A correlation of optimal heat rejection pressures in transcritical carbon dioxide cycles

In this work, a cycle simulation model has been developed to optimize the coefficient of performance (COP) of transcritical carbon dioxide air-conditioning cycles. The analysis shows that the COP of the transcritical carbon dioxide cycle varies nonmonotonically with the heat rejection pressure; a maximum COP occurs at an optimal heat rejection pressure. It is further revealed that the values of the optimal heat rejection pressure mainly depend on the outlet temperature of the gas cooler, the evaporation temperature, and the performance of the compressor. Based on the cycle simulations, correlations of the optimal heat rejection pressure in terms of appropriate parameters are obtained for specific conditions. The results are of significance for the design and control of the transcritical carbon dioxide air-conditioning and heat pump systems     

热回收式CO2制冷系统性能研究

对热回收式CO2制冷系统性能COP进行了计算和分析,结果表明:蒸发温度、气体冷却器出口CO2温度、热水加热器入口水温是影响其COP的主要因素;回热器出口过热度对COP的影响较小,对压缩机的排气温度影响较大;随着排气压力的升高,COP是否出现峰值,取决于气体冷却器入口制冷剂的特征温度;在相同工况下,蒸发温度、气体冷却器出口CO2温度、回热器出口过热度对最佳排气压力的影响较小,热水加热器入口水温是影响最佳排气压力的主要因素。     

Simulation of the optimal heat rejection pressure for transcritical CO2 expander cycle

In order to optimize and control transcritical CO₂ refrigeration cycle, a mathematical model was developed to simulate the system performance. The simulation results show that a maximum COP exists at the optimal heat rejection pressure not only for throttle valve cycle but also for expander cycle. Also, the optimal heat rejection pressures of the throttle valve cycle are greater than those of the expander cycle under the same condition. In order to further obtain correlation of the optimal heat rejection pressure for transcritical CO₂ expander cycle, it is necessary to analyze the impact degree of compressor efficiency, expander efficiency, gas cooler outlet temperature and evaporation temperature. Based on the simulation results, the values of the optimal heat rejection pressure for the expander cycle were regressed in terms of gas cooler outlet temperature and evaporation temperature at given compressor efficiency and expander efficiency. Finally, two types of polynomial correlations were obtained. One is cubic form, with an average deviation of less than 0.5% and the other is simplified form, with an average deviation of less than 1%. It is, therefore, convenient to use either correlation to simulate the performance of transcritical CO₂ expander cycle.     

Simulation of the optimal heat rejection pressure for transcritical CO<Subscript>2</Subscript> expander cycle

In order to optimize and control transcritical CO₂ refrigeration cycle, a mathematical model was developed to simulate the system performance. The simulation results show that a maximum COP exists at the optimal heat rejection pressure not only for throttle valve cycle but also for expander cycle. Also, the optimal heat rejection pressures of the throttle valve cycle are greater than those of the expander cycle under the same condition. In order to further obtain correlation of the optimal heat rejection pressure for transcritical CO₂ expander cycle, it is necessary to analyze the impact degree of compressor efficiency, expander efficiency, gas cooler outlet temperature and evaporation temperature. Based on the simulation results, the values of the optimal heat rejection pressure for the expander cycle were regressed in terms of gas cooler outlet temperature and evaporation temperature at given compressor efficiency and expander efficiency. Finally, two types of polynomial correlations were obtained. One is cubic form, with an average deviation of less than 0.5% and the other is simplified form, with an average deviation of less than 1%. It is, therefore, convenient to use either correlation to simulate the performance of transcritical CO₂ expander cycle.     

