Comparison of the heating performance of air-source heat pumps using various types of refrigerant injection

The performance degradation of air-source heat pumps cannot be avoided when they operate at both very low and high ambient temperatures. The refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures. This study measured the heating performance of air-source heat pumps in which novel vapor injection techniques of a combined flash tank and sub-cooler (FTSC) cycle and a double expansion sub-cooler (DESC) cycle were applied. The performance of these cycles was compared with that of a flash tank (FT) and a sub-cooler (SC) cycle. The average heating capacities of the FT, FTSC, and DESC cycles were higher by 14.4%, 6.0%, and 3.8%, respectively, relative to that of the SC cycle, but the average COPs for the respective cycle options were very similar.     

Performance analysis of a novel hybrid vapor injection cycle with subcooler and flash tank for air-source heat pumps

In this paper, a novel hybrid vapor injection cycle (HVIC) with subcooler and flash tank for air-source heat pumps is proposed. In the HVIC, an ejector is applied to realize the advantages of both the subcooler and flash tank vapor injection, which can efficiently reduce the irreversible thermodynamic loss and improve the system performance, especially at low ambient temperature conditions. The performance enhancement potential of HVIC is compared with conventional subcooler vapor injection cycle (SVIC) and flash tank vapor injection cycle (FTVC) using cycle simulations. The simulation results indicated that for the HVIC using R290 as refrigerant, the coefficient of performance (COP) and the volumetric heating capacity can be increased by 2.8–3.3% and 6.4–8.8% compared to the SVIC system, 1.1–2.0% and 3.2–6.0% compared to the FTVC system, respectively. In addition, improving the ejector efficiencies and adjusting the injection pressure could also enhance the performance of HVIC. Exergy analysis indicates that the total exergy destruction for the HVIC is lower than that of SVIC and FTVC due to the application of an ejector, and therefore resulting in higher exergy efficiency. Overall, the HVIC could be more effective to enhance heating capacity and system efficiency.     

Transient and steady-state experimental investigation of flash tank vapor injection heat pump cycle control strategy

Recent research on vapor injection technique has been mostly focused on performance improvement using different system configurations. The flash tank cycle typically shows better performance than the internal heat exchanger cycle. However, the flash tank cycle control strategy is not yet clearly defined. In this study, a novel cycle control strategy is proposed for an R-410A vapor injection flash tank heat pump system and its feasibility was experimentally investigated. The proposed novel cycle control strategy utilized an electronic expansion valve (EEV) for the upper-stage expansion and a thermostatic expansion valve for the lower-stage expansion, and applied an electric heater in the vapor injection line to introduce superheat to the injected vapor by providing a control signal to the upper-stage EEV. Both transient and steady-state system behaviors were studied. The proposed cycle control strategy was found to be able to provide reliable control to the system.     

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.     

Analysis and optimisation of different two-stage transcritical carbon dioxide cycles for heating applications

Increased interest in the environmental impact of refrigeration technology is leading toward design solutions aimed at improving the energy efficiency and use of eco-friendly refrigerants with low GWP. The aim of this paper is to theoretically analyse R744 air to water heat pump cycles for heating applications up to 80 °C. This work studies the following cycle configurations: two-stage with injection (with and without intermediated cooling between compressors) and a single-stage circuit coupled with an auxiliary circuit. Internal heat transfer among the different streams of refrigerant is included, and the cycles have been optimised with regards to COP in terms of the intermediate conditions and gas cooler pressure. Finally, these cycles have been compared and analysed among each other and with a subcritical injection cycle working with R134a and a single-stage R744 cycle. The improved cycle with R744 can represent a global improvement of 15% in terms of COP.     

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.     

Transient modeling of a flash tank vapor injection heat pump system – Part I: Model development

This two-part article explores the dynamic behavior of a flash tank vapor injection heat pump system from a numerical simulation perspective. Part I provides a first-principles model describing the transient heat transfer and flow phenomena of the system with detailed modeling techniques for each component. The vapor injection scroll compressor is analyzed with the internal heat transfer between the refrigerant and metallic parts taken into account. Lumped-parameter models are developed for the flash tank and expansion devices. Heat exchangers are modeled using a finite volume approach and accounting for the complex tube circuitry. The separated flow model without interfacial exchange is utilized for two-phase flows in order to incorporate an appropriate void fraction model so that a more accurate prediction for refrigerant mass distribution can be achieved. The modular nature of the component models allows flexibility in the system configuration. Transient simulations are carried out for start-up and shut-down operations. A detailed comparison of model predictions against experimental data is presented in the companion paper.     

