Variable Speed Refrigeration System Research Articles

설비공학 분야별 최근 연구 동향: 2021년 설비공학논문집 발표논문에 대한 종합적 고찰

This article reviews the papers published in the Korean Journal of Air-Conditioning and Refrigeration Engineering in 2021. The purpose of this article is to understand the status of current research issues in the areas of heating, cooling, ventilation, sanitation, and indoor environments of buildings and plant facilities. Summary of this paper is follows.<BR> (1) The topics of the paper in the field of building mechanical system were the energy consumption, indoor environment analysis, building energy saving, ventilation system, renewable energy system and law of mechanical facilities.<BR> (2) The topics of the research in the field of indoor environment were the IAQ, ventilation, air cleaning, cooling/heating load, renewable energy, zero-energy building, building energy performance etc.<BR> (3) The research topics in the field of refrigeration in 2021 include robust temperature control of variable speed refrigeration system, R32 liquid-injection showcase vapor compression system, hybrid dessicant dehumidifying system, compressor torque calculation by differential pressure measurement, FAPO zeolite adsorbent, compression-adsorption hybrid refrigeration system and so on.<BR> (4) The topics of the papers published in the field of heat were heat transfer, a heat exchanger and a battery, etc. <BR> (5) The topics of the papers published in the field of heat transfer were the air flow rate inside drum of clothes dryer, the dynamic behavior and thermal performance of a solar water heater, a higher energy density storage system using sensible heat or phase change of water, factors affecting performance characteristics of falling film evaporation using R-1233zd(E) refrigerant and the underground seawater intake system in a power plant.

가변속 냉동시스템의 견실 온도 제어를 위한 구조화 불확실성 기반의 μ-Synthesis 제어기 설계

This paper investigates μ-synthesis robust control for a variable speed refrigeration system to enhance robustness against model uncertainty and external disturbance. As the practical transfer function model obtained by experiments to design a controller inevitably contains model uncertainties such as unmodeled dynamics and modeling errors, a robust controller is necessary to track references in the system. Target temperature and superheat were simultaneously controlled in the system by adjusting rotating speed of compressor and opening angle of an electronic expansion valve respectively. Moreover, the μ-synthesis controller treats model uncertainty as structured perturbation; it is designed using structured singular value. It is shown that the μ-synthesis approach which takes structured uncertainties into account provides robust control performance against both dynamic perturbations including modeling errors and external disturbances. The validity of the proposed method was demonstrated by simulations and experiments.

유전 알고리즘을 이용한 최적 서보 제어 기반의 다입 · 출력 가변속 오일쿨러 시스템의 정밀한 온도제어

A variable speed refrigeration system (VSRS) has been applied to various industrial fields because of its excellent energy saving performance and partial load response capability. In this paper, an optimal servo controller for a variable speed oil cooler system was investigated to obtain precise temperature control performance against disturbance and model uncertainty. A state space model for controller design was derived from empirical transfer function. The controller was designed as a servo type to follow the step command strictly while minimizing an evaluation function of the secondary form for state variable and input energy. Especially, the weighting matrix, a key parameter in an optimal servo controller design, was automatically designed to meet design specifications using genetic algorithm (GA). The validity of the suggested controller was confirmed through MATLAB-based simulations and actual experiments. Moreover, the optimality and ease of design of the proposed optimal servo controller were demonstrated by comparing results with a conventional PI controller.

칼만 상태 관측기와 피드포워드 제어기를 갖는 슬라이딩 모드 제어 기반의 가변속 냉동시스템의 강인한 온도 제어

This paper describes sliding mode control using a state observer with Kalman filter and feedforward controller for a variable speed refrigeration system (VSRS) to ensure robust control performance against model uncertainty and disturbance. The sliding mode control was designed using a state space model obtained from a practical transfer function model, which was obtained by conducting experiments near operation point. The control input of SMC was achieved from estimation of state variables by a state observer with Kalman filter. Additionally, the feedforward controller was designed to minimize the steady state error due to saturation function. Simulations and experiments were conducted to verify the control performance of the suggested SMC with the feedforward controller and the state observer with Kalman filter. The results of simulations and experiments showed that the proposed controller was capable of robust temperature control for VSRS despite stepwise changes of reference and external heat load.

