Performance optimization of transcritical CO2 refrigeration cycle with thermoelectric subcooler

Use of thermoelectric subcooler is one of the techniques to improve the performance of transcritical CO₂ cycle. Thermodynamic analyses and optimizations of transcritical CO₂ refrigeration cycle with thermoelectric subcooler are presented in this paper. Further, the effects of various operating parameters on cycle performances are studied. It is possible to optimize current supply, discharge pressure, and CO₂ subcooling simultaneously based on maximum cooling COP for thermoelectrically enhanced transcritical CO₂ refrigeration cycle to get best performance. Results show that thermoelectric current supply, COP improvement, and discharge pressure reduction increase with increase in cycle temperature lift, with maximum values of 11 A, 25.6%, and 15.4%, respectively, for studied ranges. Use of thermoelectric subcooler in CO₂ refrigeration system not only improves the cooling COP, also reduces the system high‐side pressure, compressor pressure ratio, and compressor discharge temperature, and enhances the volumetric cooling capacity. Component‐wise irreversibility distribution shows similar trend with basic CO₂ cycle, although values are lower leading to higher second law efficiency. Cooling capacity may be enhanced by increasing the current supply for the same thermoelectric configuration with penalty of COP. Study reveals that thermoelectrically enhanced CO₂ refrigeration cycle yields significant performance improvement especially for higher‐cycle temperature lift. Copyright © 2011 John Wiley & Sons, Ltd.     

Theoretical study of a transcritical ejector refrigeration cycle with refrigerant R143a

In this paper, a transcritical ejector refrigeration cycle (TERC) using refrigerant R143a as working fluid is proposed to improve the performance of the ejector refrigeration systems driven by low-grade thermal energy. This method adopts an adequate combination of thermal and mechanical energy through the operation of the transcritical process for generator to enhance the performance of the conventional ejector refrigeration cycle (ERC) at the cost of additional driving mechanical energy. The performance characteristics of the TERC are investigated based on theoretical simulations. The TERC is also compared with the conventional ERC using refrigerant R134a. The study shows that when utilizing the low-grade thermal energy, the TERC yields significant increase in COP by adding auxiliary mechanical energy of the cycle pump and has a higher potential in making effective use of the low-grade thermal energy with gradient temperature, such as solar energy gained by a flat plate or evacuated tube solar collector. This also indicates that the TERC is an attractive alternative to the ejector refrigeration systems driven by low-grade thermal energy. Further experimental work for the TERC may be launched in the near future to verify practical applications.     

Incorporation of an organic Rankine cycle in a transcritical booster CO2 refrigeration system

The objective of this work is the investigation of an alternative refrigeration system, which combines a supermarket CO₂ booster system with an organic Rankine cycle. The waste heat after the high-pressure compressor is used in order to feed the organic Rankine cycle for electricity production. The working fluid in the organic Rankine cycle is selected to be the octamethyltrisiloxane (MDM), which has a low global warming potential, and so all the used working fluids in the system are environmentally friendly. The system is studied in the transcritical region for ambient temperatures from 27°C to 40°C. In every case, the system is optimized in order to minimize the overall electricity demand. According to the final results, the system coefficient of performance (COP) can be enhanced from 4.83% to 7.60%, while the electricity consumption can be reduced from 4.60% to 7.03% compared with the conventional booster system. Higher enhancements are found in cases with higher ambient temperatures. Furthermore, a preliminary financial study indicates that the examined idea is viable with the simple payback period to be 5.45 years. The present study is conducted by using a homemade model written in Engineering Equation Solver.     

跨临界制冷循环的适用条件及在CO2制冷系统中的应用研究

在热力学第一、第二定律的基础上研究了跨临界制冷循环若干适用条件。详细分析了跨临界制冷循环对环境温度、制冷压力和温度、回热温差的要求，得出了启动危机、最小制冷高低压差等概念，将这些成果用于CO2制冷剂的分析。获得了相应的数据。可供研究CO2跨临界制冷系统应用。     

Exergy analysis of transcritical carbon dioxide refrigeration cycle with an expander

In this paper, a comparative study is performed for the transcritical carbon dioxide refrigeration cycles with a throttling valve and with an expander, based on the first and second laws of thermodynamics. The effects of evaporating temperature and outlet temperature of gas cooler on the optimal heat rejection pressure, the coefficients of performance (COP), the exergy losses, and the exergy efficiencies are investigated. In order to identify the amounts and locations of irreversibility within the two cycles, exergy analysis is employed to study the thermodynamics process in each component. It is found that in the throttling valve cycle, the largest exergy loss occurs in the throttling valve, about 38% of the total cycle irreversibility. In the expander cycle, the irreversibility mainly comes from the gas cooler and the compressor, approximately 38% and 35%, respectively. The COP and exergy efficiency of the expander cycle are on average 33% and 30% higher than those of the throttling valve cycle, respectively. It is also concluded that an optimal heat rejection pressure can be obtained for all the operating conditions to maximize the COP. The analysis results are of significance to provide theoretical basis for optimization design and operation control of the transcritical carbon dioxide cycle with an expander.     

