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Experimental study on thermophysical properties of molten salt nanofluids prepared by high-temperature melting
Solar Energy Materials and Solar Cells 191 (2019) 209–217
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Experimental study on thermophysical properties of molten salt nanofluids prepared by high-temperature melting
T
⁎
Xia Chena,b, Yu-ting Wua,b, , Lu-di Zhanga,b, Xin Wangc, Chong-fang Maa,b a Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China b Key Laboratory of Heat Transfer and Energy Conversion, Beijing Education Commission, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China c College of Mechanical Engineering and Applied Electronics Technology, Beijing University of Technology, Beijing 100124, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molten salt nanofluid High-temperature melting Thermophysical properties
Heat storage is a key technology in solar thermal power, helps solar thermal power eliminate the instability in energy supply for the grid, and provides continuous and stable high-quality electrical energy to improve power system efficiency and extend system life. Molten salt is an important material for heat storage and heat transfer in solar thermal power. In recent years, nanofluid technology has been applied to investigate heat storage and transfer materials, such as molten salt, to improve heat capacity. However, the nanofluid technology is hampered by the easy agglomeration of nanoparticles in molten salts, and this phenomenon degrades the performance of the molten salt nanofluids. To reduce or prevent agglomeration, a two-step method with high-temperature melting in preparing molten salt nanofluids is developed recently. Molten salt nanofluids obtained by high-temperature melting show good stability and long time to agglomerate. This paper presented the study on the thermophysical properties of molten salt nanofluids prepared by high-temperature melting method. The molten salt nanofluids possessed low-melting point salt as base liquid and SiO2 with a diameter of 20 nm as nanoparticles. The specific heat of this type of molten salt nanofluid was tested, and the optimum concentration of nanoparticles was determined. Results showed that the specific heat capacities of the molten salt nanofluid samples with different mass fractions of nanoparticles were higher than those of pure molten salt. The average specific heat of molten salt nanofluid with a mass fraction of 0.5% was the highest at 1.950 J/(g K), which was 24.5% more than that of pure molten salt. The thermophysical properties of molten salt nanofluids with a mass fraction of 0.5% and prepared by high-temperature melting were experimentally tested and compared with those of pure molten salt. These properties included melting point, primary crystallization point, thermal conductivity, viscosity, latent heat, and density. The molten salt nanofluids showed good performance for heat storage and transfer.
1. Introduction The current world faces the dual pressures of rapid increases in energy demand and serious environmental pollution. Vigorous development of renewable energy, such as solar energy, is an important way to solve the shortage of energy and reduce haze, acid rain, and other disastrous weather conditions caused by fossil fuels, and such reduction is an important part of current social energy strategy. Solar thermal power generation technology is based on heat storage, can eliminate the instability in energy supply for the grid, and provide continuous and stable high-quality electrical energy to improve power system efficiency
and extend system life. Molten salts are considered a potential heat transfer and storage medium in solar thermal power because of their advantages of wide temperature range, high heat capacity, low cost, and pollution-free characteristic [1,2]; these salts are also recognized as a realistic large-scale high-temperature heat storage and transfer medium in the international arena. At present, molten salts as a heat storage and transfer medium are successfully used in several solar thermal power plants [3,4]. Molten salts are a type of regenerative heat storage and transfer working fluid used in engineering and technology, and enhancing the heat storage capacity and thermal performance of these materials have been mainly explored.
⁎ Corresponding author at: Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China. E-mail address: [email protected] (Y.-t. Wu).
https://doi.org/10.1016/j.solmat.2018.11.003 Received 10 December 2017; Received in revised form 4 November 2018; Accepted 5 November 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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Thermal stability of molten salt nanofluids prepared by high-temperature melting was good because high-temperature melting directly adds the nanoparticles into the molten salt base liquid with mechanical mixing under high temperature conditions. During this process, the nanoparticles are dispersed in the molten salt base liquid and promote the formation of an electrical double layer on the surface of the nanoparticles, so that the nanoparticles are charged; moreover, the nanoparticles are not easily agglomerated under the mutual repulsion of the same charge [18]. In the present study, molten salt nanofluids were prepared using high-temperature melting based on previous studies [18] to furtherly investigate the thermophysical properties of molten salt nanofluids prepared by high-temperature melting. The specific heat of molten salt nanofluids was tested, and the optimum concentration of nanoparticles was determined. The thermophysical properties of molten salt nanofluids (including melting point, primary crystallization point, thermal conductivity, viscosity, latent heat, and density) with the optimum fraction were also experimentally tested and compared with those of pure molten salt.
Specific heat is one of the important thermophysical parameters of molten salt. Improving the specific heat of molten salt can play a key role in improving the heat storage capacity and reducing the heat storage cost of solar thermal power generation systems. In recent years, the development of the technology of water nanofluids as the representative of nanofluids has increased the attention on improving heat storage and transfer capacity by adding nanoparticles [5–8]. Previous studies have found that adding nanoparticles into molten salts significantly increases the specific heat of molten salts. Bharath [9] added SiO2 nanoparticles with different particle sizes (5, 10, 30, and 60 nm) into solar salts and studied the effect of nanoparticle size on the specific heat characteristics of molten salts. Ramaprasath [10] added nano-SiO2 with different sizes (5, 10, 30, and 60 nm) into HitecXL salt and studied the specific heat characteristics of molten salt nanofluids. The results showed that adding 5, 10, 30, and 60 nm SiO2 increases the specific heat capacities of molten salt nanofluids by 28%, 34%, 19%, and 30%, respectively. Ho and Pan [11] added different proportions of Al2O3 nanoparticles on the basis of Hitec salt and studied the optimized specific heat characteristics of molten salts. The optimal concentration of Al2O3 nanoparticles is 0.063 wt%. Tiznobaik et al. [12] obtained a high-temperature nanofluid material by dispersing four sizes of SiO2 nanoparticles in binary carbonate. The addition of nanoparticles increases the specific heat of the original binary carbonate by 25%. Wu et al. [13,14] prepared a low-melting point mixed molten salt and prepared molten salt nanofluids with SiO2, Al2O3, and TiO2 as nanoparticles. The specific heat of the molten salt nanofluids increased by nearly 20% than that of pure molten salt. The thermal conductivity of molten salt nanofluids is unclear unlike their specific heat that obviously increases. Existing findings indicate that high electrical conductivity of nanoparticles and large specific surface area mean improved thermal conductivity of molten salt nanofluids [15,16]. The two-step method with ultrasonic dispersion is the most widely used method for preparing nanofluids. Currently, the limitation of this method is the possible particle sedimentation and aggregation with respect to time. Nanoparticles are of high particle surface energy owing to large specific surface area and the activity of particle surfaces; accordingly, nanoparticles easily agglomerate together to form large aggregates with a few weak interfacial interfaces. Once nanoparticle aggregates are formed, they are difficult to disassemble without external effect. When aggregates grow to a certain size, they precipitate as a result of gravity and lose the characteristics of nanofluids. Nanoparticles mainly interact with water molecules in water and with molten salt ions in molten salts. These materials are suspended in molten salts, thereby changing the electrical properties to form an electrical double layer according to DLVO theory [17]. The gravitation of agglomeration between nanoparticles is greatly reduced through the repulsion between the electric double layers. As a result, nanoparticles disperse. On the basis of the above-mentioned theory, authors have proposed a two-step method with high-temperature melting in preparing molten salt nanofluids in a previous study [18]. The base solution was a lowmelting point molten salt, and the nanoparticles were SiO2 with a diameter of 20 nm. The specific heat was measured, and the stability of the molten salt nanofluids was studied. The results show that the average specific heat of molten salt nanofluids prepared by high-temperature melting was 1.789 J/(g K), which was close to that of molten salt nanofluids prepared by ultrasonic dispersion and 16.4% higher than that of pure molten salt. The molten salt nanofluids prepared by ultrasonic dispersion showed poor thermal stability under high-temperature condition of 450 °C, and the average specific heat decreased by 8.5% after only 200 h. The thermal stability of molten salt nanofluids prepared by high-temperature melting showed a highly stable performance in long-time experiments. The variation of specific heat was less than 5% after 2000 h under the same high-temperature experimental condition. The two-step method with high-temperature melting is stable and reliable for preparing molten salt nanofluids [18].
2. Method 2.1. Molten salt nanofluid preparation Authors previously developed a low-melting point quaternarymixed nitrate, namely, Ca(NO3)2-KNO3-NaNO3-LiNO3 (2:6:1:2 in mass ratio). The study results showed that the low-melting point molten salt is suitable for solar thermal power generation. This molten salt possesses a melting point of approximately 85.4 °C and a maximum operating temperature of 600 °C. When the temperature is below 300 °C, the thermal conductivity is 0.53 W/(m K) and the specific heat is 1.519 kJ/ (kg K). Moreover, analysis of the experimental results on the convective heat transfer and long-term stability of molten salt showed that the risk of blockage of the said molten salt is greatly reduced and the corrosion to the pipeline is decreased [19]. In the present study, the abovementioned low-melting point molten salt was used as the base solution. Furthermore, the frequently used nanoparticles for preparing nanofluids were used as SiO2 nanoparticles with a diameter of 20 nm. SiO2 nanoparticles with different mass fractions were mixed with molten salt to prepare molten salt nanofluids by using high-temperature melting. The concrete steps for the two methods are discussed in the subsections below [18]. (1) Low-melting point molten salt and SiO2 nanoparticles were weighed using an electronic balance, and the low-melting point molten salt was placed in a muffle furnace and heated to 400 °C (higher than its melting point of 300 °C). (2) SiO2 nanoparticles were slowly added into the high-temperature molten salt at 400 °C, and the molten salt liquid was stirred evenly with a stirring bar during the adding process. (3) Magnetic stirrer was rotated at 600 r/min, and the mixture of molten salt and SiO2 nanoparticles was stirred by a magnetic stirrer until the mixture cooled and solidified (about 15 min) to obtain a composite nanometer molten salt.
