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Journal of Energy Storage 22 (2019) 137–143
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Experimental investigation of specific heat capacity improvement of a binary nitrate salt by addition of nanoparticles/microparticles
T
Ming Liua, John Severinoa, Frank Brunoa,b, Peter Majewskib,
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a b
Barbara Hardy Institute, School of Engineering, University of South Australia, Mawson Lakes Boulevard, Mawson Lakes, SA5095, Australia Future Industries Institute, University of South Australia, Mawson Lakes Boulevard, Mawson Lakes, SA5095, Australia
ARTICLE INFO
ABSTRACT
Keywords: Concentrating solar power Solar salt Nanoparticles Particles Specific heat capacity Enhancement
Several recent studies reported anomalous enhancement of specific heat capacity by doping minute concentration (e.g. 1 wt.%) of nanoparticles into the eutectic salt. In this work, silica particles with three different sizes and nano-sized silicon nitride and silicon carbide particles were dispersed in solar salt with particle concentrations of 0.5–7 wt.%. To prepare the composite material, a conventional two-step aqueous solution method was used and the drying approach was modified to suit large-scale production. The specific heat capacities of the composites, pure solar salt and particles were measured using differential scanning calorimetry in a temperature range of 300–400 ᵒC and the results were compared. Silicon nitride and silicon carbide nanoparticles have marginal effect on the specific heat capacity of solar salt over the examined concentration of 0.5–3 wt.%. The specific heat capacity of silica composites with 3 wt.% of silica were enhanced by 4.3–9.7% over that of the pure solar salt. However, when 5 wt.% and 7 wt.% of silica (30 μm) were dispersed, the specific heat capacity was reduced by 10.6% and 9%, respectively. Material characterization of the composite material proved the formation of sodium silicate due to reaction of silica and sodium nitrate, which is probably correlated to the reduction of specific heat capacity.
1. Introduction Concentrating solar power (CSP) technologies generate electricity by focusing the solar radiation beam onto a small area and capturing the solar energy in the form of heat. The CSP installed capacity has increased significantly over the last decade. By the end of 2015, the CSP market had a total capacity of 4.9 GW worldwide and it is projected to provide 12% of global electricity by 2050 [1]. With the current molten salt thermal energy storage (TES) technology, CSP is able to mitigate the short load fluctuation and operate over longer periods. The molten salt is a mixture of sodium nitrate and potassium nitrate with a weight ratio of 60:40 (known as solar salt). By alternately operating the solar salt in a hot tank at 565 ᵒC and a cold tank at 290 ᵒC, the TES system can be charged and discharged, respectively [2]. Considerable research effort has been made by researchers to reduce the high cost of CSP generated electricity, which is one of the main obstructions of wider deployment of CSP technology [3]. Recent studies have reported experimental results of solar salt with specific heat capacity enhanced by doping nanoparticles (NPs) [4–8]. The increase in the specific heat capacity could significantly reduce the cost of TES and hence decrease the cost of generated electricity. The composite material
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containing NPs smaller than 100 nm is defined as a nanofluid. Previous studies in regards to solar salt nanofluid were reviewed and are summarized in Table 1. According to previous experimental results, the addition of NPs in solar salt causes anomalous enhancement or reduction in specific heat capacity. It is probably due to the type, concentration and size of NPs and the synthesis method and there is no evidence to prove the cause of inconsistency. A two-step aqueous solution method developed by Shin and Banerjee [12] was widely used to synthesize the nanofluid and this method was slightly varied in different studies. Lu and Huang [4] synthesized the nanofluids based on a volume ratio of 0.9%, 2.7% and 4.6%, which makes it difficult to compare this result to others without knowing the density of NPs. If the density of NPs and solar salt are 3970 and 1794 kg/m3, respectively, then the equivalent weight ratio will be 2.0%, 5.8% and 9.6% as quoted in the article. Silica and alumina are the mostly investigated NPs doped in the solar salt. Chieruzzi et al. [5] found 1.0 wt.% of silica-alumina NP could improve the specific heat capacity of solar salt by 22.5%. The work by Andreu-Cabedo et al. [7] and Schuller et al. [9] presented a consistent trend: the former showed an enhancement of 25.03% with 1.0 wt. % silica NP [7] and the latter showed an enhancement of 30.6% with
Corresponding author. E-mail address: [email protected] (P. Majewski).
https://doi.org/10.1016/j.est.2019.01.025 Received 14 September 2018; Received in revised form 24 January 2019; Accepted 25 January 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Summary of previous published studies in molten nitrate salt based nanofluids. Ref.
NPs and concentrations
Preparation method
Results
Lu and Huang, 2013 [4]
Alumina: 13 & 90 nm; 0.9, 2.7 & 4.6 vol.%.
Chieruzzi et al., 2013 [5]
Silica (7 nm), alumina (13 nm), titania (20 nm) & silica-alumina (2-200 nm); 0.5, 1.0 & 1.5 wt.% Silica (5, 10, 30 & 60 nm); 1.0 wt.%
mix solar salt & NP + water (1:1) disperse in ultrasonic bath (100 min) dry in hot plate (105 °C for 12 h) melt in oven (300 ᵒC for 40 min) mix solar salt & NP + water (1:100) disperse in ultrasonic bath (100 min) dry in hot plate (200 °C for 2 h)
cp: -2 – -13%; reduction increases with increasing NP concentration and decreasing NP size cp: -1.4 – -19.3% (0.5 & 1.5 % silica, alumina & titania NPs); maximum cp: +22.5 % (1.0 % silica-alumina) cp: +10 % (5 nm), +13 % (10 nm, +21 % (30 nm), +28 % (60 nm) cp: +3.41 % (0.5 % NP), +25.03 % (1.0 % NP), +2.0 % (1.5 % NP), +3.69 % (2.0 % NP)
Dudda and Shin, 2013 [8] Andreu-Cabedo et al., 2014 [7]
Silica (12 nm); 0.5, 1.0, 1.5 and 2.0 wt.%
Schuller et al., 2015 [9] Lasfargues et al., 2015 [10] Hu et al., 2017 [6]
Alumina (40 nm); 0.125, 0.25, 0.5, 0.75, 1.0, 1.5 and 2.0 wt.% Copper oxide (29 nm) & titanium oxide (34 nm); 0.1, 0.5, 1.0 and 1.5 wt.% Alumina (20 nm); 0.5, 1.0, 1.5 and 2.0 wt. %
Chieruzzi et al., 2017 [11]
Silica (7 nm), alumina (13 nm) & silicaalumina (2-200 nm); 1.0 wt.%
mix solar salt & NP + water (1:100) disperse in ultrasonic bath (200 min) dry in hot plate (200 °C for 7 h) mix solar salt + water (1:10) disperse NP using ultrasound probe (5 min) dry in hot plate (100 °C for 1 h) mix solar salt & NP + water disperse in ultrasonic bath (120 min) dry in hot plate (90 °C) mechanical mixing using a ball-mill (room temperature) mix NP + water (1:4000) disperse in ultrasonic bath (60 min) add solar salt disperse in ultrasonic bath (60 min) dry in vacuum oven (110 °C for 7 h) high temperature mixing using a twin screw microcompounder rotor speed of 100 and 200 rpm mixing time of 15 and 30 min
cp: +30.6 % (1.0 % NP) cp: +10.5% (0.1 % copper oxide); cp: +1.9 – +8.3%; enhancement increases with increasing NP concentration cp: +18.6% (silica-alumina, 30 min at 200 rpm); -28.1 – 2.6% (silica); -7.8 – 2.8% (alumina);
Table 2 Specifications and properties of the particles used in this study.
