Variable Air Volume Research Articles

Deep-learning-based fault detection and diagnosis of air-handling units

This study proposed a real-time fault diagnostic model for air-handling units (AHUs); the model used deep learning to improve the operational efficiency of AHUs and thereby reduce the energy consumption of HVAC—heating, ventilating, and air conditioning—systems in buildings. Additionally, EnergyPlus simulation software was employed to establish different types of fault operation behavior data to serve as references for deep learning, thus reducing the complexity of data preprocessing, retaining data completeness, and improving the reliability of the diagnostic model.The proposed deep neural network fault diagnostic model can serve as a reference for this research field; the model features five hidden layers, each comprising 200 neurons. Additionally, this study tested abnormal faults commonly observed in AHUs, including failure to control two-way hydronic valves and variable air volume box dampers as well as supply air temperature sensors exhibiting measurement error. After performing diagnosis with data that had not been used in the training or verification process, the diagnostic results indicated that the diagnostic model exhibited 95.16% accuracy.

Reducing university air handling unit energy usage through controls-based energy efficiency measures

With heating, ventilation, and air conditioning (HVAC) consuming the largest share of commercial building energy usage in the U.S., identifying HVAC energy efficiency measures can have a large impact. Thus, this research provides results from a University of Dayton case study of twelve total air handling units (AHUs) in five diverse buildings – academic, athletics, administrative, place of worship, and library – that were analyzed for energy savings through three energy efficiency measures: reducing AHU static pressure (SP), scheduling AHUs off overnight, and identifying and fixing economizer controls. First, variable air volume (VAV) damper position distributions were created to identify over-pressurized AHUs and step down their SP accordingly. Eleven of the twelve AHUs were determined to be over-pressurized and lowering their SP to final setpoints ranging 0.8 – 1.2 inches water-gauge resulted in average fan energy savings of 33%. Next, nine of the twelve AHUs were found to be serving unoccupied zones overnight and were in turn scheduled off for seven to twelve hours per night depending on building usage, resulting in 35% average energy savings. Lastly, AHU fraction outdoor air was calculated and compared to ideal economizer fraction outdoor control. All economizers were identified as fully or partially malfunctioning and if fixed could result in an average of 17% cooling savings.

PIV measurement and CFD simulations of an air terminal device with a dynamically adapting geometry

Air quality is an important factor for human well-being, and it is crucial that when using mechanical ventilation system, air is properly distributed so it reaches every user of a ventilated zone. To improve ventilation systems, and in consequence the air quality, this paper focuses on studying the impact of an air terminal device (ATD) with a dynamically changing geometry on the effectiveness of a variable air volume (VAV) ventilation system. VAV system characterizes a change in the airflow magnitude through the system when the heat gains lower, which may be a risk for human health as air may not reach the furthest parts of a zone. To combat this threat, an ATD with a dynamically changing geometry was installed. As a ventilation quality indicator, the air throw was taken under consideration. Thanks to the new ATD, the steady air throw should be maintained despite the changes in flow in the ventilation system. To achieve this, the research was divided into two stages. The first stage included a series of computational fluid dynamics simulations that considered the alteration of the airflow and the air terminal device diameter. Afterwards, verifying laboratory measurements were conducted on a laboratory stand using a particle image velocimetry (PIV) system that included an air terminal device with a changing geometry. The results of the simulations and the PIV measurements showed that the changes in the geometry of the air terminal device improve the ventilation effectiveness of a VAV system, allowing the system to maintain a constant air throw despite the changing airflow magnitude through the system.

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.

Too many Cooks in the kitchen?: A review of noise and vibration challenges in mixed-use buildings for the session: Structure-borne noise in buildings and what we can do about it

Several projects with rooftop structural vibration and/or noise transmission through structures, and footfall impact issues will be shared, along with a review of recommendations and outcomes of the example projects. We plan to discuss noise/vibration challenges such as: (1) Design, or re-design, of Heating, Ventilation, and Air-conditioning (HVAC) and kitchen exhaust fan (KEF) on rooftops and within building envelopes. (2) Finding noise leaks in structure. (3) Is it the MRI machine’s vibration, or the Variable Air Volume (VAV) box causing noise and vibration in the office upstairs? (4) Discussion of resonance of different floor/ceiling assemblies.