2.2. Experimental reagents and instruments KNO3, NaNO3, Ca(NO3)2, and LiNO3 of analytical grade were used as nitrates in this experiment. SiO2 with a purity of 99.9% and a particle size of 20 nm was used as the nano-powder. The experimental reagents used in the experiment and their related parameters are listed in Tables 1 and 2. The experimental apparatuses are listed in Table 3.
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differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis, the instrument can provide simultaneous TG and differential thermal signals at the same temperature. DSC is a thermal analysis method that measures the relation of the power difference between a sample and a reference material to temperature at a programmed temperature, and the operation principle of method is shown in Fig. 1. Samples and reference materials were placed in two identical crucibles. Each crucible possessed separate heating and temperature sensing elements and was monitored by two systems: one was for controlling the rise rate of temperature and the other was for compensating the temperature difference between samples and reference materials. When the temperature difference between the sample and reference material appears as a result of the thermal effect in the heating process, the current flowing into the compensation heating wire will change through the differential thermal amplifying circuit and compensated amplifier. When the sample absorbs heat, the compensation amplifier immediately increases the current at the sample side. Conversely, when the sample releases heat, the current at the reference material side increases until the heat on both sides balances and no temperature difference exists. The relation of the amount of heat required to maintain these isothermal conditions to time or temperature, phase transition temperature, and melting and freezing point can be obtained from the appearance or disappearance of peaks in the DSC curve. TG analysis is a technique that obtains the relationship between the mass and temperature of a substance by means of a thermobalance under the control temperature. Fig. 2 shows the schematic of the basic structure of a thermobalance, which is an important part of automatic and continuous weighing and recording. The temperature of sample can be changed by a temperature program in the weighing process, and the atmosphere around the sample can be controlled or adjusted. The curve of the relationship between the sample quality and temperature under the programmed temperature, which is TG curve, can be obtained by TG analysis. The temperature and sensitivity of the instrument were calibrated first before the experiment, and the accuracy of measuring the melting point of the instrument was verified using the bismuth and aluminum standards. Aluminum oxide crucible was used in the experiment to prevent the adhesion or reaction between standards and metal crucible. Figs. 3 and 4 show the melting points of bismuth and aluminum, respectively. Table 4 lists the error analysis results of the melting points of bismuth and aluminum. The results show that the relative errors of the melting points of bismuth measured by three measurements are 0.63%, 0.66%, and 0.77%, respectively. The melting points of aluminum are 0.67%, 0.68%, and 0.55%. The errors are not more than 1%, and the accuracy is very high.
Table 1 Physical parameters of reagents. Name
Melting point °C
Density g/cm3
Latent value J/g
Content ％
KNO3 NaNO3 Ca(NO3)2
337 310 αtype 42.7 βtype 39.7 anhydrous 561 255
2.11 2.26 αtype 1.89 βtype 1.82 2.50 2.38
115 173 153 153 145 371
≥ 99.0 ≥ 99.0 ≥ 99.0
LiNO3
≥ 99.0
Table 2 Physical parameters of nanoparticles. Name
Diameter Nm
Specific surface area m2/g
Pattern
Fill density g/ cm3
Purity %
SiO2
20
120
Sphere
0.08
≥ 99.0
Table 3 Experimental instruments. Name
Parameters
Electronic balance
Type: ML204 Maximum weighing range: 220 g Readability: 0.0001 g Weighing technology: MonoBloc Linear error: 0.2 mg Type: SX-G16103 Rated power: 8k W Rated voltage: 380 V Rated temperature: 1000 °C Type: STA-449F3 Temperature: − 150 to 2000 °C Heating/cooling rate: 0.001–50 K/min Weighing capacity: 35,000 mg Weighing resolution: 0.1 μg DSC resolution: < 1 μW Type: SU-8020 Two electronic resolutions: 1.0 nm, 1.3 nm Acceleration voltage: 0.1–30 kV Observation rate: 20–8,000,000 (Negative output) 60–2,000,000 (Monitor output) Type: LFA 457 Temperature: 125–500 °C Heating/cooling rate: 0.01–50 K/min Laser energy: 18.5 J/pulse Thermal diffusivity range: 0.01–1000 mm2/s Thermal conductivity range: 0.1–2000 W/mK Type: NZ-A Measurement range: 0.1–10 cP Temperature range: room temperature − 1500 °C Accuracy: + 3.0% Repetition Rate: + 5.0% Type: RTW-10 Maximum temperature: 1500 °C Temperature measurement accuracy: ± 0.5 °C Maximum heating power: 10 kW Type: ZNCL-BS140 Speed range: 50–1800 r/min Motor power: 40 W Weight: 2200 ± 200 g
Muffle furnace
Synchronous thermal analyzer
Field emission scanning electron microscopy
Laser thermal conductivity analyzer
High-temperature melt viscosity tester
Melt physical property tester
Magnetic stirring apparatus
2.3.2. Specific heat and latent heat of fusion The latent heat of fusion and the specific heat of molten salt nanofluids were measured by a synchronous thermal analyzer. The melting latent heat of the sample can be obtained by integrating the area of the melting peak of the DSC curve of the sample. By comparing the heat capacity of the known standard sample and the specific heat of unknown samples with the DSC curve, the specific heat of the unknown sample can be calculated. Sapphire (α-AL2O3) was used as the standard sample in the experiment, and the calculation formula is expressed as follows:
Cp, st =
Hms Cp, s, hmst
(1)
where Cp,st is the specific heat of the sample to be tested, J/(g·K); Cp,s is the specific heat of the standard sample, J/(g K); mst is the mass of the sample to be tested, g; ms is the mass of the standard sample, g; H is the DSC signal difference between the reference material and reference empty crucible, μV; and h is DSC signal difference between the sample to be tested and reference empty crucible, μV. Temperature and sensitivity corrections were performed before the
2.3. Experimental method 2.3.1. Melting point A synchronous thermal analyzer (STA-449F3) was used to measure the melting point of molten salt nanofluids. When combined with 211
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Fig. 1. Schematic of differential scanning calorimetry. a. Thermocouple; b. Compensated thermoelectric wire; c. Crucible; d. Electric stove.
Fig. 2. Basic structure of thermal balance. Fig. 4. Melting point of Al.
of LiNO3 measured in the aluminum crucible and the literature value is low, and the maximum deviation is less than 6%; by contrast, the specific heat of LiNO3 measured in the platinum and rhodium crucibles is different from those in the literature. The relative error is approximately 10%, and the maximum relative error is approximately 16%. These results are due to the difficult measurement of the specific heat of the samples at high temperatures. Specifically, accurately measuring specific heat must ensure that the quality of the sample during the test is constant. The liquid sample easily evaporates and decomposes, thereby causing the change in quality. The aluminum crucible can be sealed; thus, the quality of samples does not change maximally and the relative error of the specific heat remains small. The aluminum crucible was used to measure the specific heat of the samples. Fig. 3. Melting point of Bi.
2.3.3. Density The density of molten salt nanofluids can be calculated by [21]
experiment. The standard samples of Bi and Al were used to verify the accuracy of the simultaneous thermal analyzer in measuring latent heat of fusion. Aluminum oxide crucible was also used to prevent the adhesion or reaction of Bi or Al with the metal crucible in the melting process. The results of the latent heat of fusion of Bi and Al are shown in Figs. 5 and 6. The measured latent heats of fusion of Bi and Al were compared with the theoretical values, and the results are shown in Table 5. The relative errors of latent heat of fusion for Bi measured in the three measurements are 4.99%, 2.96%, and 4.71%. The relative errors of latent heat of fusion for Al are 5.26%, 5.50%, and 4.42%. The guide samples of LiNO3 in aluminum and platinum–rhodium crucibles were used to verify the accuracy of the synchronous thermal analyzer in measuring the specific heat. The specific heat values of LiNO3 are shown in Table 6, which were compared with the values in [20]. The results show that the relative error between the specific heat
−1
∅ ⎞ ⎛ 1-∅v + v⎟ , ρnf =⎜ ρb ρp ⎝ ⎠
(2)
where ρnf is the density of molten salt nanofluids; ∅v is the volume fraction of nanoparticles; ρ b and ρp are the densities of base fluid and nanoparticles, respectively. The volume fraction of nanoparticles can be calculated from the mass fraction because of the difficulty in accurately determining the particle volume and is expressed as follows:
∅v =
ρp ∅m ρp ∅m + ρ b (1-∅m )
,
where ∅m is the mass fraction of nanoparticles. 212
(3)
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Table 4 Error analysis of melting point of Bi and Al. Measurement number
1 2 3
Bi
Al
Theoretical value °C
Measured value °C
Absolute Error °C
Relative Error %
Theoretical value °C
Measured value °C
Absolute Error °C
Relative Error %
271.4 271.4 271.4
273.1 273.2 273.5
1.7 1.8 2.1
0.63 0.66 0.77
660.3 660.3 660.3
664.7 664.8 663.9
4.4 4.5 3.6
0.67 0.68 0.55
Table 6 Specific heat of LiNO3.
