal high pressure under ideal conditions to an optimal high pressure under real conditions occurred upon the emergence of non-ideal factors. Conventional exergy analysis results indicated that exergy destruction within the throttling process accounts for the largest share (36%) of overall exergy destruction. However, advanced exergy analysis results suggested that irreversibilities in the evaporator and gas cooler are the main sources of exogenous exergy destructions in the compressor and throttling process, and their optimization potentials are typically 2–4 times greater than those of compressor and throttling process. Thus, the two heat exchange processes provide significant opportunities for transcritical automotive CO2 refrigeration cycle performance improvement.

Energy analysis and performance comparison of transcritical CO2 supermarket refrigeration cycles

Using carbon dioxide as a refrigerant gains importance as a result of restrictions applied to high-GWP (Global Warming Potential) refrigerants. Although CO2 is an environment-friendly refrigerant, its low critical temperature and high operating pressure lead to high energy consumption, especially in commercial refrigeration. As a result, research and improvements are being made to reduce the energy consumption of transcritical CO2 refrigeration cycles. This paper presents thermodynamic analysis and pressure optimizations to obtain maximum performance for transcritical booster, parallel compression, and ejector expansion supermarket refrigeration cycles. Effects of different operation conditions were also investigated and cycles were compared to each other in terms of performance. It was obtained that up to 47% Coefficient of Performance (COP) improvement can be achieved using ejector expansion cycle with the help of expansion work recovery in the ejector. It was also shown that ejector expansion cycle outperforms other CO2-only cycles without ejector in all conditions.

Thermodynamic study of carbon dioxide transcritical refrigeration cycle with dedicated subcooling and cascade recooling

Stimulated by the fast development of cold chain commodity market and a large amount of demand in cold storages, the application of CO2 refrigeration system is to enhance remarkably. Owing to the low critical temperature of CO2, the dedicated subcooling technology is widely-used in the CO2 transcritical refrigeration cycle to increase the system performance. In this regard, reducing the compressor power of subcooling subcycle and extending the working range of heat source temperature is highly important for the mechanical subcooling technology and absorption one, respectively. Motivated by such purpose, the CO2 transcritical cycle with dedicated subcooling and cascade recooling is presented and the thorough investigation is performed thermodynamically. The proposed system is compared with layouts with dedicated mechanical subcooling and dedicated absorption one at first. Subsequently, the effect of recooling capacity ratio, the critical parameter for the system performance, is analyzed in-depth. Finally, the optimum recooling capacity ratio for different conditions is fitted by the neural network approach. Compared to layouts with dedicated mechanical subcooling and dedicated absorption one, the secondary compressor work goes down by 28.5% and the lower limit of heat source temperature is extended by 4 °C, respectively in the proposed system. The paper is favorable for the development and performance improvement of CO2 transcritical refrigeration systems.

Performance analysis of multi-refrigerant multi-variable environment refrigeration system based on marine cold chamber

This paper is based on the engineering equation solver to calculate the performance of refrigeration system under multi-refrigerant multivariable environment, which can simulate the operating performance of the system under different working conditions and the view of each state point, and provide help for the design and optimization of trans-critical CO2 refrigeration cycle refrigeration units. Using thermodynamic analysis, the system performance is observed by using the equations of conservation of energy, conservation of momentum and conservation of mass as the basis for the change of evaporator pressure, gas cooler pressure and intermediate pressure in the system; the state of each position in the ejector part of the system is calculated; after the calculation is completed, the pressure-enthalpy diagram of the system can be plotted; according to the different data parameters selected, the COP of the system can be plotted with the change of certain data parameters for different parameters. The results showed that the calculation method used is suitable for different refrigerants, compared with the traditional calculation and selection, the advantage of the EES software is that it can use the laws of thermodynamics for calculation and iteration.

Research and application progress of transcritical CO2 refrigeration cycle system: a review

AbstractCO2 refrigerant and its transcritical cycle system have become the research focus in the refrigeration field due to their advantages of environmentally friendly, safe and low environmental temperature adaptability. This paper summarizes the research progress on the modified methods for the defects of basic transcritical CO2 refrigeration cycle system in recent years. Meanwhile, the technical status of transcritical CO2 refrigeration technology in commercial supermarket refrigeration, heat pump system, automobile air conditioning and artificial ice rink is discussed in detail. Finally, the development of transcritical CO2 refrigeration cycle system is prospected and the key problems to be solved are put forward.

