Feb-2025

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

Article Summary

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 heat capacity of 0.48 kJ/kg·K, whereas iron has a slightly lower specific heat capacity of 0.452 kJ/kg·K. These materials are effective due to their ability to absorb and retain heat efficiently (Khatod, et al., 2022). On the other hand, liquid storage materials contain substances like therminol, engine oil, ethanol, butane, and propane. For example, water has a specific thermal heat capacity of 4.19 kJ/kg·K at 20-100°C, ethanol has 2.4 kJ/kg·K up to 78°C, butanol has 2.4 kJ/kg·K up to 118°C, and engine oil has 1.88 kJ/kg·K up to 160°C.

Sensible heat storage mediums made of solid materials have an advantage over liquids due to their ability to handle larger temperature fluctuations. Unlike liquids, solid thermal storages do not change phase, meaning they do not melt or flow, which eliminates the risk of leakage from the storage container. Conventional thermal energy storage methods, such as those using molten salt, have been widely adopted. However, these systems are expensive, have a limited lifespan, and often rely on materials that are harmful to the environment.

Latent heat storage, also referred to as phase change heat storage, operates by absorbing and releasing thermal energy during a material’s phase transition. This type of storage has a higher energy density compared to conventional sensible heat storage due to the substantial enthalpy changes occurring during the phase transition. Typically, latent heat storage systems are lighter and more compact, which can result in reduced costs compared to sensible heat storage systems.

Latent heat storage utilising phase change materials has diverse applications, including solar thermal storage, enhancing energy efficiency in buildings, and recycling waste heat. Salt hydrates, for example, have latent heat capacities of 115.5-280 kJ/kg and phase change temperatures of 29-117°C. Paraffin wax stores about 202 kJ/kg of heat and changes phases at 40-45°C. Paraffin C14 has the highest latent heat of 228 kJ/kg. Salt eutectics, which are salt mixtures, have heat capacities of 74-790 kJ/kg and phase change at 13-767°C. Carbonate salts are used for high-temperature applications, with heat capacities of 142-509 kJ/kg and phase change temperatures of 732-1,330°C. These materials are chosen for their thermal properties to improve energy storage efficiency at different temperatures.

Thermochemical heat storage utilises reversible chemical reactions to store and release energy efficiently. Energy is stored as chemical compounds produced by an endothermic reaction. When these compounds recombine in an exothermic reaction, the stored energy is released as needed. The amount of heat stored is equal to the reaction’s enthalpy. For example, zeolite has a heat density of about 200 kWh/m³, and silica gel has about 230 kWh/m³. Table 1 presents the advantages and limitations of sensible, latent, and thermochemical heat storage systems.

Sand batteries can store large amounts of heat at high temperatures, making them ideal for applications needing stable and efficient heat retention. They help solve renewable energy challenges like intermittency and variability by storing excess energy and releasing it when needed, improving the energy grid’s efficiency and reliability. Developing sand battery technology can lead to better thermal energy storage systems, providing a scalable and practical solution for both industrial and residential needs. This article explores the potential of sand batteries to contribute to a more sustainable and resilient energy future.

Sand battery benefits
Sand, used as sensible heat storage and commonly referred to as a ‘sand battery’, represents an innovative approach to energy storage. This method utilises the unique thermal properties of sand to absorb, store, and release heat efficiently. It offers a cost-effective and scalable solution for thermal energy storage, making it a promising technology for various applications, including renewable energy integration and industrial heat management.