Exergetic analysis of transcritical CO2 residential air-conditioning system based on experimental data

The experimental system for the transcritical CO₂ residential air-conditioning with an internal heat exchanger was built. The effects of working conditions on system performance were experimentally studied. Based on the experimental dada, the second law analysis on the transcritical CO₂ system was performed. The effects of working conditions on the total exergetic efficiency of the system were investigated. The results show that in the studied parameter ranges, the exergetic efficiency of the system increases with the increases of gas cooler side air inlet temperature, gas cooler side air inlet velocity and evaporating temperature. And it will decrease with the increases of evaporator side air inlet temperature and velocity. Then, a complete exergetic analysis was performed for the entire CO₂ transcritical cycle including compressor, gas cooler, expansion valve, evaporator and internal heat exchanger under different working conditions. The average exergy loss in gas cooler is the highest one under all working conditions which is about 30.7% of the total exergy loss in the system. The second is the average exergy loss in expansion valve which is about 24.9% of the total exergy loss, followed by the exergy losses in evaporator and compressor, which account for 21.9% and 19.5%, respectively. The exergy loss in internal heat exchanger is the lowest one which is only about 3.0%. So in the optimization design of the transcritical CO₂ residential air-conditioning system more attentions should be paid to the gas cooler and expansion valve.     

Transcritical CO2 heat pump systems: exergy analysis including heat transfer and fluid flow effects

This paper presents the exergetic analysis and optimization of a transcritical carbon dioxide based heat pump cycle for simultaneous heating and cooling applications. A computer model has been developed first to simulate the system at steady state for different operating conditions and then to evaluate the system performance based on COP as well as exergetic efficiency, including component wise irreversibility. The chosen system includes the secondary fluids to supply the heating and cooling services, and the analyses also comprise heat transfer and fluid flow effects in detail. The optimal COP and the exergetic efficiency were found to be functions of compressor speed, ambient temperature and secondary fluid temperature at the inlets to the evaporator and gas cooler and the compressor discharge pressure. An optimization study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on heat transfer area has been conducted. The exergy flow diagram (Grassmann diagram) shows that all the components except the internal heat exchanger contribute significantly to the irreversibilities of the system. Unlike a conventional system, the expansion device contributes significantly to system irreversibility. Finally, suggestions for various improvement measures with resulting gains have been presented to attain superior system performance through reduced component irreversibilities. This study is expected to offer useful guidelines for system design and its optimisation and help toward energy conservation in heat pump systems based on transcritical CO₂ cycles.     

Optimization of ejector-expansion transcritical CO2 heat pump cycle

Optimization studies along with optimum parameter correlations, using constant area mixing model are presented in this article for ejector-expansion transcritical CO₂ heat pump cycle with both conventional and modified layouts. Both the energetic and exergetic comparisons between valve, turbine and ejector-expansions-based transcritical CO₂ heat pump cycles are also studied for simultaneous cooling and heating applications. Performances for conventional layouts are presented by maximum COP, optimum discharge pressure and corresponding entrainment ratio and pressure lift ratio of ejector, whereas for modified layout by maximum COP, optimum discharge pressure and corresponding pressure lift ratio. The optimization for modified layout can be realized for certain entrainment ratio, evaporator and gas cooler exit temperature combinations. Considering the trade-off between the system energetic and exergetic performances, and cost associated with expansion devices, the ejector may be the promising alternative expansion device for transcritical CO₂ heat pump cycle.     

Simulation of the optimal heat rejection pressure for transcritical CO<Subscript>2</Subscript> expander cycle

In order to optimize and control transcritical CO₂ refrigeration cycle, a mathematical model was developed to simulate the system performance. The simulation results show that a maximum COP exists at the optimal heat rejection pressure not only for throttle valve cycle but also for expander cycle. Also, the optimal heat rejection pressures of the throttle valve cycle are greater than those of the expander cycle under the same condition. In order to further obtain correlation of the optimal heat rejection pressure for transcritical CO₂ expander cycle, it is necessary to analyze the impact degree of compressor efficiency, expander efficiency, gas cooler outlet temperature and evaporation temperature. Based on the simulation results, the values of the optimal heat rejection pressure for the expander cycle were regressed in terms of gas cooler outlet temperature and evaporation temperature at given compressor efficiency and expander efficiency. Finally, two types of polynomial correlations were obtained. One is cubic form, with an average deviation of less than 0.5% and the other is simplified form, with an average deviation of less than 1%. It is, therefore, convenient to use either correlation to simulate the performance of transcritical CO₂ expander cycle.     

