Variable Volume Air Conditioning Research Articles

Room temperature and humidity decoupling control of common variable air volume air-conditioning system based on bilinear characteristics

Variable air volume (VAV) air-conditioning (AC) systems are widely employed to achieve a comfortable room thermal and humid environment depending on its better regulation performance and energy efficiency. In the single coil VAV AC system, conventional proportional-integral (PI) control algorithm is usually adopted to track the set-points of the room temperature and humidity by regulating the supply air flow rate and the chilled water flow rate, respectively. However, the control performance is usually not good due to the high coupling of the heat and mass transfer in the air-handling unit (AHU). A model-based control method is developed to realize the decoupling control of the room temperature and humidity according to the bilinear characteristics of the temperature and humidity variation. In this control method, a bilinear room temperature controller is used to track the room temperature set-point based on the real-time cooling load, while a room humidity controller is used to track the room humidity set-point depending on the real-time humidity load. The control performance was validated in a simulated VAV AC system. The test results show that comparing with the conventional PI control, the room temperature and humidity are controlled much more robustly and accurately by using the proposed model-based control method.

Assessing the energy performance of VAV and VRF air conditioning systems in an office building located in the city of Florianópolis

Abstract The objective of this study is to analyze the energy performance of two types of water-cooled air conditioning systems, variable air volume (VAV) and variable refrigerant flow (VRF), in terms of their cooling energy use through building simulation. These systems were designed to operate in an office building located in the city of Florianópolis, Brazil. The analysis involved the application of two building use schedules: a) constant and b) variable. Moreover, an analysis of the coefficient of performance (COP) and partial load ratio (PLR), and the percentage of operating hours for each range of cooling COP and PLR for each air conditioning system, allowed the system cooling efficiency to be assessed and the results to be related to the annual energy consumption. The coefficient of performance of VAV and VRF is 6.7 and 5.0, respectively, but the VRF system presented the lowest energy consumption for both schedules. The difference in the cooling energy consumption values for the VRF and VAV systems, for the variable schedule compared with the constant schedule, is mainly influenced by the partial load performance during the hottest period of the year.

Climate Compensation and Indoor Temperature Optimal Measuring Point Energy Saving Control in VAV Air-Conditioning System

Temperature measuring point is the key to room environment control. Temperature measuring points and climate changes are directly related to the room control effect. It is of great theoretical and practical significance to study the temperature measuring points and control strategy based on climate compensation. In this study, first, the climate compensation concept in a heating system was introduced into a variable air volume (VAV) air-conditioning system. The heating load was modeled as a function of supply air temperature by analyzing the heat exchange. Based on each control link of subsystems, a climate compensation scheme is proposed to determine the optimal set-point of the supply air temperature. At the same time, a layout of multiple temperature measuring points of an air-conditioned room was studied. Furthermore, the optimal indoor temperature measuring point was determined using an adaptive weighted fusion method. Finally, simulation results show that the proposed method has better control effects on indoor temperature adjustment compared with the traditional method. The optimal supply air temperature in summer and winter was determined according to the proposed climate compensation scheme, and the supply air temperature was controlled using an improved single-neuron adaptive control strategy. Experimental results show that the maximum energy saving can reach up to 35.5% in winter and 6.1% in summer.

Optimal Control Strategy for Variable Air Volume Air-Conditioning Systems Using Genetic Algorithms

This study is aimed at developing a real-time optimal control strategy for variable air volume (VAV) air-conditioning in a heating, ventilation, and air-conditioning (HVAC) system using genetic algorithms and a simulated large-scale office building. The two selected control variables are the settings for the supply air temperature and the duct static pressure to provide optimal control for the VAV air-conditioning system. Genetic algorithms were employed to calculate the optimal control settings for each control variable. The proposed optimal control conditions were evaluated according to the total energy consumption of the HVAC system based on its component parts (fan, chiller, and cold-water pump). The results confirm that the supply air temperature and duct static pressure change according to the cooling load of the simulated building. Using the proposed optimal control variables, the total energy consumption of the building was reduced up to 5.72% compared to under ‘normal’ settings and conditions.

