Outdoor Air Fraction Research Articles

A practical real-time control strategy for high-performance building HVAC operation concerning potential pandemic outbreaks in post-pandemic era

The lessons learned from the COVID-19 pandemic emphasize the need for building HVAC systems to be adaptable and prepared to adjust their operation for infection risk mitigation for future pandemics. Most guidelines suggest increasing outdoor air fraction in HVAC systems during pandemics. However, this approach is not energy-efficient due to the conditioning of additional outdoor air, and many practical systems face limitations in introducing more outdoor air than normal due to capacity or design constraints. Therefore, there is an urgent need for an alternative solution for HVAC system operation during pandemics that is energy-efficient, can effectively mitigate infection risks and does not require extensive retrofits to existing systems. To meet this need, this study proposes a practical real-time control strategy for high-performance HVAC operation in response to potential pandemic outbreaks. First, a supply air temperature (SAT) reset scheme for infection risk mitigation is proposed based on Mamdani fuzzy inference theory. Then, a coordinated control scheme is proposed to coordinate SAT reset with chilled water temperature adjustment for energy-efficiency enhancement. This strategy does not require system equipment retrofits but only changes in control logic to transition to pandemic operation. The effectiveness of the proposed strategy is validated in a high-fidelity Modelica-based simulation environment under various weather conditions. Results show that, compared to normal operation, the proposed strategy can significantly reduce new infection cases by 32∼46 % without increasing energy use, and may even achieve slight energy savings of 9∼13 %, while maintaining expected occupant comfort.

Quantification of how mechanical ventilation influences the airborne infection risk of COVID-19 and HVAC energy consumption in office buildings.

This paper presents an EnergyPlus-based parametric analysis to investigate the infection risk of Coronavirus Disease 2019 (COVID-19) under different mechanical ventilation scenarios for a typical medium-sized office building in various climate zones. A Wells-Riley (WR) based Gammaitoni-Nucci (GN) model was employed to quantitatively calculate the airborne infection risk. The selected parameters for the parametric analysis include the climate zone, outdoor air fraction, fraction of infectors, quanta generation rate, and exposure time. The loss and deposition of particles are not considered. The results suggest that the COVID-19 infection risk varies significantly with climate and season under different outdoor air fraction scenarios since the building heating and cooling load fundamentally impacts the supply airflow rate and thus directly influences the amount of mechanical ventilation, which determines the dilution ratio of contaminants. This risk assessment identified the climate zones that benefit the most and the least from increasing the outdoor air fraction. The climate zones such as 1A (Honolulu, HI), 2B (Tucson, AZ), 3A (Atlanta, GA), and 7 (International Falls, MN) are the most energy-efficient locations when it comes to increasing the outdoor air fraction to reduce the COVID-19 infection risk. In contrast, the climate zones such as 6A (Rochester, MN) and 6B (Great Falls, MT) are the least energy-efficient ones. This paper facilitates understanding a widely recommended COVID-19 risk mitigation strategy (i.e., increase the outdoor airflow rate) from the perspective of energy consumption.Electronic Supplementary MaterialSupplementary material is available for this article at 10.1007/s12273-022-0937-5 and is accessible for authorized users.

Associations between outdoor air pollution, ambient temperature and fraction of exhaled nitric oxide (FeNO) in university students in northern China - A panel study

