Transcritical CO2 Ejector Expansion Research Articles

Comprehensive performance evaluation on a transcritical CO2 ejector-expansion heat pump system

In order to reduce irreversible loss and recover expansion work for improving automotive thermal system performance, this study experimentally evaluated the performances and characteristics of a CO2 ejector-expansion (EJE) heat pump system for electric vehicles under typical cooling and heating operating modes, examined the adaptability of the EJE system with a fixed-dimension ejector, and compared the EJE system with the basic-regenerative (REG) and vapor-injection (INJ) systems in the − 30 °C to 50 °C range. The results show that the increasing electronic expansion valve (EXV) opening leads to an increasing entrainment ratio and a decreasing lift ratio in both two modes. The maximum COP is 2.00 at 35 °C/27 °C (out-cabin/in-cabin air temperature) and 1.85 at 0 °C/20 °C. The COP improvement reaches a maximum of 21.6 % and 31.0 % at the ejector design temperature of 45 °C, compared to the REG and INJ systems, respectively. However, the heating performance of the ejector designed at a high temperature significantly decreases as the ambient temperature goes down. At − 30 °C/20 °C, the EJE system performs worse with a COP degradation of 6.1 % compared to the REG system.

Self-optimized efficient operation of trans-critical CO2 ejector-expansion heat pump water heater system with two-stage evaporation

In application scenarios with lower ambient temperatures and higher sink temperatures, the trans-critical CO2 heat pump water heater system using an ejector instead of a conventional expansion device offers better performance, and the higher pressure differential allows for partial pressure loss recovery through the ejector. However, the complexity of discharge pressure and ejector matching affects the further application of the system. Therefore an optimization method called Extremum seeking control method is proposed to achieve the lowest power consumption implementation under off-design conditions. The ESC controller applied to the system uses the compressor power consumption as the feedback signal and the discharge pressure is used as operating input set-point. The dynamical model of a trans-critical CO2 heat pump water heater built on Modelica/Dymola is used as the study object and optimized for three scenarios: fixed, sloped step profile, and actual ambient temperature. The simulation results show that the ESC applicable to the proposed system can optimize the matching of discharge pressure and ejector driving nozzle throat area to obtain the best performance. Compared with the benchmark system, the extremum seeking control for actual ambient temperature conditions results in a 5.7% increase in system efficiency.

Enhancing energy efficiency and sustainability in ejector expansion transcritical CO2 and lithium bromide water vapour absorption refrigeration systems

Enhancing energy efficiency and sustainability in ejector expansion transcritical CO2 and lithium bromide water vapour absorption refrigeration systems

Advanced exergy analysis of a solar-driven cascade ejector system with heat storage

Solar-driven cascade ejector system can not only promote the utilization reliability of the solar energy, but also improve the performance of solar energy systems. This paper proposes a novel solar-driven cascade ejector system which integrates a solar ejector heat storage subsystem with R290 as refrigerant and a transcritical CO2 ejector-expansion refrigeration subsystem. The conventional and advanced exergy analysis methods are utilized to investigate the system’s exergetic performance, and explore the interactions among the system components and their improvement potential. The effects of compressor efficiency, area ratio and efficiency of ejectors on the exergetic performance of system and components are investigated. The analysis results show that the transcritical CO2 ejector-expansion refrigeration subsystem has higher exergy efficiency and its components have small exogenous exergy destruction. The endogenous exergy destruction and the avoided exergy destruction of cascade system account for 88.85% and 41.46%, respectively. Besides, 66.35% of exogenous exergy destruction of middle heat exchanger is caused by the compressor and CO2 ejector, and to improve the performance of CO2 ejector and CO2 evaporator would decrease the exergy destruction of compressor. CO2 ejector and R290 ejector have the superior improvement potential in each subsystem compared to other components. The rise of ejector area ratio would increase the endogenous exergy destruction of ejector. For the system with high-performance compressor, R290 ejector efficiency should be attracted more attention to reduce its avoided endogenous exergy destruction.

