二氧化碳制冷技术

CO2作为一种天然工质，是目前CFCs工质替代的一个重点研究方向。根据CO2作为制冷剂的相关热物理和化学性质及CO2制冷循环，说明采用CO2作制冷剂、采用跨临界循环的优越性。介绍CO2制冷循环系统关键设备——压缩机、膨胀机、气体冷却器／蒸发器的研究进展情况，并对采用CO2作制冷剂的汽车空调、热泵系统的应用进行综述，指出今后研究的发展方向。     

CO2制冷特性和应用分析

介绍了CO2制冷剂的的历史和发展;分析总结了CO2的物理化学性质和制冷特性;阐述了亚临界循环、跨临界循环等4种制冷循环的基本原理;介绍了CO2制冷技术在汽车空调、热泵系统和复叠式制冷系统等方面的应用。     

应用于展示柜的CO_2蒸气压缩式制冷系统循环的分析

近年来,制冷剂对臭氧层的破坏和全球温室效应等环保问题日益突出,CO2作为理想的制冷剂开始重新得到重视。本文介绍理论的跨临界CO2蒸气压缩式制冷循环,对实验室现有CO2跨临界制冷系统进行一系列研究,并与常用制冷剂的循环进行对比,认为CO2制冷循环取代传统制冷循环应用于展示柜可大大提高制冷效率。     

二氧化碳跨临界循环制冷的开发与研究进展

由于近年来制冷剂对臭氧层的破坏和全球温室效应等环保问题日益突出,CO2作为理想的制冷剂开始重新得到重视。给出了CO2跨临界制冷循环的典型流程和特点;对国内外针对超临界CO2特性的、CO2制冷循环、CO2制冷设备的开发以及CO2跨临界循环系统安全性与可靠性等四个方面的研究现状进行了综述,指出了二氧化碳跨临界循环制冷今后的发展趋势。     

CO_2跨临界制冷循环气体冷却器研究现状

由于臭氧层的破坏和温室效应的加剧,HFCs和HCFCs制冷剂将逐步被替代。零ODP,低GWP的CO2是非常理想的替代制冷剂,CO2跨临界制冷循环以优异的热力学性能在汽车空调领域、民用住宅领域都有应用。在CO2跨临界循环系统中气体冷却器起着重要作用,对整个系统的能效有着显著的影响。本文总结近十年来CO2跨临界制冷循环气体冷却器在结构设计、计算模拟方面的研究进展。紧凑、高效、廉价以及更精确的模拟计算方法是气体冷却器未来的研究方向。     

CO2跨临界循环及其在汽车空调系统方面的研究状况

介绍了CO2制冷剂的物理特性,分析了CO2跨临界制冷循环的特点,综述了目前国内外CO2跨临界循环在汽车空调方面的研究状况,并指出了一些亟待解决的问题.     

CO2制冷剂及其跨临界循环系统的开发与研究

从CO2制冷剂的研究背景出发,通过对其作为制冷剂的历史回顾,主要介绍了CO2制冷剂再开发的目的、特点以及应用和研究方向;对国内在该领域的开发与研究进行了归纳和总结,指出了CO2跨临界循环系统今后的研究和发展方向.     

二氧化碳跨临界压缩机研究进展

介绍CO2制冷剂跨临界循环的优势，回顾CO2压缩机的发展历史，对已开发的各种类型的CO2压缩机进行分析，总结CO2压缩机发展的关键技术。     

低温冷库制冷领域使用CO2作为制冷剂的应用概述

对天然制冷工质CO2(R744)应用于低温大容量制冷工况下的循环形式进行了叙述,在给定工况下进行了循环性能计算,并与传统工质进行了比较.指出CO2跨临界循环与传统制冷剂循环相比有明显优势,适合应用于低温冷库工况.     

二氧化碳跨临界循环制冷的研究进展

给出了CO2跨临界制冷循环的典型流程和特点;对国内外针对超临界CO2特性的研究、CO2制冷循环的研究、CO2制冷设备的研究与开发以及CO2跨临界循环系统安全性与可靠性的研究现状等4个方面进行了综述,指出了二氧化碳跨临界循环制冷的发展趋势.     