Supplier Purity Structure Shape Surface area (m2/g) Density (g/l)
Silica 250nm
Silica 500 nm
Silica 30 μm
Silicon nitride 15-30nm
Silicon carbide < 80nm
Fiber Optic Center Inc. > 99.9 % Amorphous Perfectly spherical (Non-porous) 2-6 1800
Geltech Inc. > 99.9 % Amorphous spherical 6.22 2000
Evonik Resource Efficiency GmbH ≥ 99.0 % Amorphous granulated 270-330 280
US Research Nanomaterials Inc. ≥ 99.0 % Amorphous spherical 50-120 50
US Research Nanomaterials Inc. ≥ 99.0 % Beta cubic 25-50 50
Fig. 1. Specific heat capacity of nanoparticles investigated in this work and pure eutectic salt. Table 3 Average specific heat capacity of nanoparticles and pure eutectic salt (σ=standard deviation). Cp (J/(g∙K)) st
1 run 2nd run 3rd run Average σ
Silica 250nm
Silica 500 nm
Silica 30 μm
Silicon nitride
Silicon carbide
Eutectic salt (batch1)
Eutectic salt (batch2)
Eutectic salt (batch3)
1.578 1.639 1.533 1.583
1.502 1.530 1.504 1.512
0.943 0.966 0.972 0.961
1.373 1.381 1.379 1.377
1.468 1.491 1.497 1.485
1.639 1.564 1.531 1.578 0.037
1.613 1.600 1.605
1.551 1.557 1.540
138
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Fig. 2. The specific heat capacity in the temperature range of 300–400 ᵒC for the nanofluids and the pure eutectic salt with (a) 0.5 wt.% particles; (b) 1.0 wt.% particles; (c) 1.5 wt.% particles; (d) 2.0 wt.% particles and (e) 3.0 wt.% particles.
1.0 wt. % alumina NP [9]. Dudda and Shin [8] reported an increasing trend in specific heat capacity with increasing size of 1.0 wt.% silica NPs from 5 to 60 nm. Hu et al. [6] observed a slightly enhanced specific heat capacity (1.9–8.3 %) of alumina based nanofluids and the increment rises with increasing NP concentration up to 2.0 wt.%. Mica with 0.5, 1 and 2 wt.% was added into a nitrate salt eutectic and the specific
heat capacity of nanofluids were enhanced by 13–19 % [13]. The dispersed particles were imaged using transmission electron microscopy and the observed size varied from a few nanometers to a few micrometers (˜45 μm was reported by the manufacturer) [13]. This could indicate that particles larger than 100 nm is still capable of enhancing the specific heat capacity. 139
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doped samples. This structure has a significant mass fraction in the sample and is therefore concluded to be responsible for enlarging the specific heat capacity. However, the material composition of the nanostructure has not been identified so far. Tiznobaik et al. [22] proved the enhancement of specific heat capacity is less sensitive to the material of nanoparticle itself, probably more to the size, shape and mass concentration of the nanoparticles. In this work, five different nanoparticles/microparticles were investigated as additives in the solar salt with various particle concentrations from 0.5 wt.% to 7 wt.%. Among them, it is the first attempt to study the effect of silicon nitride and silicon carbide NPs on the specific heat capacity of solar salt. Silica with larger particle size (over 100 nm) was used since previous studies listed in Table 1 focused on the nano-sized particles. Instead of fast drying the aqueous solution using a hotplate, a slow-drying method was adopted. The specific heat capacity of the synthesized composites were measured for a temperature range of 300 ᵒC – 400 ᵒC by using a differential scanning calorimetry (DSC).
Table 4 Average specific heat capacity of nanoparticles and pure eutectic salt in liquid state. Material
Concentration (wt. %)
Cp_exp (J/(g∙K))
Enhancement (%)
Cp_model (Eq. (1)) (J/(g∙K))
Eutectic salt + silica 250nm
0.5 1.0 1.5 2.0 3.0 0.5 1.0 1.5 2.0 3.0 0.5 1.0 1.5 2.0 3.0 5.0 7.0 0.5 1.0 1.5 2.0 3.0 0.5 1.0 1.5 2.0 3.0
1.572 1.564 1.515 1.547 1.649 1.571 1.610 1.514 1.572 1.731 1.602 1.655 1.570 1.577 1.645 1.411 1.436 1.503 1.503 1.563 1.616 1.547 1.564 1.563 1.560 1.607 1.544
−0.38 −0.89 −3.99 −1.97 4.50 −0.44 2.03 −4.06 −0.38 9.70 1.52 4.88 −0.51 −0.06 4.25 −10.58 −9.00 −4.75 −4.75 −0.95 2.41 −1.97 −0.89 −0.95 −1.14 1.84 −2.16
1.578 1.578 1.578 1.578 1.578 1.578 1.577 1.577 1.577 1.576 1.575 1.572 1.569 1.566 1.559 1.547 1.535 1.577 1.576 1.575 1.574 1.572 1.578 1.577 1.577 1.576 1.575
Eutectic salt + silica 500 nm Eutectic salt + silica 30 μm
Eutectic salt + Silicon nitride Eutectic salt + Silicon carbide
2. Methodology 2.1. Nanofluids synthetization Industrial grade sodium nitrate (NaNO3, > 99.0 wt. %) and potassium nitrate (KNO3, > 99.0 wt. %) were supplied by ACE Chemicals (Australia). The raw materials were dried in a furnace at 150 ℃ for 2 h prior to mixing in the required weight ratio. To achieve a homogenous sample, the mixture was melted in a furnace at 350 ℃ for 6 h. The formed eutectic salt was manually ground into powder by using a mortar and pestle and the obtained sample was stored under dry conditions. Five nanoparticles/microparticles were selected to be synthesized with the eutectic nitrate salt. They include: silica (SiO2) of three particle sizes of 250 nm (Fiber Optic Center Inc.), 500 nm (Geltech Inc.) and 30 μm (AEROPERL® 300/30, Evonik Resource Efficiency GmbH), silicon nitride (Si3N4, US Research Nanomaterials Inc.) and silicon carbide (SiC, US Research Nanomaterials Inc). The specifications and physical properties of those particles provided by the manufacturer are summarized in Table 2. A conventional two-step aqueous solution method was used in the synthetization process. The weight concentration of each particle was 0.5%, 1%, 1.5%, 2% and 3%, respectively and another two samples were prepared containing 5 wt.% and 7 wt.% of AEROPERL® 300/30 silica. A total of 200 mg of composite was synthesized for each particle and concentration and 27 samples were prepared. The required amount of powder particle and salt mixture were dissolved in 20 ml of distilled water in a glass beaker and sonicated for 100 min in an ultrasonicator. To evaporate the water, the aqueous solution was placed inside a furnace at 60 ℃ for 24 h. This drying process is much longer compared to the traditional two-step aqueous method as presented in Table 1. This process was investigated because it is more likely that the drying process takes longer in large quantity production. However, the aqueous solution method has the drawback of consuming a large quantity of water, time and energy. The procedure of using mechanical mixing as presented in [10,11] should be explored in order to prepare larger quantity of samples. Silicon nitride and silicon carbide were treated with HCl due to their hydrophobic characteristic before dispersing into water. To identify the potential reaction between silica and sodium nitrate, 10 g of composite with 5 wt.% of AEROPERL® 300/30 silica was prepared and subject to X-ray diffraction examination.
Fig. 3. Specific heat capacity enhancement dependence with particle concentration.
