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Lithium nitrate purity influence assessment in ternary molten salts as thermal energy storage material for CSP plants
Journal Pre-proof Lithium nitrate purity influence assessment in ternary molten salts as thermal energy storage material for CSP plants
Mauro Henríquez, Luis Guerreiro, Ángel G. Fernández, Edward Fuentealba PII:
S0960-1481(19)31566-6
DOI:
https://doi.org/10.1016/j.renene.2019.10.075
Reference:
RENE 12444
To appear in:
Renewable Energy
Received Date:
19 December 2018
Accepted Date:
13 October 2019
Please cite this article as: Mauro Henríquez, Luis Guerreiro, Ángel G. Fernández, Edward Fuentealba, Lithium nitrate purity influence assessment in ternary molten salts as thermal energy storage material for CSP plants, Renewable Energy (2019), https://doi.org/10.1016/j.renene. 2019.10.075
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
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Lithium nitrate purity influence assessment in ternary molten salts as thermal energy storage material for CSP plants Mauro Henríquez1,2, Luis Guerreiro3, Ángel G. Fernández1*, Edward Fuentealba1 1 Centro
2
de Desarrollo Energético Antofagasta (CDEA), Universidad de Antofagasta, Av. Universidad de Antofagasta 02800, Antofagasta, Chile
Universidad de Almería, Crta. De Sacramento s/n, E04120 La Canada de San Urbano, Almería, Spain 3
Univ. Evora, Renewable Energies Chair, Palacio Vimioso, 7002 Evora, Portugal
* Corresponding
author Email: [email protected]
Abstract
The addition of lithium nitrate is assumed to improve the performance of molten salts, extending the work temperature range. This paper presents an evaluation of the influence of different degrees of purity of LiNO3, in a ternary mixture with composition 30wt%LiNO3 + 13wt%NaNO3 + 57wt% KNO3 including a Chilean mixture obtained from the Atacama Desert brines. In addition, the use of synthetic lithium nitrate obtained from the chemical synthesis using Li2CO3 and HNO3, was incorporated in the comparison. The melting point results of the 30wt% LiNO3
+ 13wt% NaNO3 + 57wt% KNO3 mixture for different purities (128 ° C, 124 ° C), show a reduction of 92 to 96 ° C with respect to the 223 ° C of the solar salt and thermal stability results show maximum temperature are around 594 and 596 ° C, which means that this mixture could work at maximum operating temperatures similar to those of solar salt. Finally, it was also determined that for the use of the proposed ternary mixture would mean a reduction of 35% in the volume of inventory with respect to solar salt since the proposed ternary salt presents an improvement about 14-21% in the heat capacity. Keywords: Thermal Energy Storage; Lithium Nitrate; Concentrated Solar Power 1.
Introduction During the last years, Chile has been one of the most attractive solar markets due to
its excellent solar conditions, in particular the Atacama Desert, which presents an annual 1
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global radiation value of 2571 kWh/m2 and an index of direct normal irradiance (DNI) of 3389 kWh/m2. This location is among the best worldwide for capturing and storing solar energy [1] and thus Concentrated Solar Power (CSP) rises as a promising solution. These features are of major interest when considering the extensive saline deposits that are present in this region of north Chile with a great potential to be used as energy storage material [2]. The current second-generation CSP technology was developed from the pilot project called Solar Two with a capacity of 10 MWe developed in the Mojave desert in the USA by the Department of Energy (DOE) that incorporated a thermal storage system of two tanks with salts melted (60% de NaNO3 and 40% de KNO3), which was a significant improvement to the initial project called Solar One, which considered a thermal storage system in a single tank filled with rocks and sand, using oil as heat transfer fluid (HTF) [3]. In 2012, the US Department of Energy published the objective of reducing it to 6 c/kWh by 2020 through its Sunshot Vision Study. If the behavior of technology is analyzed in recent years the cost for a CSP plant has dropped from 20,6 ¢/kWh in 2010 to 12 ¢/kWh in 2015 and 10 ¢/kWh in 2017 [4-5]. The aim fixed for Chile by 2025 is to achieve an LCOE of 5 ¢/kWh and the usage of molten salts containing lithium could be an interesting option in order to reach this objective. In particular, the low melting point of the ternary molten salt containing LiNO3, could increase the operating temperature gradient, and reduce the volume of salts required in the storage tank, compared to solar salt. Additionally, the lower melting point could directly influence the cost reduction [6,7]. This optimization could involve other advantages like reducing heat tracing costs, pumping power costs, and the risk of salt freezing. Moreover further cost reductions are foreseen in the receiver as well as in the power block since Li based salt proposed is less corrosive [89]. 1.1 Literature review: In the last years, different authors have investigated the most important thermal properties regarding the use of LiNO3 as TES material in CSP plants [6, 10-12]. However, a more accurate review of this material is necessary since some results presented in the literature are contradictory. 2