Simulation of the optimal heat rejection pressure for transcritical CO2 expander cycle

In order to optimize and control transcritical CO₂ refrigeration cycle, a mathematical model was developed to simulate the system performance. The simulation results show that a maximum COP exists at the optimal heat rejection pressure not only for throttle valve cycle but also for expander cycle. Also, the optimal heat rejection pressures of the throttle valve cycle are greater than those of the expander cycle under the same condition. In order to further obtain correlation of the optimal heat rejection pressure for transcritical CO₂ expander cycle, it is necessary to analyze the impact degree of compressor efficiency, expander efficiency, gas cooler outlet temperature and evaporation temperature. Based on the simulation results, the values of the optimal heat rejection pressure for the expander cycle were regressed in terms of gas cooler outlet temperature and evaporation temperature at given compressor efficiency and expander efficiency. Finally, two types of polynomial correlations were obtained. One is cubic form, with an average deviation of less than 0.5% and the other is simplified form, with an average deviation of less than 1%. It is, therefore, convenient to use either correlation to simulate the performance of transcritical CO₂ expander cycle.     

Cycle parameter optimization of vortex tube expansion transcritical CO2 system

Optimization studies along with optimum parameter correlations are presented in this article for a vortex tube expansion transcritical CO₂ refrigeration cycle with two cycle layouts based on the Maurer model (1999) and the Keller model (1997). A simple thermodynamic model is proposed and used for vortex tube analysis. Finally, the COP improvement and effect on optimum discharge pressure by using vortex tube in transcritical CO₂ cycle instead of expansion valve are presented. The results show that the effect of cold mass fraction and inlet water temperature to desuperheater (used to cool hot gas from vortex tube) on the cycle optimization is negligible. The Maurer model is better than the Keller model in terms of moderately more COP improvement and lower cost due to less components. The use of a vortex tube is more effective for higher gas cooler exit temperature for both models. Results show that the vortex tube expansion transcritical CO₂ cycle for the Maurer model can give higher COP improvement for lower cooling temperature applications; however the trend is reverse for the Keller model.     

Effects of operating parameters on the performance of a CO2 air conditioning system for vehicles

This paper deals with the effects of the operating parameters on the cooling performance that can be applied for a transcritical CO₂ automotive air conditioning system. The experimental conditions of the performance tests for a CO₂ system and components such as a gas cooler and an evaporator were suggested to compare with the performance of each at the standardized test conditions. This research presents experimental results for the performance characteristics of a CO₂ automotive air conditioning system with various operating conditions such as different gas cooler inlet pressures, compressor speeds and frontal air temperatures/flow rates passing through the evaporator and the gas cooler. Experimental results show that the cooling capacity was more than 4.9 kW and coefficient of performance (COP) was more than 2.4, at each optimum pressure of gas cooler inlet during idling condition. Also, the cooling capacity was about 7.5 kW and COP was about 1.7 at the optimum pressure of gas cooler inlet during driving condition when air inlet temperatures of gas cooler and evaporator were 45 °C and 35 °C, respectively. Therefore, we concluded that the automotive air conditioning system using CO₂ refrigerant has good performance. This paper also deals with the development of optimum high pressure control algorithm for the transcritical CO₂ cycle to achieve the maximum COP.     

