Hydrothermal model for small-scale linear Fresnel absorbers with non-uniform stepwise solar distribution

The study of the hydrothermal behaviour of water-steam is a key topic in direct steam generation through solar concentrators. The hydrothermal behaviour is affected by the non-uniformity in the solar flux distribution along the axis of the linear absorber. The non-uniformity produces non-illuminated regions at the absorber ends (especially in small Linear Fresnel Collector prototypes or in non-rectangular configurations of Fresnel systems), and partially illuminated regions due to the gradual contribution of the mirror rows to the reflected solar radiation in the absorber. The aim of this paper is to present a new general hydrothermal model for linear absorbers that accounts for this non-uniformity and allows simulating Linear Fresnel systems of any size, mirror field geometry, and working conditions. The present model considers both, one phase flow-for heating/cooling of the liquid or the superheated steam-and two-phase flow-for describing boiling or condensation mechanisms-using a homogeneous mix model. The model was validated against experimental data from prototypes under different working conditions. Infrared thermography was used to measure the absorber temperature profile. A good agreement between experimental and predicted datasets was found, which confirmed the reliability of the model.