Exergy analysis of transcritical carbon dioxide refrigeration cycle with an expander

In this paper, a comparative study is performed for the transcritical carbon dioxide refrigeration cycles with a throttling valve and with an expander, based on the first and second laws of thermodynamics. The effects of evaporating temperature and outlet temperature of gas cooler on the optimal heat rejection pressure, the coefficients of performance (COP), the exergy losses, and the exergy efficiencies are investigated. In order to identify the amounts and locations of irreversibility within the two cycles, exergy analysis is employed to study the thermodynamics process in each component. It is found that in the throttling valve cycle, the largest exergy loss occurs in the throttling valve, about 38% of the total cycle irreversibility. In the expander cycle, the irreversibility mainly comes from the gas cooler and the compressor, approximately 38% and 35%, respectively. The COP and exergy efficiency of the expander cycle are on average 33% and 30% higher than those of the throttling valve cycle, respectively. It is also concluded that an optimal heat rejection pressure can be obtained for all the operating conditions to maximize the COP. The analysis results are of significance to provide theoretical basis for optimization design and operation control of the transcritical carbon dioxide cycle with an expander.     

Theoretical performance analysis of heat pump water heaters using carbon dioxide as refrigerant

The aim of this paper is to simulate the performance of an air source heat pump water heater using carbon dioxide (CO₂) as a working fluid. The heat pump water heating system consists of a compressor, a gas cooler, an expansion device and an evaporator. The computer simulation model has been developed by using the heat transfer data and the thermodynamic properties of CO₂. The effects on the heat pump performance by the operating parameters such as the compressor rotational speed, the inlet water temperature at the gas cooler, the inlet air temperature at the evaporator and the mass flow rate ratio of water to refrigerant were presented. For rated capacities of a 4 kW compressor with a 10 kW gas cooler and a 6 kW evaporator, the coefficient of performance is found to be between 2.0 and 3.0. The mass flow rate ratio of water and CO₂ between 1.2 and 2.2 is the most suitable value for generating hot water temperature above 60°C at 15–25°C ambient air temperature. Copyright © 2007 John Wiley & Sons, Ltd.     

A simplified method to evaluate the energy performance of CO2 heat pump units

The prediction of the performances of CO₂ transcritical heat pumps demands accurate calculation methods, where a particular effort is devoted to the gas cooler modelling, as the correlation between high pressure and gas cooler outlet temperature strongly affects the cycle performance. The above-mentioned methods require a large amount of input data and calculation power. As a consequence they are often useless for the full characterisation of heat pumps which are sold on the market.A simplified numerical method for the performance prediction of vapour compression heat pumps working in a transcritical cycle is presented, based only on performance data at the nominal rating conditions. The proposed procedure was validated against experimental data of two different tap water heat pumps. For the considered units, simulation results are in good agreement with the experimental ones. The deviations range from −6.4% to +1.7% and from −3.8% to +5.8% for the COP_H of the air/water heat pump and the water/water heat pump, respectively. The heating capacity deviations stayed within −5.5% and +1.7% range and within −5.0% and +7.9% range for the same units.The proposed mathematical model appears to be a reliable tool to be used by the refrigeration industry or to be implemented into dynamic building-plant energy simulation codes. Finally, it represents a useful instrument for the definition of tailored approximated optimal high pressure curve considering the operating characteristics of the specific CO₂ transcritical unit. It could also be implemented on board of a real unit control system where it could be used as model coupled to computational intelligence algorithms for pressure optimisation.     

Theoretical and experimental studies on optimum heat rejection pressure for a CO2 heat pump system

The experimental and simulation researches have been conducted to investigate the relationships between optimum heat rejection pressure and other related operating parameters for a transcritical CO₂ heat pump system with two throttle valves. It proved that it is relatively reliable to control the heat rejection pressure of the CO₂ system with two expansion valves in series. The experimental results also show similar trends with those from simulation, under widely different operating conditions. Thus both the simulation and experimental results meet here: for a transcritical CO₂ cycle, there exists an optimal heat rejection pressure, under which the system can reach the maximum heating coefficient of performance (COP). Furthermore, the research also reveals that the optimal heat rejection pressure mainly depends on the refrigerant outlet temperature of gas cooler whereas the evaporating temperature and the performance of the given compressor have smaller effect on the optimum heat rejection pressure. Based on the experimental data, a correlation of the optimal heat rejection pressure with respect to mainly involved parameters is obtained for specific conditions.     