Equipment design and installation features to disperse refrigerant releases in rooms—part II: determination of procedures

There is always a risk of leakage of refrigerant into a room that refrigeration and air conditioning equipment occupies. Mitigation of build-up of flammable concentrations from leakage through appropriate equipment construction and installation criteria minimises the potential for ignition. This paper is the first part of an investigation into design and installation measures to disperse leaked flammable refrigerant. It mainly describes the experiments and provides an analysis of the data. The paper describes a purpose built test facility, which was used to carry out experiments to study the dispersion of carbon dioxide to simulate leaked refrigerant. By measuring carbon dioxide concentrations and making flow visualisation, the effects of parameters such as equipment airflow and installation height were observed. The observed trends provide guidance for designing refrigeration and air conditioning equipment, which helps to ensure rapid dispersion of flammable concentrations in the event of a leak of flammable refrigerant. A second paper (Part II) discussed the development of numerical correlations, which are used in the resulting design procedure.     

The use of natural refrigerants: a complete solution to the CFC/HCFC predicament

The ideal refrigeration or heat pump cycle for a given purpose is defined by the boundary conditions of the application and is completely independent of the refrigerant used. The real cycle should approach the theoretical ideal as closely as practically possible. The thermodynamic and heat transfer properties of the refrigerant are important in this respect. Natural substances such as ammonia, propane and carbon dioxide are often better than the present halocarbons in this regard. By using simple methods of safety it is possible to use these three natural fluids for practically all conventional refrigeration and heat pump systems.     

Experiment and simulation on the performance of an autocascade refrigeration system using carbon dioxide as a refrigerant

The main purpose of this study is to investigate the performance of an autocascade refrigeration system using zeotropic refrigerant mixtures of R744/134a and R744/290. One of the advantages of this system is the possibility of keeping the highest pressure of the system within a limit by selecting the composition of a refrigerant mixture as compared to that in the vapor compression system using pure carbon dioxide. Performance test and simulation have been carried out for an autocascade refrigeration system by varying secondary fluid temperatures at evaporator and condenser inlets. Variations of mass flow rate of refrigerant, compressor power, refrigeration capacity, and coefficient of performance (COP) with respect to the mass fraction of R744 in R744/134a and R744/290 mixtures are presented at different operating conditions. Experimental results show similar trends with those from the simulation. As the composition of R744 in the refrigerant mixture increases, cooling capacity is enhanced, but COP tends to decrease while the system pressure rises.

Résumé

The main purpose of this study is to investigate the performance of an autocascade refrigeration system using zeotropic refrigerant mixtures of R744/134a and R744/290. One of the advantages of this system is the possibility in keeping the highest pressure of the system within a limit by selecting the composition of a refrigerant mixture as compared to that in the vapor compression system using pure carbon dioxide. Performance test and simulation have been carried out for an autocascade refrigeration system by varying secondary fluid temperatures at evaporator and condenser inlets. Variations of mass flow rate of refrigerant, compressor power, refrigeration capacity, and coefficient of performance (COP) with respect to the mass fraction of R744 in R744/134a and R744/290 mixtures are presented at different operating conditions. Experimental results show similar trends with those from the simulation. As the composition of R744 in the refrigerant mixture increases, cooling capacity is enhanced, but COP tends to decrease while the system pressure rises.     

CO_2低温冷冻冷藏系统的节能分析

二氧化碳既可以作为制冷剂,也可以作为载冷剂应用于低温冷冻冷藏工程,这对解决制冷剂替代问题和节能减排都很有意义。本文比较常规载冷剂系统、二氧化碳载冷剂系统和二氧化碳/氨复叠制冷系统的特点和运行能耗。结果表明,二氧化碳制冷剂和载冷剂系统具有制冷机组性能系数高、循环泵耗功小、管路和设备保温散热少及空气冷却器霜层较薄等特点。     

Experimental studies of thermodynamic properties of R744-oil-mixtures up to 140 °C and 150 bar

The knowledge of the thermodynamic properties of pure refrigerants and refrigeration oils is a precondition for developing a refrigeration cycle. In addition, the miscibility of the refrigerant in the oil and the influence of the working fluids on the properties of the mixtures are to be known. For the refrigerant carbon dioxide, several types of refrigeration oils with different miscibility/solubility and viscosity properties can be used. In MAC applications for passenger cars, PAG oils are preferred.In this paper, new methods and equipment to measure the thermodynamic properties miscibility, density, vapour pressure, solubility, and viscosity are discussed and examples of experimental data are shown for a R744-oil-mixture. At first the miscibility gap was examined by visual observation of the mixtures. Then the other properties were measured in the miscible range from 70 to 100% oil up to 140 °C and up to 150 bar.     

