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nitrites salts for concentrated solar power
Solar Energy 137 (2016) 385–392
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Accurate viscosity measurement of nitrates/nitrites salts for concentrated solar power Yuan Jin a,b, Jinhui Cheng a, Xuehui An a, Tao Su a, Peng Zhang a,⇑, Zhong Li a a b
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China College of Sciences, Shanghai University, Shanghai 200444, China
a r t i c l e
i n f o
Article history: Received 14 September 2015 Received in revised form 27 July 2016 Accepted 23 August 2016
Keywords: Viscosity Nitrates/nitrites salts Concentrating solar power Molten salts
a b s t r a c t Hitec (NaNO3-NaNO2-KNO3) and Solar salt (NaNO3-KNO3) are two of molten salts extensively used in Concentrated Solar Power (CSP). Viscosity plays an essential role in process of heat transfer for CSP system. However, it has some certain deviation and unsatisfactory errors among different reports; additionally, it still lacks the data close to melting point, which also makes another barrier for their application. In this work, accurate viscosity data of Hitec and Solar salt with 2.5% uncertainty were systematically measured using an optimized rotational coaxial cylinder in a wide temperature range that fill in the blanks of the existing data. Meanwhile, relevant viscosity were explicitly summarized and particularly analyzed based on the experimental data. Viscosity of the specific components of emerging ternary nitrates (Ca (NO3)2-NaNO3-KNO3, LiNO3-NaNO3-KNO3 and Ca(NO3)2-LiNO3-KNO3 mixtures) having potential applications for CSP systems were measured in the high temperature areas for the first time. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Molten salts have been widely used to be as one of ideal heat transfer and thermal storage media in solar power fields because of large specific heat capacity, low viscosity, wide temperature range and good compatibility to alloys (Yang and Garimella, 2010; Frank et al., 2012; Peng et al., 2010; Chen et al., 2011). Thermo-physical properties of molten salts dramatically impact their heat transfer and thermal storage performances. For example, high boiling and low melting point can improve the efficiency of power generation; large specific heat capacity can reduce the usage amount of molten salt; viscosity can affect the velocity distribution of molten salts fluids and determines their boundary layer thickness, which can influence the efficiency of heat transfer. There was one point to be mentioned, viscosity is also the source of the flow resistance that could increase the energy consumption of pump power (Kearney et al., 2003). Therefore, it is very essential to obtain precise experimental data of thermo-physical properties, especially viscosity, to evaluate heat transfer and thermal storage performance, and to provide significant parameters for the safety design of the solar power systems. The practical determination of molten salt viscosity is very difficult, due to the obstacles in high-temperature (more than 1000 K)
⇑ Corresponding author. E-mail address: [email protected] (P. Zhang). http://dx.doi.org/10.1016/j.solener.2016.08.037 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
experimental measurement caused by the special features of molten salts, such as low detection limit (lower than 1 mPa s), serious material corrosion and accurate measurement of sample temperature (Nunes et al., 2003). Only few techniques, including capillary method, torsional vibration method, and rotational method, have been proposed. The application of capillary method at high temperature, using both the Ostwald and Ubbelohde design, is dramatically complicated due to the possibility of a change of the capillary diameter caused by corrosion, or by recrystallization of molten salts on the surface of the capillary material (Cohen and Jones, 1957). Furthermore, the oscillating-cup method has extensive application to ionic melts where the viscosities fall into the range 0.5–10 mPa s, and is in general not feasible for systems of viscosities larger than 10 mPa s. For rotational method, especially rotating cylinder technique with a large measuring range (0.1–106 mPa s), the high accuracy and reproducibility can be easily achieved because of its definite physical meaning, mathematical simplification, direct and precisely measurement of the sample temperature (Mill, 1995; Cohen and Jones, 1957). Molten nitrates/nitrites, one kind of heat transfer and thermal storage media, have been extensively employed in concentrating solar power (CSP) systems (Siegel et al., 2011; Serrano-López et al., 2013; Yang and Garimella, 2010; Coscia et al., 2013). Molten nitrates/nitrites based heat transfer fluids (HTFs) are commonly used in modern CSP systems with the first molten salt power tower systems launched in 1984. These pioneering systems were the
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Y. Jin et al. / Solar Energy 137 (2016) 385–392
THEMIS tower in France and Molten salt electric experiment in the United States. As previously described, another important advantage of utilizing nitrates/nitrites salts in the power tower systems is their capability for thermal energy storage. These successful CSP systems include the Eurelios power project in the Italy and the Solar Two project in the United States (Dunn et al., 2012; Vignarooban et al., 2015). So far, Hitec (NaNO3-NaNO2-KNO3, 7–40–53 wt.%) and Solar Salt (NaNO3-KNO3, 60–40 wt.%), have been successfully applied in CSP (Boerema et al., 2012; Singh, 1985; Yang and Garimella, 2010). In addition, in order to improve the heat transfer and thermal storage performance and reduce the system cost, some emerging ternary nitrates (Ca(NO3)2-NaNO3-KNO3, LiNO3-NaNO3-KNO3 and Ca(NO3)2-LiNO3-KNO3 mixtures) which have great potential applications for CSP systems were prepared (Bradshaw, 2010). Based on the usage as working fluid in CSP, these molten salts have received attention, especially the viscosity playing an important role in calculations or simulations for thermal exchange and storage system design. For Hitec, initial measurements of the viscosity was performed by Kirst et al. (1940) using an Ostwald viscometer, and then by Gaune (1982) and Chen et al. (2011) using an oscillating right-circular cylinder viscometer and oscillation cup viscometer respectively. In addition, the data have been also reported in the open literatures (Geiringer, 1962; Singh, 1985; Coastal Chemical Co., 2011; McDuffie et al., 1963; Yang and Garimella, 2010; Bohlmann, 1972). Molten NaNO3-KNO3 with a eutectic point at 45.7 wt.%, NaNO3 and 54.3 wt.% KNO3 (mol composition: NaNO3-KNO3, 50–50 mol.%) has been extensively studied, whereas, the commercial Solar salt with a composition (NaNO3-KNO3, 60– 40 wt.%) has sparse data reported, because the viscosities of the two compositions were supposed to be basically the same (Serrano-Lopez et al., 2013). Viscosity of the equimolar mixture NaNO3-KNO3 has been measured by Murgulescu and Zuca (1969) utilizing an improved damped oscillating sphere method and by Coscia et al. (2012) with a Rheometric ARES Rheometer using a fixed Couette, and the data of viscosity from Mar et al. (1982) and Nissen (1982) were also presented. Different compositions of the emerging ternary nitrates system have been investigated because different phase diagrams have been published in the open literature (Menzies and Dutt, 1911; Jänecke, 1942; Levin et al., 1956; Gomez et al., 2013), however, few sporadic viscosity data have been obtained (Menzies and Dutt, 1911; Jänecke, 1942; Bergman et al., 1955; Levin et al., 1956; Kearney et al., 2003; Brosseau et al., 2004; Bradshaw, 2010; St. Laurent et al., 2000; Siegel et al., 2011). The viscosity data of these molten nitrates/nitrites mixtures used for CSP systems are still insufficient although they have been studied by many researchers with experimental measurements and calculations. Firstly, viscosity data of molten nitrates/nitrites have particularly large deviation among different researchers reported in open literatures in that the low viscosity measurement of molten nitrates/nitrites are difficult under high temperature. For example, for Hitec, the maximum deviation was more than 50% (at 700 K). Moreover, the measurement errors were wrongly defined or not referred in some experimental data. Secondly, the viscosity
data of these molten nitrates/nitrites mixtures are seriously insufficient. Many viscosity data are lacking under condition close to melting point, which can provide significant parameters for the safety analysis of molten salt thermo-hydraulic loop under abnormal working conditions. In addition, viscosity data of the emerging ternary nitrates, especially the components in this work, are still nonexistent at high temperature. In present paper, accurate viscosity measurements of the molten nitrates/nitrites were systematically conducted using an optimized rotational coaxial cylinder in a wide temperature range. The viscosity data of extensively used molten Hitec and Solar salt were detailed summarized and reassessed based on the experimental data. The viscosity of the emerging ternary nitrates was systematically analyzed. The reliability and accuracy of these viscosity data were analyzed and evaluated. These reliable viscosity results fill in the blanks of the existing data, which is beneficial to the design of CSP systems. 2. Experimental procedure 2.1. Sample preparation Table 1 shows the components of binary and ternary nitrates/ nitrites molten salts for measurements in this paper. The molten salt mixtures were prepared from NaNO3, NaNO2, KNO3, LiNO3 and Ca(NO3)2-4H2O salts (Analytical reagent). These salts were separately dried at 400 K under argon atmosphere for 24 h, and then mixed in graphite crucibles. The mixed nitrates/nitrites were heated to 500 K, and keep this temperature for 24 h in the argon atmosphere. For calcium nitrate containing salts, other than the separately dried, the salts were heated to 620 K under argon atmosphere for 24 h to allow the water of hydration of the calcium nitrate constituent to evolve slowly. 2.2. Measurement apparatus The viscometer used in this work is based on coaxial cylinder method, shown in Fig. 1. The viscometer mainly consists of four fundamental systems: the measuring unit, the spindle and crucible, a high temperature furnace, and other auxiliary system. This measuring unit is improved on the basis of Brookfield DVIII. With an aim to guarantee the turbulence nonexistence in the sample liquids during measurement process, suitable rotational speed and the sizes of spindle and crucible were selected through calculations and pre-experiments. The crucible with inner diameter of 29 mm and the depth of 220 mm, and the spindle with diameter of 26 mm and length of 150 mm were made of graphite. Under such conditions, the measurement range was 1.6–16 mPa s in a speed of 30 Revolutions Per Minute (RPM). In order to get the exact temperature of molten sample, a K-type thermocouple was put into the sample fluid. For rotating cylinder viscometer, the viscosity is obtained by formula (2) derived from formula (1) which is deduced by using . SMC ¼ r12 R12 120ðh þ cÞ. The standard sample with viscosity
Table 1 Compositions and melting point of the binary and ternary nitrates/nitrites mixtures system in this work. Composition Hitec Commercial solar salt Equimolar NaNO3-KNO3 LiNO3-NaNO3-KNO3 Ca(NO3)2-NaNO3-KNO3 a Ca(NO3)2-LiNO3-KNO3
LiNO3 (wt.%)
23.4 21.7
NaNO3 (wt.%)
NaNO2 (wt.%)
KNO3 (wt.%)
7 60 45.7 17.3 16
40
53 40 54.3 59.3 48 58.3
Ca(NO3)2 (wt.%)
Melting point (K)
36 20
415 511 495 413 406 390
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Y. Jin et al. / Solar Energy 137 (2016) 385–392 Table 2 The testing results of Cannon S6 oils (certified viscosity standard values). Temperature (K)
Measured (mPa s)
Standard (mPa s)
Deviation (%)
298 313 323 353
7.76 4.91 3.76 2.02
7.897 4.914 3.765 2.003
1.73 0.08 0.13 +0.85
researchers always choose potassium nitrate as viscosity reference fluid to test and verify the reliability of this high temperature viscometer. In this work, the viscosity of KNO3 molten salt was measured and the results are shown in Table 3. Compared with previous data, the deviation of the values obtained is within the error ranges. This indicates that the viscometer is reliable, and feasible for the molten salts nitrates/nitrites mixtures. This optimized viscometer is a kind of traditional standard method, which can work within the whole ranges just through calibrating the SMC using a Newtonian fluid of known viscosity. The input parameters used to calculate the viscosity are SMC, M and RPM, accurate temperature measurement of the sample liquids is necessary for the fitting curves. 2.3. Uncertainty analysis
Fig. 1. The viscometer based on rotating cylinder method.