Similar studies have been carried out with other inorganic salts as base materials, such as eutectic carbonate salts (Li2CO3-K2CO3) [14,15], eutectic ternary nitrate salts (LiNO3–NaNO3–KNO3) [16,17], eutectic quaternary nitrate salts (Ca(NO3)2·4H2O-KNO3-NaNO3-LiNO3) [18,19] and eutectic chloride salts (BaCl2, NaCl, CaCl2, and LiCl) [20]. Shin and Banerjee reported 118-124% enhancement in the specific heat capacity of carbonate salt with 1.5 wt. % silica NP [14]. It indicated that the enhanced specific heat capacity of solar salt can be potentially augmented through optimization of additives and synthetization method. To investigate the mechanism for specific heat enhancement, Shin and Banerjee [14,21] and Tiznobaik et al. [22] performed material characterization studies using scanning electron microscopy (SEM) and they compared the SEM images of pure salt samples and nanofluid samples. They found nanostructures of a higher density phase in the NP
2.2. Specific heat measurement The specific heat capacity of the particles, pure eutectic nitrate salt and composites were measured using a DSC (DSC 8000, PerkinElmer Inc.). Each sample was sealed in a 50 μl aluminum pan and lid and the sample mass was kept at 10–15 mg. The areas method was used to 140
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Fig. 4. XRD pattern of a sample with 5 wt. % silica in sola salt after annealing at 300 °C.
determine the specific heat capacity and the details concerning this method was explained in Ferrer et al.’s work [23]. In their work, the accuracy of the three commonly used DSC methods (dynamic, isostep and areas methods) were compared and the areas method was proved to be the most proper method for molten salt measurement with a very low relative errors (< 3%) [23]. The specific heat capacity in the temperature range of 300–400 ᵒC was measured. Although the solar salt is stable at higher temperature [24], the measurement becomes more difficult due to the creep and decomposition issues [25]. The same procedure was applied to measure the specific heat capacity of particles, pure eutectic salt and composites. Three independent batches of eutectic salt was prepared and tested. The measurement was repeated at least three times for each sample to ensure the repeatability of the instrument and the average value was obtained.
20 nm and 90 nm were reported to be 1.30 J/(g∙K) [4], 1.12 J/(g∙K) [6] and 1.10 J/(g∙K) [4], respectively, which are all higher than that of bulk alumina with a heat capacity of 0.96 J/(g∙K) in a similar temperature range [26]. The specific heat capacity of silicon nitride and silicon carbide NPs are measured to be 1.377 and 1.485 J/(g∙K), respectively. These values are slightly lower than the eutectic salt, but significantly higher than heat capacity values reported for bulk silicon carbide [27] and bulk silicon nitride [28]. 3.2. Specific heat capacity of composites The specific heat capacity of the composites are presented in Fig. 2ae for particle concentrations of 0.5 wt.%, 1 wt.%, 1.5 wt.%, 2 wt.% and 3 wt.%, respectively, and for various temperatures. The average values and the change in percentage compared to that of the pure eutectic salt are listed in Table 4 and presented in Fig. 3. The two red dash lines in Fig. 2a-e are the specific heat capacity of the eutectic salt ± 2σ (standard deviation), which means 95% of the measured values are in between those two lines and the variation is due to the experimental error. Any measured heat capacity values above or below the lines are confidently believed to be higher or lower than that of the pure eutectic salt. The DSC results were compared with those predicted by a simple mixing model in Eq. (1) based on the thermal equilibrium between two compounds, which are shown in Table 4 as well. The predicted specific heat capacity is extremely close to that of the pure eutectic since the concentration of particles is very low.
2.3. Material characterization The powdered sample was positioned on a flat plate sample holder and the analysis was performed on a PANalytical Empyrean X-ray diffractometer (XRD) equipped with high-resolution theta (Θ)–theta (Θ) goniometer using Cu K-α radiation of wavelength 1.54 Å and PIXcel3D detector. Scans were conducted from 5-90ᵒ 2Θ at 0.01ᵒ 2-Θ intervals with a count time of 5 s/point. The X-ray conditions were set to 40 kV and 40 mA. 3. Results and discussion
ms cp, s + mp cp, p
3.1. Specific heat capacity of particles and pure eutectic salt
cp, mp =
In Fig. 1, the specific heat capacity of each investigated particle and the pure eutectic salt is plotted against the temperature. The average value over the examined temperature range was summarized in Table 3. As shown in Fig. 1, the temperature effect on the heat capacity values of the particles and the salt is not significant. The specific heat capacity of this eutectic salt was measured in previous published studies and the values are 1.59 and 1.60 J/(g∙K) [4], 1.48 J/(g∙K) [7], 1.648 J/ (g∙K) [5] and 1.472 J/(g∙K) [8] in the liquid state. The value of 1.58 ± 0.04 J/(g∙K) obtained from this work is in good agreement with those reported values, which verifies the measurement accuracy. The specific heat capacity of the silica decreases from 1.583 J/(g∙K) to 0.961 J/(g∙K) with increasing particle size. Similar trends on alumina NPs has been reported by Lu and Huang [4] and Hu et al. [6]. The specific heat capacity of alumina NPs with an average size of 13 nm,
where m and cp stand for mass and specific heat capacity, respectively. Subscripts mp, p, and s denote composite, particle, and eutectic salt, respectively. As it is shown in Fig. 2a-e, most of the composite samples have very similar specific heat capacity to that of the pure eutectic salt. The specific heat capacity of the eutectic salt doping with 3 wt.% of 500 nm silica is 9.7% higher than that of the pure salt (in Fig. 2e). According to the mixing model, the specific heat capacity of the composite material is marginally reduced or increased depending on the specific heat capacity of the particle. The simple mixing theory failed to predict the increase/decrease in the composite. Shin and Banerjee [14] proposed a new model (in Eq. (2)) to explain the enhancement by accounting for the effect of the compressed layer and the model was found to be in good agreement with the experiment result in their work. 141
ms + m p
(1)
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cp, mp =
ms cp, s + mp cp, p + mcl cp, cl ms + mp + mcl
capacity of sodium silicate with 0.77 J/(g∙K) is lower than that of pure solar salt. Overall, thermodynamic considerations indicate that the system of silica plus sodium nitrate is not in thermodynamic equilibrium. Within the temperature range studied in this publication, reactions between the compounds that affect the heat capacity of the samples cannot be excluded.
(2)
where subscript cl denotes compressed layer. It can be noticed that at a particle concentration of 0.5 wt.% and 1.5 wt.%, almost all the composite materials have lower specific heat capacity than the pure eutectic salt. Silicon nitride and silicon carbide NPs fails to improve the specific heat capacity in almost all the concentrations. AEROPERL®300/30 silica (30 μm) has a large surface area of 300 ± 30 m2/g and a high level of porosity with mesoporous volume of 1.83 cm3/g and macro-pore volume of 2.45 cm3/g according to the manufacturer’s specification. The samples with 1 wt. % and 3 wt. % of AEROPERL®300/30 shows approximately 5% enhanced specific heat capacity. However, a reduction was found when the concentration of the same particle increases to 5 wt.% and 7 wt.%. A nanoparticle (AEROSIL®300), which has similar surface area as AEROPERL®300/30 and an average primary particle size of 7 nm, was dispersed into three concentrations: 0.5, 1 and 1.5 wt.% in Chieruzzi et al.’s work [5]. The same solar salt was used as the base material and the average specific heat capacity in the range of 250–300 ᵒC was compared. The negligible effect: 0.8 and −1.4% enhancement was found in the samples containing 1.0 and 1.5 wt.% of NPs, respectively, however, 19.3% reduction showed in the sample with the addition of 0.5 wt.% NP [5]. So far, there is no discussion on the mechanism of specific heat capacity reduction.