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Siegel et al.[13] was one of the first authors reporting this material, highlighting its suitable characteristics improving the operational temperature range, although the main problem associated with lithium is its price. Indeed, lithium nitrate is more expensive than potassium, calcium and sodium nitrate, thus, cost is one of the main reasons why lithium based salts have not been used in any commercial application, as well as the even more costly AgNO3 and CsNO3 nitrates [14]. Cordaro et al. [14] developed a diagram of experimental ternary phases for the LiNO3NaNO3-KNO3 components and the first study documenting the eutectic point of this mixture dates back to 1964 [15], in which it was determined that 17.3wt% NaNO3 + 59.4wt% KNO3 + 23.3wt% LiNO3 composition had the lowest melting point. The exact eutectic point has not yet been determined due to the measurement being strongly influenced by the quality of the salt used and the types of impurities present. Wang et al. [1617] established the eutectic composition in 25.9wt% LiNO3 + 20.6wt% NaNO3 + 54.1wt% KNO3, obtaining that thermal stability depends only on the LiNO3, since this component becomes unstable and thermally decomposes forming Li2O, and Li2O2, and that the other two components remain unchanged. Olivares et al. [18] studied the thermal behavior of the mixture composed by 30wt% LiNO3 + 18wt% NaNO3 + 52wt% KNO3, using the DSC/TGA, determining that the melting points obtained using different cover atmospheres of argon, nitrogen and oxygen were, 121 °C, 122 °C and 120 °C, respectively, so this parameter were not influenced by the cover atmosphere used. Mantha et al. [19] performed a simulation model based on thermodynamic principles to predict the eutectic temperature and composition of 25.92 wt.% LiNO3-20.01 wt.%NaNO354.07 wt.% KNO3 ternary salt system, this calculation was validated using DSC. Bradshaw and Meeker [20] evaluated four ternary Li/Na/KNO3 mixtures with 12%, 20%, 27% and 30% LiNO3 as shown in table 1, where they determined their respective melting points and the maximum decomposition temperature. Table 1: Melting point and thermal stability temperature of ternary mixtures [20].
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% LiNO3
% NaNO3
% KNO3
Melting point (°C)
12 20 27 30
18 28 33 18
79 51 40 52
200 150 160 120
Temperature stability (upper limit) °C 550 550 550 550
Another important thermophysical property investigated by Yuan Jin, Bradshaw and Coscia [21,23] is the viscosity of nitrate molten salt mixtures used in the CSP industry. This parameter allows to evaluate the thermal power and performance of thermal solar plants, since it directly influences heat transfer affecting buoyancy of molten salt mixtures. Viscosity is the property that offers resistance to the flow, which directly affects the design of the pumps and the piping of the system. As it was mentioned before, another important parameter to be fixed in these molten salts containing LiNO3 is the thermal stability. Mohammad et al. [7] determined the melting point for the mixture of 29.63wt% LiNO3 + 13.23wt%NaNO3 + 57.14 KNO3 experimentally and through simulations using FactSage software, where the values of 122.8°C and 120.84°C respectively were obtained. This author also studied the thermal decomposition in different inert atmospheres obtaining the results shown in table 2. Table 2: Thermal decomposition of ternary 29.63wt % LiNO3 + 13.23wt % NaNO3 + 57.14wt % KNO3 at different scanning rate and cover gas [7].
Sweep
Scanning rate
Gas Type
(0C/min)
Argon
10
Air Oxygen
TG rapid weight loss
TG onset
temperature
temperature (0C)
working range (0C)
545
517
422
5
533
485
410
10
600
490
477
beginning temperature (0C)
4
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In addition, in this research, it is proposed the use of LiNO3 with different purity in order to check the influence of the commercial grade of LiNO3 to be used. This purity content could be the key for a less expensive Chilean LiNO3 for future CSP applications. It is important to highlight that one of the mixtures tested was obtained directly from brines extracted in the Salar de Atacama, in the north of Chile. The mixture selected for the thermal study was 30wt%LiNO3 + 57wt%KNO3 + 13wt%NaNO3, since this formulation presents the most interesting physicochemical properties, according the literature review. 1.2 Influence of impurities: It is known that impurities tend to drive corrosion but in this case our study is only focusing on the thermal properties usually used for economical and predictive studies. The impurity content of the salt analyzed is shown in table 3. Depending on the procedure to obtain large amounts of these salts and their impurity content, the final cost for the CSP industry also varies as shown in table 3. It is important to point out that synthetic salts are used only for laboratory scale tests since they are not economically interesting for commercial applications due to their extremely high cost. Table 3: Chemical composition of ternary 30wt%LiNO3 + 13wt%NaNO3 + 17wt% KNO3 and cost estimation Component
Origin
Purity (%)
Mois ture (%)
Cl(%)
Perclo rate (%)
Mg (%)
Na (%)
Sulphate (%)
Carbonate (%)
USD/Ton
NaNO3
Natural deposit
99.5
0.01
0.04
0.01
0.02 5
0.05 4
0.242
0.028
600
KNO3
Natural deposit
99.28
0.26
0.05 6
0.01
0.02 5
0.05 4
0.237
0.041
800
LiNO3 Todini Company
Synthetic (Large amounts available s)
99
0.8
-
-
-
0.2
0.05
-
15000
LiNO3 98%
Synthetic
98
0.4
0.00 2
-
0.01
0.2
0.03
-
N.A.