go was used to calibrate the Spindle multiplier constant (SMC) through formula (3). If the spindle speed could be obtained, the sample viscosity can be computed by measuring the torque (M).
g¼
M 1 1 2 2 4pðh þ cÞx r R
ð1Þ
g¼
SMC M RPM
ð2Þ
SMC ¼
RPM go M
ð3Þ
where h is the length of spindle (m), r is the radius of spindle (m), x is the angular velocity (rad/s); and c is the correction factor of the spindle (m); R is the radius of the Crucible (m). M is moment of torque (N mm); The spindle multiplier constant (SMC) is calculated using the formula (3), where go is a known viscosity of the Newtonian fluid (mPa s); RPM is revolutions per minute (r/min). The measurement process was as follows: the molten salt sample was heated to target temperature and keep it at least 60 min. After that, move down the spindle until desired setting position, guaranteeing that the spindle immerge into the sample completely, then adjust the speed to 30 RPM. Finally, decrease temperature gradually and record the temperature and viscosity. A low temperature standard sample certified by NIST, Cannon S6 viscosity standard oil, was chosen to verify the accuracy of the instrument in measuring ranges. The results of the evaluation are shown in Table 2. The deviation between the measured values and the standard values are less than ±2%. Viscosity of potassium nitrate (KNO3) has been reported by many authors through different techniques (Timidei et al., 1970; Janz et al., 1968; Yoshiyoki et al., 1980; Zuca, 1970; Ohta et al., 1975; Tolbaru et al., 1998; Wellman et al., 1966; Zuca and Borcan, 2002), and the results show good agreement. So the
To analyze the uncertainty of the experiment data, three steps should be performed. Firstly, the uncertainty sources (x1 ; x2 ; x3 ; . . .) must be confirmed. Secondly, the standard uncertainty (lx1 ; lx2 ; lx3 ; . . .) is calculate, including two types, i.e. Type A and Type B. Type A is based on statistics, generally, standard deviation is the uncertainty. For Type B, the uncertainty is evaluated on the basis of experience including calibration certificates, certificate of identification, and so on. Finally, the expanded uncertainty is represented as the standard uncertainty multiplied by a confidence factor, which is evaluated according to Eq. (4) (Li, 2014).
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi XN @f 2 XN @f @f 2 U¼k ðuxi Þ þ 2 16i
ð4Þ
where U is the combined uncertainty; uxi and uxj represent the standard uncertainty components caused by xi and xj, respectively; f represents the function of parameters measured in this work; eij is the correlation coefficient between xi and xj; k is the confidence factor, generally, k takes the value of 2 (confidence level = 95%). Firstly, uncertainty sources of viscometer include SMC, M, RPM, T and the standard sample named S6 (a standard viscosity oil certified by NIST) according to formula (1)–(3). Secondly, the standard uncertainty of each source (SMC, M, RPM, T and S6) is calculated as followed: (1) The standard uncertainty of SMC is calculated by using the S6 oil whose viscosity is 7.897 mPa s in 298 K, uncertainty is 0.1%. The standard error was calculated by Bessel functions
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 1 ðXi XÞ s1 ¼ ¼ 0:0083 N1
l1 ¼ s1
.pffiffiffiffi N ¼ 0:00262
The relative uncertainty is:
l10 ¼ lX1 ¼ 0:00262 ¼ 0:5%. 0:54
(2) The standard uncertainty from M was computed (type A) by using molten salt samples.
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Y. Jin et al. / Solar Energy 137 (2016) 385–392
Table 3 The testing results of potassium nitrate. Temperature (K)
Measured viscosity (mPa s)
Literature value (mPas) Ohta, T.
713.9 703.7 693.5 683.3 672.1 663 652.8 641.5 631.2 621.9 611.6 a
1.79 1.84 1.95 2.03 2.12 2.20 2.35 2.47 2.58 2.77 2.87
a
1.80 1.85 1.93 2.01 2.10 2.19 2.31 2.45 2.60 2.77 2.88
Deviation range (mPa s)
Wellman, R.E.
b
1.75 1.82 1.90 1.98 2.08 2.17 2.29 2.44 2.58 2.73 2.91
Janz, G.J.
b
1.72 1.80 1.89 1.99 2.10 2.20 2.32 2.47 2.61 2.75 2.92
Timidei, A. 1.73 1.81 1.90 1.99 2.10 2.20 2.32 2.47 2.61 2.75 2.91
b
0.07 to +0.01 0.04 to +0.01 0.06 to 0.02 0.05 to 0.02 0.04 to 0.02 0.03 to 0 0.06 to 0.03 0.02 to 0 0 to +0.02 0.04 to 0 +0.01 to +0.05
Oscillation cup method. Capillary method.
b
The standard error s2 was calculated by Bessel functions
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 1 ðXi XÞ s2 ¼ ¼ 11:11 N1
l2 ¼ s2
.pffiffiffiffi N ¼ 3:93 N mm
The relative uncertainty is:
3:93 l20 ¼ lX2 ¼ 368:44 ¼ 1:1%.
(3) The standard uncertainty of RPM (type B, calibration certificates): The precision of stepping motor is less than 0.034%. The impact on the viscosity test is:
l3 ¼ 0:0102 r=min
l4 ¼ 0:020 mPa s
(5) Standard uncertainty identification)
l5 from S6 (type B, certificate of
l5 ¼ 0:00663 mPa s
Finally, the expanded uncertainty is calculated according to Eq. (4). SMC, M, RPM, T and S6 are independent, so eij is 0.
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 2 2 @f @f @f 2 2 l þ l þ l þ l þ l 5 4 @x1 1 @x2 2 @x3 3 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 ¼ 2 0:0332 þ 0:0707 þ 0:0023 þ 0:02 þ 0:006632
U¼k
¼ 0:162 mPa s The sample viscosity is 6.63 mPa s, so the relative uncertainty 0:162 ¼ 2:5% (see Tables 4–6). 6:63 The first parameter SMC was calculated from the experiment according to type A evaluation method, and estimated to be less than 0.5%. The standard uncertainty of torque was 1.1% obtained from the experiment data according to type A evaluation method,
is:
Table 4 The data of KNO3 from each investigation were fitted to these equations by a least squares technique. Author
Fitting curves
This work
g ¼ 0:16315 expð1:409 103 =T þ 2:134 105 =T 2 Þ g ¼ 35:27437 expð5:797 103 =T þ 2:621 106 =T 2 Þ g ¼ 1:37436 expð1:566 103 =T þ 1:24 106 =T 2 Þ g ¼ 0:05647 expð2:584 103 =T 1:038 105 =T 2 Þ g ¼ 0:04492 expð2:918 103 =T 2:237 105 =T 2 Þ
Wellman, R.E. Janz, G.J. Timidei, A.
3. Results and discussion 3.1. Hitec
(4) Standard uncertainty l4 from T (type B, calibration certificates): The precision of the thermocouple is less than 0.5 K. The impact on the viscosity test is:
Ohta, T.
Furthermore, the synchronous motor performed 0.034% on the basis of its precision introductions. The temperature was directly measured every time to obtain the actual temperature. Uniform temperature profiles of the K-type thermocouple within 0.5 K, and the standard uncertainty estimated to be less than 0.3% according to type B evaluation method. The standard uncertainty from S6 was 0.1%. So it is considered that the uncertainty accompanied by the measurement in high temperature is estimated to be less than 2.5% (listed in Table 7).
Hitec is a ternary mixture of alkali-nitrates/nitrites and is commonly used as HTF and thermal storage material in solar thermal systems. The advantage of Hitec is that its melting point (415 K) is much lower and the thermal conductivity is almost twice that of organic HTFs (Boerema et al., 2012). The thermal stability results show that Hitec salt is stable with very limited decomposition at temperatures below 773 K (Peng et al., 2010). The experimental results from Coastal Chemical Co. LLC show that the seamless stainless steel tubing had no visible corrosion attacks after being immersed in continuous flow of Hitec for 10 months at 772–822 K. Viscosity of Hitec has been measured through different methods including rotating cylinder method, capillary method and oscillation cup method, and the results are shown in Fig. 2. (Coastal Chemical Co., 2011; Kirst et al., 1940; Geiringer, 1962; Singh, 1985; Gaune, 1982; Cohen and Jones, 1957; McDuffie et al., 1963; Wu et al., 2012; Yang and Garimella, 2010; Bohlmann, 1972). Comprehensive analysis of the viscosity results of Hitec from different open literatures were presented in Table 8. Even if experimental data obtained by different authors are available under the normal operating temperature (470–770 K), a sizeable disagreement was commonly found, and the maximum deviation was more than 50% (at 700 K). It can be seen obviously from Fig. 2 that the viscosity data of Hitec is lack close to melting point (<465 K) which can provide significant parameters for the safety analysis of CSP systems under abnormal condition, while large deviations will appear if extending the fitting curves directly from working areas to the melting point. Therefore, it is necessary to measure the viscosity in this temperature range. For viscosity measurement, the error is an essential part of the viscosity data. However, to be best of our knowledge, there are some reported data without test error or with a wrong error regarding to Hitec (listed in Table 8). Such as, density (the error more than 2%) is essential in process of data for oscillation cup
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Y. Jin et al. / Solar Energy 137 (2016) 385–392 Table 5 Calculation of the standard uncertainty from SMC by using the S6 oil. Number
1
2
3
4
5
6
7
8
9
10
SMC Xi X
0.549 0.009
0.531 0.009
0.542 0.002
0.547 0.007
0.539 0.001
0.537 0.003
0.531 0.009
0.528 0.012
0.547 0.007
0.551 0.011
Table 6 Calculation of the standard uncertainty from M by using molten salt samples. Number
1
2
3
4
5
6
7
8
M Xi X
372.88 4.48
381.38 12.98
366.22 2.18
360.88 7.52
352.42 15.98
384.37 15.97
370.87 2.47
358.55 9.85
work, and can be fitted by Eq. (5), Adjusted R2 = 99.94% (R is regression coefficient):
Table 7 Error of viscosity caused by the deviation of physical parameters. Sources
Uncertainty (%)
Spindle multiplier constant Moment of torque Revolutions per minute Temperature S6 sample Total
0.5 1.1 0.034 0.3 0.1 2.5
g ¼ 1:149 exp 810:896=T þ 7:806 105 =T 2
ð5Þ
The maximum of measured value is 14.2 mPa s (in 420 K) within the scope of the instrument test. The maximum of measured temperature is 695 K, which can basically satisfy the engineering application in CSP systems. The results nearly coincide well with the sole Coastal Chem. Brochure data (USA) in the low temperature areas (<465 K). To evaluate the deviation, the averaged value of reported data including the results of our works is used as a reference in Fig. 3. As shown in Fig. 2, the tendency of the g T curve shows a good agreement among different reported data, although it has some certain deviation among different reports. 3.2. Binary nitrate salts
Fig. 2. Viscosity of Hitec varying with temperature.
