The use of phase change materials (PCMs) to increase solar domestic hot water (SDHW) system efficiency has been already studied by different ways. Some studies place the storage material in the water tank, others directly in the solar thermal collector. However both of them show that the effectiveness of such a use is not relevant. This paper is devoted to a new approach: the PCM is placed in the heat transfer fluid solar loop from the SDHW system. This configuration is studied under different weather conditions and system parameters. On the contrary to previous results, this parametric study highlights a significant increase of the system efficiency due to the PCM. Then, a genetic algorithm allows proposing an optimized system configuration. (c) 2012 Elsevier Ltd. All rights reserved.