Performance limit for economized cycles with continuous refrigerant injection

While economizing can improve the performance of vapor compression cycles, the cost of multi-stage compressors has limited its implementation to large-scale applications. However, the development of compressors with injection ports presents new opportunities for economizing in smaller-scale applications. In addition to eliminating the need for a costly multi-stage compressor, multiple injection ports can be added at relatively low cost to further improve cycle performance. In the current study, a model was developed to study the effect of the number of injection ports on performance and determine the limit to performance with injection. The model predicts that continuous injection, in which economized refrigerant is injected at an infinite number of ports to maintain saturated vapor in the compressor, provides very significant performance improvements for air-conditioning and refrigeration applications. At standard operating conditions, the COP increases between 18% and 51% depending on the application, with higher temperature lift cycles benefiting most significantly.     

Modeling of a novel spool compressor with multiple vapor refrigerant injection ports

A model of a novel rotary spool compressor has been developed to explore the effect of multiple injection ports on compressor and cycle performance. The thermodynamic model includes the effects of heat transfer and leakage and is numerically solved to predict the compressor power consumption and mass flow rate. Saturated vapor injection is modeled assuming that the injection pressures and the timing of the injection process can be controlled.The model predicts that adding a single injection port will provide a 12% increase in the cycle coefficient of performance (COP) when the compressor runs at 1907 rpm with R-22 evaporating at ?7 °C, condensing at 49 °C, and 15 °C of superheat. Adding a second, non-optimized injection port increases the COP by 16% compared to the cycle without injection. The model is used to investigate the effect of injection pressure, port location, and port diameter on cycle performance.     

Experimental analysis of a novel cooling system driven by liquid refrigerant pump and vapor compressor

This study introduced a novel energy saving cooling system, i.e. a combined cycle coupled with a traditional vapor compression cycle with a pumped liquid two-phase cooling cycle. The system has two operation modes, i.e. the compression cycle mode driven by compressor and the pump cycle mode driven by refrigerant pump. A multi-purpose test bench was constructed to experimentally evaluate the performance of the integrated cycle system under various operation conditions. The effects of cycle working condition and the shift temperature between the two operation modes on the overall cycle performance were investigated in detail. It is found that the novel cycle system has a higher EER compared to the traditional compressor system when the ambient temperature is relatively low. The further experimental results and comparative annual energy saving analysis also indicated that the proper shift temperature is about −5 °C from the system EER and cooling capacity point of view.     

Effects of vapor injection techniques on the heating performance of a CO2 heat pump at low ambient temperatures

The objective of this study is to investigate the effects of vapor injection techniques on the heating performance of a CO₂ heat pump. The performances of the flash tank vapor injection (FTVI), sub-cooler vapor injection (SCVI) and FTVI with a suction line heat exchanger (FTSX) cycles were measured and analyzed with variations of the outdoor temperature, compressor frequency, and injection mass flow rate. At the outdoor temperature of −15 °C and compressor frequency of 55 Hz, the heating capacity and COP of the optimized SCVI cycle were 12.1% and 12.7% higher than those of the optimized FTVI cycle, respectively, because the total mass flow rate in the SCVI cycle was higher than that in the FTVI cycle by the large temperature and pressure differences in the sub-cooler of the SCVI cycle. In addition, the optimum injection flow rate ratios in the vapor injection CO₂ cycles yielding the maximum COP were determined at various compressor frequencies.     

Experimental testing of an oil-flooded hermetic scroll compressor

In this work, a residential air conditioning compressor designed for vapor injection has been modified in order to inject large quantities of oil into the working chamber in order to approach an isothermal compression process. The compressor was tested with oil injection mass flow fractions of up to 45%. At an evaporating temperature of ?10 °C and condensing temperature of 30 °C, the overall isentropic efficiency was up to 70% at the highest oil injection rate. Overall, over the testing envelope investigated, there are no significantly negative effects experienced for the compressor and the compressor isentropic efficiency and refrigerant mass flow rate improve monotonically as the oil injection rate is increased.     