Fig. 5. Latent heat of fusion of Bi.
Temperature °C
Cp-a J/ (g K)
Cp-pr J/ (g K)
Cp-l J/ (g K)
Relative error between Cp-a and Cp-l %
Relative error between Cp-pr and Cp-l %
67 87 107 127 147 167 187 207 227 317 327 337 347 357 367 377
1.367 1.418 1.429 1.465 1.474 1.49 1.524 1.572 1.741 1.995 2.049 2.042 2.05 2.064 2.045 2.055
1.218 1.245 1.251 1.274 1.306 1.332 1.369 1.408 1.476 1.815 1.871 1.981 1.915 1.932 1.952 1.976
1.34 1.39 1.41 1.45 1.5 1.57 1.59 1.63 1.67 2.06 2.08 2.06 2.07 2.11 2.1 2.09
2.01 2.01 1.35 1.03 − 1.73 − 5.10 − 4.15 − 3.56 4.25 − 3.16 − 1.49 − 0.87 − 0.97 − 2.18 − 2.62 − 1.67
− 9.10 − 10.43 − 11.28 − 12.14 − 12.93 − 15.16 − 13.90 − 13.62 − 11.62 − 11.89 − 10.05 − 3.83 − 7.49 − 8.44 − 7.05 − 5.45
Note: Cp-a, specific heat of LiNO3 measured in aluminum crucible. Cp-pr, specific heat of LiNO3 measured in platinum–rhodium crucible. Cp-l, specific heat of LiNO3 from [20].
the measurement. The suspension system then freely vibrates by means of inertia. The amplitude gradually decreases as a result of the internal friction within the liquid and the friction between the liquid and crucible wall. The logarithmic decay rate can be calculated by measuring the amplitude cycle and changes. On this basis, the viscosity can be obtained using internal formula. The measurement accuracy of the high-temperature viscometer was verified using pure water, and the results are shown in Fig. 8. The results show that the error in the measured value is small. To analyze the deviation between the measured value of pure water and the theoretical value, the relative errors between the two were calculated, and the results are shown in Table 7. The maximum relative error is approximately 6.48%, and the accuracy of the high-temperature melt viscosity tester meets the experimental requirements. The viscosity of Hitec salts was also tested to verify the measurement accuracy, and the results are shown in Fig. 9. The viscosity of Hitec salt measured in this study agrees well with that in literature and can meet the requirements of the viscosity measurement of molten salt nanofluids in the experiment.
Fig. 6. Latent heat of fusion of Al.
2.3.4. Viscosity The rotary vibration method was used to measure the viscosity of molten salt nanofluids. Fig. 7 shows the schematic of the instrument. The external graphite crucible is connected with a molybdenum rod driven by a stepping motor, and the stepping motor drives the suspending wire, the reflecting mirror, the inertia disk, the molybdenum rod, and the crucible to form a suspension system. The suspension system is driven by a stepping motor and rotates left and right during
2.3.5. Thermal diffusivity and thermal conductivity The thermal diffusivity of molten salt nanofluids was measured
Table 5 Error analysis of latent heat of fusion of Bi and Al. Measurement number
1 2 3
Bi
Al
Theoretical value J/g
Measured value J/g
Relative Error %
Theoretical value J/g
Measured value J/g
Relative Error %
53.3 53.3 53.3
50.64 51.72 50.79
4.99 2.96 4.71
397.0 397.0 397.0
376.12 375.15 379.44
5.26 5.50 4.42
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Fig. 7. Schematic of the experimental system for viscosity measurement. 1. Nitrogen; 2. Circulating water cooling tank; 3. Vacuum pump; 4. Circulating water pump; 5. Furnace; 6. Crucible; 7. Stepping motor and suspension system; 8. Laser measurement system; 9. Temperature control system; 10 Stepping motor and power supply; 11. Data processing and computing system; 12. Laser launcher and receiving device.
using a laser thermal conductivity analyzer. The basic method of laser thermal conductivity analyzer is flicker method. The laser source or flash xenon lamp emits a beam of light pulses in an instant at a certain set temperature and illuminates uniformly under the surface of the sample. Thus, the surface temperature rises instantaneously after absorbing energy. Then, the energy transfers to a cold side (upper surface) as a way of one-dimensional heat conduction. The infrared detector is used to continuously measure the corresponding temperature rise in the central part of the upper surface of the sample to obtain the temperature rise (detector signal) curve. The light pulse width is infinitely small under the ideal condition. The conduction of heat inside the sample is the ideal one-dimensional heat transfer from the lower surface to the upper surface, and no transverse heat flow exists. External measurement environment is the ideal adiabatic condition without heat loss. The thermal diffusivity can be obtained with half heating time as follows: Fig. 8. Viscosity of pure water [22].
α = 0.1388 × d 2/ t50,
where α is the thermal diffusivity, d is vessel thickness, and t50 is the half heating time. Thermal conductivity is calculated as follows:
Table 7 Error of viscosity pure water. Temperature/°C
20 29 40 50 59 69
(4)
Measured value /mPa s
Reference value/ mPa s
Relative error/ %
0.97 0.8 0.65 0.56 0.5 0.44
1.005 0.821 0.656 0.549 0.474 0.413
3.48 2.55 0.91 − 2.00 − 5.46 − 6.48
λ = α∙ρ∙Cp,
(5)
where λ is thee thermal conductivity, ρ is the density, and Cp is the specific heat. 3. Results and discussion 3.1. Specific heat Low-melting point molten salt was used as the base solution, and SiO2 with a diameter of 20 nm was chosen as nanoparticles. Molten salt nanofluids with different mass fractions were prepared by high-temperature melting method, and they are shown in Table 8. The specific heat of the above-mentioned molten salt nanofluids was measured, and it is shown in Fig. 10. The specific heat capacities of samples with different mass fractions Table 8 Molten salt nanofluids with different mass fractions prepared by high-temperature melting.
Fig. 9. Viscosity of Hitec [23,24].
214
Order number
Mass fraction
Order number
Mass fraction
Y1 Y2 Y3 Y4 Y5
0.1% 0.25% 0.5% 0.75% 1%
Y6 Y7 Y8 Y9
1.25% 1.5% 2% 3%
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Fig. 10. Specific heat of molten salt nanofluids with different mass fractions prepared by high-temperature melting.
Fig. 12. DSC curve of pure molten salt [25].
prepared by high-temperature melting method are higher than those of pure molten salt in the temperature range of 200–350 °C. The specific heat of samples Y1–Y9 are 1.729–1.795, 1.779–1.903, 1.865–2.031, 1.789–1.823, 1.798–1.822, 1.649–1.698, 1.743–1.808, 1.634–1.720, 1.603–1.660 J/(g K), respectively. The mass fraction is closely related to the specific heat of molten salt nanofluids. When the mass fraction of nanoparticles is less than 0.5%, the specific heat of molten salt nanofluids increases with the mass fraction of nanoparticles. The specific heat is 1.748 J/(g K) under the mass fraction of 0.1% and is 11.6% higher than that of pure molten salt. When the proportion is higher than 0.5%, the specific heat of molten salt nanofluids decreases with the mass fraction of nanoparticles. The specific heat of molten salt nanofluids is 1.622 J/(g K), which is the lowest, when the mass ratio is 3%. Compared with that of pure molten salt, the specific heat increase rate is only 3.6%. The specific heat of molten salt nanofluids with the mass fraction of 0.5% is the highest with an average of 1.950 J/(g K), which is 24.5% higher than that of pure molten salt. The best mass fraction of SiO2 nanoparticles to improve the specific heat of molten salt nanofluids prepared by hightemperature melting method is 0.5%. The thermal properties of the molten salt nanofluid sample (Sample 3) at a mass ratio of 0.5% will be thoroughly investigated below.
