Decarbonisation Technology - February 2025 Issue

Sand: an innovative approach to storing sensible heat Sand batteries can help solve renewable energy challenges like intermittency and variability by storing excess energy and releasing it when needed

S Sakthivel and Atul Choudhari Tata Consulting Engineers

Introduction A variety of advanced energy storage systems are available, each leveraging different principles to store energy efficiently. Mechanical energy storage systems encompass various technologies. These include pumped hydro energy storage with a round-trip efficiency of 70-80%, flywheel energy storage with a cycle efficiency of 85-90%, compressed air energy storage, and gravity energy storage. Thermal energy storage systems absorb and store heat in different forms, such as sensible heat storage, latent heat storage, thermochemical energy storage, and pumped thermal energy storage. Chemical energy storage methods include hydrogen and synthetic natural gas. Electrochemical energy storage systems comprise a range of batteries such as lithium-ion, sodium-sulphur, lead-acid, solid-state, nickel- cadmium, sodium-ion, and metal-air batteries. Additionally, flow battery energy storage systems, including vanadium redox, polysulphide bromide, and zinc-bromine batteries, as well as emerging technologies like paper and flexible batteries, are part of this category. Electrical energy storage systems include electrostatic and magnetic energy storage, which stores energy in either electric fields or magnetic fields (Mitali, et al., 2022) . Each system has unique advantages and applications, contributing to a robust and diversified energy storage landscape. Among these different systems, thermal energy storage is pivotal for several compelling reasons. It can be used to balance renewable energy by storing excess power during periods of abundance, such as sunny or windy days, and

releasing it when production is low, such as on cloudy or calm days. This capability ensures a steady and reliable energy supply, mitigating the inherent variability of renewable sources. Additionally, thermal storage systems enhance grid stability and efficiency by storing thermal energy during off-peak periods and discharging it during peak demand times, effectively reducing the load on the grid during high-demand periods. Thermal energy storage also plays a significant role in cutting CO₂ emissions as it can be used to facilitate energy production or utilisation during periods when it is most cost-effective and environmentally friendly. Thermal energy storage Thermal energy storage systems have emerged as a highly cost-effective solution (Sunku Prasad, et al., 2019) . Sub-categories include sensible heat storage (such as water, molten salt, rock, and sand). There is also latent heat storage, which uses phase change materials such as salt hydrates, metal alloys, or organics like paraffin waxes, as well as thermochemical, ice, and eutectic storage systems (Ali, et al., 2024) . Sensible heat storage is the simplest and most economical storage method, which makes use of a material’s sensible heat capacity. Sensible heat storage materials can be either solid or liquid. Solid storage materials offer various options, each with unique thermal properties. For instance, sand has a specific heat capacity of 0.703-0.8 kJ/kg·K and a thermal conductivity of 0.2-0.7 W/m·K, with a bulk density of 1,800 kg/m3 (Tetteh, et al., 2024) . Aluminum, known for its high thermal conductivity of 237 W/m·K, is another excellent choice. Steel has a specific