Increasing the flexibility of conventional power plants is one key challenge for the massive integration of highly volatile renewable energy resources into the German and European power system. Flexible power plants ensure the security of supply by compensating fluctuations in the electrical grid caused by intermittent renewable energy production from wind and sun. In this paper, the future development of the German electricity market is predicted and evaluated in terms of the future flexibility demand of coal-fired power plants based on simulation studies for four distinct years. As the flexibility measures for coal-fired power plants generally use inherent capabilities of the existing power plant systems, they are usually restricted by limiting factors. Here, the limiting factors for reducing the minimum load and increasing load change rates are discussed and potential measures to mitigate their influence are presented. An alternative way to increase the flexibility lies in properly integrating thermal energy storages. This integration is presented and evaluated using a dynamic power plant model in EBSILON®Professional. Simulations show that by integrating this storage concept the electrical minimum load can be reduced while simultaneously increasing the load change rate.