Thermo-physical properties of Al2O3-SiO2/PAG composite nanolubricant for refrigeration system

Thermal conductivity and viscosity of the Al₂O₃-SiO₂/PAG composite nanolubricants for 0.02 to 0.1% volume concentrations at a temperature range of 303 to 353 K were investigated. Al₂O₃ and SiO₂ nanoparticles were dispersed in the Polyalkylene Glycol (PAG 46) lubricant using the two-step method of preparation. Thermal conductivity and viscosity were measured using KD2 Pro Thermal Properties Analyzer and LVDV-III Rheometer, respectively. The result shows that the thermal conductivity and viscosity of composite nanolubricants increase with volume concentration and decreases with temperature. Composite nanolubricants behave as Newtonian in the range of the temperatures and volume concentrations studied. The highest thermal conductivity increment is 2.41% at 0.1% concentration and temperature of 303 K. A maximum value of 9.71% in viscosity at 0.1% concentration is observed at temperature of 333 K. A new correlation model to predict the properties of composite nanolubricants has been proposed for applications in refrigeration systems.     

Comparitive analysis of an automotive air conditioning systems operating with CO2 and R134a

This paper evaluates performance merits of CO₂ and R134a automotive air conditioning systems using semi-theoretical cycle models. The R134a system had a current-production configuration, which consisted of a compressor, condenser, expansion device, and evaporator. The CO₂ system was additionally equipped with a liquid-line/suction-line heat exchanger. Using these two systems, an effort was made to derive an equitable comparison of performance; the components in both systems were equivalent and differences in thermodynamic and transport properties were accounted for in the simulations. The analysis showed R134a having a better COP than CO₂ with the COP disparity being dependent on compressor speed (system capacity) and ambient temperature. For a compressor speed of 1000 RPM, the COP of CO₂ was lower by 21% at 32.2°C and by 34% at 48.9°C. At higher speeds and ambient temperatures, the COP disparity was even greater. The entropy generation calculations indicated that the large entropy generation in the gas cooler was the primary cause for the lower performance of CO₂.     

Cooling performance of a variable speed CO2 cycle with an electronic expansion valve and internal heat exchanger

The cooling performance of a CO₂ cycle must be improved to develop a competitive air-conditioning system with the conventional air-conditioners using HFCs. In this study, the cooling performance of a variable speed CO₂ cycle was measured and analyzed by varying the refrigerant charge amount, compressor frequency, EEV opening, and length of an internal heat exchanger (IHX). The basic CO₂ system without the IHX showed the maximum cooling COP of 2.1 at the compressor discharge pressure of 9.2 MPa and the optimum normalized charge of 0.282. The cooling COP decreased with the increase of compressor frequency at all normalized charges. The optimum EEV opening increased with compressor frequency. Simultaneous control of EEV opening and compressor frequency allowed optimum control of the compressor discharge pressure. The optimal compressor discharge pressure of the modified CO₂ cycle with the IHX was reduced by 0.5 MPa. The IHX increased the cooling capacity and COP of the CO₂ cycle by 6.2–11.9% and 7.1–9.1%, respectively, at the tested compressor frequencies from 40 to 60 Hz.     

高温压缩机变转速的复叠式热泵系统性能实验研究

为研究变转速压缩机对复叠式热泵系统的影响,本文搭建了高温压缩机变转速的复叠式热泵实验台。通过实验研究了不同运行工况下系统排气温度、中间温度、制热量、功率及COP随高温压缩机转速的变化规律。结果表明:在冷凝温度为46℃,蒸发温度为-35℃～-10℃时,压缩机运行安全可靠;在冷凝温度为46℃,蒸发温度为-25℃,高温压缩机转速从1 200 r/min增至6 000 r/min,制热量提升了129. 7%,低温压缩机功率减少43.4%; COP随高温压缩机转速的增加呈先增大后减小的趋势,存在最大COP和对应的最佳高温压缩机转速。     

Performance evaluation of a CO2 heat pump system for fuel cell vehicles considering the heat exchanger arrangements