Cost
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LiNO3 99,995%
Synthetic
99.995
0.01
0.00 1
-
0.00 5
0.00 5
0.01
-
N.A.
LiNO3 Atacama Desert
Natural deposit
99.7
10.4
0.00 2
-
-
-
0.024
0.264
N.A.
Stern [24] and Fernandez et al. [25] reported the conventional method to produce LiNO3, combining lithium carbonate or lithium hydroxide with nitric acid (HNO3). The reactions for this process are the following: Li2CO3 + 2HNO3 ↔ 2LiNO3 + H2O + CO2
(1)
LiOH + HNO3 ↔ LiNO3 + H2O
(2)
Using these reactions, the production of LiNO3 at industrial scale could be possible in the Atacama Desert deposits and it could be the key for a successful development of the next generation of CSP plants. 2.
Experimental procedure: All ternary mixtures were prepared with LiNO3 of different purity. Commercial
(98%) and extra-pure LiNO3 (99.995 %) compounds were purchased from Merck KGaA, Todini Company (99%), Atacama Desert (99.7%) and the NaNO3 (99.5%), KNO3 (99.5%) was provided by SQM. In order to evaluate the transformation of Li2CO3 from Atacama to LiNO3, an X- ray microdifractometer Bruker™ D8 Discover® was used at University of Evora (Portugal). To determine their heat capacity using DSC Q-20 equipment, the mixture was dehydrated in a pyrex crucible for 2 h and heat up to 300°C in an air atmosphere furnace. After that, the mixture was cooled down slowly inside the oven. The crucibles that were used in the DSC analysis are made of aluminium and were hermetically sealed. Tests were conducted in an inert atmosphere of nitrogen (50 ml/min) and at a heating ramp of 10°C /min, previously calibrated with indium (DSC analysis) and sapphire for Modulated DSC study. This measurement was obtained using the ASTM standard E1269 [26], obtaining the Cp value from the average of 5 analyses in the same sample. The typical sample size involved in the 6
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experiments was 10 mg. The standard uncertainty for heat Capacity (J / (g °C)) were 4*10-2 (J / (g °C)) for LiNO3 98 %, 2*10-2 (J / (g °C)) for LiNO3 99.995 %, 2*10-2 (J / (g °C)) for LiNO3 99 %, and 2*10-2 (J / (g °C)) for LiNO3 99.7 Atacama Desert. The viscosities of molten salt mixtures were measured using a Brookfield DV-III viscometer (Brookfield Engineering, Middleboro, MA), taking measurements at 150, 175, 200 and 300°C. The molten salt mixtures were contained in a stainless steel crucible that was heated in a furnace maintained at constant temperature by a Brookfield Thermosel controller up to 300ºC. All measurements were performed at a single rotational speed, 149 rpm, because molten nitrates are Newtonian fluids. RHEOCALC 32 was the software that allows to record the values of viscosity, shear stress, speed gradient, temperature and torque percentage for each RPM. Thermogravimetric analysis (TGA) was performed using a Mettler Toledo TGA-DSC 1 LF/894 STARe in the temperature range 25°C to 600 °C at a heating rate of 10 K/min. A mass of approximately 10 mg to 18 mg was also sealed in a 40-ml aluminium pan. For the heat capacity and thermal stability tests, a calibration process of the equipment is previously required using certified reference materials as the In (Temperature 156.59°C ± 0.01) obtaining an error between 0.01 and 0.006 % respectively. 3. Results and discussion: 3.1
Synthesis of lithium nitrate
Producing LiNO3 directly from the lithium-rich brine solutions of the Atacama Desert could mean a considerable reduction in production costs. However, thermodynamically (HSC chemistry table 4) it is not possible to obtain this compound directly using the LiCl from salt reservoirs and NaNO3. The conventional process for producing LiNO3 could be through lithium carbonate or lithium hydroxide with nitric acid, according to reaction 1 and 2. However, this process requires high purity processed raw materials. Table 4 shows results of Gibbs free energy for the production of LiNO3 from LiCl (a) and NaNO3, it is observed that for all the temperature ranges evaluated ∆G > 0, which confirms the fact that this process can not happen spontaneously.
