Heat transfer analysis of encapsulated phase change materials

Solar energy is receiving a lot of attention recently since it is a clean, renewable, and sustainable energy. Solar energy is used for space heating, power generation and other applications. A major limitation however is that it is available for only about 2000 h a year in many places. Therefore it is critical to find ways to store solar thermal energy for the off hours. Sensible heat of material has been used for storing thermal energy but due to material properties this type of thermal storage has limitations. Using encapsulated phase change materials is potentially a better way to store thermal energy with the associated reversible heat transfer. The present work deals with certain aspects of storing solar thermal energy in high temperature phase change materials with melting points above 400 °C. The objective is the storage of large amounts of solar energy (～600 MWh). Two kinds of encapsulated capsules are considered; zinc encapsulated in nickel and eutectic salt mixtures (57 mol% NaCl and 43 mol% MgCl₂) in stainless steel encapsulation. Diffusion and phase change computations are reported here in the form of temperature profiles of the phase changing and encapsulated materials for spherical capsules. The time for heating and melting during charging (storage of thermal energy into capsulated phase change material) and the time for cooling and solidification during discharging (retrieval of thermal energy) are presented for both zinc–nickel and salt–stainless steel systems. As per expectations, the time for heat transfer is much shorter for liquid heat transfer media compared to those for gases. Moreover, the heat transfer times are shorter with smaller sizes of capsules.