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).