Variable Air Volume Terminal Unit Research Articles

Bilinear observer-based robust adaptive fault estimation for multizone building VAV terminal units

In this paper, we propose a novel bilinear observer-based robust fault detection, isolation and adaptive fault estimation methodology to precisely estimate a class of actuator faults, namely bias in damper position and lock-in-place faults, in Variable-Air-Volume (VAV) terminal units of Heating Ventilation and Air-Conditioning (HVAC) systems. The proposed adaptive fault estimator is robust in the sense that the fault estimates are not affected by the unmeasured disturbance variable and that the effects of measurement noises on fault estimates are attenuated. The fault estimation algorithm with the integrated building control system improves occupants comfort and reduces the operation, maintenance, and utility cost, thereby reducing the impact on the environment. The effectiveness of the methodology for adaptive estimation of multiple or single VAV damper faults is successfully demonstrated through different simulation scenarios with SIMBAD (SIMulator of Building And Devices), which is being used in industries for testing and validation of building energy management systems.

Data-driven Optimal Control of Single Duct VAV Terminal Unit

The purpose of this study is to propose a control method that optimizes the set point of the variable air volume (VAV) terminal unit using real-time prediction model of indoor thermal and air environment and energy consumption. Indoor thermal environment and air environment are predicted through indoor load and carbon dioxide (CO2) concentration. A prediction model was developed through operation data of the VAV terminal unit by artificial neural network (ANN) algorithm. The developed prediction model was used for real-time set point control of the VAV terminal unit. The optimal control of VAV terminal unit can be expressed in 3 step. First, it predicts the current indoor load and CO2 concentration. Second, all supply temperatures and supply air flow rate that can be provided in the predicted condition are repeatedly simulated. Finally, the set point of the minimum energy consumption is applied to the control. The evaluation of the proposed control method was compared with the dual maximum control logic. Comparative evaluation was performed using TRNSYS. It was confirmed that the proposed control method reduced the reheat coil energy consumption by about 20% and the air supply fan energy consumption by about 17% compared to the existing control method.

Optimization of supply air flow and temperature for VAV terminal unit by artificial neural network

The purpose of this paper is to optimize the supply air flow rate and temperature of the variable air volume (VAV) terminal unit by artificial neural network (ANN). In general, the setpoint of the VAV terminal unit are supply air flow rate and supply air temperature. These setpoints are determined by the design value, and are calculated by the maximum indoor heating/cooling load and the ventilation requirements. The setpoints calculated based on the design value does not cause any problems in maintaining the indoor environment. However, energy consumption cannot be optimized because it is based on the maximum design value. In order to improve the existing control method, it is important to apply the setpoints according to the real time indoor conditions. The real time indoor condition can be utilized for control through prediction model. In this study, the indoor load, indoor air quality and energy consumption were predicted and used for VAV system control. In addition, the predictive model was developed with the ANN algorithm, and the process of selecting input data and optimizing the predictive model was performed. The performance of the ANN based optimal control algorithm (suggested CASE) for the VAV terminal unit in the target building was compared with those of the dual maximum control algorithm (existing CASE), one of the VAV terminal unit control algorithms. The comparison of heating season showed that the ANN based control algorithm of VAV terminal unit reduced 16.7% of supply fan energy consumption and 19.5% of reheat coil energy consumption compared to the existing CASE using the fixed setpoint.

MPC solution for optimal load shifting for buildings with ON/OFF staged packaged units: Experimental demonstration, and lessons learned