A novel bi-evaporator cooling system via integration of absorption refrigeration cycle for waste energy recovery from an ejector-expansion trans-critical CO2 (EETRCC) cycle: Proposal and optimization with environmental considerations

In this paper an integrated refrigeration system is designed, in which the waste heat from an ejector-expansion trans-critical CO2 (EETRCC) cooling scheme is used for the generation of cooling not only above but also below the temperature of zero degrees Celsius. Waste heat usage was accomplished via employment of an absorption refrigeration system in conjunction with a CO2trans-critical refrigeration cycle. The absorption refrigeration cycle generates excess cooling above zero temperature utilizing the waste heat from ejector expansion system. Appropriate thermodynamic models are employed for feasibility assessment of integrated refrigeneration system, along with economic considerations to estimate produced cooling price. The results revealed significant better performance of novel integrated framework compared to standalone EETRCC cycle. In the best-case scenario, the levelized cost of cooling for the former is obtained as 105.3 $/GJ and, while the latter produces cooling with a levelized cost of 120.9 $/GJ. Also, it has been concluded that, when the temperature of the second evaporator is raised, the exergy efficiency of the system drops by 3 %, which results in increment of unit cost of the cooling by 6.34 %. In addition, highest exergy destructions are found to be 27.53 kW in the gas cooler and 15.57 kW in the generator.

Thermodynamic analysis of a novel multi-target temperature transcritical CO2 ejector-expansion refrigeration cycle with vapor-injection

In the present paper, a multi-target temperature ejector-expansion subcooler vapor-injection refrigeration cycle is proposed in order to improve the performance of natural fluid CO2 refrigeration cycles. A thermodynamics model to simulate a two-phase ejector cycle integrating a vapor-injection is first established from the energetic and exergetic perspectives. The behavior of the new cycle is then analyzed, and the adjustment direction of the cooling capacity and the evaporating temperature is pointed out. Furthermore, the new cycle is also compared with the subcooler vapor-injection refrigeration cycle and the ejector-expansion refrigeration cycle. The obtained results show that the behavior of the evaporators is affected by the bypass mass ratio and the entrainment ratio, and the two parameter determines the direction of adjusting the cooling capacity distribution and the evaporating temperature. The COP of the new cycle is about 17.9–29.4% and 2.8–7.8% higher than that of subcooler vapor-injection refrigeration cycle and ejector-expansion refrigeration cycle; the exergy efficiency of new cycle is about 14–28.2% and 3.7–11.3% higher than that of subcooler vapor-injection refrigeration cycle and ejector-expansion refrigeration cycle; and it is also found that the performance of the new cycle is improved more significantly under low cooling temperature and high ambient temperature conditions.

Structure and control co-optimization for an ejector expansion heat pump coupled with thermal storages

Optimal design and control are often coupled tasks for improving energy system performances. There is a lack of study on the structure and control co-optimization of heat pump coupled with thermal energy storages, which can enhance the penetration levels of intermittent renewables in integrated energy systems, thus we proposed a coupled dynamic optimization method for the structure and control co-optimization of this kind of dynamic energy system, and applied it successfully to a transcritical CO2 ejector expansion heat pump coupled with hot and cold water storages for thermal energy charging. Based on a complicated nonlinear dynamic system model considering thermal stratification in thermal energy storages, the structure and control co-optimization solutions were obtained for this coupled system during energy charging by using the co-optimization method with genetic algorithm. The detailed structures of gas cooler, evaporator, hot and cold energy storage tanks were optimized, then a dynamic optimal control strategy was developed for the whole charging process. The overall coefficient of performance can be increased by 23.2% and the total power consumption can be reduced by 17.6% using this newly developed co-optimization strategy, compared to the constant control parameter strategy. This study would be helpful for a structure and control co-optimization of other dynamic or stable energy systems.

