Glass encapsulated phase change materials for high temperature thermal energy storage

A new encapsulation method for high temperature phase change materials (PCM) is developed. Nitrate salts and metals are used as the PCM core with melting temperatures in the 300-400 °C range. Borosilicate is used as encapsulating material based on its high thermal resistance, non-reactivity and optical properties. Its transparency combined with the transparency of some PCM in the molten state allows the analysis of the melting process through visual observation. The volume expansion of the PCM is managed through a void space inside the capsules. The capsule design, fabrication, and testing is described in detail. The PCM melting and solidification process is identified using a combination of visual and infrared images. The experimental observations are complemented by a finite difference method to solve the energy equations simulating the transient melting/freezing process inside a spherical PCM. The model analyzes the effect of the convective heat transfer coefficient on the PCM capsule melting and freezing starting times and the duration of the PCM melting process. Boundary conditions are set to match those in the experimental rig developed. Results show that the main system parameters can be qualitatively assessed and adequately determined to describe the experimental observations.