A New Phase Change Material Based on Potassium Nitrate with Silica and Alumina Nanoparticles for Thermal Energy Storage

Abstract

In this study different nanofluids with phase change behavior were developed by mixing a molten salt base fluid (KNO 3 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 (SiO 2 ), alumina (Al 2 O 3 ), and a mix of silica-alumina (SiO 2 -Al 2 O 3 ) 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 KNO 3 . 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.

Anomalous Increase in Specific Heat of Binary Molten Salt-Based Graphite Nanofluids for Thermal Energy Storage

“…have been performed to identify the mechanism underlying the specific heat increase in the molten salt nanofluids [18][19][20][21][22]. Chieruzzi et al enhanced the specific heat of a binary nitrate (NaNO 3 -KNO 3 ) by dispersing various nanoparticles (Al 2 O 3 , SiO 2 , TiO 2 , and SiO 2 -Al 2 O 3 ) [23,24]. The enhanced specific heat of the molten salt nanofluids was explained in various ways, e.g., electron microscope images and molecular dynamics simulation [25][26][27][28][29][30].…”

Heat Exchange Analysis on Latent Heat Thermal Energy Storage Systems Using Molten Salts and Nanoparticles as Phase Change Materials

“…In a recent study, another HSM was developed through water solution method by using KNO 3 as molten salt (melting point of 334°C) and 1 wt.% of SiO 2 , Al 2 O 3 , and a mixture of SiO 2 -Al 2 O 3 as nanoparticles [23]. The HSM has an increased specific heat with silica and silica-alumina nanoparticles (+9.5% and +4.7%, respectively).…”

“…However, some studies performed by University of Perugia reported an increase of 15% mainly with silica-alumina nanoparticles in nitrate mixture [22] and also in KNO 3 as molten salt having a melting point of 334°C (the latent heat increased by 12%). In both studies, the NEPCMs were produced in water solution [23]. It was shown that the stored heat of molten salts as NaNO 3 -KNO 3 [22] and KNO 3 [23] is increased with the addition of 1.0 wt.% of SiO 2 -Al 2 O 3 and SiO 2 .…”

“…In both studies, the NEPCMs were produced in water solution [23]. It was shown that the stored heat of molten salts as NaNO 3 -KNO 3 [22] and KNO 3 [23] is increased with the addition of 1.0 wt.% of SiO 2 -Al 2 O 3 and SiO 2 . Mixing of nanoparticles and the salt mixture at high temperature in a twin screw microcompounder [21].…”

Thermal energy storage enhancement of a binary molten salt via in-situ produced nanoparticles

“…Introducing nanoparticle into a base fluid to improve its thermophysical properties, termed as 'nanofluid', has been extensively studied in the past [4][5][6][7][8]. Recently, similar concept was used to disperse suitable particles in solar salts to improve their thermal energy storage capacity [9][10][11][12][13][14][15][16][17], hence termed as 'nano-salt' here to consider the dispersion in both solid and liquid phases suitable for TES applications.. A majority of the work [18][19][20][21][22][23][24] showed that the specific heat of nano-salt was increased, except the experimental results of Lu et al [20]. For example, Shin and Banerjee [4] dispersed silica nanoparticles into alkali metal chloride salt eutectics, and reported a c p enhancement of 14.5% at 1 wt % particle concentration.…”