Thermodynamic Analysis of Transcritical CO2 Ejector Expansion Refrigeration Cycle with Dedicated Mechanical Subcooling

The new configuration of a transcritical CO2 ejector expansion refrigeration cycle combined with a dedicated mechanical subcooling cycle (EMS) is proposed. Three mass ratios of R32/R1234ze(Z) (0.4/0.6, 0.6/0.4, and 0.8/0.2) were selected as the refrigerants of the mechanical subcooling cycle (MS) to further explore the possibility of improving the EMS cycle’s performance. The thermodynamic performances of the new cycle were evaluated using energetic and exergetic methods and compared with those of the transcritical CO2 ejector expansion cycle integrated with a thermoelectric subcooling system (ETS). The results showed that the proposed cycle presents significant advantages over the ETS cycle in terms of the ejector performance and the system energetic and exergetic performances. Taking the EMS cycle using R32/R1234ze(Z) (0.6/0.4) as the MS refrigerant as an example, the improvements in the coefficient of performance and system exergy efficiency were able to reach up to 10.27% and 15.56%, respectively, at an environmental temperature of 35 °C and evaporation temperature of −5 °C. Additionally, the advantages of the EMS cycle were more pronounced at higher environmental temperatures.

Experimental investigation on the performance of transcritical CO2 ejector–expansion heat pump water heater system

The effects of hot-water outlet temperature, compressor discharge pressure, compressor rotation speed, and expansion valve opening on the ejector and overall system performance were investigated for tap water outlet temperatures ranging from 50 to 90 °C. The coefficient of performance (COP) of the ejector–expansion heat pump system reaches 4.6 when the tap water outlet temperature is 70 °C, which is 10.3% higher than the corresponding basic cycle. Compared to the basic system, adding an ejector is more effective for the generation of high-temperature hot water. The COP of the ejector–expansion heat pump system increases; however, the COP improvement ratio ΔCOP decreases with the increase in the discharge pressure because of lower pressure lift ratio and ejector efficiency. The COP and ΔCOP increase when the compressor rotation speed decreases under our test conditions; simultaneously, the heat capacity decreases.

Comprehensive experimental study on a transcritical CO2 ejector-expansion refrigeration system

A comprehensive experimental study on the ejector and cooling performances of a transcritical CO2 ejector-expansion system is presented. The ejector performance of the entrainment ratio, pressure lift ratio, and efficiency were investigated under various primary flow pressure, secondary flow pressure, and back pressure conditions. A new coefficient of mass balance, β, was introduced based on the liquid mass balance in and/or out of the vapor–liquid separator, which was able to estimate the deviation between current operating state and a steady state. The experiment results show that the coefficient of performance (COP) ratio of the ejector-expansion refrigeration system to the basic system decreases from 18.9% to −11% with the coefficient of mass balance increasing from −0.1 to 0.1. Both the entrainment ratio and COP decrease with an increase in the coefficient of mass balance.

Experimental investigation on a transcritical CO2 ejector expansion refrigeration system with two-stage evaporation

Adding a two-stage evaporator in the transcritical CO2 ejector expansion refrigeration system can not only regulate the quality equilibrium of system refrigerant but also improve the system performance. This paper presents an experimental study on the transcritical CO2 ejector expansion refrigeration system with two-stage evaporation under variable operating conditions. Some key performance indicators such as gas cooler pressure, mass flow rate, entrainment ratio and pressure lift ratio of ejector, cooling capacity and COP of system were evaluated through varying the volumetric flow rate of the second evaporator chilled water Vte,wa, area ratio of ejector At/Amix, compressor speed N and expansion valve opening EV. The present results indicated that the second evaporator played a significant role in improving the system performance, and such improvement was more evident for smaller entrainment ratio. Greater gas cooler pressure, pressure lift ratio of ejector and cooling capacity of system could be obtained by increasing the volumetric flow rate Vte,wa, and compressor speed. The small area ratio of ejector was conducive to high gas cooler pressure, great entrainment ratio of ejector as well as large cooling capacity and COP of system. This study provides an approach for the improvement of ejector expansion refrigeration system.

Research on CO2 ejector component efficiencies by experiment measurement and distributed-parameter modeling

In this study we combine the experimental measurement data and the theoretical model of ejector to determine CO2 ejector component efficiencies including the motive nozzle, suction chamber, mixing section, diffuser as well as the total ejector efficiency. The ejector is modeled utilizing the distributed-parameter method, and the flow passage is divided into a number of elements and the governing equations are formulated based on the differential equation of mass, momentum and energy conservation. The efficiencies of ejector are investigated under different ejector geometric parameters and operational conditions, and the corresponding empirical correlations are established. Moreover, the correlations are incorporated into a transient model of transcritical CO2 ejector expansion refrigeration cycle (EERC) and the dynamic simulations is performed based on variable component efficiencies and fixed values. The motive nozzle, suction chamber, mixing section and diffuser efficiencies vary from 0.74 to 0.89, 0.86 to 0.96, 0.73 to 0.9 and 0.75 to 0.95 under the studied conditions, respectively. The response diversities of suction flow pressure and discharge pressure are obvious between the variable efficiencies and fixed efficiencies referring to the previous studies, while when the fixed value is determined by the presented correlations, their response differences are basically the same.