Predication control for indoor temperature time-delay using Elman neural network in variable air volume system

Aiming at the prediction control for indoor temperature time-delay in variable air volume (VAV) air conditioning system, this paper presents an indoor temperature prediction control method based on Elman neural network multi-step prediction model. Firstly, this paper introduces basic control principles of pressure-dependent and pressure-independent VAV terminal through comparable analysis and points out significance of indoor temperature prediction control based on pressure-dependent VAV terminal. Then, Elman neural network multi-step prediction model and corresponding indoor temperature prediction control method for pressure-dependent VAV terminal are proposed based on the fundamental principle of periodic prediction control for time-delay system. Finally, the effect of proposed prediction control method is validated by the experimental study according to the test data of supply air volume regulating process, provided that the supply air volume control loop adopts constant static pressure control method. Experimental results indicate the proposed indoor temperature prediction control method based on pressure-dependent VAV terminal could change the conventional regulating mode of the VAV air conditioning system, which will be benefit for improving the control stability of indoor temperature control loop and other corresponding control loops.

Fault diagnosis and energy consumption analysis for variable air volume air conditioning system: a case study

Several common faults and their causes in variable air volume (VAV) air conditioning system are presented, and the principle of fault diagnosis of air conditioning system is briefly described. The VAV air conditioning system was modeled in TRNSYS, and five typical faults of the cooling mode were simulated. The comparative analysis of the respective under normal operation and fault operation had been made, and the impact of each fault on the energy consumption was also analyzed. The actual operating characteristics of air conditioning system was then evaluated. Further, some parameters under fault operations were compared with those under the normal operation, from which the changing characteristics of parameters could be discovered, and the characteristics can be used to diagnose faults. The simulation results demonstrate that fault can affect the energy consumption of VAV air conditioning system, and the impact of each fault is different. In addition, monitoring the change of energy consumption and operation parameters is helpful in fault diagnosis, and the effective fault diagnosis has great significance to energy-saving of the air conditioning system.

Novel cylindrical induction controller and its application in VAV air conditioning system in an office building

High performance variable air volume (VAV) air-conditioning systems can be as energy efficient as systems based on water or refrigerant, while first and life-cycle costs of such systems are significantly lower. The paper presents experimental test results of a novel cylindrical induction VAV controller and simulation results of energy consumption and thermal comfort in an office building with such controllers. A method for estimation of the minimum induced airflow required to avoid cold air dumping and a method for induction controllers and diffusers selection and sizing are proposed. To compare the VAV system with induction controllers and two other popular air conditioning systems, in terms of energy consumption and thermal comfort in an office building, the IDA Indoor Climate and Energy simulation tool was applied. The comparison was made for moderate climate conditions of Krakow, Poland, typical of Central and East Europe. The results showed that the total consumption of the electric energy used in cooling season for cooling and media transportation was similar when using a VAV system with induction controllers, a system with air chilled beams or a fan coils system. Similar thermal conditions were also observed.

Experimental study on a novel fuzzy control method for static pressure reset based on the maximum damper position feedback

In variable air volume (VAV) systems, conventional control methods reset the supply air static pressure based on the exact mathematics model of the controlled objects which are often difficult to be founded, because the VAV controlled systems have the characteristics of nonlinear, multivariable and long delay times. In order to develop a simple inference process for real-time applications, this paper presents a novel fuzzy control method for static pressure reset with no using of the mathematics model. The proposed fuzzy control method is developed based on the maximum damper position feedback using Mamdani-type fuzzy rule and functioning fuzzy subset inference (FFSI). This study provides the detail implement process of the proposed fuzzy control method. Supported by the experimental data, the proposed method is compared with optimal fixed changing step method in an integrated control test rig for a VAV air conditioning system. Experimental results show that the proposed method has better energy efficient and control performance. A minimum of 7% energy saving of the proposed fuzzy control method is shown through experimental study.

