Nitrate salts doped with CuO nanoparticles for thermal energy storage with improved heat transfer

Molten salts possess significant potential for use as heat transfer fluids (HTFs) and/or thermal storage media in advanced high-temperature concentrating solar power (CSP) plants. However, the thermal performance of these materials is hindered by typically low thermal conductivity—on the order of 1W/m-K in the solid phase and less in the liquid phase. Much work has been done to improve the thermal conductivity of HTFs through the addition of nanoparticles of higher conductivity, such as metallic nanoparticles or nanoparticulate graphite. These nanofluids display improved thermal conductivity and otherwise behave similarly to the pure HTFs. This investigation proposes such a system, focusing on the nitrate salts: potassium nitrate, sodium nitrate, and the potassium–sodium nitrate eutectic (54 weight percent potassium nitrate), with melting points of 334°C, 306°C, and 222°C, respectively. Attention is also paid to use of these materials as latent heat thermal energy storage (TES) materials—i.e., phase change materials (PCMs). The nitrate salt melt is a highly oxidative environment, so the use of carbonaceous materials or elemental metals may be hampered by degradative effects. Hence, this investigation specifically examines cupric oxide (CuO) nanoparticle-enhanced nitrate salts systems. The thermophysical properties (e.g., thermal diffusivity, latent heat) of the salt-nanoparticle systems are measured. Further, temperature-variant FTIR spectroscopy is used to determine any potential degradation of the salt after thermal cycling. The suitability of the enhanced nitrate salts systems, in regard to the chemical stability of the additive and the improvement of the thermal performance of the system relative to the pure salts, is demonstrated.