For conventional power plants, the integration of thermal energy storage opens up a promising opportunity to meet future technical requirements in terms of flexibility while at the same time improving cost-effectiveness. In the FLEXI- TES joint project, the flexibilization of coal-fired steam power plants by integrating thermal energy storage (TES) into the power plant process is being investigated. In the concept phase at the beginning of the research project, various storage integration concepts were developed and evaluated. Finally, three lead concepts with different storage technologies and integration points in the power plant were identified. By means of stationary system simulations, the changes of net power output during charging and discharging as well as different storage efficiencies were calculated. Depending on the concept and the operating strategy, a reduction of the minimum load by up to 4% of the net capacity during charging and a load increase by up to 5% of the net capacity during discharging are possible. Storage efficiencies of up to 80% can be achieved.