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Table 4: Evaluation of LiNO3 formation with LiCl + NaNO3 with HSC Chemistry software T
NaNO3 + LiCl(a)=LiNO3 +NaCl ∆S ∆G
∆H C
kcal
cal/K
kcal
0.000 100.000 200.000 300.000 400.000 500.000 600.000 700.000 800.000 900.000 1000.000
4.077 3.171 2.194 6.711 2.450 1.600 0.538 -0.754 -2.289 2.659 0.480
1.987 -0.852 -3.157 5.399 -1.872 -3.046 -4.335 -5.733 -7.233 -2.551 -4.331
3.535 3.489 3.687 3.616 3.709 3.954 4.322 4.825 5.472 5.652 5.995
K
Log(K)
1E-003 9E-003 2E-002 4E-002 6E-002 8E-002 8E-002 8E-002 8E-002 9E-002 9E-002
-2.828 -2.044 -1.703 -1.379 -1.204 -1.118 -1.082 -1.084 -1.115 -1.053 -1.029
In the present research, the lithium nitrate codified as Atacama Desert, has been obtained following reaction 1 using material from Chilean salt reservoirs.. XRD analyzes for sample 1 and 2 were performed to confirm obtaining LiNO3 from Li2CO3 produced in the Atacama Desert., reaching a higher ratio LiNO3/Li2CO3 (Table 5). Table 5: Results of tests to obtain LiNO3 from the laboratory. Atacama Desert code
Weight
Volume
Weight
Ratio
Li2CO3 [g]
HNO3 [cc]
LiNO3 [g]
(LiNO3/Li2CO3)
01
73.89
139
110
1.5
02
73.89
139
131.41
1.8
, X-ray diffraction analyzes have been performed (Figure 1 and 2). Table 6 summarizes the compounds found by XRD of the samples synthesized.
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Figure 1: XRD study in sample Atacama Desert 01 of LiNO3
Figure 2: XRD study in sample Atacama Desert 02 of LiNO3
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Table 6: Summary of XRD analysis in samples 1 and 2 Component LiNO3 LiNO3 *3H2O Total 3.2
% Sample 1 92.15 7.85 100
% Sample 2 96.41 3.59 100
Thermal storage properties measurement 3.2.1 Melting point and Heat Capacity The DSC analysis obtained for the Li based salt with a composition of 30wt% LiNO3
+ 57wt% KNO3+13wt% NaNO3 (Figure 3), using LiNO3 with 98% of purity, showed an initial signal at 85.33°C corresponding to solid-solid type transformation from the mixture, together with the signal corresponding to the fusion of the mixture at 128.27°C. The energy required to melt the mixture was 121.5 J/g. The crucibles that were used for these assays were hermetically sealed; consequently, the water vapour released was retained in the upper part of the crucibles. When the sample was cooled, a lower solidification point was observed (77.24ºC). Consequently, for all the cycles conducted in this study, the same solidification point was obtained. The reason of this important reduction in the solidification point could be explained as being a sub-cooling problem. This behaviour is an important drawback for latent heat storage using phase change materials [27-30], however it could be a key factor when sensible heat storage systems are proposed.
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Figure 3: DSC test of ternary 30wt%LiNO3 (98%) + 13wt%NaNO3 + 57wt%KNO3
Heat capacity measurement for the ternary mixture with LiNO3 (98%) were conducted through modulated differential scanning calorimetry, MDSC study showed that heat capacity of the salt at 390°C was 1.718 J/g °C (Figure 4).
Figure 4: MDSC test of ternary 30wt%LiNO3 (98%) + 13wt%NaNO3 + 57wt%KNO3 at 390ºC
As it was mentioned before, LiNO3 codified as Atacama, was obtained through reaction 1, using Salar de Atacama brines and a similar thermal study was developed in this 11
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case. The results of the thermal analysis of the ternary lithium mixture with a composition of 30wt% LiNO3 + 57wt% KNO3+13wt% NaNO3 (Figure 5) showed a melting point of 124.14°C and the energy required to melt the mixture was 118.5 J/g. The heat capacity (Figure 6) measured was 1.740 J/g °C at 390ºC as shown in Table 6. A similar behaviour was found regarding the solidification point obtained: 76.48ºC.
Figure 5: DSC test of ternary 30wt%LiNO3 (Atacama Desert) + 13wt%NaNO3 + 57wt%KNO3
Figure 6: MDSC test of ternary 30wt%LiNO3 (Atacama Desert) + 13wt%NaNO3 + 57wt%KNO3
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It is important to highlight the reduction in the solidification points obtained, since this will be the reference temperature to take into account in the solar thermal industry in order to avoid salt freezing in the system. This fact means a significant reduction of the lower working temperature, with the consequent reduction of the costs associated to maintain the salt liquid (heat tracing costs). Current technology using “Solar Salt” presents melting and solidification points around 223ºC [31] with an operation security temperature above 290ºC.
3.2.2 Thermal stability mixture with different LiNO3 purity grades In the thermal stability analysis of the ternary Li/K/Na nitrate mixture for different qualities of LiNO3, the following results can be observed: from Figures 8 to 11 four zones were defined where zone 1 and 2 correspond to the loss of supramolecular and intramolecular water mass respectively, zone 3 is a stable zone where there is no mass loss and zone 4 that corresponds to the loss of mass by decomposition of nitrate salts. Results are shown in Table 7, which is in accordance with results reported by other authors [7, 32]. Table 7: Thermal Stability Zones for the ternary mixture. Zone 1 (Supramolecular water losses)
Zone 2 (intramolecular water losses)
Zone 3 (Stability)
Zone 4 (decomposition)
Total Weight loss (wt%)
Ternary Mixture
T °C
Weight loss (wt%)
T °C
Weight loss (wt%)
T °C
∆T
T °C
Weight loss (1- 3 wt%)
Li 98% (Merck)
29 -123
1.69%
123-373
1.624%
373-424
51
424-596
1.75%
5.1
Li 99 % (Todini)
40 - 100
1.39%
100-264
0.068%
264-368
104
368-596
2.84%
4.3
Li 99,7 % (Atacama Desert)
31-135
1.19%
135-326
0.35%
326-475
149
475-594
2.03%
3.6
Li 99,995% (Merck)
35 -132
1.86%
132-343
0.60%
343-446
103
446 -596
1.19%
3.6
The highest weight loss (wt%) of all ternary mixtures is 5.1% and it corresponds to the ternary mixture composed by LiNO3 98%, with the highest amount of impurities according to table 3. In turn, this mixture appears with the highest weight loss in zone 2 with 1.62%, more than all the other LiNO3 mixtures (Figure 7). The second mixture with the 13
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highest loss is Li 99 % (Todini) with 4.3% followed by Li Atacama Desert and the Lithium with a purity of 99.995%.
