A novelty for thermal energy storage utilizing the principle of solid to solid phase change in a lithium sulfate at elevated temperatures

The novelty presented in this research includes a demonstration of storing thermal energy in a solid material Li2SO4 by exploiting a fundamental principle of phase to phase change while remaining in solid form at elevated temperatures, above 500 degrees C, using suitable heat transfer fluid; specifically, sodium-potassium eutectic alloy NaK. Analysis revealed that during the process of energy storage the solid storage material essentially remained within a narrow range of temperature which was best described as a temperature step wave traveling in the storage medium at near constant speed. The analysis also revealed the ease in scalability of the proposed constructional model to meet both rate of heat generation and storage capacity for various applications; i.e., in the area of solar energy. Specifically, it is shown that the rate of energy storage remains basically constant and that the duration of the thermal energy storage is linearly related to the length of storage material for the parameters considered herein. Also, since the temperature of the storage medium remains within a small range while storing thermal energy the implication is that the internal entropy generation is accordingly minimal. Additionally, for the storage material to be in the solid form it further renders the storage systems ease of handling and operation.