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How varying condenser coolant flow rate affects chiller performance: Thermodynamic modeling and experimental confirmation
Abstract
Although most commercial chillers are designed and installed to operate at constant coolant flow
rates, one can alter the power consumption of their compressors by modifying the design to incorporate
variable coolant flow rates. The issue addressed here is how to model that explicit flow-rate dependence
within an analytic chiller model. We expand our earlier thermodynamic model to explicitly account for
the influence of condenser coolant flow rate, and validate model predictions against an extensive set of
experimental measurements from a large commercial centrifugal chiller.
... Gordon et al. [20] highlighted that the condenser water flow rate could be a control variable in improving the energy performance of chiller systems. They established an analytic semi-empirical chiller model to study variations of chiller COP (coefficient of performance) at different condenser water flow rates. ...
... One major task along with the model development was to ascertain which modelling approach would provide the most accurate simulation results with regard to the validation tests using manufacturer-supplied data and field-monitored data. Compared with other three public domain models -DOE-2 model [25], Gordon-Ng model [20] and ASHRAE Primary toolkit model [26], the modified model provided more accurate prediction over the chiller power, especially under low load conditions with variable condenser water flow or variable speed drives. The improvement in the accuracy of the modified model is due to the inclusion of the condenser water temperature along with the chiller part load ratio in the EIRFPLR function which expresses chiller efficiency under part load conditions. ...
... Assuming that the motor's heat dissipation (Q m ) accounts for 5% of E cc as given by Eq. (19), the degree of superheat (T evsh ) can then be calculated by using Eq. (20), ...
This study investigates how the environmental performance of
water-cooled chiller systems can be optimized by applying load-based
speed control to all the system components. New chiller and cooling
tower models were developed using a transient systems simulation program
called TRNSYS 15 in order to assess the electricity and water
consumption of a chiller plant operating for a building cooling load
profile. The chiller model was calibrated using manufacturer's
performance data and used to analyze the coefficient of performance when
the design and control of chiller components are changed. The
NTU-effectiveness approach was used for the cooling tower model to
consider the heat transfer effectiveness at various air-to-water flow
ratios and to identify the makeup water rate. Applying load-based speed
control to the cooling tower fans and pumps could save an annual plant
operating cost by around 15% relative to an equivalent system with
constant speed configurations.
... Gordon et al. [20] highlighted that the condenser water flow rate could be a control variable in improving the energy performance of chiller systems. They established an analytic semi-empirical chiller model to study variations of chiller COP (coefficient of performance) at different condenser water flow rates. ...
... One major task along with the model development was to ascertain which modelling approach would provide the most accurate simulation results with regard to the validation tests using manufacturer-supplied data and field-monitored data. Compared with other three public domain models -DOE-2 model [25], Gordon-Ng model [20] and ASHRAE Primary toolkit model [26], the modified model provided more accurate prediction over the chiller power, especially under low load conditions with variable condenser water flow or variable speed drives. The improvement in the accuracy of the modified model is due to the inclusion of the condenser water temperature along with the chiller part load ratio in the EIRFPLR function which expresses chiller efficiency under part load conditions. ...
... Assuming that the motor's heat dissipation (Q m ) accounts for 5% of E cc as given by Eq. (19), the degree of superheat (T evsh ) can then be calculated by using Eq. (20), ...
This study investigates the energy performance of chiller and cooling tower systems integrated with variable condenser water flow and optimal speed control for tower fans and condenser water pumps. Thermodynamic-behaviour chiller and cooling tower models were developed to assess how different control methods of cooling towers and condenser water pumps influence the trade-off between the chiller power, pump power, fan power and water consumption under various operating conditions. Load-based speed control is introduced for the tower fans and condenser water pumps to achieve optimum system performance. With regard to an example chiller system serving an office building, the optimal control coupled with variable condenser water flow could reduce the annual system electricity use by 5.3% and operating cost by 4.9% relative to the equivalent system using constant speed fans and pumps with a fixed set point for cooling water temperature control.
... For grey box applications of components in heating systems, see e.g. (Jonsson and Palsson, 1994;Weyer et al., 2000) for heat exchanger modelling, (Madsen and Holst, 1995) for modelling building heat dynamics, and (Gordon et al., 2000) for modelling of air conditioners. ...
... A detailed model is the Dahl model. It is a commonly used model for describing forces arising from friction F h and described in (Dahl, 1968). The model states the relationship between the stretch and strain of a given material. ...
This paper presents a model of a thermostatic valve based on first principles and where the hysteresis effect is modelled using an adaptive model for friction compensation. The grey box modelling approach is applied, i.e. both physical interpretation and statistical methods are used to build and validate the suggested model. The model performance is illustrated using empirical data and issues concerning the modelling of hysteresis in valves are discussed.
... For grey box applications of components in heating systems, see e.g. Jonsson and Palsson (1994); Weyer et al. (2000) for heat exchanger modeling, Madsen and Holst (1995) for modeling building heat dynamics, and Gordon et al. (2000) for modeling of air conditioners. ...
... The model has been extended to apply to the case of variable condenser flow rate in Gordon et al. (2000). Though the final functional form is non-linear, the parameter estimation can be done using methods such as OLS and EIV. ...
... Gordon et al. [7] and Hydeman et al. [8] developed chiller models to study variations of chiller COP (coefficient of performance) at different condenser water flow rates. No analysis was A water-cooled chiller system in an air-conditioned hotel can take up about one-quarter of the total electricity consumption and considerable amounts of water in the heat rejection process. ...
... Graves [11] reformulated the Gordon-Ng model [7] in order to optimize a system designed with two chiller-pump pairs and one cooling tower. The chiller model was coupled with an Ntueffectiveness (Ntu-no. of transfer units) model [12] for evaluating the cooling tower performance. ...