Small and medium-sized commercial buildings (SMCB) are significant demand response resources, and it is important to develop grid-responsive control algorithms that exploit those resources and create financial benefits for building owners and HVAC service providers. Furthermore, unlike large-sized commercial buildings, there is an opportunity to have universally applicable control solutions for many SMCBs since those buildings have a consistent HVAC system configuration: SMCBs are commonly served by multiple-staged air conditioning units controlled by their own thermostats. Despite the demand response potential and scalability, however, very few control solutions are available for SMCBs. Typical model predictive control (MPC) and heuristic control approaches for cooling load shifting that lower thermostat setpoints before an electric price jump are suitable mainly for large-sized commercial buildings where a continuous capacity modulation is possible, e.g., via dampers in variable air volume terminal units. However, those approaches can cause undesired, high peaks for SMCBs due to the nature of ON/OFF unit staging and narrow thermostat deadbands. This could discourage the use of advanced grid-responsive controls for SMCBs due to the concern of high demand charges, and has to be resolved. This paper presents a MPC solution that overcomes this challenge. It has a hierarchical MPC structure where an upper level MPC is responsible for electrical load shifting in response to an electric price signal while a lower level MPC is responsible for coordinating compressor stages to eliminate unnecessary peaks and follows the setpoints determined by the upper level MPC. Two one-month, comprehensive laboratory tests have been carried out to demonstrate load shifting and cost savings for the algorithm. Interesting trade-offs between energy efficiency and load flexibility were observed and are discussed, and lessons learned for applying MPCs for SMCBs are also presented.

New Optimal Supply Air Temperature and Minimum Zone Air Flow Resetting Strategies for VAV Systems

Buildings account for a large portion of the total energy use in the US; therefore, improving the operation of typical variable-air-volume (VAV) systems in buildings can provide a tremendous economic opportunity. ASHRAE Guideline 36 recommends a resetting strategy for supply air temperature (SAT) for VAV systems based on outside air temperature. However, this strategy may not produce optimal performance, particularly when simultaneous cooling and heating occurs in zones. In addition, there is no strategy recommended in the Guideline to reset the zone minimum airflow set point in a single-duct VAV terminal unit with reheat, although this setpoint has a great impact on zone reheat requirements and ventilation efficiency. Thus, this paper introduces new strategies to reset both the SAT and zone minimum airflow rate set points to improve the efficiency of typical VAV systems. The strategies were tested under various conditions through experiments performed in fully instrumented VAV systems located in the HVAC lab at the University of Cincinnati. The experiments were conducted on a chilled-water VAV system that serves three controlled zones with hot-water reheat VAV boxes controlled by a typical commercial BACnet web-based building automation system BAS. The simulation studies were performed using the building energy simulation software EnergyPlus to evaluate the strategies at a larger scale in various locations. The simulation results show that the proposed resetting strategies can provide fan energy savings between 1.6% and 5.7% and heating load savings between 7.7% to 33.7%, depending on the location. The laboratory testing shows that the proposed strategies can provide stable control performance in actual systems as well as achieving the anticipated reheat and fan energy savings. The result offers significant improvements that can be implemented in the Guideline for single-duct VAV system operation and control.

Inverse model-based detection of programming logic faults in multiple zone VAV AHU systems

Implementation of sequences of operation for heating, ventilation, and air conditioning (HVAC) systems into a building automation system (BAS) is a critical process for building performance. Mistakes are commonly made while interpreting the sequences of operation from the mechanical design, converting them into BAS programs, and making ad-hoc changes to the BAS programs to address occupant complaints. This paper presents a novel inverse model-based fault detection and diagnostics (FDD) method for the discovery of BAS programming logic faults in variable air volume (VAV) air handling unit (AHU) systems. Using widely available BAS trend data types, the method first trains inverse models characterizing the heat and air mass balance of a VAV AHU system’s mixing box, heating and cooling coils, VAV terminal units, and zones. Then, the expected operational behaviour of the VAV AHU system is constructed by using ASHRAE Guideline 36 sequences of operation and the trained inverse models. Deviations from the expected operation are treated as the symptoms for the detection of programming logic faults. The method is demonstrated with BAS trend data from a 41-zone VAV AHU system in Ottawa, Canada. Two faults in supply air temperature and pressure reset programs are detected, significantly increasing fan electricity and heating energy use.

Optimal Control Method of Variable Air Volume Terminal Unit System

This study reviewed the existing studies on the types of variable air volume (VAV) terminal units, control and operation methods, prediction models, and sensor calibration methods. As a result of analyzing the existing research trends on the system type, characteristics, and control method of VAV terminal units studies such as theoretical verification and energy simulation were conducted to improve the existing control methods, reset the set value using a mathematical model, and add a monitoring sensor for the application of control methods. The mathematical model used in the study of VAV terminal unit control methods was used to derive set values for minimum air volume, supply temperature, ventilation requirements, and indoor comfort. The mathematical model has a limitation in collecting input information for professional knowledge and model development, and development of a building environment prediction model using a black box model is being studied. The VAV terminal unit system uses a sensor to control the device, and when an error occurs in the sensor, indoor comfort problems and energy waste occur. To solve this problem, sensor calibration techniques have been developed using statistical models, mathematical models, and Bayesian statistical models. The possibility of developing a method for calibrating the variable air volume terminal unit sensor using the prediction model was confirmed. In conclusion, the VAV terminal unit system is one of the most energy efficient systems. The energy saving potential of current VAV systems can still be improved through control methods, the use of predictive models, and sensor calibration methods.

