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.