Experimental and Numerical Testing of an Induction Variable Air Volume (VAV) Controller with Two Damper Blades

A VAV (Variable Air Volume) air conditioning system without reheat of the air supplied is generally regarded as energy efficient. Re-heat can be avoided when induction VAV controllers are used instead of conventional controllers since primary airflow can be reduced to about 10–20% of its nominal value without a draught risk. The aim of the present tests was to determine the induction and efficiency characteristics of a compact square induction VAV controller with two damper blades and to find relationships between flow ratio and governing independent variables such as: inlet airflow, inlet pressure and terminating elements flow resistance. The aim was also to find a method of flow ratio interpolation for any values of independent variables within the range of tests. A controller of new construction with two damper blades was tested experimentally in a physical model using the tracer gas method and numerically using the CFD method. In order to approximate and interpolate the induced airflow for any values of governing independent variables, a set of relevant relationships was found. CFD calculation results can be approximated using a set of equations with the expanded uncertainty of 5 m3/h. This method can be implemented in the codes for VAV controller selection. The tested construction meets the most important requirement for an induction VAV controller i.e. for the lowest inlet airflow the amount of induced airflow is higher than the required minimum value (NV=2–3). When comparing efficiency characteristics of the tested model of induction VAV controller with test results of jet pumps, it can be concluded that the controller of the tested construction may reach similar efficiency and thus the new construction seems appropriate. There is a satisfactory agreement between CFD and measurement results. CFD method can be successfully used for optimization of the induction VAV controller construction.

A Review of Static Pressure Reset Control in Variable Air Volume Air Condition System

For the sake of energy saving of variable air volume (VAV) system, this paper presents the a review of static pressure control around the optimization problem of static pressure reset in VAV air conditioning system. Then, main control methods of static pressure reset are described, and existing problems are analyzed and concluded. Finally, it is pointed out that the critical technology and the development trend of static pressure reset control. This overview is not intended to be an exhaustive survey on this topic, and any omission of other works is purely unintentional.

Design of Control Scheme for Variable Air Volume Air-Conditioning System

The air quantity of variable air volume system for the rooms and the total air quantity of the system changes with the change of room load. Combined with the system composition in the laboratory, the paper determines the control scheme of the variable air volume system, that is, indoor temperature-control, indoor positive pressure control, air distribution static pressure control, air-supply temperature control and new air volume control. The dotted lines with arrows mean the output signals from the control unit to actuator, and the solid lines with arrows represent the input signals from the actuator to the control unit.

Design of Terminal Controller of VAV Air-Conditioning System Based on BAS3000 DDC

The terminal device (VAVBOX) is the key part in variable air volume (VAV) air conditioning system, by which we can adjust air output, compensate indoor load change, keep room temperature to be constant. The control effect of VAV system depends largely on the performance of terminal device and the coordination between terminal device and the whole system. In this paper, BAS - 3520 DDC produced by Advantech corporation is used to design VAV air conditioning pressure independent terminal controller. Experiments show that the terminal controller based on DDC can well satisfy the system control requirement, the transition time and overshoot of the indoor temperature achieve good improvement. At the same time , with Webaccess introduced, VAV BOX operation condition can be monitored by host computer, what makes the daily management of the VAV BOX more intelligent.

Development and Application of the Control Technology for VAV Air-Conditioning System

The variable air volume (VAV) air-conditioning system with its superior energy saving effect has been applied in many developed countries, also drawn more and more attention in China. Due to the characteristics of VAV air-conditioning system are nonlinear, large time lag, strong coupling, multi-variable and multi-disturbance, the control complexity of the VAV system increase greatly. The energy conservation and comfort of the VAV air-conditioning system can not fully been embodied in application. The research on the VAV system has become the hot spot in the science and technology workers, many new methods and strategies have emerged. From the three aspects: the air system, the water system and the large scale system, the paper describes the control strategies, the control methods and the research progress about the VAV air-conditioning system.