method and capillary method, while the error from density if often ignored. To solve these questions, accurate viscosity data of Hitec with 2.5% uncertainty in a wide range of 420 (close to melting point) to 695 K were given with an interval almost ten degrees in this
The phase diagram for the NaNO3-KNO3 system indicate that there exists a eutectic point of 45.7 wt.%, NaNO3 and 54.3 wt.% KNO3 (50–50 mol%) at temperatures of 495 K (Coscia et al., 2012). In order to reduce the cost in modern CSP systems, the cheaper commercial mixture Solar Salt (NaNO3-KNO3, 60–40 wt.%) was prepared. It is used in the Solar Two central receiver system located in California and several other solar plants in Spain (Kuravi et al., 2013). It melts at 511 K and remains in thermally stable liquid phase at temperatures up to 873 K. Viscosity of the equimolar binary NaNO3-KNO3 eutectic salt has been studied by many researchers with experiment, meanwhile, the data and fitting curves of viscosity parameters were also presented in Fig. 4 and Table 9. Commercial compositions Solar salt has similar thermo-physical properties with the equimolar component salt, and the differences of viscosity between equimolar and commercial compositions are negligible in previously reported literatures (Serrano-Lopez et al., 2013).
Table 8 The viscosity of Hitec varying with temperature can be fitted by the following equation, where g is viscosity, mPa s; T absolute temperature, K. Method
Temperature (K)
Fitting curves
Error
Rotational coaxial cylinder method
420–695
2.5%
Rotating method Ostwald viscosimeter
423–773 455–780
Oscillating viscosimeter
457–755
g ¼ 1:149 expð810:896=T þ 7:806 105 =T 2 Þ g ¼ ðT 273Þ^ 2:104 10^ 5:7374 g ¼ 67:57 103 expð2247:11=TÞ h i g ¼ 0:5631 exp 146:9794T 1 þ ð57:4265 104 T 2 Þ g ¼ 0:75484 expð6418=RTÞ g ¼ 1000 exp ½4:343 2:0143ðlnðT 273Þ 5:011Þ
Oscillation cup method Oscillation cup method a b c
523–723 520–773
Error: the reliability of the test data was defined using different methods, such as error, deviation or uncertainty. Accuracy. Maximum error.
a
Reference This work Coastal Chem. Brochure Kirst et al. (1940) Gaune (1982)
±5.0% 0.5% c
b
Chen et al. (2011) Costal Chem. Co.
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Y. Jin et al. / Solar Energy 137 (2016) 385–392
Fig. 3. Viscosity of Hitec deviation compared with the average values. Fig. 5. Viscosity of multi-component nitrate varying with temperature.
g ¼ 5:103 exp 2:575 103 =T þ 1:305 106 =T 2
ð7Þ
As shown in Fig. 4, the negligible differences were found for viscosity between the commercial compositions and the equimolar compositions (less than 0.2 mPa s). The experimental values of the equimolar component salt are in good agreement with that of Murgulescu and Zuca (1969), Nissen (1982) and Mar et al. (1982). Apparently, the equation given by Coscia et al. (2012) shows the high viscosity behavior of nitrates in low temperature areas. 3.3. Viscosity of the multi-component nitrate salts
Fig. 4. Viscosity of binary nitrate salts varying with temperature.
Accurate viscosity of these binary nitrate salts were systematically measured with an interval almost ten degrees in this work. For the equimolar salt, experimental data was given in the range of 506 K close to melting point (495 K) to 713 K, and can be fitted by Eq. (6), Adjusted R2 = 99.96%:
g ¼ 2:551 exp 1:65 103 =T þ 1:022 106 =T 2
ð6Þ
For the commercial compositions, experimental data was given in the range of 516 K close to melting point (511 K) to 720 K, and can be fitted by Eq. (7), Adjusted R2 = 99.72%:
Multi-component nitrate molten salts mixtures formulated from the nitrates of sodium, potassium, lithium and calcium display liquidus temperatures lower than that of Hitec. It can be observed that the addition of a third salt (LiNO3) to the constituents of Solar Salt (NaNO3 + KNO3) increases the thermal stability of the multi-component nitrate salt, moreover, the addition of Ca(NO3)2 into the mixture reduces the melting points and the economic cost of these mixtures (Fernandez et al., 2014). Viscosity of these nitrates/nitrites salts with varying temperature was given in Fig. 5. Fitting curves equations in Table 10. For Hitec XL (Ca(NO3)2-NaNO3-KNO3), different phase diagrams have been published, and the ternary diagram has one ternary eutectic and one ternary peritectic point (Menzies and Dutt, 1911; Jänecke, 1942; Levin et al., 1956). The eutectic point is the minimum of temperature where a liquid temperature exists (406 K) and the peritectic point is the second intersection point
Table 9 The viscosity of Binary nitrate salts varying with temperature can be fitted by the following equation, where g is viscosity, mPa s; T absolute temperature, K. Method
Temperature (K)
Fitting curves
Error
Rotational coaxial cylinder method
516–720
Rotational coaxial cylinder method
506–713
Rheometric ARES rheometer Oscillation cup method
495–820 548–873
g ¼ 5:103 expð2:575 10 =T þ 1:305 10 =T Þ g ¼ 2:551 expð1:65 103 =T þ 1:022 106 =T 2 Þ g ¼ expð2210=T 2:48Þ g ¼ 22:714 0:12 ðT 273Þ þ ð2:281 104 Þ ðT 273Þ2
Damped oscillating sphere method
523–723
g ¼ 0:4384 expð9:2163=T þ 6:52 105 =T 2 Þa
3
6
2
b
Reference
2.5%
Commercial solar saltd
2.5%
Equimolar NaNO3-KNO3d
1%
c
Coscia et al., 2012 Nissen (1982)
ð1:474 107 Þ ðT 273Þ3
a b c d
The data from Murgulescu were fitted to this equation. Error: the reliability of the test data was defined using different methods, such as error, deviation or uncertainty. Uncertainty. This work.
Murgulescu and Zuca (1969)
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Table 10 The viscosity of these molten salts nitrates/nitrites varying with temperature can be given by the following equation, where g is viscosity, mPa s; T absolute temperature, K. Composition LiNO3-NaNO3-KNO3 Ca(NO3)2-NaNO3-KNO3 Ca(NO3)2-LiNO3-KNO3
Viscosity (mPa s) g ¼ 3:355 exp 2:038 103 =T þ 1:109 106 =T 2 g ¼ 19:376 exp 4:215 103 =T þ 1:997 106 =T 2 g ¼ 1:226 exp 8:479 102 =T þ 9:317 105 =T 2
Temperature range (K) 418–645 503–670 465–705
close to 433 K (Gomez et al., 2013). Ternary mixture Hitec XL in this work (the ternary eutectic) was considered to be the best composition by Gomez et al. (2013). Viscosity data of Hitec XL was firstly measured at the high temperature (503–670 K), which has been reported by Bradshaw (2010) in low temperature areas (420–500 K) using a Brookfield DV-II+ viscometer ever. Through Fig. 5, the experimental results trend nearly to coincide with the data of Bradshaw (2010), and the fitting curve equation was given by Eq. (8), Adjusted R2 = 99.87%:
g ¼ 19:376 exp 4:215 103 =T þ 1:997 106 =T 2
ð8Þ
The eutectic point of LiNO3-NaNO3-KNO3 mixture salt melts is at about 413 K, while Ca(NO3)2-LiNO3-KNO3 mixture salt melts at about 390 K (Mcmurdie and Hall, 1949). The viscosity data in low temperature areas have been also reported by Bradshaw (2010), and the viscosity data function relationships has also been given. Low viscosity data of these multi-component nitrates were firstly systematically recorded in this work. For emerging ternary candidate LiNO3–NaNO3–KNO3 mixture salts, viscosity shows excellence. The maximum of measured value performed 15.5 mPa s in 418 K (close to melting point), and the viscosity data were given approaching to 645 K. The fitting curve equation was given by Eq. (9), Adjusted R2 = 99.96%:
g ¼ 3:355 exp 2:038 103 =T þ 1:109 106 =T 2
ð9Þ
For emerging ternary candidate Ca(NO3)2-LiNO3-KNO3 mixture salt, the melting temperature is much lower than that of even Hitec XL. Viscosity data of this ternary mixture were recorded from 465 to 705 K with an interval almost ten degrees. Obviously, it is less than that of Hitec XL. The fitting curve equation was given by following Eq. (10), Adjusted R2 = 99.76%:
g ¼ 1:226 expð8:479 102 =T þ 9:317 105 =T 2 Þ
ð10Þ
3.3.1. Contrastive analysis Compared with these molten nitrates/nitrites in Fig. 6, the viscosity of binary nitrate salts, ternary Hitec and multi-component LiNO3–NaNO3–KNO3 salt shows the same trend and excellent properties. Due to the presence of calcium nitrate, the viscosity of ternary mixtures Ca(NO3)2-LiNO3-KNO3 salt and Hitec XL increases dramatically in low temperature range (<650 K). However, the viscosity is no longer affected intensely by the increase of calcium nitrate in higher temperature, and the viscosity of all these salts behaves seem that will be the same (above 700 K). This may due to the presence of divalent Ca ion which can increase the coulomb force or lead to complexation, further investigation on it will be conducted in the near future. Main advantage of these ternary nitrates mixture is that their melting points are lower compared to that of the Solar Salt and Hitec. The problem of freezing is easier to be controlled with lower melting points. Also, these ternary nitrates mixture show temperature stability up to 773 K (Gomez et al., 2013; Fernandez et al., 2014), suggesting potential suitability as a HTF in CSP systems. Due to the addition of LiNO3 salt, ternary LiNO3–NaNO3–KNO3 salt seems more feasible in CSP systems. Unfortunately, LiNO3 salt is
Fig. 6. Contrastive viscosity analysis of these salts varying with temperature.
relatively expensive that may be a barrier for large scale application. Costs of the industrial grade nitrate salts follow the order of Li > K > Ca > Na in the Chinese market. Although the addition of calcium nitrate to molten salt increases the viscosity, the cost of calcium nitrate containing salts is lower than Li/K nitrates salts and the viscosity of these calcium nitrate containing salts are below 5 mPa s over 590 K which can meet the requirements of engineering application. So they also have feasibility as heat transfer media in CSP systems.