BackgroundNorthern China has severe air pollution, especially in winter. Fractional exhaled nitric oxide (FeNO) is an established biomarker of airway inflammation. AimTo study associations between ambient temperature, air pollution and FeNO in university students in northern China. MethodsWe performed a panel study in 67 university students without asthma diagnosis in the city of Taiyuan. FeNO was measured 6 times, over one heating season. Outdoor PM10, PM2.5, SO2, NO2 and O3 were measured at a fixed location in the campus. SO2, NO2 and O3 were measured 7 days (24 h/day) before the FeNO test. PM2.5 and PM10 were measured at different lag times (lag 1 day to lag 7 days). Temperature and carbon monoxide (CO) data were collected from a nearby monitoring station (lag 7). Linear mixed models were applied to study associations between exposure and FeNO, adjusting for gender, age, current smoking, height and furry pet or pollen allergy. ResultsThe overall geometric mean (GM) of FeNO was 17.2 ppb. GM of FeNO was lowest (12.9 ppb) in January and highest (20.0 ppb) in April. The range of lag 7 pollution was 105.0–339.0 μg/m3 for PM10, 72.0–180.0 μg/m3 for PM2.5, 36.0–347.0 μg/m3 for SO2, 26.0–69.0 μg/m3 for NO2, 31.0–163.0 μg/m3 for O3 and 0.93–3.14 mg/m3 for CO. The lag 7 temperature ranged from −4.5 to 20.1 °C. FeNO was consistently higher at higher outdoor temperature (p < 0.001). In multi-pollutant models with temperature adjustment, PM10, PM2.5 and SO2 were associated with FeNO (all p-values <0.001). In contrast, CO was negatively associated (protective) with FeNO (p < 0.001). Associations between exposure and FeNO were similar in men and women. ConclusionPM10, PM2.5 and SO2 and outdoor temperature can be associated with airway inflammation, measured as FeNO, in young adults in northern China while CO could be negatively associated with FeNO.

Assessment of university classroom ventilation during the COVID-19 pandemic

Ventilation plays an important role in mitigating the risk of airborne virus transmission in university classrooms. During the early phase of the COVID-19 pandemic, methods to assess classrooms for ventilation adequacy were needed. The aim of this paper was to compare the adequacy of classroom ventilation determined through an easily accessible, simple, quantitative measure of air changes per hour (ACH) to that determined through qualitative “expert judgment” and recommendations from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), and the American Conference of Governmental Industrial Hygienists (ACGIH)®. Two experts, ventilation engineers from facilities maintenance, qualitatively ranked buildings with classrooms on campus with regard to having “acceptable classroom ventilation.” Twelve lecture classrooms were selected for further testing, including a mix of perceived adequate/inadequate ventilation. Total air change per hour (ACH) was measured to quantitatively assess ventilation through the decay of carbon dioxide in the front and rear of these classrooms. The outdoor ACH was calculated by multiplying the total ACH by the outdoor air fraction. The classrooms in a building designed to the highest ASHRAE standards (62.1 2004) did not meet ACGIH COVID-19 recommendations. Four of the classrooms met the ASHRAE criteria. However, a classroom that was anticipated to fail based on expert knowledge met the ASHRAE and ACGIH criteria. Only two classrooms passed stringent ACGIH recommendations (outdoor ACH > 6). None of the classrooms that passed ACGIH criteria were originally expected to pass. There was no significant difference in ACH measured in the front and back of classrooms, suggesting that all classrooms were well mixed with no dead zones. From these results, schools should assess classroom ventilation considering a combination of classroom design criteria, expert knowledge, and ACH measurements.

Using Co-simulation between EnergyPlus and CONTAM to evaluate recirculation-based, demand-controlled ventilation strategies in an office building

A coupled energy, airflow, and contaminant transport building model was developed using co-simulation between EnergyPlus and CONTAM. The model was used to analyze different strategies to control supply air delivery and return air recirculation rates including the use of demand-controlled ventilation (DCV) strategies. Strategies were evaluated for their effects on indoor pollutant concentrations and energy use of an office building in Trondheim, Norway. Typically, office buildings in Norway employ 100% outdoor air ventilation systems. Measurements in the office building served as the basis to develop the coupled model. The same building was also simulated with the outdoor conditions of Beijing.The results showed that all the simulated DCV strategies yielded reductions in energy use compared to a baseline, schedule-based strategy. Using recirculation of return air was also an energy efficient measure which increased the otherwise low indoor humidity levels in Trondheim. Using CO2-based DCV may result in increased levels of indoor particulate (PM2.5) from outdoors but using PM2.5 monitoring in the ventilation control strategies reduced indoor concentration of PM2.5 and energy usage. However, the low outdoor PM2.5 levels in Trondheim may not justify its use in this location. The Beijing case revealed that the indoor levels of PM2.5 can be reduced below the World Health Organization requirement of annual average of 10 μg/m3 using PM2.5 control.Co-simulation results revealed that it is possible to both reduce energy use and improve IAQ by controlling the outdoor air fraction based on multiple pollutants while also considering local outdoor environments.