4. Conclusion The investigation of the effect of additions of silicon carbide, silicon nitride, and silica to solar salt show that the specific heat capacity can be affected by the additional materials. While the addition of silicon carbide and silicon nitride does not result in an increase of the specific heat capacity of the eutectic salt, the addition of silica can both increase and also decrease the specific heat capacity at various concentrations. While the study does not contradict the model presented by Shin and Banerjee [14], thermodynamic considerations clearly indicate that silica and sodium nitrate are not in thermodynamic equilibrium at the temperatures studied here and the formation of sodium silicate has occurred. Therefore, phase reactions between silica and sodium nitrate during the analysis may affect the specific heat capacity of the composite samples. Acknowledgements This research was performed as part of the Australian Solar Thermal Research Initiative (ASTRI), a project supported by the Australian Government, through the Australian Renewable Energy Agency (ARENA). The authors also acknowledge the University of South Australia who has funded this research through the Research Themes Investment Scheme – Seed Funding scheme.
3.3. Mechanism The results of this study do not correlate with the results reported by Shin and Banerjee [14]. In the case of NPs of silicon carbide and silicon nitride, a slight increase in specific heat capacity can be observed with the addition of 2 wt.% for both materials. Even then, the results are within the standard deviation of the measurements. If the model in Eq. (2) presented by Shin and Banerjee [14] is correct, it can be expected that an increase in heat capacity can be observed independent of the type of nanoparticle. However, in this study, an increase in heat capacity was observed at 2 wt.%, if at all, and not at 1 wt.% as reported by Shin and Banerjee [14]. The results of the samples with silica particles show a greater difference from the results presented by Shin and Banerjee [14]. An increase in specific heat capacity outside the standard deviation can be observed with the addition of 3 wt.%. This may indicate that, in addition to the model of Shin and Banerjee [14], other factors may influence the heat capacity of the composite material. One possibility could be thermodynamic phase reactions between the solar salt and silica. Marshall [29] reported silica solubility in high concentrated sodium nitrate solutions up to temperatures of 300 ᵒC. At 300 ᵒC the silica solubility in sodium nitrate was reported to be about 0.025 mol at a sodium nitrate molality of 1, i.e. 1.73% silica in sodium nitrate. It cannot be excluded that the dissolution of silica into molten sodium nitrate affect its specific heat capacity. Moreover, Hoshino et al. [30] reported about the formation of sodium silicate, Na2SiO3, by sodium nitrate and silica following the reaction:
References [1] S. Teske, J. Leung, L. Crespo, M. Bial, E. Dufour, C. Richter, Solar Thermal Electricity Global Outlook 2016, in, European Solar Thermal Electricity Association (ESTELA), Greenpeace International and SolarPACES, 2016 pp. 114. [2] T. Bauer, N. Breidenbach, N. Pfleger, D. Laing, M. Eck, Overview of molten salt storage systems and material development for solar thermal power plants, World Renewable Energy Forum, American Solar Energy Society, Denver, Colorado, USA, 2012. [3] M. Liu, N.H. Steven Tay, S. Bell, M. Belusko, R. Jacob, G. Will, W. Saman, F. Bruno, Review on concentrating solar power plants and new developments in high temperature thermal energy storage technologies, Renew. Sustain. Energy Rev. 53 (2016) 1411–1432, https://doi.org/10.1016/j.rser.2015.09.026. [4] M.-C. Lu, C.-H. Huang, Specific heat capacity of molten salt-based alumina nanofluid, Nanoscale Res. Lett. 8 (2013), https://doi.org/10.1186/1556-276X-8-292 292. [5] M. Chieruzzi, G.F. Cerritelli, A. Miliozzi, J.M. Kenny, Effect of nanoparticles on heat capacity of nanofluids based on molten salts as PCM for thermal energy storage, Nanoscale Res. Lett. 8 (2013), https://doi.org/10.1186/1556-276X-8-448 448-448. [6] Y. Hu, Y. He, Z. Zhang, D. Wen, Effect of Al2O3 nanoparticle dispersion on the specific heat capacity of a eutectic binary nitrate salt for solar power applications, Energy Convers. Manage. 142 (2017) 366–373, https://doi.org/10.1016/j. enconman.2017.03.062. [7] P. Andreu-Cabedo, R. Mondragon, L. Hernandez, R. Martinez-Cuenca, L. Cabedo, J.E. Julia, Increment of specific heat capacity of solar salt with SiO2 nanoparticles, Nanoscale Res. Lett. 9 (2014) 582, https://doi.org/10.1186/1556-276X-9-582. [8] B. Dudda, D. Shin, Effect of nanoparticle dispersion on specific heat capacity of a binary nitrate salt eutectic for concentrated solar power applications, Int. J. Therm. Sci. 69 (2013) 37–42, https://doi.org/10.1016/j.ijthermalsci.2013.02.003. [9] M. Schuller, Q. Shao, T. Lalk, Experimental investigation of the specific heat of a nitrate–alumina nanofluid for solar thermal energy storage systems, Int. J. Therm. Sci. 91 (2015) 142–145, https://doi.org/10.1016/j.ijthermalsci.2015.01.012. [10] M. Lasfargues, Q. Geng, H. Cao, Y. Ding, Mechanical dispersion of nanoparticles and its effect on the specific heat capacity of impure binary nitrate Salt mixtures, Nanomaterials 5 (2015) 1136, https://doi.org/10.3390/nano5031136. [11] M. Chieruzzi, G.F. Cerritelli, A. Miliozzi, J.M. Kenny, L. Torre, Heat capacity of nanofluids for solar energy storage produced by dispersing oxide nanoparticles in nitrate salt mixture directly at high temperature, Sol. Energy Mater. Sol. Cells 167 (2017) 60–69, https://doi.org/10.1016/j.solmat.2017.04.011. [12] D. Shin, D. Banerjee, Effects of silica nanoparticles on enhancing the specific heat capacity of carbonate salt eutectic (work in progress), Int. J. Struct. Changes in Solids – Mech. Appl. 2 (2010) 25–31.
