Theoretical and applied aspects of designing a high-temperature thermal energy storage system based on graphite material

The relevance of studying this problem is because classical thermal insulation materials have limited thermal resistance, which complicates their use in energy storage systems with heat transfer fluids operating at temperatures up to 2000°C. Developing design solutions for thermal energy storage systems with multi-layer insulation ensures an optimal balance between the level of heat loss and economic efficiency. Therefore, the aim of this article is to conduct a technical and economic analysis of design solutions for a thermal energy storage system with a high-temperature working medium. The article proposes an algorithm for calculating the storage system design, based on solving heat transfer equations in cylindrical geometry, considering conduction, radiation exchange, and convection in interlayer gaps. The algorithm allows for a comprehensive assessment of heat losses and the total cost of materials (graphite, tungsten, basalt wool, argon) depending on the temperature of the graphite accumulating material, the thickness of the insulation, and the required thermal capacity of the storage system. To improve the accuracy of calculations, an iterative procedure is used, considering the temperature dependencies of the thermophysical properties of the materials. Within the study, a comparative analysis of three sizes of thermal energy storage systems, "M", "S" and "B" with stored thermal energy ranging from 805 to 39680 MWt, was conducted. It was found that with significant heat losses because of the use of a thin layer of insulation, the most reasonable operation of the storage system is at a temperature of the graphite cylinder between 1800–2000°C. To eliminate heat losses, the thickness of the insulation is increased, primarily using basalt wool, and it is also economically beneficial to reduce the temperature of the graphite to 800–1200°C. The practical use of the proposed calculation algorithm allows determining the optimal ratios between the volume of graphite, the operating temperature, and the total cost of the thermal storage system of a capacity. The introduced dimensionless parameter, which shows the ratio of the external radius of the insulation to the radius of the graphite cylinder, determines the key ranges of optimal temperature for storage systems of different sizes.