In the course of the “Energiewende” the German electricity market is undergoing major changes. The stateaided priority of renewable generation has led to a significant decline in electricity prices. This reduces the profit margin of cogeneration units, but on the other hand it increases the economic impact of flexibilization potential like heat storage systems. In this work, a 100 MWel combined-cycle power plant supplying heat and power to a paper mill is investigated. Currently the plant is operated heat-controlled and is therefore unable to react to changing electricity spot prices. With the integration of a heat storage, the plant is enabled to switch to power-controlled mode. To evaluate the technical impact of the storage, the plant and a thermochemical MgO/Mg(OH)2 storage are modelled using the stationary process simulation tool EBSILON. Different operation modes are investigated and results are used to derive a mixed integer linear programming model to optimize the operation of the plant/storage system. Taking into account measurements of the heat demand and EPEX spot prices, a benefit of some 100k €/a is possible compared to normal operation mode.