Energy Conservation Potential of Economizer Controls Using Optimal Outdoor Air Fraction Based on Field Study

In commercial buildings, HVAC systems are becoming a primary driver of energy consumption, which already account for 45% of the total building energy consumption. In the previous literature, researchers have studied several energy conservation measures to reduce HVAC system energy consumption. One of the effective ways is an economizer in air-handling units. Therefore, this study quantified the impact of the outdoor air fraction by economizer control type in cooling system loads based on actual air-handling unit operation data in a hospital. The optimal outdoor air fraction and energy performance for economizer control types were calculated and analyzed. The result showed that economizer controls using optimal outdoor air fraction were up to 45% more efficient in cooling loads than existing HVAC operations in the hospital. The energy savings potential was 6–14% of the differential dry-bulb temperature control, 17–27% of the differential enthalpy control, 8–17% of the differential dry-bulb temperature and high-limit differential enthalpy control, and 16–27% of the differential enthalpy and high-limit differential dry-bulb temperature control compared to the no economizer control. The result of this study will contribute to providing a better understanding of economizer controls in the hospital when the building operates in hot-humid climate regions.

Performance Simulation and Analysis of Occupancy-Based Control for Office Buildings with Variable-Air-Volume Systems

Variable-air-volume (VAV) systems are used in many office buildings. The minimum airflow rate setting of VAV terminal boxes has a significant impact on both energy consumption and indoor air quality. Conventional controls usually have the terminal’s minimum airflow rate at a constant (e.g., 30% or more of the terminal design airflow rate), irrespective of the occupancy status, which may cause problems, such as excessive simultaneous heating and cooling, under ventilation, and thermal comfort issues. This paper examines the potential of energy savings from occupancy-based controls (OBCs). The sensed occupancy information, either occupant presence or people count, is used to determine the airflow rate of terminal boxes, the thermostat setpoints, and the lighting control. Using EnergyPlus, a whole-building energy modeling software, the energy savings of OBC strategies are evaluated for representative existing medium office buildings in the U.S. The simulation results show that the conventional OBC, based on occupant presence sensing, can save 8% of whole-building energy use in Miami (hot climate) for systems without air-side economizer and about 13% in both Baltimore (mixed climate) and Chicago (cold climate). Comparatively, the advanced OBC, based on people counting, can save 8% in Miami to 23% in Baltimore for systems with economizers. The outdoor-air fraction of the supply air from air-handling units significantly affects the potential energy savings from the advanced OBC strategy. In addition to energy savings, the advanced OBC satisfies the zone ventilation during all occupied hours over the whole year.

Development of Inverse Greybox Model-Based Virtual Meters for Air Handling Units

Energy submetering at the equipment level provides a tool to identify energy use anomalies and detect operational inefficiencies. While physical meters can be costly and difficult to install, virtual meters (VMs) overcome practical issues associated with physical meters and provide insights into critical unmeasured quantities. This article introduces an inverse greybox model-based virtual metering method to estimate the energy in an air handling unit (AHU). Models that represent the components typically found in AHUs are formulated using a data set from a highly instrumented AHU and combined into an integrated greybox model. The use of the integrated model to create VMs is demonstrated by using a data set from an independent AHU located in a large office building in Ottawa, ON, Canada. Model parameters are estimated by using the genetic algorithm and used in creating VMs that can estimate the heat supplied/extracted at the AHU level. In addition, the model is used to estimate a monthly average outdoor air fraction used by the AHU. The potential of the component models and VMs to detect operational inefficiencies and support operational decisions is demonstrated through illustrative examples. Note to Practitioners-This article presents a novel virtual metering algorithm to estimate the heating and cooling energy at the air handling unit (AHU) level. This virtual metering algorithm fills a gap in the literature and provides a tool that will help detect and interpret energy use anomalies, identify operational inefficiencies, and guide on-going commissioning of building energy systems. Facility managers, operators, and other stakeholders can use the insights gained from virtual metering to improve building operational performance. Future planned research includes developing virtual meters to characterize energy flows at the zone level and visualization methods to make inverse modeling results more accessible to different building stakeholders.

