Thermodynamic analyses on an ejector enhanced CO2 transcritical heat pump cycle with vapor-injection

In this paper, an ejector enhanced vapor injection CO₂ transcritical heat pump cycle with sub-cooler (ESCVI) for heating application in cold regions is proposed. The thermodynamic analysis using energetic and exegetic methods is carried out to predict the performance characteristics of the ejector enhanced cycle, and then compared with those of the conventional vapor injection heat pump cycle with sub-cooler (SCVI). The simulation results demonstrate that the ejector enhanced cycle exhibits better performance than the conventional vapor injection cycle under the specified operating conditions. The improvements of the maximum system COP and volumetric heating capacity could reach up to 7.7% and 9.5%, respectively. Exergetic analysis indicates that the largest exergy destruction ratio is generated at the compressor followed by the evaporator and gas cooler. Additionally, the exergy efficiency of the ejector is introduced to quantify the effectiveness of the exergy recovery process, which may be a new criterion to evaluate the performance of the ejector enhanced vapor compression cycle.     

Development of an ejector cooling system with thermal pumping effect

This paper presents a feasibility study of an ejector cooling system (ECS) that utilizes a multi-function generator (MFG) to eliminate the mechanical pump. The MFG serves as both a pump and a vapor generator. The MFG is designed based on the pressure equilibration between high and low pressures through heating and cooling process. In this design, an ECS that contains no moving components and is entirely powered by heat can be practicable. A prototype using refrigerant R141b as working fluid was constructed and tested in the present study. The experimental results showed that the system coefficient of performance (COP_o) was 0.218 and the cooling capacity was 0.786 kW at generating temperature (T_G) 90 °C, condensing temperature (T_C) 32.4 °C and evaporating temperature (T_E) 8.2 °C. While taking into account the extra heat needed for the MFG operation, the total coefficient of performance (COP_t) is 0.185. It is shown that a continuous operation for the generation of cooling effect in an ECS with MFG can be achieved. This cooling machine can be very reliable since there is no moving part.     

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.     

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.     

R32经济器系统涡旋压缩机中间补气参数的分析与优化

为了优化带经济器的R32空气源热泵系统的制热性能,结合涡旋压缩机的结构以及实际运行特点,本文利用MATLAB建立了系统的数学模型,并通过实验数据验证了仿真结果,研究了在不同环境温度下系统的补气压力、准一级压缩内容积比对相对喷气量的影响.研究结果表明:经济器系统较普通热泵系统,更适宜在环境温度低于-10 ℃的工况下运行;...     

Performance evaluation of a vapor injection heat pump system for electric vehicles

Battery-powered electric vehicles (EV) need an efficient electric heating system for extending the driving mileage. An air-source heat pump system offers an economical alternative for EV because it consumes less energy than a heating system using Joule heat and it can use the same components as an air conditioning system for cooling. However, its performance degradation is inevitable at very low ambient temperatures. Although vapor refrigerant injection is known as a good technology to overcome this problem in residential heat pump systems, the number of vapor injection heat pump studies for EV applications is quite limited. In this paper, considering the characteristics of EV application, the configuration of a vapor injection heat pump system for EV is introduced, and it was modeled, using a scroll compressor geometry-based thermodynamic analysis. The performance was estimated for cold ambient regions under the EV operational conditions.     

低温热泵用涡旋压缩机性能的试验研究

制定低温热泵用涡旋压缩机试验方案,对研制的原型机进行性能测试。试验结果表明：在冷凝温度不变的情况下,随着蒸发温度的降低,原型机的制热量有所减少,但减少的速度低于普通热泵系统用涡旋压缩机;压缩机的电功率有所增加,但增加的幅度不大,且压缩机的排气温度也有所降低,故在低温工况下采用准二级压缩热泵用涡旋压缩机比采用普通热泵用涡旋压缩机可以更有效地提高空气源热泵的低温制热性能,是寒冷地区用小型空气源热泵比较适宜采用的压缩机。     

Investigation of the heat pump water heater using economizer vapor injection system and mixture of R22/R600a

This paper presents the experimental study of a heat pump water heater (HPWH) using economizer vapor injection system and mixture of R22/R600a. Performances of HPWH using economizer vapor injection system are compared with that at different mixed mass ratios of R22/R600a. Study demonstrates that the heating capacity and energy efficiency ratio (EER) of the unit increased, and the discharge temperature of compressor decreased when using vapor injection and mixing refrigerant of R22/R600a. It is also found that the HPWH unit with economizer vapor injection system has a better performance for the high outlet water temperature under lower temperature conditions at 15% mass ratio of R600a for the mixing refrigerant. In addition, fundamental and practical influence of vapor injection pressure on the HPWH performance has been investigated experimentally. The simplified model is proposed for predicting the optimal vapor injection pressure of compressor using the mixing refrigerant R22/R600a.     

