Modeling of Occupant-Generated CO₂ Dynamics in Naturally Ventilated Classrooms

Abstract

A numerical method is presented to estimate the concentration of occupant-generated CO₂ for the (time-varying) occupancy typically found in nonforced ventilated elementary school classrooms. Here, the governing mass balance was solved numerically and compared to experimental measurements in order to estimate the respiration and (time-varying) infiltration rates. For the cases studied, we estimate an average CO₂ generation rate per child as 404 mg/min⁻¹. This is similar to estimates found in the literature for the age and activity level of elementary students, the classroom occupants. The average estimated infiltration rates were found to be larger than those measured from the decay of the tracer gas SF₆ under closed-door static conditions. The in-use infiltration rates were increased by additional air exchange due to people entering and leaving the room. In addition, we show that the difference (or error) between the instantaneous concentration of CO₂ and the time-averaged value recorded by a data-logging CO₂ monitor varies depending on the infiltration rate and sampling time. Therefore, the time interval selected for averaging may increase the overall error of the apparent CO₂ concentration. We conclude that the methods used to measure air exchange rates in naturally ventilated rooms underestimate the actual ventilation rate of a room under “in-use” conditions. However, even with the addition of uncontrolled outdoor air, the concentration of CO₂ in the classrooms studied was higher than recommended to meet air quality objectives.

Keywords:

• CO₂
• IAQ
• model
• natural ventilation
• tracer gas decay
• ventilation

ACKNOWLEDGMENTS

We gratefully acknowledge financial support of the British Columbia Health Research Foundation ST#10(98) and the Natural Science and Engineering Research Council of Canada.

Notes

^A Air exchange rate, closed doors.

^B Air exchange rate, open doors.

^C Volumetric flow rate, closed doors.

^D Volumetric flow rate, open doors.

^E Flow required to meet ASHRAE standard based on number of students present in the classroom.

^A Average measured CO₂ concentration for school day.

^B Average CO₂ concentration predicted by model.

^C Respiration rate.

^D Estimated magnitude of the air exchange rate disturbance ×100.

^E Magnitude of the air exchange rate averaged over entire day ×100.

^F Standard deviation.