Abstract
In this study different nanofluids with phase change behavior were developed by mixing a molten salt base fluid (KNO3 selected as phase change material) with nanoparticles using the direct synthesis method. The thermal properties of the nanofluids obtained were investigated. Following the improvement in the specific heat achieved, these nanofluids can be used in concentrating solar plants with a reduction of storage material. The nanoparticles used (1.0 wt.%) were silica (SiO2), alumina (Al2O3), and a mix of silica-alumina (SiO2-Al2O3) with an average diameter of 7, 13, and 2–200 nm respectively. Each nanofluid was prepared in water solution, sonicated, and evaporated. Measurements of the thermophysical properties were performed by DSC analysis, and the dispersion of the nanoparticles was analyzed by SEM microscopy. The results obtained show that the addition of 1.0 wt.% of nanoparticles to the base salt increases the specific heat of about 5–10 % in solid phase and of 6 % in liquid phase. In particular, this research shows that the addition of silica nanoparticles has significant potential for enhancing the thermal storage characteristics of KNO3. The phase-change temperature of potassium nitrate was lowered up to 3 °C, and the latent heat was increased to 12 % with the addition of silica nanoparticles. These results deviated from the predictions of theoretical simple mixing model used. The stored heat as a function of temperature was evaluated for the base salt, and the nanofluids and the maximum values obtained were 229, 234, 242, and 266 J/g respectively. The maximum total gain (16 %) due to the introduction of the nanoparticles (calculated as the ratio between the total stored heat of the nanofluids and the base salt in the range of temperatures 260–390 °C) was also recorded with the introduction of silica. SEM and EDX analysis showed the presence of aggregates in all nanofluids: with silica nanoparticles they were homogenously present while with alumina and silica-alumina also zones with pure salt could be detected.
Highlights
Carbon dioxide is responsible for major man-made greenhouse effect, making it the most important contributor to climate change
The different nanoparticles induce a change in the shape of the heat flow curve of the base salt that modifies the values of the heat of fusion and the melting point of the nanofluids
The latent heat of the pure KNO3 was compared to the literature data so the differential scanning calorimeter (DSC) measurement reliability was verified
Summary
Carbon dioxide is responsible for major man-made greenhouse effect, making it the most important contributor to climate change. The carbon dioxide emissions could still grow in the coming years as a result of the increased demand for world energy. In the absence of new policies, carbon dioxide emissions from the energy sector would increase by 61 % over 2011 levels by 2050 [1]. Policy choices and market developments that bring the share of fossil fuels in primary energy demand down to just under. To limit these emissions, it is necessary to make better use of the produced thermal energy by increasing the energy efficiency of industrial processes (heat recovery) and buildings and by increasing the use of renewable sources such as solar energy [3]. A key technological issue for solar thermal power plants and industrial waste heat recovery is to integrate an economic storage of thermal energy (Thermal Energy Storage–TES) [4,5,6], with the overall objective to increase the solar contribution, to improve efficiency, and to reduce the levelized energy cost (LEC).
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