Analysis of lighting conditions of indoor living walls: Effects on CO2 removal

Vertical greening systems, or living walls, are becoming increasingly used indoors for improving the sustainability of buildings, including for the mitigation of excess CO2 levels, derived from human respiration. However, light provision within indoor environments is often insufficient for the efficient functioning of many plant species, leading to low photosynthetic CO2 removal rates, and the need for supplementary light sources. In this study, we investigated the performance of supplementary lighting employed for indoor living wall systems, and whether optimised lighting conditions could lead to improved CO2 removal. In situ trials with several medium-large indoor living walls were performed to sample the lighting scenarios currently employed. We concluded that the majority of plants in existing systems were exposed to suboptimal lighting and will have a net-zero CO2 removal efficiency. Sealed chamber experiments using two common living wall plant species were conducted to explore the effect of varying lighting conditions on CO2 removal efficiency. Comparisons on optimal and “best case” in situ conditions were carried out, showing that CO2 removal efficiency was significantly correlated with both leaf and stem angles, which suggest phototropism may influence in situ CO2 removal. After a ten-day experimental period, the highest CO2 removal efficiency for both test plant species was observed at 200 μmol m−2 s−1 light flux density (~10500 lux) at 15° from the vertical growing surface. Our results indicate that most current lighting systems are inadequate for healthy plant photosynthesis and CO2 removal, and that modified lighting systems could improve this performance. The estimation of the CO2 removal ability of a 5 m2 passive living wall decreases from an ACH of 0.21 h−1, achieved in an optimal light exposure condition, to only 0.03 h−1 when plants are exposed to sub-optimal conditions. To reduce maintenance costs, technical guidelines for indoor living wall lighting should be established, and lighting suppliers should recognise the developing niche market for specialised indoor living wall lighting.