CO2-heat pump water heater: characteristics, system design and experimental results

CO₂ is one of the few non-toxic and non-flammable working fluids that do not contribute to ozone depletion or global warming, if leaked to the atmosphere. Tap water heating is one promising application for a trans-critical CO₂ process. The temperature glide at heat rejection contributes to a very good temperature adaptation when heating tap water, which inherits a large temperature glide. This, together with efficient compression and good heat transfer characteristics of CO₂, makes it possible to design very efficient systems. A heating-COP of 4.3 is achieved for the prototype when heating tap water from 9°C to 60°C, at an evaporation temperature of 0°C. The results lead to a seasonal performance factor of about 4 for an Oslo climate, using ambient air as heat source. Thus, the primary energy consumption can be reduced with more than 75% compared with electrical or gas fired systems. Another significant advantage of this system, compared with conventional heat pump water heaters, is that hot water with temperatures up to 90°C can be produced without operational difficulties.     

中间补气量对经济器热泵系统性能的影响

对带经济器的热泵系统进行了理论分析,建立了整个系统的数学模型,进行了实验研究,研究分析了低温工况下中间补气量对各系统性能的影响。结果表明：在低温环境下,系统的制热量、压缩机功率都会随中间补气量的增加而增加,排气温度随中间补气量的增加而显著减小,系统COP先增加后减小,根据实验结果可知最优的中间补气量,约占环境温度下系统总质量流量的10-14%。     

Theoretical study of a thermoelectric-assisted vapor compression cycle for air-source heat pump applications

This paper proposes a thermoelectric-assisted vapor compression cycle (TVCC) for applications in air-source heat pump systems which could enhance the heating capacity of the system. Performances of TVCC are calculated and then compared with that of basic vapor compression cycle (BVCC). The simulation results show that when coefficients of performance (COPs) of the two cycles are almost equal, the TVCC under maximum COP condition of the thermoelectric modules still performs better than BVCC by 13.0% in heating capacity through selecting the appropriate intermediate temperature. In addition, the TVCC can also achieve an improvement of 16.4%–21.7% in both the heating COP and capacity when compared with the BVCC with an assistant electric heater that is provided with the equivalent power input of thermoelectric heat exchanger. Thus, the TVCC could be beneficial to the applications in small heat pumps if there is always need for auxiliary electric heat.     

Modelling the performance of a transcritical CO2 heat pump for high temperature heating

A prototype transcritical CO₂ heat pump was constructed for heating water to temperatures greater than 65°C while providing refrigeration at less than 2°C. The heating capacity was 115 kW at an evaporation temperature of +0.3°C and a hot water temperature of 77.5°C, with a heating coefficient of performance (COP) of 3.4. Performance data is presented for each of the compressor, the gas cooler, and the recuperator as well as for the overall heat pump system. Equipment performance data was incorporated into a computer model to enable parametric investigations of heat pump performance. Model predictions showed that the hot water temperature could be increased from 65 to 120°C with a relatively small reduction in heating capacity and heating COP of 33 and 21%, respectively. Model predictions also highlight the potential for significant capacity improvements by eliminating the recuperator in favour of a larger gas cooler.     

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.     

电动客车用余热回收型热泵的换热器优化及性能实验研究

本文针对热泵空调系统在冬季低温工况下制热能力衰减问题,通过换热器设计优化,研发出基于喷射补气的余热回收型热泵空调系统,并进行了性能实验研究.结果表明:研制的准二级压缩电动客车热泵空调系统在低温条件下具有较好的制热性能.在环境温度为-20℃,车内温度为20℃,余热量为1.8 kW的制热工况下,相比于无余热回收工况,系统制...     