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.     

Exergy analysis of Joule–Thomson cryogenic refrigeration cycle with an ejector

In this paper, exergy method is applied to analyze the ejector expansion Joule–Thomson (EJT) cryogenic refrigeration cycle. The exergy destruction rate in each component of the EJT cycle is evaluated in detail. The effect of some main parameters on the exergy destruction and exergetic efficiency of the cycle is also investigated. The most significant exergy destruction rates in the cycle are in the compressor and ejector. The ejector pressure ratio and compressor isothermal efficiency have a significant effect on the exergetic efficiency of the EJT cycle. The exergy analysis results show the EJT cycle has an obvious increase in the exergetic efficiency compared to the basic Joule–Thomson refrigeration cycle. A significant advantage from the use of the ejector is that the total exergy destruction of the EJT cycle can be reduced due to much more decreasing of the exergy destruction rates in the compressor and expansion valve. The exergy analysis also reconfirms that applying an ejector is a very important approach to improve the performance of the Joule–Thomson cryogenic refrigeration cycle.     

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.     

Performance of the two-phase ejector expansion refrigeration cycle

This paper is a continuation of the authors’ previous work. The paper presents new experimental data of the system performance of the two-phase ejector refrigeration cycle (TPERC). The TPERC uses a two-phase ejector as an expansion device while the conventional refrigeration cycle (CRC) uses an expansion valve. The TPERC enables the evaporator to be flooded with refrigerant, resulting in a higher refrigerant-side heat transfer coefficient. The experimental study shows that the TPERC gives a higher cooling capacity and a higher coefficient of performance. Moreover, the pressure ratio and the discharge temperature of the compressor of the TPERC are lower than those of the CRC.     

两相流引射循环系统实验研究及引射器性能数值模拟

用ANSYS CFX软件对两相流引射制冷循环中的引射器内部流动进行了数值模拟,分析了混合室直径和喷嘴喉部直径对引射器性能的影响;根据数值模拟结果加工了引射器试件,对R134a两相流引射器及引射循环制冷系统性能进行了实验研究,探讨了固定工况条件下引射器喷嘴喉部直径和混合室直径的优化匹配。实验与模拟结果均表明,在固定工况条件下,存在使引射比及COP分别达到最大的最佳混合室直径和喷嘴喉部直径组合。在冷凝温度为55℃、蒸发温度为3℃的工况下,当混合室直径为16mm、喷嘴喉部直径为2.0mm时引射器的引射比达到最大值,而两相流引射循环制冷系统的COP在混合室直径为16mm、喉部直径为1.7mm时最高,模拟与实验结果的变化趋势是一致的,但二者的引射比值误差较大。     

Performance characteristics of HCFC-123 ejector refrigeration cycles

Ejector refrigeration systems can use low grade thermal energy, at temperatures as low as 60°C, to provide space cooling. Since this waste energy is widely available and the cost of its supply is negligible in many cases, cooling costs can be lower than conventional systems, which makes the method very attractive. The present study describes a computer simulation model for ejector refrigeration systems that was developed using an existing ejector theory. This model allows for internal irreversibilities within the ejector to be included and caters for the addition of a regenerator and a precooler for improving the system coefficient of performance. The study shows that HCFC-123 is a suitable replacement for CFC-11 in this application. Results also indicate that the use of a variable geometry ejector can maintain the optimum performance of refrigeration systems when operating conditions change.     

A review on two-phase ejector as an expansion device in vapor compression refrigeration cycle

This paper presents a comprehensive review of two-phase ejector as an expansion device in vapor compression refrigeration cycle over the past two decades. It also covers research opportunities that are still open in the field of two-phase ejectors as an expansion valve. The studies of the application of ejector as an expansion device are relatively scarce compared to the application of ejector as heat-driven refrigeration system. A better understanding of two-phase flow in the ejector is necessary to optimize energy saving of the system. This paper also presents effects of geometric parameters of the ejector as an expansion valve on the performance of vapor compression refrigeration cycle. In addition, the effect of working fluid on the two-phase expansion refrigeration system is covered. The authors predict that the challenge of future research on design of two-phase ejector is how to generate a pressure rise in diffuser for minimum compressor work and optimum COP improvement.     