A study on the sensor calibration method using data-driven prediction in VAV terminal unit

In this paper, sensor calibration method using data-driven prediction models are developed to eliminate sensor error in the variable air volume (VAV) system. Indoor sensible loads and indoor carbon dioxide (CO2) concentrations, which are the main factors necessary for sensor operation in a VAV system, were predicted using a data-driven model (artificial neural network). Using this prediction model, we developed a method to calibrate the sensor error by using it to derive a system model of the sensor that we want to calibrate. As a result of the performance evaluation of the indoor sensible load prediction model, MBE was −1.8% and Cv(RMSE) was 3.4%. The performance evaluation of CO2 prediction models showed that MBE was −3.2% and Cv(RMSE) was 5.4%. In verification of sensor calibration method, it was confirmed that the error occurring in the sensor can be corrected through the application and verification of the method and procedure for calibrating the VAV terminal unit sensor using the prediction model. In addition, it was confirmed that the error can be corrected both in the case of a single error (CASE1 ∼ CASE3) in which an error occurs in one sensor and in the case of two or more multiple errors (CASE4 ∼ CASE7). In addition, the calibration of the sensor data was able to solve practical difficulties such as sensor replacement.

Multi-sensor information fusion based control for VAV systems using thermal comfort constraints

The conventional control methods of variable air volume (VAV) air conditioning systems usually assume that the indoor air is well mixed, and consider each building zone as one node with homogeneous temperature distribution. The average temperature is subsequently used as the controlled parameter in the VAV cascade control process, which might cause uneven temperature distribution and unsatisfactory thermal comfort. This paper presents a coupled simulation of computational fluid dynamics (CFD) and building energy simulation (BES) for the VAV system in an office building located in Shanghai for the purpose of simulating the building, the VAV control system, and indoor thermal environment simultaneously. An external interface is developed to integrate the CFD and BES models based on quasi-dynamic coupling approach. Based upon the developed co-simulation platform, the novel VAV control method is further proposed by fusing information from multiple sensors. By adding two temperature sensors to constrain the thermal comfort of the occupied zone, the supply air temperature setpoint of the VAV terminal unit can be reset in real time. The novel control method is embedded into the co-simulation platform and compared with the conventional VAV control approach. The results illustrate that the temperature distribution under the proposed method is more uniform. At most times of the typical test day, the air diffusion performance indexes (ADPIs) for the proposed method are above 80%, while the ADPIs for the conventional control method are between 60% and 80%. Due to multi-sensor information fusion, the proposed VAV control approach has better ability to ensure the indoor thermal comfort.

Development of Air Flow Rate Prediction Model Using Multiple Regression in VAV Terminal Unit

Accurate measurement of air flow rate is essential in automatic building control using the variable air volume (VAV) system. In order to solve the problems of the existing air flow measurement method and improve the accuracy of air flow control, this study developed a data-based multiple regression air flow prediction model. The independent variables used in the development of the predictive model were selected as the factors used for control and monitoring when operating with variable air flow rate in the existing air conditioning system. Data collection and correlation between independent variables and air flow rate of the terminal unit were analyzed. Using the IBM SPSS statistics version 25, an air flow rate prediction model was developed using multiple regression analysis. Reliability of model was evaluated by comparing the measured airflow. The relative error of −9.3% to 10.4% is shown when comparing the estimated air flow rate by the developed model with the measured air flow rate.

