Due to global energy shortages and the unstable energy supply, as well as to concerns over environmental pollution, renewable energies have drawn the attention of researchers as a way to obtain sustainable energy sources. Concentrated solar power energy (CSP) stands out among the renewable energies that have been studied thanks to its advancing development and annual production, reaching 6,2 GW in 2020 (REN21, 2021). Nevertheless, as a result of the sun's intermittency, it is necessary to use a profitable storage system with which is possible to increase the efficiency and conservation of energy as well as the competitiveness of CSP compared to traditional sources. To improve the thermodynamic cycles of solar thermal energy, the use of heat transfer fluids (HTF) based on organic substances such as oils has been replaced with the use of molten salts such as nitrates and nitrites, given their high thermal stability, low vapor pressure, chemical inertness, availability, and profitability. However, these salts have low specific heat capacity and low thermal conductivity, so they are still under study (Serrano-López et al., 2013).

The use of solid nanoparticles (less than 100 nm in size) to obtain nanofluids with molten salt as base fluid -thereby improving the thermal properties of the molten salt- has been studied by several authors (Chieruzzi et al., 2017a; Hu et al., 2017; Zhang et al., 2017). Ho and Pan (2017) showed that the optimum amount to be added to commercial HITEC salt in order to obtain the best properties was 0,063 wt.% of alumina nanoparticles, through which an increase in the specific heat capacity of 19,9% is obtained. In this study, it was shown that, at higher concentrations of alumina, there is a decrease in the specific heat capacity of the salt. Schuller et al. (2015), on the other hand, proved that there is a parabolic relation between the specific heat and the mass fraction of alumina nanoparticles added to nitrate salts, obtaining a maximum specific heat increase of 30,6% with a 0,78 wt.% of alumina.

Although there are several reports on the use of solid additives to improve the properties of the salts used in CSP (Muñoz-Sánchez, Nieto-Maestre, Imbuluzqueta, et al., 2017; Schuller et al., 2015), only some reports (Betts, 2011; Muñoz-Sánchez et al., 2017) show the influence of the method on measuring thermal properties, specifically heat capacity. Regarding the synthesis method typically used for the preparation of nanofluids based on molten salts, the most common is the two-step method, the well-known dissolution method, which includes water as a dispersion medium in the process. For this reason, recent research has focused on improving the method by making some modifications without affecting the thermal properties of the nanofluid (Chen et al., 2019; Chieruzzi et al., 2017b; Navarrete et al., 2020). In this vein, different proposed methods including the ball mill dry method and a spray drying method were evaluated by Navarrete et al. (2020). Similarly, Chieruzzi et al. (2017b) determined an increase in the specific heat capacity of up to 18,6% with 1 wt.% of SiO₂-Al₂O₃ in Solar Salt at high temperatures using a twin-screw micro-compounder. None of these reports used commercial HITEC salt as base salt, and they do not propose an entirely new synthesis method.

Therefore, considering the importance of developing new synthesis methods as well as the current controversy regarding the most suitable method to measure the specific heat capacity (Cp) of molten salt, especially with nanoparticles that are molten salt-based nanofluids (MSBNFs), this work focuses on contributing to the understanding of this property, mainly concerning the commercial molten HITEC salt in comparison with MSBNFs with 0,5, 1,0, and 1,5 wt.%. Alumina nanoparticles were synthesized by means of a new two-step method. The specific heat capacity was evaluated with both scanning differential calorimetry (DSC) according to the ASTM E1269 (ASTM International, 2011) and modulated scanning differential calorimetry (MDSC) according to the ASTM E2716 (ASTM International, 2015). On the other hand, the melting temperature was measured via DSC, and the thermal stability with and without nanoparticles was evaluated through thermogravimetric analysis (TGA).

