Secondary Chilled Water Research Articles

Systematic analysis method of sensor and actuator faults propagation and impacts for optimally controlled complex building chilled water systems

The effectiveness of the commonly used optimal control strategy of a secondary chilled water system usually suffers from the health conditions of numerous sensors and actuators involved. This paper presents a systematic analysis method of sensor and actuator faults propagation and impacts for optimally controlled complex building chilled water systems. Through fault modelling and fault injection, the proposed method aims to analyze the sensor and actuator fault propagation behaviours between the control strategy and the chilled water system. Case studies have shown the results that the errors of sensors and actuators significantly affect the pump control performance and indoor thermal environment. The pump control performance is more sensitive to three sensors: the supply air temperature sensor, the indoor air temperature sensor and the valve opening signal sensor of the terminal at the critical hydraulic loop. The indoor thermal environment is more sensitive to the indoor air temperature sensor. Compared to the baseline case without errors, the maximum deviation could reach 29.20% and 11.11% (in Celsius) for the chilled water pump energy consumption and indoor temperature respectively, under 10% error of related sensors. Actuator fault, like stuck valve, would result in a deviation of up to 42.46% in the pump energy consumption and up to 16.87% (in Celsius) in the indoor temperature. For applications, the proposed method is hoping to acquire general symptoms under various types of sensor faults for the fault diagnosis and contribute to enhancing the robustness of the optimal chilled water pump control strategy.

Heat Transfer Analysis of Cu–Water Nanofluid in a District Cooling Chilled Water Loop

Abstract Nanofluids consist of nanoparticles made of materials with high thermal conductivity suspended in a base fluid such as water. In theory, the presence of thermally conductive nanoparticles in a base fluid improves the heat transfer performance of the resulting nanofluid. This article numerically investigates the impact of nanoparticles on the energy performance of a district cooling system. The current work focuses on using Cu–water nanofluid as the working fluid for the secondary chilled water loop. It examines the effect of varying the nanoparticles concentration, nanofluid flowrate, and return temperature on the system energy performance. The numerical model is built using the engineering equation solver (EES) and validated using operational data obtained from the McQuay chilled water system operating in one of the university central facility plants. In the current numerical model, the Reynolds number in the shell side of the heat exchanger is varied between 2200 and 8800 at a volume fraction of 0.02. The result shows that for a fixed cooling capacity of 280 kW, the Cu–water nanofluid reduced the mass flowrate by 4.8% and the corresponding pump work input by 33.6%. This improved energy performance of the circulating water reduced the overall chiller system work input by 3.8% and increased the corresponding system coefficient of performance (COP) by 3.9%. The current findings reveal the potential impact and opportunity of nanofluids on the effectiveness of the district cooling system chiller water loop and the associated overall energy performance.

Liquefied natural gas re-gasification cold energy hybrid system integration in gas-steam combined cycle power plant model: Enhancement in power generation and performance

• Hybrid system integration that uses a secondary chilled water circuit is proposed. • Significant enhancement of power output occurs at higher ambient temperature. • The hybrid system produces higher cooling duty. • The efficiency gain is at best marginal and depends on the integration approach. The liquefied natural gas (LNG) cold energy utilization for inlet air cooling (IAC) to augment combined cycle power output is widely recognized as a feasible and sustainable alternative. However, issues with flexible operation and hybrid system integration still remain in warm climate region. To take the edge off from such kind of issues and to enhance the power generation and the performance, few possible opportunities are explored for a gas-steam combined cycle power plant (CCPP) with thermal integration of LNG re-gasification cold energy. Three integration approaches, such as 1st gas turbine IAC, 2nd condenser water cooling (CWC) in steam turbine cycle and 3rd combined approach of both IAC and CWC are proposed. Thermodynamic modeling and simulation-based method is adapted for CCPP model identification and thermodynamic performance comparison. Moreover, the results are established as a function of relative humidity and ambient temperature along with off design power generation of the proposed system. The result shows that the CCPP relative power improved from 1.01% to18.48% (for third integration approach) and 0.17% to 0.217% (for second integration approach), in the ambient temperature range of 13–45 °C and relative humidity of 90–40% respectively, compared to that of non-integrated CCPP. In third integration approach the relative efficiency enhancement is observed in the range of 0.05–0.40%, particularly in higher range of ambient conditions. In second integration approach the CCPP efficiency enhanced from 0.17% to 0.22% in ambient temperature range of 13–45 °C and relative humidity 40–90%.

A COMPARISON STUDY FOR ACTIVE CHILLED BEAM AND VARIABLE AIR VOLUME SYSTEMS FOR AN OFFICE BUILDING

Comparison in terms of energy and cost is carried out between two different air distribution systems, namely, Variable Air Volume (VAV) and Active Chilled Beam (ACB) based on a virtual building. The cooling load calculation was performed to determine the total load demand and the system design was carried out to determine the system capacity. Energy and cost analyses were carried out to determine which system gives the better saving. From the results analysis, it shows that the ACB is more energy-saving than the VAV, especially at full load and normal part-load conditions because of the reduced AHU fan’s capacity. In terms of cost, the ACB has a higher initial cost than the VAV where it is mainly contributed by the secondary chilled water pipe, heat exchanger and the beam itself. However, the operation cost is lower for ACB, and for long term use, the ACB is more cost-saving than the VAV.

