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The throttling process, as an important part of the heat pump system, plays a crucial role in the efficient and reliable operation of the whole system. This paper, taking the quasi two-stage compression high-temperature heat pump with green refrigerant HP-1 as the research subject, established the mathematical models of the circulatory system and electronic expansion valve by using MATLAB and considering the influence of the opening of electronic expansion valve and thermodynamic properties of the new green refrigerant. It simulated the matching characteristics of electronic expansion valve opening and flow rate under variable operating conditions, and fitted the HP-1 dimensionless flow coefficient correlation by power-law distribution using experimental data. The research results show that the electronic expansion valve with an elliptical conical body structure adapts to the throttling characteristics of the HP-1 high-temperature heat pump system under variable operating conditions. When the evaporating temperature varies from 50 ℃ to 90 ℃ and the condensing temperature varies from 60 ℃ to 120 ℃, the opening adjustment range of this type of valve body is from 49.8% to 69.8% for the main throttle valve, and from 41.5% to 56.0% for the injection throttle valve. The relative deviation of the fitted correlation results and the actual test data is between -7.8% and +7.5%, and the flow coefficient correlation can accurately predict the flow characteristics of the electronic expansion valve with a similar body structure. The selection of favorable electronic expansion valve matching refrigerant properties and the optimization of the electronic expansion valve control system are essential for the actual operating performance. This study provides a good research foundation for the selection of electronic expansion valves and the optimization of the control system for the HP-1 high temperature heat pump.

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Highlights • On-site testing data of a screw chiller were obtained in a cinema under varying load. • The developed model was validated with testing data. • COP of the screw chiller system was predicted with ANN model. Abstract An artificial neural (ANN) model is developed under on-site operating conditions, and it is different within a laboratory. On-site testing carried out to gather amount of data is for the purpose of training and prediction. Back propagation algorithm (BP) is applied to establish the transient coefficient of performance (COP) prediction model. The most suitable network structure is selected by comparison and cross-validation method, and the selected model is trained through setting the suitable network parameters. The error between the predicted results and the experimental data is almost within ±5.0% and the maximum error is 5.8%. Additionally, the validated model is used to evaluate the performance of a water-cooled variable frequency screw water chiller under various operating conditions and different openness of electronic expansion valve (EXV). It is valuable to apply neutral network on water-cooled screw chillers with maximum efficiency when satisfying a building cooling load instantly under necessary conditions. [ABSTRACT FROM AUTHOR]

Highlights • There are several gray-box mass flow rate reference models in the literature. • Model applications to two walk-in refrigeration systems resulted in some deviations. • The model deviations were attributed to several factors. • The difficulty in the calculation of TXV diaphragm force causes some deviations. • The presence of a liquid line receiver appears to cause some deviations. Abstract Thermostatic expansion valves (TXVs) are widely used in air-conditioning and commercial refrigeration systems. Accurate predictions of the refrigerant mass flow rate across a refrigeration cycle are desired in many engineering processes and applications such as a computer simulation model, for control purposes, or for fault detection and diagnostics (FDD). Several researchers have developed mass flow models for TXVs. This paper reviews and analyzes four gray-box TXV mass flow models in the literature and evaluates and compares their performance. Three of the models were developed for air-conditioners, but in the current work their applicability to refrigeration system modeling is assessed, with the use of normal and faulted laboratory data from a commercial walk-in cooler and a freezer. Model regression parameters are calculated during the training process with the use of parameter estimation techniques and a training dataset. The models are then used to predict the mass flow rates for several validation data points. Each of the models is found to perform well for the majority of the data points for the freezer, typically with errors below 10%, but application of the models to the cooler's dataset resulted in relatively large errors. The source of the model deviations includes the difficulty associated with the calculation of the TXV diaphragm force and the thermodynamic states of a two-phase refrigerant at the inlet of the valve. The presence of a liquid line receiver, which are common in refrigeration equipment, appears to cause some model deviation. [ABSTRACT FROM AUTHOR]

