Thermodynamic analysis of different two-stage transcritical carbon dioxide cycles

The aim of this paper is the thermodynamic evaluation and optimisation of different two-stage transcritical carbon dioxide cycles. Five different cycles are studied: basic single-stage cycle, single-throttling with two-stage compression cycle, split cycle, phase separation cycle and single-stage cycle coupled with a gas cooling circuit. Each basic cycle is analysed for the effect of internal heat transfer between different streams of refrigerants. In the case of two-stage compression, intermediate cooling between the compressor stages is present. An analysis on the Plank cycle for intermediate pressure higher than critical one is performed. Each cycle is optimised with regards to energy performance, calculating the optimal values of both the upper and the intermediate pressures. In the case of split cycle, the ratio of the mass flow rate in the main stream to the one in the auxiliary stream is also optimised.     

Two-stage transcritical carbon dioxide cycle optimisation: A theoretical and experimental analysis

The aim of this paper is to investigate, both experimentally and theoretically, the potential of improving the cycle efficiency through two stage compression with intermediate cooling, at operating conditions typical of air conditioning. The experimental set-up consists of two closed loop air circuits acting as heat sink and heat source for gas-cooler and evaporator, respectively. The tested refrigerating circuit includes two tube-and-fin heat exchangers as gas-cooler and evaporator, a back-pressure valve as throttling device and a double-stage semi-hermetic compound, two-piston, reciprocating compressor equipped with oil separator and intercooler. A full set of thermocouples, pressure transducers and flow-meters allows measurement of all the main parameters of the CO₂ cycle, enabling to perform heat balance both air and refrigerant side. Tests were run at fixed evaporation pressure, evaporator outlet superheating and gas-cooler outlet temperature, varying the gas-cooler outlet pressure in the range 8–11 MPa. The optimal gas-cooler pressure for this application as well as the effect of the intercooler efficiency on the cycle performance were investigated.

A FORTRAN code for the simulation of an improved two-stage cycle was validated against the experimental results; a theoretical analysis performed with this code is proposed for optimisation and energy performance evaluation of such a cycle.     

Natural refrigerant-based subcritical and transcritical cycles for high temperature heating

Theoretical analyses of subcritical/transcritical heat pumps using four natural refrigerants, carbon dioxide, ammonia, propane and isobutane have been carried out for high temperature heating applications at different heating outlet temperatures and heat sources using computer models. The compressor discharge pressures have been optimized for transcritical and subcritical (with near critical operation of condenser) cycles. Results show that for subcritical heat pumps, use of subcooling is efficient for heating applications with a gliding temperature. Results also show that although propane yields better coefficient of performance (COP) in low temperature heating applications, ammonia performs the best in high temperature heating applications. Finally, design aspects of major components of all the four heat pumps for high temperature heating have been discussed, particularly with reference to suitability of available lubricants to the newly evolved operating conditions.     

跨临界二氧化碳汽车空调系统的研究与分析

随温室效应日益加剧,二氧化碳制冷剂重新回到人们的视野,引起广泛关注。介绍了跨临界二氧化碳汽车空调系统的组成及热力循环模型,并阐述了系统主要部件压缩机、气体冷却器、蒸发器、节流机构工作性能的影响因素和研究发展方向。     

Historical and present developments of ejector refrigeration systems with emphasis on transcritical carbon dioxide air-conditioning applications

This paper gives an overview of historical and present developments on how ejectors can be utilized to improve the performance of air-conditioning and refrigeration systems. Research on ejector refrigeration cycles that utilize low-grade energy sources to produce cooling is summarized. Another major class of ejector refrigeration cycles that is described tries to recover expansion work by means of a two-phase ejector. This particular approach appears to be very promising for transcritical carbon dioxide (CO₂, R744) systems with inherently large throttling losses. The paper further presents the latest analytical and experimental results of a comprehensive study carried out to investigate possible performance improvements of transcritical R744 two-phase ejector systems. Relevant operational parameters were varied and effects on performance resulting from different ejector geometries were studied as well. Two-phase mixing shock waves inside the ejector were detected by recoding static wall pressure distributions.     

跨临界二氧化碳汽车空调特性分析

本文建立跨临界循环汽车空调装置的数学模型,通过样机实验数据对模型进行检验,用此模型分析了跨临界循环汽车空调装置的变工况特性,以及吸气回热结构对于装置工作性能的影响。本文的研究结果可用于指导跨临界循环汽车空调装置的设计改进。     

A study of transcritical carbon dioxide flow through adiabatic capillary tubes

This paper advances a study of the transcritical expansion of carbon dioxide (R-744, CO₂) through adiabatic capillary tubes. The influence of both operating conditions (inlet and exit pressures, inlet temperature) and tube geometry (capillary diameter and tube length) on the CO₂ mass flow rate was experimentally evaluated using a purpose-built testing facility with a strict control of the measured variables. A dimensionless correlation to predict the refrigerant mass flow rate as a function of tube geometry and operating conditions was developed. In addition, a theoretical model was put forward based on the mass, energy and momentum conservation principles. The model results were compared with experimental data, when it was found that the model predicts 95% of the measured refrigerant mass flow rate within an error band of ±10%. The model was also employed to advance the knowledge about the transcritical carbon dioxide flow through adiabatic capillary tubes.     

