A transient finite volume model was used to predict collection and retention of heat for rectangular cross section solar collector/storage systems when filled with water and various concentrations of phase change material (PCM) slurries with a 65 degrees C phase change temperature. It was found that the PCM slurry systems collected heat marginally less effectively than water filled stores. Retention of heat at higher temperatures in the PCM slurry systems may allow higher solar savings fractions to be realised depending on heat demand patterns. (c) 2006 Elsevier Ltd. All rights reserved.