Abstract:
Concentrating solar power (CSP) plants with Thermal energy storage (TES) system offer a power supply technology equivalent to the conventional power plants. Increasing the storage capacity of TES materials can enhance the efficiency of CSP plants. One of the most commonly used TES materials is the binary carbonate salt eutectic mixture consisting of Li2CO3 and K2CO3. In this study, with a view to increasing the storage capacity of this material, two different carbon allotropes - multiwalled carbon nanotube (MWCNT) and graphite nanoparticle (GNP) - in three different weight percentages (2 wt.%, 4wt.% and 6 wt.%) are added individually with the eutectic mixture. The dispersion homogeneity, purity of the phase, and homogeneous suspension ability of the prepared nanocomposites samples is examined and verified using standard techniques such as Scanning Electron Microscope (SEM), Energy-dispersive Spectroscopy (EDS), and Uv-vis spectrophotometry, respectively. The thermal properties, including melting temperature, latent heat of fusion, specific heat capacity, and thermal stability of the prepared samples are measured using the Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA).
It is observed that the melting point temperature and latent heat of fusion decrease with nanoparticle addition. An increase in the concentration of both type of nanoparticles decreases the latent heat of fusion of the base material and the decrement is higher in case of GNP inclusion. However, no definite trend is observed in the decrement of melting point temperature. Specific heat capacity (Cp) is enhanced by the doping of the salt with MWCNT and GNP in both the solid and liquid phases. The obtained results are compared with the theoretical prediction. The highest rate of enhancement of the heat capacity among all the samples, is found to be 25% for 6% GNP-based nanofluid at 520°C - 560°C. Furthermore, 6% GNP-based nanocomposite exhibits higher stability than all other samples employed in this study. Therefore, GNP is identified as the most potent organic nanoparticle to be used as an additive with solar salts.
In the present study, it is also revealed that the agglomeration of the nanoparticle plays the most crucial role in the enhancement of specific heat capacity and thermal stability of the base material. Finally, compressed layer theory is employed to explain the possible relation between the agglomeration of nanoparticles and the enhanced specific heat capacity.