Total weigh loss (wt%)
5.0% 4.0% 3.0% 2.0% 1.0% 0.0% Li 98% (Merck) Li 99 % (Todini)
Li 99,7 Li 99,995% (Atacama (Merck) Desert) Ternary mixtures with different purity of LiNO3
Figure 7: Total weight loss of the eutectic mixture of 30 % LiNO3–13 % NaNO3–57 % KNO3 ternary system.
In the case of the LiNO3 99.995%, presents the largest water loss temperature, from an initial temperature of 35 °C until 132 °C in zone 1 (Figure 11), showing supramolecular water losses in this zone. Zone 4 is focusing on the maximum decomposition temperature that the proposed ternary mixtures can reach. The maximum temperatures obtained are around 594 and 596°C. In this zone the decomposition could be given by the formation of lithium oxides [16] in zone 4, due to the instability of LiNO3, and the evolution at high temperature of such gases N2, NO, O2, N2O, and NO2 [7,32].
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Figure 8: Thermal ternary salt decomposition curve of 30% LiNO3 + 13% NaNO3 + 57% KNO3 mixture with 98 % LiNO3 purity
Figure 9: Thermal ternary salt decomposition curve of 30% LiNO3 + 13% NaNO3 + 57% KNO3 mixture with 99 % LiNO3 purity
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Figure 10: Thermal ternary salt decomposition curve of 30% LiNO3 + 13% NaNO3 + 57% KNO3 mixture with 99, 7 % LiNO3 purity
Figure 11: Thermal ternary salt decomposition curve of 30% LiNO3 + 13% NaNO3 + 57% KNO3 mixture with 99,995 % LiNO3 purity
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The use of the ternary molten salts incorporating lithium nitrate proposed in this research, would reduce the lowest temperature point by almost 100ºC when comparing with Solar Salt. Table 8 shows a summary of the main thermal properties obtained for LiNO3 ternary mixture, using different LiNO3 purity grades compared with solar salt [33]. Table 8: Thermal results obtained in ternary 30wt% LiNO3 + 13wt% NaNO3 + 57wt% KNO3 using different LiNO3 purity grades Mixture
Melting point (°C)
Descomposition temperature (°C)
Solidification point (°C)
Heat Capacity (J / (g °C))
(% wt ) Heat Capacity/ Heat Capacity solar salt
Li 98
128
-
92
596
77.24
1.718
+14.5
Li 99
125
-
95
596
76.05
1.824
+21.6
124
-
96
76.48
1.740
+16.0
Li 99,995
128
-
92
596
75.13
1.828
´+21.8
Solar Salt
223
565
-
1.50
0
Li 99,7 Atacama Desert
Melting point (°C)- Melting point solar Salt (°C)
0
596
It is observed that the heat capacity of the different ternary mixtures at different qualities of lithium nitrate has an increase between 14 to 21% compared to the conventional binary solar salt, which increases the storage capacity of the lithium mixture for a similar volume using solar salt.
3.2.3 Viscosity study Viscosity evaluation of different lithium ternary mixtures with a composition of 30 wt.% LiNO3 + 57 wt. % KNO3 + 13 wt. % NaNO3, has been carried out, using the different LiNO3 grades previously reported. Samples were measured from 150°C to 300 °C and tests were performed with controlled shear rates from 2.94 rpm to 191 rpm and the reported viscosities were obtained at a shear rate of 149 rpm (Table 9).
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Table 9: Viscosity results of ternary molten nitrate salt mixtures composed by 30% LiNO3 57% KNO3, and 13% NaNO3. Viscosity (cP) T ( °C)
Viscosity (cP) LiNO3
Viscosity (cP) LiNO3
Viscosity (cP)
LiNO3 99.7
99.995 %
99 %
LiNO3 98 %
Atacama Desert
Viscosity (cP) Solar salt
150
20.4
20.4
20.8
19.3
-
170
14.51
13.81
12.62
13.5
-
200
10.01
9.78
9.75
8.6
6.7
230
9.76
8.74
9.61
7.2
5.51
260
9.84
8.55
8.59
5.7
4.83
300
8.71
7.13
6.75
5.4
4.68
The viscosity measurements are shown in Figure 12. Viscosity results for different lithium nitrate grades drops from 21 cP, at 150 °C until 10 cP at 200°C. Above 200°C the viscosity values showed a slight reduction until 300ºC.