that of pure molten salt [25], respectively. The starting point of the melting peak was used as the melting point of the sample following the standard accepted by the International Association for Thermal Analysis. From Fig. 11, melting point, melting peak and melting point of termination are found to be 106.8 °C, 125.0 °C, and 132.1 °C, respectively. The latent heat of fusion is 100.6 J/g. Compared with those of pure molten salt (Fig. 12), melting point, peak point, and melting point of termination are slightly improved. The specific comparison results are shown in Table 9. The decomposition temperature of molten salt nanofluid is 610 °C compared with that of pure molten salt. The result insignificantly changes when using temperature under liquid state. Thus, a wide range of temperature can be used. The latent heat of molten salt nanofluid is 100.6 J/g, which is slightly lower than that of pure salt. 3.3. Density The density of molten salt nanofluid with a mass fraction of 0.5% can be calculated using Eq. (2), and the results are listed in Table 10. The density of molten salt nanofluid prepared by high-temperature melting decreases with the increase in temperature. In particular, density ranges from 1.795 g/cm3 to 1.985 g/cm3 in the temperature range of 150–500 °C, and this density range is slightly higher than that of pure molten salt [25]. The fitting formula of the density of molten salt nanofluid with temperature is expressed as follows:
3.2. Melting point and latent heat Figs. 11 and 12 show the melting point of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5% and
ρ = 2.066 − 5.4 × 10−4t
150 °C
≤
t
≤
500 °C
(6)
where ρ is the density of molten salt nanofluid and t is the temperature. 3.4. Thermal diffusivity and thermal conductivity Fig. 13 shows the thermal diffusivity of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%. In particular, thermal diffusivity ranges from 0.098 to 0.141 mm2/s in the temperature range of 150–350 °C. The fitting formula of the thermal diffusivity of molten salt nanofluid with temperature is expressed as follows:
α = 0. 106 + 1.071 × 10−4t
50°C ≤ t ≤ 350°C
(7)
where α is the thermal diffusivity of molten salt nanofluid and t is the temperature. The thermal conductivity of molten salt nanofluid prepared by hightemperature melting with a mass fraction of 0.5% can be calculated using Eq. (5), and the results are shown in Fig. 14. The thermal conductivity of molten salt nanofluid presents a linear relationship with temperature as it increases with the increase in temperature. The average thermal conductivity is nearly 0.528 W/(m K), which is similar
Fig. 11. DSC curve of molten salt nanofluid prepared by high-temperature melting method with a mass fraction of 0.5%. 215
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Table 9 Thermoproperties of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5% and those of pure molten salt. Sample
Melting point °C
Decomposition temperature °C
Latent heat of fusion J/g
Pure molten salt [25] Molten salt nanofluid
96.8 106.8
612.0 610.0
106.9 100.6
Table 10 Density of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%. Sample
Molten salt nanofluid Pure molten salt
Density (g/cm3) 150 °C
200 °C
250 °C
300 °C
350 °C
400 °C
450 °C
500 °C
1.985 1.953
1.958 1.924
1.931 1.896
1.904 1.868
1.877 1.839
1.850 1.811
1.823 1.782
1.795 1.754
Fig. 15. Viscosity of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%.
Fig. 13. Thermal diffusivity of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%.
3.5. Viscosity Fig. 15 shows the viscosity of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%. In particular, viscosity varies from 0.72 to 2.20 mPa s in the temperature range of 150–450 °C. The viscosity of molten salt nanofluids decreases with the increase in temperature, and the measurement of the viscosity of molten salt nanofluids exhibits a good relationship with temperature, as shown below.
η = 2.19 + 2.25 × 10−3t − 1.19 × 10−5t 2, R − Square = 0.95048
(9)
3.6. Sensible heat storage cost The unit price can be calculated on the basis of Eq. (10).
PC = Fig. 14. Thermal conductivity of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%.
(10)
where Mi is the mass fraction of each component and PCi is the unit price of each component ($/kg). Integral average specific heat and sensible heat storage value can be calculated by Eqs. (11) and (12).
to that of solar salt (average thermal conductivity of 0.520 W/(m K)) [26] and 50.9% more than that of Hitec salt (average thermal conductivity of 0.350 W/(m K)) [27]. The fitting formula of the thermal conductivity of molten salt nanofluid with temperature is expressed as follows:
λ=0.396+4.087×10-4t 200℃≤t ≤350℃
∑ Mi×PCi ,
t
Cp̅ =
∫t1 2 Cp dt t2-t1
,
(11)
t
Qsensible=
(8)
where λ is the thermal conductivity of molten salt nanofluid and t is the temperature.
∫t12 Cp̅ dt 3600
,
(12)
where t1 is the minimum operating temperature (°C) and t2 is the maximum operating temperature (°C). 216
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Table 11 Sensible heat storage costs.
Molten salt nanofluid Pure molten salt [25]
PC $/kg
Average specific heat capacity J/(g·K)
Qsensible kW·h/kg
TC $/(kW·h)
1.6 1.6
1.95 1.57
0.217 0.19
7.2 8.2
References
Sensible heat storage cost can be calculated on the basis of Eq. (13).
TC =
PC , Qsensible
[1] F. Zaversky, J. García-Barberena, M. Sánchez, D. Astrain, Transient molten salt twotank thermal storage modeling for CSP performance simulations, Sol. Energy 93 (2013) 294–311. [2] O. Garbrecht, F. Al-Sibai, R. Kneer, K. Wieghardt, CFD-simulation of a new receiver design for a molten salt solar power tower, Sol. Energy 90 (2013) 94–106. [3] U. Hermann, D.W. Kearney, Survey of thermal energy storage for parabolic trough power plants, J. Sol. Energy Eng. 124 (2) (2002) 145–152. [4] D. Kearney, B. Kelly, U. Herrmann, et al., Engineering aspects of a molten salt heat transfer fluid in a trough solar field, Energy 29 (2004) 861–870. [5] S. Witharana, I. Palabiyik, Z. Musina, et al., Stability of glycol nanofluids - the theory and experiment, Powder Technol. 239 (2013) 72–77. [6] C. Wang, J. Yang, Y.L. Ding, Phase transfer based synthesis and thermophysical properties of Au/Therminol VP-1 nanofluids, Progress. Nat. Sci. Mater. Int. 23 (3) (2013) 338–342. [7] Z. Haddad, C. Abid, H.F. Oztop, A. Mataoui, A review on how the researchers prepare their nanofluids, Int. J. Therm. Sci. 76 (2014) 168–189. [8] N.A.C. Sidik, H.A. Mohammed, O.A. Alawi, S. Samion, A review on preparation methods and challenges of nanofluids, Int. Commun. Heat. Mass Transf. 54 (2014) 115–125. [9] D. Bharath, Effect of Nanoparticle Dispersions in Binary Nitrate Salt as Thermal Energy Storage Material in Concentrated Solar Power Applications, The University of Texas, Austin, 2013https://rc.library.uta.edu/uta-ir/handle/10106/11833. [10] D. Ramaprasath, Utlization of Molten Nitrate Salt Nanomaterials for Heat Capacity Enhancement in Solar Power Applications, The University of Texas, Austin, 2013. [11] M.X. Ho, C. Pan, Optimal concentration of alumina nanoparticles in molten Hitec salt to maximize its specific heat capacity, Int. J. Heat. Mass Transf. 70 (2014) 174–184. [12] H. Tiznobaik, D.H. Shin, Enhanced specific heat capacity of high-temperature molten salt-based nanofluids, Int. J. Heat. Mass Transf. 57 (2013) 542–548. [13] Y.T. Wu, N. Ren, T. Wang, et al., Experimental study on optimized composition of mixed carbonate salt for sensible heat storage in solar thermal power plant, Sol. Energy 85 (2011) 1957–1966. [14] N. Ren, Y.T. Wu, C.F. Ma, L.X. Sang, Preparation and thermal properties of quaternary mixed nitrate with low melting point, Sol. Energy Mater. Sol. Cells 127 (2014) 6–13. [15] S.F. Ahmed, M. Khalid, W. Rashmi, et al., Recent progress in solar thermal energy storage using nanomaterials, Renew. Sustain. Energy Rev. 67 (2017) 450–460. [16] O. Arthur, M.A. Karim, An investigation into the thermophysical and rheological properties of nanofluids for solar thermal applications, Renew. Sustain. Energy Rev. 55 (2016) 739–755. [17] Z.Q. Chen, G.X. Wang, G.Y. Xu, Colloid and Interface Chemistry, Higher Education Press, Beijing, 2001. [18] Xia Chen, Yuting Wu, Ludi Zhang, et al., Experimental study on the specific heat and stability of molten salt nanofluids prepared by high-temperature melting, Solar Energy Materials and Solar Cells. Accepted. DOI: 10.1016/j.solmat.2017.11.021. [19] N. Ren, Y.T. Wu, C.F. Ma, et al., Preparation and thermal properties of quaternary mixed nitrate with low melting point, Sol. Energy Mater. Sol. Cells 127 (2014) 6–13. [20] D.M. Blake, L. Moens, M.J. Hale, et al., New heat transfer and storage fluids for parabolic trough solar thermal electric plants, in: Proceedings of the 11th Solar PACES International Symposium On concentrating Solar Power and Chemical Energy Technologies, Zurich, Switzerland, September 4–6, 2002. [21] B.X. Wang, L.P. Zhou, X.F. Peng, Surface and size effects on the specific heat capacity of nanoparticles, Int. J. Thermophys. 27 (2006) 139–151. [22] Z.J. Cai, T.Y. Long, Fluid Mechanics Pump and Fan, China Architecture & Building Press, Beijing, 1999. [23] P.G. Gaune, Viscosity of potassium nitrate-sodium nitrite-sodium nitrate mixtures, J. Chem. Eng. Data 27 (2) (1982) 151–153. [24] J. Singh, Y.R. Mayhew, Heat transfer fluids and systems for process and energy applications, Int. J. Heat Fluid Flow 7 (2) (1986) 154 https://www.sciencedirect. com/science/article/pii/0142727X86900640. [25] Nan Ren, Study on Preparation and Thermophysical Properties of Mixed Molten Salts for Heat Transfer and Storage (Ph.D. thesis), Beijing University of Technology, Beijing, 2015. [26] L.L.C. Coastal Chemical Co, Hitec Heat Transfer Salt, Brenntag Company, Houston, 2002, pp. 1–10. [27] V.M.B. Nunes, M.J.V. Lourenco, F.G.V. Santos, et al., Importance of accurate on viscosity and thermal conductivity in molten salts applications, J. Chem. Eng. Data. 48 (3) (2003) 446–450.