4. Conclusions Viscosity data of molten Hitec and Solar salt were explicitly summarized and reassessed on the basis of the experimental data. Experimental deviation and errors were discussed, and it presented a good result for our experimental data compared with the reported ones. In addition, the problem of viscosity data being lack close to melting point was solved in this paper. Viscosity of the certain components emerging ternary nitrates Hitec XL in the range of 503–670 K, LiNO3–NaNO3–KNO3 salt in the range of 418–645 K and Ca(NO3)2-LiNO3-KNO3 salt in the range of 465– 705 K were measured with an interval of almost ten degrees using this optimized rotational coaxial cylinder viscometer for the first time. The viscosity of lithium nitrate containing salts shows similarity with Hitec and Solar salt, and the addition of lithium nitrate to molten salt had relatively little effect on viscosity. However, it is apparent for calcium nitrate containing salts that calcium nitrate will increase the viscosity of the molten salt.
Acknowledgement The authors are grateful to acknowledge the financial supported by the ‘‘Strategic Priority Research Program” of Chinese Academy of Sciences, Grant No. XD02002400, Youth Innovation Promotion
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Association CAS and the National Natural Science Foundation of China (21406256). References Bergman, A.G., Rassonskaya, I.S., Shmidt, N.E., 1955. Izvest. Sektora. Fiz.-Khim. Anal., Inst. Obshch. Neorg. Khim. Tr. Fiz. Inst. Akad. Nauk SSSR 26, 156–163. Boerema, N., Morrison, G., Taylor, R., Rosengarten, G., 2012. Liquid sodium versus Hitec as a heat transfer fluid in solar thermal central receiver systems. Sol. Energy 86 (9), 2293–2305. Bohlmann, E.G., 1972. Heat Transfer Salt for High Temperature Steam Generation. Reactor Chemistry Division. Bradshaw, R.W., 2010. Viscosity of Multi-Component Molten Nitrate Salts-Liquidus to 200 °C, Sandia National Laboratory, Technical Report No. SAND2010-1129, Livermore, CA. Brosseau, D.A., Hlava, P.F., Kelly, M.J., 2004. Testing of thermocline filler materials and molten-salt heat transfer fluids for thermal energy storage systems used in Parabolic Trough Power Plants. Technical Report No. SAND2004-3207. Chen, Y.C., Wu, Y.T., Ren, N., Ma, C.F., 2011. Experimental study of viscosity characteristics of high-temperature heat transfer molten salts. Sci. China Technol. Sci. 54 (11), 3022–3026. Cohen, S.I., Jones, T.N., 1957. Viscosity Measurements on Molten Fluoride Mixtures. Oak Ridge National Laboratory, ORNL-2278.. Coscia, K., Neti, S., Oztekin, A., Nelle, S., Mohapatra, S., Elliott, T., 2012. The thermophysical properties of the NaNO3-KNO3, LiNO3-NaNO3, and LiNO3-KNO3 systems. In: ASME International Mechanical Engineering Congress and Exposition. IMECE2011. November 11–17, Denver, Colorado, USA. Coscia, K., Nelle, S., Elliott, T., Mohapatra, S., Oztekin, A., Neti, S., 2013. Thermophysical properties of LiNO3-NaNO3-KNO3 mixtures for use in concentrated solar power. J. Sol. Energy Eng. 135 (3), 1–5. Dunn, R.I., Hearps, P.J., Wright, M.N., 2012. Molten-salt power towers: newly commercial concentrating solar storage. Proc. IEEE 100 (2), 504–515. Fernandez, A.G., Ushak, S., Galleguillos, H., Perez, F.J., 2014. Development of new molten salts with LiNO3 and Ca(NO3)2 for energy storage in CSP plants. Appl. Energy 119, 131–140. Frank, G.F.Q., Yang, X.P., Ding, Z., Zuo, Y.Z., Shao, Y.Y., Jiang, R.H., Yang, X.X., 2012. Thermocline stability criterions in single-tanks of molten salt thermal energy storage. Appl. Energy 97, 816–821. Gaune, P.G., 1982. Viscosity of potassium nitrate–sodium nitrite–sodium nitrate mixtures. J. Chem. Eng. Data 27 (2), 151–153. Geiringer, P.L., 1962. Handbook of Heat Transfer Media (Heat Transfer Salt). RE Krieger Pub Co., Huntington. NY, pp. 208–213. Gomez, J.C., Calvet, N., Starace, A.K., Glatzmaier, G.C., 2013. Ca(NO3)2-NaNO3-KNO3 molten salt mixtures for direct thermal energy storage systems in parabolic trough plants. J. Sol. Energy Eng. 135 (2), 021016. HITEC Heat Transfer Salt. Costal Chemical Co., L.L.C., Brenntag Company.. Jänecke, E., 1942. The quaternary system Na, K, Ca, Mg//NO3 and its subsystems. Angew. Phys. Chem. 48 (9), 453–512. Janz, G.J., Dampier, F.W., Lakshminarayan, G.R., 1968. Molten salts: volume 1, electrical conductance, density and viscosity data. Nat. Stand Ref Data Ser, NBS (US) 15, 25–27. Kearney, D., Herrmann, U., Nava, P., Kelly, P., Mahoney, R., Pacheco, J., Cable, R., Potrovitza, N., Blake, D., Price, H., 2003. Assessment of a molten salt heat transfer fluid in a parabolic trough solar field. J. Sol. Energy Eng. 125 (2), 170– 176. Kirst, W.E., Nagle, W.M., Castner, J.B., 1940. A new heat transfer medium for high temperatures. Trans. Am. Inst. Chem. Eng. 36 (3), 371–394. Kuravi, S., Trahan, J., Goswami, D.Y., Rahman, M.M., Stefanakos, E.K., 2013. Thermal energy storage technologies and systems for concentrating solar power plants. Prog. Energy Combust. 39 (4), 285–319.
Levin, E.M., McMurdie, H.F., Hall, F.P., 1956. Phase Diagrams for Ceramists, vol. 1.. The American Ceramic Society, Columbus, OH. Li, D.S., 2014. Fundamental of Metrology. China machine press, Beijing (China) (in Chinese). Mar, R.W., Bradshaw, R.W., Carling, R.W., Goods, S.H., Nagelberg, A.S., Nissen, D.A., 1982. Progress report: molten nitrate salt technology development, Sandia National Laboratories, SAND82-8220. McDuffie, H.F., McCoy, H.E., Robertson, R.C., Dunlap, S., Thoma, R.E., 1963. Assessment of Molten Salts as Intermediate Coolants for LMFBR’s. Oak Ridge National Laboratory. Mcmurdie, H.F., Hall, F.P., 1949. Phase diagrams for ceramists: supplement NO. 1. J. Am. Ceram. Soc. 32, 154–164. Menzies, A.W.C., Dutt, N.N., 1911. The liquidus surface of the ternary system composed of the nitrates of potassium, sodium, and calcium. J. Am. Chem. Soc. 33 (8), 1366–1375. Mill, K.C., 1995. Viscosity of Molten Slags. Slag Atlas, Düsseldorf, Germany, second ed., pp. 349–402. Murgulescu, I.G., Zuca, S., 1969. Viscosity of binary mixtures of molten nitrates as a function of ionic radius-II. Electrochim. Acta 14, 519–526. Nissen, D.A., 1982. Thermophysical properties of the equimolar mixture NaNO3KNO3 from 300 to 600 °C. J. Chem. Eng. Data 27 (3), 269–273. Nunes, V.M.B., Lourenco, M.J.V., Santos, F.J.V., de Castro, C.A.N., 2003. Importance of accurate data on viscosity and thermal conductivity in molten salts applications. J. Chem. Eng. Data 48 (3), 446–450. Ohta, T., Borgen, O., Brockner, W., 1975. High-temperature viscometer for fluid liquids part I: on-line computer-facilitated torsion pendulum. Berichte der Bunsengesellschaft fur physikalische chemie 79 (4), 335–344. Peng, Q., Ding, J., Wei, X.L., Yang, J.P., Yang, X.X., 2010. The preparation and properties of multi-component molten salts. Appl. Energy 87 (9), 2812–2817. Serrano-Lopez, R., Fradera, J., Cuesta-Lopez, S., 2013. Molten salts database for energy applications. Chem. Eng. Process. 73, 87–102. Siegel, N.P., Bradshaw, R.W., Cordaro, J.B., Kruizenga, A.M., 2011. Thermophysical property measurement of nitrate salt heat transfer fluids. In: ASME 5th International Conference on Energy Sustainability, Washington, pp. 439–446. Singh, J., 1985. Heat Transfer Fluids and Systems for Process and Energy Applications: Molten Salts. M Dekker Ins, New York, pp. 223–240. St. Laurent, S.J., Kolb, W.J., Pacheco, J.E., 2000. Thermocline Thermal Storage Tests for Large-Scale Solar Thermal Power Plants. Sandia National Laboratory, Albuquerque, NM, Technical Report No. SAND2000-2059C. Timidei, A., Lederman, G., Janz, G.J., 1970. Accurate molten salt viscometry. Instrum. Sci. Technol. 3 (2), 309–320. Tolbaru, D., Borcan, R., Zuca, S., 1998. Viscosity measurements on molten salts with an oscillating cup viscometer: viscosity of molten KNO3 and NaCl. Berichte der Bunsengesellschaft fur physikalische chemie 102, 1387–1392. Vignarooban, K., Xu, X., Arvay, A., Hsu, K., Kannan, A.M., 2015. Heat transfer fluids for concentrating solar power systems – a review. Appl. Energy 146, 383–396. Wellman, R.E., Dewitt, R., Ellis, R.B., 1966. Viscosity of potassium nitrate. J. Chem. Eng. Data 11, 156–158. Wu, Y.T., Chen, C., Liu, B., Ma, C.F., 2012. Investigation on forced convective heat transfer of molten salts in circular tubes. Int. Commun. Heat Mass 39, 1550– 1555. Yang, Z., Garimella, S.V., 2010. Thermal analysis of solar thermal energy storage in a molten-salt thermocline. Sol. Energy 84 (6), 974–985. Yoshiyoki, A., Kosugiyama, O., Miyajima, H., 1980. Determination of the viscosity of molten potassium nitrate with an oscillating cup viscometer. J. Chem. Soc. 76, 2531–2541. Zuca, S., 1970. Viscosity of some molten nitrates. Rev. Roum. Chim. 15 (9), 1277– 1286. Zuca, S., Borcan, R., 2002. Viscosity of binary mixtures of molten nitrates as a function of ionic radius-III⁄. Electrochim. Acta 11 (15), 1817–1825.