A novel CO₂ heat pump system was provided for use in fuel cell vehicles, when considering the heat exchanger arrangements. This cycle which had an inverter-controlled, electricity-driven compressor was applied to the automotive heat pump system for both cooling and heating. The cooling and heating loops consisted of a semi-hermetic compressor, supercritical pressure microchannel heat exchangers (a gas cooler and a cabin heater), a microchannel evaporator, an internal heat exchanger, an expansion valve and an accumulator. The performance characteristics of the CO₂ heat pump system for fuel cell vehicles were analyzed by experiments. Results for steady and transient state performance were provided for various operating conditions. Furthermore, experiments to examine the arrangements of a radiator and an outdoor heat exchanger were carried out by changing their positions for both cooling and heating conditions. The arrangements of the radiator and the outdoor heat exchanger were tested to quantify cooling/heating effectiveness and mutual interference. The improvement of heating capacity and coefficient of performance (COP) of the CO₂ heat pump system was up to 54% and 22%, respectively, when using preheated air through the radiator instead of cold ambient air. However, the cooling capacity quite decreased by 40–60% and the COP fairly decreased by 43–65%, for the new radiator-front arrangement.     

A combined-cycle refrigeration system using ejector-cooling cycle as the bottom cycle

A combined-cycle refrigeration system (CCRS) that comprises a conventional refrigeration and air-conditioning system using mechanical compressor (RAC/MC) and an ejector-cooling cycle (EJC) is proposed and studied. The EJC is driven by the waste heat from the RAC/MC and acts as the bottom cycle of the RAC/MC. A system analysis shows that the COP of a CCRS is significantly higher than a single-stage refrigeration system. Improvement in COP can be as high as 18.4% for evaporating temperature of the RAC/MC T_e at −5°C. A prototype of the CCRS was built and tested in the present study. Experimental results show that at T_e=−4.5°C, COP is improved by 14% for a CCRS. For T_e at 5°C, COP can be improved by 24% for a CCRS with higher condensing temperature of the RAC/MC. The present study shows that the CCRS using the ejector-cooling cycle as the bottom cycle of the RAC/MC is viable. Further improvement in COP is possible since the prototype is not designed and operated at an optimal condition.     

Experimental evaluation of a CO2 transcritical refrigeration plant with dedicated mechanical subcooling

CO₂ transcritical refrigeration cycles require optimization to reach the performance of conventional solutions at high ambient temperatures. Theoretical studies demonstrated that the combination of a transcritical cycle with a mechanical subcooling cycle improves its performance; however, any experimentation with CO₂ has been found. This work presents the energy improvements of the use of a mechanical subcooling cycle in combination with a CO₂ transcritical refrigeration plant, experimentally. It tested the combination of a R1234yf single-stage refrigeration cycle with a semihermetic compressor for the mechanical subcooling cycle, with a single-stage CO₂ transcritical refrigeration plant with a semihermetic compressor. The combination is evaluated at two evaporating levels of the CO₂ cycle (0 and −10 °C) and three heat rejection temperatures (24, 30 and 40 °C). The optimum operating conditions and capacity and COP improvements are analysed with maximum increments on capacity of 55.7% and 30.3% on COP.     

Theoretical study on a frost-free refrigerated display cabinet

This study deals with a frost-free refrigerated display cabinet, in which frosting can be retarded by dehumidifying the air before it enters the evaporator of the display cabinet via a desiccant-coated heat exchanger. Because the desiccant can be regenerated via the condensation heat of the refrigerant (which is exhausted into ambient air in conventional refrigerated display cabinets), the proposed system can achieve high energy efficiency. Calculation shows that the coefficient of performance (COP) of this system is 3.1 and the daily electricity consumption of the compressor is 10.5 kWh when the temperature (T_RA) and relative humidity (RH_RA) of the return air entering the evaporator are 12 °C and 0.6, respectively. Furthermore, it is found that T_RA and RH_RA significantly affect the COP: COP increases by about 115% when T_RA varies from 8 to 14 °C, and increases by 35% when RH_RA varies from 0.5 to 0.8.     

Performance evaluation of a two-stage CO2 cycle with gas injection in the cooling mode operation

The cooling performance and reliability of a transcritical CO₂ cycle can be significantly improved by using a multi-stage compressor with gas injection because the CO₂ cycle has a large pressure difference across a compressor. The objective of this study is to investigate the performance and operating characteristics of a two-stage CO₂ cycle with gas injection. In this study, the performances of a two-stage CO₂ cycle with gas injection (called as “two-stage gas injection cycle”) were measured as the amount of refrigerant charge, first- and second-stage compressor frequencies, and first- and second-stage EEV openings were varied in the cooling mode operation. The cooling COP of the two-stage gas injection cycle was maximally enhanced by 16.5% over that of the two-stage non-injection cycle in the experiments. In addition, when the first- and second-stage EEV openings were increased, the compression ratio decreased and this in turn, improved the cooling COP of the two-stage gas injection cycle. However, when the first-stage EEV opening was increased, the mass flow rate through the evaporator decreased, and this in turn, decreased the cooling capacity of the two-stage gas injection cycle. Therefore, in the two-stage gas injection cycle, an optimum control of both EEV openings is required.     