Performance analysis of a modified dual-ejector and dual-evaporator transcritical CO2 refrigeration cycle for supermarket application

This paper proposes a modified dual-ejector and dual-evaporator transcritical CO2 refrigeration cycle for supermarket application. Based on the conventional dual-evaporator transcritical CO2 refrigeration cycle, the modified cycle introduces two ejectors and a flash tank. The first liquid-vapor ejector sucks partial throttling flash vapor flowing to the low-temperature evaporator into the high-temperature evaporator; the second vapor-vapor ejector takes the vapor from the high-temperature evaporator as the primary flow to entrain the vapor from the low-temperature evaporator, and then the compressor suction pressure is lifted. Furthermore, the introduction of the first ejector increases the primary flow of the second ejector, and then improves the ability of the second ejector to lift the suction pressure of the compressor. The performance comparison between the two cycles is conducted by adopting energy and exergy analysis methods, and the efficiencies of the two ejectors are also analyzed. The comparison results demonstrate that the use of dual-ejector reduces the compressor pressure ratio by up to 19.1% under a typical working condition, and the improvements of the COP and exergy efficiency could reach up to 15.9–27.1% and 15.5–27.5% under all given working conditions. In addition, the modified cycle has an optimal gas cooler pressure of around 8.15 MPa, which is lower than the 8.3 MPa of the conventional cycle. The performance comparison results state clearly the energy-saving potential of the modified cycle and its application prospect in supermarket refrigeration.

Ejector optimization and performance analysis of electric vehicle CO2 heat pump with dual ejectors

The performance of fixed ejector is limited by working conditions, so it is difficult to meet the operating requirements of electric vehicle heat pump system under varying conditions and wide temperature range. In this paper, a transcritical CO2 refrigeration cycle with dual ejectors in parallel (DEP) for electric vehicle heat pump system is established and its operation condition weight statistics method in all climate is proposed. On this basis, a fixed ejector optimization method based on the genetic algorithm is proposed, which takes the increment of integrated part load value (ΔIPLV) compared to the conventional transcritical CO2 refrigeration cycle (CON) system as the evaluation index. The performance analysis results show that under the cooling conditions, the coefficient of performance (COP) of DEP system with the optimized ejector for cooling is increased by 17.32%–23.42% compared to the CON system, and the COP of the transcritical CO2 refrigeration cycle with a single ejector (SEJ) system is increased by 7.31%–9.47%. In the heating mode, the COP of DEP system with the optimized ejector for heating is increased by 18%–19.79%, while the COP of SEJ system with the unoptimized ejector is decreased by 0.07%–2.43%.

Characteristics of CO2-ionic liquids/PEG200 as new working pairs for absorption-compression refrigeration system

Absorption-compression refrigeration system using CO2-Ionic Liquids (ILs) as working pairs could avoid the high operating pressure in transcritical CO2 refrigeration cycle. This work proposed using PEG200 as the additive to improve the physical properties of the pure ILs, making the absorption-compression refrigeration system being more efficient and less corroded. The effects of PEG200 on the CO2-ILs absorption and desorption, the viscosity and the corrosion behavior on stainless steel were investigated experimentally. Results show that PEG200 has positive effect on these key parameters at temperature of 293.15 K to 343.15 K and pressures up to 10.0 MPa. The addition of PEG200 to [emim][FAP] could increase the CO2 solubility by 10.17% maximally, reduce the viscosity by 24.43% maximally, and accelerate the desorption rate. The CO2 solubility increases with the pressure increasing, and decreases with the temperature increasing. The viscosity of the mixed solvent decrease with the increase of the temperature and pressure. The addition of PEG200 to [emim][Tf2N] effectively weakens the corrosion on the 316L stainless steel, the corrosivity of ILs/PEG200 is less than that of the pure ILs. This work shows ILs with PEG200 added have better physical properties as the CO2 absorbent, providing a new approach of selection for CO2 absorption.

Synergistic effect of geometric parameters on CO2 ejector based on local exergy destruction analysis

Ejector is valuable to improve the transcritical CO2 refrigeration cycle. The improvement on geometric parameters is significant for the performance enhancement of CO2 ejector. In this paper, numerical simulation based on a homogeneous equilibrium model is used to implement the sensitivity analysis on the key geometric parameters. With the simulation, the influence mechanisms of the key geometric parameters on the macro-performance under overall operation conditions of the ejector are discovered based on the analysis of flow field structure and local exergy destruction distribution. The results show that the outlet area of primary nozzle, convergent angle of pre-mixer and nozzle exit position are strongly coupled. A sensitivity matrix of these three key geometric parameters to local exergy destruction distribution is obtained to evaluate their synergism quantitatively. This work would provide a new way to guide the optimization on the pre-mixer of the ejector considering the coupling effect of geometric parameters.