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Solar Energy journal homepage: www.elsevier.com/locate/solener
Accurate viscosity measurement of nitrates/nitrites salts for concentrated solar power Yuan Jin a,b, Jinhui Cheng a, Xuehui An a, Tao Su a, Peng Zhang a,⇑, Zhong Li a a b
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China College of Sciences, Shanghai University, Shanghai 200444, China
a r t i c l e
i n f o
Article history: Received 14 September 2015 Received in revised form 27 July 2016 Accepted 23 August 2016
Keywords: Viscosity Nitrates/nitrites salts Concentrating solar power Molten salts
a b s t r a c t Hitec (NaNO3-NaNO2-KNO3) and Solar salt (NaNO3-KNO3) are two of molten salts extensively used in Concentrated Solar Power (CSP). Viscosity plays an essential role in process of heat transfer for CSP system. However, it has some certain deviation and unsatisfactory errors among different reports; additionally, it still lacks the data close to melting point, which also makes another barrier for their application. In this work, accurate viscosity data of Hitec and Solar salt with 2.5% uncertainty were systematically measured using an optimized rotational coaxial cylinder in a wide temperature range that fill in the blanks of the existing data. Meanwhile, relevant viscosity were explicitly summarized and particularly analyzed based on the experimental data. Viscosity of the specific components of emerging ternary nitrates (Ca (NO3)2-NaNO3-KNO3, LiNO3-NaNO3-KNO3 and Ca(NO3)2-LiNO3-KNO3 mixtures) having potential applications for CSP systems were measured in the high temperature areas for the first time. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Molten salts have been widely used to be as one of ideal heat transfer and thermal storage media in solar power fields because of large specific heat capacity, low viscosity, wide temperature range and good compatibility to alloys (Yang and Garimella, 2010; Frank et al., 2012; Peng et al., 2010; Chen et al., 2011). Thermo-physical properties of molten salts dramatically impact their heat transfer and thermal storage performances. For example, high boiling and low melting point can improve the efficiency of power generation; large specific heat capacity can reduce the usage amount of molten salt; viscosity can affect the velocity distribution of molten salts fluids and determines their boundary layer thickness, which can influence the efficiency of heat transfer. There was one point to be mentioned, viscosity is also the source of the flow resistance that could increase the energy consumption of pump power (Kearney et al., 2003). Therefore, it is very essential to obtain precise experimental data of thermo-physical properties, especially viscosity, to evaluate heat transfer and thermal storage performance, and to provide significant parameters for the safety design of the solar power systems. The practical determination of molten salt viscosity is very difficult, due to the obstacles in high-temperature (more than 1000 K)
⇑ Corresponding author. E-mail address: [email protected] (P. Zhang). http://dx.doi.org/10.1016/j.solener.2016.08.037 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
experimental measurement caused by the special features of molten salts, such as low detection limit (lower than 1 mPa s), serious material corrosion and accurate measurement of sample temperature (Nunes et al., 2003). Only few techniques, including capillary method, torsional vibration method, and rotational method, have been proposed. The application of capillary method at high temperature, using both the Ostwald and Ubbelohde design, is dramatically complicated due to the possibility of a change of the capillary diameter caused by corrosion, or by recrystallization of molten salts on the surface of the capillary material (Cohen and Jones, 1957). Furthermore, the oscillating-cup method has extensive application to ionic melts where the viscosities fall into the range 0.5–10 mPa s, and is in general not feasible for systems of viscosities larger than 10 mPa s. For rotational method, especially rotating cylinder technique with a large measuring range (0.1–106 mPa s), the high accuracy and reproducibility can be easily achieved because of its definite physical meaning, mathematical simplification, direct and precisely measurement of the sample temperature (Mill, 1995; Cohen and Jones, 1957). Molten nitrates/nitrites, one kind of heat transfer and thermal storage media, have been extensively employed in concentrating solar power (CSP) systems (Siegel et al., 2011; Serrano-López et al., 2013; Yang and Garimella, 2010; Coscia et al., 2013). Molten nitrates/nitrites based heat transfer fluids (HTFs) are commonly used in modern CSP systems with the first molten salt power tower systems launched in 1984. These pioneering systems were the
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THEMIS tower in France and Molten salt electric experiment in the United States. As previously described, another important advantage of utilizing nitrates/nitrites salts in the power tower systems is their capability for thermal energy storage. These successful CSP systems include the Eurelios power project in the Italy and the Solar Two project in the United States (Dunn et al., 2012; Vignarooban et al., 2015). So far, Hitec (NaNO3-NaNO2-KNO3, 7–40–53 wt.%) and Solar Salt (NaNO3-KNO3, 60–40 wt.%), have been successfully applied in CSP (Boerema et al., 2012; Singh, 1985; Yang and Garimella, 2010). In addition, in order to improve the heat transfer and thermal storage performance and reduce the system cost, some emerging ternary nitrates (Ca(NO3)2-NaNO3-KNO3, LiNO3-NaNO3-KNO3 and Ca(NO3)2-LiNO3-KNO3 mixtures) which have great potential applications for CSP systems were prepared (Bradshaw, 2010). Based on the usage as working fluid in CSP, these molten salts have received attention, especially the viscosity playing an important role in calculations or simulations for thermal exchange and storage system design. For Hitec, initial measurements of the viscosity was performed by Kirst et al. (1940) using an Ostwald viscometer, and then by Gaune (1982) and Chen et al. (2011) using an oscillating right-circular cylinder viscometer and oscillation cup viscometer respectively. In addition, the data have been also reported in the open literatures (Geiringer, 1962; Singh, 1985; Coastal Chemical Co., 2011; McDuffie et al., 1963; Yang and Garimella, 2010; Bohlmann, 1972). Molten NaNO3-KNO3 with a eutectic point at 45.7 wt.%, NaNO3 and 54.3 wt.% KNO3 (mol composition: NaNO3-KNO3, 50–50 mol.%) has been extensively studied, whereas, the commercial Solar salt with a composition (NaNO3-KNO3, 60– 40 wt.%) has sparse data reported, because the viscosities of the two compositions were supposed to be basically the same (Serrano-Lopez et al., 2013). Viscosity of the equimolar mixture NaNO3-KNO3 has been measured by Murgulescu and Zuca (1969) utilizing an improved damped oscillating sphere method and by Coscia et al. (2012) with a Rheometric ARES Rheometer using a fixed Couette, and the data of viscosity from Mar et al. (1982) and Nissen (1982) were also presented. Different compositions of the emerging ternary nitrates system have been investigated because different phase diagrams have been published in the open literature (Menzies and Dutt, 1911; Jänecke, 1942; Levin et al., 1956; Gomez et al., 2013), however, few sporadic viscosity data have been obtained (Menzies and Dutt, 1911; Jänecke, 1942; Bergman et al., 1955; Levin et al., 1956; Kearney et al., 2003; Brosseau et al., 2004; Bradshaw, 2010; St. Laurent et al., 2000; Siegel et al., 2011). The viscosity data of these molten nitrates/nitrites mixtures used for CSP systems are still insufficient although they have been studied by many researchers with experimental measurements and calculations. Firstly, viscosity data of molten nitrates/nitrites have particularly large deviation among different researchers reported in open literatures in that the low viscosity measurement of molten nitrates/nitrites are difficult under high temperature. For example, for Hitec, the maximum deviation was more than 50% (at 700 K). Moreover, the measurement errors were wrongly defined or not referred in some experimental data. Secondly, the viscosity
data of these molten nitrates/nitrites mixtures are seriously insufficient. Many viscosity data are lacking under condition close to melting point, which can provide significant parameters for the safety analysis of molten salt thermo-hydraulic loop under abnormal working conditions. In addition, viscosity data of the emerging ternary nitrates, especially the components in this work, are still nonexistent at high temperature. In present paper, accurate viscosity measurements of the molten nitrates/nitrites were systematically conducted using an optimized rotational coaxial cylinder in a wide temperature range. The viscosity data of extensively used molten Hitec and Solar salt were detailed summarized and reassessed based on the experimental data. The viscosity of the emerging ternary nitrates was systematically analyzed. The reliability and accuracy of these viscosity data were analyzed and evaluated. These reliable viscosity results fill in the blanks of the existing data, which is beneficial to the design of CSP systems. 2. Experimental procedure 2.1. Sample preparation Table 1 shows the components of binary and ternary nitrates/ nitrites molten salts for measurements in this paper. The molten salt mixtures were prepared from NaNO3, NaNO2, KNO3, LiNO3 and Ca(NO3)2-4H2O salts (Analytical reagent). These salts were separately dried at 400 K under argon atmosphere for 24 h, and then mixed in graphite crucibles. The mixed nitrates/nitrites were heated to 500 K, and keep this temperature for 24 h in the argon atmosphere. For calcium nitrate containing salts, other than the separately dried, the salts were heated to 620 K under argon atmosphere for 24 h to allow the water of hydration of the calcium nitrate constituent to evolve slowly. 2.2. Measurement apparatus The viscometer used in this work is based on coaxial cylinder method, shown in Fig. 1. The viscometer mainly consists of four fundamental systems: the measuring unit, the spindle and crucible, a high temperature furnace, and other auxiliary system. This measuring unit is improved on the basis of Brookfield DVIII. With an aim to guarantee the turbulence nonexistence in the sample liquids during measurement process, suitable rotational speed and the sizes of spindle and crucible were selected through calculations and pre-experiments. The crucible with inner diameter of 29 mm and the depth of 220 mm, and the spindle with diameter of 26 mm and length of 150 mm were made of graphite. Under such conditions, the measurement range was 1.6–16 mPa s in a speed of 30 Revolutions Per Minute (RPM). In order to get the exact temperature of molten sample, a K-type thermocouple was put into the sample fluid. For rotating cylinder viscometer, the viscosity is obtained by formula (2) derived from formula (1) which is deduced by using . SMC ¼ r12 R12 120ðh þ cÞ. The standard sample with viscosity
Table 1 Compositions and melting point of the binary and ternary nitrates/nitrites mixtures system in this work. Composition Hitec Commercial solar salt Equimolar NaNO3-KNO3 LiNO3-NaNO3-KNO3 Ca(NO3)2-NaNO3-KNO3 a Ca(NO3)2-LiNO3-KNO3
LiNO3 (wt.%)
23.4 21.7
NaNO3 (wt.%)
NaNO2 (wt.%)
KNO3 (wt.%)
7 60 45.7 17.3 16
40
53 40 54.3 59.3 48 58.3
Ca(NO3)2 (wt.%)
Melting point (K)
36 20
415 511 495 413 406 390
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Y. Jin et al. / Solar Energy 137 (2016) 385–392 Table 2 The testing results of Cannon S6 oils (certified viscosity standard values). Temperature (K)
Measured (mPa s)
Standard (mPa s)
Deviation (%)
298 313 323 353
7.76 4.91 3.76 2.02
7.897 4.914 3.765 2.003
1.73 0.08 0.13 +0.85
researchers always choose potassium nitrate as viscosity reference fluid to test and verify the reliability of this high temperature viscometer. In this work, the viscosity of KNO3 molten salt was measured and the results are shown in Table 3. Compared with previous data, the deviation of the values obtained is within the error ranges. This indicates that the viscometer is reliable, and feasible for the molten salts nitrates/nitrites mixtures. This optimized viscometer is a kind of traditional standard method, which can work within the whole ranges just through calibrating the SMC using a Newtonian fluid of known viscosity. The input parameters used to calculate the viscosity are SMC, M and RPM, accurate temperature measurement of the sample liquids is necessary for the fitting curves. 2.3. Uncertainty analysis
Fig. 1. The viscometer based on rotating cylinder method.