Studies on a two-stage transcritical carbon dioxide heat pump cycle with flash intercooling

Simulation studies on a two-stage flash intercooling transcritical carbon dioxide heat pump cycle are presented. Sub-critical and super-critical thermodynamic and transport properties of carbon dioxide are calculated employing an exclusive precision property code based on recently published correlations. Results exhibit that flash intercooling technique is not economical with CO₂ refrigerant unlike NH₃ as the refrigerant. COP is considerably lower than that of the single cycle for a given gas cooler and evaporator temperature. There is no optimum inter-stage pressure as well. However, a marginal increase in COP occurs as inter-stage pressure decreases from the classical estimate of geometric mean of gas cooler and evaporator pressure. It is observed that incorporation of desuperheating of vapour in the intercooler almost doubles the mass flow rate in the second stage which can be attributed to the large flashing that occurs in the intercooler; this increase depends on the discharge temperature from the first stage and mass flow rate of refrigerant flow in the evaporator. Compressor isentropic efficiency shows marginal influence on system performance.     

Cycle improvements to ejector‐expansion transcritical CO2 two‐stage refrigeration cycle

In this paper, a new configuration of ejector‐expansion transcritical CO₂ (TRCC) refrigeration cycle is presented, which uses an internal heat exchanger and intercooler to enhance the performance of the new cycle. The theoretical analysis on the performance characteristics was carried out for the new cycle based on the first and second laws of thermodynamics. It was found that, compared with the conventional transcritical CO₂ cycle and ejector‐expansion transcritical CO₂ cycle, the simulation results show that the coefficient of performance and second law efficiency of the new cycle were increased by about 55.5 and 26%, respectively, under the operating conditions that evaporator temperature is 10°C, gas cooler outlet temperature is 40°C and gas cooler pressure is optimum pressure. It is also concluded that the entrainment ratio for the new ejector‐expansion TRCC cycle is on average 35% higher than that of the conventional ejector‐expansion TRCC cycle. The analysis results are of significance to provide theoretical basis for design optimization of the transcritical CO₂ refrigeration cycle with an ejector‐expansion device, internal heat exchanger and intercooler. Copyright © 2007 John Wiley & Sons, Ltd.     

Particular characteristics of transcritical CO2 refrigeration cycle with an ejector

The present study describes a theoretical analysis of a transcritical CO₂ ejector expansion refrigeration cycle (EERC) which uses an ejector as the main expansion device instead of an expansion valve. The system performance is strongly coupled to the ejector entrainment ratio which must produce the proper CO₂ quality at the ejector exit. If the exit quality is not correct, either the liquid will enter the compressor or the evaporator will be filled with vapor. Thus, the ejector entrainment ratio significantly influences the refrigeration effect with an optimum ratio giving the ideal system performance. For the working conditions studied in this paper, the ejector expansion system maximum cooling COP is up to 18.6% better than the internal heat exchanger cycle (IHEC) cooling COP and 22.0% better than the conventional vapor compression refrigeration cycle (VCRC) cooling COP. At the conditions for the maximum cooling COP, the ejector expansion cycle refrigeration output is 8.2% better than the internal heat exchanger cycle refrigeration output and 11.5% better than the conventional cycle refrigeration output. An exergy analysis showed that the ejector expansion cycle greatly reduces the throttling losses. The analysis was also used to study the variations of the ejector expansion cycle cooling COP for various heat rejection pressures, refrigerant temperatures at the gas cooler exit, nozzle efficiencies and diffuser efficiencies.     

Performance characteristics of refrigeration cycle with parallel compression economization

Thermodynamic analyses and economizer pressure optimizations of ammonia, propane and isobutane‐based refrigeration cycles with parallel compression economization are presented in this article. Energetic and exergetic performance comparisons with transcritical CO₂ cycle are presented as well. Results show that the optimum economizer mass fraction as well as COP improvement increase with increase in cycle temperature lift. The expression for optimum economizer pressure has been developed. Study shows that the performance improvements using parallel compression economization are strongly dependent on the refrigerant properties as well as the operating conditions. Using parallel compression economization, carbon dioxide yields maximum COP improvement of 31.9% followed by propane (29.8%), isobutane (27.2%) and ammonia (11.3%) for studies ranges. In spite of higher COP improvement, the cooling COP as well as second low efficiency for carbon dioxide is still significantly lower than that for others. Component‐wise irreversibility distributions show the similar trends for all refrigerants except CO₂. Employing parallel compression economization in refrigeration cycle not only improves the cooling COP but also increase the compactness of evaporator. Copyright © 2010 John Wiley & Sons, Ltd.     