Flow visualization of supersonic two-phase transcritical flow of CO2 in an ejector of a refrigeration system

Supersonic two-phase flow of CO2 in an ejector was investigated in flow visualization experiments. The ejector was installed in a transcritical CO2 ejector-expansion refrigeration system with a convergent primary nozzle and a rectangular mixing chamber. The flow fields in the suction chamber and the mixing chamber of the ejector were visualized by direct photography for various operating conditions. The results showed that the liquid in the primary flow after the primary nozzle exit increased with increasing primary flow and secondary flow pressures. The expansion angle of the primary flow at the nozzle exit decreased with increasing secondary flow pressures. The primary flow was blocked in some cases at the mixing chamber entrance due to the large expansion angle which reduced the entrainment performance. The entrainment ratio was inversely related to the expansion angle. The primary and the secondary flows had a short mixing region in the mixing chamber with the mixed flow quickly becoming uniform.

Advanced exergy analyses of an ejector expansion transcritical CO2 refrigeration system

This paper presents a thermodynamic investigation on an ejector expansion transcritical CO2 refrigeration system with advanced exergy analysis. By splitting the exergy destruction into endogenous/exogenous and unavoidable/avoidable parts, more valuable information of the interactions among the system components and the components improvement potential is provided. The results indicate that the compressor with largest avoidable endogenous exergy destruction possesses the highest priority of improvement, followed by the ejector, evaporator and gas cooler. The system exergy destruction is dominantly endogenous, and 43.44% of the total exergy destruction can be avoided by improving the system components. The evaporator has a serious impact on the exogenous exergy destruction within the compressor and ejector, and its own exergy destruction is entirely belongs to endogenous part. The effects of the discharge pressure, compressor efficiency and ejector efficiency on the system exergetic performance are discussed. There is an optimal discharge pressure with respect to the minimum endogenous exergy destruction in the compressor. Avoidable endogenous exergy destruction rates of the compressor and ejector are respectively reduced by 93.6% and 81.7% when the corresponding component efficiency varies from 0.5 to 0.9.

Dynamic simulation of an improved transcritical CO2 ejector expansion refrigeration cycle

This paper presents a dynamic model of a transcritical CO2 ejector expansion refrigeration cycle with two-stage evaporation (EERC-TE), which is deemed to further improve the performance of a basic transcritical CO2 ejector expansion refrigeration cycle (EERC). The adjustments of compressor speed, the expansion valve opening, the throat area of ejector nozzle and the chilled water flow rate of evaporators are conducted as the disturbances to investigate the system transient responses. The presented results show the predictions have a good agreement with the experimental data. The change of compressor speed causes larger response of system parameters than that of the other disturbances. The expansion valve opening can result in obvious variation of the evaporator pressure and the pressure lift ratio of ejector, while the throat area of ejector nozzle leads to outstanding change of the gas cooler pressure and the ejector entrainment ratio. Moreover, these responses are compared with that of EERC and they show the similar change trends. Based on the transient results, the combination adjustment methods are proposed to improve the system performance, and the enhancement of COP of EERC-TE is more distinct and it is 2.58 times than that of EERC under the conditions of this study. In the basis of the transient simulations, flexible and diverse adjustment methods can be applied to improve the system performance and guide the system control.

