Exploration of steel slag for thermal energy storage and enhancement by Na2CO3 modification

Development of thermal storage material from recycled solid waste resources can further enhance the economic and environmental benefits of thermal energy storage system. Thermal properties of steel slag as sensible heat storage material are examined and further enhanced by Na2CO3 activation. The steel slag remains stable until 1200 °C in TG-DSC test, and the morphology kept unchanged after 200 thermal cycles (400–900 °C), indicating good thermal cyclic stability. The thermal energy storage density of steel slag is 797.9 kJ·kg−1 (400–900 °C), the thermal conductivity was measured as 0.505, 0.532 and 0.670 W·(m·K)−1 at 25, 250 and 500 °C, respectively. When the steel slag is further modified by Na2CO3, the morphology and phase of the material remained stable, and the DSC curve trend unchanged after thermal cycle. The thermal energy storage density reaches 997.0 kJ·kg−1 (400–900 °C), which is 25.3% higher than original steel slag. Even more, the thermal conductivity is 1.331, 1.323, 0.889 W·(m·K)−1 at 25, 250, and 500 °C, respectively, which is 32.7% higher than that of steel slag. Hence, the thermal properties of steel slag have been significantly improved by the developed Na2CO3 activation process.