A comparative study on the environmental impact of supermarket refrigeration systems using low GWP refrigerants

Supermarket refrigeration systems have high environmental impact due to their large refrigerant charge and high leak rates. Consequently, the interest in using low GWP refrigerants such as carbon dioxide (CO₂) and new refrigerant blends is increasing. In this paper, an open-source Life Cycle Climate Performance (LCCP) framework is presented and used to compare the environmental impact of four supermarket refrigeration systems: a transcritical CO₂ booster system, a cascade CO₂/N-40 system, a combined secondary circuit with central DX N-40/L-40 system, and a baseline multiplex direct expansion system utilizing R-404A and N-40. The study is performed for different climates within the USA using EnergyPlus to simulate the systems' hourly performance. Further analyses are presented such as parametric, sensitivity, and uncertainty analyses to study the impact of different system parameters on the LCCP.     

A comparison of refrigerants in a two-stage ejector-expansion transcritical refrigeration cycle based on exergoeconomic and environmental analysis

A comprehensive energy, exergy, exergoeconomic and environmental comparison between carbon dioxide, ethane and nitrous oxide as the refrigerants of a two-stage ejector-expansion transcritical refrigeration cycle is carried out. All of the obtained results are attained by optimizing COP subject to gas cooler and intercooler pressures in different values of gas cooler and evaporator temperatures. It is observed that the compressors operating pressure and temperature levels in the cycle for ethane are lower than other refrigerants, which leads to higher system safety and lifetime. Furthermore, the highest COP and exergy efficiency in a wide range of gas cooler temperature belongs to the ethane. The nitrous oxide refrigerant has the lowest product unit cost, which is about 4.2% lower than that of the ethane refrigerant with the highest product unit cost. Therefore, ethane is the most preferable refrigerant from energy and exergy aspects and nitrous oxide is suitable based on exergoeconomic viewpoint.     

Numerical simulation and experimental validation of vapour compression refrigeration systems. Special emphasis on CO2 trans-critical cycles

A numerical and experimental comparative study of a carbon dioxide trans-critical refrigerating system and a conventional sub-critical refrigerating cycle is presented. Attention is focussed not only on the whole refrigeration cycle, but also on the behaviour of the hermetic reciprocating compressors used in these systems. The comparative cases presented have been specially designed for small cooling capacity units with an evaporation temperature around 0 °C. A detailed numerical simulation model for hermetic reciprocating compressors performance, widely validated under conventional fluid refrigerants, has been extended to numerically obtain the CO₂ compressor prototypes behaviour. Two CO₂ compressor prototypes working with CO₂ have been experimentally tested in a specific unit, specially designed and built to analyse high-pressure single stage vapour compression trans-critical refrigerating equipments. This set-up has allowed validating a detailed numerical simulation code for the thermal and fluid-dynamic behaviour of single stage vapour compression refrigeration system working with CO₂ as fluid refrigerant. The numerical results and the experimental data obtained to validate compressors, heat exchangers and whole cycle behaviour have shown a really good agreement. Finally, the numerical and experimental comparison between the carbon dioxide system and the sub-critical conventional cycle has shown the possibility of CO₂ as fluid refrigerant under the studied working conditions.     

Literature Survey on Thermophysical Properties of Refrigerants

A bibliographic compilation is given on thermophysical properties of the environmentally acceptable hydrofluorocarbon blends R404A, R407C, and R410A. These refrigerant blends are still under investigation and meant to replace the transitional hydrochlorofluorocarbon R22 and the azeotrope R502. In a second part reliable formulations to calculate thermophysical-property surfaces of some selected well investigated fluids used in refrigeration are recommended. The fluids water, air, carbon dioxide, ammonia, R134a, R123, and R152a are subjects of that part.     

Efficiency of the indicated process of CO2-compressors

The compressor of a refrigerant compression process is the component with the major influence on the efficiency and reliability of the entire system. Due to the fluid properties of carbon dioxide (CO₂), the pressure ratio of the refrigeration process with CO₂ as the working fluid is, in relation to common refrigeration processes, rather low while the pressure difference is extremely high. From experimental and theoretical considerations it becomes obvious that at these conditions a high volumetric and energetic efficiency of the compressor may be achieved if its design is appropriate. In this paper, the effects on the efficiency of the indicated compression process of a CO₂-compressor are discussed and evaluated and a promising design concept for an efficient CO₂-compressor is derived.