go was used to calibrate the Spindle multiplier constant (SMC) through formula (3). If the spindle speed could be obtained, the sample viscosity can be computed by measuring the torque (M).
g¼
M 1 1 2 2 4pðh þ cÞx r R
ð1Þ
g¼
SMC M RPM
ð2Þ
SMC ¼
RPM go M
ð3Þ
where h is the length of spindle (m), r is the radius of spindle (m), x is the angular velocity (rad/s); and c is the correction factor of the spindle (m); R is the radius of the Crucible (m). M is moment of torque (N mm); The spindle multiplier constant (SMC) is calculated using the formula (3), where go is a known viscosity of the Newtonian fluid (mPa s); RPM is revolutions per minute (r/min). The measurement process was as follows: the molten salt sample was heated to target temperature and keep it at least 60 min. After that, move down the spindle until desired setting position, guaranteeing that the spindle immerge into the sample completely, then adjust the speed to 30 RPM. Finally, decrease temperature gradually and record the temperature and viscosity. A low temperature standard sample certified by NIST, Cannon S6 viscosity standard oil, was chosen to verify the accuracy of the instrument in measuring ranges. The results of the evaluation are shown in Table 2. The deviation between the measured values and the standard values are less than ±2%. Viscosity of potassium nitrate (KNO3) has been reported by many authors through different techniques (Timidei et al., 1970; Janz et al., 1968; Yoshiyoki et al., 1980; Zuca, 1970; Ohta et al., 1975; Tolbaru et al., 1998; Wellman et al., 1966; Zuca and Borcan, 2002), and the results show good agreement. So the
To analyze the uncertainty of the experiment data, three steps should be performed. Firstly, the uncertainty sources (x1 ; x2 ; x3 ; . . .) must be confirmed. Secondly, the standard uncertainty (lx1 ; lx2 ; lx3 ; . . .) is calculate, including two types, i.e. Type A and Type B. Type A is based on statistics, generally, standard deviation is the uncertainty. For Type B, the uncertainty is evaluated on the basis of experience including calibration certificates, certificate of identification, and so on. Finally, the expanded uncertainty is represented as the standard uncertainty multiplied by a confidence factor, which is evaluated according to Eq. (4) (Li, 2014).
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi XN @f 2 XN @f @f 2 U¼k ðuxi Þ þ 2 16i
ð4Þ
where U is the combined uncertainty; uxi and uxj represent the standard uncertainty components caused by xi and xj, respectively; f represents the function of parameters measured in this work; eij is the correlation coefficient between xi and xj; k is the confidence factor, generally, k takes the value of 2 (confidence level = 95%). Firstly, uncertainty sources of viscometer include SMC, M, RPM, T and the standard sample named S6 (a standard viscosity oil certified by NIST) according to formula (1)–(3). Secondly, the standard uncertainty of each source (SMC, M, RPM, T and S6) is calculated as followed: (1) The standard uncertainty of SMC is calculated by using the S6 oil whose viscosity is 7.897 mPa s in 298 K, uncertainty is 0.1%. The standard error was calculated by Bessel functions
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 1 ðXi XÞ s1 ¼ ¼ 0:0083 N1
l1 ¼ s1
.pffiffiffiffi N ¼ 0:00262
The relative uncertainty is:
l10 ¼ lX1 ¼ 0:00262 ¼ 0:5%. 0:54
(2) The standard uncertainty from M was computed (type A) by using molten salt samples.
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Table 3 The testing results of potassium nitrate. Temperature (K)
Measured viscosity (mPa s)
Literature value (mPas) Ohta, T.
713.9 703.7 693.5 683.3 672.1 663 652.8 641.5 631.2 621.9 611.6 a
1.79 1.84 1.95 2.03 2.12 2.20 2.35 2.47 2.58 2.77 2.87
a
1.80 1.85 1.93 2.01 2.10 2.19 2.31 2.45 2.60 2.77 2.88
Deviation range (mPa s)
Wellman, R.E.
b
1.75 1.82 1.90 1.98 2.08 2.17 2.29 2.44 2.58 2.73 2.91
Janz, G.J.
b
1.72 1.80 1.89 1.99 2.10 2.20 2.32 2.47 2.61 2.75 2.92
Timidei, A. 1.73 1.81 1.90 1.99 2.10 2.20 2.32 2.47 2.61 2.75 2.91
b
0.07 to +0.01 0.04 to +0.01 0.06 to 0.02 0.05 to 0.02 0.04 to 0.02 0.03 to 0 0.06 to 0.03 0.02 to 0 0 to +0.02 0.04 to 0 +0.01 to +0.05
Oscillation cup method. Capillary method.
b
The standard error s2 was calculated by Bessel functions
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 1 ðXi XÞ s2 ¼ ¼ 11:11 N1
l2 ¼ s2
.pffiffiffiffi N ¼ 3:93 N mm
The relative uncertainty is:
3:93 l20 ¼ lX2 ¼ 368:44 ¼ 1:1%.
(3) The standard uncertainty of RPM (type B, calibration certificates): The precision of stepping motor is less than 0.034%. The impact on the viscosity test is:
l3 ¼ 0:0102 r=min
l4 ¼ 0:020 mPa s
(5) Standard uncertainty identification)
l5 from S6 (type B, certificate of
l5 ¼ 0:00663 mPa s
Finally, the expanded uncertainty is calculated according to Eq. (4). SMC, M, RPM, T and S6 are independent, so eij is 0.
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 2 2 @f @f @f 2 2 l þ l þ l þ l þ l 5 4 @x1 1 @x2 2 @x3 3 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 ¼ 2 0:0332 þ 0:0707 þ 0:0023 þ 0:02 þ 0:006632
U¼k
¼ 0:162 mPa s The sample viscosity is 6.63 mPa s, so the relative uncertainty 0:162 ¼ 2:5% (see Tables 4–6). 6:63 The first parameter SMC was calculated from the experiment according to type A evaluation method, and estimated to be less than 0.5%. The standard uncertainty of torque was 1.1% obtained from the experiment data according to type A evaluation method,
is:
Table 4 The data of KNO3 from each investigation were fitted to these equations by a least squares technique. Author
Fitting curves
This work
g ¼ 0:16315 expð1:409 103 =T þ 2:134 105 =T 2 Þ g ¼ 35:27437 expð5:797 103 =T þ 2:621 106 =T 2 Þ g ¼ 1:37436 expð1:566 103 =T þ 1:24 106 =T 2 Þ g ¼ 0:05647 expð2:584 103 =T 1:038 105 =T 2 Þ g ¼ 0:04492 expð2:918 103 =T 2:237 105 =T 2 Þ
Wellman, R.E. Janz, G.J. Timidei, A.
3. Results and discussion 3.1. Hitec
(4) Standard uncertainty l4 from T (type B, calibration certificates): The precision of the thermocouple is less than 0.5 K. The impact on the viscosity test is:
Ohta, T.
Furthermore, the synchronous motor performed 0.034% on the basis of its precision introductions. The temperature was directly measured every time to obtain the actual temperature. Uniform temperature profiles of the K-type thermocouple within 0.5 K, and the standard uncertainty estimated to be less than 0.3% according to type B evaluation method. The standard uncertainty from S6 was 0.1%. So it is considered that the uncertainty accompanied by the measurement in high temperature is estimated to be less than 2.5% (listed in Table 7).
Hitec is a ternary mixture of alkali-nitrates/nitrites and is commonly used as HTF and thermal storage material in solar thermal systems. The advantage of Hitec is that its melting point (415 K) is much lower and the thermal conductivity is almost twice that of organic HTFs (Boerema et al., 2012). The thermal stability results show that Hitec salt is stable with very limited decomposition at temperatures below 773 K (Peng et al., 2010). The experimental results from Coastal Chemical Co. LLC show that the seamless stainless steel tubing had no visible corrosion attacks after being immersed in continuous flow of Hitec for 10 months at 772–822 K. Viscosity of Hitec has been measured through different methods including rotating cylinder method, capillary method and oscillation cup method, and the results are shown in Fig. 2. (Coastal Chemical Co., 2011; Kirst et al., 1940; Geiringer, 1962; Singh, 1985; Gaune, 1982; Cohen and Jones, 1957; McDuffie et al., 1963; Wu et al., 2012; Yang and Garimella, 2010; Bohlmann, 1972). Comprehensive analysis of the viscosity results of Hitec from different open literatures were presented in Table 8. Even if experimental data obtained by different authors are available under the normal operating temperature (470–770 K), a sizeable disagreement was commonly found, and the maximum deviation was more than 50% (at 700 K). It can be seen obviously from Fig. 2 that the viscosity data of Hitec is lack close to melting point (<465 K) which can provide significant parameters for the safety analysis of CSP systems under abnormal condition, while large deviations will appear if extending the fitting curves directly from working areas to the melting point. Therefore, it is necessary to measure the viscosity in this temperature range. For viscosity measurement, the error is an essential part of the viscosity data. However, to be best of our knowledge, there are some reported data without test error or with a wrong error regarding to Hitec (listed in Table 8). Such as, density (the error more than 2%) is essential in process of data for oscillation cup
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Y. Jin et al. / Solar Energy 137 (2016) 385–392 Table 5 Calculation of the standard uncertainty from SMC by using the S6 oil. Number
1
2
3
4
5
6
7
8
9
10
SMC Xi X
0.549 0.009
0.531 0.009
0.542 0.002
0.547 0.007
0.539 0.001
0.537 0.003
0.531 0.009
0.528 0.012
0.547 0.007
0.551 0.011
Table 6 Calculation of the standard uncertainty from M by using molten salt samples. Number
1
2
3
4
5
6
7
8
M Xi X
372.88 4.48
381.38 12.98
366.22 2.18
360.88 7.52
352.42 15.98
384.37 15.97
370.87 2.47
358.55 9.85
work, and can be fitted by Eq. (5), Adjusted R2 = 99.94% (R is regression coefficient):
Table 7 Error of viscosity caused by the deviation of physical parameters. Sources
Uncertainty (%)
Spindle multiplier constant Moment of torque Revolutions per minute Temperature S6 sample Total
0.5 1.1 0.034 0.3 0.1 2.5
g ¼ 1:149 exp 810:896=T þ 7:806 105 =T 2
ð5Þ
The maximum of measured value is 14.2 mPa s (in 420 K) within the scope of the instrument test. The maximum of measured temperature is 695 K, which can basically satisfy the engineering application in CSP systems. The results nearly coincide well with the sole Coastal Chem. Brochure data (USA) in the low temperature areas (<465 K). To evaluate the deviation, the averaged value of reported data including the results of our works is used as a reference in Fig. 3. As shown in Fig. 2, the tendency of the g T curve shows a good agreement among different reported data, although it has some certain deviation among different reports. 3.2. Binary nitrate salts
Fig. 2. Viscosity of Hitec varying with temperature.
