TA-23-C084 – Modeling Indoor Botanical Fresh Air Generation PDF

This paper presents a parametric model of the air quality and building energy impact of indoor botanical biofiltration and implements this model to elucidate scenarios wherein biofiltration may offer building energy savings in addition to indoor air quality (IAQ) improvements. The generation of fresh air with plants from inside building envelopes is an emerging technology with the capacity to improve both IAQ via biofiltration and building energy efficiency via increased air recirculation. Plants and their microbiome regeneratively filter volatile organic compounds (VOCs), particulate matter (PM), inorganic gases, and reduce CO₂ to O₂ via photosynthesis; thus, ventilation may be decoupled from heating/cooling, in theory resolving tradeoffs between IAQ and conditioning energy of outdoor air used to dilute indoor pollutants. Since its initial conception in a series of NASA experiments in the 1980s, botanical biofiltration (airflow through plants and their microorganism-rich soil) has been validated in dozens of lab-bench-scale chamber studies, and several room-scale and in-situ experiments. Numerous components of biofiltration have been characterized, such as Single Pass Filtration Efficiency (SPFE) for various pollutants, microbial dynamics, and estimates of potential HVAC savings via hand calculations or statistical methods. However, to the best of our knowledge, no integrated parametric model exists that simultaneously predicts the IAQ and energy impact of biofiltration for arbitrary building geometry, occupancy, program, construction, climate, biofilter energy consumption, etc. This paper presents a simple parametric framework that combines EnergyPlus, ASHRAE Standard 62.1 Table F-1 mass balance equations, and published values on biofiltration efficiency into an integrated single-zone model that allows rapid estimation of the IAQ and energy impact of biofiltration. A two-way flow of information between mass balance IAQ equations and EnergyPlus ventilation values yield concentrations of key indoor air pollutants (formaldehyde, benzene, toluene, CO₂) and building energy use (kWh·m²/Btu·ft²) for each hour of the year. After describing the model, we present results of a sensitivity analysis on an office shoebox that, for all ASHRAE climate zones, elucidates some parameters that significantly impact energy consumption (increase or decrease) due to biofilter operation by comparing baseline model variants to variants with simulated biofilters making 100% of the buildings’ fresh air requirements from inside. This analysis showed that IAQ is improved by biofiltration at a lower energy cost than increasing outdoor air ventilation rates; however, while building energy is reduced due to air recirculation, biofilter plug loads have greater magnitudes than HVAC energy savings in most cases we modeled, alleviating but not resolving IAQ-energy tradeoffs. As biofiltration is an emerging technology with a complex set of potential impacts ranging from the molecular level to the whole-building energy scale, we developed this tool andpresent this analysis to begin quantifying the costs and benefits of biofiltration for any given building.