Simulation comparison of VAV and VRF air conditioning systems in an existing building for the cooling season

Performance of two widely used air conditioning (AC) systems, variable air volume (VAV) and variable refrigerant flow (VRF), in an existing office building environment under the same indoor and outdoor conditions for an entire cooling season is simulated by using two validated respective models and compared. It was observed that the indoor temperatures could not be maintained properly at the set temperature by the VAV no-reheat boxes. However, it could be maintained by the VAV boxes with reheat with a significant energy consumption penalty. It was found that the secondary components (indoor and ventilation units) of the VRF AC system promised 38.0–83.4% energy-saving potential depending on the system configuration, indoor and outdoor conditions, when compared to the secondary components (heaters and the supply fan) of the VAV AC system. Overall, it was found that the VRF AC system promised 27.1–57.9% energy-saving potentials depending on the system configuration, indoor and outdoor conditions, when compared to the VAV AC system.

Simulation of a VAV air conditioning system in an existing building for the cooling mode

In this study, simulation of a variable air volume (VAV) air conditioning system in an existing office building is presented for the cooling mode. A building simulation package was used for the simulation [DOE, EnergyPlus Engineering Document, Version 1.3., U.S. Department of Energy, Washington, DC, 2005]. The design information of the existing office building, the actual number of the internal load sources such as occupants, lighting, office equipments and the manufacturer's data for the existing rooftop unit (RTU) and the VAV boxes were used in the building simulation package. The simulation results; RTU power consumption, indoor zone temperature and relative humidity, were validated with the data obtained from the on-site measurements performed in the existing office building under the same outdoor conditions. It was found that 71.1% of all simulated power consumption data falls within ±15% range from the experimental data. The reason for the existence of some data out of band is due to the possible errors associated with the difference in the solar data used for the location 40 km away from the experimental location. It was found that 90.6–94.7% of the indoor temperature experimental data fall within ±1.5 °C range from the simulated data, and 88.3–91.3% of the indoor relative humidity data fall within ±18% range from the simulated data.

Feedback linearization based control of a variable air volume air conditioning system for cooling applications

Design of a nonlinear control system for a Variable Air Volume Air Conditioning (VAVAC) plant through feedback linearization is presented in this article. VAVAC systems attempt to reduce building energy consumption while maintaining the primary role of air conditioning. The temperature of the space is maintained at a constant level by establishing a balance between the cooling load generated in the space and the air supply delivered to meet the load. The dynamic model of a VAVAC plant is derived and formulated as a MIMO bilinear system. Feedback linearization is applied for decoupling and linearization of the nonlinear model. Simulation results for a laboratory scale plant are presented to demonstrate the potential of keeping comfort and maintaining energy optimal performance by this methodology. Results obtained with a conventional PI controller and a feedback linearizing controller are compared and the superiority of the proposed approach is clearly established.

Optimization of Energy Conservation Potential for VAV Air Conditioning System using Fuzzy based Genetic Algorithm

The objective of this study is to present the test results of variable air volume (VAV) air conditioning system optimized by two objective genetic algorithm (GA). The objective functions are energy savings and thermal comfort. The optimal set points for fuzzy logic controller (FLC) are the supply air temperature (Ts), the supply duct static pressure (Ps), the chilled water temperature (Tw), and zone temperature (Tz) that is taken as the problem variables. Supply airflow rate and chilled water flow rate are considered to be the constraints. The optimal set point values are obtained from GA process and assigned into fuzzy logic controller (FLC) in order to conserve energy and maintain thermal comfort in real time VAV air conditioning system. A VAV air conditioning system with FLC installed in a software laboratory has been taken for the purpose of energy analysis. The total energy saving obtained in VAV GA optimization system with FLC compared with constant air volume (CAV) system is expected to achieve 31.5%. The optimal duct static pressure obtained through Genetic fuzzy methodology attributes to better air distribution by delivering the optimal quantity of supply air to the conditioned space. This combination enhanced the advantages of uniform air distribution, thermal comfort and improved energy savings potential. Keywords—Energy savings, fuzzy logic, Genetic algorithm, Thermal Comfort