This experimental study investigated the application of fluid flow pulsations for in-tube flow boiling heat transfer enhancement in an 8 mm smooth round tube made of copper. The fluid flow pulsations were introduced by a flow modulating expansion device and were compared with continuous flow generated by a stepper-motor expansion valve in terms of the time-averaged heat transfer coefficient. The cycle time ranged from 1 s to 7 s for the pulsations, the time-averaged refrigerant mass flux ranged from 50 kg m −2  s −1 to 194 kg m −2  s −1 and the time-averaged heat flux ranged from 1.1 kW m −2 to 30.6 kW m −2 . The time-averaged heat transfer coefficients were reduced from transient measurements immediately downstream of the expansion valves with 2 K and 20 K subcooling upstream, resulting in inlet vapor qualities at 0.05 and 0.18, respectively, and covered the saturated flow boiling range up to the dry-out inception. Averaged results of the considered range of vapor qualities, refrigerant mass flux and heat flux showed that the pulsations at low cycle time (1 s) improved the time-averaged heat transfer coefficients by 5.6% and 2.2% for the low and high subcooling, respectively. However, the pulsations at high cycle time (7 s) reduced the time-averaged heat transfer coefficients by 1.8% and 2.3% for the low and high subcooling, respectively, due to significant dry-out when the flow-modulating expansion valve was closed. Furthermore, the flow pulsations were visualized by high-speed camera to assist in understanding the time-periodic flow regimes and the effect they had on the heat transfer performance. [ABSTRACT FROM AUTHOR]

We use online convex optimization for setpoint tracking with uncertain, flexible loads. We consider full feedback from the loads, bandit feedback, and two intermediate types of feedback, partial bandit where a subset of the loads are individually observed and the rest are observed in aggregate, and Bernoulli feedback where in each round the aggregator receives either full or bandit feedback according to a known probability. We give sublinear regret bounds in all cases. We numerically evaluate our algorithms on examples with thermostatically controlled loads and electric vehicles. [ABSTRACT FROM AUTHOR]

In this article, a comparative study is presented for the transcritical cycle with expansion valve (TCEV) and transcritical cycle with vortex tube (TCVT) mainly based on the second law of thermodynamics. Natural refrigerant nitrous oxide (N2O) is used in both the cycles for analysis. The evaporator and gas cooler temperatures are varied from −55°C to 5°C and 35°C to 60°C, respectively. The effects of various operating and design parameters on the optimum heat rejection pressure, coefficient of performance (COP), exergy loss (irreversibility), and the exergetic efficiency are studied. Exergy analysis of each component in TCEV and TCVT is performed to identify the amount and locations of irreversibility. It is observed that the use of the vortex tube in place of the expansion valve reduces the total exergy losses and increases the exergetic efficiency as well as COP. The exergetic efficiency and COP of the TCVT are on average 10-12% higher compared to TCEV for the considered operating conditions. The computed values of the exergetic efficiency for TCVT using refrigerant N2O are the highest at an evaporator temperature of −55°C, and the corresponding values of exergetic efficiency and exergy losses varies between 25.35% and 15.67% and between 74.65% and 84.33%, respectively. However, COP at the same evaporator temperature of −55°C varies between 0.83 and 0.51. Furthermore, the optimum heat rejection pressure in TCVT is lower compared to that in TCEV. The results offer significant help for the optimum design and operating conditions of TCVT with refrigerant N2O. [ABSTRACT FROM AUTHOR]

Pilot-scale distributed polygeneration system driven by solar energy and its effective simulation mechanism provides promising solutions for the technology promotion and implementation, as the emerging of smart grid concepts. In this regard, this study aims to preliminary test such a system based on organic Rankine cycle with the power output of 200 kW, which is combined with cooling and heating cycle. The developed pilot system is proven to sustain the power thermal efficiency of 10% with R123 and a self-made expansion valve. Targeting a whole optimized system in practical application, a multi-time scale mechanism is proposed and consists of long-, mid- and short-term simulation with yearly, hourly and second time step, respectively. The functionality of the concept is proven by showing the model-guided optimal sequential system with hexamethyldisiloxane working fluid. It achieves a high performance ratio, efficient cost, and less land occupation, corresponding to 67.61%, $0.12 million and 3774.2 m 2 , respectively, under the long-term simulation. Rated operation decisions are correspondingly determined and present acceptable supply-and-demand matching performance at the level of mid-term modeling, with the payback time of 7.41 years. Furthermore, the system dynamic behavior is analyzed in two typical sunny and cloudy days to understand and compare its running states. The short-term model shows a steady thermal efficiency of 9.6% within 15,000 s and capture a smaller period of safety state only within 6000 s under the sunny day condition. Although the peak irradiance in the cloudy day is higher than that in the sunny day, the performance degrades dramatically due to the irradiance fluctuation. It is expected that the proposed mechanism can be extended in analyzing operational security and control strategy. [ABSTRACT FROM AUTHOR]