A water-cooled chiller system in an air-conditioned hotel can take up about one-quarter of the total electricity consumption and considerable amounts of water in the heat rejection process. This paper evaluates operating cost savings of a chiller system integrated with optimal control of cooling towers and condenser water pumps. A sophisticated chiller system model was used to ascertain how different control methods influence the annual electricity and water consumption of chillers operating for the cooling load profile of a reference hotel. It is estimated that applying load-based speed control to the cooling tower fans and condenser water pumps could reduce the annual system electricity use by 8.6% and operating cost by 9.9% relative to the equivalent system using constant speed fans and pumps with a fixed set point of 29.4 °C for cooling water temperature control. The ways to implement this advanced control for system optimization are discussed.
... Air conditioning systems are a relatively large part of industrial energy consumption [1][2]. Its energy consumption mainly includes electricity for production equipment, lighting system, wind pressure system, air conditioning system and living areas [3][4]. According to statistics, the energy consumption of large central air conditioning systems accounts for about 60% of the energy consumption of the whole building, with an annual growth rate of 5% to 6% [5][6]. ...
This paper mainly studies the energy consumption of air conditioning and refrigeration system from the perspective of the dynamic operation of air conditioning systems and selects the energy consumption of a chiller system, cooling tower system, chilled water pump system and fan coil system, which affect the energy consumption of air conditioning system, as model variables. An integrated learning method based on a multilayer perceptron is proposed to integrate spatial features with temporal features, and a spatio-temporal prediction model for air conditioning energy consumption analysis is established under the Seq2Seq framework, and then the input curves of the four variables with load change under the optimal operation of the air conditioning system under dynamic load are derived. The results show that the deviation between the simulated value and the measured, calculated value of the energy consumption analysis model of air conditioning and refrigeration equipment based on the multilayer perceptron is within 5%, which can effectively and accurately predict and reduce air conditioning energy consumption by 15% to 20%. The maximum energy-saving efficiency can reach 30% when the air conditioning system is in optimal operation under dynamic load, and the energy-saving effect of the chiller, chilled water pump and cooling water pump is relatively significant. The research results of this paper have a certain reference value for reducing the energy consumption of air conditioning systems in practice.
... The introduced method is applied for a single room as well as on a central office level. A lumped capacity model of the room developed by the authors was applied and run in parallel to the real system, in order to determine reference values resulting Among others, Gordon et al. investigated the behavior of a commercial R123 centrifugal chiller by varying the coolant flow on the secondary side of the condenser[30]. They determined, that an augmentation in coolant flow rate reduces the thermal lift in the compressor and therefore the power consumption decreases. ...
A significant amount of energy consumption in buildings is due to heating, ventilation and air-conditioning systems. Among other systems, refrigeration plants are subject of efficiency improvements. However, actual operating conditions of such plants and the performance must be known as well as any eventual optimization potential identified before enhancements can take place. Energy and exergy analyses have been widely used to assess the performance of refrigeration systems. Among others, exergy efficiency is used as an indicator to determine the system performance; however, the practical achievable values are unknown. Therefore, this work proposes a practice-oriented evaluation method for refrigeration plants, based on exergy analysis and technical standards as baseline. The identification of possible enhancements is highly relevant in practice, as measures which improve the system effectiveness most likely prevent frequent shortcomings during refrigeration plant operation. With the introduced optimization potential index (OPI), the achievable enhancements compared to the state of the art in technology and the performance are identified at a glance regardless the complexity of the system. By dividing the plant into different subsystems, each of them can be assessed individually. Laypersons can easily determine the system operating state and subsequently, if needed, initiate a detailed analysis as well as appropriate countermeasures by specialist. Moreover, modeling is seen as an appropriate method to determine additional reference values for refrigeration machines if none are available according to technical standards. Among different modeling techniques, artificial neural network models reveal the best performance for the present application. The application, functionality and purpose of the presented method is exemplified on two numerical test cases and on a real field plant as a case study. The investigation reveals an adequate operation of the studied field plant in general, where three out of seven cooling locations have performance issues. The reason should be identified in a subsequent detailed study. Overall, the auxiliary electrical exergy input shows the same magnitude as the thermal exergy input. This emphasizes the importance of minimizing the electrical energy usage, as it is the main overhead in the operating cost of refrigeration plants and also to achieve an increase in system performance. Moreover, measuring concepts of real systems are analyzed and the corresponding retrofitting costs for the application of the presented approach are identified. It is shown that a retrofit of the instrumentation can be worthwhile if the refrigeration plant already comprises a measuring concept close to the state of the art.
... Wang and Burnett [34] and Sun and Reddy [35] minimized the energy consumed by a condenser water system by changing the pump frequency and optimizing the number of cooling towers. Gordon et al. [36] took the condenser water outlet temperature as a control variable, thus reducing the energy consumed by the condenser water system by running it at partial load rate. The interaction between the power consumed by the condenser water pumps, chilled pumps, and chillers was not analyzed at the system level. ...