Algebraic solution of transcritical carbon dioxide flow through adiabatic capillary tubes

This paper outlines an algebraic model for simulating the transcritical expansion of carbon dioxide through adiabatic capillary tubes. The model was put forward based on the analytical solution of the momentum conservation equation assuming an isenthalpic expansion process. The theoretical model predictions were compared with 66 experimental data points covering different operating conditions and tube geometries. A good agreement between the experimental and calculated mass flow rates was achieved, with more than 94% of the data points lying within an error band of ±10%.     

A study of transcritical carbon dioxide flow through diabatic capillary tubes

An experimental and theoretical study of the diabatic flow of carbon dioxide through lateral capillary tube suction line heat exchangers is outlined. The influence of both operating conditions (capillary tube inlet and outlet pressures, capillary tube inlet temperature and suction line inlet temperature) and tube geometry (heat exchanger length and position, suction line diameter and capillary tube length) on the heat and mass flow rates was experimentally evaluated using a purpose-built testing facility. In total, 75 tests were carried out with heat fluxes spanning from 1 to 11 kW m⁻² and refrigerant mass flow rates ranging from 12 to 26 kg h⁻¹. In addition, the mathematical model of Hermes et al. (2008) was adapted to run with carbon dioxide as working fluid. The model was validated against experimental data, and a good agreement between the experimental and calculated mass flow rates was achieved with 85% and 98% of the data points being within ±5% and ±10% error bounds, respectively.     

Assessment on the use of common correlations to predict the mass-flow rate of carbon dioxide through capillary tubes in transcritical cycles

Capillary tubes are extensively used in small refrigeration and air-conditioning systems with synthetic refrigerants and hydrocarbons. For CO₂ transcritical applications, it has been shown that the capillary tube demonstrates an intrinsic capability of adjusting the upper pressure close to the optimal value in response to changes of gas-cooler heat sink temperature. The CO₂ flow rate through four capillary tubes of various lengths, diameters and materials was measured in a test rig. Each capillary tube was tested with inlet pressure varying from 7.5 MPa to 11 MPa and inlet temperature from 20 °C to 40 °C. Outlet pressure varied from 1.5 MPa to 3 MPa. The experimental results were validated against different numerical and approximate analytical solutions of the capillary tube equations. These models give good predictions only if the friction factor of the capillary tube is calculated accounting for its dependence on the tube roughness.     

跨临界CO2制冷系统采用透平膨胀机的可行性分析

对CO2制冷系统中应用膨胀机的研究现状进行了回顾,在此基础上对跨临界CO2制冷循环中采用透平膨胀机的可行性进行了分析.研究表明透平膨胀机完全适用于家用及商用的跨临界CO2制冷/热泵系统,并通过计算实例给出了采用不同方案和效率的透平膨胀机替代节流阀对CO2制冷系统性能的提升效果.     

Optimization of a transcritical CO2 heat pump cycle for simultaneous cooling and heating applications

Energetic and exergetic analyses as well as optimization studies of a transcritical carbon dioxide heat pump system are presented in this article. A computer code has been developed to obtain sub-critical and super-critical thermodynamic and transport properties of carbon dioxide. Estimates for irreversibilities of individual components of the system lead to possible measures for performance improvement. The COP trends indicate that such a system is more suitable for high heating end temperatures and modest cold end temperatures. Expressions for optimum cycle parameters have been developed and these correlations offer useful guidelines for optimal system design and for selecting appropriate operating conditions.     

跨临界二氧化碳热泵热水系统气冷器的仿真分析

建立了跨临界二氧化碳热泵热水系统中的气体冷却器模型，对管内二氧化碳和水侧的流动和传热进行了数值仿真；并运用该模型分析了系统运行时各有关参数对换热器性能的影响，并结合最优排气压力，研究使系统高效运行的方法，为气冷器的优化设计提供了基础。     

Simulation of a transcritical CO2 heat pump cycle for simultaneous cooling and heating applications

A steady state simulation model has been developed to evaluate the system performance of a transcritical carbon dioxide heat pump for simultaneous heating and cooling. The simulated results are found to be in reasonable agreement with experimental results reported in the literature. Such a system is suitable, for example, in dairy plants where simultaneous cooling at 4 °C and heating at 73 °C are required. The optimal COP was found to be a function of the compressor speed, the coolant inlet temperature to the evaporator and inlet temperature of the fluid to be heated in the gas cooler and compressor discharge pressure. An optimizing study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on the heat exchanger area has been carried out. Effect of heat transfer in the heat exchangers on system performance has been presented as well. Finally, a novel nomogram has been developed and it is expected to offer useful guidelines for system design and its optimisation.     

