AbstractA transient model is the basis for understanding the characteristics of dynamic systems. It must always be a tradeoff between (modeling and computational) effort and the desired output. Thus, it is necessary for complex systems to develop special models adapted to the purpose. This has been performed for the case of direct steam generation (DSG) in parabolic trough or linear Fresnel solar fields, in which there is a long section of two-phase flow. In the once-through mode (OTM), feed water from the power block is directed to the inlet of a loop. The water is then preheated, completely evaporated and superheated along the loop. This paper presents two different types of transient models for such systems.A discretized finite element model is used for detailed system characteristics and understanding. A second moving boundary model is developed which includes the combination of lumped parameters with distributed information. This fast model can especially be used for control studies and model based predictive controllers. Both models are compared against experimental data and differences regarding various system characteristics are shown. Further models like linear transfer functions in the time-domain are mentioned and an overall overview on transient DSG models is provided.