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Figure 112: Viscosity study for ternary molten nitrate salt mixtures composed by 30% LiNO3 +57% KNO3 + 13%NaNO3.
Binary solar salt mixture shows lower viscosities than the ternary mixtures, however, the lithium nitrate salts presents reasonable viscosity values from 150°C upwards, with a maximum value about 25 cP, which are in agreement with publications by other authors [22, 34]. The ternary mixtures obtained at the Atacama Desert shows a similar viscosity behavior compared with the rest of synthetic lithium nitrates and compared with the binary solar salt. For all mixtures with different qualities of LiNO3, it is observed that the tendency to decrease the viscosity is maintained as the temperature increases. A summary of the viscosity values reported in the literature for lithium ternary salts, is provided in Table 10, including the values obtained in this work [21, 14, 22, 23, 35].
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Table 10: Viscosity of ternary molten nitrate salt mixtures composed of lithium, sodium and potassium nitrate Viscosity Melting
Wt. %
Wt. %
Wt. %
Point °C
LiNO3
NaNO3
KNO3
from 133°C to 150°C
Viscosity
Viscosity
from 190°C
from 300°C
to 200°C
to 450°C
(cP)
(cP)
(cP)
Reference
128
30 (99,995)
13
57
20.4 (150°C)
10.01 (200°C)
8.71 (300°C)
125
30 (99,9)
13
57
20.4 (150°C)
9.78 (200°C)
7.13 (300°C)
128
30 (98)
13
57
20.8 (150°C)
9.75 (200°C)
6.75 (300°C)
80
29.1
22.6
48,3
25(133°C)
10(197°C)
6 (450°C)
[14]
95
20
40
40
25(133°C)
10.2(197°C)
5.8 (450°C)
[14]
105
15
42
42.5
24.5(133°C)
8.5(197°C)
4.7 (450°C)
[14]
120
37
18
45
19(150°C)
7.5(200°C)
-
[22]
170
33
33
34
19(150°C)
7.5(200°C)
-
[22]
140
30
18
52
19(150°C)
8(200°C)
-
[22]
140
23.4
17.3
59.3
13.3(133°C)
6.4(197°C)
2.8 (300°C)
[21]
127
30
21
49
20 (150°C)
10 (200°C)
4 (300°C)
[8]
Viscosity values obtained in this research are in the same range of the values reported by other authors for synthetic commercial salts. Finally, an estimation of the cost breakdown of the TES sub-system has been performed for the proposed Li ternary mixture compare with solar salt.
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4.
Estimated cost analysis for TES The estimation of the cost analysis for the Li/K/Na nitrate mixture has been based in
a previous report from NREL based on solar salt (60%NaNO3 + 40% KNO3) [4]. This study is used as a reference to extrapolate costs of the TES sub-systems. In [4], NREL carried out a cost analysis for a conventional two tank system considering a capacity of 2703 MWht based on a previous study carried out by Abengoa Solar [36] that considered three pairs of tanks with a total capacity of 8110 MWht. For the calculation of the cost of the Li/K/Na nitrate mixture, the same thermal storage capacity (2703 MWht) was used (Table 11). The operating hot salt tank temperature in Li ternary mixture was studied at 500, 550 and 565 ° C and the cold salt tank considers an operating temperature of 190 ° C, considering that the melting point temperature to avoid is 124 °C. Considering the thermal properties of the solar salt and ones obtained for the lithium mixture experimentally, a reduction in the volume of about 35% would be necessary to reach a similar TES capacity compared to solar salt. Table 11: Thermal properties of the solar salt and the ternary mixture composed by 30% LiNO3 + 57% KNO3 + 13% NaNO3
TES CAPACITY ( MWh-t) density(Kg/m3) Cp (KJ/Kg °C) T1 (°C) T2 (°C) Volume m3 % Volume respect Solar Salt
Solar Salt 2703 1808.5 1.50 288 565 12947 -
Mixture with Li/Na/K (500 °C) 2703 1784.0 1.74 190.0 500 10112 -22
Mixture with Li/Na/K (550°C) 2703 1784.0 1.74 190 550 8708 -33
Mixture with Li/Na/K (565 °C) 2703 1784.0 1.74 190.0 565.0 8359 -35
For the reference case, Abengoa reported a solar salt cost of 1100 US$/Ton along with a cost of US $ 19.6 / Kwh-t for the TES that does not consider the indirect or contingency costs. This data is provided in Table 12. 21
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Table 12: TES cost for Two tank solar salt from Abengoa [4] TES Cost for Two - Tank solar salt at 565°C from Abengoa Scaling exp. TES CAPACITY Salt Cost Cold Tank Hot Tank Salt Inventory Instrumentation Structural steel Tank insulation Electrical Concrete Foamglass Refractory Sitework Painting
1 0 0.8 0.8 1 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8
2703 MWh-t 1100 U$S/ton $4361 $10016 $30122 $212 $666 $3724 $481 $1560 $959 $531 $339 $8 $52979 $19.6
8% 19% 57% 0% 1% 7% 1% 3% 2% 1% 1% 0% MUS$ US$/Kwh-t