Development of Virtual Air Flow Sensor Using In-Situ Damper Performance Curve in VAV Terminal Unit

In this study, we developed a virtual air flow sensor using an in-situ damper performance curve to secure the stability of control of the variable air volume (VAV) terminal unit, and also established the in-situ measurement procedure. The minimum air flow rate of the VAV terminal unit was related to the energy consumption, it was important to determine the minimum air flow rate suitable for the situation of each room in terms of energy saving. However, it was difficult to set the minimum air flow rate low setpoint due to the low accuracy of the air flow sensor of the VAV terminal unit. This paper suggested a virtual air flow sensing method using an in-situ damper performance curve in the VAV terminal unit. The input factors of the virtual sensor were developed on the basis of the supply fan speed and damper opening ratio, which could be easily obtained from the existing control system. In addition, the in-situ measurement procedure of the virtual air flow sensor was developed by dividing the procedure into five steps. Finally, reliability of the virtual air flow sensor was evaluated through uncertainty analysis of input variables and relative error analysis, in comparison with the conventional air flow rate measurement method. The developed virtual air flow sensor was found to have an uncertainty of up to 8.8%, and it was also found that the closer to the maximum the values of the input variables, the lower the uncertainty. In addition, verification of relative error with respect to the measured values by the hot-wire anemometer was conducted by varying operation conditions to a total of 12 cases, and as a result, relative error was found to be up to 5.6%. In addition, the results of long-term experiment showed that relative error was within about 9.5%, and thus, the feasibility for field application and control was confirmed.

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.

A Study on a Control Method with a Ventilation Requirement of a VAV System in Multi-Zone

The objective of this study was to propose a control method with a ventilation requirement of variable air volume (VAV) system in multi-zone. In order to control the VAV system inmulti-zone, it is essential to control the terminal unit installed in each zone. A VAV terminal unit with conventional control method using a fixed minimum air flow can cause indoor air quality (IAQ) issues depending on the variation in the number of occupants. This research proposes a control method with a ventilation requirement of the VAV terminal unit and AHU inmulti-zone. The integrated control method with an air flow increase model in the VAV terminal unit, AHU, and outdoor air intake rate increase model in the AHU was based on the indoor CO2 concentration. The conventional and proposed control algorithms were compared through a TRNSYS simulation program. The proposed VAV terminal unit control method satisfies all the conditions of indoor temperature, IAQ, and stratification. An energy comparison with the conventional control method showed that the method satisfies not only the indoor thermal comfort, IAQ, and stratification issue, but also reduces the energy consumption.

A Study on the Air Flow Control of VAV Terminal Unit in Multi-Zone

The increasing social interests for energy saving has highlighted the need for more buildings with variable air volume (VAV) systems. In order to control the VAV system at multi-zone, it is essential to control terminal unit installed in each zone. This research proposes control method with ventilation requirement of VAV terminal unit in multi-zone. And the conventional and proposed control algorithms were compared through a TRNSYS simulation program. The proposed VAV terminal unit control method satisfies all the conditions of indoor thermal comfort and indoor air quality. An energy comparison with the conventional control method showed that, the method satisfies not only the indoor thermal comfort and IAQ, but also reduces the energy consumption.

A Study on the Control Method Without Stratification of Single Duct VAV Terminal Units

The objective of this study was to propose a stratification control method for the minimum air flow of a variable air volume (VAV) terminal unit. The minimum air flow rate of the VAV terminal unit is the key factor affecting thermal comfort, indoor air quality (IAQ), stratification and energy consumption depending on the operating mode of the VAV system. Therefore, selecting the proper minimum air flow is very important. In this study, a building simulation was conducted to evaluate the indoor thermal comfort, stratification, IAQ and energy consumption according to the fixed minimum air flow rate of the VAV terminal units. In addition, an algorithm was proposed considering the stratification and variable minimum air flow rate depending on the indoor load. The results of the fixed minimum air flow rate simulation showed that the minimum air flow rate considering stratification satisfies all the conditions of indoor thermal comfort, IAQ and without stratification. The proposed method was found to be more effective than the existing control method. The energy comparison with the existing control method showed that the proposed method reduces the energy consumption.