Influence of the superheat associated to a semihermetic compressor of a transcritical CO2 refrigeration plant

This work evaluates, from an energetic point of view, the effects of the superheat caused in the refrigerant by the electric motor cooling (SH_SC) in a semihermetic compressor installed in an experimental refrigerating plant, which operates with CO₂ as the working fluid in transcritical conditions. The analysis is based on 84 experimental tests which cover a wide range of operating conditions of the plant: three evaporating levels (0, ?10, ?17 °C) at four compressor speeds (1150, 1300, 1450, 1600 rpm) over a range of discharge pressures from 74.2 to 104.9 bar.The paper presents the empirical model of the compressor and its validation with the experimental measurements, which allows the behaviour of the facility to be calculated with prediction errors below 5%. With the model, the effect of the SH_SC on the energy efficiency of the plant is evaluated by comparing the actual performance of the facility with the simulated behaviour of the plant without considering the SH_SC produced by the semihermetic compressor. The maximum estimated reductions, due to the SH_SC of the compressor, are a 20% in cooling capacity and a 23% in COP.     

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.     

Experimental studies on an ammonia ejector refrigeration system

An ejector refrigeration system has been designed and developed to operate with a simulated (electric) heat source, which can be realized in practical applications by renewable energy sources like solar energy, geothermal energy, etc., or waste heat. In this paper, an experimental study on an ejector refrigeration system working with ammonia is presented. The influence of the generator, condenser, and evaporator temperatures on the ejector refrigeration system performance is presented. The entrainment ratio and COP of the system increase with increasing generator and evaporator temperatures and decrease with increasing condenser temperature.     

Performance characteristics of the ejector refrigeration system based on the constant area ejector flow model

A theoretical analysis of the ejector refrigeration system based on the constant area ejector flow model is performed. Optimised results for R-123 are presented. It is determined that the variations in condenser and evaporator temperature have a greater effect on the optimum coefficient of performance (COP) than the variation in generator temperature. At the same operating temperatures of the ejector refrigeration system, the optimum COP and area ratio determined in this study using the constant area flow model are greater than the values given in the literature for the constant pressure flow model. For the same area ratio, the COP for the system with the constant pressure ejector is relatively higher than that with the constant area ejector. In this case, however, the condenser temperature should be lowered. In addition, the refrigeration systems have almost the same COP values at lower evaporator or higher condenser temperatures.     

First and second law analysis of an ejector expansion Joule–Thomson cryogenic refrigeration cycle

In this paper, a combined first and second law approach is applied to study an ejector expansion Joule–Thomson cryogenic refrigeration cycle. The effects of the evaporator temperature, ejector pressure ratio and compressor function on the coefficient of performance (COP), exergy destruction and the exergetic efficiency have been investigated. The present study has been conducted for the evaporator and compressor temperature in the range of 75–135 and 270–330 K, respectively. The ejector pressure ratio is varied from 1.5 to 5.5. Simulation results show that COP and exergy efficiency increase with increasing evaporator temperature and ejector pressure ratio. In addition, it was found that the increase in the compressor temperature leads to the reduction in the first and second law efficiencies. Copyright © 2010 John Wiley & Sons, Ltd.     

工况对两相流引射器制冷循环系统性能的影响

引射器结构简单、无运动部件,在制冷系统中代替膨胀阀可提高系统的COP。以R134a为工质,实验研究了不同工况条件下两相流引射器制冷循环系统性能,分析了蒸发温度、冷凝温度对引射比、压力提升比、制冷量和系统COP的影响。研究发现:当冷凝温度为40℃时,随着蒸发温度的提高,引射比和压力提升比均下降,制冷量和系统COP均提高;当蒸发温度为-10℃时,随着冷凝温度的增加,引射比和压力提升比均增大,制冷量和系统COP均下降。     

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.     

A combined power and ejector refrigeration cycle for low temperature heat sources

A combined power and ejector refrigeration cycle for low temperature heat sources is under investigation in this paper. The proposed cycle combines the organic Rankine cycle and the ejector refrigeration cycle. The ejector is driven by the exhausts from the turbine to produce power and refrigeration simultaneously. A simulation was carried out to analyze the cycle performance using R245fa as the working fluid. A thermal efficiency of 34.1%, an effective efficiency of 18.7% and an exergy efficiency of 56.8% can be obtained at a generating temperature of 395 K, a condensing temperature of 298 K and an evaporating temperature of 280 K. Simulation results show that the proposed cycle has a big potential to produce refrigeration and most exergy losses take place in the ejector.