Experimental investigation of air distribution and ventilation efficiency in an ice rink arena

ABSTRACTThis study presents extensive experimental measurements in a modern Finnish ice rink arena including temperature, relative humidity, carbon dioxide, air speed, air flow and pressure difference measurements in addition to smoke tests. Furthermore, the air exchange rate (ACH), air-exchange efficiency, and mixing factor were calculated. The main aim was to determine ventilation effectiveness, vertical stratification of the air and how commonly re-circulation can be used in a modern ice rink arena representing common practice. Results show that re-circulation of return air was virtually continuous and in normal operating conditions the outdoor air fraction of the supply air was only 3.7% corresponding to ACH of 0.03 1/h. The ceiling distributed mixing ventilation was not able to mix the whole volume sufficiently, leading to two imperfectly mixed zones with an average air-exchange efficiency of 39% in the lower zone, corresponding to a mixing factor of 1.7.

Energy saving potential of an air treatment system for improved building indoor air quality in Singapore

The design of air conditioning mechanical ventilation (ACMV) system affects the building energy performance and the indoor pollutant removal process. The present study aims to reduce energy consumption on ACMV systems by employing a renewable air treatment system (ATS). The ATS is able to purify the recirculated air through the ozone-based oxidation process and air scrubbing devices. The air purification performance of primary equipment in the ATS has been studied in order to demonstrate the capability to remove indoor air pollutants. Due to the air purification process, the ATS allows a reduced supply of outdoor-air translating to a lower cooling load. In addition, the reduced outdoor-air fraction results in an improved chiller efficiency. Therefore, the ATS is capable of achieving marked energy savings because of the reduced cooling load for conditioning outdoor airflow. The proposed ATS is particular adept during a period when Singapore faces periodic bad haze situations. Activating the ATS while decreasing the outdoor-air fraction can be an attractive solution. Based on Singapore climatic condition, an energy consumption analysis has been carried out to estimate the energy saving potential of the proposed ATS with varying outdoor-air intake. The “plug-and-play” ATS can be easily integrated into any new or existing ACMV systems to realize immediate improvement in indoor air quality and building energy efficiency.

Investigating the energy performance of an air treatment incorporated cooling system for hot and humid climate

An air treatment system incorporating an air-purification unit has been proposed to reduce the energy consumption of air-conditioning system and improve indoor air quality. This system employs a lower outdoor-air fraction to realize the acceptable indoor air quality and thermal comfort resulting in marked energy savings due to the reduced cooling load for the outdoor air. The performance of the primary equipment of the air treatment system has been individually investigated. Experimental systems have been constructed to study the air-purification process and to evaluate the chiller’s performance. Experimental results have demonstrated that the proposed air-purification unit is able to remove indoor air pollutants such as volatile organic compound. A computational model has concurrently been developed to determine the performance of the air handling cooling coil which is impacted by the outdoor-air fraction and the chilled water temperature. A higher chilled water supply temperature is adopted by reducing the outdoor-air fraction without compromising the supply air cooling capacity. Having a reduced outdoor air cooling load and a better chiller’s energy efficiency operating with higher chilled water temperatures, the achievable energy savings of the air-conditioning system has been demonstrated to be up to 36% under a tropical hot and humid climate.