Energy improvements of CO2 transcritical refrigeration cycles using dedicated mechanical subcooling

In this work the possibilities of enhancing the energy performance of CO₂ transcritical refrigeration systems using a dedicated mechanical subcooling cycle are analysed theoretically. Using simplified models of the cycles, the modification of the optimum operating conditions of the CO₂ transcritical cycle by the use of the mechanical subcooling are analysed and discussed. Next, for the optimum conditions, the possibilities of improving the energy performance of the transcritical cycle with the mechanical subcooling are evaluated for three evaporating levels (5, −5 and −30 °C) for environment temperatures from 20 to 35 °C using propane as refrigerant for the subcooling cycle. It has been observed that the cycle combination will allow increasing the COP up to a maximum of 20% and the cooling capacity up to a maximum of 28.8%, being both increments higher at high evaporating levels. Furthermore, the results indicate that this cycle is more convenient for environment temperatures above 25 °C. Finally, the results using different refrigerants for the mechanical subcooling cycle are presented, where no important differences are observed.     

Assessment of R-438A as a retrofit refrigerant for R-22 in direct expansion water chiller

The present work aims to evaluate the performance characteristics of a vapor compression refrigeration system using R-438A as a retrofit refrigerant for R-22. In order to achieve this objective, a test facility is developed and experiments are performed over a wide range of chilled water inlet temperature (11:20 °C), condenser water inlet temperature (25:35 °C) and condenser water mass flow rate (363:543 kg h⁻¹). Results showed that as the chilled water inlet temperature changes from 11.5 to 20.5 °C, system COP increases from 1.78 to 2.07 at constant condenser water inlet temperature of 25.5 °C. Cooling capacity and COP of the system using R-438A are lower than R-22 by 11% and 12.5%, respectively. However, compressor discharge temperature using R-438A is slightly lower than R-22 which confirms that R-438A can be used as a retrofit refrigerant for R-22 to complete the remaining life time of the existing plants.     

Experimental study on the performance of a vapor injection high temperature heat pump

In this study, a vapor injection technique was applied in a high temperature heat pump (HTHP) for providing hot water at temperatures up to 88 °C. A prototype HTHP system with economizer vapor injection was developed and its performance was experimentally investigated under various operating conditions. Results showed that the vapor injection pressure had a large effect on heating capacity and on refrigerant temperature at the inlet of the electronic expansion valve (EEV). As the injection pressure increased from 0.82 to 0.98 MPa, the vapor injection flow ratio increased from 7.3% to 22.61% and the heating capacity increased by 7%. The system COP did not show significant change although the COP trend showed an optimal value for the injection pressure. The refrigerant temperature at the EEV inlet showed a subcooling of more than 16 °C under all studied conditions, which improved the EEV operating reliability.     

Design of experimental bench and internal pressure measurement of scroll compressor with refrigerant injection

Experiments on the inner compression process of scroll compressor with refrigerant injection can reveal the essence of refrigerant injection. The difficulty of the experiment is the design of location of measuring ports, measuring system of dynamic pressure and design of the injection system. Focusing on the dynamic pressure measurement of inner compression process during refrigerant injection, an integrated bench design method for refrigerant injection research in scroll compressor is presented in this paper. The location design of injection ports and measuring ports, frequency spectrum analysis of pressure signal, selection of the sensor type and configuration, and design of the pressure-leading system are expressed, respectively. Finally, a test bench is set up. Based on it, several elementary experiments were carried out. The results show that: this design method solves most problems in the experimental research of scroll compressor with refrigerant injection and works reliably; the refrigerant injection effects the majority of the inner compression process and should not be considered as a transient process; gas injection can increase the system performance greatly and there is an optimal injection pressure for a certain scroll compressor.     

Performance investigation of multi-stage saturation cycle with natural working fluids and low GWP working fluids

Improving the efficiency of a vapor compression cycle and using low GWP working fluids have become more important than ever due to the environmental concerns. A saturation cycle consisting of saturation compression and saturation expansion was proposed in order to improve a vapor compression cycle performance by reducing thermodynamic losses associated with single phase gas compression and isenthalpic expansion. The saturation cycle can be approached by multi-stage cycles with two-phase refrigerant injection. In this paper, the performance of saturation cycle was theoretically investigated for low GWP working fluids including natural fluids under ASHRAE standard operating conditions and extreme heating condition. The simulation results indicate that the benefit of using the multi-stage cycle is higher for the cycle with higher pressure ratio. When the saturation cycle technique (four-stage cycle) is applied, the COP improvements of D2Y60 (mixture of R32 and R1234yf), CO₂ and propane are 46.9%, 43.2% and 38.2%, respectively under extreme heating condition.     