Dynamic simulation on operating modes of ejector in transcritical CO2 ejector expansion refrigeration cycles

This article proposes an ejector model based on the real properties of CO2, which includes the critical mode and sub-critical mode (the critical mode means the primary and suction flows are both choked, and the sub-critical mode refers to only the primary flow choking). Moreover, a dynamic model of the transcritical CO2 ejector expansion refrigeration cycle is developed to simulate system responses at different ejector operational modes. The prediction results by the ejector model and system model are compared with available experimental data, respectively. Furthermore, the dynamic responses of the ejector expansion refrigeration cycle based on the entire ejector model are compared with those predicted upon the ejector model only with critical mode. The present results show the entrainment ratio provided by the ejector model coincides well with the experimental data, and most data lie within ±10% error. The system model predicts the gas cooler pressure and evaporator pressure with errors of 1.8% and 4.2% for the measured results, respectively. Moreover, the pressure and mass flow rates of the system based on the ejector model only with critical mode are higher than that by the entire ejector model. The proposed model is useful to predict performances accurately and conduct dynamic analysis reasonably.

Dynamic model of a transcritical CO2 ejector expansion refrigeration system

This paper presents a dynamic model of a transcritical CO2 ejector expansion refrigeration cycle (EERC) which is a promising refrigeration system due to its environment-benign nature and low throttling loss. The mathematical models for gas cooler, evaporator and separator are formulated by using the mass and energy conservation equations. Based on the real properties of CO2, the ejector model is developed and the variable ejector component efficiencies are taken into account because they vary with operation conditions. The compressor and expansive valve are modeled in a set of algebraic equations. The presented transient simulation results are in good agreement with the experimental data. The dynamic behaviors of the system undergoing the change of expansion valve opening and ejector area ratio are observed by the developed model, which helps to understand the characteristics of the EERC and guide the system control.

Comparative analysis of an improved two-stage multi-inter-cooling ejector-expansion trans-critical CO2 refrigeration cycle

A new two-stage multi-inter-cooling ejector-expansion system trans-critical CO2 refrigeration cycle has been recently introduced. In the present study, an internal heat exchanger has been included within this cycle for possible improvement in cooling performance. Comparative studies on the COPs of the two cycles at the same operational conditions showed that improved cycle could be more efficient alternative for the original cycle at low gas cooler pressures. Of course, internal heat exchanger increased the total exergy destruction rate, but overall improvement index increased when internal heat exchanger was used within the refrigeration cycle.

Thermodynamic study on a new transcritical CO2 ejector expansion refrigeration system with two-stage evaporation and vapor feedback

By the stability analysis of the basic transcritical CO2 ejector expansion refrigeration cycle (EERC), the paper proposed a new system which introduces another evaporator downstream the ejector to increase the gas quality into the separator and a vapor feedback valve to decrease the exceed gas into the compressor. The two new components stand for two different cycles: two-stage evaporation cycle and vapor feedback cycle. The theoretical analysis of the new system is carried out based on the first and second laws of thermodynamics to show the effect of the parameters on the system performance, such as entrainment ratio, high-side pressure, outlet temperature of gas cooler, etc. The results by the first law show that, compared with basic EERC the new system can be used in wider range of working conditions, and the COP of the two-stage evaporation cycle is 28.6% higher and the vapor feedback cycle is lower slightly. By exergy analysis at optimum high-side pressure, it is found that the exergy destruction of ejector is the greatest part. The simulation results also give the working ranges of the two cycles, which can help to analyze the system control. Hence, the improvement in the system is a promising method to reduce the restrain in basic EERC system but more study is still needed.

Effect of Suction Nozzle Pressure Drop on the Performance of an Ejector-Expansion Transcritical CO2 Refrigeration Cycle

The basic transcritical CO2 systems exhibit low energy efficiency due to their large throttling loss. Replacing the throttle valve with an ejector is an effective measure for recovering some of the energy lost in the expansion process. In this paper, a thermodynamic model of the ejector-expansion transcritical CO2 refrigeration cycle is developed. The effect of the suction nozzle pressure drop (SNPD) on the cycle performance is discussed. The results indicate that the SNPD has little impact on entrainment ratio. There exists an optimum SNPD which gives a maximum recovered pressure and COP under a specified condition. The value of the optimum SNPD mainly depends on the efficiencies of the motive nozzle and the suction nozzle, but it is essentially independent of evaporating temperature and gas cooler outlet temperature. Through optimizing the value of SNPD, the maximum COP of the ejector-expansion cycle can be up to 45.1% higher than that of the basic cycle. The exergy loss of the ejector-expansion cycle is reduced about 43.0% compared with the basic cycle.