Modelling and Performance Evaluation of an Air Handling Unit for an Air Treatment System with Regulated Outdoor-air Fraction

An air treatment system has been proposed to reduce energy consumption and improve air quality. This system employs a lower outdoor-air fraction resulting in significant energy saving due to a reduced cooling load for the outdoor air intake. A computational model has been developed to determine the performance of the air handling cooling coil. The validation of the model showed good agreement with experimental data to within 5%. The supply air condition has been theoretically studied to demonstrate the influence of outdoor-air fraction and chilled water temperature. Simulation results indicate that a higher chilled water supply temperature can be adopted by reducing the outdoor-air fraction without compromising the supply air cooling capacity. Higher chilled water supply temperatures also lead to better coefficient of performance of the chiller. The energy consumption of the chiller could be reduced by up to 48% simply by reducing the outdoor-air fraction from 40% to 10%.

Calculation of Outdoor Air Fraction through Economizer Control Types during Intermediate Season

Purpose: In this study, we examined outdoor air fraction using historical data of actual Air Handling Unit (AHU) in the existing building during intermediate season and analyzed optimal outdoor air fraction by control types for economizer. Method: Control types for economizer which was used in analysis are No Economizer(NE), Differential Dry-bulb Temperature(DT), Diffrential Enthalpy(DE), Differential Dry-bulb Temperature+Differential Enthalpy(DTDE), and Differential Enthalpy+Differential Dry-bulb Temperature (DEDT). In addition, the system heating and cooling load were analyzed by calculating the outdoor air fraction through existing AHU operating method and control types for economizer. Result: Optimized outdoor air fraction through control types was the lowest in March and distribution over 50% was shown in May. In case of DE control type, outdoor air fraction was the highest of other control types and the value was average 63% in May. System heating load was shown the lowest value in NE, however, system cooling load was shown 1.7 times higher than DT control type and 5 times higher than DE control type. For system heating load, DT and DTDE is similar during intermediate season. However, system cooling load was shown 3 times higher than DE and DEDT. Accordingly, it was found as the method to save cooling energy most efficiently with DE control considering enthalpy of outdoor air and return air in intermediate season.

Cooling energy performance analysis depending on the economizer cycle control methods in an office building

The current building tends to pursue high air-tightness for the reduction of the outside air supply to prevent an increase in the energy consumption of air conditioning, resulting in high dependency on mechanical ventilation and adverse effects on indoor air quality, occupant health and productivity. In this circumstance, many developed countries improved the ventilation regulation, but mere increase in the outdoor air introduction without considering the outdoor condition elevated the cooling energy. Therefore, study of the economizer cycle system that can cause fresh outside air introduction and energy saving has been actively conducted. In this study, we analyzed different economizer control methods, addressing mixed air temperature, outdoor air fraction and cooling demand according to the outdoor air temperature. Also, we analyzed the energy consumption of the three control types of the economizer cycle through detailed EnergyPlus simulation modeling under a variety of climatic conditions. As a result, differential enthalpy control method showed the greatest energy saving by around 10% under Korean climatic conditions compared to other methods. Differential dry-bulb control method showed 12.7% lower energy consumption than no economizer method in an intermediate period, but it did show 7.1% more energy usage during the summer period under a humid region. When properly considering the latent load, it was found that the resultant energy saving potential due to economizer operation is significant.

Impacts of duct leakage on central outdoor-air conditioning for commercial-building VAV systems

