Thermal hydraulic RELAP5 model for a solar direct steam generation system based on parabolic trough collectors operating in once-through mode

Direct steam generation in parabolic-trough solar collector requires reliable and efficient two-phase flow modelling tools. One-dimensional models based on 6 equations are a suitable approach. The present article aims to show that RELAP5 is able to simulate a single-loop system including transients caused by sudden events like solar collector defocusing or fast changes in direct solar irradiance. The numerical results have been validated with experimental data from the DISS facility located at Plataforma Solar de Almería. Six operation days have been compared using a new heat loss correlation, which has been measured on the same facility. The implemented model considers connection pipes, elbows, change of height, thermal insulation in the passive sections, collectors slope and absorbers. The numerical results are in good agreement with the experiments and it proves that RELAP5 can reproduce the underlying thermo-hydraulic phenomena. An additional numerical study of severe slugging in the connection pipes has been done, where two adjacent collectors linked by a connection pipe have been simulated at a pressure of 5 bar. Based on 374 tests, slugging phenomenon has been reported for low values of inlet mass flow rate and low quality in the entering two-phase mixture.