In the case of the TES Direct Cost of the Lithium mixture (Table 12), the scaling exponent was set by NREL in 0.8 considering a decrease in the cost of total system by economy of scale. Since the volume required for the Li-based salt is lower than the reference solar salt, a correction is made to the tank cost factor associated with the volumes of the mixtures, which is given by the following volume scale ratio C1 = (V/VSS) 0.8, which results is 0.705 and where V refers to the volume of the ternary lithium salt and VSS refers to the volume of the solar salt. The alloy and tensile requirements remains constant for hot and cold tanks, since the Alloy Multiple is set in 1. To determine the market price that lithium nitrate should have, it was considered a value of US $ 19.6.Kwh-t. which corresponds to the equivalence between the cost of the TES for two tanks with solar salt of Table 12 and the cost of TES for two tanks with nitrate mixture of Li/K/Na (Tmax 565°C) of Table 13. On the other hand, considering that the ternary mixture
22
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is composed by 30% LiNO3 + 57% KNO3 + 13% NaNO3, the estimated price of KNO3 and NaNO3 was determined, based on the price of solar salt of 1100 US $ / ton. Finally, an estimation of the TES Direct cost in Li/K/Na v/s LiNO3 Prices mixture was calculated (Figure 13). Matching the cost of TES for both mixtures to US $ 19.6/ Kwht, the price of lithium nitrate contained in the ternary mixture should have a maximum price of US$ 7520/Ton to reach the storage objectives. Above this price it would fit into a noncompetitive zone, thus the proposal to use the ternary mixture would not be competitive against the current mixture of solar salt. Table 13: TES cost for two tank Li/Na/K mixture from Atacama TES Cost for Two - Tank Li/K/Na nitrates Mixture 0,705 TES CAPACITY Salt Cost Mixture (Li/K/Na) Scaling exp Rel. Size Cold Tank 0.8 0.705 Hot Tank 0.8 0.705 Salt Inventory 1 Instrumentation 0.8 0.705 Structural steel 0.8 0.705 Tank insulation 0.8 0.705 Electrical 0.8 0.705 Concrete 0.8 0.705 Foamglass 0.8 0.705 Refractory 0.8 0.705 Sitework 0.8 0.705 Painting 0.8 0.705
2703 U$S /Ton 2689 U$S/ton 40107 $2459 5% $5647 11% $40107 76% $120 0% $375 1% $2099 4% $271 1% $879 2% $541 1% $299 1% $191 0% $5 0% $52993 MUS$ $19.6 US$/Kwh-t
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Figure 12: TES cost ternary mixture Li/K/Na v/s LiNO3 price estimation
According to the point of view of cost analysis, the results obtained in Table 4 should be evaluated at a more exhaustive level. It was obtained that for a mass unit of Li2CO3 it is possible to obtain almost twice as much LiNO3, which could mean a reduction in costs of the process to obtain lithium nitrate from lithium carbonate.
5.
Conclusions:
Lithium nitrate could be one of the most promising additives in TES materials that are under study for their application on CSP plants. The present paper confirms the excellent thermal properties evaluated for a novel mixture containing 30wt% of LiNO3 + 57 wt% KNO3 + 13 wt% NaNO3 with different purity grades for LiNO3. From the work done it is possible to obtain the following conclusions: The specific heat and melting point was detected using the DSC Q-20 for all ternary mixtures with different purity grades of LiNO3. The heat capacity is between 14% to 21% higher than 24
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the solar salt currently used. Melting point was also reduced between 92 and 96°C lower compared to solar salt. Regarding the thermal stability, it was found that for all the ternary mixtures with the different purity grades of LiNO3 evaluated, the highest weight loss (wt%) of all ternary mixtures is 5.1% corresponding to the ternary mixture composed by LiNO3 98%, with higher amount of impurities. The maximum temperatures obtained in the zone 4 (decomposition) are around 594 and 596 ° C and the decomposition could be given by the evolution at high temperature of volatile N2, NO, O2, N2O, and NO2 species. According the viscosity values, it was found that for all the ternary mixtures with the different purity grades of LiNO3 evaluated, there is not a clear relation between viscosity values and the different purity grades used. Moreover, it is observed that the tendency to decrease the viscosity is maintained as the temperature increases, behavior similar to the solar salt. Finally, regarding the cost analysis for TES, a reduction in the volume (about 35%) could be achieved to reach a similar TES capacity compared to solar salt. Matching the cost of TES for both mixtures to US $ 19.6/ Kwh-t, the price of lithium nitrate contained in the ternary mixture should have a maximum price of US$ 7520/Ton.
Acknowledgement The authors would like to acknowledge the financial support provided by CONICYT / FONDAP 15110019 "Solar Energy Research Center" SERC-Chile, FIC-R 30413089 funded by Antofagasta Government and Minera Escondida Fundation.
