The long-term global energy problem is outlined in this paper. The world has appropriate substitutes for the limited resources of oil and natural gas. This process, however, requires a careful adaptation of the present energy infrastructure. The concept of consumption and supply densities is used to explore alternative strategies for such an adaptation. In particular, the possible role of renewable energy sources for rural and urban areas is discussed. The paper elaborates on the dependence of modern urban systems on an extension of centralized supply systems.