Cooling load prediction is a critically important technology for ensuring energy conservation in cooling systems. Examples include advance air conditioning control, chiller startup, and controlling the number of chilled water pumps in operation. Many existing load prediction methods are normally arranged into three categories: energy simulation, artificial intelligence, and regression analysis. In contrast to the first two categories, regression analysis is practical and easy to implement in real applications. However, cooling load data always exhibits nonlinear, dynamic features, thus it is very difficult to accurately predict cooling loads with linear regression models. Thus, a multiple nonlinear regression (MNR) model for cooling systems in public buildings was used to accurately predict short term cooling loads in this study. The key variables in the MNR model are selected based on sensitivity analysis, and calibration methods can be used to enhance the prediction accuracy. The prediction process can be divided into three steps. The first step requires determining the weight coefficients on the input variables based on historical load and weather data. Second, the initially predicted load is obtained using the weight coefficients and predicted input variables. Finally, the initially predicted results were calibrated using relative calibration methods. The initial load prediction and final calibration are based on data from the same day, which is taken as a reference. The performance of the established MNR model is validated against a real case in Guangzhou using measured cooling load data and weather data. A case study shows that the accuracy of the final calibrated load is higher than the initially predicted load. The MNR model also produces more accurate results compared with three traditional regression models. The accurate cooling load prediction method can be used as an operation control strategy in real applications.
... A vapour compression refrigeration cycle essentially consists of an evaporator, a compressor, a condenser and an expansion device to pump the heat from a low-temperature reservoir to a higher one using electrical work input [7][8][9]. In air-conditioning application, the space to be cooled is the low-temperature reservoir whilst the ambient is the higher temperature reservoir [10,11]. The refrigerants employed in the MVC systems have evolved from CFCs and HCFCs to environmental-friendly versions such as CO 2 , Hydro-Fluoro-Olefin 1234yf etc. [12][13][14][15]. ...
The performance of a Mechanical Vapour Compression (MVC) chiller is experimentally investigated under operating conditions suitable for sensible cooling. With the emergence of the energy efficient dehumidification systems, it is possible to decouple the latent load from the MVC chillers which can be operated at higher chilled water temperature for handling sensible cooling load. In this article, the performance of the chiller is evaluated at the elevated chilled water outlet temperatures (7 – 17° C) at various coolant temperatures (28 – 32° C) and flow rates (ΔT = 4 and 5° C) for both full- and part-load conditions. Keeping the performance at the AHRI standard as the baseline condition, the efficacy of the chiller in terms of compression ratio, cooling capacity and COP at aforementioned conditions is quantified experimentally. It is observed that for each one-degree Celsius increase in the chilled water temperature, the COP of the chiller improves by about 3.5% whilst the cooling capacity improvement is about 4%. For operation at 17° C chilled water outlet temperature, the improvements in COP and cooling capacity are between 37 – 40% and 40 – 45%, respectively, compared to the performance at the AHRI standards. The performance of the MVC chiller at the abovementioned operation conditions is mapped on the chiller performance characteristic chart.
... On donne à titre d'exemple, une expression de F en évaporation [Gungor, 1987] : ...
In Europe, the energy efficiency of chillers is rated at full load for standard conditions of operation. In order to develop a European performance index that integrates the variation of operating conditions, the part load performances of chillers must be known, as well as the conditions for which chillers operate during a season. Different chillers have been tested in stationary conditions. It appears that the nowadays lack of characterization of chillers performances at reduced capacity leads to manufacturing mistakes that decrease the energy efficiency of chillers. Performances of chillers at reduced temperature and reduced capacity are modelled for the chillers that have been tested. A test loop to rate the chillers performances in dynamic conditions has been built. This loop enables to study chillers performances when cycling (on-off mode). It appears that the major efficiency loss linked to this functioning mode comes from the parasitic sleep power electric consumption. Moreover, the facility has been used also for capacity staged chillers. Control faults have been identified that also contribute to decrease the chillers performances. By coupling to the models of chillers developed, hourly load curves of dynamic building and air conditioning system simulations for an office building, seasonal performances are calculated for various climates and systems. The results emphasizes that nominal rating performance should not be used as an efficiency indicator for chillers. A method of reduction of the hourly load curves has been developed. It enables to define a seasonal performance index for European chillers.
... Refrigeration works, such as J.M. Gordon [161], provide understanding of system responses to variations in condenser conditions. Very little such experimental work can be found using ORC's though the assumption could be made that it would be highly interesting. ...
A bibliography on the topic of power cycles using low temperature differences between heat source and heat sink, including expansion devices suitable.
... To protect the environment, R123 is the recommended refrigerant to replace R11 [6,7], R134a is the suggested refrigerant to substitute R12 or R500 [6,7] and R23ea is the proposed refrigerant to replace R114 [8] in water cooled chillers while R22 will be in use till 2015 [5]. Various simulation models of vapor compression liquid chillers are developed [9][10][11][12] and experimental investigations on watercooled chillers are reported in the literature [13][14][15][16]. ...
This paper reports the results of an experimental investigation on a pilot compression chiller (4 kW cooling capacity) working with R401a and R134a as R12 alternatives. Experiments are conducted on a single-stage vapor compression refrigeration system using water as a secondary working fluid through both evaporator and condenser. Influences of cooling water mass flow rate (170–1900 kg/h), cooling water inlet temperature (27–43°C) and chilled water mass flow rate (240–1150 kg/h) on performance characteristics of chillers are evaluated for R401a, R134a and R12. Increasing cooling water mass flow rate or decreasing its inlet temperature causes the operating pressures and electric input power to reduce while the cooling capacity and coefficient of performance (COP) to increase. Pressure ratio is inversely proportional while actual loads and COP are directly proportional to chilled water mass flow rate. The effect of cooling water inlet temperature, on the system performance, is more significant than the effects of cooling and chilled water mass flow rates. Comparison between R12, R134a and R401a under identical operating conditions revealed that R401a can be used as a drop-in refrigerant to replace R12 in water-cooled chillers.
... Based on simulation results of a chiller system, they demonstrated that the speed of the condenser pumps and tower fans should be adjusted in response to the chiller load in order to achieve optimum system performance. Gordon et al. [7] established an analytic semi-empirical chiller model to study variations of chiller coefficient of performance (COP) at different condenser water flow rates. The COP is defined as the cooling capacity output in kW over the total electric power in kW of the system components. ...
