The Energiewende (German for energy transition), i.e. a low-fossil-fuel economy, is an ambitious goal. The project MOReNet solves mathematical problems of the Energiewende, which are provided by industrial partners. As a high rate of renewable energy is necessary to achieve a massive greenhouse gas reduction, the energy network of the future will consist of many decentralized energy generators and has to deal with a bidirectional energy flow. Further, the network will be characterized by a high volatility (due to dependence on external influences). The mathematical challenges are: the modeling of coupled multi-physics systems; robustification of control processes against uncertain parameters; modeling, simulation and discrete-continuous optimization under uncertainty; real-time simulation and optimization of multi-scale and hybrid systems; model reduction and adaption.