CO2跨临界双级压缩/喷射制冷循环热力学分析

建立了同时采用双级压缩和利用喷射器代替节流阀的CO2跨临界双级压缩/喷射制冷循环模型,在系统稳定运行的条件下,分析了高压压力、气体冷却器出口温度、蒸发温度和高、低压压缩机吸气过热度对循环性能的影响,并与CO2跨临界单级压缩/喷射制冷循环和双级压缩制冷循环进行了比较.结果表明:在给定条件下,双级压缩/喷射循环的性能系数明显优于其他两种循环;随着气体冷却器出口温度的升高和蒸发温度的降低,循环的性能系数分别降低了54.9%和43.2%,并且其下降速度大于双级循环的性能系数下降速度;高、低压压缩机吸气过热度升高均导致双级压缩/喷射循环性能系数降低.     

冷热组合型超市系统的设计与经济性分析

设计了冷热组合型超市系统,利用CO2跨临界循环对空间夏季供冷和冬季供热,采用R290/CO2复叠式制冷循环对食品冷冻冷藏,同时回收CO2跨临界循环高温气体散发的热量和R290/CO2复叠式制冷循环R290高温循环气体的冷凝热,实现夏季空间供冷、食品制冷的同时供应生活热水,冬季空间供暖、食品制冷的同时供应生活热水,及春秋季节食品制冷同时供应生活热水。并与供冷、供暖、食品制冷和供应生活热水分别进行的常规R404A超市系统的能效相比较,得出冷热组合型超市系统的能耗大大降低,能效明显增加,不仅节约能源,而且保护环境,是很有发展前景的绿色环保系统。     

自然工质低温复叠式制冷循环的热力学分析与比较

介绍了用于低温环境下采用自然工质CO2、NH3和R2903的复叠式制冷循环，介绍和分析了CO2、R290和NH3的物性特征，并且进行了复叠式制冷循环的热力学理论分析，通过计算得出了不同蒸发温度下的最佳低温循环的冷凝温度和最流量比。通过CO2－NH3，CO2－R290和NH3－NH3复叠式循环的比较，可以看出CO2－NH3、CO2－R290的复叠式制冷循环在低温制冷条件下有明显优势。     

CO_2跨临界制冷循环理论分析

以热力学理论为基础,对CO2跨临界制冷循环的压缩过程、放热过程、膨胀过程和吸热过程分别进行了分析,为CO2跨临界制冷系统的研究提供了理论参考。     

CO2双蒸发器压缩/喷射式跨临界制冷循环

为减小CO_2跨临界循环系统节流部分的膨胀功损失,提高系统性能,可在小型制冷系统中采用喷射器代替节流阀,部分回收工质从高压到低压过程的膨胀功。在对系统进行热力学分析的基础上,建立了CO_2跨临界压缩／喷射制冷循环的效率分析模型。计算结果表明：在合理的喷射器出口背压下,CO_2跨临界压缩／喷射制冷循环可以得到较高的循环性能。蒸发温度和气体冷却器出口温度两工况的变化对该系统性能的影响程度相对较大。在较低蒸发温度下,该系统可以明显降低压缩机出口温度,有利于系统稳定运行。     

自然工质复叠式制冷热泵系统的性能分析

介绍了以自然工质CO2为高温循环工质,R290为低温循环工质,同时制冷和供热的CO2/R290复叠式制冷热泵系统,通过对CO2/R290复叠式制冷热泵系统的性能分析,得到了气体冷却器的入口压力和出口温度,复叠式循环的蒸发温度,低温循环的冷凝温度对复叠式制冷热泵系统性能的影响,为今后的CO2/R290复叠式制冷热泵系统的优化设计和开发应用打下了一定的基础。     

环保型CO2热泵系统的理论分析与性能测试

由于CFCs和HCFCs等人工合成制冷剂对环境产生不利影响,CO2作为自然工质得到了日益重视.CO2的临界温度为31℃,一般采用跨临界循环方式,但其循环性能低于合成工质.对跨临界CO2热泵系统性能进行的热力学分析和实验测试表明:当气体冷却器出口温度一定时,跨临界CO2热泵循环存在一个最优运行压力;在相同工况下,随着蒸发温度的升高,系统的性能系数逐渐增大:气体冷却器出口温度越低,整个系统运行的效率越高.因此,在跨临界CO2热泵系统设计和运行过程中,应综合考虑蒸发温度、气体冷却器出口温度以及运行压力的影响,使系统性能最优.     