Analysis of a hybrid transcritical CO2 vapor compression and vapor ejector refrigeration system

The paper presents analysis of a hybrid transcritical CO2 (T-CO2) refrigeration cycle coupled with a heat-driven vapor ejector refrigeration cycle. The heat generated by compression of CO2 in the transcritical vapor compression refrigeration system (VCRS) is utilized to drive the secondary vapor ejector refrigeration system (VERS). The performance of the hybrid system is evaluated by maintaining identical evaporator temperatures for both T-CO2 VCRS and VERS. Cooling capacity and COP of the VERS is influenced by heat recovered from T-CO2 system, thus, limiting the choice of working fluids for the VERS. Analytical model based on conservation of mass, momentum and energy, incorporating real gas behavior is used for arriving at an optimum ejector geometry used in the VERS. Amongst the five fluids analyzed, R32 is found to be most suitable for the VERS used in the hybrid system. R32 provides the highest coefficient of performance (COP) and cooling capacity by recovering maximum heat from the generator, while, other fluids yield nearly identical performance. The proposed hybrid system using R32 provides significant improvement in system performance ranging from 10 to 50% in cooling capacity over the baseline T-CO2 system for the range of evaporating temperatures from −2.5 °C to 12.5 °C.

Energetic and exergetic studies of modified CO2 transcritical refrigeration cycles

Abstract Thermodynamic analysis including energetic and exergetic analysis is carried out employing Engineering Equation Solver for the five modified cycles: dual expansion cycle, internal heat exchanger cycle, work recovery cycle, work recovery with internal heat exchanger cycle and vortex tube expansion cycle. Contours are developed to study the effect of gas cooler temperatures and evaporator temperatures on the system performance and optimum gas cooler pressure. The modified cycle with work recovery turbine offers relatively higher COP and higher exergetic efficiency with lower compressor discharge pressure. The exergy loss in compressor, gas cooler, throttle valve and vortex tube (VT) are considerably higher than that in internal heat exchanger (IHX), evaporator and turbine. It is observed that COP of modified cycle with VT is slightly less than that with IHX, whereas the cycle with work recovery turbine brings the highest COP with the improvement of 25% at the gas cooler exit temperature of 305 K and evaporator temperature of 248 K.

Investigation of a refrigeration system based on combined supercritical CO2 power and transcritical CO2 refrigeration cycles by waste heat recovery of engine

Abstract The majority of the energy in the fuel burned in the internal combustion engines is lost in the form of waste heat. To address this issue, waste heat recovery technology has been proposed to increase the overall efficiency of engine. This paper investigates a heat driven cooling system based on a supercritical CO2 (S-CO2) power cycle integrated with a transcritical CO2 (T-CO2) refrigeration cycle, aiming to provide an alternative to the absorption cooling system. The combined system is proposed to produce cooling for food preservation on a refrigerated truck by waste heat recovery of engine. In this system, the S-CO2 absorbs heat from the exhaust gas and the generated power in the expander is used to drive the compressors in both S-CO2 power cycle and T-CO2 refrigeration cycle. Unlike the bulky absorption cooling system, both power plant and vapour compression refrigerator can be scaled down to a few kilo Watts, opening the possibility for developing small-scale waste heat driven cooling system that can be widely applied for waste heat recovery from IC engines of truck, ship and train. A new layout sharing a common cooler is also studied. The results suggest that the concept of S-CO2/T-CO2 combined cycle sharing a common cooler has comparable performance and it is thermodynamically feasible. The heat contained in exhaust gas is sufficient for the S-CO2/T-CO2 combined system to provide enough cooling for refrigerated truck cabinet whose surface area is more than 105 m2.

Research and Development of Carbon Dioxide Refrigeration Technology

With the agreement of the Kigali amendment to the Montreal Protocol, the timetable for the reduction of the mandate and the limitation of 18 controlled substances are being phased in. This has brought great challenges to the transformation, upgrading and sustainable development of China’s refrigeration and air conditioning industry. It has brought great challenges to the transformation, upgrading and sustainable development of China’s refrigeration and air conditioning industry. Carbon dioxide (CO2)is considered to be the most suitable and potential natural working medium due to its excellent environmental properties. This paper introduces the properties of CO2 refrigerant, NH3/CO2 laminated refrigeration system, CO2 secondary refrigerant refrigeration system and CO2 trans-critical refrigeration cycle system, and analyzes three representative processes of CO2 refrigeration system.