The purpose of this study was to estimate the energy losses due to duct leakage during central heating and cooling of outdoor air in commercial buildings with VAV systems. This was accomplished by calculating the required cooling and pre-heating of outdoor air with and without duct leakage in three locations. Our simplified model uses hourly weather data, along with leakage level, outdoor air fraction, and fan flow profiles to estimate heating and cooling loads associated with conditioning outdoor air for a typical office building. Energy calculations are based upon the difference in outdoor air flow due to duct leakage, combined with enthalpy differences between return air and outdoor air for cooling, and between mixed air and supply air for heating. Our results indicate that 10% pre-VAV leakage at design flow, combined with 10% post-VAV leakage flow, translates to an additional 2.04units/m2/year of electricity consumption for outside-air cooling for an office building that has a 20% outdoor air fraction in Palm Springs, CA. For the same building in Chicago with 80% heating efficiency, this leakage results in an additional 0.17scm/m2/year of natural gas consumption for pre-heating outdoor air. Based upon local utility rate structures, annual savings estimates for sealing 90% of 19% system leakage at a 20% outdoor air fraction were developed. These savings were found to vary between $1.72 and $2.80 per m2 annually when including the impacts of duct leakage on fan power, fan heat removal, and conditioning of excess outdoor air. The outdoor air conditioning savings represents roughly between 10 to 20% of this energy cost savings.

Virtual sensors for rooftop unit air-side diagnostics

In this article, virtual sensors for air-side diagnostics are developed and validated for rooftop air conditioners. The virtual air-side sensors require indoor fan differential pressure and speed measurements. Experimental data from laboratory measurements are used to evaluate the accuracy of the virtual sensors. Methodologies for improving the accuracy of air-side temperature sensors often used for outdoor-air economizer diagnostics have also been developed and evaluated. Outdoor-air temperature correction correlations improve single-point temperature inaccuracy caused by exhaust-air recirculation. A model correcting a single-point mixed-air temperature measurement for inaccuracy caused by thermal stratification and non-uniform velocity distributions at the evaporator inlet has also been described and validated. Finally, methods for estimating outdoor ventilation air fraction are presented and compared to measurements.

A Smart Mixed-Air Temperature Sensor

Accurate mixed-air temperature (MAT) measurements are notoriously difficult to obtain in packaged air-conditioning equipment for small commercial applications because of space constraints and the use of small mixing chambers. However, MAT is important for control of economizers and in diagnostic algorithms for both economizers and vapor compression equipment. This paper demonstrates that a single-point measurement of MAT can provide accurate results when combined with a correlation for bias error that depends on damper control signal and difference between outdoor and return-air temperature. The correlation could be determined using a self-calibration procedure with measurements of mixed, outdoor, return, and supply air temperatures. A system was tested in a laboratory over a wide range of outdoor conditions and damper positions, and the single-point measurement errors were reduced by about a factor of five when using the bias correction. Furthermore, errors in outdoor air fraction (OAF) determined from temperature measurements were reduced by about a factor of four.

Ventilation Control Strategy Using the Supply CO2 Concentration Setpoint

This paper proposes a new ventilation control strategy applied to multiple spaces subject to variable occupancy. The strategy specified for real-time, online ventilation control takes advantage of uninitiated air from some overventilated spaces to be used as fresh outdoor air in order to reduce system energy use while maintaining the indoor air quality (IAQ) in each space. This proposed strategy maintains a supply CO2 concentration setpoint low enough to dilute CO2 generated by full occupancy in critical zones. The supply CO2 concentration setpoint could be determined online using the monitored zone airflow rates. It is tested and evaluated by making comparisons with other known control strategies. An existing VAV system installed at the École de technologie supérieure is used to evaluate this new strategy. The outdoor air fraction and associated energy use of investigated ventilation control strategies are calculated using the VAV system component models that are developed and validated against the monitored data.

Air Handling Unit Direct Digital Control System Retrofit to Provide Acceptable Indoor Air Quality and Global Energy Optimization

The project objective was to control outdoor ventilation airflow and distribution so that ASHRAE Standard 62-1989 was satisfied while minimizing the impact of energy utilization. The base system Direct Digital Controls (DDC) are application specific, with insufficient custom programming capacity to meet the project objective by themselves. Three complex control schemes were installed. In all cases the concept was to continually reset the outdoor air (OA) fraction aspirated into the air-handling unit (AHU) as necessary to satisfy ASHRAE Standard 62-1989. A personal computer-based system, independent from the DDC controls, was used to collect data, perform the calculations, and send back new control settings.