Productive urban green façades – Biomass and bioenergy

Green façades are increasingly implemented as heat-stress counter measures, but little is known about their biomass production rates. Although urban biomass has long been considered waste, its sustainable and cost-effective potential as a renewable bioenergy resource should be acknowledged. Thus, façade greening can be regarded as climate change mitigation. In this study, seven green façades in Berlin, Germany greened with five plant species were harvested and their annual biomass production rates were measured separating herbaceous, wooden, and fruit biomass. The bioenergetic potentials of the biomass were analyzed for conversion by combustion and via biogas. The annual biomass production rates for the wall area, ground area, and the urban quarter were estimated. H. Helix, H. lupulus, P. tricuspidata, P. coccineus, and F. baldschuanica produced 0.12, 0.20, 0.20, 0.37, and 0.64 kg DM m⁻² a⁻¹ per façade area and 3.1, 4.1, 2.5, 2.6, and 12.8 kg DM m⁻² a⁻¹ per ground area, respectively. The biomass showed higher heating values of 16.2–19.8 MJ kg⁻¹ and methanogenic potentials of 126–238 L_N kg⁻¹. Combustion and methane production resulted in 1.6–18.3 MJ m⁻² a⁻¹ and 0.5–6.2 MJ m⁻² a⁻¹ per façade area, respectively. Green façades produce significantly more biomass per ground area than agricultural and silvicultural systems. This underscores the importance of green façades in promoting sustainable and ethical urban biomass and bioenergy production, while adding value to unused vertical surfaces without intensifying land-use competition.