Variable Volume Air Conditioning System Research Articles

Model-based optimal control strategy for multizone VAV air-conditioning systems for neutralizing room pressure and minimizing fan energy consumption

Control strategies for variable air volume (VAV) air-conditioning systems play a pivotal role in ensuring indoor environmental quality and energy efficiency. However, conventional approaches, such as static pressure reset (SPR) control, focus on managing indoor air temperature without considering the room pressure, which can lead to unbalanced room pressure and undesirable air leakage. Moreover, with the application of prevalent building pressure control strategies, such as airflow tracking control, to multizone VAV systems, neutralization of the room pressure is difficult across multiple zones in VAV systems. Therefore, this study introduces a model-based optimal control strategy for multizone VAV air-conditioning systems. The proposed strategy uses a multiobjective optimization framework to regulate fan frequencies and damper openings on both the supply and return sides. This holistic approach facilitates the simultaneous control of the indoor air temperature and room pressure while minimizing fan energy consumption. To assess the effectiveness of the proposed strategy, four control strategies were tested using a Python-based simulation testbed. The results demonstrate that the proposed strategy effectively maintains the indoor air temperature, neutralizes room pressure, and reduces fan energy consumption, thereby contributing to the overall efficiency of the VAV system. Moreover, the results highlight the limitations associated with combining airflow tracking control with SPR control for room pressure regulation in multizone VAV systems. This highlights the importance of adopting a model-based approach to address the complexities of concurrent room pressure and indoor air temperature control.

Global hydraulic stability analysis for dynamic regulation in multi-zone variable air volume air-conditioning system

Variable air volume air-conditioning system (VAV system) has multiple control loops that interfered with each other, which has serious impacts on its actual operation effect. To solve this problem, the index of global hydraulic stability, which takes the physical quantity of the pipe network pressure and the air flow rate as reference, is proposed based on Graph theory, and a pipe network model for the hydraulic calculation is built. Then, the simulation study based on the actual operation data is carried out in MATLAB. The distribution of the pressure and the air flow in the pipe network is obtained, and the action interferences among adjacent terminal boxes are analyzed based on global hydraulic stability under damper position regulation strategy (DP strategy) and fan frequency regulation strategy (FF strategy). Results indicate that the FF strategy can decrease the total change distance of the damper position compared with the DP strategy, and then the fluctuation of the pressure and the air flow can be reduced. The contribution of this study is to provide an evaluation index of global hydraulic stability, which can also be used in other air-conditioning systems with multiple terminals and multiple control loops.

Thermal sensation and occupancy-based cooperative control method for multi-zone VAV air-conditioning systems

Most existing multi-zone variable air volume (VAV) air-conditioning systems rely on manual adjustment or a constant temperature set point, with no consideration for users' thermal sensation. VAV terminals operate regardless of whether there is a demand for cooling in the zone, rather than according to the actual occupancy situation between multiple zones, resulting in poor indoor thermal comfort and the high energy consumption of air-conditioning systems. This paper proposes a thermal sensation and occupancy-based cooperative control (TSOCC) method for multi-zone VAV air-conditioning systems in order to provide an indoor thermal environment more in line with the real-time needs of personnel, while also increasing cooperative control between multiple zones to ensure that VAV terminals do not operate unnecessarily when zones are unoccupied. To evaluate the control performance of this method, a simulation platform is built, case simulations with different zone occupancy rates are carried out and various comparisons and analyses are made with temperature set point-based control. The results show that the TSOCC method for multi-zone VAV air-conditioning systems can provide an indoor thermal environment more in line with the thermal comfort needs of the personnel and save 6–14% of daily system energy consumption compared with the temperature set point-based control strategy, while the system's energy-saving rate increases as the zone occupancy rate decreases, marking a change from whole zone-oriented cooling to zone-oriented cooling and providing more accurate air-conditioning services for indoor personnel.