An experimental and numerical study on dynamic characteristic of linear compressor in refrigeration system

This paper presents experimental and numerical results of the dynamic characteristic and COP of a linear compressor in a refrigeration system using R600 refrigerant. The numerical analysis consists of a model and a simulation that includes the linear compressor. In this study, the dynamic characteristic of the natural frequency of the linear compressor is validated by comparing the simulation results with the experimental results. To investigate the effect of system resonance on the performance of linear compressor, COP is evaluated under evaporator pressure in the range of 48.3–63.2 kPa abs, and condenser pressure in the range of 439.0–573.3 kPa abs. Based on the results, the system resonance at the TDC was varied within a range of 3% under the test conditions. COP and its sensitivity were found to vary within 3% according to the operating frequency of the system ranging from 48.5 to 51.5 Hz.     

夏季冷热联供模式下的燃气机热泵机组性能分析

本文建立了一套以R134a为制冷剂工质、以天然气为一次能源驱动开启式压缩机做功的燃气机热泵系统,研究了当蒸发器进水温度为12～22℃,室外环境温度为24.2～35.6℃,发动机转速为1 400～2 000 r/min,夏季冷热联供模式时,空气源燃气机热泵(GEHP)机组的性能。结果表明:发动机转速和蒸发器进水温度的变化对系统性能的影响大于室外环境温度变化的影响。随着发动机转速由1 400 r/min增至2 000 r/min,系统COP_1、COP_2、PER_1、PER_2分别减小了15.5%、9.9%、18.8%、13.5%。在工况范围内,机组冷水出水温度可达6.7～19.3℃,热水出水温度可达40.7～61.7℃,考虑余热回收情况下系统PER_2可达1.14～1.45。     

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 investigation on the performance of a transcritical CO2 heat pump with multi-ejector expansion system

Carbon dioxide is becoming an interesting option also for HVAC heating appliances due to its eco-friendly characteristics. The incurrence of transcritical cycles makes the use of ejectors attractive to improve performance. Part load conditions and varied ambient temperatures imply the use of variable geometry systems of appropriate design to keep their efficiency high. This paper investigates experimentally a multi-ejector air-to-water CO₂ heat pump for heating needs, measuring the performance of the system and of each component under partial and full load conditions. A sensitivity analysis was run varying, separately or in a combined way, the following parameters or operating conditions: ejector area ratio in a range of 33.1–86.6%; compressor frequency in the range of 30–60 Hz; ambient temperature in the range of −15–12 °C according to the standard UNI EN 14511/2011. The results show the possibility to reach an optimum of the COP by varying the ejector area, once the other parameters are fixed, as a consequence of the ejector regulation on the pressures at the inlet and outlet of the compressor.     

Regenerated air cycle potentials in heat pump applications

Air (reversed Brayton) cycle has been utilized in the area of refrigeration and cryogenics for several decades, but its potentials in heat pump applications were longtime underestimated. In this paper, a thermodynamic model for the regenerated air heat pump cycle with practical compressor, expander and regenerated heat exchanger was developed. Based on the model, the relations between the system performance and the operating parameters were analyzed. The optimal heating COP (coefficient of performance) and the corresponding pressure ratio were derived. Then, air heat pump cycles (regenerated cycle and basic cycle) and vapor-compression heat pump cycles (CO₂ trans-critical cycle and R410A subcritical cycle) were numerically compared. The results indicated that the regenerated air heat pump cycle not only gets the heating capacity in line with the heating load under different operating conditions but also achieves higher COP over trans-critical CO₂ heat pump cycle in applications of large temperature difference.     