The applications of electronic expansion valves in high temperature heat pump have increased rapidly. However, correlations for predicting R245fa mass flow rate through electronic expansion valves in high temperature heat pump are very limited in open literatures. In this study, mass flow rate characteristics of R245fa refrigerant flowing through electronic expansion valves with different operational conditions are studied experimentally, and empirical correlations are proposed based on the experimental data. The results reveal that the mass flow rate increases with increasing opening degree, condensation temperature, subcooling, and orifice diameter. Based on a modified single phase orifice equation, two empirical correlations are proposed by using the power-law correlation and the polynomial fit correlation, and over 92% of the predicted data deviate from the experimental data within ±15%. [ABSTRACT FROM AUTHOR]

In many refrigeration and air-conditioning systems, the automatic controller in electronic expansion valves have been employed as a component responsible for controlling the valve opening so that the superheat at the outlet of the evaporator remains within the desired limits. In some of these controllers, the control parameters are tuned once for a certain operating point and remain unaltered, even when the operating conditions change, unless the operator changes it manually. For a strongly nonlinear plant with a dramatically time varying characteristics, linear time invariant (LTI) prediction accuracy might degrade significantly that the performance of traditional Model Predictive Controller (MPC) becomes unacceptable. This work presents an Adaptive-Model Predictive Control (AMPC) mechanism to address this degradation where the parameters are tuned continuously through recursive estimation and update approaches, making the MPC insensitive to prediction errors and to achieve the optimal superheat response. Moreover, an adaptive setpoint hunting algorithm is implemented so that the system achieves stability and improves energy efficiency simultaneously. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Besides compressor rotary speed and parameters of water flowing through gas cooler and evaporator, refrigerant charge amount and electronic expansion valve (EEV) opening are two important parameters that have significant effects on the performance of a transcritical CO2 heat pump system. In this study, the effects of refrigerant charge amount on the performance of a transcritical CO2 heat pump water heater were investigated experimentally at different EEV openings. An optimal coefficient of performance (COP) was found that corresponded to a specific refrigerant charge and a specific EEV opening. Based on the experiment, the COP peaked at charge of 1.8 kg when EEV opening was 40% of full opening. The heating capacity and the COP increased at first, reached peaks and then decreased with increase of charge amount. The COP decreased 14.95% as the CO2 charge amount was reduced by 22.2% from the optimal charge at 50% EEV opening. As EEV opening varied from 40% to 60% at the same charge amount, the heating capacity decreased more than 30%. [ABSTRACT FROM AUTHOR]

Frost formation and growth is quite important physical phenomena in evaporators since frost can directly affect both capacity and fan performance. In this study, an in-house-code was programmed to determine frost ratio on evaporator surfaces via an image processing technique. Air inlet temperatures along with pressure drop across the evaporators were monitored and both temperature and pressure breaking points were identified for evaporators with flat, hydrophilic flat and corrugated fins. Evaporator capacities and pressure drops across the evaporators were experimentally measured and compared with theoretical findings. It was revealed that the evaporator with hydrophilic fin was the most suitable selection when entire measured and calculated parameters were considered altogether. [ABSTRACT FROM AUTHOR]

In order to research the adaptivity of the capillary tube in the transcritical CO 2 refrigeration system, a separated flow model of a CO 2 capillary in which the metastable phenomenon was taken into consideration was built in the simulation of transcritical CO 2 refrigeration system, and related experiments were carried out by a plant under different gas-cooler outlet temperatures. The performances of the capillary based system were further compared with that of the EEV (electronic expansion valve) based system. The results showed that the relative COP ranged from 82% to 98% when there was a 10 °C deviation between the real gas-cooler outlet temperature and the design value. Thus the capillary based transcritical CO 2 system is promising and could achieve performance close to that of EEV based system in a wide range of gas-cooler outlet temperature. [ABSTRACT FROM AUTHOR]

The performance of a transcritical cycle is highly dependent on the rejection pressure. The optimal rejection pressure depends mainly on the inlet and outlet temperatures at the heat sink (secondary fluid). For the subcritical cycles, recent studies have demonstrated that the performance of these systems depends significantly on the degree of subcooling and its optimal value varies depending on the application. This paper presents a general methodology to calculate the optimal subcooling depending on the boundary conditions. The refrigerants R290, R134a, R1234yf and R32 are analyzed. Exergy analysis is used in order to see the subcooling effect at condenser and expansion valve independently. The optimal subcooling strongly depends on the temperature lift at the secondary fluid, and it is found when two pinch points are given in the condenser, one at the condenser outlet and another one inside the condenser (at the refrigerant dew point). [ABSTRACT FROM AUTHOR]