외란과 모델 불확실성이 존재하는 가변속 냉동시스템의 강인제어를 위한 H-infinity 제어기 설계

This paper describes the design of H-infinity controller for robust control of a variable speed refrigeration system. The suggested controller can ensure robustness against disturbance and model uncertainty by minimizing H-infinity norm of the transfer function from exogenous input to performance output and applying the small gain theorem. In particular, the controller was designed to simultaneously reduce the effects of disturbance and model uncertainty by formalizing these problems as a mixed sensitivity problem. In addition, an integrator was included to the controller to prevent the occurrence of steady state error. The validity of the proposed controller was demonstrated by computer simulations and real experiments. Moreover, the effectiveness of the proposed controller was confirmed by comparing its performance to the PI controller, which was tested under the same experimental conditions as the H-infinity controller.

Robust Temperature Control of a Variable-Speed Refrigeration System Based on Sliding Mode Control with Optimal Parameters Derived Using the Genetic Algorithm

A sliding mode control (SMC) technique based on a state observer with a Kalman filter and feedforward controller was established for a variable-speed refrigeration system (VSRS) to ensure robust control against model uncertainties and disturbances, including noise. The SMC was designed using a state-space model transformed from a practical transfer function model, which was derived by conducting dynamic characteristic experiments. Fewer parameters affecting the model uncertainty were required to be identified, which facilitated modeling. The state observer for estimating the state variables was designed using a Kalman filter to ensure robustness against noise. A feedforward controller was added to the control system to compensate for the deterioration in the transient characteristics due to the saturation function used to avoid chattering. A genetic algorithm was used to alleviate the trial and error involved in determining the design parameters of the saturation function and select optimal values. Simulations and experiments were conducted to verify the control performance of the proposed SMC. The results show that the proposed controller can realize robust temperature control for a VSRS despite stepwise changes in the reference and external heat load, and avoid the trial and error process to design parameters for the saturation function.

최적 절환 초평면을 갖는 슬라이딩 모드 제어 기반의 가변속 냉동시스템의 강인한 온도 제어기 설계

This paper deals with robust control against parameter uncertainty including disturbance for a variable speed refrigeration system (VSRS) based on a sliding mode control with an optimal switching hyper-plane. At first, a practical state-space model of VSRS was presented for applying linear transformation to a transfer function model obtained by performing experiments at near operation point. Next, a sliding mode controller (SMC) was designed to maintain robust control performance and control accuracy for the VSRS. Especially, SMC with optimal switching hyper-plane was designed by applying the optimal control principle to minimize variation of target state variable near the sliding surface. Finally, some computer simulations and real experiments for the VSRS were conducted to verify the validity of the suggested SMC. Results showed that the suggested SMC had strong robustness against model uncertainty and disturbance. Moreover, robust control performance and control accuracy by SMC were proven by comparing with PI control results under the same experimental conditions.

Effects of Evaporator and Condenser Temperatures on the Performance of a Chiller System With a Variable Speed Compressor

The operation and performance of heat-pump systems are affected by indoor and outdoor operating conditions. Power consumption and system efficiency are related to evaporator and condenser working pressures. Intelligent controllers such as a proportional integral (PI) controller improve the performance of variable speed refrigeration systems (VSRs) with electronic expansion valve (EEV). Evaporator and condenser pressures affect the system power consumption and efficiency. In this study, the influence of evaporator and condenser temperatures on the performance of a variable speed refrigeration system with an EEV was experimentally investigated at constant cooling load. The experimental system comprises of a rotary compressor, shell-and-coil condenser, EEV, and shell-and-coil evaporator for one-ton cooling capacity with refrigerant R410. Compressor speed and EEV opening are controlled by a PI controller with two control loops and the refrigerant superheat (DS) is maintained at 7°C. The results show that at constant cooling capacity, the refrigerant flow rate rises with the increase in the compressor speed. The coefficient of performance (COP) is improved with low compressor speed. The System COP is increased by 3.3% with increasing evaporator inlet water temperature for 2°C due to the reduction in the compressor speed and compression ratio. High condenser inlet water temperature promotes the refrigerant subcooling.

