This document shows the results obtained in direct steam generation for low powers in parabolic trough concentrators with bimetallic Cu-Fe wall receivers. The objective is to study its thermal behavior under transient conditions and stratified two-phase flow. Experimental results in transient state appear considering the variants of mass flows between 60 and 150 kg/h, the direction of the solar beam irradiance on the receiver is from below and on one side of the receiver, this last being the most critical, in particular in steel receivers because of low thermal conductivity. Some conclusions in this work are that the most significant deformation is in a transient state and it happens only in one part of the receiver, when it changes from the liquid phase to the steam phase during the boiling process. The bimetallic receiver reduced the transient deformation to the degree that it does not seem a problem for some critical parts of the absorber system during the direct steam generation (DSG). Whereas in steel receivers the deformation rises to 7 cm, in the Cu-Fe receivers the most drastic deformation was only of 18 mm upwards with a mass flow of 150 kg/h and wall temperature of 200 degreesC. So, the use of the bimetallic receiver is recommended for the DSG for low power applications. (C) 2003 Elsevier Ltd. All rights reserved.