Electronic Expansion Valve Opening Research Articles

Experimental investigation on dynamic performance of mode switching in an integrated system under the summer working conditions

Efficient integrated diversified energy supply systems play a pivotal role in addressing current energy challenges. Especially under the summer working conditions, the efficiency is very considerable when the system operates in the refrigeration-hot water mode. However, the fluctuation of system parameters before and after dynamic mode switching exerts substantial influence on the effective integration of system components and stable operation of the system. To achieve a nuanced comprehension of the dynamic characteristics of the integrated system during mode switching, this study utilized experimental method to comprehensively investigate six mode switching processes under summer working conditions. The variation rules of key parameters are explored and the stability of each switching process is compared. The results demonstrate that during the switching from hot water mode to refrigeration-hot water mode (Process E) and from refrigeration-hot water mode back to hot water mode (Process F), the degree of superheat exceeds 0 °C. The degree of superheat maintains stability within the range of 12.4 to 15.2 °C in Process F. The total power fluctuation values in Processes E and F approximate 100 W. Thereinto, Process F exhibits the greatest stability among the six processes, with a maximum stabilized power of approximately 1790 W, an electronic expansion valve opening degree range of 164 to 221, and a peak discharge temperature below 110 °C.

Comprehensive performance evaluation on a transcritical CO2 ejector-expansion heat pump system

In order to reduce irreversible loss and recover expansion work for improving automotive thermal system performance, this study experimentally evaluated the performances and characteristics of a CO2 ejector-expansion (EJE) heat pump system for electric vehicles under typical cooling and heating operating modes, examined the adaptability of the EJE system with a fixed-dimension ejector, and compared the EJE system with the basic-regenerative (REG) and vapor-injection (INJ) systems in the − 30 °C to 50 °C range. The results show that the increasing electronic expansion valve (EXV) opening leads to an increasing entrainment ratio and a decreasing lift ratio in both two modes. The maximum COP is 2.00 at 35 °C/27 °C (out-cabin/in-cabin air temperature) and 1.85 at 0 °C/20 °C. The COP improvement reaches a maximum of 21.6 % and 31.0 % at the ejector design temperature of 45 °C, compared to the REG and INJ systems, respectively. However, the heating performance of the ejector designed at a high temperature significantly decreases as the ambient temperature goes down. At − 30 °C/20 °C, the EJE system performs worse with a COP degradation of 6.1 % compared to the REG system.

Experimental investigation on the influence of electronic expansion valve opening on the performance of miniature refrigeration system

In the cooling of personnel and electronic equipment under specific conditions, the miniature refrigeration system needs to provide different cooling capacity to adapt to different working conditions. At present, most scholars use capillaries as throttling components. Compared with capillaries, miniature refrigeration systems using electronic expansion valves as throttling components can provide a wider range of cooling capacity adjustment. Therefore, this study constructed a miniature vapor compression refrigeration system based on electronic expansion valves as throttling devices, with a system weight of 3.0 kg and dimensions of 380 mm × 300 mm × 200 mm. The experiment investigated the impact of electronic expansion valve opening on compressor power, cooling capacity, and COP under different refrigerant charge conditions. Results indicated that at an electronic expansion valve opening of 90, within the experimental range, there was a relative change in cooling capacity of 39.85 % and a relative change in COP of 29.11 % as the refrigerant charge increased. Refrigerant charge had a significant effect on the cooling capacity and COP of the miniature refrigeration system. Under the optimal refrigerant charge of 70 g, within the experimental range, there was a relative change in cooling capacity of 88.14 % with varying electronic expansion valve opening. The use of electronic expansion valves as throttle components in miniature refrigeration systems allows for a wider range of cooling capacity adjustments, making them suitable for more cooling conditions.