Disturbance observer-based robust control against model uncertainty and disturbance for a variable speed refrigeration system

ABSTRACT Disturbance observer (DOB)-based robust control for a variable speed refrigeration system (VSRS) is described in this paper for accurate tracking of the reference temperature under model uncertainty and disturbances including external noise. In particular, a Q-filter, which is the core of a DOB, is designed by H∞ (H-infinity) norm optimization to estimate the disturbance and to compensate the model uncertainty of VSRS. Simultaneous control of the oil outlet temperature and superheat is carried out using a dual single-input single-output proportional–integral (PI) control scheme with the designed observer in the inner loop. The effectiveness of the observer is evaluated by means of computer simulations and real experiments. The results showed that settling time and steady-state error were significantly reduced when DOB was applied, compared to when DOB was not applied even in the presence of model uncertainties and disturbances. These control results show that PI control with the designed DOB can facilitate robust control even when the VSRS model contains uncertainties and unforeseen disturbances are added to the system.

Systematic design of membership functions for fuzzy logic control of variable speed refrigeration system

The systematic design of membership functions (MFs) for the fuzzy logic control (FLC) of a variable speed refrigeration system (VSRS) is presented in this paper to reduce trial and error as well as dependence on expert knowledge. In particular, the influence of MFs on the error and error change rate of controlled variables is investigated in detail to verify the validity of the designed MFs. To this end, the range of MFs obtained based on the control accuracy and experimental data for the static and dynamic states of the VSRS were used as the criterion values. Two different values from the criterion were then used to compare the control performance through simulations and experiments. Some simulations and real experiments for the VSRS were conducted to verify the validity of the designed MFs. The experimental results showed good agreement with the simulation results. The error change rate and sampling time strongly affected the control performance of the VSRS in the transient state. Moreover, the desired control accuracy at steady-state, an essential element in the design specifications, was successfully achieved using the proposed approach. The suggested method can be applied using the control accuracy and simple experimental results from the static and dynamic states of the target system without depending on iterative work when designing MFs, which will be helpful for beginners.

A study on the modeling of the minimal stable superheat for a variable speed refrigeration system

This work presents a model for variable-speed vapor compression system that is able to predict accurately the minimal stable superheat (MSS) line. Based on bubble dynamics theory, a critical stable condition is defined, which the critical operating parameters of the evaporator should satisfy. Together with the model of the refrigeration system, the MSSs depending on different operating conditions are predicted successfully. This model has been validated experimentally with steady state tests, presenting a prediction error lower than 5%. Finally, it is found that, in a wide capacity modulating range, varying superheat settings on the basis of the MSS line can lead to a higher COP.

Data Mining Approach for Analysis of Variable Speed Refrigeration System

The aim of this study is to carry out performance modeling of an experimental refrigeration system driven by variable speed compressor using Data Mining techniques with small data sets. In order to vary the capacity of the refrigeration systems, one of the best methods is controlling the rotational speed of the compressor motor with a frequency inverter. For this aim, an experimental refrigeration system is setup with a frequency inverter for controlling the speed of compressor electric motor. The experiments are made for 35 Hz to 50 Hz electric motor frequencies. Data mining technique is applied to determine the system performance parameters using actual data obtained from the measurements. From the results, it is observed that data mining procedure is suitable for forecasting the system characteristics for different compressor frequencies and cooling loads instead of making several experiments

