Lithium imide systems for high temperature heat storage in concentrated solar thermal systems

Abstract

Hydrogen is touted as one of the solutions for future energy requirements. Metal hydrides typically studied for their high hydrogen capacity can be used as a thermal storage system by taking advantage of their endothermic/exothermic reactions with hydrogen. This allows the harnessing of solar energy by utilising thermal storage to alleviate its intermittent nature. Lithium amide (LiNH2), imides (Li2NH) and nitrides (Li3N) have been widely studied for their hydrogen storage at relatively low operating temperatures, typically suited for mobile applications. However, little work has been done involving the imide to nitride reaction of lithium-based materials due to their high temperature range. The following techniques were used to characterise this system: temperature programmed desorption (TPD), temperature programmed photographic analysis (TPPA), x-ray diffraction (XRD) and pressure-composition-temperature (PCT) measurements. TPD results revealed that only a single-step reaction occurred between 100 and 600 °C. TPPA revealed that having a molten solid solution of the sample, depreciated the reversibility of hydrogen absorption and desorption. The molten sample behaved quite vigorously in TPPA measurements and consequently blocked sample filters and sintering sample cell walls; creating engineering problems at higher temperatures. The results revealed that for reactions involving Li2NH and lithium hydride (LiH), the temperature range required for thermal storage is above the melting point of the system. The diffusion and absorption of hydrogen through stainless steel would also occur in the sample cells used, resulting in further problems. The reaction pathway of Li2NH and LiH was also found to be far more complex than generally reported: XRD revealed that the expected final product of Li3N could not be identified, instead a lithium imide-nitride hydride phase (Li4‑2xN1‑xH1‑x(NH)x) was identified as the final product of this system. PCT measurements were conducted to identify the kinetic and thermodynamics of this system, but because of the molten solid-solution problem, an accurate result could not be obtained.