Mobile air cleaners (MACs) have been proposed as a supplementary solution to combat the spread of respiratory aerosols in school classrooms. To determine which, where and how to use MACs, seven small- and medium-sized MACs were selected and assessed for different settings and configurations by 1) a decay test for determining the clean air delivery rate (CADR), and 2) a perception test with a panel of subjects, together with physical measurements, of noise and air movement. The findings show that to achieve the desired CADR (appr. 1000 m 3 /h for 30 students per classroom), the key factors are the induced airflow pattern and the location of the MACs. MACs with an upward air supply toward the occupied zone showed much higher CADR (max. 775–1332 m 3 /h) than those with a horizontal air supply (max. 219–333 m 3 /h). Moreover, using multiple devices simultaneously was crucial when the room size was increased, and combining mechanical ventilation could improve aerosol removal. Achieving a sufficient CADR would always lead to a noise level above the limit of 35 dB(A), yet sometimes the rating of the panel was more than 50% acceptable. The air velocities mostly fulfilled the requirement (&lt;0.2 m/s), which aligned with the positive panel assessment. Hence, the evaluation by a panel of subjects can help to optimize the use of MACs in a classroom. 

Using mobile air cleaners in school classrooms for aerosol removal: Which, where and how

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

We perform direct numerical simulations to study the effect of increasing airflow rate on CO2 concentration in downflow and displacement ventilation in a room with one occupant. Often, CO2 concentration is used as a proxy for the concentration of bio-aerosols of respiratory droplets, and therefore, tracking the CO2 concentration in ventilation strategies can be useful to understand and quantify the risk of spread of communicable respiratory illnesses. At low to moderate airflow rates, the flow in the downflow setup is not mixed, but stratified. The CO2 concentration in the upper and lower layers is determined by the strength of the thermal plume originating from the occupant. We provide a simple theoretical model to predict the height of the stratified interface, the volumetric flux of the ascending plume, and the CO2 concentration in the lower and upper layers. At very high airflow rates, the flow is well-mixed and the average CO2 concentration in the room can be predicted with the mixing ventilation assumption. We demonstrate that at low to moderate airflow rates, displacement ventilation more effectively maintains lower CO2 concentrations in the lower layer compared to downflow ventilation. 

Effect of airflow rate on CO2 concentration in downflow indoor ventilation

The COVID-19 pandemic caused a paradigm shift in our way of using heating, ventilation, and air-conditioning (HVAC) systems in buildings. In the early stages of the pandemic, it was indeed advised to reduce the reuse and thus the recirculation of indoor air to minimize the risk of contamination through inhalation of virus-laden aerosol particles emitted by humans when coughing, sneezing, speaking or breathing. However, such recommendations are not compatible with energy saving requirements stemming from climate change and energy price increase concerns, especially in winter and summer when the fraction of outdoor air supplied to the building needs to be significantly heated or cooled down. In this experimental study, we aim at providing low-cost and low-energy solutions to modify the ventilation strategies currently used in many buildings to reduce the risk of respiratory disease transmission. We find that ultraviolet germicidal irradiation (UVGI) modules added to the HVAC system are very efficient at inactivating pathogens present in aerosols, leading to good indoor air quality even with significant indoor air recirculation. Moreover, we show that an optimal placement of the air exhaust vents relative to air supply vents can improve the ventilation efficiency, with significant consequences in terms of energy savings.

https://export.arxiv.org/pdf/2301.09136v2.pdf

Reconciling airborne disease transmission concerns with energy saving requirements: the potential of UV-C pathogen deactivation and air distribution optimization (preprint)

The COVID-19 pandemic caused a paradigm shift in our way of using heating, ventilation, and air-conditioning (HVAC) systems in buildings. In the early stages of the pandemic, it was indeed advised to reduce the reuse and thus the recirculation of indoor air to minimize the risk of contamination through inhalation of virus-laden aerosol particles emitted by humans when coughing, sneezing, speaking or breathing. However, such recommendations are not compatible with energy saving requirements stemming from climate change and energy price increase concerns, especially in winter and summer when the fraction of outdoor air supplied to the building needs to be signiﬁcantly heated or cooled down. In this experimental study, we aim at providing low-cost and low-energy solutions to modify the ventilation strategies currently used in many buildings to reduce the risk of respiratory disease transmission. We ﬁnd that ultraviolet germicidal irradiation (UVGI) modules added to the HVAC system are very eﬃcient at inactivating pathogens present in aerosols, leading to good indoor air quality even with signiﬁcant indoor air recirculation. Moreover, we show that an optimal placement of the air exhaust vents relative to air supply vents can improve the ventilation eﬃciency, with signiﬁcant consequences in terms of energy savings.

New ventilation concepts for energy saving and aerosol safety

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Reconciling airborne disease transmission concerns with energy saving requirements: the potential of UV-C pathogen deactivation and air distribution optimization (preprint)

New ventilation concepts for energy saving and aerosol safety