Theoretical analysis of LiBr/H2O absorption refrigeration systems

A computational model is developed for the parametric investigation of single‐effect and series flow double‐effect LiBr/H₂O absorption refrigeration systems. The effects of generator, absorber, condenser, evaporator and dead state temperatures are examined on the performance of these systems. The parameters computed are coefficient of performance (COP), exergy destruction rates, thermal exergy loss rates, irreversibility and exergetic efficiency. The results indicate that COP and exergetic efficiency of both the systems increase with increase in the generator temperature. There exist different optimum values of generator temperature for maximum COP and maximum exergetic efficiency. The optimum generator temperature is lower corresponding to maximum exergetic efficiency as compared to optimum generator temperature corresponding to maximum COP. The effect of increase in absorber, condenser and evaporator temperatures is to decrease the exergetic efficiency of both the systems. The irreversibility is highest in absorber in both systems. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance analysis and optimization of a new two-stage ejector-expansion transcritical CO2 refrigeration cycle

In this paper, a new two-stage configuration of ejector-expansion transcritical CO₂ (TRCC) refrigeration cycle is presented, which uses an internal heat exchanger and intercooler to enhance the performance of the new cycle. The theoretical analysis on the performance characteristics was carried out for the new cycle based on the first and second laws of thermodynamics. Based on the simulation results, it is found that, compared with the conventional two-stage transcritical CO₂ cycle, the COP and second law efficiency of the new two-stage cycle are about 12.5–21% higher than that of conventional two-stage cycle. It is also concluded that, the performance of the new two-stage transcritical CO₂ refrigeration can be significantly improved based on the presented new two-stage cycle. Hence the new two-stage refrigeration cycle is a promising refrigeration cycle from the thermodynamically and technical point of views. A regression analysis in terms of evaporator and gas cooler exit temperatures has been used, in order to develop mathematical expressions for maximum COP, optimum discharge and inter-stage pressures and entrainment ratio.     

CO2跨临界制冷循环系统[火用]经济分析

为优化CO2跨临界制冷循环系统,以达到能效和经济之间的平衡,提出?经济分析方法,并建立数学模型,模拟分析系统的COP、?效率及系统各部件的损、?经济成本等随气体冷却器出口温度升高的变化趋势。结果表明,系统的COP和?效率呈现下降趋势;各部件损除蒸发器略有下降外,压缩机、气体冷却器、节流阀的?损均有不同程度的增大;系统单位产品?成本呈现下降趋势;减小节流阀产生的?损有利于提高系统的效率。     

Modeling and simulating the transcritical CO2 heat pump system

In this paper, a mathematical model for steady-state simulation of transcritical CO₂ water-to-water heat pump system with an expander has been developed. It is used to simulate the performance of transcritical CO₂ system with CO₂ expander prototype. Simulated results are compared with experimental data to verify the accuracy of the simulation model. The comparison results show the average deviation of about 15% for COP_c(cooling coefficient of performance) and COP_h(heating coefficient of performance), about 17% for cooling and heating capacity at experimental high pressure ranges. With this model, which has been validated in a limited high pressure range, the influence of water mass flow rate and water inlet temperature of both evaporator and gas cooler on the performance of transcritical CO₂ expander system is analyzed. The results show that decreasing inlet temperature and increasing mass flow rate of cooling water cannot only increase the system performance but also reduce the optimal heat rejection pressure, at which the maximum COP (coefficient of performance) can be obtained. For chilling water, increasing its inlet temperature and mass flow rate is favorable for increasing the system performance, while the optimal heat rejection pressure does not vary very much.     