A novel variable refrigerant flow system with solar regeneration-based desiccant-assisted ventilation

Variable refrigerant flow (VRF) based air-conditioning systems provide better thermal comfort with respect to the conventional all-air systems involving either constant air volume (CAV) or variable air volume (VAV) technique. In this study, a novel VRF strategy involving potential of improved indoor air quality has been proposed by integrating solar energy with desiccant-assisted dedicated outdoor air system (DOAS). DOAS is responsible for ventilation and humidity control. An indirect evaporative cooling arrangement is incorporated in the path of supply air from desiccant for reducing the load on the VRF system. Using Energyplus simulations, thermal and electrical energy characteristics of the proposed system have been investigated for three climatic conditions. A comparative assessment of the novel system has been made against the conventional VRF and all-air VAV air-conditioning system. Results show that for warm-humid climate, electrical energy saving potential of the proposed solar energy-assisted VRF system is around 23.9 % and 9.5 % than the conventional all-air VAV and VRF systems, respectively. Under composite climate the saving offered by the proposed system with respect to VAV and VRF is determined as 13.8 % and 9.4 %, respectively. Finally, the energy saving obtained with solar energy based VRF for the hot-dry climate is 17.5 % and 11.6 % against conventional all-air VAV and VRF systems, respectively. Excellent solar fraction is observed for composite and hot-dry climates, whereas for warm-humid climate, it is determined as 0.97. A comparative analysis with conventional cooling coil assisted DOAS-VRF system also shows energy savings by the proposed system.

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.

Predictive control of multi-zone variable air volume air-conditioning system based on radial basis function neural network

The multi-zone variable air volume (VAV) air-conditioning system is a complex thermal system with large delay and nonlinearity. Due to the complex environment of multi-zone buildings and the complicated operation process of the VAV air-conditioning system, there are many difficulties in the room temperature control. This paper firstly establishes a multi-zone building model for room temperature using resistance–capacitance method. This investigation simulates and measures the dynamic response of room temperature in a three-floor building without/with air-conditioning for validation. Then a multi-zone VAV air-conditioning system room temperature predictive control model based on radial basis function (RBF) neural network (NN) is proposed. This study sets up a multi-zone VAV air-conditioning system experimental platform in the three rooms on the first floor of the building and implements the predictive control model based on the RBF neural network. The experimental results show that the predictive control model based on RBF NN is able to meet room temperature requirements. It also has strong anti-interference performance and ensures stable static pressure of the main air supply duct. The multi-zone building model can accurately simulate the temperature changes of each room when the air supply volume varies.

Adaptive predictive control method for improving control stability of air-conditioning terminal in public buildings

Due to the characteristic of large scale, multiple equipment, variable working conditions and complex debugging of air conditioning system in public buildings, the energy efficiency and operating level of the system are limited in the process of operation, and the actual operation effect will deviate from the design expectation. In addition, it is difficult to build an accurate mechanism model and to implement corresponding online control, so the commissioning for controller parameter tuning has to rely on human experience. Meanwhile, the response of indoor temperature to system regulation variables is different for different terminals due to lag characteristic, which would further affect the stability for the overall system. To solve the above problems, an adaptive predictive control method is proposed based on the physical-data driven hybrid model, which could not only realize online self-tuning of terminal controller but also improve control stability of terminal and overall system. Then, taking variable air volume air-conditioning system as the study object, the effectiveness of the proposed method is validated through comparative experiments. Results show that the proposed method can improve control stability of terminal and overall system. Achievements will have important theoretical significance and wide application prospect for improving the adaptability ability and the overall operating level of air-conditioning system in public buildings, which has complex and time-varying operation condition.

A real-time optimal control strategy for multi-zone VAV air-conditioning systems adopting a multi-agent based distributed optimization method

Determining the proper trade-off among thermal comfort, Indoor Air Quality (IAQ) and energy use is important for optimal control of air-conditioning systems. The number of optimization variables increases as systems become increasingly complex, as with multi-zone VAV (Variable Air Volume) air-conditioning systems, leading to large-scale mathematics programming challenges and inconveniences in the implementation of conventional centralized optimization strategies. This paper therefore proposes a real-time optimal control strategy adopting a multi-agent based distributed optimization method for multi-zone VAV air-conditioning systems. The proposed strategy consists of three novel schemes. First, a temperature set-point reset scheme adopts a linear rule to correlate the resetting of the temperature set-points in individual zones to simplify the optimization problem while applying proper optimization in individual zones. Second, a multi-objective optimization scheme optimizes the fresh air ratio of the supply air and the temperature set-point in the critical zone by formulating the multi-objective optimization problem. Third, a multi-agent distributed optimization scheme is developed to solve the optimization problem in a distributed manner, facilitating the deployment of local control devices of limited capacity. A TRNSYS-MATLAB co-simulation testbed is constructed to test and validate the proposed strategy. Test results show that the strategy is effective in properly balancing thermal comfort, IAQ and energy use while largely reducing programming challenges. The distributed optimization method can provide almost the same optimal outputs as conventional centralized optimization methods.