(13)
The operating temperature range of molten salt nanofluid prepared by high-temperature melting is 150–550 °C. The sensible heat storage costs of molten salt nanofluid prepared by high-temperature melting is calculated by Eqs. 10–13, and the result is showed in Table 11. The sensible heat storage cost of molten salt nanofluid prepared by hightemperature melting is 7.2 $/kW h, which is 12.2% lower than that of pure molten salt. 4. Conclusions In this study, molten salt nanofluids with different mass fractions of SiO2 nanoparticles were prepared using high-temperature melting and the specific heat was measured to determine the optimal concentration of SiO2 nanoparticles. The thermal properties of the prepared molten salt nanofluids, namely, melting point, primary crystallization point, thermal conductivity, viscosity, latent heat, and density, were experimentally tested and analyzed under the condition of optimal concentration of SiO2 nanoparticles. The conclusions were obtained as follows: 1) The specific heat capacities of the molten salt nanofluids samples prepared by high-temperature melting with different mass fractions of nanoparticles are higher than those of pure molten salt. The best mass fraction of SiO2 nanoparticles to improve the specific heat of molten salt nanofluids is 0.5%. The average specific heat of molten salt nanofluid with a mass fraction of 0.5% is the highest at 1.950 J/ (g K), which is 24.5% more than that of pure molten salt. 2) The thermal properties of the prepared molten salt nanofluids with the optimal mass ratio of SiO2 nanoparticles of 0.5% are obtained: The melting point of molten salt nanofluid is 106.8 °C, which is 10 °C higher than that of pure molten salt; The decomposition temperature of molten salt nanofluid is 610 °C, which is similar to that of pure molten salt; The latent heat of molten salt nanofluid is 100.6 J/ g, which is slightly lower than that of pure molten salt; The thermal diffusivity of molten salt nanofluid is nearly 0.181 mm2/s; The thermal conductivity is around 0.528 W/ (m K), which is a relatively high thermal conductivity compared with solar salt (average thermal conductivity of 0.520 W/(m K)) and Hitec salt (average thermal conductivity of 0.350 W/(m·K)); The viscosity of molten salt nanofluids decreases with the increase in temperature, which varies from 0.72 to 2.20 mPa s in the temperature range of 150–450 °C. 3) The sensible heat storage cost of molten salt nanofluid prepared by high-temperature melting is 7.2 $/kW h, which is 12.2% lower than that of pure molten salt. Acknowledgements This work was supported by National Natural Science Foundation of China No. 51706005, the National Basic Research Program of China (973 Program) No. 2015CB251303, Science and Technology Plan General Program of Beijing Municipal Commission of Education No. KM201610005017 and No. KM201710005009.
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Experimental study on thermophysical properties of molten salt nanofluids prepared by high-temperature melting
T
⁎
Xia Chena,b, Yu-ting Wua,b, , Lu-di Zhanga,b, Xin Wangc, Chong-fang Maa,b a Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China b Key Laboratory of Heat Transfer and Energy Conversion, Beijing Education Commission, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China c College of Mechanical Engineering and Applied Electronics Technology, Beijing University of Technology, Beijing 100124, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Molten salt nanofluid High-temperature melting Thermophysical properties
Heat storage is a key technology in solar thermal power, helps solar thermal power eliminate the instability in energy supply for the grid, and provides continuous and stable high-quality electrical energy to improve power system efficiency and extend system life. Molten salt is an important material for heat storage and heat transfer in solar thermal power. In recent years, nanofluid technology has been applied to investigate heat storage and transfer materials, such as molten salt, to improve heat capacity. However, the nanofluid technology is hampered by the easy agglomeration of nanoparticles in molten salts, and this phenomenon degrades the performance of the molten salt nanofluids. To reduce or prevent agglomeration, a two-step method with high-temperature melting in preparing molten salt nanofluids is developed recently. Molten salt nanofluids obtained by high-temperature melting show good stability and long time to agglomerate. This paper presented the study on the thermophysical properties of molten salt nanofluids prepared by high-temperature melting method. The molten salt nanofluids possessed low-melting point salt as base liquid and SiO2 with a diameter of 20 nm as nanoparticles. The specific heat of this type of molten salt nanofluid was tested, and the optimum concentration of nanoparticles was determined. Results showed that the specific heat capacities of the molten salt nanofluid samples with different mass fractions of nanoparticles were higher than those of pure molten salt. The average specific heat of molten salt nanofluid with a mass fraction of 0.5% was the highest at 1.950 J/(g K), which was 24.5% more than that of pure molten salt. The thermophysical properties of molten salt nanofluids with a mass fraction of 0.5% and prepared by high-temperature melting were experimentally tested and compared with those of pure molten salt. These properties included melting point, primary crystallization point, thermal conductivity, viscosity, latent heat, and density. The molten salt nanofluids showed good performance for heat storage and transfer.
1. Introduction The current world faces the dual pressures of rapid increases in energy demand and serious environmental pollution. Vigorous development of renewable energy, such as solar energy, is an important way to solve the shortage of energy and reduce haze, acid rain, and other disastrous weather conditions caused by fossil fuels, and such reduction is an important part of current social energy strategy. Solar thermal power generation technology is based on heat storage, can eliminate the instability in energy supply for the grid, and provide continuous and stable high-quality electrical energy to improve power system efficiency
and extend system life. Molten salts are considered a potential heat transfer and storage medium in solar thermal power because of their advantages of wide temperature range, high heat capacity, low cost, and pollution-free characteristic [1,2]; these salts are also recognized as a realistic large-scale high-temperature heat storage and transfer medium in the international arena. At present, molten salts as a heat storage and transfer medium are successfully used in several solar thermal power plants [3,4]. Molten salts are a type of regenerative heat storage and transfer working fluid used in engineering and technology, and enhancing the heat storage capacity and thermal performance of these materials have been mainly explored.
⁎ Corresponding author at: Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China. E-mail address: [email protected] (Y.-t. Wu).
https://doi.org/10.1016/j.solmat.2018.11.003 Received 10 December 2017; Received in revised form 4 November 2018; Accepted 5 November 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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Thermal stability of molten salt nanofluids prepared by high-temperature melting was good because high-temperature melting directly adds the nanoparticles into the molten salt base liquid with mechanical mixing under high temperature conditions. During this process, the nanoparticles are dispersed in the molten salt base liquid and promote the formation of an electrical double layer on the surface of the nanoparticles, so that the nanoparticles are charged; moreover, the nanoparticles are not easily agglomerated under the mutual repulsion of the same charge [18]. In the present study, molten salt nanofluids were prepared using high-temperature melting based on previous studies [18] to furtherly investigate the thermophysical properties of molten salt nanofluids prepared by high-temperature melting. The specific heat of molten salt nanofluids was tested, and the optimum concentration of nanoparticles was determined. The thermophysical properties of molten salt nanofluids (including melting point, primary crystallization point, thermal conductivity, viscosity, latent heat, and density) with the optimum fraction were also experimentally tested and compared with those of pure molten salt.
Specific heat is one of the important thermophysical parameters of molten salt. Improving the specific heat of molten salt can play a key role in improving the heat storage capacity and reducing the heat storage cost of solar thermal power generation systems. In recent years, the development of the technology of water nanofluids as the representative of nanofluids has increased the attention on improving heat storage and transfer capacity by adding nanoparticles [5–8]. Previous studies have found that adding nanoparticles into molten salts significantly increases the specific heat of molten salts. Bharath [9] added SiO2 nanoparticles with different particle sizes (5, 10, 30, and 60 nm) into solar salts and studied the effect of nanoparticle size on the specific heat characteristics of molten salts. Ramaprasath [10] added nano-SiO2 with different sizes (5, 10, 30, and 60 nm) into HitecXL salt and studied the specific heat characteristics of molten salt nanofluids. The results showed that adding 5, 10, 30, and 60 nm SiO2 increases the specific heat capacities of molten salt nanofluids by 28%, 34%, 19%, and 30%, respectively. Ho and Pan [11] added different proportions of Al2O3 nanoparticles on the basis of Hitec salt and studied the optimized specific heat characteristics of molten salts. The optimal concentration of Al2O3 nanoparticles is 0.063 wt%. Tiznobaik et al. [12] obtained a high-temperature nanofluid material by dispersing four sizes of SiO2 nanoparticles in binary carbonate. The addition of nanoparticles increases the specific heat of the original binary carbonate by 25%. Wu et al. [13,14] prepared a low-melting point mixed molten salt and prepared molten salt nanofluids with SiO2, Al2O3, and TiO2 as nanoparticles. The specific heat of the molten salt nanofluids increased by nearly 20% than that of pure molten salt. The thermal conductivity of molten salt nanofluids is unclear unlike their specific heat that obviously increases. Existing findings indicate that high electrical conductivity of nanoparticles and large specific surface area mean improved thermal conductivity of molten salt nanofluids [15,16]. The two-step method with ultrasonic dispersion is the most widely used method for preparing nanofluids. Currently, the limitation of this method is the possible particle sedimentation and aggregation with respect to time. Nanoparticles are of high particle surface energy owing to large specific surface area and the activity of particle surfaces; accordingly, nanoparticles easily agglomerate together to form large aggregates with a few weak interfacial interfaces. Once nanoparticle aggregates are formed, they are difficult to disassemble without external effect. When aggregates grow to a certain size, they precipitate as a result of gravity and lose the characteristics of nanofluids. Nanoparticles mainly interact with water molecules in water and with molten salt ions in molten salts. These materials are suspended in molten salts, thereby changing the electrical properties to form an electrical double layer according to DLVO theory [17]. The gravitation of agglomeration between nanoparticles is greatly reduced through the repulsion between the electric double layers. As a result, nanoparticles disperse. On the basis of the above-mentioned theory, authors have proposed a two-step method with high-temperature melting in preparing molten salt nanofluids in a previous study [18]. The base solution was a lowmelting point molten salt, and the nanoparticles were SiO2 with a diameter of 20 nm. The specific heat was measured, and the stability of the molten salt nanofluids was studied. The results show that the average specific heat of molten salt nanofluids prepared by high-temperature melting was 1.789 J/(g K), which was close to that of molten salt nanofluids prepared by ultrasonic dispersion and 16.4% higher than that of pure molten salt. The molten salt nanofluids prepared by ultrasonic dispersion showed poor thermal stability under high-temperature condition of 450 °C, and the average specific heat decreased by 8.5% after only 200 h. The thermal stability of molten salt nanofluids prepared by high-temperature melting showed a highly stable performance in long-time experiments. The variation of specific heat was less than 5% after 2000 h under the same high-temperature experimental condition. The two-step method with high-temperature melting is stable and reliable for preparing molten salt nanofluids [18].
