Water-cooled Centrifugal Chiller Research Articles

Adaptive neural network control of a centrifugal chiller system

In this paper, a neuro-adaptive PI control strategy for a water-cooled centrifugal chiller system is developed, and its control performance is examined. The system model consists of a flooded evaporator, a flooded condenser, a centrifugal compressor, and an electronic expansion valve. The overall system consists of three control loops: a compressor speed control loop, an inlet guide vane (IGV) control loop, and a condenser liquid level control loop. A neuro-adaptive control strategy for compressor speed control was designed. The control performance of the chiller was tested by carrying out simulation runs using an integrated building and HVAC (IB-HVAC) system model. The major details of the IB-HVAC model are described in this paper. Results show that the neuro-adaptive controller can adapt to new system dynamics of the IB-HVAC system and give good setpoint tracking responses under a wide range of operating conditions. The results show that the neuro-adaptive controller performs better than the constant gain PI controller in terms of speed of response and setpoint tracking properties.

Analyzing the carbon dioxide emissions of R134a alternatives in water-cooled centrifugal chillers using the life cycle climate performance framework

Analyzing the carbon dioxide emissions of R134a alternatives in water-cooled centrifugal chillers using the life cycle climate performance framework

Detection and Diagnosis of Multiple-Dependent Faults (MDFDD) of Water-Cooled Centrifugal Chillers Using Grey-Box Model-Based Method

This paper presents the development and use of benchmarking grey-box models for the detection and diagnosis of multiple-dependent faults (MDFDD) of a water-cooled centrifugal chiller. Models are developed using data recorded by a Building Automation System (BAS) from a central cooling plant of an institutional building. The forward residual-based fault detection model identifies a fault symptom, when the difference between the measured value of target variable and benchmarking value exceeds the corresponding threshold. For the fault diagnosis, most publications start from a known single fault and establish the impact on following variables in the system. This paper presents a rule-based backward approach. The proposed method identifies if (i) the fault symptom is correct (i.e., a variable has abnormal values), or (ii) the fault symptom is incorrect (i.e., the symptom of target variable is caused by impacts generated by other faulty variables due to the dependency between variables), or (iii) both target and regressor variables are abnormal. For testing the proposed MDFDD model, some artificial faults are inserted into the measurement data file, and results are discussed about the method potential for the application.

Harmonic Mitigation in VFD Controlled Chiller Compressor Using AC Line Reactor and DC Link Choke Combination

With the goals of a better power factor and reduction in energy use, variable frequency drives (VFDs) have been standardized in various industrial applications including Heating, Ventilation, and Air-conditioning (HVAC) systems. However, key issues related to the deterioration in the power quality associated parameters (voltage harmonic distortion and current harmonic distortion) due to the growing number of VFDs demand a wholesome analysis of the problems faced by the users. In this present work, an analysis has been performed to investigate the potential impact of using alternating current (AC) line reactor and direct current (DC) link choke combination on harmonic mitigation in a VFD driven water-cooled centrifugal chiller compressor. Using a combination of AC line reactor and DC link choke as one of the low cost solutions in harmonic mitigation of a VFD driven load in HVAC systems can be decided based on the investigation done in this research work.

Measurement Method and Experimental Analysis of Liquid Entrainment for a Flooded Evaporator of a Water-Cooled Centrifugal Chiller Based on Energy Balance

Liquid entrainment in a flooded evaporator has an important impact on the performance and safety of a water-cooled centrifugal chiller. In this paper, two methods for measuring the liquid entrainment factor in the evaporator of a centrifugal chiller based on energy balance are proposed. Method 1 involves only the heat exchange capacity of the evaporator and Method 2 involves both evaporator and condenser. The applicable conditions of the methods are discussed. Experimental measurements on the flooded evaporator of a single-stage water-cooled centrifugal chiller with refrigerant R134a show that, for a system with good thermal balance, there is little difference in the entrainment factor values obtained by the two methods. Method 2 was found to have slightly higher measurement accuracy, compared to Method 1. The uncertainty propagation analysis shows that for Method 2, the inlet and outlet water temperatures of the evaporator and condenser, motor input power, motor efficiency, transmission power loss and compressor suction and discharge temperatures are important factors. The experimental results show that the variation of the evaporator entrainment factor with refrigerant charge amount is different for different cooling capacity. At 700 and 800 refrigeration ton (RT), the entrainment factor of the test evaporator increases with the increase of refrigerant charge and the growth rate gradually accelerates. For the chiller tested, when the entrainment factor reaches 0.89% and 1.02%, respectively, at 700 ton and 800 ton, the rapid increase of the entrainment factor leads to a significant decrease in the coefficient of performance (COP) during the charging process. Based on the analysis of the experimental results, it is recommended that the maximum entrainment factor for efficient operation of the centrifugal chiller should be controlled within 1%.