Real-time multi-variable optimization of the interstage subcooling vapor-injection based transcritical CO2 HPWH: An integrated model predictive control approach

The interstage subcooling vapor-injection technique has been introduced to transcritical CO2 heat pumps to alleviate performance degradation under conditions of low ambient temperature and high water-outlet temperature. This type of solution holds considerable appeal and finds utility across a wide range of applications. However, the efficacy and efficiency of such systems are contingent upon their management, which frequently requires the implementation of suitable control systems. The primary objective is to enhance the overall system operational performance, resulting in tangible advantages in terms of economic viability, energy conservation, and environmental sustainability. To achieve these objectives, one can utilize both explicit and implicit optimization methods ranging from Model Predictive Control (MPC) to Extremum Seeking Control (ESC), each with its strengths and weaknesses. This paper compares the performance of an ad hoc integrated MPC strategy and a multi-variable ESC one. Based on the results of the system operation employing the latter strategy, a significant linear correlation was discovered between the optimal medium pressure and the optimal discharge and suction pressure. Consequently, an intrinsic self-optimization strategy for the medium pressure was developed. Subsequently, a dynamic system identification was performed on the new system, incorporating this proposed strategy. To optimize the system Coefficient of Performance (COP) while maintaining the water outlet temperature, ensuring the thermal load constraint, the MPC strategy was employed. The main focus of the MPC was to determine the optimal Electronic Expansion Valve (EEV) opening degree and water pump settings. The integrated MPC strategy was evaluated and compared to the multi-variable ESC strategy under various conditions, including fixed design conditions, changing ambient conditions, and step-change water outlet temperature. The results clearly demonstrated the effectiveness and superiority of the integrated MPC.

Operational stability influenced by lubricant oil charge in the CO2 automotive air conditioning system

In a vapor compression refrigeration system, the oil enhances the efficiency of the compressor but impairs the performance of the heat exchanger. This paper focuses on the experimental study of the influence of oil quantity on the operational stability of CO2 air conditioning system. The operational stability of the system under different oil charge quantity and operating conditions is studied. The experimental results indicate that when the oil circulation rate increases to a certain value, the adjustment of electronic expansion valve (EXV) opening or the compressor speed will cause system fluctuation and the outlet of evaporator gets close to no superheat. The critical oil circulation rate of the experimental system is about 9.2 %. The analysis of the fluctuation shows that it becomes more severe with the increasing of the refrigerant mass flow rate. When the mass flow rate increases from 210 kg∙h − 1 to 250 kg∙h − 1 at 7000 r min−1 of compressor speed, the fluctuation frequency increases from 0.072 Hz to 0.095 Hz. The occurrence of unstable phenomena has remarkable impact on system performance, the maximum difference of this system in instantaneous cooling capacity and COP during the fluctuation period is respectively 0.5 kW and 0.13.

Experimental study on operating characteristics and control strategy of CO2 heat pump in circulating heating mode

The CO2 heat pump is a promising low carbon technology for domestic hot water production due to its potential heating advantages and the natural properties of CO2. However, the change pattern of operational parameters, heating time, and energy efficiency of CO2 heat pump in circulating heating mode remains to be fully revealed. Meanwhile, the matching control strategies to improve the system performance have not been developed. Firstly, we experimentally revealed the operating characteristics of CO2 heat pump in circulating heating mode. Results demonstrated that it is quite challenging to simultaneously maintain the lower input power, shorter heating time, and higher coefficient of performance (COP) without taking control measures. It is found that the circulating heating process could be divided into stable and upward periods, depending on the water temperature at the inlet of gas cooler. The deterioration of the system performance appears during the upward period, where the instantaneous COP drops from 4.1 to 3.1. This is because input power rises, while the heating capacity drops as the water temperature at the inlet of gas cooler increases. Secondly, we experimentally developed a matching control strategy to improve the system performance during the upward period. The control method is the correlation of optimal electronic expansion valve opening, and the regulation frequency is regulating for 1 °C per rise of the water temperature at the inlet of gas cooler. As a result, the overall COP increased by 3.71%, and the overall input power reduced by 3.74% while the heating time remained constant.

Hierarchical fault diagnosis and severity identification method of building air condition systems

Faults are inevitable of building Heating, Ventilation and Air Conditioning (HVAC) system, which causing either excessive energy consumption, bad thermal comfort, or undesirable indoor air quality. Existing fault diagnosis research generally uses algorithm optimization and data processing to improve performance, but not enough consideration is given to process design and the difference of physical properties of parameters during fault. Thus, this paper proposed a two-level hierarchical fault diagnosis and severity identification method for building Variable Refrigerant Flow (VRF) system. The Extreme Gradient Boosting (XGB) was studied to train fault diagnosis model at first level, and the association rule dug out each fault feature with different severities and XGB was used to identify fault severity at second level. Three faults with ten severities were simulated to verify this method. The results indicated that the fault diagnosis accuracy of abnormal refrigerant charge, four-way valve fault and Electronic Expansion Valve (EXV) fault could reach 99.46%, 100% and 99.65% respectively. The association rule analysis found that the discharge temperature, the sub-cooler gas outlet temperature, and sub-cooler EXV opening had the highest support to three faults respectively. Based on adaptive input features, the average fault severity identification rate of three faults could reach 98.23%, 99.94% and 96.16% respectively.

