A translucent external wall system for solar space heating and daylighting is briefly presented. This system is composed of phase change material encapsulated in glass containers and a layer of transparent insulation material to reduce thermal losses to the environment. The melting process is induced by absorption of solar radiation in the bulk of the phase change material. In this article we focus on the one-dimensional mathematical-thermodynamic model of the store. The heat conduction and melting/solidification model with a term for the absorbed solar radiation employs continuous thermal properties of the phase change material. Based on this model a dynamic simulation program was developed, validated with measurements and successfully used for sensitivity analysis of system parameters. Results of these parameter studies are presented.