This work describes and evaluates a new simulation model for direct steam generation in parabolic-trough solar collectors. In direct steam generation, water is heated and evaporated through a solar field to feed a steam Rankine cycle or an industrial process. However, the behaviour of the involved multi-phase fluid poses some challenges to simulation models. The model explained in this work is based on a steady-state approach but deals with transient conditions such as start-up, shutdown and clouds in a reasonable-computing time. A new simulation tool is implemented in the TRNSYS software environment by means of new components that are suitable to be integrated into a whole solar plant model in order to carry out long-term energy production analyses with low computational resources. The main advantages of the new quasi-dynamic approach include fast computation with satisfactory accuracy; consideration of thermal inertia when addressing transient conditions; and flexibility to use different types of collector or solar field configurations. The performance of the model is validated with real experimental data obtained from the DISS solar test loop in Plataforma Solar de Almeria, Spain. This paper describes the modelling approach and summarizes the comparison of simulation results with measurements taken at the DISS facility. (C) 2015 Elsevier Ltd. All rights reserved.