Performance evaluation of wind tower natural ventilation with passive solid tube heat recovery based on CO2 levels

The use of natural ventilation strategies such as wind towers is encouraged by many governmental/non-governmental organisations to enhance indoor air quality (IAQ) and reduce energy consumption in buildings. However, their usage is restricted during unfavourable weather conditions, such as in cold/mild-cold periods, because it could lead to high ventilation heat loss and thermal discomfort. A potential solution is to incorporate passive solid tube heat recovery (THR) into the wind tower system to recover the exhaust thermal energy. The aero-thermal performance of such a system has been widely studied, but the evaluation of its impact on the indoor CO2 concentration and spatial distribution has been limited. In this work, a validated computational fluid dynamics (CFD) model of a room with a wind tower system with passive THR was employed to investigate the effects of the occupant-influenced and external environment-influenced variables on the indoor CO2 level. The results highlighted that occupant activity levels greatly affected CO2 levels in the ventilated space, with an increase of up to 44% from sedentary to medium working, based on the set conditions. For an occupancy density of 1.7–2.6 m2/person in the classroom model, the indoor above outdoor CO2 levels were within 550 ppm at outdoor wind speeds above 3 m/s. The effect of heating/non-heating seasons on the indoor CO2 levels may be negligible for locations with high wind speeds all year round. Based on the UK's year-round mean climate conditions, the CO2 level in the space ventilated by the wind tower with THR was predicted to reach Category I or II, according to EN 16798-1 (2019).