Numerical and Experimental Investigation of Organic Nanomaterials for Thermal Energy Storage and for Concentrating Solar Power Applications

Concentrating solar power (CSP) plants are used to harness solar thermal energy to generate electricity. Thermal energy storage (TES) systems enable the CSP plants to be operated during disruptions in solar energy input and furthermore to extend the operating hours of the plants to match the diurnal peak in solar energy input (that typically occurs between noon to 3 p.m.) with the peak in in daily demand for power consumption (that typically occurs between 4 p.m. to 7 p.m.). Molten salts and their eutectics are employed as TES media to store the thermal energy from the solar radiation, due to their low vapor pressure at elevated temperatures. However, the molten salts have relatively poor thermo-physical properties which are an impediment to their application in TES. In this study, the specific heat capacity of the molten salt mixture was enhanced dramatically by doping with nanoparticles. Differential scanning calorimetry was used to measure the specific heat capacity of these nanomaterial samples and the latent heat of fusion. The specific heat capacity was measured in both solid and liquid phase for these samples. Furthermore, the rheological characteristics of the nanofluids were also studied using a concentric parallel plate (corn and plate) rheometer. Binary carbonate salt mixtures were used as the base material (solvent) and mixed with organic nanoparticles such as carbon nanotubes, C60, and graphite nanoparticles. Various parameters were varied in this study: the nanoparticle materials, mass concentrations of the nanoparticles, the composition of the solvent materials (salt mixture ratios), and the synthesis protocols for the mixtures. Also, computational studies were performed using molecular dynamics (MD) simulations to estimate the interfacial thermal resistances between a nanoparticle and surrounding molten salt molecules. This enabled the estimation of the optimum nanoparticle size for performing the experimental measurements. The measurements showed that the specific heat capacity of the molten salt nanomaterials was enhanced significantly (~ 20-90%) on mixing with nanoparticles at minute concentrations (0.1-5% mass concentration). It was observed that the nanomaterial properties are strongly sensitive to small variations in the protocols implemented in this study. This was also evident for the rheological properties of the nanomaterials in the molten state (liquid phase, i.e. for "nanofluids"). Theoretical models demonstrate that the chemical properties (and to a lesser extent the thermo-physical properties) as well as composition of the solvent material plays a dominant role in determining the level of enhancement of the resultant values of the specific heat capacity when doped with nanoparticles. Chemical functionalization of nanoparticles can be used to optimize the resulting properties of the nanomaterials