Neural Network Model-Based Adaptive Control of a VAV-HVAC&R System

A dynamic system model of a two-zone variable air volume heating, ventilation and air conditioning and refrigeration (VAV-HVAC&R) system is considered. The system model consists of two environmental zones, an HVAC system and a water-cooled vapor compression chiller. Five adaptive controllers were designed to achieve good tracking control of set points of zone air temperatures, discharge air temperature, chilled water supply temperature and static pressure of the VAV-HVAC&R system. The PI controller gains were updated online using adaptive neural networks and an auto-tuning algorithm. Simulation results show that adaptive PI control gave faster response and less overshoot compared to conventional constant gain PI control. The control responses tracked set-points closely and remained stable over a typical day simulation of building operation under variable load conditions.

Indoor Air Quality Enhancement Performance of Liquid Desiccant and Evaporative Cooling-Assisted Air Conditioning Systems

The main objective of this study is to investigate the indoor air quality enhancement performance of two different liquid desiccant and evaporative cooling-assisted air conditioning systems, such as the variable air volume (VAV) system with the desiccant-enhanced evaporative (DEVap) cooler, and the liquid desiccant system with an indirect and direct evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS), compared with the conventional VAV system. The transient simulations of concentration variations of carbon dioxide (CO2), coarse particles, and fine particles (PM10 and PM2.5) in a model office space served by each system were performed using validated system models that were found in the literature. Based on the hourly thermal loads of the model space predicted by the TRNSYS 18 program, each air conditioning system was operated virtually using a commercial equation solver program (EES). The results indicated that the LD-IDECOAS provided the lowest annual indoor CO2 concentration among all the systems considered in this research, while the VAV system with DEVap cooler exceeded the threshold concentration (i.e., 1000 ppm) during the cooling season (i.e., July, August, and September). For the indoor particulate contaminant concentrations, both liquid desiccant and evaporative cooling-assisted air conditioning systems indicated lower indoor PM2.5 and PM10 concentrations compared with the reference system. The LD-IDECOAS and the VAV with a DEVap cooler demonstrated 33.3% and 23.5% lower annual accumulated indoor PM10 concentrations than the reference system, respectively. Similarly, the annual accumulated indoor PM2.5 concentration was reduced by 16% using the LD-IDECOAS and 17.1% using the VAV with DEVap cooler.

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.

Cascade control for supply air temperature in a variable air volume system

Most supply air temperature in variable air volume systems is typically set at 55F(13 °C) that is design value in these systems. However, sometimes the load of cooling varies greatly in different zones, and it means the indoor temperature of low-load zones is lower than comfortable value, even if the supply air damper is at the minimum value that is not closed for minimum outside air requirement. Meanwhile, in transition season, the indoor temperature is also lower due to low outdoor temperature. These factors provided an opportunity to apply higher supply air temperature to reduce air handling unit (AHU) cooling load and box reheating. In this paper, a cascade control method is used to raise the supply air temperature. The external control loop controls the setting value of supply air temperature with the minimum supply air volume as the control target. And the internal control loop controls the supply air temperature by regulating valve. The cascade structure can solve the problem that the time constant of the system is too large. The simulation results show that the quality of control is good and the reheating can be cancelled.

Energy savings potential of passive chilled beams vs air systems in various US climatic zones with different system configurations

The energy savings potential of passive chilled beams in various climatic zones is analyzed in this study. A passive chilled beam model, developed based on full-scale experiments, is used as a system module in a whole building simulation tool to account for the convective and radiative effects of passive chilled beams. The model was validated with measurements from a field study in an open plan office equipped with multiple passive chilled beams. In addition, a parallel field study in an adjacent identical office space equipped with an air (VAV) system was conducted to compare the resulting energy consumption with the two systems.To further study the energy savings potential and indoor thermal conditions with passive chilled beams, four different chilled beam system configurations, combined with a parallel variable air volume system, were studied using the whole building model in different US climatic zones. The results showed that using a dedicated outdoor air system, or a separate chiller for the passive chilled beams, or a desiccant wheel after the cooling coil may result in significant relative energy savings depending on the location. The last option proved to be the most efficient, providing up to 12% for hot and humid climates and up to 20% for hot and dry climates –compared to a conventional variable air volume system. Finally, the radiative-to-total cooling effect of passive chilled beams varied between 7% and 8% and has only a small impact on energy savings and indoor thermal conditions.