四种跨临界CO2压缩式热泵系统的热力性能研究

CO2是具有很大潜力的天然替代工质之一,CO2跨临界循环放热过程中具有较大温度滑移,与水侧温升过程相匹配,因此适合用于热泵热水器系统。国内外学者提出了许多提高跨临界CO2循环效率的方法,其中包括引入回热器、喷射器等设备,从不同角度对比分析在常规跨临界CO2热泵系统中引入回热器、喷射器后系统的性能变化。本文在前人工作的基础上,建立相关热力学计算模型,并进一步对四种不同形式的跨临界CO2热泵系统（常规跨临界CO2热泵系统（TCHS）、带回热器的跨临界CO2热泵系统（TCHSI）、带喷射器的跨临界CO2热泵系统（TCHSE）及带喷射器与回热器的跨临界CO2热泵系统（TCHSEI））的性能进行研究,对比分析排气压力一定的情况下四种循环的热力性能;从最优排气压力的角度出发,分析对比不同系统中气冷器出口温度变化对系统最优排气压力和制热系数的影响,以及喷射器等熵效率对系统性能的影响。以上研究为CO2压缩式热泵系统的实用化进展奠定良好的理论基础。     

A year-round dynamic simulation of a solar-driven ejector refrigeration system with iso-butane as a refrigerant

In this paper, the performance of the solar-driven ejector refrigeration system with iso-butane (R600a) as the refrigerant is studied. The effects that both the operating conditions and the solar collector types have on the system's performance are also examined by dynamic simulation. The TRNSYS and EES simulation tools are used to model and analyze the performance of a solar-driven ejector refrigeration system. The whole system is modelled under the TRNSYS environment, but the model of the ejector refrigeration subsystem is developed in the Engineering Equations Solver (EES) program. A solar fraction of 75% is obtained when using the evacuated tube solar collector. In the very hot environment, the system requires relatively high generator temperature, thus a flat plate solar collector is not economically competitive because the high amount of auxiliary heat needed to boost up the generator temperature. The results from the simulation indicate that an efficient ejector system can only work in a region with decent solar radiation and where a sufficiently low condenser temperature can be kept. The average yearly system thermal ratio (STR) is about 0.22, the COP of the cooling subsystem is about 0.48, and the solar collector efficiency is about 0.47 at T_e 15 °C, T_c 5 °C above the ambient temperature, evacuated collector area 50 m² and hot storage tank volume 2 m³.     

Hybrid vapor compression refrigeration system with an integrated ejector cooling cycle

A refrigeration system was developed which combines a basic vapor compression refrigeration cycle with an ejector cooling cycle. The ejector cooling cycle is driven by the waste heat from the condenser in the vapor compression refrigeration cycle. The additional cooling capacity from the ejector cycle is directly input into the evaporator of the vapor compression refrigeration cycle. The governing equations are derived based on energy and mass conservation in each component including the compressor, ejector, generator, booster and heat exchangers. The system performance is first analyzed for the on-design conditions. The results show that the COP is improved by 9.1% for R22 system. The system is then compared with a basic refrigeration system for variations of five important variables. The system analysis shows that this refrigeration system can effectively improve the COP by the ejector cycle with the refrigerant which has high compressor discharge temperature.     

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。     

多联式热泵驱动热管复合供热装置的实验研究

本文提出多联式热泵驱动热管的新型供热循环方式。该方式的特征是采用空气源热泵直接驱动串联式热管散热器供热。为验证该供热方式可行性,本文基于额定功率为2.5 kW的压缩机和9台热管散热器搭建了实验台,在不同工况下进行测试。测试结果表明：新型供热装置启动速度较快,当室外温度为-10 ℃时,系统运行36 min时,热管散热器表面温度即可达到45 ℃。热管散热器温度均匀性好,除1#、9#散热器存在过冷、过热现象外,其余散热器之间温差约为1 ℃。同时系统传热温差小,并能在除霜工况下正常运行。     

空气源热泵自然冷源过冷制热性能实验研究

冬季我国北方室外环境蕴含大量天然冷源,热力学分析表明热泵工质过冷释放的热量可以在蒸发器的等温吸热过程中获得补偿。为了研究大气自然冷源对热泵制热性能的影响,增设室外过冷器,搭建利用自然冷源过冷的空气源热泵实验装置。实验结果表明：当室外环境温度大于0 ℃,冷凝温度小于45 ℃的条件下,自然冷源过冷对热泵制热量与制热COP影响均较小,系统制热量维持在6.22 ~ 6.70 kW,制热COP维持在3.03,压缩机排气温度维持在103 ℃以下;当室外环境温度小于 -10 ℃,冷凝温度大于50 ℃时,随过冷度的增加,压缩机功率增加、排气温度显著增高,系统制热量呈先缓慢增加后减小趋势,制热COP降至2.3。基于上述研究提出一种空气源热泵过冷融霜新型除霜方式,融霜同时不停止制热。