Multivariate diagnosis analysis for chiller system for improving energy performance

The operating status of a large chiller system in buildings with central air-conditioning could vary at a given cooling demand and ambient condition. This study investigates the potential change of operating statuses and its influence on the system coefficient of performance (COP)—cooling energy output divided by the total electric power input. The system studied comprised four large and one small chillers. A set of primary chilled water pumps operated in different numbers to deliver the required flow rate of chilled water to the operating chillers. The actual flow rate to meet the building cooling demand was regulated by the secondary chilled water pumps. A set of evaporative cooling towers was installed to match the heat rejection required for the chillers. Comprehensive operating data were collected at 15-min intervals in 8 months. 18.46% of the total operating conditions were transitional statuses and unconventional operating statuses by switching on or off one extra cooling tower. This led to a 33.9–64.9% drop in individual system COPs. An examination of the variation of system load ratios and ambient wet bulb temperatures showed that the system COP was higher at the pair-up operation of system components even at higher wet bulb temperatures. A survival analysis indicated that the transitional statuses depended mainly on the temperature of chilled water returning to the evaporators and the temperatures of cooling water entering and leaving the condensers. Minimizing unnecessary transitional statuses and restoring the pair-up operation of system components could save electricity by 2.72%.

The problem study of district energy system in Shanghai, China

A number of district energy systems have been built recently in Yangtze River Delta where is the most developed region in China. Meanwhile, many problems around the systems are rising. Firstly, almost all energy stations adopt the redundancy design of energy load, which leads to the inefficiency of energy during part-load operation, especially. Secondly, primary and secondary chilled water could be unbalanced due to redundancy design. Furthermore, low occupancy rate could lead over supplied of energy station, and large water flow with narrow temperature difference in heating and cooling network could result in the low-energy efficiency and poor economy. In order to solve those problems, a model of district energy system was put forward in this paper, based on prototypical building load. On one hand, the model considers delayed performance of energy system and key factors of district energy system planning. On the other hand, it can make the energy supply matching to the hourly loads of every user.

Support Vector Regression and Genetic Algorithm for HVAC Optimal Operation

This study covers records of various parameters affecting the power consumption of air-conditioning systems. Using the Support Vector Machine (SVM), the chiller power consumption model, secondary chilled water pump power consumption model, air handling unit fan power consumption model, and air handling unit load model were established. In addition, it was found thatR2of the models all reached 0.998, and the training time was far shorter than that of the neural network. Through genetic programming, a combination of operating parameters with the least power consumption of air conditioning operation was searched. Moreover, the air handling unit load in line with the air conditioning cooling load was predicted. The experimental results show that for the combination of operating parameters with the least power consumption in line with the cooling load obtained through genetic algorithm search, the power consumption of the air conditioning systems under said combination of operating parameters was reduced by 22% compared to the fixed operating parameters, thus indicating significant energy efficiency.

Central Plant Control Optimization with a Thermal Chilled Water Energy Storage System: A Case Study in a High-Tech Building

The paper presents the study on the control optimization of the central cooling plant system including thermal chilled water energy storage (TES) tank in a high-tech building consists of office area, clean room and lab space. The optimized control sequences were implemented on the cooling tower staging, cooling tower fan/condenser water pump/primary chilled water pump staging and speed control, chiller staging, TES pump speed and TES modulation valves control, and secondary chilled water loop differential pressure control. The proposed control effectively resolved the problem of fluctuation in the secondary chilled water supply temperature during charging cycle and temperature increase during the discharging cycle. The number of primary chilled water pumps, condenser water pumps and cooling tower fans were also reduced to minimum after the optimization. Implementation of optimized control sequences is predicted to have an annual electricity savings of 856,125 kWh, which is about 20% of the total central plant energy consumption.

Application of Genetic Programming Method Combined with Neural Network in HVAC Optimal Operation

This study records the various air conditioning system parameters that affect power consumption and establishes system power consumption models for the chiller, the secondary chilled water pump, the air handling unit (AHU), and the cooling load of the AHU using artificial neural networks. The R2 for each of the models are as high as 0.996. Estimations for the AHU loads in the spaces where the cooling load for the AHU are satisfied and genetic programming is used to find the optimal air conditioning system parameter set for achieving minimum power consumption. These power consumption values are then set as genetic programming end points, and the mathematical symbol (+) is used as the functional ends. Finally, the computational elements of genetic programming are used to perform iterative computation. It may be concluded from the results of the experiment that the optimal parameter set obtained from the genetic programming-based search result in a minimum power consumption that complies with the loading requirements of the location of installation result in a 22% savings in term of power consumption and an average COP increase of approximately 28%, which represent very significant improvements.

