In the course of the “Energiewende,” the German electricity market is undergoing major changes. The state-aided priority of renewable generation has led to a significant decline in electricity prices. This reduces the profit margin of cogeneration units and increases the necessity of flexible operation to avoid electricity production when spot prices drop below marginal costs. In this work, a 100 MWel combined-cycle (CC) 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 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 modeled using the stationary process simulation tool ebsilon professional. Different operation modes are investigated and results are used to derive a mixed integer linear programming (MILP) model to optimize the operation of the plant/storage system. Using this method, the overall economic impact of the storage on the plant operation is quantified.