The objective of this study is to develop a high-efficiency double effect absorption cycle using multi-heat sources for tri-generation (trigen) application. The trigen system produces electricity, heating and cooling loads at the same time. The double-effect absorption refrigeration system consists of two generators, condensers, solution heat exchangers, expansion valves, an absorber and an evaporator. The cycle simulation is carried out for the H 2 O/LiBr double effect absorption cycle with multi-heat sources for parallel, serial, reverse, revised serial and revised reverse flow patterns. The absorption refrigeration system uses the high temperature steam and hot water as the multi-heat sources. A new high-efficiency cycle is selected depending on the arrangements of additional heat exchangers. This study recommends HX 2-2 cycle with two additional heat exchangers (SDHHX and DHX) as the best candidate for trigen application. It is concluded that T Hotwater has much more significant effect on the COP and Q ̇ E than T steam in the HX 2-2 cycle. It is also found that the most important UA to affect the COP is UA LG while that to affect Q ̇ E is UA A . [ABSTRACT FROM AUTHOR]

This paper presents the design and optimum scheduling of a solar CCHP (combined cool, heat and power) system which is powered by a Stirling engine in the presence of an AA-CAES (advanced adiabatic compressed air energy storage) system for a residential energy sector. An absorber and a thermal energy storage tank are employed to absorb and store all collected solar radiation for continuous energy supplied when sunlight is insufficient, and nighttime. In this paper, an ASHP (air source heat pump) that is equipped with two inside and outside air fans, an expansion valve, compressor, condenser and an evaporator is installed to cool and heat a benchmark residential building under hot and cold climate conditions, respectively. The AA-CAES system is modeled with injected and produced power constraints, storage, air balance and operation limits. A comprehensive energy and exergy based optimization is developed as a mixed-integer nonlinear program to minimize total energy procurement cost in the presence of ASHP, prime mover and AA-CAES's constraints. It is found that the use of AA-CAES results in 21.79% and 22.36% lower operational costs in the cooling and the heating modes, respectively. [ABSTRACT FROM AUTHOR]

In this study a methodology to develop a semi-empirical modelling of a classic refrigeration system is presented, which is composed by a compressor, a condenser, an expansion valve and an evaporator, and is characterized under extended operating conditions. The models are developed in a modular way, thus these can be adapted to a similar configuration related with refrigeration, air conditioning and heat pumps. The results of the modelling are compared with experimental results obtained in a refrigeration system composed by a scroll compressor, a plate heat exchanger as evaporator a louvered fin and flat tube condenser. This comparison is also used to determine some empirical parameters of the models. First the models are validated individually and then these are interconnected to develop the global model. In this approximation, the condenser imposes the condensing pressure, the evaporator the evaporating pressure and the compressor the refrigerant flow rate, among other intermediate variables. The main outputs of the model are the evaporator cooling power, the compressor power and the system COP (Coefficient of Performance). The model is validated with 76 tests developed in a steady-state, by obtaining a COP with a difference lower than 8% between the measurements and the results of the model, which is here considered as acceptable. Finally, by simulation, the model is used to determine the refrigeration system working domain. [ABSTRACT FROM AUTHOR]

This paper reports on a study on the influences of the operating characteristics of a proportional–integral (PI) controlled Electronic Expansion Valve (EEV) on the operational stability of a direct expansion (DX) air conditioning (A/C) system. Using the classical control theory, EEV's PI settings and time constant of EEV's temperature sensor were analyzed. The theoretical analysis results using the classical control theory were further verified experimentally using an experimental DX A/C system. The study results showed that a larger proportional or integral gain would lead to a high chance for the EEV–evaporator control loop to become unstable, while slowing down the rate of degree of superheat (DS) signal transfer by increasing EEV's time constants may help mitigate system's operational instability. The results confirmed that the operating characteristics of an expansion valve in a refrigeration system could impact its operational stability. [ABSTRACT FROM AUTHOR]

In this study, the influence of key parameters on the EEV mass flow rate was investigated experimentally, including the inlet temperature, inlet pressure and EEV opening. The parametric tests show that a relatively linear variation of CO 2 flow could be maintained with the decrease of inlet temperature at higher pressure and the increase of inlet pressure at lower temperature. However, due to the large density changes above the critical point, significant changes in the CO 2 flow characteristics were observed with the variations of inlet temperature at lower inlet pressure and the variations of inlet pressure at higher inlet temperature. The important role of CO 2 density can be further confirmed by an empirical correlation which incorporates the effects of density, pressure difference and EEV opening, and the maximum deviation of mass flow predictions is below 15.3%. [ABSTRACT FROM AUTHOR]