2 SiO2 + 4 NaNO3 → 2 Na2SiO3 + 4 NO + 3 O2 Although Hoshino et al. [30] studied this reaction at temperature between 600 ᵒC and 800 ᵒC, such a reaction cannot be disregarded at temperatures below 600 ᵒC. In order to verify this, a composite sample with 5 wt.% silica in solar salt was annealed for 24 h at 300 ᵒC and characterized using XRD. Fig. 4 shows the XRD pattern of a sample containing 5 wt.% silica. It is obvious that sodium silicate was formed besides sodium nitrate, potassium nitrate, and cristobalite. While the experiment cannot clarify why an increase in specific heat capacity of solar salt with 1 wt.% silica occurs, it is suggested that the formation of sodium silicate may be the reason for the decrease in heat capacity of samples with higher concentrations of silica, as the heat 142
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M. Liu et al. [13] S. Jung, D. Banerjee, Enhancement of heat capacity of nitrate salts using Mica nanoparticles, Developments in Strategic Materials and Computational Design II, John Wiley & Sons, Inc., 2011, pp. pp. 127–137. [14] D. Shin, D. Banerjee, Enhanced specific heat capacity of nanomaterials synthesized by dispersing silica nanoparticles in eutectic mixtures, J. Heat Transfer-Trans. Asme 135 (2013), https://doi.org/10.1115/1.4005163 032801. [15] H. Tiznobaik, D. Shin, Enhanced specific heat capacity of high-temperature molten salt-based nanofluids, Int. J. Heat Mass. Transf. 57 (2013), https://doi.org/10. 1016/j.ijheatmasstransfer.2012.10.062. [16] J. Seo, D. Shin, Size effect of nanoparticle on specific heat in a ternary nitrate (LiNO3–NaNO3–KNO3) salt eutectic for thermal energy storage, Appl. Therm. Eng. 102 (2016) 144–148, https://doi.org/10.1016/j.applthermaleng.2016.03.134. [17] M.X. Ho, C. Pan, Optimal concentration of alumina nanoparticles in molten hitec salt to maximize its specific heat capacity, Int. J. Heat Mass Transfer 70 (2014) 174–184, https://doi.org/10.1016/j.ijheatmasstransfer.2013.10.078. [18] W. Song, Y. Lu, Y. Wu, C. Ma, Effect of SiO2 nanoparticles on specific heat capacity of low-melting-point eutectic quaternary nitrate salt, Sol. Energy Mater. Sol. Cells 179 (2018) 66–71, https://doi.org/10.1016/j.solmat.2018.01.014. [19] X. Chen, Y.-t. Wu, L.-d. Zhang, X. Wang, C.-f. Ma, Experimental study on the specific heat and stability of molten salt nanofluids prepared by high-temperature melting, Sol. Energy Mater. Sol. Cells 176 (2018) 42–48, https://doi.org/10.1016/j.solmat. 2017.11.021. [20] D. Shin, D. Banerjee, Enhancement of specific heat capacity of high-temperature silica-nanofluids synthesized in alkali chloride salt eutectics for solar thermal-energy storage applications, Int. J. Heat Mass Transfer 54 (2011) 1064–1070, https:// doi.org/10.1016/j.ijheatmasstransfer.2010.11.017. [21] D. Shin, D. Banerjee, Enhanced thermal properties of SiO2 nanocomposite for solar thermal energy storage applications, Int. J. Heat Mass Transfer 84 (2015) 898–902, https://doi.org/10.1016/j.ijheatmasstransfer.2015.01.100.
[22] H. Tiznobaik, D. Banerjee, D. Shin, Effect of formation of “long range” secondary dendritic nanostructures in molten salt nanofluids on the values of specific heat capacity, Int. J. Heat Mass Transfer 91 (2015) 342–346, https://doi.org/10.1016/j. ijheatmasstransfer.2015.05.072. [23] G. Ferrer, C. Barreneche, A. Solé, I. Martorell, L.F. Cabeza, New proposed methodology for specific heat capacity determination of materials for thermal energy storage (TES) by DSC, J. Energy Storage 11 (2017) 1–6, https://doi.org/10.1016/j. est.2017.02.002. [24] T. Bauer, N. Pfleger, N. Breidenbach, M. Eck, D. Laing, S. Kaesche, Material aspects of solar Salt for sensible heat storage, Appl. Energy 111 (2013) 1114–1119, https:// doi.org/10.1016/j.apenergy.2013.04.072. [25] T. Bauer, D. Laing, R. Tamme, Overview of PCMs for concentrated solar power in the temperature range 200 to 350°C, Adv. Sci. Technol. 74 (2010), https://doi.org/ 10.4028/www.scientific.net/AST.74.272. [26] D.C. Ginnings, R.J. Corruccini, Enthalpy, specific heat, and entropy of aluminum oxide from 0° to 900° C, J. Res. Nat. Bur. Stand. 38 (1947) 593–600. [27] Y. Goldberg, M.E. Levinshtein, S.L. Rumyantsev, Silicon carbide (SiC), in: M.E. Levinshtein, S.L. Rumyantsev, M.S. Shur (Eds.), Properties of Advanced Semiconductor Materials: GaN, AIN, InN, BN, SiC, SiGe, John Wiley & Sons, Inc., New York, 2001, pp. pp. 93–148. [28] H.O. Pierson, 13 - covalent nitrides: properties and General characteristics, in: H.O. Pierson (Ed.), Handbook of Refractory Carbides and Nitrides, William Andrew Publishing, Westwood, NJ, 1996, pp. pp. 223–247. [29] W.L. Marshall, Amorphous silica solubilities—I. Behavior in aqueous sodium nitrate solutions; 25–300°C, 0–6 molal, Geochim. Cosmochim. Acta 44 (1980) 907–913, https://doi.org/10.1016/0016-7037(80)90280-X. [30] Y. Hoshino, T. Utsunomiya, T. Utsugi, O. Abe, Reaction of sodium nitrate and silica at High temperatures, NIPPON KAGAKU KAISHI (1980) 690–697, https://doi.org/ 10.1246/nikkashi.1980.690.
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Experimental investigation of specific heat capacity improvement of a binary nitrate salt by addition of nanoparticles/microparticles
T
Ming Liua, John Severinoa, Frank Brunoa,b, Peter Majewskib,
⁎
a b
Barbara Hardy Institute, School of Engineering, University of South Australia, Mawson Lakes Boulevard, Mawson Lakes, SA5095, Australia Future Industries Institute, University of South Australia, Mawson Lakes Boulevard, Mawson Lakes, SA5095, Australia
ARTICLE INFO
ABSTRACT
Keywords: Concentrating solar power Solar salt Nanoparticles Particles Specific heat capacity Enhancement
Several recent studies reported anomalous enhancement of specific heat capacity by doping minute concentration (e.g. 1 wt.%) of nanoparticles into the eutectic salt. In this work, silica particles with three different sizes and nano-sized silicon nitride and silicon carbide particles were dispersed in solar salt with particle concentrations of 0.5–7 wt.%. To prepare the composite material, a conventional two-step aqueous solution method was used and the drying approach was modified to suit large-scale production. The specific heat capacities of the composites, pure solar salt and particles were measured using differential scanning calorimetry in a temperature range of 300–400 ᵒC and the results were compared. Silicon nitride and silicon carbide nanoparticles have marginal effect on the specific heat capacity of solar salt over the examined concentration of 0.5–3 wt.%. The specific heat capacity of silica composites with 3 wt.% of silica were enhanced by 4.3–9.7% over that of the pure solar salt. However, when 5 wt.% and 7 wt.% of silica (30 μm) were dispersed, the specific heat capacity was reduced by 10.6% and 9%, respectively. Material characterization of the composite material proved the formation of sodium silicate due to reaction of silica and sodium nitrate, which is probably correlated to the reduction of specific heat capacity.
1. Introduction Concentrating solar power (CSP) technologies generate electricity by focusing the solar radiation beam onto a small area and capturing the solar energy in the form of heat. The CSP installed capacity has increased significantly over the last decade. By the end of 2015, the CSP market had a total capacity of 4.9 GW worldwide and it is projected to provide 12% of global electricity by 2050 [1]. With the current molten salt thermal energy storage (TES) technology, CSP is able to mitigate the short load fluctuation and operate over longer periods. The molten salt is a mixture of sodium nitrate and potassium nitrate with a weight ratio of 60:40 (known as solar salt). By alternately operating the solar salt in a hot tank at 565 ᵒC and a cold tank at 290 ᵒC, the TES system can be charged and discharged, respectively [2]. Considerable research effort has been made by researchers to reduce the high cost of CSP generated electricity, which is one of the main obstructions of wider deployment of CSP technology [3]. Recent studies have reported experimental results of solar salt with specific heat capacity enhanced by doping nanoparticles (NPs) [4–8]. The increase in the specific heat capacity could significantly reduce the cost of TES and hence decrease the cost of generated electricity. The composite material
⁎
containing NPs smaller than 100 nm is defined as a nanofluid. Previous studies in regards to solar salt nanofluid were reviewed and are summarized in Table 1. According to previous experimental results, the addition of NPs in solar salt causes anomalous enhancement or reduction in specific heat capacity. It is probably due to the type, concentration and size of NPs and the synthesis method and there is no evidence to prove the cause of inconsistency. A two-step aqueous solution method developed by Shin and Banerjee [12] was widely used to synthesize the nanofluid and this method was slightly varied in different studies. Lu and Huang [4] synthesized the nanofluids based on a volume ratio of 0.9%, 2.7% and 4.6%, which makes it difficult to compare this result to others without knowing the density of NPs. If the density of NPs and solar salt are 3970 and 1794 kg/m3, respectively, then the equivalent weight ratio will be 2.0%, 5.8% and 9.6% as quoted in the article. Silica and alumina are the mostly investigated NPs doped in the solar salt. Chieruzzi et al. [5] found 1.0 wt.% of silica-alumina NP could improve the specific heat capacity of solar salt by 22.5%. The work by Andreu-Cabedo et al. [7] and Schuller et al. [9] presented a consistent trend: the former showed an enhancement of 25.03% with 1.0 wt. % silica NP [7] and the latter showed an enhancement of 30.6% with
Corresponding author. E-mail address: [email protected] (P. Majewski).