Stability and accuracy of variable air volume box control at low flows. Part 1: Laboratory test setup and variable air volume sensor test

Variable air volume systems with direct digital controllers have been widely adopted in the HVAC system of commercial, industrial, and large residential buildings because they provide better energy efficiency and occupant comfort. Normally, a variable air volume terminal unit defines a minimum airflow rate to satisfy the space ventilation requirement and/or the proper operation of a terminal heating coil, if so equipped. However, it has been found that variable air volume terminal units often fail to perform as expected at the minimum airflow range (below 500 fpm [2.5 m/s]). Under such a flow range, the embedded airflow sensor becomes inaccurate, and the designed minimum airflow rate is less than the minimum controllable airflow rate. This results in a series of problems, including lack of ventilation, uneven airflow control, reduced damper and operator life, and energy waste. Through designed laboratory and field tests, this study (ASRHAE Research Project RP-1353) aims to identify the major factors that cause inaccuracy and instability issues in variable air volume terminal units and the relationship between the major factors and performance of the airflow sensor, controller, and terminal unit system. Laboratory tests performed in this study included a variable air volume sensor test, controller test, and system test. Four variable air volume boxes from three manufacturers and four controllers from four manufacturers were tested systematically. Two identical test beds with high accuracy (±0.5%) reference airflow meters were designed and constructed in the test facility. The size of the reference airflow measuring stations was carefully selected to provide maximum airflow measuring accuracy and maximum available system pressure drop. This article describes the laboratory test setup and summarizes the variable air volume sensor test results. A companion article summarizes the controller test, system test, and field test results. From the variable air volume sensor test, three factors, namely, inlet conditions, low variable air volume damper positions, and low airflow rates, are identified as strongly impacting variable air volume terminal unit performance.

Stability and accuracy of variable air volume box control at low flows. Part 2: Controller test, system test, and field test

This article with its companion paper (Liu et al. 2013), summarizes the findings of ASHRAE Research Project 1353 (Stability and Accuracy of VAV Box Control at Low Flows). This project aims to identify the major factors that cause the airflow measurement in a variable air volume system to be inaccurate and unstable, especially at low airflow conditions. Both a laboratory test (including variable air volume sensor test, controller test, and system test) and field test were conducted; the companion work discussed the variable air volume sensor test. In this article, findings from the controller test, system test, and field test are summarized. The controller tests involved testing of four controllers from four different manufacturers. Testing was performed for accuracy, stability, resolution, and ambient temperature effect. For the system test, the variable air volume box and the controller were operated together and tested as terminal unit systems. Two terminal units were tested, and it was found that the performance of a variable air volume terminal unit is highly dependent upon on controller performance. Zeroing and balancing at a low airflow rate 560 fpm (2.84 m/s) or 200 cfm (0.09 m3/s) for an 8-in. (0.2-m) box were effective for achieving high system accuracy at low airflow ranges. For the field tests, five variable air volume terminal units were tested in real commercial buildings. It was found that system balancing was not always an effective way to reduce the variable air volume airflow sensor error in the field due to the uncertainty of reference airflow measurement methods commonly adopted in the field testing and balancing process.

Improving airflow measurement accuracy in VAV terminal units using flow conditioners

A variable air volume (VAV) terminal unit adjusts its supply airflow rate to meet the heating or cooling load and/or the ventilation requirement of the served space. Consequently, the accuracy of the VAV airflow sensor is highly important to the VAV system operation, and an inaccuracy of the VAV airflow sensor could lead to an energy waste or insufficient ventilation. ASHRAE Research Project (RP) 1353 identified non-ideal inlet conditions, such as an elbow or kinked duct before the VAV terminal unit, as causes of observed inaccuracies of up to 45% in VAV airflow measurements. VAV airflow measurement errors are normally mitigated by on-site balancing; however, it is difficult to achieve accurate reference airflow measurements in the field because of limited straight ductwork before VAV terminal units, as well as ductwork leakage. This study explored the potential solution of using a VAV flow conditioner to regulate the velocity profile upstream of the VAV airflow sensor and increase the VAV airflow measurement accuracy. A variety of flow conditioners were evaluated with computational fluid dynamics (CFD) modeling, and a CFD-optimized prototype of a 60%-porosity K-Lab/Laws plate was fabricated and tested. For all tested inlet conditions, airflow rates, and VAV boxes, the prototype reduced the VAV airflow reading error to ±5% when it was installed immediately before the VAV box inlet, regardless of upstream duct conditions. The prototype flow conditioner had a pressure drop equivalent to that of a 2-row VAV reheat coil.