Recently, there has been an increased interest in natural fluids from industry and scientific community who see it as one of the most promising alternatives to replace the traditional CFCs and HCFCs, especially in heat pumps for residential water heating. This work presents an investigation of the behavior of the solar evaporator and expansion valve assembly of a transcritical CO 2 heat pump in transient and steady conditions. The dynamic behavior of systems using CO 2 as a refrigerant is significantly influenced by the dynamics of the heat transfer mechanisms. The solar evaporator model is based on the equations of conservation of mass, momentum, and energy. The model validation is realized by comparing simulation results and experimental data. The model is a useful tool for analyzing the behavior in transient and steady conditions simulating various operating conditions of the heat pump including solar radiation, ambient temperature, wind speed and atmospheric conditions. [ABSTRACT FROM AUTHOR]

This paper presents the energy assessment of a water/water R744 chiller/heat pump, working according to a transcritical cycle, used for winter heating, summer cooling and tap water production. The different functions (heating, cooling, hot water) are managed water side. The analysis of the R744 chiller/heat pump is based on an original simplified method, which is able to predict the energy performance of the unit based only on its performance data at the nominal rating conditions. The method was validated against experimental data. A comparison with a state-of-the-art R410A unit is presented. The monthly analysis shows that the CO 2 unit is very efficient in hot water production, but penalised in heating and cooling service. The adoption of an ejector in place of the expansion valve makes the CO 2 system reach the same energy consumption as the R410A unit, despite the presence of the water loop only in the R744 lay-out. [ABSTRACT FROM AUTHOR]

Refrigerant mass flow rate is an important measurement for monitoring equipment performance and enabling fault detection and diagnostics. This paper presents and evaluates three different virtual refrigerant mass flow (VRMF) sensors that use mathematical models to estimate flow rate using low cost measurements. The first model uses a compressor map that relates refrigerant flow rate to measurements of condensing and evaporating saturation temperature, and to compressor inlet temperature measurements. The second model uses an energy-balance method on the compressor that uses the compressor power consumption. The third model is developed using an empirical correlation for an electronic expansion valve (EEV) based on an orifice equation. The three VRMFs are shown to work well in estimating refrigerant mass flow rate for various systems under fault-free conditions with less than 5% RMS error. The combination of the three VRMFs can be utilized to detect and diagnose when the compressor and/or expansion device is not providing the expected flow. [ABSTRACT FROM AUTHOR]

The utilization of electronic expansion valves (EEVs) in refrigeration and air conditioning systems is increased for energy saving and comfort environmental. However, experimental data and refrigerant mass flow models through EEVs are very limited in open literature. In this study, a new technique using artificial neural network (ANN) is applied to depict the mass flow rates of R22 and its alternatives R407C and R410A flowing through EEVs based on the error back propagation learning algorithm. Two strategies are followed; the first is to construct individual ANN models for each refrigerant, and the second is to construct a generalized ANN model for the three investigated refrigerants. The experimental results from open literature are used to construct the ANN models. The ANN models results showed a good agreement with the corresponding experimental data. For individual models, the relative deviations for R22, R407C, and R410A are within ±0.7%, ±1.1%, and ±0.006%, respectively. While for generalized model, the relative deviations are within ±2.5%. Also the generalized model was tested out of its construction range in a predictive mode and it was found to be a reliable tool to estimate the refrigerants mass flow rates. [ABSTRACT FROM AUTHOR]

Collectively, thermostatically controlled loads (TCLs) offer significant potential for short-term demand response. This intrinsic flexibility can be used to provide various ancillary services or to carry out energy arbitrage. This study introduces an aggregate description of the flexibility of a heterogeneous TCL as a leaky storage unit, with associated constraints that are derived from the TCL device parameters and quality of service requirements. In association with a suitable TCL control strategy this enables a straightforward embedding of TCL dynamics in optimisation frameworks. The tools developed are applied to the problem of determining an optimal multi-service portfolio for TCLs. A linear optimisation model is constructed for the optimal simultaneous allocation of frequency services and energy arbitrage. In a case study, optimal service allocations are computed for eight representative classes of cold appliances and the results are validated using simulations of individual refrigerators. Finally, it is demonstrated that clustering of appliances with similar capabilities can significantly enhance the flexibility available to the system. [ABSTRACT FROM AUTHOR]

A transient response model for vapor compression refrigeration system has been developed in the paper. The system model contains four sub-models representing condenser, evaporator, compressor and electronic expansion valve (EEV). The condenser and the evaporator are developed based on the moving-boundary formulation. Through linearization, these dynamic models are transformed into state-space representation which is expressed in the form of matrix. The compressor and EEV adopt steady models because their thermal inertia is much smaller compared with the heat exchangers (i.e., condenser and the evaporator). The system model has been validated by experiment in terms of step change of EEV opening degree and heat load as well as ramp increase of inlet temperature of coolant oil of condenser. The results show that the model simulations have a good agreement with the experimental data. The simulation errors compared with the experimental data are mostly less than 10%. Since the model proposed in this study is expressed in the form of state-space matrix, they are featured by strong portability and high computation efficiency. It allows us to investigate the thermal dynamic characteristics of a refrigeration system under any complicated conditions and develop excellent control schemes. [ABSTRACT FROM AUTHOR]