Chiller systems are commonly used to provide cooling energy in commercial buildings but with considerable electricity and water consumption. This paper discusses the environmental benefits of a sustainable chiller system with full variable speed control under climate change. Hourly weather data of climate change scenarios in 2020, 2050 and 2080 were predicted by using the program CCWorldWeatherGen, based on a typical meteorological year weather file for Hong Kong. The building simulation program EnergyPlus was used to model a full variable speed chiller system and a conventional system operating for a reference office building. Simulation results show that the environmental performance of the sustainable system is superior to the conventional ones with annual electricity saving of 25.7–31.5%, water saving of 1.8–2.2% and carbon emissions reduction of 25.6–30.3% under the three climate change scenarios. The sustainable system would bring an improvement in the annual average coefficient of performance even under global warming. The significance of this study is to promote the use of full variable speed control in chiller systems and elaborate on techniques in designing and operating the systems with enhanced environmental performance.
... The tendency of the COP to increase at decreased evaporator water flow rates and to decrease at decreased condenser water flow rates has been reported [11,13,14]. These trends will be evaluated in the context of this energy saving intervention. ...
Cooling systems consume up to 25% of the total electricity used on deep level mines. These systems are integrated with the water reticulation system to provide chilled service water to the mine as well as cooling for mine ventilation air. Although there is definite potential for demand-side management on these systems, it is critical that the service delivery be maintained so as not to adversely affect productivity. An energy saving strategy based on variable water flow was developed for the unique demands of integrated mine cooling systems. The strategy is based on matching the evaporators with the demand of chilled water; condensers adapting to the heat load; and the bulk air cooler matching the demand of ventilation air requirements. In this paper, a case study is presented in which the savings and consequences of implementing the developed energy saving strategy are investigated. It is shown that a decrease of 31.5% in overall electrical energy usage is possible without affecting the service delivery or performance of the cooling system.
... For grey box applications of components in heating systems, see e.g. (Jonsson and Palsson, 1994; Weyer et al., 2000) for heat exchanger modelling, (Madsen and Holst, 1995) for modelling building heat dynamics, and (Gordon et al., 2000) for modelling of air conditioners. ...
This paper presents a model of a thermostatic valve based on first principles and where the hysteresis effect is modelled using an adaptive model for friction compensation. The grey box modelling approach is applied, i.e. both physical interpretation and statistical methods are used to build and validate the suggested model. The model performance is illustrated using empirical data and issues concerning the modelling of hysteresis in valves are discussed.
... The indirect measurement can be written as 1= I where/denotes the chiller model. Many models in different forms are available for such applieation (Wang et al. 2000;Gordon et al. 2000), Here, a simple inverse chiller model is adopted (Wangetal. 2000); ...
This paper presents a general framework of utilizing a fused measurement of the building instantaneous cooling load to improve the reliability of the chiller sequencing control in building automation systems. The fused measurement is obtained by combining the complementary advantages of two different approaches to measuring the building cooling load. One approach is the direct measurement, which calculates the building cooling load directly, using the differential water temperature and water flow rate measurements. The other is the indirect measurement, which calculates building cooling load based on chiller models using the instantaneous chiller electrical power input, etc. The combination strategy is tested using the field data collected from the central plant of the air-conditioning system in a high-rise building in Hong Kong. The confidence degree associated with the fused measurement is systematically evaluated. Periodic update of the fusion algorithm parameters is also developed to improve the performance of the fusion strategy and the chiller sequencing control.
... This gives the flexibility that the condensing pressure does not have to be kept at an artificially high level at low outdoor-air temperature, thereby enabling the use of varying set-point condensing-pressure control. A varying set-point condensing pressure control, often incorporated as an optional feature, modulates the heat rejection airflow in response to a outdoor ambient temperature to improve an individual chiller's COP at low ambient-temperature conditions [11,24,25]. ...
Optimizing system configuration is always an interest of building designers. By statistical analysis of the design data of 50 commercial buildings in Hong Kong and the performance data of 186 chiller models, it is ascertained that the chiller capacity and the fraction of full-load capacity have little influence on the system's energy performance. Following on this, this paper presents an evaluation of the energy performance of a multiple-chiller system consisting of 2-10 equally sized chillers. Such an analysis was based on performance data from three major manufacturers. It is found that the energy efficiency of multiple-chiller system improves with a higher number of chillers, and the maximum saving is estimated to be 9.5%. Based on the results of the study, a simplified model relating energy use with number of chillers has been established. The model can help designers more quickly determine how the energy efficiency can be weighted against other factors, such as the additional plant room space and the financial implications.
... TAYLOR [4] stated that using variable speed chillers and/or variable flow, primaryonly pumping systems is a viable means to eliminate significant degradation in system performance in the part load operation while accommodating the low "delta-T" syndrome of chilled water circuits. GORDON et al [5] established an analytic semi-empirical chiller model to study variations of the coefficient of performance (COP) of the chiller at different condenser water flow rates and highlighted that the condenser water flow rate could be a control variable in improving the energy performance of chiller systems. ...
The optimum control strategy and the saving potential of all variable chiller plant under the conditions of changing building
cooling load and cooling water supply temperature were investigated. Based on a simulation model of water source chiller plant
established in dynamic transient simulation program (TRNSYS), the four-variable quadratic orthogonal regression experiments
were carried out by taking cooling load, cooling water supply temperature, cooling water flow rate and chilled water flow
rate as variables, and the fitting formulas expressing the relationships between the total energy consumption of chiller plant
with the four selected parameters was obtained. With the SAS statistical software and MATHEMATICA mathematical software, the
optimal chilled water flow rate and cooling water flow rate which result in the minimum total energy consumption were determined
under continuously varying cooling load and cooling water supply temperature. With regard to a chiller plant serving an office
building in Shanghai, the total energy consumptions under different control strategies were computed in terms of the forecasting
function of cooling load and water source temperature. The results show that applying the optimal control strategy to the
chiller plant can bring a saving of 23.27% in power compared with the corresponding conventional variable speed plant, indicating
that the optimal control strategy can improve the energy efficiency of chiller plant.
