Researchers at Penn State University are studying steelmaking waste as a possible low-cost material for storing solar energy as heat. Early tests found that electric arc furnace slag showed the most promising results among three types of steel slag tested for high-temperature thermal energy storage.
The research is part of a three-phase project focused on finding cheaper materials for thermal energy storage (TES). The first-stage results were published in the journal Solar Energy, with researchers examining the structure, stability and heat-related properties of different steel slags.
Solar power produces electricity only when sunlight is available. Cloudy weather and the day-night cycle can reduce or stop solar generation. Thermal energy storage could help by storing heat when energy is available and releasing it later when needed.
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TES works somewhat like a battery, but instead of storing electricity directly, it stores energy as heat. The stored heat can then be released when power or heat is needed. However, high-temperature TES systems can be expensive because of the materials and equipment involved.
The Penn State team is looking at steel slag as a possible alternative. Slag is a rock-like waste material produced when impurities are removed during steelmaking. Large amounts of this material are generated near steel plants, making it an industrial waste stream that could potentially be reused.
The researchers tested three types of steel slag: ladle metallurgy furnace slag, basic oxygen furnace slag and electric arc furnace (EAF) slag. Each type has a different chemical and physical structure because it comes from a different stage of steel production.
The team used several tests to study the materials. They examined their structure, chemical phases, thermal behavior and signs of degradation. These tests helped researchers understand how each material might behave when exposed to high temperatures.
EAF slag performed best in the early evaluation. Researchers found that it had relatively low mass loss, little detectable free lime and a more stable response to heat. Its structure also appeared less likely to experience expansion caused by moisture-related reactions.
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However, the researchers are not yet saying that EAF slag is ready for commercial thermal energy storage systems. The current results are based on indirect indicators of thermal and physical stability rather than complete performance testing.
The team still needs to directly measure properties such as thermal conductivity, specific heat capacity and density. Long-term testing through repeated heating and cooling cycles will also be needed to understand whether the material can maintain its performance over time.
The project could have benefits beyond energy storage if the researchers can successfully reuse steelmaking waste. Turning industrial byproducts into useful materials could reduce the amount of waste sent for storage while also lowering the need for newly mined raw materials.
The researchers plan to validate the early findings and use computer simulations to study properties that are difficult to measure directly. After that, they can begin exploring ways to modify the materials for practical thermal energy storage applications.
The work highlights a different approach to renewable energy storage: using existing industrial waste rather than relying only on specially engineered storage materials. If later testing confirms the early results, steel slag could become a useful material for storing heat from solar and other energy systems.