Modeling and simulation of the transcritical CO2 heat pump system

In this study, a CO₂ transcritical cycle model without imposing any excessive constraints such as fixed discharge pressure and suction pressure is developed. The detailed geometrical variation of the gas cooler and the evaporator have been taken into account. The model is validated with the experimental measurements. Parametric influences on the CO₂ system with regard to the effect of dry bulb temperature, relative humidity, inlet water temperature, compressor speed, and the capillary tube length are reported. The COP increases with the dry bulb temperature or the inlet relative humidity of the evaporator. Despite the refrigerant mass flowrate may be increased with the inlet water temperature, the COP declines considerably with it. Increasing the compressor speed leads to a higher heating capacity and to a much lower COP. Unlike those of the conventional sub-critical refrigerant, the COP of the transcritical CO₂ cycle does not reveal a maximum value against the capillary tube length.     

Surface tension measurement of oil/refrigerant mixture by maximum bubble pressure method

Lubrication oil used in refrigerant compressors forms oil mist in the compressor shell. Some of the oil mist is discharged into a refrigeration cycle with refrigerant and causes degradation of heat transfer in heat exchangers. Since the generation of the oil mist is related to the Weber number, it is necessary to measure the surface tension of the oil/refrigerant mixture before discussing the oil mist generation in the compressor. In this study, the maximum bubble pressure method was adapted to measure the surface tension of PAG (polyalkylene glycol) oil/CO₂ mixture. The density of the mixture needed for the measurement was also carried out. It was found that the surface tension of PAG/CO₂ mixture sharply decreases with an increase in the concentration of the refrigerant in the mixture. The surface tension of the mixture under 10 MPa and 100 °C is estimated to be 14.6 mN m⁻¹ by an extrapolation.     

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.     

CO_2热泵热电池储能性能实验研究

CO_2热泵热电池系统由跨临界二氧化碳水源热泵与蓄冷蓄热装置组成,其在储能过程中系统的效率会逐渐降低。本文实验研究了CO_2热泵热电池的储能性能,分析了储冷罐、储热罐循环水体积流量、压缩机频率和电子膨胀阀开度对储能效率的影响。结果表明:低循环水流量既可使储能罐获得良好的温度分层,又能获得较大的换热量;压缩机频率越高,系统效率越大;同时电子膨胀阀开度也影响系统的储能效率。当压缩机频率为50 Hz,电子膨胀阀开度为330脉冲,储冷罐、储热罐循环水体积流量分别为0.2 m~3/h、0.1 m~3/h时,总体COP最大,为5.49。同时数学拟合了系统COP与储冷罐、储热罐出水温度、控制参数的关联式,提出了一种基于遗传算法的优化控制策略,系统总COP可达6.29。     

Experimental study on cooling performance of air conditioning system with dual independent evaporative condenser

Evaporative condenser is an energy efficient and environmentally friendly air conditioning equipment. This paper proposed an air conditioning system using dual independent evaporative condenser and investigated the cooling performance. Many factors, such as evaporator water inlet temperature, compressor frequency, air dry-bulb temperature, air velocity and water spray rate, which influenced the cooling performances of air conditioning system with evaporative condenser have been investigated. The results indicated that cooling capacity and coefficient of performance (COP) increased significantly with the increasing of evaporator water inlet temperature (12–25 °C), the air velocity (2.05–3.97 m s⁻¹) and the water spray rate (0.03–0.05 kg m⁻¹ s). However, COP decreased with the increasing ambient air dry-bulb temperature (31.2–35.1 °C) and the compressor frequency (50–90 Hz). Furthermore, the heat transfer coefficient (K₀) was 232–409 W m⁻² K⁻¹ in different air velocity and water spray rate.     

Scattering effect of SiO2 particles on correlated color temperature uniformity of multi-chip white light LEDs

This study proposes the addition of SiO₂ scattering particles into the phosphor layer of a multi-chip white light LED (MCW-LED) for enhancing its performance. It is demonstrated by computer simulations that SiO₂ particles can bring significant effects on the correlated color temperature (CCT) uniformity and luminous flux. Through the simulation experiments, it was found that the MCW-LED with hybrid SiO₂ structure with SiO₂ particle size around 1 μm can achieve higher color uniformity than other package structures. We investigated the influence of SiO₂ (quartz) concentration on the CCT and the output flux of the MCW-LED with CCTs 7000, 7700, and 8500 K, respectively. Compared to an LED package without SiO₂, SiO₂ packages can shrink the CCT deviation about 48%. In this study, it was demonstrated that the inclusion of about 5–10% SiO₂ can produce an MCW-LED with higher CCT uniformity and optimal lumen output.