Carbon capture from indoor air as an alternative ventilation technique

Abstract

A polluted indoor space is a serious problem for the occupants especially for office workers who require high attentiveness levels. Indoor pollutants, especially carbon dioxide (CO2), reduce the workers performance by increasing sickness risk and impairing cognitive abilities needed to make informed decisions. Typically, indoor air quality is improved by diluting indoor contaminants with fresh outdoor air. However, the outdoor air should be dehumidified and cooled before it is entrained into the office, especially in hot and humid climates. Nonetheless, this solution is energy intensive since conventional systems use vapor compression-based air-conditioning. This increases the building electricity consumption as well as its carbon footprint. To mitigate the danger of global warming emerging from the increased carbon emissions, sustainable ventilation techniques must be conceived. The use of the recirculated indoor air, characterized by lower temperature and humidity levels than the outdoor air, could reduce the ventilation load. However, the indoor air suffers from increased CO2 levels generated by the occupant. To overcome this problem, adsorption-based CO2 removal from the room air can be a good solution. Recently, this system has become more promising due to the emerging of new generation of solid adsorbents, the metal-organic frameworks (MOF). They can be produced to exhibit high capacity and affinity towards carbon dioxide and can be regenerated at low temperature energy such as solar and waste energy. CO2 capture by adsorption reduces the ventilation load by reducing the outdoor air requirement to modest levels needed to maintain healthy levels of VOCs and O2. A sustainable cooling system is developed using MOF-packed adsorption beds for CO2 capture to treat the indoor air and resupply it to the space. A numerical model simulating the heat and mass transfer in the adsorbent bed is developed and used to size the adsorption system for a case study of a typical office in the hot and humid climate of Beirut, Lebanon. The proposed ventilation system reduced the outdoor air requirements and ventilation load by 72.6 % and 36 %, respectively during the peak load month of August.