3种跨临界二氧化碳热泵热水器系统的实验研究

研制了跨临界二氧化碳热泵热水器实验台,在一定蒸发温度下,分别对常规系统、带回热器系统及带喷射器和回热器系统这3种跨临界二氧化碳热泵热水器系统在制取不同温度热水时的性能进行了实验研究,分析了制热系数、制热量、压缩机耗功量、压缩机压比等参数随热水出水温度的变化趋势,并分析了回热器、喷射器的工作性能,实验结果表明:和常规系统相比,引入回热器后系统制热系数可以提高约10％,引入喷射器和回热器后系统制热系数可以提高约20％.     

A perspective on the optimisation of hard carbon and related coatings for engineering applications

Hard carbon coatings hold the key to improved performance for many types of products. However the achievement of these improvements requires the selection of the appropriate type of carbon coating and therefore the correct process and appropriate deposition parameters. The huge range of properties achievable in carbon coatings is mainly due to the ability of carbon to form different types of interatomic bonds, to take up different sites, and to adopt different structures. In addition to intrinsic material properties, other factors must also be considered for each application, such as the adhesion level achievable and coating cost. This complex situation explains why the number of applications for hard carbon films is still more limited than originally expected. Despite the considerable progress achieved during the last decade in hard coating technologies, practical results often appear conflicting, with differences in properties occurring even within the same types of coatings. Furthermore, the many different deposition systems and processes which have been developed introduce further complications in regard to (for example) achievable coating uniformity and deposition rates. Thus, there is often confusion in the use of certain fundamental principles, especially regarding the growth mechanisms and the effects which produce more dense homogeneous and stable coating materials. This is especially true for the improved properties of tetrahedral amorphous carbon films, which are different from previously reported diamond-like carbon materials, and can be created by adapting and improving existing industrial processes, to offer advantages compared to earlier coatings, and hence possibilities for important new applications. This paper discusses issues relating to intrinsic material properties, and practical aspects such as adhesion, to provide a framework for the development, selection and use of hard carbon coatings in practical situations.     

Graphene and carbon black filled conductive nanocomposite films for heating element applications

Graphene has ultra-high electrical and thermal conductivity, which makes graphene as the most encouraging fillers for thermally conductive composites. Graphene and/or carbon black filled conductive polymer composite (CPC) films used as heating element are smarter than the traditional heating elements due to less environmental pollution, ease of application on many surfaces and possess the merits of lightweight. In this study, we investigated mainly the production, characterization and industrial application of graphene/carbon black reinforced styrene acrylic copolymer emulsion matrix composite films deposited on polyvinyl chloride for flexible heating element. After that, the films were dried at room temperature for 24 h in air. Structural and surface properties of the CPC films were characterized by X-ray diffraction and scanning electron microscopy. Temperature, time and voltage relation of the produced composite films were investigated. Heating and electrical properties of the CPC films were determined by using a thermal camera and 4-point probe measurement system, respectively. The electrical resistivity of the CPC films decreases from ~?10⁸ to 10¹ Ω cm with increasing the filler content or using a combination of two fillers. Graphene and carbon black filled conductive polymer composites to be considered as candidates for flexible heating element applications exhibited good electrical and heating properties thanks to synergistic effect of fillers.     

Transcritical carbon dioxide small commercial cooling applications analysis

This paper presents a project to develop an R744 commercial single door bottle cooler that is cost competitive and matches the performance of typical cost optimised R404A and R134a systems. Compressors with different displacement and efficiency values are evaluated for refrigerating systems with fin and tube and steel wire-on-tube gas coolers. Capillary tubes are tested. A methodology to properly sizing them and to optimize the combination of capillary tube and refrigerant charge is developed. The problem of optimal cycle high pressure is addressed and Liao’s approximated solution questioned. Tests demonstrate that the CO₂ energy consumption systems are higher than traditional ones especially at ambient temperatures above 25 °C. Carbon dioxide appears to be a feasible option for stand-alone refrigerating equipment in terms of total equivalent warming impact (TEWI) compared to HFC refrigerants with actual single stage R744 compressor technology, only if the refrigeration units operate at medium-low gas cooler inlet temperature.     

Selection of heating cycles for thermoluminescent detectors

Conclusions Of the two methods of detector heating considered in this study, the first method gives a higher reproducibility for than the second. The amplitude reading method and the second of the detector heating methods considered above are completely incompatible.The reproducibility of in the course of the first method of heating improves as the heat capacity of the detector C₂ decreases (assuming that C₁=const).For a given construction of HE and detector there exists an optimal heating rate, at which the highest degree of reproducibility is attained.Translated from Izmeritel'naya Tekhnika, No. 9, pp. 74–75, September, 1971.