Key wordschiller plant–control strategy–variable speed–cooling water flow rate–chilled water flow rate
... Gordon et al. [7] highlighted that the condenser water flow rate could be a control variable in improving the energy performance of chiller systems. They established an analytic semi-empirical chiller model to study variations of chiller COP (coefficient of performance) at different condenser water flow rates. ...
There are increasing views on implementing all-variable speed chiller plants in place of conventional constant speed plants. Supporters of these views claim that all-variable speed chiller systems can operate much more efficiently at part load in response to changes in building cooling load. This paper introduces load-based speed control for all-variable speed plants to optimize their environmental performance. Thermodynamic-behaviour chiller system models were developed to perform environmental assessment (in terms of annual electricity and water consumption) for typical constant speed and all-variable speed chiller systems operating for the cooling load profile of a local office building. Operating cost differences between the two systems were calculated and compared in an economic analysis. Applying load-based speed control to the variable speed chiller plant can decrease the annual total electricity use by 19.7% and annual water use by 15.9% relative to the corresponding constant speed plant. The significance of this study is to provide more insights into how to make chiller systems more sustainable.
... Varying water volume through packaged chiller evaporators and condensers is an advanced technique used to improve an air-conditioning system's efficiency by reducing pump motor power. An increase in the water flow rate may improve the chiller's COP and decrease the chiller's energy consumption [22,23]. However, it inevitably increases the energy use of pumps. ...
A large cooling system of residential building in Changsha City in China was investigated in the summer of 2003. The relationships among the controlled variables, uncontrolled variables and the chillers' performance were obtained empirically with the test data. A model of optimal operation for the system was established based on these empirical relationships. A parameter, system coefficient of performance (SCOP), is presented to analyze the effect of energy saving of the cooling system. The results show that the energy saving is likely to reach as high as 10% by applying the optimal model to the cooling system.
The emergence of building condenser water systems with all-variable speed pumps and tower fans allows for increased efficiency and flexibility of chiller plants in partial load operation but also increases the control complexity of condenser water systems. This study aims to develop an integrated modeling technique for evaluating and optimizing the energy performance of such a condenser water system. The proposed system model is based on the semi-physical semi-empirical chiller, pump, and cooling tower models, with capabilities of fully considering the hydraulic and thermal interactions in the condenser water loop, being solved analytically and much faster than iterative solvers and supporting the explicit optimization of the pump and tower fan frequency. A mathematical approach, based on the system model and constrained optimization technique, is subsequently established to evaluate the energy performance of a typical dual setpoint-based variable speed strategy and find its energy-saving potential and most efficient operation by jointly optimizing pumps and tower fans. An all-variable speed chiller plant from Wuhan, China, is used for a case study to validate the system model’s accuracy and explore its applicability. The results showed that the system model can accurately simulate the condenser water system’s performance under various operating conditions. By optimizing the frequencies of pumps and tower fans, the total system energy consumption can be reduced by 12%–13% compared to the fixed dual setpoint-based strategy with range and approach setpoints of 4 °C and 2 °C. In contrast, the energy-saving potential of optimizing the cooling tower sequencing is insignificant. A simple joint speed control method for optimizing the pumps and tower fans emerged, i.e., the optimal pump and fan frequency are linearly correlated (if both are non-extremes) and depend on the chiller part load ratio only, irrespective of the ambient wet-bulb temperature and chilled water supply temperature. It was also found that the oversizing issue has further limited the energy-saving space of the studied system and results in the range and approach setpoints being inaccessible. The study’s findings can serve as references to the operation optimization of all-variable speed condenser water systems in the future.
The emergence of increasingly affordable variable-speed drive technology has changed the approach used to control chilled water systems equipped with these drives. The purpose of this research was to develop an integrated chilled water modeling technique that can determine the optimal system setpoints and estimate the energy saving potential of chiller system. The chiller system equipped with Variable Frequency Drives (VFDs) on cooling tower fans and condenser water pumps. To accomplish the objective, physical component models of the centrifugal chiller, cooling tower and condenser water pump were established with the goal of incorporating the system’s condenser water flow rate and cooling tower fan speeds as optimization variables. Furthermore, a cooling load prediction algorithm was developed using a multiple non-linear regression model to approximate the building’s cooling load subject to a range of environmental conditions. The inputs and outputs of the individual component models were linked to estimate how adjusting the cooling tower fan and condenser water pump speed would influence the system’s comprehensive performance. The overall system model was then optimized using a generalized reduced gradient optimization algorithm to determine the potential energy savings through speed control with VFDs and to ascertain a control logic strategy for the building automation system to operate the heating and cooling system. A case-study was performed on a single chiller system at a museum and the model was calibrated according to logged data collected over four months. Results showed that for the system analyzed, the energy saving of optimizing the cooling tower fan system was found to be 12–15%, while the energy saving potential of optimizing the condenser water pump with the cooling tower fan was negligible. Additionally, comparing different cooling tower fan control strategies showed that a wet-bulb approach-based cooling tower control strategy was shown to have the highest correlation to the optimized fan speed with an R² of 0.924.
