Low-melting-temperature binary molten nitrate salt mixtures for solar energy storage

Abstract

Thermal analysis of the binary system (Al(NO3)3)–(Cu(NO3)2) of different ratios was performed by using differential scanning calorimetry and thermogravimetric analysis. The eutectic temperature of the binary salt mixture was determined to be 65 °C. Moreover, the degradation temperature, specific heat, latent heat of fusion and thermal stability were reported, respectively. Further, this work focuses on synthesising low-temperature salt mixtures comprised of aluminium nitrate non-hydrate (Al(NO3)3) and copper nitrate hydrate (Cu(NO3)2) of varying compositions (60:40, 50:50, 55:45 and 57.5:42.5), respectively, which are optimised using Design of Expert v13. The performance of the binary salt mixture was determined from the results obtained from thermal stability measurements after the exposure of salt mixture to 120 h at a higher temperature. The results showed that the novel binary molten salt mixtures exhibit decomposition at 215 °C and the latent heat was found to increase from 17.7 to 20.73 J g−1 with the increase in aluminium nitrate composition with excellent repeatability after three sets of experiments. The average specific heat of binary molten salt mixture was observed from 2.73 to 2.13 J g−1 K−1. The optimal molar ratio of binary molten salts is found to be 60:40 (Al/Cu) from the obtained results. Field emission scanning electron microscopy and energy-dispersive X-ray showed a uniform distribution of both the salts in the mixture. The average specific heat of the optimal molar ratio was 21.8% higher compared to other molar ratios, indicating the most promising material to be used in solar applications.