Regenerator performance using freezing concentration for heat source tower heat pump system

Freeze concentration has a good regeneration effect and is easy for latent heat utilization. The electronic expansion valve opening has a great impact on the icing process. Hence, to investigate variations in the freezing process and gain the optimum regeneration effect, a solution regenerator was tested under different system-side control strategies. The solution volume flow was 5 m3/h and the electronic expansion valve opening degree (OEEV) was from 100% to 50%. For ice-covering growth, ice in low opening degree spent a long time to cover the plate. The area of ice under 100% to 65% OEEV was 1.43 times that under 50% OEEV. Peak velocities from 100% to 50% OEEV all appeared in the early stage of icing development. Moreover, the low opening degree made the ice distribute unevenness. The percentage slope of 100% OEEV was 1.23% and that of 50% OEEV was 3.46%. In addition, decreasing ice thickness uniformity was beneficial to the separation process. The separation ratio from 100% to 50% OEEV was 49.51%, 51.67%, 57.88%, and 74.54%, respectively. Both the low average thickness growth rate and high separation ratio were beneficial to increase the regeneration concentration. The effective specific ice production rate was from 100% to 50% OEEV was 5.99 kg/kWh, 6.18 kg/kWh, 6.94 kg/kWh, and 6.88 kg/kWh, respectively. Results showed that 65% OEEV operation was optimum for the solution regenerator unit using freeze concentration.

Research on the adaptive proportional-integral control method of a direct-expansion photovoltaic-thermal heat pump system

An adaptive proportional-integral control method (APCM) of a direct expansion photovoltaic-thermal heat pump (DX-PVTHP) system is proposed. First, the APCM was adopted and the proportional and integral factors were determined by experiments. Then, a universal engineering model combined the 10-factor model and temperature prediction model is built. The initial electronic expansion valve (EEV) opening during start-up under different working conditions is predicted and the EEV opening under dramatic irradiance changes is corrected under the APCM. Multiple experiments were conducted to evaluate the effectiveness of the APCM and test the performance of DX-PVTHP system under the APCM. The results reveal that the stability of superheat improved under typical cloudy skies and the maximum overshoot value was 3.2 °C. The overshoot time percentage of superheat was 27.54% with a decrease of 36.12% relative to the conventional control method. Moreover, the average coefficient of performance and photoelectric conversion efficiency were 3.51 and 13.25% in household mode, and 3.16 and 13.78% in commercial mode. Furthermore, the comprehensive coefficient of heating and electricity generation (CHE) is proposed to evaluate the comprehensive coefficient of the DX-PVTHP system. The CHE of the system was obtained 34.92% in household mode and 32.83% in commercial mode.

Experimental investigation of cooling performance of a CO2 heat pump system with an integrated accumulator heat exchanger for electric vehicles: Impact of refrigerant charge and valve opening

The electric vehicle is drawing particular attentions due to the ever-increasing awareness of environmental pollution protection. For the environmental friendly CO2 automotive heat pump system, the investigation of refrigerant charge amount and valve opening on the system and component performance is still limited, especially for a system with an integrated accumulator heat exchanger. In addition, there is still a lack of oil circulation rate investigation on a CO2 vehicle system. In this study, an experimental investigation of refrigerant charge amount and valve opening for system/component performance and oil circulation performance has been systematically conducted for a cooling mode. There exists an optimal refrigerant charge amount corresponding to a maximum value of the system coefficient of performance, with an optimal charge value of 550 g for the electronic expansion valve opening from 30% to 50%, and with 500 g for the opening above 50%. Lifting the refrigerant charge amount from 450 g to 700 g for each valve opening provides a fall of ~35 °C for the discharge temperature and a growth of ~20 bar for the discharge pressure, respectively, and more pronounced at the low valve opening domains. Increasing the valve opening can accelerate the compressor efficiency, but the impact of the refrigerant charge amount may not carefully follow a similar trend. Lifting the refrigerant charge amount can lead to a significant rise in oil circulation rate, especially under high valve opening domains with the exception of 600 g charge level. Under the low refrigerant charge level, the low-pressure accumulator inlet port is the superheated state, and the oil circulation rate tends to be dropped with the increase of valve opening; while under the high charge level with the inlet to be two-phase state, the oil circulation rate curve behaves as a growing trend. This study is beneficial for the engineering community to have a deep understanding on the electric vehicle system thermal management to maintain the vehicle reliability, and provide safety and comfort benefits.

