Transient 2-D Heat Transfer Analysis of Encapsulated Phase Change Materials for Thermal Energy Storage

Solar energy is receiving a lot of attention since it is a clean, renewable, and sustainable energy. A major limitation however is that it is available for only about 2,000 hours a year in many places and thus it is essential to find ways to store solar thermal energy for the off hours. The present work deals with heat transfer aspects of storing solar thermal energy in high temperature phase change materials with melting points above 300 °C. Two-dimension transient heat transfer analysis is conducted to investigate thermal energy storage using encapsulated phase change material (EPCM) for concentrated solar power (CSP) applications. Sodium nitrate, NaNO3, is considered as the phase change material (PCM) encapsulated by stainless steel in a cylindrical shaped capsule. Stream function-vorticity formulation is employed to study the effect of buoyancy-driven convection in the molten salt on the total charging and discharging times for various sizes of PCM capsulated. Simulations are also conducted for a horizontally placed rod inside a flow channel. Storage times are calculated for laminar and turbulent flows of heat transfer fluids transferring heat into EPCM. It is shown that the buoyancy-driven convection in the molten PCM enhances internal heat transfer inside the capsule and hence helps to slightly shorten the total heat transfer times during both charging and discharging processes. Flow characteristics of the heat transfer fluid have profound effect on the nature of phase change process inside the EPCM rod.