Control Strategies in Multi-Zone Air Conditioning Systems

In a commercial building, a significant amount of energy is used by the ventilation systems to condition the air for the indoor environments to satisfy the required quantity (temperature and humidity) and quality (amount of fresh air). For many years, Variable Air Volume (VAV) systems have been considered as the most efficient solutions by balancing the airflow volume based on the demand making them energy efficient when compared with the traditional Constant Air Volume (CAV) systems. However, the setpoints in VAV systems are often misread by the sensors due to stratification and formation of pollutant pockets and responding to design levels that overestimate the real-time demand conditions, which result in waste of energy, thermal discomfort and unhealthy air. In general, VAV devices are expensive, complicated and prone to failures and they are used only in medium and large projects. More recently, new technologies have evolved to solve this issue. In one of the new solutions, VAV motors terminals are replaced with flaps which are simpler and less expensive thus, they can be implemented in a wider range of projects. In systems, balancing and supplying the optimal airflow to reduce the energy consumption while delivering ideal thermal and Indoor Air Quality (IAQ) levels are the main challenges. In this paper, a comparison of the recent technologies with traditional VAV systems is presented to be used as a guild line for researchers and designers in the field of Heating Ventilation Air Conditioning (HVAC).

Energy-saving potential of dedicated outdoor-air system assisted by vacuum-based membrane dehumidifier

The proposed research presents a dedicated outdoor-air system (DOAS) integrated with a vacuumbased membrane dehumidifier (VMD). The primary objective of this study was to evaluate the energy-saving potential of the proposed VMD–DOAS combination. VMD–DOAS comprised a membrane-energy exchanger (MEE), dew-point indirect evaporative cooler (DP-IEC), and VMD. VMD possessed a characteristic by virtue of which the dehumidification process was isothermal; i.e., no temperature change was observed during the VMD process. While VMD served to control the dry-air supply, the required target temperature (i.e., 17 °C) was maintained via DP-IEC operation. The remaining sensible heat of the conditioned zone was controlled by the ceiling radiant cooling panel (CRCP). The load of the conditioned zone was driven by TRANSYS 18, and an engineering equation solver (EES) was used for evaluating the energy-saving potential of the proposed system with CRCP by comparing it against the variable-air-volume (VAV) system. Results of this study demonstrated that the proposed DOAS with CRCP consumed 37% less operating energy compared to the VAV system. This observed energy-saving potential of the proposed system was driven by reducing the dehumidification load and subsequent energy recovery by MEE.

Model predictive control for buildings with active one-pipe hydronic heating

One-pipe hydronic heating systems in their active (decentralized pumping) form promise great benefits over traditional two-pipe variable volume systems, and even more so over variable air volume systems. The heat exchanger units are connected in series, which presents its challenges and opportunities. This paper presents a model predictive controller capable of harnessing as many benefits as there is in the system when used in a building. A case study on a small office building illustrates the capabilities and validates the concept.

Modular modeling of air-conditioning system with state-space method and graph theory

To improve the modeling efficiency of HVAC system, the modular models for HVAC components are to be established by using the state-space method in this study. The component modular models are used to establish a dynamic system model for a VAV (variable-air-volume) system, and the dynamic system model has been validated by transient response experiments. As an example of model application, the ARIMA-based predictive PI controller for room temperature has been investigated with the system model, which is shown to have the better control effect than the traditional PI controller, and this has been validated by experiments as well. The work in this study will be of significance to the dynamic modeling and control algorithm study of air conditioning systems.

Rule-based fuzzy control method for static pressure reset using improved Mamdani model in VAV systems

In variable air volume air(VAV) conditioning systems, conventional methods for static pressure reset depends on accurate mathematics models which are often difficult to be built. On the contrary, controls, which are based on a series of control rules derived from the operating and commissioning experience, have the capacity to achieve better effects. A rule-based fuzzy control method (RBFCM) for static pressure reset is proposed using improved Mamdani model and functioning fuzzy subset inference method, which reflects the physical characteristics of the controlled object and is convenient for the engineers to understand and use. The development and implementation of the proposed RBFCM are necessary to demonstrate in detail. Through comparative experiments, the proposed RBFCM is compared with the conventional fixed-step method and itself with different actual domain in a control experiment platform for the VAV air conditioning system. Experimental results indicate that the proposed RBFCM has better control performances and outstanding energy-saving potential if the actual domain can be selected reasonably. A minimum of 33% energy saving rate of the proposed RBFCM with the optimal actual domain is shown through experimental study.