To evaluate the performance of the ejector-expansion refrigeration cycle (EERC) using zeotropic mixtures, a numerical study is conducted. A constant-pressure two-phase ejector model for zeotropic mixtures is established. The effects of both the fluid composition and the working conditions are investigated. Mixture R134a/R143a is selected as the working and the simulation results reveal that, the cycle COP increases first and then decreases as MF t (the mass fraction of R134a) increases in the researched condition. The COP gets a maximum value of 4.18 with MF t of 0.9 and yields a minimum value of 3.66 with MF t of 0.5. With mixture 0.9/0.1, the COP improvement reaches a maximum value of 10.47%. This improvement rises at high condensing temperature or low evaporating temperature. The exergy analysis shows that the compressor and ejector contribute the most exergy destruction, and the cycle exergy efficiency achieves a maximum value with MF t of 0.7. [ABSTRACT FROM AUTHOR]

A multi-ejector expansion pack, intended as a substitute for a standard high-pressure electronic expansion valve (HPV), was designed, manufactured and experimentally investigated. Four different ejector cartridges were sized to enable a discrete opening characteristic with a binary profile for a parallel-compression R744 system. The system is rated for 70 kW at a 35 °C gas cooler outlet temperature and a −3 °C evaporation temperature. High values of ejector efficiency, exceeding 0.3 over a broad operation range, were recorded for all four of the cartridges tested under vapour compression conditions. The applicability of the multi-ejector pack as a main flashing device was verified experimentally. Similar profiles of the discharge pressure control error were recorded for both alternative options: expansion purely in the HPV vs. HPV-assisted expansion in the multi-ejector pack. [ABSTRACT FROM AUTHOR]

Refrigerant mass flow rate through electronic expansion valve (EEV) makes significant sense for refrigeration system intelligent control and energy conservation. Objectives of this study were to present experimental data of R134a mass flow rate through EEV and to develop models for EEV mass flow rate prediction via two approaches: dimensionless correlation based on Buckingham π -theorem and artificial neural network (ANN) model based on dimensionless parameters. The database utilized for model training and test was comprised of our experimental data and data available in open literatures including R22, R407C, R410A and R134a. Compared with three existing dimensionless correlations, the proposed dimensionless correlation and ANN model demonstrated higher accuracy. The proposed dimensionless correlation gave mean relative error (MRE) of 6.60%, relative mean square error of (RMSE) 12.05 kg h −1 and correlation coefficient (R 2 ) of 0.9810. The ANN model with the configuration of 8-6-1 showed MRE, RMSE and R 2 of 3.97%, 7.59 kg h −1 and 0.9924, respectively. [ABSTRACT FROM AUTHOR]

The use of storage-enhanced heat recovery room air-conditioner (SEHRAC) to supply hot water and provide space cooling for energy saving has been confirmed effective, but the heat recovery process can lead to significant refrigerant pressure fluctuations. Whether an expansion device can function properly despite the fluctuations is decisive to the successful use of SEHRAC. Capillary tube (CT) and thermostatic expansion valve (TEV) are the two most used expansion devices in SEHRAC, but a suitable type has never been identified in extant literature. In this study, laboratory experiments were conducted to enable a side-by-side comparison of the two expansion devices. A prototype SEHRAC, which could be switched between the CT and TEV systems, was designed and setup in a test facility for two identical sets of experiments. For each set of experiments, a series of tests under different outdoor temperatures were conducted. It was found that an overall better performance was achieved by the TEV system. The space cooling and water heating capacities were on average 16.3–19.4% and 18.5–23.4% larger than the CT system. The overall coefficient of performance was also found 12.5–20.9% higher. The results confirmed that TEV is preferred for use in SEHRAC. [ABSTRACT FROM AUTHOR]

The paper presents the results of experimental investigations of the ORC system equipped with the expansion valve simulating microturbine operation. The aim of the research was to verify the correct functioning of the installation components and to access the technical condition of the thermal insulation in the heating and cooling mode with regard to the design assumptions. The research was conducted for different values of medium flow rates. The images were acquired using the thermal imaging camera FLIR E50, which is equipped with a specialized software. On the basis of the experimental results, the heat losses have been estimated, together with the places which were the major source of loss (the so-called thermal bridges). It was shown experimentally that the application of a thermal imaging camera can constitute a simple and fast thermal diagnostics method for installations of this type. [ABSTRACT FROM AUTHOR]