An air handling unit fault isolation method by producing additional diagnostic information proactively

As summarized in ASHRAE project RP-1312, an air handling unit (AHU) might suffer from 31 types of faults. Among them, 11 types of faults are of the most serious. Due to the limitation of initial cost, AHUs are always equipped with a limited amount of sensors that are vital for the purpose of automatic control. The diagnostic information is very poor. Therefore, it is challenging to isolate these faults. In this study, a proactive AHU fault isolation method is proposed to overcome this problem. Its basic idea is to introduce dynamic disturbances to the faulty AHUs to produce additional diagnostic information. The dynamic disturbances are generated through proactive actions such as resetting set-points or control signals in building management systems or AHU controllers. A list of proactive fault isolation rules is proposed to isolate these serious faults based on the additional diagnostic information. Evaluations are made on a simulated variable air volume air-conditioning system. Results show that the proposed method can isolate the serious faults of AHUs effectively.

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.

Static pressure set-point reset smoothing scheme based-on improved iterative learning control and variable trajectory for air-conditioning system

Variable static pressure is the main strategy for air volume adjustment in variable air volume air-conditioning system and can offer huge energy savings compared with constant air volume systems. When parameter set-points reset with load, dynamic processes are excited. Some processes take a long time to reach a new stable state with large overshoots. Improved iterative learning control (ILC) algorithm is studied to ameliorate transition processes of the static pressure reset control loop. A mathematical model of the static pressure control loop is established, and a new desired trajectory is given to improve the system tracking performance. Simulation and experimental studies are performed on traditional Proportion Integration Differentiation (PID) control, Proportion Differentiation (PD) type ILC, fuzzy gain PD type ILC and variable speed-integral PID type ILC. Results show that the system has the smallest overshoot and the shortest adjustment time with the fuzzy gain PD type ILC. Meanwhile the system can be betterly stabilized near the set-point with smallest evaluation index and the desired trajectory can be tracked with better dynamic performance. Studies show that the dynamic process is smoothed by improved ILC and variable trajectory. It is of great significance to enhance the whole system stability.

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.

Matching of air conditioning power and PV panel output power based on dynamic controlling of air conditioning air supply volume

ABSTRACT This paper presents a dynamic control strategy of air-conditioning air supply volume based on statistical data of the spatial and temporal distribution of occupants in the building, aiming at reducing overheating in buildings under local heat island effect. Nowadays, energy consumption in air-conditioning system is relatively high when considering the limited number of occupants and chiller plants operating inefficiently because the traditional central air-conditioning system was not designed to operate at different cooling load demands. Therefore, variable air volume (VAV) air-conditioning system is applied in this paper with corresponding wind volume control method. To address the above issue, in this paper: (1) A spatiotemporal distribution of occupants in the building is analyzed, and the relationship between occupant distribution and characteristics of the thermal dissipating load is established; (2) A strategy for dynamic controlling of the air supply volume in time and space is proposed; (3) A distributed photovoltaic system supplies power to cooling load of the building, and a strategy is proposed for optimizing the match between cooling power and photovoltaic (PV) output power using competitive swarm optimization algorithm; and (4) Air blower total amount of wind control method is adopted in VAV air-conditioning system for dynamic controlling of the air supply volume. This paper uses a simulation to verify the feasibility and optimality of the above strategies. The simulation results show that the building energy consumption is greatly reduced by 39.52%, the PV accommodation is improved as 77.14%, and four control indicators are well satisfied due to the air blower total amount of wind control method.

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.

