Rapid population growth and industrialization in the last decades resulted in an enormous stress on ecosystems, resources, political systems and economies. In urban areas, these stresses are especially high, since our cities consume large amounts of resources (e.g., water, energy, nutrients) and convert these to low value waste streams (linear metabolism). Apart from that, local renewable resources (rain, solar, wind etc.) are often neglected in current supply schemes. Unlike self-sufficient natural systems, which make use of sustainable inputs and recycling mechanisms, cities at the current stage cannot be considered sustainable.