Slags as Thermal Energy Storage Media for Concentrated Solar Power and Renewable Energy Integration

The growing demand for low-cost, high-performance thermal energy storage (TES) materials has prompted interest in repurposing metallurgical slags in concentrated solar power (CSP) systems. This review critically evaluates the potential of various steel, copper, and aluminum slags as high-temperature TES media in packed-bed configurations. Emphasis is placed on the thermophysical properties of steel slags, for example, with reported thermal conductivities ranging from 1.6 to 1.9 W/m K and specific heat capacities of up to 1.5 J/g K, making them competitive with conventional materials, such as molten salts. The review also explores numerical modeling approaches such as the Schumann model, local thermal non-equilibrium (LTNE), and continuous solid-phase frameworks to capture heat transfer behavior in slag-based TES systems. Additionally, system-level integration strategies, particularly direct and indirect packed-bed designs, are compared to conventional two-tank molten salt systems in terms of performance, cost, and environmental benefits. Notably, steel slags offer thermal stability above 1000°C, economic savings of up to 40% over commercial fillers, and significant CO₂ reductions through circular material reuse. Case studies and simulations validate slag's long-term performance and scalability in CSP and industrial waste heat recovery applications. The review identifies research gaps in slag characterization, compatibility with heat transfer fluids, and modeling fidelity. This work contributes a comprehensive roadmap for advancing slag-based TES technologies, providing insights for research in designing next-generation, cost-effective CSP systems.