https://doi.org/10.1016/j.est.2019.01.025 Received 14 September 2018; Received in revised form 24 January 2019; Accepted 25 January 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Summary of previous published studies in molten nitrate salt based nanofluids. Ref.
NPs and concentrations
Preparation method
Results
Lu and Huang, 2013 [4]
Alumina: 13 & 90 nm; 0.9, 2.7 & 4.6 vol.%.
Chieruzzi et al., 2013 [5]
Silica (7 nm), alumina (13 nm), titania (20 nm) & silica-alumina (2-200 nm); 0.5, 1.0 & 1.5 wt.% Silica (5, 10, 30 & 60 nm); 1.0 wt.%
mix solar salt & NP + water (1:1) disperse in ultrasonic bath (100 min) dry in hot plate (105 °C for 12 h) melt in oven (300 ᵒC for 40 min) mix solar salt & NP + water (1:100) disperse in ultrasonic bath (100 min) dry in hot plate (200 °C for 2 h)
cp: -2 – -13%; reduction increases with increasing NP concentration and decreasing NP size cp: -1.4 – -19.3% (0.5 & 1.5 % silica, alumina & titania NPs); maximum cp: +22.5 % (1.0 % silica-alumina) cp: +10 % (5 nm), +13 % (10 nm, +21 % (30 nm), +28 % (60 nm) cp: +3.41 % (0.5 % NP), +25.03 % (1.0 % NP), +2.0 % (1.5 % NP), +3.69 % (2.0 % NP)
Dudda and Shin, 2013 [8] Andreu-Cabedo et al., 2014 [7]
Silica (12 nm); 0.5, 1.0, 1.5 and 2.0 wt.%
Schuller et al., 2015 [9] Lasfargues et al., 2015 [10] Hu et al., 2017 [6]
Alumina (40 nm); 0.125, 0.25, 0.5, 0.75, 1.0, 1.5 and 2.0 wt.% Copper oxide (29 nm) & titanium oxide (34 nm); 0.1, 0.5, 1.0 and 1.5 wt.% Alumina (20 nm); 0.5, 1.0, 1.5 and 2.0 wt. %
Chieruzzi et al., 2017 [11]
Silica (7 nm), alumina (13 nm) & silicaalumina (2-200 nm); 1.0 wt.%
mix solar salt & NP + water (1:100) disperse in ultrasonic bath (200 min) dry in hot plate (200 °C for 7 h) mix solar salt + water (1:10) disperse NP using ultrasound probe (5 min) dry in hot plate (100 °C for 1 h) mix solar salt & NP + water disperse in ultrasonic bath (120 min) dry in hot plate (90 °C) mechanical mixing using a ball-mill (room temperature) mix NP + water (1:4000) disperse in ultrasonic bath (60 min) add solar salt disperse in ultrasonic bath (60 min) dry in vacuum oven (110 °C for 7 h) high temperature mixing using a twin screw microcompounder rotor speed of 100 and 200 rpm mixing time of 15 and 30 min
cp: +30.6 % (1.0 % NP) cp: +10.5% (0.1 % copper oxide); cp: +1.9 – +8.3%; enhancement increases with increasing NP concentration cp: +18.6% (silica-alumina, 30 min at 200 rpm); -28.1 – 2.6% (silica); -7.8 – 2.8% (alumina);
Table 2 Specifications and properties of the particles used in this study.
Supplier Purity Structure Shape Surface area (m2/g) Density (g/l)
Silica 250nm
Silica 500 nm
Silica 30 μm
Silicon nitride 15-30nm
Silicon carbide < 80nm
Fiber Optic Center Inc. > 99.9 % Amorphous Perfectly spherical (Non-porous) 2-6 1800
Geltech Inc. > 99.9 % Amorphous spherical 6.22 2000
Evonik Resource Efficiency GmbH ≥ 99.0 % Amorphous granulated 270-330 280
US Research Nanomaterials Inc. ≥ 99.0 % Amorphous spherical 50-120 50
US Research Nanomaterials Inc. ≥ 99.0 % Beta cubic 25-50 50
Fig. 1. Specific heat capacity of nanoparticles investigated in this work and pure eutectic salt. Table 3 Average specific heat capacity of nanoparticles and pure eutectic salt (σ=standard deviation). Cp (J/(g∙K)) st
1 run 2nd run 3rd run Average σ
Silica 250nm
Silica 500 nm
Silica 30 μm
Silicon nitride
Silicon carbide
Eutectic salt (batch1)
Eutectic salt (batch2)
Eutectic salt (batch3)
1.578 1.639 1.533 1.583
1.502 1.530 1.504 1.512
0.943 0.966 0.972 0.961
1.373 1.381 1.379 1.377
1.468 1.491 1.497 1.485
1.639 1.564 1.531 1.578 0.037
1.613 1.600 1.605
1.551 1.557 1.540
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Fig. 2. The specific heat capacity in the temperature range of 300–400 ᵒC for the nanofluids and the pure eutectic salt with (a) 0.5 wt.% particles; (b) 1.0 wt.% particles; (c) 1.5 wt.% particles; (d) 2.0 wt.% particles and (e) 3.0 wt.% particles.
1.0 wt. % alumina NP [9]. Dudda and Shin [8] reported an increasing trend in specific heat capacity with increasing size of 1.0 wt.% silica NPs from 5 to 60 nm. Hu et al. [6] observed a slightly enhanced specific heat capacity (1.9–8.3 %) of alumina based nanofluids and the increment rises with increasing NP concentration up to 2.0 wt.%. Mica with 0.5, 1 and 2 wt.% was added into a nitrate salt eutectic and the specific
heat capacity of nanofluids were enhanced by 13–19 % [13]. The dispersed particles were imaged using transmission electron microscopy and the observed size varied from a few nanometers to a few micrometers (˜45 μm was reported by the manufacturer) [13]. This could indicate that particles larger than 100 nm is still capable of enhancing the specific heat capacity. 139
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doped samples. This structure has a significant mass fraction in the sample and is therefore concluded to be responsible for enlarging the specific heat capacity. However, the material composition of the nanostructure has not been identified so far. Tiznobaik et al. [22] proved the enhancement of specific heat capacity is less sensitive to the material of nanoparticle itself, probably more to the size, shape and mass concentration of the nanoparticles. In this work, five different nanoparticles/microparticles were investigated as additives in the solar salt with various particle concentrations from 0.5 wt.% to 7 wt.%. Among them, it is the first attempt to study the effect of silicon nitride and silicon carbide NPs on the specific heat capacity of solar salt. Silica with larger particle size (over 100 nm) was used since previous studies listed in Table 1 focused on the nano-sized particles. Instead of fast drying the aqueous solution using a hotplate, a slow-drying method was adopted. The specific heat capacity of the synthesized composites were measured for a temperature range of 300 ᵒC – 400 ᵒC by using a differential scanning calorimetry (DSC).
