A hybrid transient model for simulation of air-cooled refrigeration systems: Description and experimental validation

In this paper are described a hybrid dynamic model for transient simulation of refrigeration systems as well as dynamic experiments that have been performed on an air/water heap pump. The machine under consideration is made of an evaporator, a condenser, an expansion valve, a variable speed scroll compressor and a receiver. The refrigerant and second fluid flows in heat exchangers are approximated by a cascade of Continuous Stirred Tank Reactors (CSTRs). This model is quite flexible since a unique structure is used for the evaporator and the condenser models according to different boundary conditions. This is due to the use of a switching procedure between different configurations based on a phase stability test that is designed to ensure the continuity of the system simulation. An analytical thermodynamic model of the refrigerant based on an equation of state is used. Good agreement between simulation results and experimental data is achieved.     

State-space model for dynamic behavior of vapor compression liquid chiller

A state-space model has been developed to investigate the dynamic behaviors of refrigeration system. By using vector-matrix notation and linearization, the ordinary differential equations describing the chiller's dynamic thermal behaviors are transformed into a representation form of the state space. The model has been validated by a series of dynamic response experiments of a chiller with the refrigerant R-134a. The average errors of the model results compared with the experimental data are all less than 10% in the experimental cases. Afterward, the model is used to investigate the transient behaviors of the exit cold carrier temperature of evaporator under different A disturbances. The state-space model developed in this study helps us have a good idea of the dynamic performance of a refrigeration system under transient conditions and design optimized control strategies for a high-energy-efficiency HVAC system.     

多回路制冷空调综合型实验系统的研制

制冷系统是多输入、多输出的系统,而且各操作参数相互关联.随着环境和室内热负荷的变化,制冷空调系统通常是在变工况条件下运行.为全面研究制冷系统的实际工作特性,研制开发了变频蒸气压缩式多回路制冷空调综合性实验系统,通过调节连接蒸发器和冷凝器的空气回路可以在任何时间模拟实现全年的运行工况,而不受季节的影响.详细介绍了实验系统的设计原理、调节方法、技术参数和数据采集系统.利用该实验系统既可获得参数间的动态关联特性,还可进行单体设备的性能测试以及不同控制策略的比较实验,为制冷系统动态仿真研究、各部件的合理匹配和优化运行控制策略提供实验基础.     

Dynamic simulation and optimal matching of a small-scale refrigeration system

This paper describes a principle and method of optimal matching to reduce energy consumption in small-scale refrigeration systems, based on systems analysis. A knowledge of the dynamic characteristics of a refrigeration system is important for predicting the performance of the system. A simulation model of a refrigeration system consisting of a compressor, an evaporator, a condenser and a capillary tube has been established to illustrate optimal matching. For each component a mathematical model has been developed, in which the concept of transient and distributive is introduced. On the basis of dynamic simulation, a method of optimal matching to minimize power consumption is recommended. To test the reliability of the theoretical models, an experiment was carried out on a small-scale refrigeration system. The experimental data were compared with the theoretical results and it is shown that the theory is valid.     

制冷系统变工况性能数值模拟

采用分布参数法建立制冷系统数学模型,在建立冷凝器和蒸发器两相区模型时采用分相流模型,并考虑流型变化对制冷剂流动换热过程的影响。利用所建模型,分析电子膨胀阀开度、压缩机转速、冷凝器风速等参数变化对系统性能的影响,为控制规律的研究提供依据。     

Transient heat pump behaviour: a theoretical investigation

A detailed mathematical model of vapour compression heat pumps is described. Model equations of the various heat pump components are given. The component models include the condenser, evaporator, accumulator, expansion device, and compressor. A brief discussion of the modelling techniques is presented, as is the solution methodology. Preliminary simulation results are also illustrated. The model developed predicts the spatial values of temperature and enthalpy as functions of time for the two heat exchangers. The temperatures and enthalpies in the accumulator, compressor and expansion device are modelled in lumped-parameter fashion. Pressure responses are determined by using continuity satisfying models for both the condenser and evaporator. The summary provides a list of future work anticipated in the area of dynamic heat pump modelling.     

Transient heat pump behaviour: a theoretical investigationComportement de la pompe à chaleur en régime transitoire: recherche théorique

A detailed mathematical model of vapour compression heat pumps is described. Model equations of the various heat pump components are given. The component models include the condenser, evaporator, accumulator, expansion device, and compressor. A brief discussion of the modelling techniques is presented, as is the solution methodology. Preliminary simulation results are also illustrated. The model developed predicts the spatial values of temperature and enthalpy as functions of time for the two heat exchangers. The temperatures and enthalpies in the accumulator, compressor and expansion device are modelled in lumped-parameter fashion. Pressure responses are determined by using continuity satisfying models for both the condenser and evaporator. The summary provides a list of future work anticipated in the area of dynamic heat pump modelling.     