Synthesis

Synthesis of ternary nitrate salt (HITEC)

Sodium nitrate (AR ≥ 99,5% purity, Millipore Corporation), sodium nitrite, and potassium nitrate (AR ≥ 99,0% purity, Millipore Corporation) were selected to prepare the eutectic ternary nitrate salt known as HITEC. The eutectic composition in the NaNO₃-NaNO₂-KNO₃ ternary salt system was synthesized by means of weighted amounts of 7, 40, and 53 mol.%, respectively, and mixing them uniformly in a mortar. The mixtures were then heated in a furnace at a constant heating rate of 5 °C/min from room temperature to 150 °C and held for 1 h until complete fusion was achieved to form the eutectic salts. After that, a heating rate of 3 °C/ min to 270 °C and holding for 48 h were used to eliminate the water in the mixture. Finally, two heating and cooling cycles were carried out in a furnace in order to ensure homogeneity in the mixture. Finally, the molten salts were cooled rapidly and pulverized for future experiments.

Synthesis of molten salt-based nanofluids (MSBNFs)

To obtain a homogenous mixture between the molten salt and the alumina nanoparticles, a new synthesis method of the nitrate-nitrites molten salt-based nanofluids was developed. Hence, after moisture elimination over 48 h at 270 °C in a furnace, the HITEC molten salt was homogenized and pulverized using a mortar for 20 min and an Emax high-energy mill for 15 min at 300 rpm. Next, the pulverized molten salt was mixed with the alumina nanoparticles (0,5, 1,0, and 1,5 wt.%), which had been previously suspended in butanol and sonicated for at least one hour, using magnetic agitation for 15 min. Finally, the mixture was subjected to two heating and cooling cycles in order to ensure the homogeneity of the MSBNFs and the complete elimination of butanol, using cycles between 160 and 270°C with 1 h holds. Figure 1 presents the new synthesis method, and Table 1 shows some thermal properties of HITEC and alumina.

Figure 1 Source: Authors

Table 1 ^*

(Vignarooban et al., 2015)

Characterization

Transmission electron microscopy (TEM)

The size and shape of alumina nanoparticles were observed with an electron transmission microscope (Tecnai F20 Super Twin, with a field emission source and a resolution of 0,1 nm in 200 Kv TMP). Considering the low solubility of alumina in butanol, the particles were dispersed in it. Then, an aliquot was placed on a copper grid (Lacey carbon mesh 200) and heated long enough to ensure solvent elimination.

Structural analysis by scanning electron microscopy (SEM)

The samples were fixed on graphite tape, and a thin gold coating (Au) was made using DENTON VACUUM Desk IV equipment. The analysis was performed in a high-vacuum scanning electron microscope (JEOL JSM 6490 LV) to obtain high-resolution images. A secondary electron detector was used to evaluate the morphology and topography of the samples. The elemental analysis was carried out by energy-dispersive X-ray spectroscopy (EDS; INCA PentaFETx3, Oxford Instruments). SEM images were taken from the pristine alumina and the MSBNFs after the DSC and MDSC measurements in order to verify the presence of the nanoparticles inside the molten salt, as well as to evaluate the agglomeration.

Differential scanning calorimetric (DSC) analysis

The melting point of the base fluid and the MSBNFs was measured using a differential scanning calorimeter (Q200, TA Instruments, Inc.). An aluminum Tzero hermetic pan/ lid (TA instruments) was used to place the samples in the DSC under a nitrogen atmosphere, and these were analyzed using TA Universal Analyzer 2000, version 4,5 A. The weighting amount for each mixture was around 20 mg for both the base fluid and the MSBNFs. The measurement started at room temperature and increased linearly by 10 °C/min up to 400 °C, reaching equilibrium at 40 °C. The test had a sampling interval every 0,10 s/pt, and two heating and cooling runs were performed to obtain a homogeneous result.