CO2跨临界热泵高温化途径分析

CO2作为一种环境友好的自然工质,以其为循环工质的跨临界热泵制热能力突出.建立CO2跨临界增压和CO2跨临界热泵理论分析模型,研究不同增压过程对热泵系统COP、气冷器中水的出口温度及质量流量的影响规律.结果表明,2种热泵高温化方案均会提升压缩机等熵效率、功耗和压缩机出口工质温度,且提升了气冷器出口水温,但COP和热水的...     

Transcritical CO2 heat pump systems: exergy analysis including heat transfer and fluid flow effects

This paper presents the exergetic analysis and optimization of a transcritical carbon dioxide based heat pump cycle for simultaneous heating and cooling applications. A computer model has been developed first to simulate the system at steady state for different operating conditions and then to evaluate the system performance based on COP as well as exergetic efficiency, including component wise irreversibility. The chosen system includes the secondary fluids to supply the heating and cooling services, and the analyses also comprise heat transfer and fluid flow effects in detail. The optimal COP and the exergetic efficiency were found to be functions of compressor speed, ambient temperature and secondary fluid temperature at the inlets to the evaporator and gas cooler and the compressor discharge pressure. An optimization study for the best allocation of the fixed total heat exchanger inventory between the evaporator and the gas cooler based on heat transfer area has been conducted. The exergy flow diagram (Grassmann diagram) shows that all the components except the internal heat exchanger contribute significantly to the irreversibilities of the system. Unlike a conventional system, the expansion device contributes significantly to system irreversibility. Finally, suggestions for various improvement measures with resulting gains have been presented to attain superior system performance through reduced component irreversibilities. This study is expected to offer useful guidelines for system design and its optimisation and help toward energy conservation in heat pump systems based on transcritical CO₂ cycles.     

Study on carbon dioxide gas cooler heat transfer process under supercritical pressures

In carbon dioxide transcritical air‐conditioning and heat pump systems, the high‐pressure‐side heat exchanger operating at supercritical pressures is usually called as gas cooler. The carbon dioxide gas cooler displays much difference from the traditional heat exchangers employing constant property fluids. The commonly used logarithmic mean temperature difference (LMTD) and effectiveness—heat transfer unit (ε‐NTU) fail for the gas cooler design calculation as the carbon dioxide properties change sharply near the critical or pseudo‐critical point in the heat transfer processes. The new effective heat transfer temperature difference expression for variable fluid property derived by the authors is verified by numeric simulation of the carbon dioxide gas cooler. Moreover, the available correlated models for the cooled carbon dioxide supercritical heat transfer are used to simulate the gas cooler. Detail analysis is made for the deviations among the different models, and for the distributions of local convective coefficient, heat flux, and local temperature of carbon dioxide along the flow path in the gas cooler. Copyright © 2002 John Wiley & Sons, Ltd.     

Transcritical CO2 refrigerator and sub‐critical R134a refrigerator: A comparison of the experimental results

This paper describes experiments comparing a commercial available R134a refrigeration plant subjected to a cold store and a prototype R744 (carbon dioxide) system working as a classical ‘split‐systems’ to cool air in residential applications in a transcritical cycle. Both plants are able to develope a refrigeration power equal to 3000 W. The R744 system utilizes aluminium heat exchangers, a semi‐hermetic compressor, a back‐pressure valve and a thermostatic expansion valve. The R134a refrigeration plant operates using a semi‐hermetic reciprocating compressor, an air condenser followed by a liquid receiver, a manifold with two expansion valves, a thermostatic one and a manual one mounted in parallel, and an air cooling evaporator inside the cold store. System performances are compared for two evaporation temperatures varying the temperature of the external air running over the gas‐cooler and over the condenser. The refrigeration load in the cold store is simulated by means of some electrical resistances, whereas the air evaporator of the R744 plant is placed in a very large ambient. The results of the comparison are discussed in terms of temperature of the refrigerants at the compressor discharge line, of refrigerants mass flow rate and of coefficient of performance (COP). The performances measured in terms of COPs show a decrease with respect to the R134a plant working at the same external and internal conditions. Further improvements regarding the components of the cycle are necessary to use in a large‐scale ‘split‐systems’ working with the carbon dioxide. Copyright © 2009 John Wiley & Sons, Ltd.     