Investigation on variable flow control in existing water-cooled chiller plant of high-rise commercial building in subtropical climate

An all-variable-speed chiller plant has been advocated so that the chillers and their associated pumps can work at reduced speed during part-load conditions for energy saving purposes. In this new building project, the savings merit is certainly attractive. However, an existing chiller plant or retrofit project, which is constrained by the performance of installed equipment and the configuration of a hydronic circuit, may not benefit from the an ideal extent of savings. As such, an existing chiller plant serving a typical commercial building was used to evaluate this. Through year-round dynamic simulation, it is found that there were about 5%, 6%, and 8% energy savings of the whole chiller plant when applying variable flow control on secondary chilled water pumps only, both primary and secondary chilled water pumps, and all chilled and condenser water pumps, respectively. If constant-speed centrifugal chillers were substituted by the variable-speed ones, an additional 4% savings could be achieved, which was, however, less than that incurred from the associated pumps. The payback periods of different variable flow control strategies were well below 1 year, indicating the techno-economical feasibility for wider application in existing chiller plants in subtropical climates.

Diagnosis of the low temperature difference syndrome in the chilled water system of a super high-rise building: A case study

The low delta-T syndrome exists in many large primary–secondary chilled water systems, which results in the degradation of the system overall energy performance. This paper presents a method and a case study on diagnosing the low delta-T problem resulted from the deficit flow that frequently occurred in the chilled water system of a super high-rise building at its early operation stage. The history operation data during the days when deficit flow and low delta-T syndrome occurred are analyzed. The improper set-point of outlet water temperature on the secondary side of heat exchangers is finally diagnosed as the fault that resulted in the deficit flow and low delta-T syndrome. Diagnosis of this fault was also validated in the in situ experimental tests. The deficit flow could be eliminated if temperature set-point was reset higher. Compared with the original set-point of outlet water temperature on the secondary side of heat exchangers, 87.67kW (72.37%) of the total power of pumps on primary and secondary sides of heat exchangers could be saved in the test cases when higher set-points were used.

A fault-tolerant and energy efficient control strategy for primary–secondary chilled water systems in buildings

Primary–secondary chilled water systems with decoupled bypass in building air-conditioning often cannot work in healthy condition as desired in practical operation, due to excess secondary flow demand resulting in deficit flow in the bypass line. This paper presents a fault-tolerant and energy efficient control strategy for secondary chilled water pump systems to solve this operation and control problem providing enhanced operation performance and energy efficiency of chilled water systems. The strategy employs the flow-limiting technique that ensures the water flow of secondary loop not exceed that of the primary loop while still maintaining highest possible delivery capacity of cooling to terminals. The strategy is also integrated with a differential pressure set-point optimizer to determine the optimal set-point. The performance of this strategy is evaluated in a simulated real-life environment representing the chilled water system in a super high-rise building by comparing it with two conventional control strategies. Results show that the proposed strategy can effectively eliminate the deficit flow at both starting and normal operation periods. The energy saving in the secondary chilled water pumps can be up to over 70% and 50% at system starting and normal operation periods respectively compared with the other strategies.

Optimal Piping Design for Enhanced Energy Saving in District Cooling System – A Case Study

This study performs a numerical investigation into the power consumption characteristics of various primary-secondary chilled water circuit designs in a district cooling system (DCS) installed in six buildings located within the same block in Taipei, Taiwan. An E20-II model is created of the DCS and a series of simulations are performed to determine the primary-secondary chilled water piping design which maximizes the energy saving obtained in the DCS over the course of a typical year. It is shown that the use of a region-pumping system or a boost-pumping system reduces the power consumption of the secondary chilled water circuit by 26.5% and 29.9%, respectively, compared to that of a common-pumping system. In addition, the results show that for a practical chilled water system in which the temperature differential on the primary side is 5.0oC while that on the secondary side is 6.5oC, an average monthly energy saving of around 5~7% is obtained compared to a DCS with equal temperature differentials in the primary-secondary circuits provided that the flow rate and the temperature differential of the chilled water in the primary circuit are a little less than those in the secondary circuit.

Performance enhancement of a complex chilled water system using a check valve: Experimental validation

Abstract The large primary–secondary chilled water systems often suffer from the excess flow demand and low chilled water temperature difference, which is known as the low ΔT central plant syndrome, during operation. This paper presents an approach for experimental validation of the possible utility of a check valve (i.e., putting a one-direction check valve in the chilled water by-pass line) to solve this operational problem and enhance the overall system performance, prior to a check valve is really installed. The experimental tests were carried out on the complex central chiller plant in a super high-rise building using a simulated check valve through fully closing one of the butterfly valves in the by-pass line when the system operated with a significant excess flow demand and experienced the low ΔT syndrome. The results from the tests showed that the system operational performance can be improved when the simulated check valve was used. Compared to that without using the check valve, about 9.2% of the total energy of the chillers and secondary water pumps was saved in the test period when the simulated check valve was used.

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.