This paper develops a practical miniature and portable vapor compression refrigerator. The dimensions of the refrigerator are 190 mm × 190 mm × 100 mm and its weight is 2.75 kg. It integrates a miniature variable speed rotary compressor with a miniature electronic expansion valve (EXV) operating in pulse width modulation (PWM) mode. Besides, a new type of miniature microchannel plate-fin evaporator is developed with vacuum bonding technology and a brazed aluminum parallel flow microchannel condenser is applied. Experiments are conducted to explore the operating performance of the prototype miniature refrigerator, especially the effects of EXV duty cycle and working conditions on the cooling performance. Experiments find that the operation of the system is smooth and reliable; the results show that it has a cooling capacity of 260 W which is high enough for cooling one person with moderate working load, where the ambient temperature is as high as 50 °C and the inlet cold water temperature of the evaporator is 24 °C, respectively. Under these operation conditions, the system COP can reach 1.62 and the reversible efficiency is 0.324. [ABSTRACT FROM AUTHOR]

Effects of water mal-distribution on thermostatic-expansion-valve (TEV) stability and its controlled two-circuit evaporator performance were investigated experimentally and numerically. 7 sets of water mal-distribution were obtained with Circuit 2 water percentage ( F 2 ) from 36.9% to 9.1%. Results showed that the TEV control-loop was stable with F 2 from 36.9% to 27.0%, and the overall evaporator capacity decreased by 3.4%. However, hunting occurred with F 2 from 27.0% to 9.1% due to the evaporator outlet superheat smaller than the TEV minimal stable signal (MSS), hence system parameters such as evaporator pressures and temperatures oscillated over time. Hunting introduced a sharp decrease in the refrigerant flowrate to the evaporator, thus the overall evaporator capacity decreased by 14.1%. With water mal-distribution rate increased, the decrease in overall evaporator capacity was mainly attributed to Circuit 2 with smaller overall heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Electronic expansion valves (EEV) are employed increasingly in transcritical CO 2 systems. In order to investigate the mass flowrate characteristics of supercritical CO 2 flowing through an EEV, the effects of EEV outlet pressure on the mass flowrate of CO 2 is studied under constant EEV inlet pressure and inlet temperature. The two-phase flow patterns of supercritical CO 2 flowing through an EEV are revealed, and the conventional choked flow judgment formulas is proved effective for supercritical CO 2 . Based on Bernoulli Equation with expansion factor, the mass flowrate correlation of supercritical CO 2 flowing through an EEV is presented. The influence of the expansion coefficient on the mass flowrate is considered in the correlation in which the correction coefficient is only affected by the EEV opening, the changing rate of the density and the pressure difference in the entrance region. The mass flowrate predicted by the correlation in this paper shows a good agreement with experimental data. The maximum relative errors are within 10%. The average and standard deviations are 0.76% and 5.9%, respectively. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes and evaluates the development of generalised steady state and transient mathematical models for thermostatic expansion valves (TEVs) of the types used in commercial refrigeration systems. The model is of a generalised nature, because it is not necessary to input performance or geometrical data for a particular valve to operate the model. However, if required, the models can provide an accurate correlation of valve manufacturer's data. Derivations are provided and validating data is presented. The mathematical models described in the paper form part of computer software that simulates the thermal operation of whole refrigeration systems. The software is titled Vapour Compression Refrigeration System (VCRS) simulator, and by way of example the paper presents results from the VCRS simulator which are used to aid a discussion of operating faults, such as hunting and under damping and their possible causes, where these can be attributed to the expansion valve. [Copyright &y& Elsevier]

Abstract: When an air source heat pump (ASHP) operates in heating mode, frost can be accumulated on the surface of its finned outdoor coil. Frosting deteriorates the operation and energy efficiency of the ASHP and periodic defrosting becomes necessary. Currently the most widely used standard defrosting method for ASHPs is reverse cycle defrost. On the other hand, electronic expansion valves (EEVs) are commonly used in heat pump/refrigeration systems, including ASHP units, as throttle regulators of refrigerant flow. This paper reports on an experimental investigation of the performance of a reverse cycle defrost operation for a 6.5kW experimental heating capacity residential ASHP whose outdoor coil had four-parallel refrigerant circuits, and with an EEV as the refrigerant flow throttle regulator. The detailed descriptions of the experimental ASHP unit and the experimental procedures of ASHP are firstly presented. This is followed by reporting the comparative experiments of two control strategies for the EEV to investigate their effects on defrosting performance: the EEV being fully open and the EEV being regulated by a degree of refrigerant superheat (DS) controller. Experimental results revealed that when the EEV was regulated by the DS controller during defrosting, a higher defrosting efficiency and less heat wastage would be resulted in. [Copyright &y& Elsevier]