超临界补气增焓高温CO2热泵热风机实验研究

针对工业上循环加热工况下,CO₂热泵气体冷却器出口温度过高、能效显著降低的问题,文章提出了带有超临界补气的高温CO₂热泵循环,并对系统进行了实验研究。主要分析了主路膨胀阀开度、压缩机频率、气体冷却器的风机频率对出风温度、性能系数COP等参数的影响。实验结果表明:出风温度随着主路膨胀阀开度的增大呈现降低趋势而COP呈升高趋势;降低气体冷却器的风机频率对出风温度的提升效果最为明显,风机频率每调低1%,出风温度最大提升5.07%。以得到最高出风温度为目标的实验数据表明,该系统在气体冷却器出口温度为75℃以上时,出风温度可达130℃以上,对应COP为1.40~1.50。     

CO2跨临界制冷循环系统[火用]经济分析

为优化CO2跨临界制冷循环系统,以达到能效和经济之间的平衡,提出?经济分析方法,并建立数学模型,模拟分析系统的COP、?效率及系统各部件的损、?经济成本等随气体冷却器出口温度升高的变化趋势。结果表明,系统的COP和?效率呈现下降趋势;各部件损除蒸发器略有下降外,压缩机、气体冷却器、节流阀的?损均有不同程度的增大;系统单位产品?成本呈现下降趋势;减小节流阀产生的?损有利于提高系统的效率。     

Experimental study on HFC125 critical heat pump

A critical cycle heat pump with HFC125 was studied experimentally. The experimental result indicates that the heat pump with HFC125 can use the general components of the conventional heat pump well. Hot water with wide-range temperature can be conveniently got by the critical heat pump system through water flow control. The COP_h of the critical cycle drops a little when the temperature of outlet water rises from 60 °C to 75 °C. And adding heat recovering exchanger cannot improve the performance of the cycle, but can reduce the working pressure of the cycle. Comparing with the CO₂ trans-critical heat pump, HFC125 critical heat pump has a better performance of refrigeration, lower working pressure, which is especially suitable for dual-function of supplying hot water and refrigeration in the civil and industrial buildings.     

Development and validation of static simulation model for CO2 heat pump

A simulation model for the CO₂ heat pump water heater was developed and validated in this study. Component models of the gas cooler, evaporator, compressor, and expansion valve were constructed with careful consideration for the heat transfer performances. To validate the simulation model, experiments were carried out using an actual CO₂ heat pump water heater (water heating capacity: 22.3 kW; hot-water temperature: 90 °C). In simulations and experiments, the effects of the inlet water temperature and outside air temperature on the system characteristics were discussed. As a result, the average difference in COP between the simulation results and experimental results is 1.5%.     

Exergy assessment of an optimized capillary tube‐based transcritical CO2 heat pump system