The increasing demand for cooling energy, the growth rates of refrigeration systems and therising temperatures justify targeted energy efficiency measures in the field of air conditio-ning and process cooling to achieve national and international climate protection targets. Anenergy-efficient and energy-cost-saving control system for refrigeration supply systems is onemeasure for the reduction of greenhouse gas emissions.In this dissertation, the most cost-effective, energy- and climate-efficient operating modes ofrefrigeration supply technologies in a system are determined. A predictive simulation basedoptimization is used to incorporate the complexity of the interactions and boundary conditi-ons into the result. The efficiency of cooling systems depends on the part load performance ofthe chillers and their condensation temperature, which are often not measured continuously.The energy data collected remains unused and the manufacturer’s data deviates from the ac-tual performance. Due to the individual operating characteristics of the chillers, self-learningperformance models are used for the predictive optimization.For the technical verification of the predictive simulation-based optimization, a test benchwith an air-cooled and a water-cooled compression chiller and two cold water reservoirs wasinstalled in the laboratory of the University of Kassel. The laboratory results show that adirect coupling between the predictive simulation-based optimization and the machine controlfor a real refrigeration supply system is technically possible and that there is an energy sa-ving potential of almost 30 % and because of a variable energy price an additional cost savingpotential of 5 %.To determine the energy savings potential and to evaluate industrial implementation strate-gies of the predictive simulation-based optimization, the methodology is applied to a plasticsprocessing and a meat processing plant. The results for the plastics industry show that thecontrol strategy of the optimization together with the installation of a cold water storagetank with a significant volume, a free cooler and a prognosis horizon of 48 hours reducesthe electrical energy demand by more than 43 % compared to the reference case. Procuringelectricity via the power exchange leads to an increase in electrical energy demand, but atthe same time between energy costs are reduced by 9.3 % and 12 %The case study of the meat processing plant shows that the results of the optimization can beused to implement an expert control strategie. The outside temperature controlled setpointshift of the cooling water temperature and the methodology of the cooling demand ramp forthe optimal switch-on times of the compression chillers are easy to implement. The data ofthe energy monitoring system of the meat processing plant show that the use of this controlhas already led to an efficiency increase of 9.5 %.
This article reviews the current research on the development and implementation of automated fault detection and diagnostics (AFDD) technology for building systems. This article first examines the fundamentals of AFDD and its special requirements on building systems to provide a theoretical formulation of the problem. Some infrastructural barriers for scalable AFDD implementation are discussed. Then it reviews various methods used by previous researchers and real-life products in a typical AFDD workflow. The article compares different methods for feature generation and fault detection and fault diagnostics, and proposes ways to improve their current limitations from other research disciplines. The authors also discuss potential research topics to inspire further developed of new AFDD methodologies and make them more applicable, including: 1) fault assessments and improved information delivery; 2) benchmarking AFDD performance; 3) better interoperability of AFDD methods; and 4) cases of fault propagation and simultaneous faults.
This study investigates the energy performance of chiller and cooling tower systems integrated with variable-frequency control for cooling tower fans and condenser water pumps. With regard to an example chiller system serving an office building, Chiller and cooling towers models were developed to assess how different variable-frequency control methods of cooling towers fans and condenser water pumps influence the trade-off between the chiller power, pump power and fan power under various operating conditions. The matching relationship between the cooling tower fans frequency and condenser water pumps frequency at optimal energy consumption of the system is introduced to achieve optimum system performance.
This paper presents an improved ripple bee swarm optimization (IRBSO) algorithm to solve the problem of the economic dispatch of chiller plants (EDCP). Using the characteristics of biological communities, different movement models are adopted to search within the feasible solution space. This paper uses non-linear ripple weight factors and self-adaption repulsion factor to improve the BSO and proposes the influence of parameters on the IRBSO method to more effectively search the feasible space. For all bee swarms, the efficiency of searching movement in the solution space improves, and the capacity of information discovery and mining increases. This paper utilizes the test cases to verify the proposed IRBSO, including EDCP problems for a single day and a single week. Compared with other methods, the results of the proposed IRBSO exhibit higher accuracy and stability, making it suitable for the operation planning of multiple chiller systems.
The energy saving control target's hourly coefficient of system performance and hourly optimization strategy were proposed according to the loading characteristics of subway ventilation and air conditioning (SVAC) system. The strategy was used to simulate a new subway station and to analyze the energy saving effect of the optimization scheme. Results show that in a typical summer day, the amount of energy consumption in this scheme decreases by 9.4% compared with the normal scheme, so the optimization scheme has an obvious effect on the saving of energy.
In typical commercial buildings, water-cooled chilled water plants use a significant amount of energy. They account for between 10% and 20% of the overall facilities usage and serve roughly one third of the commercial floor space. From literature and experience the annual average "wire-to-water" efficiency* of these plants ranges from as low as 0.3 kW/ton (12 COP) up to 1.2 kW/ton (2.9 COP) or more. This efficiency is strongly influenced by three factors: the equipment efficiency (chillers, towers and pumps); the system configuration (e.g., the pumping scheme and types of control valves); and control sequences (e.g., equipment staging and setpoints). This article presents a parametric analysis technique to optimize the control sequences of chilled water plants that has been successfully applied to dozens of projects. This optimization technique is based on peer reviewed research and validated through measurement and verification techniques. Unfortunately, the techniques used are not presently available in standard off-the-shelf simulation tools.