Thermodynamic Analysis of a CO2 Refrigeration Cycle with Integrated Mechanical Subcooling

Different alternatives are being studied nowadays in order to enhance the behavior of transcritical CO2 refrigeration plants. Among the most studied options, subcooling is one of the most analyzed methods in the last years, increasing cooling capacity and Coefficient Of Performance (COP), especially at high hot sink temperatures. A new cycle, called integrated mechanical subcooling cycle, has been developed, as a total-CO2 solution, to provide the subcooling in CO2 transcritical refrigeration cycles. It corresponds to a promising solution from the point of view of energy efficiency. The purpose of this work is to present, for the first time, thermodynamic analysis of a CO2 refrigeration cycle with integrated mechanical subcooling cycle from first and second law approaches. Using simplified models of the components, the optimum operating conditions, optimum gas-cooler pressure, and subcooling degree are determined in order to obtain the maximum COP. The main energy parameters of the system were analyzed for different evaporation levels and heat rejection temperatures. The exergy destruction was analyzed for each component, identifying the elements of the system that introduce more irreversibilities. It has been concluded that the new cycle could offer COP improvements from 11.7% to 15.9% in relation to single-stage cycles with internal heat exchanger (IHX) at 35 °C ambient temperature.

Theoretical research of the performance of a novel enhanced transcritical CO2 refrigeration cycle for power and cold generation

Three different refrigeration cycles, namely the vapor compression cycle, the combined refrigeration cycle and the new combined refrigeration cycle for power and cold, are investigated. The principles of mass and energy conservation are applied to every component of the systems, and the resulted linear system of equations was numerically solved. It was found that the new proposed cycle had a lower optimum gas cooler pressure than other refrigeration cycles, and this would enhance the lifetime and safety of the system. Besides, it was found that the coefficient of performance of the new cycle is higher than that of the vapor compression and combined refrigeration cycles 110% and 50%, respectively. The exergy efficiency was approximately 78% and 58% higher than that of the vapor and combined refrigeration cycles, respectively. Furthermore, the exergoeconomic results of the new cycle showed that the total unit cost was lower by 16.6% and 13% than that for the vapor compression and combined refrigeration cycles, respectively.

A numerical contrast on the adjustable and fixed transcritical CO2 ejector using exergy flux distribution analysis

An adjustable ejector with a needle is deemed as an effective device to improve the performance of the transcritical CO2 refrigeration cycle with a wide operating conditions. In this paper, a homogeneous CFD model is built up to compare the performance of the adjustable and fixed ejectors with the same key structural sizes. By validating the numerical results with the experimental data, the k-ε RNG and k-ε Realizable are considered to predict the performance of the CO2 ejector well. Furthermore, an exergy analysis model is proposed to obtain the exergy fluxes of the primary and secondary flows are obtained, respectively. The results discover that the main exergy transfer (interaction between the primary and secondary flows) locates in the suction chamber. Meanwhile, the higher exergy loss is observed in the suction chamber of the adjustable ejector due to flow separation along the needle and higher viscous dissipation of oblique shock wave, which results in 5%–11% lower entrainment ratio than that of the fixed ejector. However, the higher velocity in the fixed ejector results in higher friction of the wall and more intense flow separation in the diffuser which causes higher exergy loss in the mixer and diffuser. Accordingly, the exergy efficiency of the adjustable ejector is only 0.5% lower than that of the fixed ejector. In spite of the special case in this paper, the adjustable ejector is considered to be able to achieve a similar exergy efficiency with the fixed ejector.

CO2 Transcritical Refrigeration Cycle with Dedicated Subcooling: Mechanical Compression vs. Absorption Chiller

The objective of this paper is the comparison of two dedicated subcooling methods, after the gas cooler, in a CO2 transcritical refrigeration system. The use of vapor compression refrigeration with R134a for subcooling is the first method, and the second is the use of an absorption chiller that operates with a LiBr-H2O working pair. The examined systems are compared energetically and exegetically with the reference transcritical CO2 refrigeration cycle without subcooling. The analysis is conducted for different operating scenarios and in every case, the system is optimized by selecting the proper temperature and pressure levels. The analysis is performed with a developed and validated model in Engineering Equation Solver. According to the final results, the use of the absorption chiller is able to decrease the system electricity consumption by about 54% compared to the simple transcritical cycle, while the decrease with the mechanical subcooling is 41%. Both systems with dedicated subcooling are found to have an important increase in the system exergy performance compared to the simple transcritical cycle. However, the system with the mechanical subcooling is found to be the best choice exegetically, with a small difference from the system with the absorption chiller.

Performance of a new two-stage transcritical CO2 refrigeration cycle with two ejectors

The present study describes a new two-stage transcritical CO2 refrigeration cycle with two ejectors (NERC). At this studied cycle (NERC), vapor compression is performed in two stages and each vapor compression line includes separate ejector. Thermodynamic and exergetic analysis of the proposed cycle are performed using Engineering Equation Solver (EES) software. The new cycle (NERC) is compared with a conventional cycle. It is found that depended on conditions the new cycle (NERC) improves the COP from 20% to 80% compared to the conventional cycle. The effects of main cycle parameters i.e. the high-side pressure of a compressor (discharge pressure), intermediate pressure, the outlet temperature of gas cooler and evaporator temperature on COP are studied.