一种变频空调电控箱体降温模块的研究

本文根据变频热泵机组系统特点和制冷循环热传导原理设计了一种变频空调电控箱体的降温模块,该降温模块的U型散热装置与制冷剂配管接触弧度为3/4圆弧结构,接触点切线成45°斜角,放置在空调系统中出储液器到进入电子膨胀阀之间的管路部分,使制冷剂经过该散热装置的温度处于40℃～50℃之间最佳温度范围。在环境温度43℃、出水温度15℃的制冷工况下,通过实验分析和验证了增加该散热装置和自然冷却两种状态下系统的性能。结果表明:加装制冷剂散热装置机组运行时变频器功率器件产生的热量可以通过制冷剂散热装置带走,降温效果较为明显。同时,通过理论计算和实验测试对比了变频器功率发热部件温度与制冷剂进口温度之间温度差变化情况,验证了加装制冷剂散热装置的效果。     

余热驱动的吸收-压缩复叠制冷循环性能及经济性分析

本文提出一种新型吸收-压缩复叠制冷循环,该循环由内燃机余热驱动的高温级复合吸收-压缩制冷循环与动力驱动的低温级CO2亚临界压缩制冷循环复叠构成。对不同制冷工质对在此循环中的性能进行了对比分析;并进一步研究了关键参数对复叠制冷循环性能的影响规律;最后进行了经济性分析。结果表明：该循环的理论性能优于两级复叠制冷循环;在冷凝温度为40 ℃、蒸发温度为-35 ℃条件下,R124-DMAC/R744性能优异,COP可达2.864,是较为理想的工质对,且年总成本较低为15 150.14 美元。     

基于镍钛片的弹热冰箱设计与仿真研究

弹热制冷通过单轴应力驱动弹热工质发生相变而产生制冷作用,由于其具有较高的理论制冷效率且温室效应为零,因此被认为是一种替代传统蒸气压缩式制冷的新型固态相变制冷技术。本研究团队从提升弹热原型机紧凑型角度出发,已研发了两代基于拉伸镍钛丝的弹热冰箱。为进一步提升弹热冰箱的制冷性能,本文使用镍钛片替换镍钛丝作为弹热工质,设计了第三代弹热冰箱,并构建系统仿真模型。通过仿真预测了第三代弹热冰箱的性能,结果表明：相较于前两代弹热冰箱,性能均有提升。在绝热工况下,第三代弹热冰箱将能够实现12℃的制冷温差,在热端散热工况下,弹热冰箱能实现11.4℃的冷端温降以及零温差下0.59 W/g的制冷功率。通过增加拉伸应变、更换潜热更大厚度更薄的弹热工质以及增大电机转速,弹热冰箱在32℃环温条件下能够维持10.7℃的冷藏温度。     

Comparative experimental study of low GWP alternative for R134a in a walk-in cold room

The environmental problems induced by the ongoing increase in the global worming potential (GWP) pose a significant interest among researchers. It was found that, the currently used refrigerants are with high GWP (National Refrigerants Inc., 2004), so that it becomes necessary to search for alternatives to these refrigerants that can properly operate on the same systems but with low GWP. Therefore a walk-in cold room working with vapor compression cycle is constructed and tested in this paper. The performance of R134a refrigerant with high GWP is compared to another low GWP refrigerant R1234ze in a trail to provide a solution of the problem of high GWP of refrigerants currently used in cold rooms. The results obtained in this study have shown that, the cooling capacity of R1234ze was lower than that of R134a by 2% to 13%. The lowest evaporating temperature that could be reached for R1234ze is −13 °C while the lowest temperature of R134a is −30 °C. Regarding the power consumption, R1234ze has lower power consumption than R134a by about 9% to 15% therefore it can be concluded that R 1234ze can be recommended to be used at high and medium evaporating temperature after carrying out the suitable modifications on the refrigeration cycle.     

Thermodynamische Berechnungen für CO<Subscript>2</Subscript>-Kältemaschinenprozesse mit ein- oder zweistufiger Verdichtung in Klimaanlagen

For the design of an (automotive) air-conditioning system which employs R744 (carbon dioxide, CO₂) as a refrigerant, thermodynamic calculations are performed for vapour compression refrigeration processes with single-stage or double-stage compression and throttle-valve expansion. The process parameters are varied systematically for an evaporation temperature of 0?°C and a CO₂-cooler exit-temperature (lowest CO₂ temperature in heat transfer to the ambient) between 15?°C and 55?°C. Their influence upon the coefficient of performance, the specific and the volumetric refrigeration capacities is described. For the double-stage compression cycle, the ratio of the high-pressure and low-pressure mass flows is determined, too. The double-stage compression with interstage cooling and internal heat exchanger in the low- or high-pressure cycle shows the potential for a substantial increase in coefficient of performance relative to the single-stage compression cycle. Thermodynamically optimized intermediate pressure, high pressure and temperature change in the internal heat exchangers are presented for CO₂-cooler exit-temperatures of 25 and 40?°C. The increase in energy efficiency is paid for by additional invest and the more complex operation of the CO₂ refrigeration unit.The results are not only valid for air conditioning systems but can also be applied in the design and operation for other applications of CO₂ refrigeration systems with an evaporation temperature of 0?°C.     

