An Attempt to Model Ventilation Rate in Classrooms Based on the Measurement of Relative Humidity

Indirect CO2-based measurement of the ventilation rate is a well-established method based upon a balance equation of the CO2 generated by people and dispersed by infiltration and ventilation. In principle, ventilation rate can also be estimated by water vapour mass balance when storage terms are properly modelled. This work aims to benchmark the CO2-based model and the water vapour-based model to estimate of ventilation rate in classrooms. The case study is a secondary school in Morlupo, Rome. Here, four naturally ventilated classrooms and the adjacent spaces were monitored for a two-week period (indoor temperature and relative humidity RH, CO2 concentration, occupancy, outdoor temperature and RH). The ventilation rate for each classroom was estimated using the indirect CO2-based method and then fed to an energy model developed in TRNSYS. Buffer effects for moisture were estimated using a single-layer Equivalent Penetration Depth Model. The simulated humidity ratio was compared to the measured one and input parameters for the storage models were tweaked until convergence using an optimization algorithm. Such process was repeated for 2 of the 4 classrooms. Then, the tuned parameters identified for the storage model were used as input on the remaining 2 classrooms and the ventilation rate obtained using the watervapour based method was compared to the results of the CO2-based method. Results show that the water vapourbased method significantly underestimates the air changes per hour, calling for an in-depth analysis of storage buffer terms.