For flexible power generation of biogas plants and effective heat utilization, efficient heat storage technologies will play a decisive role. For this purpose, a process is being developed in the ThermoFlex project, based on controlled operation of a secondary digester in thermophilic temperature ranges after a mesophilic main digester. This allows effective, flexible heat storage in the secondary digester for digester heating and external heat utilization at an acceptable temperature level without additional storage. The process biology of the secondary digester tolerates controlled temperature variations. An overview of the application potential and design of the ThermoFlex process is given. Batch and semicontinuous fermentation tests at various temperatures are described.