Natural energy sources like sun and wind as well as waste heat from industrial processes are characterized by a fluctuating energy supply. To balance supply and demand in renewable energy systems, thermal energy storage (TES) is an important concept. This can be realized by storages containing numerous capsules filled with an appropriate phase change material (PCM). For applications it is essential to optimize the heat flow during the charging and discharging processes. The author presents a method for numerical simulation of these processes. He has developed a multi-scale approach combining detailed modelling of single capsules and a coarser simulation of a whole storage system. Numerical results from simulation are compared to measured data from experiments.