Heating energy performance and part load ratio characteristics of boiler staging in an office building

Commercial buildings account for significant portions of the total building energy in South Korea and thus a variety of research on the boiler operation related to heating energy in office buildings has been carried out thus far. However, most of the researches have been conducted on the boiler itself, i.e., the part load ratio characteristics and the corresponding gas energy consumption patterns are not analyzed in the existing studies. In this study, the part load ratio and the operating characteristics of gas boiler have been analyzed within an office building equipped with the conventional variable air volume system. In addition, the gas consumption among different boiler staging schemes has been comparatively analyzed. As a result, significant portions of total operating hours, heating load and energy consumption has been found to be in a part load ratio range of 0 through 40% and thus energy consumption is significantly affected by boiler efficiency at low part load conditions. This suggests that boiler operation at the part load is an important factor in commercial buildings. In addition, utilizing sequential boiler staging scheme can save a gas usage of about 7%. For annual heating energy saving, applying the sequential control boiler with a 3:7 proportion staging is considered to be the optimal control algorithm for maximum efficiency of boilers.

A proactive fault detection and diagnosis method for variable-air-volume terminals in building air conditioning systems

A proactive fault detection and diagnosis (FDD) method is proposed for variable-air-volume (VAV) terminals in this study to overcome the problem of diagnostic information poor. The basic idea is to produce additional diagnostic information proactively through introducing specific disturbances to the suspected VAV terminal. The dynamic responses of the VAV terminal, which can be observed from the measurements and control signals, are helpful in isolating faults which are challenging for the conventional passive FDD methods. A list of proactive actions is designed to introduce the smallest disturbances into indoor environment. Therefore, the proposed method can work during office hours. Without loss of generality for practical applications, the proactive actions change the zone air temperature set-point only. The proposed method can be implemented in both building management systems and VAV terminal controllers. Evaluations are made on a simulated VAV system on Dymola platform. Results show that the method can improve FDD performance significantly.

Optimization of BuildingsEnergy Consumption by Designing Sliding Mode Control for Multizone VAV Air Conditioning Systems

Variable air volume (VAV) is the most common installation among heating ventilating and air conditioning (HVAC) systems. To maintain the comfort level and lessen energy utilization, there is a pressing need for its effective control. In this study, a lumped parameter model composed of multizone VAV is considered, and sliding mode control (SMC) is designed to guarantee robust operation in the presence of uncertainties. For comparison of the proposed controller performance, a proportional integral derivative (PID) controller is additionally designed. The indoor temperature of zones is controlled by positioning the supply air dampers. Tracking objectives of controllers are inspected via two practical cases of desired temperature setpoints including (a) sinusoidal waveform and (b) the combination of steps. Results obtained using SMC ensure the robust operation of the VAV system against parametric uncertainties. In addition, SMC is more energy efficient than PID in terms of overshoot and settling time.

Analysis of thermo-active foundations for office buildings

Ground source heat pumps (GSHPs) have been widely used to heat and cool buildings. However, the installation costs of GSHPs with vertical boreholes can be significant and may result in reducing their cost-effectiveness relative to other conventional heating and cooling systems. As alternatives to GSHPs, thermos-active foundations (TAFs) have lower installation costs but similar thermal performance. This paper provides a simplified analysis approach using the G-function method specific to TAF systems. In particular, G-functions have been determined with various thermal pile shapes to predict the short-term and the long-term performance of TAFs. As part of analysis outlined in the paper, the G-functions are integrated in a whole-building simulation tool to compare the energy efficiency and cost effectiveness of TAFs and GSHPs to conventional VAV systems for office buildings in three US climates. It is found that TAFs have slightly better energy efficiency but are significantly more cost effective than both GSHPs and variable air volume (VAV) systems in all the US climates considered in the analysis.

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.