To the group of solar thermal power systems belongs the solar tower where a field of two-axis tracking heliostats reflect the sun arrays to the receiver aperture. The solar radiation is concentrated at the receiver surface and concentration ratios up to 1000 can be reached. In the receiver high temperatures are reached which can generate steam for a Rankine cycle or ultimately even high enough temperatures to use the heat in a gas turbine system of a combined cycle power plant. Germany’s first and only solar tower power plant for experimental and demonstration purposes has been erected in the town of Juelich. The central receiver plant, which has been designed with an open volumetric receiver, will supply the grid with a nominal power of 1.5 MWe. A further upgrade to a hybrid system by combining the solar operation with combustion process for natural gas or biogas is taken into account. The advantage of the solar-fossil hybrid power plants, compared to solar-only systems, lies in low additional investment costs due to an adaptable solar share and reduced technical and economical risks. For that reason an advanced software tool library is developed for the modelling of such hybrid power plants. From this library components are used to develop a simulation model for the calculation of a small hybrid power plant. This tool enables the calculation of the energy output of not only individual operating points but also of a whole year. In this paper a simulation model for one hybrid concept with a burner of the solar tower power plant of Juelich is described. The newly developed additions to the model library have been validated with results from another software tool. Different operation strategies for this special hybrid concept and simulation results for parts of the small solar tower power plant are described and discussed.