A Model and Multi-Mode Control of a Centrifugal Chiller System: A Computer Simulation Study

In this paper, a model of a water-cooled centrifugal chiller system is developed and its control performance is studied. The overall system consists of three control loops: (i) a compressor speed control; (ii) an inlet guide vane control and (iii) a condenser liquid-level control. The model is validated by comparing the simulation results with the experimental data under steady state conditions. The results match to within 10% accuracy. A multi-mode control strategy was designed to simulate the operation of the centrifugal chiller as it transitions between start-up, speed control mode, IGV control mode and shutdown mode during a typical day operation. COP of the chiller was studied by conducting simulation runs under a wide range of operating conditions. The results showing the coefficient of performance of the system at different cooling loads as a function of cooling water temperature under IGV control mode and compressor speed control mode are presented.

Analysis of centrifugal chillers with oil-free magnetic bearings for enhancing building energy performance

This article examines the actual operating performance of centrifugal chillers with oil-free magnetic bearings (hereafter oil-free chillers) to analyze energy saving potentials for commercial buildings. A chiller system retrofit for a shopping arcade involved replacing three existing constant speed water-cooled centrifugal chillers by three oil-free centrifugal chillers of the same capacity. The coefficient of performance was calculated based on operating data logged at 1-h intervals for over 1 year before and after replacement. Oil-free chillers with variable speed control operated at a higher coefficient of performance under part load conditions, reducing the total electricity consumption of the shopping arcade by 9.6%. If the average technical efficiency in data envelopment analysis was improved to 1 from 0.6 in the existing case, the average system coefficient of performance could increase from 6.2 to 10.3. The improvement involved controlling the temperature of supply chilled water at a lower boundary curve and operating an equal percentage of capacity among chillers. It is economically viable to design an all-variable speed chiller system with oil-free chillers instead of the conventional constant speed system for a reference office building in a subtropical climate.

Fault detection and diagnosis of chiller using Bayesian network classifier with probabilistic boundary

False alarm rate (FAR) is an important evaluating indicator when chiller fault detection and diagnosis (FDD) system is applied on site, and higher FAR will not be accepted by users and manufacturers. When we use Bayesian network classifier (BNC) to operate FDD process, it is hard to keep the FAR within an acceptable range when the class node contained the normal operating condition. So a practical chiller FDD methodology is proposed. At first, a probabilistic boundary limit under the normal operating condition is integrated in the BNC to reduce the FAR in chiller FDD process. Then, we add the site information to the BNC to solve the problem that missed detection rate (MDR) rises caused by the reduction of FAR. Evaluation of the methodology was made on a 90-ton water-cooled centrifugal chiller reported in ASHRAE RP-1043. The results show that the FAR decreases from 22.7% to 4.7% after integrating the probabilistic boundary, and the MDR of refrigerant overcharge and refrigerant leakage decreases from 27.6% and 9.4–7.6% and 4.5% respectively after adding the site information.

Empirical modeling of the impacts of faults on water-cooled chiller power consumption for use in building simulation programs

Empirical models of four chiller faults that can be applied within existing building models to study overall impacts are developed in this paper. The faults include overcharging, excess oil, non-condensables in refrigerant and water-side condenser fouling. A single generalized model structured was developed for these faults that forces predicted fault impacts to be zero with no fault and increase with increasing fault level. The models were trained and tested using available laboratory data for a water-cooled centrifugal chiller where all four faults were artificially introduced. The fault model behavior was studied and then they were integrated in hospital models from DOE commercial reference building models (Deru et al., 2011) and simulations were performed in different climates. The simulation results showed maximum increases of building electricity consumption, electricity peak demand and water consumption of the hospitals due to faults of 4.7%, 7.8% and 1.8% respectively. The fault impacts were found to be more severe in hotter and more humid climates.