2. Method 2.1. Molten salt nanofluid preparation Authors previously developed a low-melting point quaternarymixed nitrate, namely, Ca(NO3)2-KNO3-NaNO3-LiNO3 (2:6:1:2 in mass ratio). The study results showed that the low-melting point molten salt is suitable for solar thermal power generation. This molten salt possesses a melting point of approximately 85.4 °C and a maximum operating temperature of 600 °C. When the temperature is below 300 °C, the thermal conductivity is 0.53 W/(m K) and the specific heat is 1.519 kJ/ (kg K). Moreover, analysis of the experimental results on the convective heat transfer and long-term stability of molten salt showed that the risk of blockage of the said molten salt is greatly reduced and the corrosion to the pipeline is decreased [19]. In the present study, the abovementioned low-melting point molten salt was used as the base solution. Furthermore, the frequently used nanoparticles for preparing nanofluids were used as SiO2 nanoparticles with a diameter of 20 nm. SiO2 nanoparticles with different mass fractions were mixed with molten salt to prepare molten salt nanofluids by using high-temperature melting. The concrete steps for the two methods are discussed in the subsections below [18]. (1) Low-melting point molten salt and SiO2 nanoparticles were weighed using an electronic balance, and the low-melting point molten salt was placed in a muffle furnace and heated to 400 °C (higher than its melting point of 300 °C). (2) SiO2 nanoparticles were slowly added into the high-temperature molten salt at 400 °C, and the molten salt liquid was stirred evenly with a stirring bar during the adding process. (3) Magnetic stirrer was rotated at 600 r/min, and the mixture of molten salt and SiO2 nanoparticles was stirred by a magnetic stirrer until the mixture cooled and solidified (about 15 min) to obtain a composite nanometer molten salt.
2.2. Experimental reagents and instruments KNO3, NaNO3, Ca(NO3)2, and LiNO3 of analytical grade were used as nitrates in this experiment. SiO2 with a purity of 99.9% and a particle size of 20 nm was used as the nano-powder. The experimental reagents used in the experiment and their related parameters are listed in Tables 1 and 2. The experimental apparatuses are listed in Table 3.
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differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis, the instrument can provide simultaneous TG and differential thermal signals at the same temperature. DSC is a thermal analysis method that measures the relation of the power difference between a sample and a reference material to temperature at a programmed temperature, and the operation principle of method is shown in Fig. 1. Samples and reference materials were placed in two identical crucibles. Each crucible possessed separate heating and temperature sensing elements and was monitored by two systems: one was for controlling the rise rate of temperature and the other was for compensating the temperature difference between samples and reference materials. When the temperature difference between the sample and reference material appears as a result of the thermal effect in the heating process, the current flowing into the compensation heating wire will change through the differential thermal amplifying circuit and compensated amplifier. When the sample absorbs heat, the compensation amplifier immediately increases the current at the sample side. Conversely, when the sample releases heat, the current at the reference material side increases until the heat on both sides balances and no temperature difference exists. The relation of the amount of heat required to maintain these isothermal conditions to time or temperature, phase transition temperature, and melting and freezing point can be obtained from the appearance or disappearance of peaks in the DSC curve. TG analysis is a technique that obtains the relationship between the mass and temperature of a substance by means of a thermobalance under the control temperature. Fig. 2 shows the schematic of the basic structure of a thermobalance, which is an important part of automatic and continuous weighing and recording. The temperature of sample can be changed by a temperature program in the weighing process, and the atmosphere around the sample can be controlled or adjusted. The curve of the relationship between the sample quality and temperature under the programmed temperature, which is TG curve, can be obtained by TG analysis. The temperature and sensitivity of the instrument were calibrated first before the experiment, and the accuracy of measuring the melting point of the instrument was verified using the bismuth and aluminum standards. Aluminum oxide crucible was used in the experiment to prevent the adhesion or reaction between standards and metal crucible. Figs. 3 and 4 show the melting points of bismuth and aluminum, respectively. Table 4 lists the error analysis results of the melting points of bismuth and aluminum. The results show that the relative errors of the melting points of bismuth measured by three measurements are 0.63%, 0.66%, and 0.77%, respectively. The melting points of aluminum are 0.67%, 0.68%, and 0.55%. The errors are not more than 1%, and the accuracy is very high.
Table 1 Physical parameters of reagents. Name
Melting point °C
Density g/cm3
Latent value J/g
Content ％
KNO3 NaNO3 Ca(NO3)2
337 310 αtype 42.7 βtype 39.7 anhydrous 561 255
2.11 2.26 αtype 1.89 βtype 1.82 2.50 2.38
115 173 153 153 145 371
≥ 99.0 ≥ 99.0 ≥ 99.0
LiNO3
≥ 99.0
Table 2 Physical parameters of nanoparticles. Name
Diameter Nm
Specific surface area m2/g
Pattern
Fill density g/ cm3
Purity %
SiO2
20
120
Sphere
0.08
≥ 99.0
Table 3 Experimental instruments. Name
Parameters
Electronic balance
Type: ML204 Maximum weighing range: 220 g Readability: 0.0001 g Weighing technology: MonoBloc Linear error: 0.2 mg Type: SX-G16103 Rated power: 8k W Rated voltage: 380 V Rated temperature: 1000 °C Type: STA-449F3 Temperature: − 150 to 2000 °C Heating/cooling rate: 0.001–50 K/min Weighing capacity: 35,000 mg Weighing resolution: 0.1 μg DSC resolution: < 1 μW Type: SU-8020 Two electronic resolutions: 1.0 nm, 1.3 nm Acceleration voltage: 0.1–30 kV Observation rate: 20–8,000,000 (Negative output) 60–2,000,000 (Monitor output) Type: LFA 457 Temperature: 125–500 °C Heating/cooling rate: 0.01–50 K/min Laser energy: 18.5 J/pulse Thermal diffusivity range: 0.01–1000 mm2/s Thermal conductivity range: 0.1–2000 W/mK Type: NZ-A Measurement range: 0.1–10 cP Temperature range: room temperature − 1500 °C Accuracy: + 3.0% Repetition Rate: + 5.0% Type: RTW-10 Maximum temperature: 1500 °C Temperature measurement accuracy: ± 0.5 °C Maximum heating power: 10 kW Type: ZNCL-BS140 Speed range: 50–1800 r/min Motor power: 40 W Weight: 2200 ± 200 g
Muffle furnace
Synchronous thermal analyzer
Field emission scanning electron microscopy
Laser thermal conductivity analyzer
High-temperature melt viscosity tester
Melt physical property tester
Magnetic stirring apparatus
2.3.2. Specific heat and latent heat of fusion The latent heat of fusion and the specific heat of molten salt nanofluids were measured by a synchronous thermal analyzer. The melting latent heat of the sample can be obtained by integrating the area of the melting peak of the DSC curve of the sample. By comparing the heat capacity of the known standard sample and the specific heat of unknown samples with the DSC curve, the specific heat of the unknown sample can be calculated. Sapphire (α-AL2O3) was used as the standard sample in the experiment, and the calculation formula is expressed as follows:
Cp, st =
Hms Cp, s, hmst
(1)
where Cp,st is the specific heat of the sample to be tested, J/(g·K); Cp,s is the specific heat of the standard sample, J/(g K); mst is the mass of the sample to be tested, g; ms is the mass of the standard sample, g; H is the DSC signal difference between the reference material and reference empty crucible, μV; and h is DSC signal difference between the sample to be tested and reference empty crucible, μV. Temperature and sensitivity corrections were performed before the
2.3. Experimental method 2.3.1. Melting point A synchronous thermal analyzer (STA-449F3) was used to measure the melting point of molten salt nanofluids. When combined with 211
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Fig. 1. Schematic of differential scanning calorimetry. a. Thermocouple; b. Compensated thermoelectric wire; c. Crucible; d. Electric stove.