Table 4 Average specific heat capacity of nanoparticles and pure eutectic salt in liquid state. Material
Concentration (wt. %)
Cp_exp (J/(g∙K))
Enhancement (%)
Cp_model (Eq. (1)) (J/(g∙K))
Eutectic salt + silica 250nm
0.5 1.0 1.5 2.0 3.0 0.5 1.0 1.5 2.0 3.0 0.5 1.0 1.5 2.0 3.0 5.0 7.0 0.5 1.0 1.5 2.0 3.0 0.5 1.0 1.5 2.0 3.0
1.572 1.564 1.515 1.547 1.649 1.571 1.610 1.514 1.572 1.731 1.602 1.655 1.570 1.577 1.645 1.411 1.436 1.503 1.503 1.563 1.616 1.547 1.564 1.563 1.560 1.607 1.544
−0.38 −0.89 −3.99 −1.97 4.50 −0.44 2.03 −4.06 −0.38 9.70 1.52 4.88 −0.51 −0.06 4.25 −10.58 −9.00 −4.75 −4.75 −0.95 2.41 −1.97 −0.89 −0.95 −1.14 1.84 −2.16
1.578 1.578 1.578 1.578 1.578 1.578 1.577 1.577 1.577 1.576 1.575 1.572 1.569 1.566 1.559 1.547 1.535 1.577 1.576 1.575 1.574 1.572 1.578 1.577 1.577 1.576 1.575
Eutectic salt + silica 500 nm Eutectic salt + silica 30 μm
Eutectic salt + Silicon nitride Eutectic salt + Silicon carbide
2. Methodology 2.1. Nanofluids synthetization Industrial grade sodium nitrate (NaNO3, > 99.0 wt. %) and potassium nitrate (KNO3, > 99.0 wt. %) were supplied by ACE Chemicals (Australia). The raw materials were dried in a furnace at 150 ℃ for 2 h prior to mixing in the required weight ratio. To achieve a homogenous sample, the mixture was melted in a furnace at 350 ℃ for 6 h. The formed eutectic salt was manually ground into powder by using a mortar and pestle and the obtained sample was stored under dry conditions. Five nanoparticles/microparticles were selected to be synthesized with the eutectic nitrate salt. They include: silica (SiO2) of three particle sizes of 250 nm (Fiber Optic Center Inc.), 500 nm (Geltech Inc.) and 30 μm (AEROPERL® 300/30, Evonik Resource Efficiency GmbH), silicon nitride (Si3N4, US Research Nanomaterials Inc.) and silicon carbide (SiC, US Research Nanomaterials Inc). The specifications and physical properties of those particles provided by the manufacturer are summarized in Table 2. A conventional two-step aqueous solution method was used in the synthetization process. The weight concentration of each particle was 0.5%, 1%, 1.5%, 2% and 3%, respectively and another two samples were prepared containing 5 wt.% and 7 wt.% of AEROPERL® 300/30 silica. A total of 200 mg of composite was synthesized for each particle and concentration and 27 samples were prepared. The required amount of powder particle and salt mixture were dissolved in 20 ml of distilled water in a glass beaker and sonicated for 100 min in an ultrasonicator. To evaporate the water, the aqueous solution was placed inside a furnace at 60 ℃ for 24 h. This drying process is much longer compared to the traditional two-step aqueous method as presented in Table 1. This process was investigated because it is more likely that the drying process takes longer in large quantity production. However, the aqueous solution method has the drawback of consuming a large quantity of water, time and energy. The procedure of using mechanical mixing as presented in [10,11] should be explored in order to prepare larger quantity of samples. Silicon nitride and silicon carbide were treated with HCl due to their hydrophobic characteristic before dispersing into water. To identify the potential reaction between silica and sodium nitrate, 10 g of composite with 5 wt.% of AEROPERL® 300/30 silica was prepared and subject to X-ray diffraction examination.
Fig. 3. Specific heat capacity enhancement dependence with particle concentration.
Similar studies have been carried out with other inorganic salts as base materials, such as eutectic carbonate salts (Li2CO3-K2CO3) [14,15], eutectic ternary nitrate salts (LiNO3–NaNO3–KNO3) [16,17], eutectic quaternary nitrate salts (Ca(NO3)2·4H2O-KNO3-NaNO3-LiNO3) [18,19] and eutectic chloride salts (BaCl2, NaCl, CaCl2, and LiCl) [20]. Shin and Banerjee reported 118-124% enhancement in the specific heat capacity of carbonate salt with 1.5 wt. % silica NP [14]. It indicated that the enhanced specific heat capacity of solar salt can be potentially augmented through optimization of additives and synthetization method. To investigate the mechanism for specific heat enhancement, Shin and Banerjee [14,21] and Tiznobaik et al. [22] performed material characterization studies using scanning electron microscopy (SEM) and they compared the SEM images of pure salt samples and nanofluid samples. They found nanostructures of a higher density phase in the NP
2.2. Specific heat measurement The specific heat capacity of the particles, pure eutectic nitrate salt and composites were measured using a DSC (DSC 8000, PerkinElmer Inc.). Each sample was sealed in a 50 μl aluminum pan and lid and the sample mass was kept at 10–15 mg. The areas method was used to 140
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Fig. 4. XRD pattern of a sample with 5 wt. % silica in sola salt after annealing at 300 °C.
determine the specific heat capacity and the details concerning this method was explained in Ferrer et al.’s work [23]. In their work, the accuracy of the three commonly used DSC methods (dynamic, isostep and areas methods) were compared and the areas method was proved to be the most proper method for molten salt measurement with a very low relative errors (< 3%) [23]. The specific heat capacity in the temperature range of 300–400 ᵒC was measured. Although the solar salt is stable at higher temperature [24], the measurement becomes more difficult due to the creep and decomposition issues [25]. The same procedure was applied to measure the specific heat capacity of particles, pure eutectic salt and composites. Three independent batches of eutectic salt was prepared and tested. The measurement was repeated at least three times for each sample to ensure the repeatability of the instrument and the average value was obtained.
20 nm and 90 nm were reported to be 1.30 J/(g∙K) [4], 1.12 J/(g∙K) [6] and 1.10 J/(g∙K) [4], respectively, which are all higher than that of bulk alumina with a heat capacity of 0.96 J/(g∙K) in a similar temperature range [26]. The specific heat capacity of silicon nitride and silicon carbide NPs are measured to be 1.377 and 1.485 J/(g∙K), respectively. These values are slightly lower than the eutectic salt, but significantly higher than heat capacity values reported for bulk silicon carbide [27] and bulk silicon nitride [28]. 3.2. Specific heat capacity of composites The specific heat capacity of the composites are presented in Fig. 2ae for particle concentrations of 0.5 wt.%, 1 wt.%, 1.5 wt.%, 2 wt.% and 3 wt.%, respectively, and for various temperatures. The average values and the change in percentage compared to that of the pure eutectic salt are listed in Table 4 and presented in Fig. 3. The two red dash lines in Fig. 2a-e are the specific heat capacity of the eutectic salt ± 2σ (standard deviation), which means 95% of the measured values are in between those two lines and the variation is due to the experimental error. Any measured heat capacity values above or below the lines are confidently believed to be higher or lower than that of the pure eutectic salt. The DSC results were compared with those predicted by a simple mixing model in Eq. (1) based on the thermal equilibrium between two compounds, which are shown in Table 4 as well. The predicted specific heat capacity is extremely close to that of the pure eutectic since the concentration of particles is very low.