Experimental study of electronic expansion valve opening on the performance of electric vehicle heat pump system at different compressor speeds

The good matching of the electronic expansion valve (EXV) opening and compressor speed in an electric vehicle heat pump air conditioning system is significant for improving the whole system performance. The matching of EXV opening and compressor speed and its effect on system performance was experimentally investigated and analyzed. The results showed that at various compressor speeds, the condensing pressure increased and the evaporating pressure decreased to a small extent with the decrease of EXV opening, thus increasing the pressure ratio. The condensing pressure had a jump phenomenon at a specific EXV opening, and the corresponding jump opening was different at each compressor speed. The COP reached the optimal values of 2.4, 2.6, and 2.1 at the EXV opening of 110, 100, 90 steps, respectively, due to the synergistic effect between compressor speed and EXV opening. Moreover, it was found that the changing trend of COP was consistent with that of condenser outlet temperature. The compressor speed and EXV opening had a more significant impact on condensing pressure than evaporating pressure. The influence of compressor speed on each performance parameter was greater than that of EXV opening, while the EXV opening significantly influenced the subcooling degree. Under different compressor speed conditions, changing the single adjustment step of EXV according to the influence of EXV on each parameter can realize the rapid and accurate control of the heat pump system performance, which can provide a reference for the formulation of relevant control strategies and the reduction of the system energy consumption.

An experimental study on improving defrosting performances of air source heat pump unit based on hot-gas bypass method

ABSTRACT A novel hot-gas bypass defrosting method for an air source heat pump (ASHP) unit has been presented in a previous work, which can satisfy defrosting and uninterrupted heating simultaneously. To further improve the novel hot-gas bypass defrosting performances, the effects of electronic expansion valve (EEV) on both the defrosting and uninterrupted heating performances are experimentally investigated. The results showed that the defrosting period and consumption decreased by 64.7% from 510 to 180 s and 19.7% from 1133.8 to 910.2 kJ when the opening of a 500-step EEV increased from 120-step to 480-step. Although the defrosting period for EEV opening of 480-step was the shortest, the period of blowing cold air was the longest which lasted for around 60 s. Besides, the percentage of consumed energy amount used for defrosting and the average heating capacity during defrosting period ranged from 65.2%~81.7% and 1178 ~ 903 W, respectively, with five EEV openings. The maximum average COP was 0.57 with EEV opening of 120-step, while the minimum value was 0.36 with EEV opening of 280-step. The results can help to optimize the defrosting control strategy of the ASHP units using the novel hot-gas bypass defrosting method and facilitate the development of ASHP technology.

Experimental study on the performance of trans-critical CO2 two-stage compression refrigeration system with and without an ejector at low temperatures

A trans-critical CO2 two-stage compression refrigeration system with and without an ejector at low temperatures was set up and the effects of gas cooler pressure, gas cooler outlet temperature, and electronic expansion valve opening on the performance of the gas cooler and the refrigeration system were investigated experimentally. At a gas cooler pressure of 91 bar, an outlet temperature of 33 °C, and an electronic expansion valve opening of 95%, the COP of a trans-critical CO2 ejection-expansion two-stage compression refrigeration system reaches a peak of 0.78, which is 2.28 times higher than that of a trans-critical CO2 two-stage compression refrigeration system under the same operating conditions. This experiment also obtained the optimal operating range of the ejector. The effect of gas cooler pressure on COP is the most obvious under the conditions studied in this paper, followed by the electronic expansion valve opening and finally the gas cooler outlet temperature. However, the higher COP can be obtained for the electronic expansion valve opening condition, and the corresponding ∆COP is higher. There exists both an optimal intermediate pressure and an optimal gas cooler pressure to maximize the COP.

