Following the formulation of certain strategic policy goals, such as reducing greenhouse gases (GHGs) and including more renewable sources (RES) as part of the energy mix in several parts of the world, the practical translation and actual implementation of these goals have led to the introduction of substantial volumes of intermittent renewable electric sources. Because affordable bulk storage for electricity is still lacking, demand and supply need to be (instantaneously) balanced. The resulting challenge that intermittent renewable power sources pose to the controllability of the electric power system requires greater flexibility from other parts of the system, as well as flexibility through interaction with other energy sectors such as the heating sector, the natural gas sector, and the transportation sector.@en