Towards Productive Cities: Environmental Assessment of the Food-Energy-Water Nexus of the Urban Roof Mosaic

Abstract

Cities are rapidly growing and need to look for ways to optimize resource consumption. Metropolises are especially vulnerable in three main systems, often referred to as the FEW (i.e., food, energy, and water) nexus. In this context, urban rooftops are underutilized areas that might be used for the production of these resources. We developed the Roof Mosaic approach, which combines life cycle assessment with two rooftop guidelines, to analyze the technical feasibility and environmental implications of producing food and energy, and harvesting rainwater on rooftops through different combinations at different scales. To illustrate, we apply the Roof Mosaic approach to a densely populated neighborhood in a Mediterranean city. The building-scale results show that integrating rainwater harvesting and food production would avoid relatively insignificant emissions (13.9-18.6 kg CO₂ eq/inhabitant/year) in the use stage, but their construction would have low environmental impacts. In contrast, the application of energy systems (photovoltaic or solar thermal systems) combined with rainwater harvesting could potentially avoid higher CO₂ eq emissions (177-196 kg CO₂ eq/inhabitant/year) but generate higher environmental burdens in the construction phase. When applied at the neighborhood scale, the approach can be optimized to meet between 7% and 50% of FEW demands and avoid up to 157 tons CO₂ eq/year. This approach is a useful guide to optimize the FEW nexus providing a range of options for the exploitation of rooftops at the local scale, which can aid cities in becoming self-sufficient, optimizing resources, and reducing CO₂ eq emissions.

Keywords: industrial ecology; life cycle assessment (LCA); rainwater harvesting; resource self‐sufficiency; solar energy; urban agriculture.

Figure 1

Steps proposed for assessing the Roof Mosaic approach

Figure 2

Layout (left‐hand side) and diagram (right‐hand side) of the system represented by S.F1 where water and tomatoes come from rooftop system (AS = alternative systems). Energy (electricity and hot water) comes from conventional systems in this scenario. The rest of the scenarios are available in figure A.5 and A.6 in the supporting information on the Web

Figure 3

Environmental impacts of the four scenarios of alternative production on the rooftop and the required conventional systems to meet the total demand

Figure 4

Analysis of the indicators of eight different combinations proposed at the neighborhood scale. The best environmental performance indicator is in bold, and the darker the green color, the larger the number of outperforming indicators