The specific heat capacity of HITEC molten salt and the MSBNFs was measured using two different methods. The first method involved using the traditional differential scanning calorimeter (DSC; Q200, TA Instrument, Inc.) according to the ASTM E1269 standard (ASTM International, 2011). The samples were heated to 250 °C and held at this temperature for 5 min in order to obtain a stable heat flux signal, after which they were again heated to 350 °C at a rate of 20 °C/min. The specific heat of the sample was obtained according to (1), where m_ref and m_Sample are the standard reference and sample masses, respectively:

As a second method to measure the Cp of all samples, which involved the modulated differential scanning calorimeter (MDSC), was used according to the ASTM E2716 standard (ASTM International, 2015), with the same equipment used for the DSC analysis. This kind of method uses a sinusoidal temperature oscillation instead of the traditional linear ramp, which simultaneously provides the heat capacity of the sample and the heat flow. The sample was heated to 250 °C and held at this temperature for 5 min in order to obtain a stable heat flux signal. After that, the amplitude was ± 0,50 °C every 130 s. Finally, the sample was heated again to 350 °C at 2 °C/min. The temperature evaluated regarding the specific heat capacity for both methods was 300 °C, as it is the temperature at which the salt is commonly maintained in parabolic trough plants.

Thermogravimetric analysis

The thermal stability of pure eutectic salt and the MSBNFs was determined by thermogravimetric analysis (TGA; Q500, TA Instruments, Inc.) under a constant stream of nitrogen at a flow rate of 50 ml/min, a temperature ramp of 15°C/min in the range 25-800 °C, and an isotherm for 5 min. A platinum crucible was used, with weights between 7 and 10 mg for both the HITEC salt and the nanofluid.

Morphology of alumina

The alumina used to synthesize the MSBNFs was obtained from clay minerals by means of an acid leaching process, followed by alkaline precipitation and an acidification process, to finally calcine and obtain mainly delta-gamma alumina with 98,5% purity. The complete procedure was reported by Botero et al. (2020). Figure 2a and 2b show the SEM and TEM images of the pristine alumina nanoparticles used to elaborate the MSBNFs, respectively, and Figure 2c shows the size distribution (average size of 5,1 nm). The regular shape can be observed in the SEM and TEM images.

Figure 2 Source: Authors

In this study, a two-step method which replaces water with butanol and uses an Emax high-energy mill to prepare molten salt-based nanofluids is proposed. HITEC molten salt was used as the base fluid with different mass fractions of Al₂O₃ nanoparticles. The thermal properties, namely melting point, specific heat capacity, and thermal stability, of the commercial molten salt and the MSBNFs were experimentally tested. The conclusions obtained are presented in this section.

According to the results, it was possible to demonstrate the viability of the synthesis of the molten salt-based nanofluids through the aforementioned method, as evidenced in the increase in specific heat capacity of up to 0,53% when measured by DSC and of 4,57% when evaluated by MDSC for 1,0 wt.% of alumina nanoparticles. The thermal stability was not affected and a melting point reduction was obtained for this sample. Similarly, a good distribution of the nanoparticles was evident in all mass fractions of alumina, especially in 1,0 wt.%, which showed a Cp higher than the other compositions evaluated. Hence, it may be stated that a better distribution of smaller aggregates tends to be correlated with an increase in specific heat capacity.

The specific heat capacity (Cp) values of both the HITEC commercial molten salt and the nanofluids were evaluated by two different methods: traditional differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (MDSC). The enhancement percentage measured by the DSC method with all proportions of nanoparticles was not significant in comparison with the Cp of the pure HITEC salt. This behavior could indicate that there is no effect caused by the addition of this type of nanoparticle, which contradicts recent studies. On the other hand, the MDSC method showed a greater variation in the enhancement percentage, thus indicating that there is a positive effect attributable to the nanoparticles, in addition to a greater sensitivity of this technique to small changes in the sample, especially in non-homogeneous samples such as nanofluids. This establishes the contribution of the two phases that make up the nanofluid, the molten salt as the base fluid and the solid nanoparticles. For this reason, the MDSC technique could be more appropriate for measuring this property in this type of sample. Similarly, the heating rate used in each of the techniques can influence the sensitivity to determine changes in nanofluids.

The good distribution of the nanoparticles, together with an increase in specific heat capacity without affecting thermal stability and a reduction in the melting point, demonstrate that the proposed two-step method is viable for manufacturing molten salt-based nanofluids while avoiding the use of water and therefore its subsequent elimination, thus reducing costs and production times, without having a great influence on the dispersion and homogenization of the nanofluid. Hence, the storage capacity of the nanofluid is increased to improve the thermal storage systems of CSP plants.