2.3. Material characterization The powdered sample was positioned on a flat plate sample holder and the analysis was performed on a PANalytical Empyrean X-ray diffractometer (XRD) equipped with high-resolution theta (Θ)–theta (Θ) goniometer using Cu K-α radiation of wavelength 1.54 Å and PIXcel3D detector. Scans were conducted from 5-90ᵒ 2Θ at 0.01ᵒ 2-Θ intervals with a count time of 5 s/point. The X-ray conditions were set to 40 kV and 40 mA. 3. Results and discussion
ms cp, s + mp cp, p
3.1. Specific heat capacity of particles and pure eutectic salt
cp, mp =
In Fig. 1, the specific heat capacity of each investigated particle and the pure eutectic salt is plotted against the temperature. The average value over the examined temperature range was summarized in Table 3. As shown in Fig. 1, the temperature effect on the heat capacity values of the particles and the salt is not significant. The specific heat capacity of this eutectic salt was measured in previous published studies and the values are 1.59 and 1.60 J/(g∙K) [4], 1.48 J/(g∙K) [7], 1.648 J/ (g∙K) [5] and 1.472 J/(g∙K) [8] in the liquid state. The value of 1.58 ± 0.04 J/(g∙K) obtained from this work is in good agreement with those reported values, which verifies the measurement accuracy. The specific heat capacity of the silica decreases from 1.583 J/(g∙K) to 0.961 J/(g∙K) with increasing particle size. Similar trends on alumina NPs has been reported by Lu and Huang [4] and Hu et al. [6]. The specific heat capacity of alumina NPs with an average size of 13 nm,
where m and cp stand for mass and specific heat capacity, respectively. Subscripts mp, p, and s denote composite, particle, and eutectic salt, respectively. As it is shown in Fig. 2a-e, most of the composite samples have very similar specific heat capacity to that of the pure eutectic salt. The specific heat capacity of the eutectic salt doping with 3 wt.% of 500 nm silica is 9.7% higher than that of the pure salt (in Fig. 2e). According to the mixing model, the specific heat capacity of the composite material is marginally reduced or increased depending on the specific heat capacity of the particle. The simple mixing theory failed to predict the increase/decrease in the composite. Shin and Banerjee [14] proposed a new model (in Eq. (2)) to explain the enhancement by accounting for the effect of the compressed layer and the model was found to be in good agreement with the experiment result in their work. 141
ms + m p
(1)
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cp, mp =
ms cp, s + mp cp, p + mcl cp, cl ms + mp + mcl
capacity of sodium silicate with 0.77 J/(g∙K) is lower than that of pure solar salt. Overall, thermodynamic considerations indicate that the system of silica plus sodium nitrate is not in thermodynamic equilibrium. Within the temperature range studied in this publication, reactions between the compounds that affect the heat capacity of the samples cannot be excluded.
(2)
where subscript cl denotes compressed layer. It can be noticed that at a particle concentration of 0.5 wt.% and 1.5 wt.%, almost all the composite materials have lower specific heat capacity than the pure eutectic salt. Silicon nitride and silicon carbide NPs fails to improve the specific heat capacity in almost all the concentrations. AEROPERL®300/30 silica (30 μm) has a large surface area of 300 ± 30 m2/g and a high level of porosity with mesoporous volume of 1.83 cm3/g and macro-pore volume of 2.45 cm3/g according to the manufacturer’s specification. The samples with 1 wt. % and 3 wt. % of AEROPERL®300/30 shows approximately 5% enhanced specific heat capacity. However, a reduction was found when the concentration of the same particle increases to 5 wt.% and 7 wt.%. A nanoparticle (AEROSIL®300), which has similar surface area as AEROPERL®300/30 and an average primary particle size of 7 nm, was dispersed into three concentrations: 0.5, 1 and 1.5 wt.% in Chieruzzi et al.’s work [5]. The same solar salt was used as the base material and the average specific heat capacity in the range of 250–300 ᵒC was compared. The negligible effect: 0.8 and −1.4% enhancement was found in the samples containing 1.0 and 1.5 wt.% of NPs, respectively, however, 19.3% reduction showed in the sample with the addition of 0.5 wt.% NP [5]. So far, there is no discussion on the mechanism of specific heat capacity reduction.
4. Conclusion The investigation of the effect of additions of silicon carbide, silicon nitride, and silica to solar salt show that the specific heat capacity can be affected by the additional materials. While the addition of silicon carbide and silicon nitride does not result in an increase of the specific heat capacity of the eutectic salt, the addition of silica can both increase and also decrease the specific heat capacity at various concentrations. While the study does not contradict the model presented by Shin and Banerjee [14], thermodynamic considerations clearly indicate that silica and sodium nitrate are not in thermodynamic equilibrium at the temperatures studied here and the formation of sodium silicate has occurred. Therefore, phase reactions between silica and sodium nitrate during the analysis may affect the specific heat capacity of the composite samples. Acknowledgements This research was performed as part of the Australian Solar Thermal Research Initiative (ASTRI), a project supported by the Australian Government, through the Australian Renewable Energy Agency (ARENA). The authors also acknowledge the University of South Australia who has funded this research through the Research Themes Investment Scheme – Seed Funding scheme.
3.3. Mechanism The results of this study do not correlate with the results reported by Shin and Banerjee [14]. In the case of NPs of silicon carbide and silicon nitride, a slight increase in specific heat capacity can be observed with the addition of 2 wt.% for both materials. Even then, the results are within the standard deviation of the measurements. If the model in Eq. (2) presented by Shin and Banerjee [14] is correct, it can be expected that an increase in heat capacity can be observed independent of the type of nanoparticle. However, in this study, an increase in heat capacity was observed at 2 wt.%, if at all, and not at 1 wt.% as reported by Shin and Banerjee [14]. The results of the samples with silica particles show a greater difference from the results presented by Shin and Banerjee [14]. An increase in specific heat capacity outside the standard deviation can be observed with the addition of 3 wt.%. This may indicate that, in addition to the model of Shin and Banerjee [14], other factors may influence the heat capacity of the composite material. One possibility could be thermodynamic phase reactions between the solar salt and silica. Marshall [29] reported silica solubility in high concentrated sodium nitrate solutions up to temperatures of 300 ᵒC. At 300 ᵒC the silica solubility in sodium nitrate was reported to be about 0.025 mol at a sodium nitrate molality of 1, i.e. 1.73% silica in sodium nitrate. It cannot be excluded that the dissolution of silica into molten sodium nitrate affect its specific heat capacity. Moreover, Hoshino et al. [30] reported about the formation of sodium silicate, Na2SiO3, by sodium nitrate and silica following the reaction:
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2 SiO2 + 4 NaNO3 → 2 Na2SiO3 + 4 NO + 3 O2 Although Hoshino et al. [30] studied this reaction at temperature between 600 ᵒC and 800 ᵒC, such a reaction cannot be disregarded at temperatures below 600 ᵒC. In order to verify this, a composite sample with 5 wt.% silica in solar salt was annealed for 24 h at 300 ᵒC and characterized using XRD. Fig. 4 shows the XRD pattern of a sample containing 5 wt.% silica. It is obvious that sodium silicate was formed besides sodium nitrate, potassium nitrate, and cristobalite. While the experiment cannot clarify why an increase in specific heat capacity of solar salt with 1 wt.% silica occurs, it is suggested that the formation of sodium silicate may be the reason for the decrease in heat capacity of samples with higher concentrations of silica, as the heat 142
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