Cascaded latent heat thermal energy storage with molten salt/nanoparticles composite

Solar energy application with latent heat thermal energy storage (LHTES) attracts remarkable attention recently, and molten salts as phase change materials (PCMs) can be the potential media. The appropriate choosing of multiple PCMs is very significant for the performance improvement of the LHTES unit. In the present study, HITEC salt, eutectic solar salt and pure sodium nitrate with the volume ratios of 1:1:1 were used as the PCMs, and the oil was adopted as the HTF. An enthalpy-porosity model was developed to numerically investigate the heat transfer characteristics of the shell-tube LHTES unit during the charging (melting) process. Various structures infiltrated with the PCMs were studied to optimize the performance of the LHTES unit, while different mass flow rates and inlet temperatures of oil were carried out simultaneously. The results showed that different heat transfer characteristics and time-durations of the charging process can be found with the diversity of layouts of three PCMs, whereas the best solution was to adapt the temperature gradient to the phase change temperatures of the PCMs. Furthermore, aluminum oxide nanopowder (Al2O3) was doped into the salts in order to alleviate the poor thermal performance of the LHTES unit with pure PCMs, considering the scheme of series numerical studies. Temperature evolutions showed that the charging process were enhanced by the nanoparticles with high thermal conductivity.