The energy efficiency improvement of the refrigeration system to improve the operation quality makes it unavoidable to strive for the refrigeration system operation. Nevertheless, the processes taking place in it should be as accurate as possible to describe the underlying physical and mathematical model development and refinement. The experimental investigation of any refrigeration system is usually very complicated, mainly due to the financial costs and the large number of variables involved. The use of numerical models can reduce the costs and also facilitate understanding the phenomena related to the problem. The article aims to present and analyze the behavior components of the vaporcompression refrigeration system in case of various refrigerants. Refrigerants included in the present analysis are R22/R134a/R407C/R410A. The simulation program is based upon steady state mathematical models of the refrigeration circuit including the compressor, heat exchangers and thermostatic expansion valve. The simulation results have been presented in a graphic. [ABSTRACT FROM AUTHOR]

Abstract: Liquefied Natural Gas (LNG) and chemical processing are energy-intensive facilities, such that any enhancement of their efficiency will result in abundant reduction of energy consumption and green house gas emissions. To enhance LNG plant energy efficiency, the potential of various options for improving liquefaction cycle efficiency is investigated in this study. After developing models for the LNG process using ASPEN software, four expansion loss recovery options are simulated. The simulation results show that the compressor power reduction, expansion work recovery, and LNG production increase can be achieved as much as 2.68MW, 3.82MW, and 1.24%, respectively, by replacing conventional expansion processes with expanders. By implementing all of the enhancements, the power consumption per unit mass of LNG could be reduced by 7.07% and 3.68%, with and without considering deduction of the recovered power from the total required power, respectively. Therefore, the expansion work recovery is an important option to be implemented in LNG plants. [Copyright &y& Elsevier]

Abstract: In this paper, the results of a research project aimed at deriving simple, high-performance, auto-tuning, robust control algorithms for evaporators controlled by means of EEVs (Electronic Expansion Valves) are reported. Control design is performed by resorting to a detailed virtual prototyping environment described in . The proposed control scheme consists of two nested loops. In the inner loop, the plant is connected in feedback to a PID controller. It is assumed that the structure of the process model is given but its parameters are unknown. The outer loop is composed of a parameter estimator and an adaptation algorithm that updates the parameters of the PID controller on the basis of the result of a system identification procedure. Performance of the proposed controller is evaluated in the virtual prototyping environment by means of simulations. [Copyright &y& Elsevier]

In this study, control algorithm, based on fuzzy logic, for compressor speed and opening percentage of electronic expansion valve (EEV), was investigated. The main aim of this study is to increase efficiency of the chiller system, having variable speed scroll compressor (VSSC) and EEV. Current chiller system has an air cooled condenser, a shell-tube liquid evaporator, and a vertical scroll compressor. Flow of the refrigerant R134a was controlled by EEV in the system. Chiller systems are designed for working at maximum load conditions; on the contrary, these plants work under partial load conditions most of their life cycle by means of thermostatic control. Refrigeration capacity modulation methods, such as variable speed compressors, allow to match continuously the compressor refrigeration capacity to the load, determining an energy saving according to classical thermostatic control. In this study, compressor speed was controlled with fuzzy logic algorithm according to chiller water output temperature and EEV opening percentage was controlled with fuzzy logic algorithm according to refrigerant superheat value at outlet of the evaporator. Compressor speed has been changed between 30 and 60 Hz and EEV opening percentage has been changed between 10 and 45% of the maximum opening percentage. In fact, the chiller plant was operated as on/off at 50 Hz fixed speed by a thermostat. During the study, it has been operated as variable speed by adding PWM inverter. In the present study, fuzzy logic controlled variable speed chiller system exhibited 17% coefficient of performance increase according to the thermostatic controlled fixed speed system. Also, better water temperature control was obtained via fuzzy controlled VSSC. Copyright © 2009 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: This paper discusses the strategy of improving the efficiency of air-source heat pumps by adding a small amount of heat to the sensor of the thermostatic expansion valve (TXV). TXV heating retards the closing of the valve and boosts energy efficiency in heating mode. Test results demonstrate that appropriate TXV heating achieves an improvement in coefficient of performance (COP) and thermal comfort. The required heating power is no more than 40W and the additional equipment cost is less than $20 at manufacturer cost (2006). Thus, the strategy of TXV heating is both technologically practical and low cost. [Copyright &y& Elsevier]