Decarbonisation Technology - February 2025 Issue

increases until it reaches a threshold where the energy is fully stored. Charging times can vary based on the specific type of sand battery and the temperature of the heat source. During the discharging process, when thermal energy is required, the sand is exposed to a heat sink or another device that can extract the stored heat (by using water). As the sand’s temperature drops, the stored energy is released as heat. A typical sand battery operation is depicted in Figure 1 . The heat stored within the sand can be used to heat water, which can be used in various heating applications, such as building heating systems. Sand can be maintained at approximately 500°C for several months using resistive heating, a technique that involves in-situ heating by passing an electric current through a resistive element. This process converts electrical energy directly into thermal energy, ensuring consistent and efficient heating of the sand bed over extended periods. The resistive heating method leverages the inherent properties of resistance units to generate and sustain high temperatures, making it a reliable and effective approach for long-term thermal energy storage. This method effectively harnesses thermal energy for practical use, optimising the energy storage and release process for efficient heating solutions. A recent study ( Tetteh, et al., 2024) demonstrated that adding metallic chips (such as aluminum, brass, and stainless-steel scrap) to sand significantly improves its thermal properties. In particular, brass-sand layers have significantly better thermal conductivity, whereas blends of sand and aluminum chips delivered well-balanced thermal performance. These configurations effectively mitigate inherent sand constraints, thereby enhancing the overall thermal efficiency of the packed bed. The study found that mixing sand with 20% (by volume) aluminum and brass chips significantly increased the maximum heat rate (ºC/min), achieving 1.7 times and 1.65 times the heat rate of pure sand, respectively. M/S Polar Night Energy has designed and commercialised a sand-based thermal energy storage system in Western Finland. M/S Polar claims their sand battery has a heating power of 100 kW with a capacity of 8 MWh and commenced utilisation in 2022. Furthermore, M/S Polar has a 3 MWh test pilot facility in Hiedanranta, Tampere, which

is integrated with a local district heating grid, supplying heat to multiple buildings. Sand batteries offer several advantages that make them an attractive energy storage solution. Their low capital and operational costs enhance accessibility and contribute to the cost- effectiveness of renewable energy systems. With proper maintenance, they have a long lifespan, ensuring reliable heat storage and release over many years, making them a durable and sustainable option. Furthermore, they are highly scalable, so can store large amounts of thermal energy. This scalability is essential for managing variable energy production from renewable sources, ensuring a consistent and reliable energy supply during peak demand periods ( Jose, 2023 ). However, sand has several limitations. Sand batteries are less efficient than other energy storage technologies due to heat dissipation during charging and discharging, which leads to energy losses. Additionally, they gradually lose heat over time, impacting their energy storage capacity and requiring periodic recharging for optimal performance. Current research is focused on improving their efficiency and minimising energy losses. Enhancing insulation and containment strategies is essential to address this issue effectively. Engineering design challenges Designing sand-based batteries involves tackling several critical research and engineering challenges. Particle size and packing density significantly influence the battery’s porosity and heat storage capacity, necessitating precise control over particle size distribution and packing configuration. Ensuring material compatibility between sand and other battery components is crucial to prevent degradation or chemical reactions that could compromise performance. Maintaining cyclic stability over numerous charge-discharge cycles is essential, requiring careful management of thermal expansion and contraction to prevent mechanical failure. Enhancing heat transfer mechanisms within the sand medium is vital for enabling rapid charging and discharging processes while maintaining uniform temperature distribution. Optimising sand’s thermal conductivity is also paramount to