CO2跨临界低温地热源水-水热泵的实验研究

研究用CO2跨临界循环热泵系统开发低温地热资源，详细介绍了CO2跨临界循环热泵系统的运行机理、装置特点、热源选取。通过低温地热源的模拟实验，给出实验最优工况的调节方法，这对开发应用CO2热泵系统用能效率有着重要作用。     

跨临界CO2制冷循环火用分析

基于热力学第二定律,对跨临界CO2制冷循环过程的损失及火用效率进行理论分析,发现节流过程火用损失最大,循环火用效率为25%。提高蒸发温度和降低冷却终了温度是提高循环火用效率的有效途径;升高冷却压力,可以降低节流过程火用损失,但是对循环火用效率影响不大。     

CO2跨临界热泵高温化途径分析

CO2作为一种环境友好的自然工质,以其为循环工质的跨临界热泵制热能力突出.建立CO2跨临界增压和CO2跨临界热泵理论分析模型,研究不同增压过程对热泵系统COP、气冷器中水的出口温度及质量流量的影响规律.结果表明,2种热泵高温化方案均会提升压缩机等熵效率、功耗和压缩机出口工质温度,且提升了气冷器出口水温,但COP和热水的...     

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.     

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.     

Construction and testing of a wet-compression absorption carbon dioxide refrigeration system for vehicle air conditioner

The environmental benefits of the transcritical carbon dioxide (CO₂) refrigeration cycle are considerable. But its application is greatly challenged by the high operation pressure, which could be as high as 120 bar. A wet-compression absorption (WCA) CO₂ refrigeration cycle was constructed by adding a non-volatile liquid into a CO₂ refrigeration cycle. CO₂ is highly soluble in the liquid and easily absorbed and desorbed by it. In the WCA CO₂ refrigeration cycle, the high side pressure was less than 35 bar, which was tremendously reduced compared to the transcritical CO₂ refrigeration cycle.In this paper, following a thermodynamic analysis of working fluid, a WCA CO₂ refrigeration demonstrator plant was constructed within the restricted physical and operational envelope of an existing vehicle refrigeration unit. This unique plant operated satisfactorily, delivering sustainable cooling for refrigerated vehicle. The relationship between system performance and the cycle ratio and IHX (internal heat exchanger) efficiency was tested. The components used in the demonstrator were entirely based on existing components and not optimized and considerable potential exists for efficiency improvements.     

Design and experimental analysis of a carbon dioxide transcritical chiller for commercial refrigeration

Carbon dioxide is an interesting solution for commercial refrigeration and in perspective for air-conditioning systems. In this paper a newly developed carbon dioxide transcritical air cooled chiller for refrigerating propylene glycol down to −8 °C supply temperature is described. The aim of the project was at optimising the cycle energy efficiency while assuring reliable operation and simple management of the unit. The carbon dioxide optimal pressure issue is addressed with an innovate system architecture and control logic. Using a flash tanks and two electronic valves, the optimal cycle upper pressure was maintained in transcritical operation mode. The managing of the valves allows the refrigeration machine efficiency improvement when the gas cooler inlet air allows subcritical working conditions. A simulation model of the chiller was developed and its results validated with experimental data. A measurement campaign was carried out, testing the chiller at external temperatures ranging from 18 to 35 °C, the unit energy efficiency ranging from 3.1 to 2.0.     

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.     

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.     

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 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.     

Research on performance of mixed absorption refrigeration for solar air-conditioning

A novel lithium bromide/water mixed absorption refrigeration cycle that is suitable for the utilization of solar air-conditioning and can overcome the drawbacks of low system overall efficiency of traditional solar absorption refrigeration air-condition systems is presented. The accessorial high pressure generator was added in the cycle. The lithium bromide solution flowing out from the high pressure generator was mixed with the solution from the low pressure absorber to increase lithium bromide solution concentration and decrease pressure in the high pressure absorber. The performance of a mixed absorption refrigeration cycle was analyzed. The theoretical analysis shows that the highest COP is 0.61, while the highest available temperature difference of heat resource is 33.2°C. The whole coefficient of performance of the solar air-conditioning using mixed absorption cycle is 94.5% higher than that of two-stage absorption. The advantages of solar air-conditioning can be markedly made use of by the cycle.