method and capillary method, while the error from density if often ignored. To solve these questions, accurate viscosity data of Hitec with 2.5% uncertainty in a wide range of 420 (close to melting point) to 695 K were given with an interval almost ten degrees in this
The phase diagram for the NaNO3-KNO3 system indicate that there exists a eutectic point of 45.7 wt.%, NaNO3 and 54.3 wt.% KNO3 (50–50 mol%) at temperatures of 495 K (Coscia et al., 2012). In order to reduce the cost in modern CSP systems, the cheaper commercial mixture Solar Salt (NaNO3-KNO3, 60–40 wt.%) was prepared. It is used in the Solar Two central receiver system located in California and several other solar plants in Spain (Kuravi et al., 2013). It melts at 511 K and remains in thermally stable liquid phase at temperatures up to 873 K. Viscosity of the equimolar binary NaNO3-KNO3 eutectic salt has been studied by many researchers with experiment, meanwhile, the data and fitting curves of viscosity parameters were also presented in Fig. 4 and Table 9. Commercial compositions Solar salt has similar thermo-physical properties with the equimolar component salt, and the differences of viscosity between equimolar and commercial compositions are negligible in previously reported literatures (Serrano-Lopez et al., 2013).
Table 8 The viscosity of Hitec varying with temperature can be fitted by the following equation, where g is viscosity, mPa s; T absolute temperature, K. Method
Temperature (K)
Fitting curves
Error
Rotational coaxial cylinder method
420–695
2.5%
Rotating method Ostwald viscosimeter
423–773 455–780
Oscillating viscosimeter
457–755
g ¼ 1:149 expð810:896=T þ 7:806 105 =T 2 Þ g ¼ ðT 273Þ^ 2:104 10^ 5:7374 g ¼ 67:57 103 expð2247:11=TÞ h i g ¼ 0:5631 exp 146:9794T 1 þ ð57:4265 104 T 2 Þ g ¼ 0:75484 expð6418=RTÞ g ¼ 1000 exp ½4:343 2:0143ðlnðT 273Þ 5:011Þ
Oscillation cup method Oscillation cup method a b c
523–723 520–773
Error: the reliability of the test data was defined using different methods, such as error, deviation or uncertainty. Accuracy. Maximum error.
a
Reference This work Coastal Chem. Brochure Kirst et al. (1940) Gaune (1982)
±5.0% 0.5% c
b
Chen et al. (2011) Costal Chem. Co.
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Y. Jin et al. / Solar Energy 137 (2016) 385–392
Fig. 3. Viscosity of Hitec deviation compared with the average values. Fig. 5. Viscosity of multi-component nitrate varying with temperature.
g ¼ 5:103 exp 2:575 103 =T þ 1:305 106 =T 2
ð7Þ
As shown in Fig. 4, the negligible differences were found for viscosity between the commercial compositions and the equimolar compositions (less than 0.2 mPa s). The experimental values of the equimolar component salt are in good agreement with that of Murgulescu and Zuca (1969), Nissen (1982) and Mar et al. (1982). Apparently, the equation given by Coscia et al. (2012) shows the high viscosity behavior of nitrates in low temperature areas. 3.3. Viscosity of the multi-component nitrate salts
Fig. 4. Viscosity of binary nitrate salts varying with temperature.
Accurate viscosity of these binary nitrate salts were systematically measured with an interval almost ten degrees in this work. For the equimolar salt, experimental data was given in the range of 506 K close to melting point (495 K) to 713 K, and can be fitted by Eq. (6), Adjusted R2 = 99.96%:
g ¼ 2:551 exp 1:65 103 =T þ 1:022 106 =T 2
ð6Þ
For the commercial compositions, experimental data was given in the range of 516 K close to melting point (511 K) to 720 K, and can be fitted by Eq. (7), Adjusted R2 = 99.72%:
Multi-component nitrate molten salts mixtures formulated from the nitrates of sodium, potassium, lithium and calcium display liquidus temperatures lower than that of Hitec. It can be observed that the addition of a third salt (LiNO3) to the constituents of Solar Salt (NaNO3 + KNO3) increases the thermal stability of the multi-component nitrate salt, moreover, the addition of Ca(NO3)2 into the mixture reduces the melting points and the economic cost of these mixtures (Fernandez et al., 2014). Viscosity of these nitrates/nitrites salts with varying temperature was given in Fig. 5. Fitting curves equations in Table 10. For Hitec XL (Ca(NO3)2-NaNO3-KNO3), different phase diagrams have been published, and the ternary diagram has one ternary eutectic and one ternary peritectic point (Menzies and Dutt, 1911; Jänecke, 1942; Levin et al., 1956). The eutectic point is the minimum of temperature where a liquid temperature exists (406 K) and the peritectic point is the second intersection point
Table 9 The viscosity of Binary nitrate salts varying with temperature can be fitted by the following equation, where g is viscosity, mPa s; T absolute temperature, K. Method
Temperature (K)
Fitting curves
Error
Rotational coaxial cylinder method
516–720
Rotational coaxial cylinder method
506–713
Rheometric ARES rheometer Oscillation cup method
495–820 548–873
g ¼ 5:103 expð2:575 10 =T þ 1:305 10 =T Þ g ¼ 2:551 expð1:65 103 =T þ 1:022 106 =T 2 Þ g ¼ expð2210=T 2:48Þ g ¼ 22:714 0:12 ðT 273Þ þ ð2:281 104 Þ ðT 273Þ2
Damped oscillating sphere method
523–723
g ¼ 0:4384 expð9:2163=T þ 6:52 105 =T 2 Þa
3
6
2
b
Reference
2.5%
Commercial solar saltd
2.5%
Equimolar NaNO3-KNO3d
1%
c
Coscia et al., 2012 Nissen (1982)
ð1:474 107 Þ ðT 273Þ3
a b c d
The data from Murgulescu were fitted to this equation. Error: the reliability of the test data was defined using different methods, such as error, deviation or uncertainty. Uncertainty. This work.
Murgulescu and Zuca (1969)
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Table 10 The viscosity of these molten salts nitrates/nitrites varying with temperature can be given by the following equation, where g is viscosity, mPa s; T absolute temperature, K. Composition LiNO3-NaNO3-KNO3 Ca(NO3)2-NaNO3-KNO3 Ca(NO3)2-LiNO3-KNO3
Viscosity (mPa s) g ¼ 3:355 exp 2:038 103 =T þ 1:109 106 =T 2 g ¼ 19:376 exp 4:215 103 =T þ 1:997 106 =T 2 g ¼ 1:226 exp 8:479 102 =T þ 9:317 105 =T 2
Temperature range (K) 418–645 503–670 465–705
close to 433 K (Gomez et al., 2013). Ternary mixture Hitec XL in this work (the ternary eutectic) was considered to be the best composition by Gomez et al. (2013). Viscosity data of Hitec XL was firstly measured at the high temperature (503–670 K), which has been reported by Bradshaw (2010) in low temperature areas (420–500 K) using a Brookfield DV-II+ viscometer ever. Through Fig. 5, the experimental results trend nearly to coincide with the data of Bradshaw (2010), and the fitting curve equation was given by Eq. (8), Adjusted R2 = 99.87%:
g ¼ 19:376 exp 4:215 103 =T þ 1:997 106 =T 2
ð8Þ
The eutectic point of LiNO3-NaNO3-KNO3 mixture salt melts is at about 413 K, while Ca(NO3)2-LiNO3-KNO3 mixture salt melts at about 390 K (Mcmurdie and Hall, 1949). The viscosity data in low temperature areas have been also reported by Bradshaw (2010), and the viscosity data function relationships has also been given. Low viscosity data of these multi-component nitrates were firstly systematically recorded in this work. For emerging ternary candidate LiNO3–NaNO3–KNO3 mixture salts, viscosity shows excellence. The maximum of measured value performed 15.5 mPa s in 418 K (close to melting point), and the viscosity data were given approaching to 645 K. The fitting curve equation was given by Eq. (9), Adjusted R2 = 99.96%:
g ¼ 3:355 exp 2:038 103 =T þ 1:109 106 =T 2
ð9Þ
For emerging ternary candidate Ca(NO3)2-LiNO3-KNO3 mixture salt, the melting temperature is much lower than that of even Hitec XL. Viscosity data of this ternary mixture were recorded from 465 to 705 K with an interval almost ten degrees. Obviously, it is less than that of Hitec XL. The fitting curve equation was given by following Eq. (10), Adjusted R2 = 99.76%:
g ¼ 1:226 expð8:479 102 =T þ 9:317 105 =T 2 Þ
ð10Þ
3.3.1. Contrastive analysis Compared with these molten nitrates/nitrites in Fig. 6, the viscosity of binary nitrate salts, ternary Hitec and multi-component LiNO3–NaNO3–KNO3 salt shows the same trend and excellent properties. Due to the presence of calcium nitrate, the viscosity of ternary mixtures Ca(NO3)2-LiNO3-KNO3 salt and Hitec XL increases dramatically in low temperature range (<650 K). However, the viscosity is no longer affected intensely by the increase of calcium nitrate in higher temperature, and the viscosity of all these salts behaves seem that will be the same (above 700 K). This may due to the presence of divalent Ca ion which can increase the coulomb force or lead to complexation, further investigation on it will be conducted in the near future. Main advantage of these ternary nitrates mixture is that their melting points are lower compared to that of the Solar Salt and Hitec. The problem of freezing is easier to be controlled with lower melting points. Also, these ternary nitrates mixture show temperature stability up to 773 K (Gomez et al., 2013; Fernandez et al., 2014), suggesting potential suitability as a HTF in CSP systems. Due to the addition of LiNO3 salt, ternary LiNO3–NaNO3–KNO3 salt seems more feasible in CSP systems. Unfortunately, LiNO3 salt is
Fig. 6. Contrastive viscosity analysis of these salts varying with temperature.
relatively expensive that may be a barrier for large scale application. Costs of the industrial grade nitrate salts follow the order of Li > K > Ca > Na in the Chinese market. Although the addition of calcium nitrate to molten salt increases the viscosity, the cost of calcium nitrate containing salts is lower than Li/K nitrates salts and the viscosity of these calcium nitrate containing salts are below 5 mPa s over 590 K which can meet the requirements of engineering application. So they also have feasibility as heat transfer media in CSP systems.
