Direct Steam Generation in parabolic trough solar collectors is one of the most promising alternatives for replacing the use of thermal oil in solar power plants and process heat. The main advantages are: elimination of steam generator heat exchangers; use of a non-toxic fluid; operation with power cycle higher temperatures, therefore with higher efficiency. Nevertheless, modelling the two-phase flow heat transfer is a complex task, and there is a lack of modular simulation tools that can easily be replicable for different configurations. In this work, a quasi-dynamic model developed for once-through direct steam generation, using Ebsilon professional software is presented and its results are assessed against experimental results from a test campaign carried out at the DISS test facility. The modelling methodology consists in splitting the solar field into individual components for which modelling and performance are assessed at a detailed level. Quasi-dynamic simulations are feasible by the combination of dynamic components and a time-series, where calculations are carried out for each timestep. The model is highly versatile, both at the system configuration and simulation levels. The model performance was evaluated by comparing simulation and experimental results for different operation stages, i.e. start-up, cool-down, steady and transient solar radiation. (C) 2020 Elsevier Ltd. All rights reserved.