Fig. 2. Basic structure of thermal balance. Fig. 4. Melting point of Al.
of LiNO3 measured in the aluminum crucible and the literature value is low, and the maximum deviation is less than 6%; by contrast, the specific heat of LiNO3 measured in the platinum and rhodium crucibles is different from those in the literature. The relative error is approximately 10%, and the maximum relative error is approximately 16%. These results are due to the difficult measurement of the specific heat of the samples at high temperatures. Specifically, accurately measuring specific heat must ensure that the quality of the sample during the test is constant. The liquid sample easily evaporates and decomposes, thereby causing the change in quality. The aluminum crucible can be sealed; thus, the quality of samples does not change maximally and the relative error of the specific heat remains small. The aluminum crucible was used to measure the specific heat of the samples. Fig. 3. Melting point of Bi.
2.3.3. Density The density of molten salt nanofluids can be calculated by [21]
experiment. The standard samples of Bi and Al were used to verify the accuracy of the simultaneous thermal analyzer in measuring latent heat of fusion. Aluminum oxide crucible was also used to prevent the adhesion or reaction of Bi or Al with the metal crucible in the melting process. The results of the latent heat of fusion of Bi and Al are shown in Figs. 5 and 6. The measured latent heats of fusion of Bi and Al were compared with the theoretical values, and the results are shown in Table 5. The relative errors of latent heat of fusion for Bi measured in the three measurements are 4.99%, 2.96%, and 4.71%. The relative errors of latent heat of fusion for Al are 5.26%, 5.50%, and 4.42%. The guide samples of LiNO3 in aluminum and platinum–rhodium crucibles were used to verify the accuracy of the synchronous thermal analyzer in measuring the specific heat. The specific heat values of LiNO3 are shown in Table 6, which were compared with the values in [20]. The results show that the relative error between the specific heat
−1
∅ ⎞ ⎛ 1-∅v + v⎟ , ρnf =⎜ ρb ρp ⎝ ⎠
(2)
where ρnf is the density of molten salt nanofluids; ∅v is the volume fraction of nanoparticles; ρ b and ρp are the densities of base fluid and nanoparticles, respectively. The volume fraction of nanoparticles can be calculated from the mass fraction because of the difficulty in accurately determining the particle volume and is expressed as follows:
∅v =
ρp ∅m ρp ∅m + ρ b (1-∅m )
,
where ∅m is the mass fraction of nanoparticles. 212
(3)
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Table 4 Error analysis of melting point of Bi and Al. Measurement number
1 2 3
Bi
Al
Theoretical value °C
Measured value °C
Absolute Error °C
Relative Error %
Theoretical value °C
Measured value °C
Absolute Error °C
Relative Error %
271.4 271.4 271.4
273.1 273.2 273.5
1.7 1.8 2.1
0.63 0.66 0.77
660.3 660.3 660.3
664.7 664.8 663.9
4.4 4.5 3.6
0.67 0.68 0.55
Table 6 Specific heat of LiNO3.
Fig. 5. Latent heat of fusion of Bi.
Temperature °C
Cp-a J/ (g K)
Cp-pr J/ (g K)
Cp-l J/ (g K)
Relative error between Cp-a and Cp-l %
Relative error between Cp-pr and Cp-l %
67 87 107 127 147 167 187 207 227 317 327 337 347 357 367 377
1.367 1.418 1.429 1.465 1.474 1.49 1.524 1.572 1.741 1.995 2.049 2.042 2.05 2.064 2.045 2.055
1.218 1.245 1.251 1.274 1.306 1.332 1.369 1.408 1.476 1.815 1.871 1.981 1.915 1.932 1.952 1.976
1.34 1.39 1.41 1.45 1.5 1.57 1.59 1.63 1.67 2.06 2.08 2.06 2.07 2.11 2.1 2.09
2.01 2.01 1.35 1.03 − 1.73 − 5.10 − 4.15 − 3.56 4.25 − 3.16 − 1.49 − 0.87 − 0.97 − 2.18 − 2.62 − 1.67
− 9.10 − 10.43 − 11.28 − 12.14 − 12.93 − 15.16 − 13.90 − 13.62 − 11.62 − 11.89 − 10.05 − 3.83 − 7.49 − 8.44 − 7.05 − 5.45
Note: Cp-a, specific heat of LiNO3 measured in aluminum crucible. Cp-pr, specific heat of LiNO3 measured in platinum–rhodium crucible. Cp-l, specific heat of LiNO3 from [20].
the measurement. The suspension system then freely vibrates by means of inertia. The amplitude gradually decreases as a result of the internal friction within the liquid and the friction between the liquid and crucible wall. The logarithmic decay rate can be calculated by measuring the amplitude cycle and changes. On this basis, the viscosity can be obtained using internal formula. The measurement accuracy of the high-temperature viscometer was verified using pure water, and the results are shown in Fig. 8. The results show that the error in the measured value is small. To analyze the deviation between the measured value of pure water and the theoretical value, the relative errors between the two were calculated, and the results are shown in Table 7. The maximum relative error is approximately 6.48%, and the accuracy of the high-temperature melt viscosity tester meets the experimental requirements. The viscosity of Hitec salts was also tested to verify the measurement accuracy, and the results are shown in Fig. 9. The viscosity of Hitec salt measured in this study agrees well with that in literature and can meet the requirements of the viscosity measurement of molten salt nanofluids in the experiment.
Fig. 6. Latent heat of fusion of Al.
2.3.4. Viscosity The rotary vibration method was used to measure the viscosity of molten salt nanofluids. Fig. 7 shows the schematic of the instrument. The external graphite crucible is connected with a molybdenum rod driven by a stepping motor, and the stepping motor drives the suspending wire, the reflecting mirror, the inertia disk, the molybdenum rod, and the crucible to form a suspension system. The suspension system is driven by a stepping motor and rotates left and right during
2.3.5. Thermal diffusivity and thermal conductivity The thermal diffusivity of molten salt nanofluids was measured
Table 5 Error analysis of latent heat of fusion of Bi and Al. Measurement number
1 2 3
Bi
Al
Theoretical value J/g
Measured value J/g
Relative Error %
Theoretical value J/g
Measured value J/g
Relative Error %
53.3 53.3 53.3
50.64 51.72 50.79
4.99 2.96 4.71
397.0 397.0 397.0
376.12 375.15 379.44
5.26 5.50 4.42
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Fig. 7. Schematic of the experimental system for viscosity measurement. 1. Nitrogen; 2. Circulating water cooling tank; 3. Vacuum pump; 4. Circulating water pump; 5. Furnace; 6. Crucible; 7. Stepping motor and suspension system; 8. Laser measurement system; 9. Temperature control system; 10 Stepping motor and power supply; 11. Data processing and computing system; 12. Laser launcher and receiving device.
using a laser thermal conductivity analyzer. The basic method of laser thermal conductivity analyzer is flicker method. The laser source or flash xenon lamp emits a beam of light pulses in an instant at a certain set temperature and illuminates uniformly under the surface of the sample. Thus, the surface temperature rises instantaneously after absorbing energy. Then, the energy transfers to a cold side (upper surface) as a way of one-dimensional heat conduction. The infrared detector is used to continuously measure the corresponding temperature rise in the central part of the upper surface of the sample to obtain the temperature rise (detector signal) curve. The light pulse width is infinitely small under the ideal condition. The conduction of heat inside the sample is the ideal one-dimensional heat transfer from the lower surface to the upper surface, and no transverse heat flow exists. External measurement environment is the ideal adiabatic condition without heat loss. The thermal diffusivity can be obtained with half heating time as follows: Fig. 8. Viscosity of pure water [22].
α = 0.1388 × d 2/ t50,
where α is the thermal diffusivity, d is vessel thickness, and t50 is the half heating time. Thermal conductivity is calculated as follows:
Table 7 Error of viscosity pure water. Temperature/°C
20 29 40 50 59 69
(4)
Measured value /mPa s
Reference value/ mPa s
Relative error/ %
0.97 0.8 0.65 0.56 0.5 0.44
1.005 0.821 0.656 0.549 0.474 0.413
3.48 2.55 0.91 − 2.00 − 5.46 − 6.48
λ = α∙ρ∙Cp,
(5)
where λ is thee thermal conductivity, ρ is the density, and Cp is the specific heat. 3. Results and discussion 3.1. Specific heat Low-melting point molten salt was used as the base solution, and SiO2 with a diameter of 20 nm was chosen as nanoparticles. Molten salt nanofluids with different mass fractions were prepared by high-temperature melting method, and they are shown in Table 8. The specific heat of the above-mentioned molten salt nanofluids was measured, and it is shown in Fig. 10. The specific heat capacities of samples with different mass fractions Table 8 Molten salt nanofluids with different mass fractions prepared by high-temperature melting.
Fig. 9. Viscosity of Hitec [23,24].
214
Order number
Mass fraction
Order number
Mass fraction
Y1 Y2 Y3 Y4 Y5
0.1% 0.25% 0.5% 0.75% 1%
Y6 Y7 Y8 Y9
1.25% 1.5% 2% 3%
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Fig. 10. Specific heat of molten salt nanofluids with different mass fractions prepared by high-temperature melting.