4. Conclusions Viscosity data of molten Hitec and Solar salt were explicitly summarized and reassessed on the basis of the experimental data. Experimental deviation and errors were discussed, and it presented a good result for our experimental data compared with the reported ones. In addition, the problem of viscosity data being lack close to melting point was solved in this paper. Viscosity of the certain components emerging ternary nitrates Hitec XL in the range of 503–670 K, LiNO3–NaNO3–KNO3 salt in the range of 418–645 K and Ca(NO3)2-LiNO3-KNO3 salt in the range of 465– 705 K were measured with an interval of almost ten degrees using this optimized rotational coaxial cylinder viscometer for the first time. The viscosity of lithium nitrate containing salts shows similarity with Hitec and Solar salt, and the addition of lithium nitrate to molten salt had relatively little effect on viscosity. However, it is apparent for calcium nitrate containing salts that calcium nitrate will increase the viscosity of the molten salt.
Acknowledgement The authors are grateful to acknowledge the financial supported by the ‘‘Strategic Priority Research Program” of Chinese Academy of Sciences, Grant No. XD02002400, Youth Innovation Promotion
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Association CAS and the National Natural Science Foundation of China (21406256). References Bergman, A.G., Rassonskaya, I.S., Shmidt, N.E., 1955. Izvest. Sektora. Fiz.-Khim. Anal., Inst. Obshch. Neorg. Khim. Tr. Fiz. Inst. Akad. Nauk SSSR 26, 156–163. Boerema, N., Morrison, G., Taylor, R., Rosengarten, G., 2012. Liquid sodium versus Hitec as a heat transfer fluid in solar thermal central receiver systems. Sol. Energy 86 (9), 2293–2305. Bohlmann, E.G., 1972. Heat Transfer Salt for High Temperature Steam Generation. Reactor Chemistry Division. Bradshaw, R.W., 2010. Viscosity of Multi-Component Molten Nitrate Salts-Liquidus to 200 °C, Sandia National Laboratory, Technical Report No. SAND2010-1129, Livermore, CA. Brosseau, D.A., Hlava, P.F., Kelly, M.J., 2004. Testing of thermocline filler materials and molten-salt heat transfer fluids for thermal energy storage systems used in Parabolic Trough Power Plants. Technical Report No. SAND2004-3207. Chen, Y.C., Wu, Y.T., Ren, N., Ma, C.F., 2011. Experimental study of viscosity characteristics of high-temperature heat transfer molten salts. Sci. China Technol. Sci. 54 (11), 3022–3026. Cohen, S.I., Jones, T.N., 1957. Viscosity Measurements on Molten Fluoride Mixtures. Oak Ridge National Laboratory, ORNL-2278.. Coscia, K., Neti, S., Oztekin, A., Nelle, S., Mohapatra, S., Elliott, T., 2012. The thermophysical properties of the NaNO3-KNO3, LiNO3-NaNO3, and LiNO3-KNO3 systems. In: ASME International Mechanical Engineering Congress and Exposition. IMECE2011. November 11–17, Denver, Colorado, USA. Coscia, K., Nelle, S., Elliott, T., Mohapatra, S., Oztekin, A., Neti, S., 2013. Thermophysical properties of LiNO3-NaNO3-KNO3 mixtures for use in concentrated solar power. J. Sol. Energy Eng. 135 (3), 1–5. Dunn, R.I., Hearps, P.J., Wright, M.N., 2012. Molten-salt power towers: newly commercial concentrating solar storage. Proc. IEEE 100 (2), 504–515. Fernandez, A.G., Ushak, S., Galleguillos, H., Perez, F.J., 2014. Development of new molten salts with LiNO3 and Ca(NO3)2 for energy storage in CSP plants. Appl. Energy 119, 131–140. Frank, G.F.Q., Yang, X.P., Ding, Z., Zuo, Y.Z., Shao, Y.Y., Jiang, R.H., Yang, X.X., 2012. Thermocline stability criterions in single-tanks of molten salt thermal energy storage. Appl. Energy 97, 816–821. Gaune, P.G., 1982. Viscosity of potassium nitrate–sodium nitrite–sodium nitrate mixtures. J. Chem. Eng. Data 27 (2), 151–153. Geiringer, P.L., 1962. Handbook of Heat Transfer Media (Heat Transfer Salt). RE Krieger Pub Co., Huntington. NY, pp. 208–213. Gomez, J.C., Calvet, N., Starace, A.K., Glatzmaier, G.C., 2013. Ca(NO3)2-NaNO3-KNO3 molten salt mixtures for direct thermal energy storage systems in parabolic trough plants. J. Sol. Energy Eng. 135 (2), 021016. HITEC Heat Transfer Salt. Costal Chemical Co., L.L.C., Brenntag Company.
Levin, E.M., McMurdie, H.F., Hall, F.P., 1956. Phase Diagrams for Ceramists, vol. 1.. The American Ceramic Society, Columbus, OH. Li, D.S., 2014. Fundamental of Metrology. China machine press, Beijing (China) (in Chinese). Mar, R.W., Bradshaw, R.W., Carling, R.W., Goods, S.H., Nagelberg, A.S., Nissen, D.A., 1982. Progress report: molten nitrate salt technology development, Sandia National Laboratories, SAND82-8220. McDuffie, H.F., McCoy, H.E., Robertson, R.C., Dunlap, S., Thoma, R.E., 1963. Assessment of Molten Salts as Intermediate Coolants for LMFBR’s. Oak Ridge National Laboratory. Mcmurdie, H.F., Hall, F.P., 1949. Phase diagrams for ceramists: supplement NO. 1. J. Am. Ceram. Soc. 32, 154–164. Menzies, A.W.C., Dutt, N.N., 1911. The liquidus surface of the ternary system composed of the nitrates of potassium, sodium, and calcium. J. Am. Chem. Soc. 33 (8), 1366–1375. Mill, K.C., 1995. Viscosity of Molten Slags. Slag Atlas, Düsseldorf, Germany, second ed., pp. 349–402. Murgulescu, I.G., Zuca, S., 1969. Viscosity of binary mixtures of molten nitrates as a function of ionic radius-II. Electrochim. Acta 14, 519–526. Nissen, D.A., 1982. Thermophysical properties of the equimolar mixture NaNO3KNO3 from 300 to 600 °C. J. Chem. Eng. Data 27 (3), 269–273. Nunes, V.M.B., Lourenco, M.J.V., Santos, F.J.V., de Castro, C.A.N., 2003. Importance of accurate data on viscosity and thermal conductivity in molten salts applications. J. Chem. Eng. Data 48 (3), 446–450. Ohta, T., Borgen, O., Brockner, W., 1975. High-temperature viscometer for fluid liquids part I: on-line computer-facilitated torsion pendulum. Berichte der Bunsengesellschaft fur physikalische chemie 79 (4), 335–344. Peng, Q., Ding, J., Wei, X.L., Yang, J.P., Yang, X.X., 2010. The preparation and properties of multi-component molten salts. Appl. Energy 87 (9), 2812–2817. Serrano-Lopez, R., Fradera, J., Cuesta-Lopez, S., 2013. Molten salts database for energy applications. Chem. Eng. Process. 73, 87–102. Siegel, N.P., Bradshaw, R.W., Cordaro, J.B., Kruizenga, A.M., 2011. Thermophysical property measurement of nitrate salt heat transfer fluids. In: ASME 5th International Conference on Energy Sustainability, Washington, pp. 439–446. Singh, J., 1985. Heat Transfer Fluids and Systems for Process and Energy Applications: Molten Salts. M Dekker Ins, New York, pp. 223–240. St. Laurent, S.J., Kolb, W.J., Pacheco, J.E., 2000. Thermocline Thermal Storage Tests for Large-Scale Solar Thermal Power Plants. Sandia National Laboratory, Albuquerque, NM, Technical Report No. SAND2000-2059C. Timidei, A., Lederman, G., Janz, G.J., 1970. Accurate molten salt viscometry. Instrum. Sci. Technol. 3 (2), 309–320. Tolbaru, D., Borcan, R., Zuca, S., 1998. Viscosity measurements on molten salts with an oscillating cup viscometer: viscosity of molten KNO3 and NaCl. Berichte der Bunsengesellschaft fur physikalische chemie 102, 1387–1392. Vignarooban, K., Xu, X., Arvay, A., Hsu, K., Kannan, A.M., 2015. Heat transfer fluids for concentrating solar power systems – a review. Appl. Energy 146, 383–396. Wellman, R.E., Dewitt, R., Ellis, R.B., 1966. Viscosity of potassium nitrate. J. Chem. Eng. Data 11, 156–158. Wu, Y.T., Chen, C., Liu, B., Ma, C.F., 2012. Investigation on forced convective heat transfer of molten salts in circular tubes. Int. Commun. Heat Mass 39, 1550– 1555. Yang, Z., Garimella, S.V., 2010. Thermal analysis of solar thermal energy storage in a molten-salt thermocline. Sol. Energy 84 (6), 974–985. Yoshiyoki, A., Kosugiyama, O., Miyajima, H., 1980. Determination of the viscosity of molten potassium nitrate with an oscillating cup viscometer. J. Chem. Soc. 76, 2531–2541. Zuca, S., 1970. Viscosity of some molten nitrates. Rev. Roum. Chim. 15 (9), 1277– 1286. Zuca, S., Borcan, R., 2002. Viscosity of binary mixtures of molten nitrates as a function of ionic radius-III⁄. Electrochim. Acta 11 (15), 1817–1825.