Experimental study on the heating performance of transcritical CO2 heat pump for electric buses

In this paper, a transcritical heat pump using CO2 as the refrigerant was developed, and the heating performance test experiment was carried out in the standard bus air conditioning performance enthalpy difference laboratory to explore the influence of outdoor temperature, compressor speed and electronic expansion valve (EEV) opening on the performance of the heat pump system. Operating at −25 °C/20 °C (outdoor temperature is −25 °C, indoor temperature is 20 °C), the heating capacity (Q) of the CO2 heat pump is 10.5 kW, the coefficient of performance (COP) is 1.24, indicating that the system can also effectively save energy under ultra-low temperature conditions. As the compressor speed rises, the COP of system drops sharply, so the compressor operating frequency should be reduced as much as possible under the premise of satisfying the Q and outlet air temperature (Ta.out). Increasing the opening of the EEV can effectively reduce the compressor exhaust temperature (Tcom.out) and pressure(Pcom.out) to ensure the safe and stable operation of the system.

Design and analysis of electric vehicle thermal management system based on refrigerant-direct cooling and heating batteries

A thermal management system (TMS) based on R134a refrigerant is proposed, which not only meet the thermal requirements of cabin, but also refrigerant-directly cool and heat battery. Compared with the traditional electric vehicle (EV) TMSs, an electronic expansion valve (EXV) is equipped after the battery cooling/heating plate in the refrigerant branch circuit. The mainly working modes of the TMS and their control methods are described in detail. The paper applies mathematical one-dimension simulation method to analyze the feasibility of mainly working modes, including battery preheating mode, mixed heating mode, low-temperature mixed heating mode. The results show that the TMS is completely feasible to balance the heat distribution between cabin and battery by controlling the EXVs opening. In the battery pre-heating mode, the relationship between refrigerant state of cooling/heating plate outlet and the battery temperature uniformity is analyzed, and a control strategy to improve the battery temperature uniformity is proposed. In low temperature environment, when the TMS switches from cabin heating mode to mixed heating mode, the cabin air temperature will drop sharply. This is because the battery temperature is too low, that pressure of TMS is too low to maintain the heating demand of the cabin.

Experimental investigation on improving defrosting performance of air source heat pump through vapor injection

During the defrosting process in cold regions, the air source heat pump with vapor injection encounters some problems, e.g. long defrosting time and low defrosting efficiency. To solve these problems, a new defrosting strategy through injecting medium-pressure vapor refrigerant into compressor during defrosting process was proposed based on its special structure. Five groups of experiments having different openings of injection electronic expansion valve were designed and conducted to investigate the effect of this defrosting strategy. Results indicate that an optimal opening of injection electronic expansion valve exists during defrosting process with vapor injection. For the experimental air source heat pump, it is 50%. At the opening of less than 50%, the experimental unit can run safely and defrosting performance improves gradually with the opening increase. When the opening increases to greater than 50%, the defrosting process is unsteady. At the optimal opening of injection electronic expansion valve, the defrosting time and power consumption are respectively decreased by 20.61% and 17.98%, while the defrosting efficiency is improved by 6.22%.

Performance improvement of heat pumps by optimizing refrigerant charge using novel variable liquid-line length system

• A variable liquid-line length system (VLLS) is proposed and validated with experiments. • The effects of the VLLS in heat pumps are investigated with thermodynamic perspective. • The VLLS enables a heat pump to operate with the optimum refrigerant charge. • The VLLS enhances COP and maximum capacity of a heat pump for cooling and heating. Heat pumps are often operated with nonoptimal amounts of refrigerant charge under various operating conditions; this results in excessive energy consumption by the air-conditioning systems in buildings. This paper proposes a novel variable liquid-line length system (VLLS) to improve the performance of heat pumps by optimizing the amount of refrigerant charge under various operating conditions. The feasibility of the VLLS is experimentally verified by proving that changes in the condensing and evaporating pressures afford the same effects as those of varying the amount of refrigerant charge. The performance improvement in the heat pump using the VLLS was evaluated by varying the heat sink and source temperatures under a fixed electronic expansion valve (EEV) opening and fixed superheat. The results indicate that the VLLS with a fixed EEV opening enhances the maximum capacity and coefficient of performance (COP) of the heat pump by appropriately controlling superheat and subcooling under various operating conditions. Furthermore, the heat pump adopting the VLLS with a fixed superheat exhibits substantial improvements in the heating and cooling COPs by varying the optimal amount of refrigerant charge.