Theoretical and experimental studies on optimum heat rejection pressure for a CO2 heat pump system

The experimental and simulation researches have been conducted to investigate the relationships between optimum heat rejection pressure and other related operating parameters for a transcritical CO₂ heat pump system with two throttle valves. It proved that it is relatively reliable to control the heat rejection pressure of the CO₂ system with two expansion valves in series. The experimental results also show similar trends with those from simulation, under widely different operating conditions. Thus both the simulation and experimental results meet here: for a transcritical CO₂ cycle, there exists an optimal heat rejection pressure, under which the system can reach the maximum heating coefficient of performance (COP). Furthermore, the research also reveals that the optimal heat rejection pressure mainly depends on the refrigerant outlet temperature of gas cooler whereas the evaporating temperature and the performance of the given compressor have smaller effect on the optimum heat rejection pressure. Based on the experimental data, a correlation of the optimal heat rejection pressure with respect to mainly involved parameters is obtained for specific conditions.     

The performance analysis of a two‐stage transcritical CO2 cooling cycle with various gas cooler pressures

A theoretical analysis of a two‐stage transcritical CO₂ cooling cycle is presented. The effect of a two‐stage cycle with intercooling process on the system coefficient of cooling performance is presented for various gas cooler pressures. However, the performance comparison between one‐stage and two‐stage cycles is presented for same operating conditions. Gas cooler pressure, compressor isentropic efficiency, gas cooler efficiency, intercooling quantity and refrigerant outlet temperature from the gas cooler are used as variable parameters in the analysis. It is concluded that the performance of the two‐stage transcritical CO₂ cycle is approximately 30% higher than that of the one‐stage transcritical CO₂ cycle. Hence, the two‐stage compression and intercooling processes can be assumed as valuable applications to improve the transcritical CO₂ cycle performance. Copyright © 2008 John Wiley & Sons, Ltd.     

Irreversibility minimization of heat exchangers for transcritical CO2 systems

Irreversibility analyses of both evaporator and gas cooler of a CO₂ based transcritical heat pump for combined cooling and heating, employing water as the secondary fluid, have been reported. The analysis includes both operational and material associated irreversibilities. Optimization of heat exchanger tube diameter and length and effect of design parameters on overall system performance is also presented. Results clearly show that higher heat transfer coefficient can be achieved by reducing the diameter only to a limited extent due to rapid increase in pressure drop. The minimum possible diameter depends on mass flow rate (capacity) and division of flow path. The right combination of optimum diameter and length depends on the number of passes, capacity and operating parameters. It is noteworthy that due to higher pressure drop occurring in the evaporator compared to the gas cooler, zero temperature approach is attained before the optimum length is reached in case of the evaporator. Presented results are expected to help choose effective heat exchanger size in terms of diameter, length and number of passes.     

A transcritical CO2 heat pump for simultaneous water cooling and heating: Test results and model validation

Working prototype of a transcritical CO₂ heat pump system for simultaneous cooling and heating of water is designed and developed based on numerical simulation studies. System behaviour and performance of the system have been studied experimentally for various operating parameters such as system pressure, water mass flow rate, water inlet temperature and expansion valve opening. Finally, the system simulation model predictions have been validated by the test data. Test results show the effect of water mass flow rate to be modest for both evaporator and gas cooler, whereas the effect of water temperature at the inlet to the gas cooler on system performance is significant. The expansion valve opening has a significant effect as well near the full valve closing condition (up to 20°). Validation of the simulation model shows reasonably good agreement (a maximum deviation of 15%) with the test data exhibiting fairly similar trends. Copyright © 2008 John Wiley & Sons, Ltd.     

Performance optimization of transcritical CO2 cycle with parallel compression economization

Being a low critical temperature fluid, CO₂ transcritical system offers low COP for a given application. Parallel compression economization is one of the techniques to improve the COP for transcritical CO₂ cycle. An optimization study of transcritical CO₂ refrigeration cycle with parallel compression economization is presented in this paper. Further, performance comparisons of three different COP improvement techniques; parallel compression economization alone, parallel compression economization with recooler and multistage compression with flash gas bypass are also presented for chosen operating conditions. Results show that the parallel compression economization is more effective at lower evaporator temperature. The expression for optimum discharge pressure has been developed which offers useful guideline for optimal system design and operation. Study shows that the parallel compression with economizer is promising transcritical CO₂ cycle modifications over other studied cycle configurations. A maximum improvement of 47.3% in optimum COP is observed by employing parallel compression economization for the studied ranges.