Techno-Economic and Environmental Assesment of Compressor Retrofitting Procedure

In this study, retrofitting of a fixed speed refrigeration system to a variable speed refrigeration system is investigated. Economical and technical benefits of the retrofitting procedure are also studied. Refrigeration systems are usually designed to overcome maximum load conditions. However, they work under partial load in most of their life cycle. Hence, in this study a variable speed compressor that equalizes the cooling load to the demand that assures significant energy saving is considered. As a result of this study, the retrofitted variable speed compressor performed 17% decreased energy consumption versus the thermostatic controlled fixed speed system. Also, the retrofitting procedure provided considerable payback time. From the result, 10750 kilograms less carbon emission per year as an environmental benefit within retrofitting procedure is obtained.

가변속 냉동시스템의 상태방정식 모델링과 최적제어

Abstract This paper deals with precise analytical state equation modeling of a variable speed refrigeration system (VSRS)for optimum control in state space. The VSRS is described as multi-input and multi-output (MIMO) system, which hastwo controlled variables and two control inputs. First, the Navier-Stokes equation and mass flow rate were applied to eachcomponent of the basic refrigeration cycle to build a dynamic model. The dynamic model, represented by a differentialequation, was transformed into the state equation formula. Next, a full-order state observer was built to estimate all of thestate variables to compose an optimum control system. Then, an optimum controller was designed to minimize an evaluationfunction that has input energy and control error. Finally, simulations and experiments were conducted to verify the validityof the proposed modeling and designed optimum controller to regulate target temperature and superheat in a 1RT oil coolersystem. The results show that the proposed method, state equation modeling and optimum control, is efficient to ensure optimal control performance of the VSRS.

MODELING OF A VAPOR-COMPRESSION CHILLER FOR PERFORMANCE STUDY

A variable speed refrigeration system was designed to supply chilled liquid for cooling high-power electronic devices to maintain the temperature at an acceptable level. Fin-plate heat exchangers were adopted to save space. The paper presents the simulation of the refrigeration system to study its steady performance. The simulation model was developed by using a detailed thermodynamic model and containing a series of heat transfer correlations for specific parameters. The cooling capacity of the refrigeration system under different working conditions is investigated. The simulation results keep in agreement with experimental data. The cooling capacity increases with the rise of cooled oil inlet temperature. Besides, condenser cooling liquid inlet temperature affects the cooling capacity greatly. The cooling capacity and the coefficient of performance (COP) of the system under different motor speeds are studied subsequently. The simulation results have been validated by experiments. The mean relative error of the cooling capacity and the COP between simulation results and experimental data is 12.6% and 4.8%, respectively. The results can be used to develop control strategy for regulating refrigeration flow rate to offer adequate cooling capacity and supply cooled oil of constant temperature.

An Empirical Model for a Variable Speed Refrigeration System

This paper deals with an empirical dynamic model for decoupling control of the variable speed refrigeration system (VSRS). To cope with inherent complexity and nonlinearity in system dynamics, the model parameters are first obtained based on experimental data. In the study, the dynamic characteristics of indoor temperature and superheat are assumed to be first-order model with time delay. While the compressor frequency and opening angle of electronic expansion valve are varying, the indoor temperature and the superheat exhibit interfering characteristics each other in the VSRS. Thus, each decoupling model has been proposed to eliminate such interference. Finally, the experiment and simulation results indicate that the proposed model offers more tractable means for describing the actual VSRS comparing to other models currently available.

Development and evaluation of transfer functions for a variable speed compressor

In this study, transfer functions of electronic expansion valve and variable speed scroll compressor in a chiller system are developed based on an experimental analysis. Dynamic behaviours of water temperature at chiller output and refrigerant superheat at evaporator outlet have either the first- or the second-order linear models, respectively. Different from the existing modelling studies in literature novel models are presented for variable speed refrigeration system with scroll compressor and liquid type evaporator. The scroll compressor used in the current study is not produced for variable speed operation however it is operated with some limitations in the range of 30 to 50 Hz through an inverter. Similar restrictions are presented for the electronic expansion valve as 10 per cent to 35 per cent opening range because of operational limitations. To validate transfer functions of the variable speed scroll compressor and electronic expansion valve, mean square error values for all transfer functions are calculated. The mean square error values vary between 0.0126 and 0.3116. In addition, all transfer functions are designed and tested using corresponding proportional–integral–derivative controllers in order to see their applicability. The results indicate that energy savings are a consequence of operating the chiller at smaller frequencies than the basic 50 Hz while partial load occurred.