Hybrid vapor compression refrigeration system with an integrated ejector cooling cycle

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

Modeling and experimental evaluation of an automotive air conditioning system with a variable capacity compressor

A steady state computer simulation model has been developed for refrigeration circuits of automobile air conditioning systems. The simulation model includes a variable capacity compressor and a thermostatic expansion valve in addition to the evaporator and micro channel parallel flow condenser. An experimental bench made up of original components from the air conditioning system of a compact passenger vehicle has been developed in order to check results from the model. The refrigeration circuit was equipped with a variable capacity compressor run by an electric motor controlled by a frequency converter. Effects on system performance of such operational parameters as compressor speed, return air in the evaporator and condensing air temperatures have been experimentally evaluated and simulated by means of developed model. Model results deviate from the experimentally obtained within a 20% range though most of them are within a 10% range. Effects of the refrigerant inventory have also been experimentally evaluated with results showing no effects on system performance over a wide range of refrigerant charges.     

螺杆冷水机组动态仿真

给出了螺杆冷水机组动态过程模型，其中，壳管式冷凝器和满液式蒸发器被分成制冷剂侧、管壁、壳体以及水侧等四个控制容积，针对每个控制容积建立了质量或能量平衡的微分方程，由于螺杆压缩机和电子膨胀阀具有较小的热惯性，因而采用稳态模型描述压缩过程和膨胀过程，通过“自适应步长”方法实现了系统仿真，仿真结果与测试数据吻合良好。     

An NTU-ε model for alternative refrigerants: Un modèle NTU-ε pour les frigorigènes de remplacement

This paper presents a steady state simulation model to predict the performance of alternative refrigerants in vapour compression refrigeration/heat pump systems. The model is based on the NTU-ε method in analysing the heat exchangers following an elemental approach. The model extends its applicability to new refrigerants including hydrocarbons and uses a large database of REFPROP package for refrigerant properties. The main inputs to the model include the physical details of the heat exchangers, compressor efficiency, mass flow rates of heat transfer fluids and their inlet temperatures to the evaporator and the condenser, the pressure drops across the heat exchangers and the capacity of either the evaporator or condenser (in kW). The model results are validated with a wide range of experimental data of HCFC22 and propane (HC290) on a heat pump test facility for a number of parameters, e.g. coefficient of performance, condenser capacity, mass flow rate of the refrigerant and compressor discharge temperature. Although the model is currently tested for pure refrigerants using compact brazed plate (counter flow type) heat exchangers, it can also be applied to mixture of refrigerants as well as to other types of heat exchangers.     

Dynamic simulation of electronic expansion valve controlled refrigeration system under different heat transfer conditions

This work presents a new dynamic lumped parameter model that is able to simulate the oscillation of electronic expansion valve (EEV) controlled refrigeration system due to unsuitable control or abrupt change of external parameters. Based on bubble dynamics theory, the heat transfer characteristic is used to divide the evaporator into the stable and unstable conditions, and a matching switch criteria is presented. Together with the model of main components, the dynamic behavior of oscillation of system parameters is simulated successfully. Model validation against experimental data demonstrates the capabilities of the modeling approach in predicting the instability of system control loop. This study provides the basis for further developing advanced control algorithm which will ensure that an EEV-controlled system refrigeration system has better stability.     

压缩制冷空调装置动态仿真研究

对压缩制冷空调装置的动态仿真开展了初步研究,建立了包括压缩机、冷颖器、毛细管、蒸发器等四个模块在内的较完整的单压缩制冷循环的物理数学模型,并应用于开机过程的动脉模拟,仿真结果与文献中的实验结果基本吻合,同时根据仿真数据预测某些参数的变化情况,提示了系统的动态特性。     

板翅式蒸发器液冷系统设计

为了研究板翅式换热器在沸腾相变换热情况下的传热及压降特性,设计一套采用氟里昂蒸气压缩循环的液体冷却试验系统,其中板翅式换热器作为蒸发器,R22作为制冷剂,热水作为被冷却液体提供制冷剂蒸发所需热量。对该系统的运行工况、参数范围及系统各部件（包括压缩机、冷凝器、毛细管、板翅式蒸发器）进行设计及选择,并介绍所搭建的试验台。     

A model-driven multivariable controller for vapor compression refrigeration systems

A model-driven controller for vapor compression refrigeration systems is presented herein. Mathematical sub-models were developed for each of the system components: heat exchangers (condenser and evaporator), variable-speed compressor and variable-orifice electric expansion device. The overall system simulation model was used to design a MIMO controller based on the linear-quadratic Gaussian method using a state observer of the Kalman filter type. A purpose-built testing apparatus comprised of a variable-speed compressor and a pulse-width modulated expansion valve was used to collect data for the system identification and model validation exercises. It was found that the model reproduces the experimental trends of the working pressures in conditions far from the operation point (±30%) with a maximum deviation of ±5%. Additional experiments were also performed to verify the ability of the controller of tracking reference changes and rejecting thermal load disturbances as high as 15%.     