The operating stability of centralized air supplying and dust removing system (CASDRS) is the guarantee of efficiency and the quality of the cigarette making process. The pressure changing in the network, the value of the flow rate of the branch and especially the system resistance balance are the main evaluate indexes of the system. In this paper, a simulation model of CASDRS is built, as an evaluate tool of the system to analyze stability and energy saving potential. The simulation toolbox MATLAB/simulink is employed to analysis the resistance balance of the system, a pipe-net model is developed to the ventilation system. Through the connection of the blocks in the simulink toolbox, the operation of the matrixes which including constraining parameter input matrixes, the connection of the matrix and the calculation of the element of the matrix and the matrix itself, the system simulation model is obtained. The sensitivity result is given as the input parameters varied hourly and the result about energy consumption is more accurate.
eGovernment deploys governmental information and services for citizens and general society. As the Internet is being used as underlying platform for information exchange, these services are exposed to data tampering and unauthorised access as main threats against citizen privacy. These issues have been usually tackled by applying controls at application level, making authentication stronger and protecting credentials in transit using digital certificates. However, these efforts to enhance security on governmental web sites have been only focused on what malicious users can do from the outside, and not in what insiders can do to alter data straight on the databases. In fact, the lack of security controls at back-end level hinders every effort to find evidence and investigate events related to credential misuse and data tampering. Moreover, even though attackers can be found and prosecuted, there is no evidence and audit trails on the databases to link illegal activities with identities. In this article, a Salting-Based Authentication Module and a Database Intrusion Detection Module are proposed as enhancements to eGovernment security to provide better authentication and auditing controls.
Utility plants are widely used to meet cooling, heating, and, in many cases, electricity needs of large manufacturing plants and commercial facilities. Efforts to increase energy efficiency and reduce environmental footprint of these facilities have gained significant traction in recent years. In this paper, we present an optimization problem derived by employing accurate models for each process unit and simulation studies that underscore some key challenges in optimal operation of utility plants. We also demonstrate the value of a model-based approach to systematic optimization of the utility plant operation. We show that even for relatively simple systems the decision for optimal operation is nonintuitive.
Ventilation and air-conditioning system (VAC) is the most energy-saving potential system in the metro. This paper analyzes the passenger traffic, air-conditioning load and station air supply on the initial, recent and long-term phase of metro station. And it proposes that it is necessary to run chilled-water pumps, air handing unit (AHU) fans and back/exhaust fans with frequency conversion technology (FCT). Then it uses the thermodynamic method to analyze the impact of running chilled-water pumps with FCT. The results show that running chilled-water pumps with FCT can reduce the total power consumption of system, although increases chiller energy consumption. Then the temperature and velocity fields of the platform and station hall are simulated by CFD software according to the variable air volume. And the results show that under the condition of running the VAC system with FCT, temperature and velocity fields distribution are both in the comfortable range. Finally, by taking a typical summer day for example, this paper analyzes the energy savings of chilled-water pumps, AHU fans and back/exhaust fans on the initial, recent and long-term phase, and the calculation results show that the respective total energy savings are 1103.4 kWh, 1064.3 kWh and 926.2 kWh, and the respective total power saving ratio is 73.4%, 71.2% and 59.5%.
Selecting the model is an important and essential step in model based fault detection and diagnosis (FDD). Several factors must be considered in model evaluation, including accuracy, training data requirements, calibration effort, generality, and computational requirements. All modeling approaches fall somewhere between pure first-principles models, and empirical models. The objective of this study was to evaluate different modeling approaches for their applicability to model based FDD of vapor compression air conditioning units, which are commonly known as chillers. Three different models were studied: two are based on first-principles and the third is empirical in nature. The first-principles models are the Gordon and Ng Universal Chiller model (2nd generation), and a modified version of the ASHRAE Primary Toolkit model, which are both based on first principles. The DOE-2 chiller model as implemented in CoolTools{trademark} was selected for the empirical category. The models were compared in terms of their ability to reproduce the observed performance of an older chiller operating in a commercial building, and a newer chiller in a laboratory. The DOE-2 and Gordon-Ng models were calibrated by linear regression, while a direct-search method was used to calibrate the Toolkit model. The ''CoolTools'' package contains a library of calibrated DOE-2 curves for a variety of different chillers, and was used to calibrate the building chiller to the DOE-2 model. All three models displayed similar levels of accuracy. Of the first principles models, the Gordon-Ng model has the advantage of being linear in the parameters, which allows more robust parameter estimation methods to be used and facilitates estimation of the uncertainty in the parameter values. The ASHRAE Toolkit Model may have advantages when refrigerant temperature measurements are also available. The DOE-2 model can be expected to have advantages when very limited data are available to calibrate the model, as long as one of the previously identified models in the CoolTools library matches the performance of the chiller in question.
Energy saving is one of the most important issues in high-tech manufacturing industries, such as semiconductor and electronics, because large chilled water systems are used to satisfy big cooling load requirements. In this paper, a new optimal integrity scheme based on a two-stage strategy, including a scheduling stage and an operating stage, is proposed to minimize the system energy consumption within a future time period. Instead of a lag scheme used in the general method, a forecasting scheme consisting of a series of optimal schemes at each sub-time period is also proposed for the two-stage design. The performance of the proposed method is examined through an industrial case. The cost of the proposed method is much less than that of the conventional method, so the proposed method is cost-efficient in applications of large air-conditioning systems.
An inverse cycle machine is a system that needs a given form of energy, to transfer thermal energy from a source at low temperature
side, to a sink at high temperature side.
Classical linear regression analysis by the method of Ordinary Least Squares (OLS) considers errors to affect the dependent variable only, while it is implicitly assumed that the independent or regressor variables are free of error. This assumption is frequently unrealistic, because measurement inaccuracies in the regressor variables are sometimes very large. If these variables are correlated, which is often the case, model coefficients identified by OLS will be biased. This issue may be of secondary concern if the model is used for predictive purposes only, but it is problematic if the model coefficients are to be interpreted as physical quantities and used for diagnostic purposes. A modeling approach that can overcome this deficiency is the Error-InVariable (EIV) regression approach, which can provide unbiased parameter estimates even in the presence of errors in the regressor variables (under the assumption that the errors are unbiased and normally distributed) and when the regressor variables are correlated. This paper describes this method, and, based on precise chiller performance data measured in a laboratory, illustrates the benefit and advantage of the EIV method over the OLS method in the framework of the Gordon and Ng (GN) chiller model. It is found that biased parameter estimates are very likely to occur when OLS is used for identification using the GN model even when well-maintained field instrumentation is used. This bias in parameter estimates can be minimized or even eliminated when the EIV method is used instead.