Static balance duct design method and temperature dependent variable air volume systems

In hot and humid tropical countries, the air-conditioning system has a high energy usage in commercial buildings. This study introduces a static balance duct design method for a temperature dependent variable air volume system. Ducts are looped together, which cause the air to self-balance and minimize the pressure loss across the duct network. The static balance duct network can easily identify the critical zone of highest pressure loss. This zone appears at the longest duct length from the air source and is always consistent for different load conditions. The air supplied to this critical zone is controlled by the zone temperature sensor, which directs feedback to the variable frequency drive fan of the air-handling unit. The rest of the variable air volume terminal units are zone temperature controlled. The duct looping design mitigates the problems in pressure variations and complements the temperature dependent control strategy. An open office case study is carried out to demonstrate the merit of the static balance duct design method. An actual installation was completed in a shopping mall. The recorded temperature readings verified that the static balance duct network and temperature dependent variable air volume air-conditioning system is operational and functional. Approximately 27% energy savings is achieved compared to the constant air volume system. Practical application: The static balance duct design method is a combination of equal friction duct design method and a loop duct network. This static balance duct network can identify the critical zone of highest pressure loss and this zone is always consistent. In this temperature-dependent variable air volume (TDVAV), the variable air volume (VAV) terminal unit at the critical zone is eliminated. The zone temperature sensor at the critical zone provides direct feedback to the variable frequency drive fan. All other terminal units are temperature dependent. The static balance TDVAV system enhances energy savings, improves the performance of VAV system and reduces the faults for VAV systems.

Dynamic simulation of a VAV system based on dynamic PMV control

The predicted mean vote (PMV) index is taken as the control target in an air-conditioning system is in line with the requirements of the indoor thermal environment. In addition, with the emergence of “air conditioning disease”, people began to pay more attention to the study on the dynamic thermal comfort of indoor thermal environment. Firstly, the dynamic PMV is proposed, and the amplitude and period of the dynamic PMV are confirmed by experiments and questionnaire surveys. Then, a simulation model of variable air volume (VAV) air-conditioning system based on dynamic PMV control is built by Transient System Simulation Program (TRNSYS). Simultaneously, the simulation models of the VAV air-conditioning system based on constant temperature control and static PMV control are also built by TRNSYS. The simulation results show that the comfort and energy saving of the VAV air-conditioning system based on dynamic PMV control are more prominent.

An advanced simulation test bed for the stability analysis of variable air volume air-conditioning control system. Part 1: Optimal simplified model of building envelope for room thermal performance prediction

This paper and its future companion papers, were aimed to develop a set of efficient and accurate enough modular tools to build a simulation test bed for the stability analysis of interacting VAV (variable air volume air-conditioning system) control loops. By providing the correlation equations between the heat flow and temperature of the two opposite surfaces, the wall model was expected to link adjacent zones effectively and efficiently. In part 1, the development of an optimal simplified 3R2C (composed of three resistances and two capacitances) thermal network model of building envelope, i.e., the wall model, was descripted. The verification showed that the simplification of one-dimensional heat conduction through a typical wall of heavy construction was acceptable. The accuracy of the optimal simplified 3R2Cmodel was satisfying, and outperformed that of the optimal 4R3C model as well as two 3R2C models which were normally used in practical applications. First and foremost, the room dimensions of the test bed with the boundary conditions in numerical scheme were designed under the principle of similarity. After validation of the numerical scheme with the high-cited scaled experiment, a series of numerical test cases simulating the potential oscillations in practical applications, were conducted. Then, the long neglected concerned frequency range within which heat flow transfer through extern wall, was determined. Finally, the part of solid wall was separated with the part of indoor air, and was developed into a flexible wall module of 3R2C model based on frequency domain regression.

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.

Impact of the duct static pressure reset control strategy on the energy consumption by the HVAC system

This article addresses different duct static pressure control strategies which could be implemented in variable air volume air-conditioning systems (VAV). Two pressure reset control strategies are compared to the commonly used control solution based on the “Constant static pressure” method. First pressure reset control strategy, known as PID Control, uses signals from VAV boxes controllers to reset duct static pressure in a way that one of the VAV dampers is maintained almost entirely open. Second strategy decreases static pressure setpoint until an adjustable number of pressure requests occur. As a response to the certain amount of requests, static pressure setpoint is increased. This strategy is called Trim & Respond. Both static pressure reset control strategies described in this paper are considered to have more significant potential for energy savings than the “Constant static pressure” method. In order to validate this potential, several simulations for different control strategies were carried out and the obtained results are compared and analysed. The theoretical limit of the energy savings - set of the optimal control actions, was estimated with Nelder-Mead algorithm and also presented in this article. General description of the static pressure control strategies Constant static pressure, PID Control and Trim & Respond is given.