Parametric study of a room air conditioner during defrosting cycle based on a modified defrosting model

Reverse cycle defrosting is widely used in room air conditioner (RAC) system. The objective of this study is to investigate system transients during defrosting from a numerical simulation perspective and provide control guidance for maximizing defrosting performance without wet compression. For this purpose, a system-level defrosting model for RAC system is developed in line with its practical application. A modified first-principle hot gas defrosting model is developed to eliminate the mass non-conservation error. Model validation against experimental data demonstrates the model could predict the system transients within reasonable accuracy. Wet compression during defrosting under basic control strategy is captured in the simulation and experiments. The model is applied to evaluate the effect of compressor frequency and electronic expansion valve (EEV) opening on defrosting. The results show that wet compression could be eliminated by decreasing EEV opening. The compressor frequency has little impact on the time point when wet compression occurs, however, it could weaken wet compression by reducing defrosting duration. Based on the operation maps, the EEV opening and compressor frequency are recommended in the range of 40–60 % and 70–90 Hz, respectively, which could guarantee safe compression with short defrosting duration and low energy consumption.

The effects of the opening of the electronic expansion valve in the high-stage cycle on the performance of a cascade heat pump water heater

The opening of the electronic expansion valve (EEV) in the high-stage cycle of a cascade heat pump not only affects the suction and discharge statuses of the compressor, but also regulates the intermediate temperature, thereby, determines the system performance. In this study, we conducted experimental analyses on a cascade heat pump water heater that was equipped with an EEV in the high-stage cycle (high-stage EEV) and an inverter compressor in the low-stage cycle (low-stage compressor) to investigate the effects of the opening of the high-stage EEV on the suction and discharge statuses of the high-stage compressor and the system performance. Our results indicated that the optimum opening of the high-stage EEV maximized the coefficient of performance, and the increase in the frequency of the low-stage compressor from 150 to 225 Hz increased the optimum opening of the EEV from 37.5% to 50%. Moreover, the decrease in the opening of the high-stage EEV and the increase in the frequency of the low-stage compressor increased the heating capacity of the system by 4.88W/% and 30.14W/Hz, respectively. Furthermore, the opening of the high-stage EEV had little influence on the suction superheat of the high-stage compressor, specifically, the suction superheats ranged from 6.0 to 9.5 °C at all of the EEV openings (32.5–75.0%). Nevertheless, low EEV openings increased the discharge pressure of the high-stage compressor approaching the safe operating limit of the compressor. Hence, the control on the opening of the high-stage EEV should be aimed to optimize the system performance and reduce the discharge pressure of the high-stage compressor.

Energy and exergy analyses of a transcritical CO2 air conditioning system for an electric bus

Abstract The cooling performance of a transcritical CO2 air conditioning system for an electric bus was investigated using energy and exergy analyses. The effects of the working conditions, such as the compressor speed, electronic expansion valve (EEV) opening, inlet temperature and flow rate of indoor air and the inlet temperature of outdoor air on the energy performance were experimentally investigated. Exergy analysis of the system was performed based on the experimental data. Experimental results showed a coefficient of performance (COP) of 2.88 and a cooling capacity of 31.4 kW at a condition of 35 °C/27 °C (an outdoor air temperature of 35 °C and an indoor air temperature of 27 °C). As the outdoor air temperature increased to 45 °C, a COP of 1.87 was achieved with a cooling capacity of 22.8 kW. Therefore, the system possessed a good cooling performance in hot climates. Increasing the indoor air flow rate could improve the exergy efficiency of the system and increasing the indoor and outdoor air temperatures could reduce the exergy efficiency of the system. Among the exergy destructions of the components, the values obtained for the gas cooler and EEV accounted for significant proportions, offering great opportunities for improving the system performance. In extremely hot climates the exergy destruction of EEV comprised the greatest percentage and further research should be performed on the significant throttling loss of the expansion process.