Performance Evaluation of Oil-free Chillers for Building Energy Performance Improvement

This paper investigates how to improve the energy performance of central air-conditioning systems by using oil-free chillers. Three conventional water-cooled centrifugal chillers of equal capacity were replaced by three oil-free chillers of the same capacity in an existing system. Operating data for performance evaluation were logged at 1-hr intervals for over one year before and after the replacement. The superior coefficient of performance (COP) of oil-free chillers with variable speed control brought an energy saving of 9.6% in the total electricity consumption of a shopping arcade when they operated for a wide range of system cooling demands. Results of data envelopment analysis gave an average technical efficiency of 0.6 which ascertains an opportunity for performance enhancement. The system COP can be further improved by tightening the control in the temperature of supply chilled water and implementing even load sharing among chillers to meet the overall system capacity.

An intelligent chiller fault detection and diagnosis methodology using Bayesian belief network

A generic intelligent fault detection and diagnosis (FDD) strategy is proposed in this study to simulate the actual diagnostic thinking of chiller experts. A three-layer Diagnostic Bayesian Network (DBN) is developed to diagnose chiller faults based on the Bayesian Belief Network (BBN) theory. The structure of the DBN is a graphical and qualitative illustration of the intrinsic causal relationships among causal factors in Layer 1, faults in Layer 2 and fault symptoms in Layer 3. The parameters of the DBN represent the quantitative probabilistic relationships among the three layers. To diagnose chiller faults, posterior probabilities of the faults under observed evidences are calculated based on the probability analysis and the graph theory. Compared with other FDD strategies, the proposed strategy can make use of more useful information of the chiller concerned and expert knowledge. It is effective and efficient in diagnosing faults based on uncertain, incomplete and conflicting information. Evaluation of the strategy was made on a 90-ton water-cooled centrifugal chiller reported in ASHRAE RP-1043.

Consistent initialization of system of differential-algebraic equations for dynamic simulation of centrifugal chillers

For simulation of centrifugal chiller system with differential algebraic equations, consistent initial conditions are difficult to obtain due to the two-phase refrigerant cycle and the connections between several components. Existing rigorous initialization methods are not suitable for the particular problems in centrifugal chiller simulation, such as the surge instability. In this paper, the dynamic model of a water-cooled centrifugal chiller is developed in Modelica with Dymola and the TLK/IfT Library (TIL). A preprocessing scheme and a direct initialization method are proposed for the consistent initialization of chiller DAE system. Simulation results have shown the effectiveness of the proposed modelling framework and the initialization method.

Energy saving potential for a high-rise office building

Within the context of this study, a walk-through energy audit was performed for a typical office building in Hong Kong. The focus of energy saving opportunities was mainly on mechanical ventilation and air conditioning (MVAC) and lighting. The emphasis was mainly on more energy-efficient equipment, a better control of the equipment, indoor air quality and human behaviour. Among others, the measures considered to reduce MVAC energy were replacing the air-cooled reciprocation chillers with water-cooled centrifugal chillers; installing variable speed drives (VSDs) for the secondary chilled water pump motors; maintaining the indoor air quality within reasonable limits while monitoring the indoor CO2 concentration to adjust the fan speed of primary air-handling units (PAUs) by VSDs and re-adjusting the temperature setpoint to use less energy for air conditioning; and encouraging environmentally friendly human behaviour within the built environment, especially in hot and humid countries such as Hong Kong. The measures to reduce lighting energy were mainly replacing the lamps with energy-efficient lamps and preventing over-illumination. The results led to the conclusion that an annual saving of around 48% of MVAC and 62% of lighting energy is possible for this building. The measures discussed are also applicable to other similar office buildings in Hong Kong or similar areas, especially to old buildings. Considering that a tremendous amount of electrical energy is consumed by office and other commercial buildings, any measures to reduce the energy use in these sectors will contribute significantly to the reduction of CO2 emissions and preservation of the environment.

Sensor-fault detection, diagnosis and estimation for centrifugal chiller systems using principal-component analysis method

An online strategy is developed to detect, diagnose and validate sensor faults in centrifugal chillers. Considering thermophysical characteristics of the water-cooled centrifugal chillers, a dozen sensors of great concern in the chiller-system monitoring and controls were assigned into two models based on principal-component analysis. Each of the two models can group a set of correlated variables and capture the systematic trends of the chillers. The Q-statistic and Q-contribution plot were used to detect and diagnose the sensor faults, respectively. In addition, an approach based on the minimization of squared prediction error of reconstructed vector of variables was used to reconstruct the identified faulty-sensors, i.e., estimate their bias magnitudes. The sensor-fault detection, diagnosis and estimation strategy was validated using an existing building chiller plant while various sensor faults were introduced.