Sand as a thermal energy storage material for solar thermal technologies

Sand, an inexpensive and abundantly available natural geomaterial, holds promise as a thermal energy storage (TES) material in diverse solar thermal systems such as concentrated solar power, solar heating, and solar gasification. Sand possesses the ability to capture solar radiation during daylight hours, preserving it as heat on both a daily and seasonal basis, and then releasing it when demand is high. Although sand exhibits lower thermal conductivity and specific heat capacity when compared to molten salt and engineered TES materials, it compensates for these drawbacks by withstanding high temperatures and being economically advantageous.

For sand to effectively function as a TES medium, it needs to acquire a high energy density. This entails a considerable specific heat capacity and resilience to substantial temperature variations. Optimal for this purpose, pure quartz sand stands out due to its elevated specific heat capacity, high thermal conductivity, and its ability to maintain integrity without agglomerating or degrading even at temperatures exceeding 1000°C. Impurities adversely affect the energy density of sand. The presence of clays, carbonates, and feldspars has been identified as leading to premature agglomeration, early degradation, and/or reduced specific heat capacity in sands. At a temperature of 600°C, clays were observed to enhance the tendency of sand to agglomerate, with agglomeration increasing with rising temperature and pressure. Below 800°C, carbonate minerals undergo decarbonization, resulting in mass loss and alterations in grain-size distribution. For instance, calcite grains transform into lime powder, which exhibits an extremely low specific heat capacity. Below 1200°C, feldspars undergo vitrification, leading to agglomeration that hinders fluidization and the flow of sand through the system. To avoid these complications, it is imperative to limit impurities to less than 2%.