A novel method was put forward for improving the energy efficiency of air-cooled water chiller plant operating on part load conditions. The conventional multiple-chiller plant was proposed to be integrated into one refrigeration cycle, by connecting those separate compressors, condensers and evaporators in parallel, respectively. The integrated multiple-chiller plant uses the electronic expansion valve to control refrigerant flow, achieving variable condensing temperature control. A prototype composed of four reciprocating compressors (including one variable-speed compressor), with total nominal cooling capacity of 120 kW was simulated and experimented. Both the simulative and experimental results indicated that applying this novel energy-saving method, the air-cooled chiller plant could get a significant performance improvement on various part load ratio (PLR) conditions, due to the apparent decrease of condensing temperature and some increase of evaporating temperature. Under the same outdoor temperature of 35 °C, when the PLR decreased from 100% to 50%, the COP increased by about 16.2% in simulation and 9.5% in experiment. Also, the practical refrigeration output ratio of the system was 55% on the condition of 50% PLR.
A review of reverse cycle machines thermodynamics is proposed. It is shown how starting from equilibrium thermodynamics at the beginning, a new appraisal was proposed since 1980, and developed until now. Particular emphasis is put on vapour compression reverse machines (mainly refrigerating machines and heat pumps), but also on three or quadrithermal configurations, that appear as the most promising alternative to vapour compression machines today. However other machines alternatives are considered shortly. The main features of this review are: the usefulness of thermodynamics to develop more significant upperbounds for various optimization criterions concerning reverse cycle machines and their possible interrelation. The importance of irreversibilities in characterization of reverse cycle machines is enlighten too. Complementary studies are proposed in the field of exergoeconomy and environmental concern.
The study presents modelling and analysis of air-cooled chiller system in an office building at Central Queensland University in Rockhampton, Australia. EnergyPlus, building energy simulation software, has been used to model and to simulate the energy savings. Base case cooling energy has been compared with measured data. The simulated results show a reasonable agreement with the measured data. As a passive cooling means, the effect of economiser usages and pre-cooling have been simulated and analysed to assess annual demand savings for an energy intensive office building at Rockhampton, Australia. It was found that implementation of the pre-cooling and economiser system could save 115 kW/m2/month and 72 kW/m2/month total cooling energy and 26 kW/m2/month and 42 kW/m2/month chiller energy, respectively.
This paper investigates the robustness of a diagnostic model for the performance evaluation
of reciprocating chillers. The model, which is described by three adjustable parameters of the
chiller, is verified by experimental results with three purpose-built system configurations. It is found
that the model is both flexible and accurate with respect to changes in the system layout and the
predictions of chiller coefficients of performance (COP) are well within the experimental uncertainties.
We develop a simple analytic diagnostic model for reciprocating chillers. With only a handful of non-intrusive, in situ measurements, one can then ascertain quantitatively how chiller performance changes with time or after a prescribed modification. We derive how reciprocating chillers can be characterized by just three parameters with clear physical significance. We then verify the correspondence between theory and reality with detailed experimental measurements. It is also demonstrated how this model can be used to establish optimal operating conditions for reciprocating chillers, and to evaluate potential improvements that stem from changes in operating conditions or the distribution of heat exchanger inventory. Again, comparisons with actual performance data from commercial chillers are provided. We give quantitative expression to the contribution to chiller efficiency of internal dissipation from compression and throttling, and consequently reinforce the fact that endoreversible chiller models are far off the mark.
The efficiency of chillers (refrigeration and heat pump devices) is limited by the dissipation from their principal components: compressor, throttler, and heat exchangers at the condenser and evaporator. Developing a generalized finite-time thermodynamics model for reciprocating chillers, we derive analytic formulae for how the fixed finite resources of cycle time and heat exchanger inventory should be allocated so as to optimize chiller performance. Our predictions for optimal operating schemes are compared with detailed experimental data from two different commercial chillers. The agreement between theory and actual performance data attests to the empirical wisdom that has evolved in chiller manufacture. Besides quantitatively documenting the individual sources of irreversibility, we show how the limitations of currently-available chiller components affect optimal chiller design, as well as how potential steps to improve chiller efficiency can be evaluated within a universal thermodynamic framework.
From detailed experimental measurements on commercial reciprocating chillers, the loss mechanisms that dominate chiller performance can be identified, quantified and incorporated into a general irreversible thermodynamic model for predicting chiller behavior. The data can also be used to demonstrate the weaknesses and inaccuracies of a host of endoreversible chiller models that have been presented, where the primary sources of internal dissipation have been ignored. We quantitatively establish the dominant contributions to chiller performance of internal irreversibilities, such as fluid friction in the compressor and evaporator, and of finite-rate heat transfer at the heat exchangers. Heat leaks are measured experimentally and shown to be close to negligible. The empirical wisdom that has evolved in the commercial production of reciprocating chillers, namely, that rated capacity operation corresponds to near maximum efficiency, is explained in terms of a general thermodynamic model. Taking account of constraints of heat exchanger size and cost, we use experimental data to show that simple thermodynamic modeling can account for the optimal designs that are produced by the chiller industry.
- S Salinas
S. Salinas, private communication, Supersymmetry Services Pte Ltd, Singapore, 1997.