Fig. 12. DSC curve of pure molten salt [25].
prepared by high-temperature melting method are higher than those of pure molten salt in the temperature range of 200–350 °C. The specific heat of samples Y1–Y9 are 1.729–1.795, 1.779–1.903, 1.865–2.031, 1.789–1.823, 1.798–1.822, 1.649–1.698, 1.743–1.808, 1.634–1.720, 1.603–1.660 J/(g K), respectively. The mass fraction is closely related to the specific heat of molten salt nanofluids. When the mass fraction of nanoparticles is less than 0.5%, the specific heat of molten salt nanofluids increases with the mass fraction of nanoparticles. The specific heat is 1.748 J/(g K) under the mass fraction of 0.1% and is 11.6% higher than that of pure molten salt. When the proportion is higher than 0.5%, the specific heat of molten salt nanofluids decreases with the mass fraction of nanoparticles. The specific heat of molten salt nanofluids is 1.622 J/(g K), which is the lowest, when the mass ratio is 3%. Compared with that of pure molten salt, the specific heat increase rate is only 3.6%. The specific heat of molten salt nanofluids with the mass fraction of 0.5% is the highest with an average of 1.950 J/(g K), which is 24.5% higher than that of pure molten salt. The best mass fraction of SiO2 nanoparticles to improve the specific heat of molten salt nanofluids prepared by hightemperature melting method is 0.5%. The thermal properties of the molten salt nanofluid sample (Sample 3) at a mass ratio of 0.5% will be thoroughly investigated below.
that of pure molten salt [25], respectively. The starting point of the melting peak was used as the melting point of the sample following the standard accepted by the International Association for Thermal Analysis. From Fig. 11, melting point, melting peak and melting point of termination are found to be 106.8 °C, 125.0 °C, and 132.1 °C, respectively. The latent heat of fusion is 100.6 J/g. Compared with those of pure molten salt (Fig. 12), melting point, peak point, and melting point of termination are slightly improved. The specific comparison results are shown in Table 9. The decomposition temperature of molten salt nanofluid is 610 °C compared with that of pure molten salt. The result insignificantly changes when using temperature under liquid state. Thus, a wide range of temperature can be used. The latent heat of molten salt nanofluid is 100.6 J/g, which is slightly lower than that of pure salt. 3.3. Density The density of molten salt nanofluid with a mass fraction of 0.5% can be calculated using Eq. (2), and the results are listed in Table 10. The density of molten salt nanofluid prepared by high-temperature melting decreases with the increase in temperature. In particular, density ranges from 1.795 g/cm3 to 1.985 g/cm3 in the temperature range of 150–500 °C, and this density range is slightly higher than that of pure molten salt [25]. The fitting formula of the density of molten salt nanofluid with temperature is expressed as follows:
3.2. Melting point and latent heat Figs. 11 and 12 show the melting point of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5% and
ρ = 2.066 − 5.4 × 10−4t
150 °C
≤
t
≤
500 °C
(6)
where ρ is the density of molten salt nanofluid and t is the temperature. 3.4. Thermal diffusivity and thermal conductivity Fig. 13 shows the thermal diffusivity of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%. In particular, thermal diffusivity ranges from 0.098 to 0.141 mm2/s in the temperature range of 150–350 °C. The fitting formula of the thermal diffusivity of molten salt nanofluid with temperature is expressed as follows:
α = 0. 106 + 1.071 × 10−4t
50°C ≤ t ≤ 350°C
(7)
where α is the thermal diffusivity of molten salt nanofluid and t is the temperature. The thermal conductivity of molten salt nanofluid prepared by hightemperature melting with a mass fraction of 0.5% can be calculated using Eq. (5), and the results are shown in Fig. 14. The thermal conductivity of molten salt nanofluid presents a linear relationship with temperature as it increases with the increase in temperature. The average thermal conductivity is nearly 0.528 W/(m K), which is similar
Fig. 11. DSC curve of molten salt nanofluid prepared by high-temperature melting method with a mass fraction of 0.5%. 215
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Table 9 Thermoproperties of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5% and those of pure molten salt. Sample
Melting point °C
Decomposition temperature °C
Latent heat of fusion J/g
Pure molten salt [25] Molten salt nanofluid
96.8 106.8
612.0 610.0
106.9 100.6
Table 10 Density of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%. Sample
Molten salt nanofluid Pure molten salt
Density (g/cm3) 150 °C
200 °C
250 °C
300 °C
350 °C
400 °C
450 °C
500 °C
1.985 1.953
1.958 1.924
1.931 1.896
1.904 1.868
1.877 1.839
1.850 1.811
1.823 1.782
1.795 1.754
Fig. 15. Viscosity of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%.
Fig. 13. Thermal diffusivity of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%.
3.5. Viscosity Fig. 15 shows the viscosity of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%. In particular, viscosity varies from 0.72 to 2.20 mPa s in the temperature range of 150–450 °C. The viscosity of molten salt nanofluids decreases with the increase in temperature, and the measurement of the viscosity of molten salt nanofluids exhibits a good relationship with temperature, as shown below.
η = 2.19 + 2.25 × 10−3t − 1.19 × 10−5t 2, R − Square = 0.95048
(9)
3.6. Sensible heat storage cost The unit price can be calculated on the basis of Eq. (10).
PC = Fig. 14. Thermal conductivity of molten salt nanofluid prepared by high-temperature melting with a mass fraction of 0.5%.
(10)
where Mi is the mass fraction of each component and PCi is the unit price of each component ($/kg). Integral average specific heat and sensible heat storage value can be calculated by Eqs. (11) and (12).
to that of solar salt (average thermal conductivity of 0.520 W/(m K)) [26] and 50.9% more than that of Hitec salt (average thermal conductivity of 0.350 W/(m K)) [27]. The fitting formula of the thermal conductivity of molten salt nanofluid with temperature is expressed as follows:
λ=0.396+4.087×10-4t 200℃≤t ≤350℃
∑ Mi×PCi ,
t
Cp̅ =
∫t1 2 Cp dt t2-t1
,
(11)
t
Qsensible=
(8)
where λ is the thermal conductivity of molten salt nanofluid and t is the temperature.
∫t12 Cp̅ dt 3600
,
(12)
where t1 is the minimum operating temperature (°C) and t2 is the maximum operating temperature (°C). 216
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Table 11 Sensible heat storage costs.
Molten salt nanofluid Pure molten salt [25]
PC $/kg
Average specific heat capacity J/(g·K)
Qsensible kW·h/kg
TC $/(kW·h)
1.6 1.6
1.95 1.57
0.217 0.19
7.2 8.2
References
Sensible heat storage cost can be calculated on the basis of Eq. (13).
TC =
PC , Qsensible
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(13)
The operating temperature range of molten salt nanofluid prepared by high-temperature melting is 150–550 °C. The sensible heat storage costs of molten salt nanofluid prepared by high-temperature melting is calculated by Eqs. 10–13, and the result is showed in Table 11. The sensible heat storage cost of molten salt nanofluid prepared by hightemperature melting is 7.2 $/kW h, which is 12.2% lower than that of pure molten salt. 4. Conclusions In this study, molten salt nanofluids with different mass fractions of SiO2 nanoparticles were prepared using high-temperature melting and the specific heat was measured to determine the optimal concentration of SiO2 nanoparticles. The thermal properties of the prepared molten salt nanofluids, namely, melting point, primary crystallization point, thermal conductivity, viscosity, latent heat, and density, were experimentally tested and analyzed under the condition of optimal concentration of SiO2 nanoparticles. The conclusions were obtained as follows: 1) The specific heat capacities of the molten salt nanofluids samples prepared by high-temperature melting with different mass fractions of nanoparticles are higher than those of pure molten salt. The best mass fraction of SiO2 nanoparticles to improve the specific heat of molten salt nanofluids is 0.5%. The average specific heat of molten salt nanofluid with a mass fraction of 0.5% is the highest at 1.950 J/ (g K), which is 24.5% more than that of pure molten salt. 2) The thermal properties of the prepared molten salt nanofluids with the optimal mass ratio of SiO2 nanoparticles of 0.5% are obtained: The melting point of molten salt nanofluid is 106.8 °C, which is 10 °C higher than that of pure molten salt; The decomposition temperature of molten salt nanofluid is 610 °C, which is similar to that of pure molten salt; The latent heat of molten salt nanofluid is 100.6 J/ g, which is slightly lower than that of pure molten salt; The thermal diffusivity of molten salt nanofluid is nearly 0.181 mm2/s; The thermal conductivity is around 0.528 W/ (m K), which is a relatively high thermal conductivity compared with solar salt (average thermal conductivity of 0.520 W/(m K)) and Hitec salt (average thermal conductivity of 0.350 W/(m·K)); The viscosity of molten salt nanofluids decreases with the increase in temperature, which varies from 0.72 to 2.20 mPa s in the temperature range of 150–450 °C. 3) The sensible heat storage cost of molten salt nanofluid prepared by high-temperature melting is 7.2 $/kW h, which is 12.2% lower than that of pure molten salt. Acknowledgements This work was supported by National Natural Science Foundation of China No. 51706005, the National Basic Research Program of China (973 Program) No. 2015CB251303, Science and Technology Plan General Program of Beijing Municipal Commission of Education No. KM201610005017 and No. KM201710005009.
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