Using a solar air heater to ventilate classrooms during the winter season in New Zealand: a potential alternative solution to assist during COVID 19 outbreaks

Ninety per cent of New Zealand classrooms are naturally ventilated by opening windows. Achieving a suitable ventilation level will rely on teachers. A survey showed that less than half of the teachers opened windows during teaching time. Due to the high occupant density in classrooms and a low natural ventilation rate, it is challenging to provide adequate ventilation during the southern hemisphere winter months (June to September). From 9 am to 3 pm, school hours align well with the optimum solar radiation, providing opportunities for solar ventilation. A crossover intervention study was performed to investigate the effect of operating a 3 m2 roof-mounted solar air heater (SAH) on classroom ventilation. This study was carried out in six primary schools from June to September 2014. In each school, two adjacent classrooms were randomly assigned to a treatment group (SAH installed and operated) or a control group (SAH installed but not operated). The outlet air velocity from the SAH was monitored at a 10-min interval. Classroom carbon dioxide (CO2) level was monitored at a 2-min interval. The classroom hourly ventilation rate was estimated using CO2 as the tracer gas. The hourly CO2 levels in the treatment classrooms ranged from 551 ppm to 4992 ppm, with a mean (standard deviation) value of 1309 (619) ppm. In the control classrooms, the hourly CO2 levels ranged from 550 ppm to 4830 ppm, with a mean (SD) value of 1405 (702) ppm. The CO2 levels in the treatment classrooms were statistically significantly lower than in the control classrooms (p < 0.01). The mean volumetric flow rate of the outlet air was around 34.0 m3/h with a mean incoming temperature of 28.9 °C. The mean air changes per hour (ACH) in treatment and control classrooms ranged from 1.3/h to 3.4/h and 1.1/h to 3.2/h, respectively. Overall, the flow rate coming from the SAH was insufficient to keep a CO2 level below the 1000 ppm threshold. However, increasing the SAH area could provide alternative classroom ventilation during the COVID 19 pandemic. A follow-up project is currently investigating the use of nanofluid coating and improving the collector design to increase the efficiency of the SAH.