Ejector expansion refrigeration system: Ejector design and performance evaluation

Use of a two-phase flow ejector as an expansion device in vapor compression refrigeration systems is one of the efficient ways to enhance its performance. The present work aims to design a constant-area two phase flow ejector and to evaluate performance characteristics of the ejector expansion refrigeration system working with R134a. In order to achieve these objectives, a simulation program is developed and effects of operating conditions and ejector internal efficiencies on the system performance are investigated using EES software. Comparison between present results and published experimental data revealed that the developed model can predict the system COP with a maximum error of 2.3%. The system COP increased by 87.5% as evaporation temperature changed from −10 °C to 10 °C. Finally, correlations to size ejector main diameters as a function of operating conditions, system cooling capacity and ejector internal efficiencies are reported.     

补气增焓低温多联机制热性能实验研究

低温空气源热泵产品普遍采用中间补气技术解决空气源低温多联机在低温环境中能力衰减严重的问题.本文研究了低温多联机中间补气增焓系统,通过多联机性能实验测试对系统压力、温度等参数进行测试,并对测试数据进行拟合分析,确定了补气增焓技术的控制要素:流经室外换热器的制热主路制冷剂的质量流量与状态是多联机补气增焓控制中的主要因素;中...     

Recent advances in vapor compression cycle technologies

This paper comprehensively reviews the recent studies on advanced vapor compression cycle technologies. These technologies are categorized in three groups: subcooling cycles, expansion loss recovery cycles, and multi-stage cycles. The subcooling cycle research is focused on a suction-line heat exchanger, thermoelectric subcooler and mechanical subcooler. The expansion loss recovery cycles are mainly focused on utilizing an expander and ejector. The multi-stage cycle research includes a vapor or liquid refrigerant injection cycle, two-phase refrigerant injection cycle. All these advanced vapor compression cycle technology options are reviewed, and their effects are discussed. In recent years, the research and development have been made to improve the performance of the VCC. This paper presents the improved cycle options and their comprehensive review. From the review results, several future research needs were suggested.     

A study of working fluids for heat driven ejector refrigeration using lumped parameter models

This paper studies the influence of working fluids over the performance of heat driven ejector refrigeration systems performance by using a lumped parameter model. The model used has been selected after a comparison of different models with a set of experimental data available in the literature. The effect of generator, evaporator and condenser temperature over the entrainment ratio and the COP has been investigated for different working fluids in the typical operating conditions of low grade energy sources. The results show a growth in performance (the entrainment ratio and the COP) with a rise in the generator and evaporator temperature and a decrease in the condenser temperature. The working fluids have a great impact on the ejector performance and each refrigerant has its own range of operating conditions. R134a is found to be suitable for low generator temperature (70–100 °C), whereas the hydrocarbons R600 is suitable for medium generator temperatures (100–130 °C) and R601 for high generator temperatures (130–180 °C).     

Refrigerant injection for heat pumping/air conditioning systems: Literature review and challenges discussions

This paper reviews the major research on refrigerant injection techniques in detail. Liquid and vapor refrigerant injection techniques are discussed and compared. The current research on refrigerant injection techniques falls into two categories: system level research and component level research. The system level research is focused on low ambient temperature heating, heat pump water heating, high ambient temperature cooling, cycle comparison, and control strategy development. Internal heat exchanger and flash tank cycles are the two typical cycles for refrigerant injection. These two cycles are discussed and compared in detail. The component level research is focused on employing different types of compressors, variable speed compressors, the injection process, and the flash tank. Different types of compressors employing refrigerant injection are presented. Based on the literature study, the potential future research directions are presented and discussed. The flash tank cycle control strategy and refrigerant charge management strategy are worth further research efforts. Compressor design can be improved in order to optimize the performance with refrigerant injection. The appropriate design of flash tanks plays a vital role in achieving appropriate two-phase flow patterns in the flash tank. Computational Fluid Dynamics (CFD) modeling can be a useful tool to facilitate the design of the flash tank.