Development of Novel Thermal Analysis Techniques For Characterization Of Salt Based Phase Change Materials For Thermal Energy Storage Applications

Abstract

Solar energy storage technologies have proved to be promising in terms of providing an uninterrupted power supply. Latent heat thermal energy storage systems in the form of salt phase change materials have been successfully used in concentrated solar plants for energy storage applications due to their superior energy storage parameters and functionalities. In the present work, to characterize the salt phase change materials, the development, suitability, and reliability of a characterization method devoid of the limitations of the conventional thermal characterization techniques have been researched and proposed. A new approach based on the solution to inverse heat conduction problem for assessing solidification parameters of phase change materials (PCMs) salts in steel mold with furnace cooling has been proposed. The method estimates the steel mold -salt interfacial heat flux, and it is used to calculate the latent heat and phase change parameters of salt PCMs using calorimetry-based energy balance equations. This method is more accurate than the conventional Computer-Aided Cooling Curve Analysis (CACCA) techniques. It eliminates the drawbacks of baseline fitting calculations and errors introduced due to the improper selection of solidification points. Pure Salt PCMs such as potassium nitrate (KNO3) and solar salt mixture of 60 wt%NaNO3 and 40 wt%KNO3 were used for the validation of this technique in the work. The solidification parameters such as rate of cooling, time taken for solidification and latent heat of the PCMs were determined and were found to be in close accordance with the reported literature data. A quantitative method for this determination of phase change parameters of salt-based PCMs has also been proposed. This technique involves estimating mold-salt interfacial heat flux by solving Fourier’s law of heat conduction within the salt and using it to calculate the phase change enthalpy of salt PCMs. Pure salt PCMs such as potassium nitrate (KNO3), sodium nitrate (NaNO3)), and solar salt mixture (60wt.%NaNO3+40wt.%KNO3) were used for validation of this technique Further, the effects of the addition of MWCNT carbon-based nanostructures on thermal energy storage (TES) parameters of lithium-based eutectic binary, ternary and quaternary salts were investigated. The addition of nanoparticles showed no effect on the solidification time and temperatures of the PCMs. In binary and ternary salts, lower concentrations of nanoparticles showed a positive impact on the TES parameters. LiK and LiT with 0.1% MWCNT showed an enhancement in the latent heat values of 27.6% and 19.28% respectively. However, the effects were limited at higher MWCNT weight concentrations. It was observed that the addition of nanoparticles had no significant effect on the TES parameters of the quaternary salts. In addition, the micrographic studies to analyze the agglomeration at higher concentrations of nanoparticles and wettability studies were also performed.