Effect of nanoparticle dispersions in binary nitrate salt as thermal energy storage material in concentrated solar power applications

Despite the huge improvements in the field of solar energy to produce electricity, the high cost of production has been one of the major problems in this field. Concentrated solar power (CSP) uses a number of mirrors to concentrate solar radiant energy onto a single focal point. Electrical power is produced when the concentrated light is converted to heat, which drives a heat engine connected to an electrical power generator. This heat is first stored in a thermal energy storage (TES) and transferred to a heat engine to be converted to electricity and thus TES can provide heat even after the sunset for extended hour of power production. Hence, the key for reducing the cost of electricity mainly relies on the operating temperature of TES, since it will determine the thermal to electric conversion efficiency. Increasing TES operating temperature can enhance the cycle efficiency and as a result the cost of electricity can be reduced. However, traditional TESs such as paraffin wax and fatty acid are likely to decompose at high temperatures (300~400 °C). Molten salts can achieve higher temperatures leading to higher efficiency and lower costs. Hence, they have caught the attention of researchers as a potential substitute for traditional TESs. Molten salts can achieve higher temperatures leading to higher efficiency and lower cost However, poor thermo-physical properties of molten salts were one of the most challenging problems. It has been recently found that doping of nanoparticles in molten salts significantly enhance their thermo-physical properties. In this study, eutectic of sodium nitrate and potassium nitrate at 60:40 by weight were chosen as the base molten salt and silica nanoparticles were used to enhance the specific heat capacity of the salts. A modulated differential scanning calorimeter (MDSC) was employed to measure the specific heat capacity of the TESs. Different sizes (5, 10, 30 and 60 nanometers) of nanoparticles were considered to investigate if the size of the nanoparticle had an effect on the specific heat capacity. It was seen that the doping of nanoparticles enhanced the specific heat capacity by approximately 27% for 60nm. Material characterization was carried out using the Scanning Electron Microscope (SEM) to explore the cause of the enhanced specific heat capacity and it was found that the nano-engineered molten salts were filled with distinct nanostructures. It was observed that as the amount of nanostructures increased the enhancement of specific heat capacity also increased. This finding would lead to decrease in amount of TES used in the power plants which leads to a decrease in the size of the thermal storage tank and eventually reduces this total cost.