Numerical simulation of a variable speed refrigeration system

This work presents two numerical models to simulate the transient and steady state behavior of a vapor compression refrigeration system. The condenser and the evaporator were divided into a number of control volumes. Time dependent partial differential equations system was obtained from the mass, energy and momentum balances for each control volume. As the expansion valve and the compressor both have very small thermal inertia, the steady state models were applied for these components. Transient and steady state models numerical predictions were compared and good agreement was found. Further simulations were performed with the objective of verifying the possibility of controlling the refrigeration system and the superheating of the refrigerant in the evaporator outlet by varying the compressor speed and the throttling valve sectional area. The results indicate that the proposed models can be used to formulate an algorithm for controlling a refrigeration system.     

An elemental NTU- model for vapour-compression liquid chillers

This paper presents a steady-state model for predicting the performance of vapour-compression liquid chillers over a wide range of operating conditions. The model overcomes the idealisations of previous models with regard to modelling the heat exchangers. In particular, it employs an elemental NTU- methodology to model both the shell-and-tube condenser and evaporator. The approach allows the change in heat transfer coefficients throughout the heat exchangers to be accounted for, thereby improving both physical realism and the accuracy of the simulation model. The model requires only those inputs that are readily available to the user (e.g. condenser inlet water temperature and evaporator water outlet temperature). The outputs of the model include system performance variables such as the compressor electrical work input and the coefficient of performance (COP) as well as states of the refrigerant throughout the refrigeration cycle. The methodology employed within the model also allows the performance of chillers using refrigerant mixtures to be modelled. The model is validated with data from one single screw chiller and one twin-screw chiller where the agreement is found to be within ±10%.     

Effects of accumulator heat exchangers on the performance of a refrigeration system

An accumulator heat exchanger (AHX) consists of an accumulator and an inner heat exchanger (IHX) contained in a shell. The AHX has been used in multi-air-conditioners to obtain system reliability and high performance by providing liquid refrigerant into expansion devices and preventing wet-compression. Energy is exchanged between the evaporator exit and the condenser exit in the AHX. In this study, the heat transfer characteristics of the AHX were investigated experimentally, and the effects of the AHX on the performance of a refrigeration system using R22 were measured. The operating characteristics of the refrigeration system with the AHX were considerably different from those without the AHX. The AHX system showed higher refrigerant flow rate than the non-AHX system at a constant EEV (electronic expansion valve) opening because of higher subcooling, resulting in better performance and reliability of the refrigeration system. At 50% EEV opening, the cooling capacity and COP of the AHX system were higher than those of the non-AHX system by 7.5% and 3.2%, respectively.     

Comparison of the transient behavior of microchannel and fin-and-tube evaporators

The development of control algorithms for refrigeration systems requires models capable of simulating transient behavior with sensible computational time and effort. The most pronounced dynamics in these systems are found in the condenser and the evaporator, especially the transient behavior of the evaporator is of great importance when designing and tuning controllers for refrigeration systems. Various so called moving boundary models were developed for capturing these dynamics and showed to cover the important characteristics. A factor that has significant influence on the time constant and nonlinear behavior of a system is the amount of refrigerant charge in the evaporator which is considerably reduced when microchannel heat exchangers are utilized. Here a moving boundary model is used and adapted to simulate and compare the transient behavior of a microchannel evaporator with a fin-and-tube evaporator for a residential air-conditioning system. The results are validated experimentally at a test rig.     

A novel special distributed method for dynamic refrigeration system simulation

A novel dynamic mathematical model based on spatially distributed approach has been developed and validated in this paper. This model gives good agreement in predicting the system COP and other parameters. The validated model has been used to enhance the prediction of the micro variations of superheat and sub-cooling. The novel spatial distributed model for the condenser and evaporator in refrigeration system, calculates the two-phase region in gas and liquid field separately since the gas and liquid in the two-phase region have different velocities. Previous researchers have used a pre-defined function of the void fraction in their spatially distributed model, based on experimental results. This approach results in the separate solution of the mass and energy equations, and less calculation is required. However, it is recognized that the mass and energy equations should be coupled during solving for more accurate solution. Based on the energy and mass balance, the spatial distribution model constructed here solves the velocity, pressure, refrigerant temperature, and wall temperature functions in heat exchangers simultaneously. A novel iteration method is developed and reduces the intensive calculations required. Furthermore, the condenser and evaporator models have shown a parametric distribution along the heat exchanger surface, therefore, the spatial distribution parameters in the two heat exchangers can be visualised numerically with a two-phase moving interface clearly shown.