Control of flash gas bypass MAC system with emphasis on start-ups and transients

Most of the previous research on flash gas bypass (FGB) focused on performance improvement in steady state and demonstrated that compared to direct expansion mode (DX), FGB mode have better performance. However, the control strategy of flash gas bypass system and dynamic behavior during start-ups and transients were not yet clearly defined and investigated. In this paper, a novel control strategy has been proposed for an automobile air conditioning system operating in flash gas bypass mode with R134a as the refrigerant. The proposed control strategy utilized an electronic expansion valve (EV) for the control of subcooling from condenser outlet and a bypass valve (BV) for superheat from compressor inlet. Both start-up and transient system behaviors were studied. The experimental results showed that the proposed cycle control strategy was found to be able to provide reliable control to the system. In addition, proper sizing of bypass valve and flash gas bypass tank have also been studied.     

Optimum cycle control of a two-stage injection heat pump with a double expansion sub-cooler

The refrigerant injection technique has rapidly developed in recent years due to its outstanding performance at low ambient temperatures, and various control methods for a heat pump with injection have been presented. However, most studies on cycle control have been theoretical, and practical control methods for cycle optimization based on experimental results hardly have been presented. In this study, an optimum cycle control method was proposed for a refrigerant injection heat pump with a double expansion sub-cooler based on the intermediate pressure and the injection ratio. The optimum sub-cooler pressure ratio was proposed from 0.4 to 0.7 in view of the heating capacity, and from 0.7 to 0.8 in view of the COP. The optimum injection ratio increased from 0.1 to 0.3 with an increase in the compression ratio.     

滚动转子式补气压缩机在热泵系统中的实验研究

介绍了滚动转子式补气压缩机的设计,并将其在热泵系统中进行了实验研究。分析了不同制热工况下滚动转子式补气压缩机的性能,对比了带闪发器与过冷器的经济器热泵系统、滚动转子式与涡旋式补气压缩机的性能。结果表明:随着室外环境温度的下降,滚动转子式补气压缩机补气后制热量提升比例逐步增大;滚动转子式补气压缩机制热实验中,带闪发器系统的制热量较高;在超低温制热工况下滚动转子式补气压缩机制热量提升18%左右,与涡旋式补气压缩机相比制热量相当,性能略高。     

带闪蒸型经济器的风冷螺杆热泵机组性能的实验研究

研究了带闪蒸型经济器风冷螺杆热泵机组对制热性能的影响,理论分析了补气压力的变化对机组制热性能的影响。研究表明,带闪蒸型经济器热泵螺杆机组制热量随经济器的补气压力的减小而增大,压缩机功率也随经济器的补气压力的减小而增大,压缩机的COP随着经济器的补气压力的升高先升高再降低,存在最佳效率的补气压力点。实验测试了比最佳补气压力点偏高的经济器补气压力对机组性能的影响,理论计算结果和实测数据吻合良好。     

On-field measurement method of vapor injection heat pump system

This paper examines air-to-air injection heat pumps (HPs) and proposes a method to determine both heating capacity and coefficient of performance (COP) in situ, allowing analyses of already-installed setups and long-term observation. Due to the uncertain nature of air enthalpy measurements, this method instead uses refrigerant enthalpies calculated from pipe contact temperatures and steady-state energy balances to determine the COP. However, these energy balances are highly dependent on the hypothesis that the working fluid remains strictly monophasic in certain locations, which is not always true in practice. A parametric variable analysis is conducted on the refrigerant vapor quality to calculate the deviation in the COP due to diphasic conditions at the compressor inlet, injection port, and condenser and flash tank outlets. A final in situ COP uncertainty is presented, due to both the measurement and vapor quality uncertainties.     