Experimental assessment of energy storage via variable speed compressor

In this study, usage of a variable speed refrigeration system in latent heat thermal energy storage (LHTES) system is investigated to increase energy storage efficiency. Four different compressor speed control cases are compared to obtain a constant heat transfer fluid (HTF) temperature at the inlet of the energy storage tank. These control cases are (i) control with evaporation temperature, (ii) control with ethylene glycol temperature at the outlet section of evaporator, (iii) control with suction pressure of the compressor and (iv) on/off control. By means of the experimental analysis the best control strategy is obtained as control with Case (ii), in terms of stability of inlet temperature of heat transfer fluid, variations of energy efficiency of LHTES and coefficient of performance (COP) of the system. While the Case (ii) provides the most stabile inlet temperature of HTF, Case (iv) represents the worst stability. Besides, the highest energy efficiency (99.0%), exergy efficiency (87.0%) and COP (2.05) values observed in Case (ii). Additionally, the time period to reach the set value is nearly 50 min in Case (i), Case (iii) and Case (iv) on the other hand this unstable initial time period becomes nearly 25 min for Case (ii). As a result, variable speed compressor should be controlled with Case (ii) to stabilize ethylene glycol–water solution temperatures as well as increase efficiency and COP of the system.

Thermodynamic analysis of variable speed refrigeration system using artificial neural networks

This study presents thermodynamic performance modeling of an experimental refrigeration system driven by variable speed compressor using artificial neural networks (ANNs) with small data sets. Controlling the rotational speed of compressor with a frequency inverter is one of the best methods to vary the capacity of refrigeration system. For this aim, an experimental refrigeration system was designed with a frequency inverter mounted on compressor electric motor. The experiments were made for different compressor electric motor frequencies. Instead of several experiments, the use of ANNs had been proposed to determine the system performance parameters based on various compressor frequencies and cooling loads using results of experimental analysis. The backpropagation learning algorithm with two different variants was used in the network. In order to train the neural network, limited experimental measurements were used as training and test data. The best fitting training data set was obtained with eight neurons in the hidden layer. The results showed that the statistical error values of training were obviously within acceptable uncertainties. Also the predicted values were very close to actual values.

Sistema De Refrigeração Com Velocidade Variável E Estágio De Entrada De Elevado Fator De Potência

Este trabalho propõe a análise e implementação de um novo sistema de acionamento eletrônico aplicado à sistemas de refrigeração. A proposta consiste em substituir no sistema de refrigeração convencional o motor monofásico de indução por um motor trifásico de indução, e o estágio de entrada convencional por um estágio apresentando correção ativa do fator de potência. O protótipo do sistema de refrigeração proposto é composto por um motor trifásico de 0,5HP, um compressor do tipo Bitzer IY (Belt-Drive) aberto, um inversor em ponte convencional e um préregulador boost monofásico. O novo sistema opera em conformidade com as restrições impostas pelas normas internacionais IECs (Harmônicas / Flicker / Interferências Eletromagnéticas) propiciando elevado rendimento, alto fator de potência e reduzidos níveis de interferências eletromagnéticas (IEM). Com o objetivo de verificar a viabilidade e performance da regulação da temperatura no sistema de refrigeração proposto o inversor de tensão do estágio de saída foi acionado por uma estratégia de controle convencional escalar tensãofreqüência (V/f), e por uma estratégia de controle vetorial simplificada com orientação pelo estator.