A capillary tube‐based CO₂ heat pump is unique because of the transcritical nature of the system. The transcritical cycle has two independent parameters, pressure and temperature, unlike the subcritical cycle. A comparative study for various operating conditions, based on system COP and exergetic efficiency, of a capillary tube and a controllable expansion valve‐based transcritical carbon dioxide heat pump systems for simultaneous heating and cooling at 73 and 4°C, respectively, is presented here. Two optimized capillary tubes having diameter of 1.5 and 1.6 mm are compared with an equivalent controllable throttle valve. Heat transfer and fluid flow effects are included in the gas cooler and evaporator model and capillary tube employs the homogeneous flow model to simulate two‐phase flow. Subcritical and supercritical thermodynamic and transport properties of CO₂ are calculated employing a precision in‐house property code. Optimization of effective distribution of total heat exchanger area ratio between gas cooler and evaporator is investigated. The exergetic efficiency is better in case of the capillary tube than that of a controllable throttle valve‐based system. Capillary tube‐based system is shown to be quite flexible regarding changes in ambient temperature, almost behaving to offer an optimal pressure control just like the controllable expansion valve yielding both, maximum system COP and maximum exergetic efficiency. Relatively at a smaller diameter, the capillary tube exhibits better exergetic efficiency. Capillary tube length is the critical parameter that influences system optimum conditions. The exergy flow diagram exhibits that compressor, gas cooler and capillary tube contribute a larger share, in that order, to system irreversibility. It is fairly established in this study that a capillary tube can be a good engineering option for small capacity systems in lieu of an expansion valve, which has been thought of as the only possible solution to attain the pressure optimization, an important feature of all transcritical CO₂ systems. Copyright © 2009 John Wiley & Sons, Ltd.     

Defrosting method adopting dual hot gas bypass for an air-to-air heat pump

A novel dual hot gas bypass defrosting (DHBD) method is developed to remove frost from the outside heat exchanger (HEX) of an air-to-air heat pump. The proposed method adopts two bypass lines of hot gas from the compressor: one is connected to the inlet of the outdoor HEX, and the other is connected to the outlet of the exchanger. We compare the dynamic performance and defrosting time of the conventional reverse cycle defrosting (RCD), hot gas bypass cycle defrosting (HGBD), and DHBD methods using a medium air-to-air 16 kW heat pump. The salient feature of the DHBD method is its ability to prevent a sharp decrease in the compressor outlet temperature at the melting frost stage after the HGBD process begins. Due to the additional bypass, the DHBD method sustained a higher compressor outlet pressure and reduced the defrosting time by 36% compared to the HGBD method. Compared to RCD, the defrosting time was comparable (126%); however, the amenity characteristics of the DHBD method were superior than those of the RCD method. The proposed DHBD method can overcome the main disadvantages of the RCD and HGBD methods, and showed excellent performance for an air-to-air heat pump in a defrosting operation.     

供暖用CO2空气源热泵变频运行性能研究

采用压缩机变频、设置回热器与气液分离器辅助加热等技术途径,设计与构建一种供暖用CO2空气源热泵系统。在此基础上,建立响应面模型对供暖用CO2空气源热泵的压缩机运行频率进行优化,以提高供暖用CO2空气源热泵的低温性能。响应曲面法分析结果表明,低温环境下压缩机合理升频运行可有效提高供暖用CO2空气源热泵制热量,虽压缩比增大,但仍能保证压缩机稳定运行。为提高供暖用CO2空气源热泵的性能系数(COP),在低温环境下压缩机可分段变频运行。当环境温度依次为-5、-10及-15℃时,COP最大时对应的压缩机运行频率分别为55、58及60 Hz。     

Experimental investigation of integrated refrigeration system (IRS) with gas engine,compression chiller and absorption chiller

An integrated refrigeration system (IRS) with a gas engine, a vapor-compression chiller and an absorption chiller is set up and tested. The vapor-compression refrigeration cycle is operated directly by the gas engine. The waste heat from the gas engine operates the absorption refrigeration cycle, which provides additional cooling. The performance of the IRS is described. The cooling capacity of the IRS is about 596 kW, and primary energy ratio (PER) reaches 1.84 at air-conditioning rated conditions. The refrigerating capacity of the prototype increased and PER of prototype decreased with the increase of the gas engine speed. The gas engine speed was preferably regulated at part load condition in order to operate the prototype at high-energy efficiency. The refrigerating capacity and PER of the prototype increased with the increase of the outlet temperature of chilled water or the decrease of the inlet temperature of cooling water. The integrated refrigeration chiller in this work saves running costs as compared to the conventional refrigeration system by using the waste heat.