Design and simulation of a heat pump for simultaneous heating and cooling using HFC or CO2 as a working fluid

This article presents a Heat Pump for Simultaneous heating and cooling (HPS) designed for hotels, luxury dwellings and smaller office buildings. The main advantage of the HPS is to carry out simultaneously space heating and space cooling in a dual mode. The ambient air is used as a balancing source to run a heating or a cooling mode. The HPS also participates to domestic hot water preparation all year round. The second advantage is that, during winter, some energy recovered by subcooling of the refrigerant is stored at first in a cold water tank that is not used for cooling. This energy is used subsequently as a cold source at the water evaporator in order to improve the average coefficient of performance and to run a defrosting sequence at the air evaporator. Two refrigerants are studied: HFC R407C and carbon dioxide. HFCs provide good performance, but new restrictive regulations on F-gases lead us to study low-GWP refrigerants as well. Highly efficient models of compressors and heat exchangers have been defined. Annual simulations show that CO₂ is a refrigerant which adapts rather well to the operation of the HPS thanks to the higher amount of energy available by subcooling and the large temperature glide at heat rejection used for DHW production.     

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...     

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.     

Performance improvement of air source heat pump using gas-injected rotary compressor through port on blade

Gas injection has been a crucial technology to avoid the serious degradation of air source heat pumps in low ambient temperature. A novel injection structure on the blade for rotary compressors has been put forward in previous research to overcome the drawback of traditional injection structures. Based on a verified numerical model, the thermodynamic performance of an air source heat pump with the new gas-injected rotary compressor is investigated. The results indicate that, compared to the air source heat pump with the regular single-stage rotary compressor, the proposed injection structure can enhance heating capacity and COP of the air source heat pump by 23.1–28.2% and 4.5–8.1%, respectively.     

Scroll compressors using gas and liquid injection: experimental analysis and modelling

The first part of this paper presents an experimental analysis of different hermetic scroll compressors using different methods of injection: the first one is without injection, the second one uses vapor injection and the third one liquid injection. The analysis reveals the influence of these methods on the compressor behavior. A simplified model of the scroll compressor is then proposed that takes into account not only the different internal processes but also the refrigerant injection. It assumes that the refrigerant mass flow rate is affected by a heating-up due to a uniform wall temperature. This fictitious wall is supposed to gain heat from the electromechanical losses and from the discharged gas and to loose heat to the suction and to the ambient. The compression step is considered isentropic up to the adapted pressure and then at constant volume until the discharge pressure. The model is able to compute variables of first importance like the mass flow rate, the electric power and the discharge temperature, as well as secondary variables as suction heating-up, discharge cooling-down, and ambient losses. The validation based on 45 experimental results shows excellent results.     

Analysis of a hybrid compression—absorption cycle using lithium bromide and water as the working fluid

Hybrid refrigeration cycles which combine a mechanical compressor and an absorption cycle in such a way that they share a single evaporator were analysed. The motivation for the investigation of hybrid cycles was the need to more efficiently utilize the output of an internal combustion engine. The hybrid cycles make efficient use of both the work and the heat output of an engine. Performance calculations are reported for a promising cycle which utilizes LiBr-H₂O as the working fluid. For this working fluid, the refrigerant is water. Owing to the potential sensitivity of the absorption cycle components to oil contamination, the cycle was analysed assuming an oil-free steam compressor (screw design). Although oil-free steam compressors are available, they are used only sparingly in the industry. The capital cost for such a compressor is very high and the isentropic efficiency of the available units is low. This combination of high cost and low performance results in poor economics for the hybrid cycle based on the available technology. However, the cycle has significant potential from a thermodynamic viewpoint and it provides an incentive for compressor manufacturers to refine the oil-free steam compression technology.     

外绕微通道冷凝器空气源热泵热水器仿真与优化

空气源热泵热水器比燃气或电热水器更为节能。本文提出了一种外绕微通道冷凝器,可以减少制冷剂充灌量、提高换热效率、降低成本、提高安全性。建立了热泵热水器的准稳态系统模型,制冷剂侧采用稳态模型,水箱和水侧采用动态模型。通过实验证明了该系统模型可以准确地预测时变的系统功耗、水温,以及系统时均COP。通过仿真分析,发现水箱隔热层可以缩短水的加热时间、并提高系统COP约9.2%。增大冷凝器也可提升系统COP,但几乎不改变水的加热时间。最后提出了微通道冷凝器的三流程优化设计方法。     

二氧化碳制冷和热泵循环

二氧化碳制冷剂由于它的制热性能系数高和对大气的全球气候变暖潜能值小,现在获得了愈来愈多的应用,尤其是在热泵热水器和汽车空调领域。本文论述二氧化碳制冷剂的各种制冷和热泵循环。