Experimental Analysis of a Genetic-Fuzzy Inverter DX VAV A/C System for Automatically Ventilated Buildings

In recent years, the quest has been focused on energy efficient building design. To achieve this in terms of high efficiency air conditioning schemes for hot climate cooling, the combination of variable refrigerant volume (VRV) with variable air volume (VAV) systems have become popular. In this paper, attention is focused on achieving good thermal comfort and indoor air quality (IAQ) combined with energy savings by using multi-zone VAV air conditioning (A/C) that incorporates a genetic based fuzzy logic controller (FLC). Experimental analysis based on a combined demand controlled ventilation (DCV) with economizer cycle (EC) was performed on an inverter driven multi-zone direct expansion (DX) VAV A/C system integrated with a fuzzy logic controller and optimized by a genetic algorithm (GA). The opening of the VAV box damper was controlled using the fuzzy logic controller. Based on the test results, the proposed fuzzy logic operated system maintained supply air temperature close to 13°C and an occupant zone at a consistent temperature of around 24 °C. In DCV mode, the concentration of CO2 was maintained between 950 ppm and 1040 ppm while, when the system was operated under a combined DCV-EC ventilation scheme, the CO2 concentration was maintained at between 350 ppm and 970 ppm. The experimental results suggested that CO2 concentration obtained experimentally was well within the permissible limit for the varying load conditions. Under the combined DCV-EC mode of ventilation, using a fuzzy-genetic algorithm, the maximum power obtained for the supply air fan and variable speed compressor was 415 W and 3.4 kW respectively. The compressor was totally turned OFF during the economizer cycle thus contributing to total power savings. The energy savings potential of the proposed fuzzy controlled multi-zone DX VAV A/C system yielded 70% and 89% under DCV and combined DCV economizer cycle ventilation modes respectively when compared with a constant air volume (CAV) A/C system operated under the DCV technique. The test results suggested that it was feasible for this fuzzy control methodology, integrated with the developed genetic algorithm, to provide a proper control of IAQ, thermal comfort and energy conservation.

Performance improvement of VAV air conditioning system through feedforward compensation decoupling and genetic algorithm

VAV (variable air volume) control system has the feature of multi-control loops. While all the control loops are working together, they interfere and influence each other. This paper designs the decoupling compensation unit in VAV system in the method of feedforward compensation. This paper also designs the controller parameters of VAV system by means of inverse deducing and the genetic algorithm. Experimental results demonstrate that the combination of the feedforward compensation decoupling and the controller optimization by genetic algorithm can improve the performance of the VAV control system.

Development of a dynamic model for a DX VAV air conditioning system

Abstract A direct expansion (DX) variable air volume (VAV) air conditioning (A/C) system consists of a VAV air distribution subsystem and a DX refrigeration plant. This paper reports on the development of a representative and complete dynamic mathematical model for the DX VAV A/C system having a variable speed compressor and pressure independent VAV terminals. The model is component based and takes into account the dynamic behaviors of both the DX refrigeration plant and the VAV air distribution subsystem simultaneously. The dynamic model has been developed based on the principles of mass and energy conservation and using correlations describing the operational performance of some of the system’s components, which are either field tested or available from manufacturers. An experimental rig has been set up for the DX VAV A/C system, and experimental work has been conducted to obtain the system’s responses to the step change of compressor speed. The dynamic model developed was then validated using the experimental data obtained. Steady state and transient responses for five major operating parameters obtained from both the model and the experiment were compared and found to be in good agreement.