Austrian biomass CHP-plants (solid biomass) significantly contribute to the countrys heat supply and, therefore, are an integral part of the renewable energy mix. In order to obtain a subsidy for these renewable energy systems, Austrian law stipulates a fuel utilisation efficiency rate of more than 60%. This thesis provides an overview over the different concepts which can be applied to combined heat and power generation. Subsequently, the fuel utilisation efficiency rates of existing CHP-systems will be determined and analysed by evaluating the results of a questionnaire that was sent to the operators of the respective systems. On the basis of the results of the questionnaire and the fact that the majority of these renewable energy systems are operated using a steam power process, an exemplary calculation will show which fuel utilisation efficiency rates can be achieved with different operating concepts and which criteria are decisive in reaching a high fuel utilisation efficiency rate. Finally, a specific system (steam power process with extraction-condensation turbine) will be simulated. Based on this specific case, operating mode modifications and optimisations will be presented in order to increase the fuel utilisation efficiency rate. The technical limitations of the respective measures will be displayed.