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Declaration of interest statement: We declare to be aware of the interest statement of this document and not conflicto of interest has been reported
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Highlights:
Lithium based salts present a higher work temperature range compared to solar salt Higher purities show a decrease in viscosity with the temperature 35% of reduction could be achieved using LiNO3, to reach the current TES capacity Melting point shows a reduction close to 90°C with respect to solar salt The heat capacity increases between 14 to 21% respect solar salt
Mauro Henríquez, Luis Guerreiro, Ángel G. Fernández, Edward Fuentealba PII:
S0960-1481(19)31566-6
DOI:
https://doi.org/10.1016/j.renene.2019.10.075
Reference:
RENE 12444
To appear in:
Renewable Energy
Received Date:
19 December 2018
Accepted Date:
13 October 2019
Please cite this article as: Mauro Henríquez, Luis Guerreiro, Ángel G. Fernández, Edward Fuentealba, Lithium nitrate purity influence assessment in ternary molten salts as thermal energy storage material for CSP plants, Renewable Energy (2019), https://doi.org/10.1016/j.renene. 2019.10.075
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
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Lithium nitrate purity influence assessment in ternary molten salts as thermal energy storage material for CSP plants Mauro Henríquez1,2, Luis Guerreiro3, Ángel G. Fernández1*, Edward Fuentealba1 1 Centro
2
de Desarrollo Energético Antofagasta (CDEA), Universidad de Antofagasta, Av. Universidad de Antofagasta 02800, Antofagasta, Chile
Universidad de Almería, Crta. De Sacramento s/n, E04120 La Canada de San Urbano, Almería, Spain 3
Univ. Evora, Renewable Energies Chair, Palacio Vimioso, 7002 Evora, Portugal
* Corresponding
author Email: [email protected]
Abstract
The addition of lithium nitrate is assumed to improve the performance of molten salts, extending the work temperature range. This paper presents an evaluation of the influence of different degrees of purity of LiNO3, in a ternary mixture with composition 30wt%LiNO3 + 13wt%NaNO3 + 57wt% KNO3 including a Chilean mixture obtained from the Atacama Desert brines. In addition, the use of synthetic lithium nitrate obtained from the chemical synthesis using Li2CO3 and HNO3, was incorporated in the comparison. The melting point results of the 30wt% LiNO3
+ 13wt% NaNO3 + 57wt% KNO3 mixture for different purities (128 ° C, 124 ° C), show a reduction of 92 to 96 ° C with respect to the 223 ° C of the solar salt and thermal stability results show maximum temperature are around 594 and 596 ° C, which means that this mixture could work at maximum operating temperatures similar to those of solar salt. Finally, it was also determined that for the use of the proposed ternary mixture would mean a reduction of 35% in the volume of inventory with respect to solar salt since the proposed ternary salt presents an improvement about 14-21% in the heat capacity. Keywords: Thermal Energy Storage; Lithium Nitrate; Concentrated Solar Power 1.
Introduction During the last years, Chile has been one of the most attractive solar markets due to
its excellent solar conditions, in particular the Atacama Desert, which presents an annual 1
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global radiation value of 2571 kWh/m2 and an index of direct normal irradiance (DNI) of 3389 kWh/m2. This location is among the best worldwide for capturing and storing solar energy [1] and thus Concentrated Solar Power (CSP) rises as a promising solution. These features are of major interest when considering the extensive saline deposits that are present in this region of north Chile with a great potential to be used as energy storage material [2]. The current second-generation CSP technology was developed from the pilot project called Solar Two with a capacity of 10 MWe developed in the Mojave desert in the USA by the Department of Energy (DOE) that incorporated a thermal storage system of two tanks with salts melted (60% de NaNO3 and 40% de KNO3), which was a significant improvement to the initial project called Solar One, which considered a thermal storage system in a single tank filled with rocks and sand, using oil as heat transfer fluid (HTF) [3]. In 2012, the US Department of Energy published the objective of reducing it to 6 c/kWh by 2020 through its Sunshot Vision Study. If the behavior of technology is analyzed in recent years the cost for a CSP plant has dropped from 20,6 ¢/kWh in 2010 to 12 ¢/kWh in 2015 and 10 ¢/kWh in 2017 [4-5]. The aim fixed for Chile by 2025 is to achieve an LCOE of 5 ¢/kWh and the usage of molten salts containing lithium could be an interesting option in order to reach this objective. In particular, the low melting point of the ternary molten salt containing LiNO3, could increase the operating temperature gradient, and reduce the volume of salts required in the storage tank, compared to solar salt. Additionally, the lower melting point could directly influence the cost reduction [6,7]. This optimization could involve other advantages like reducing heat tracing costs, pumping power costs, and the risk of salt freezing. Moreover further cost reductions are foreseen in the receiver as well as in the power block since Li based salt proposed is less corrosive [89]. 1.1 Literature review: In the last years, different authors have investigated the most important thermal properties regarding the use of LiNO3 as TES material in CSP plants [6, 10-12]. However, a more accurate review of this material is necessary since some results presented in the literature are contradictory. 2
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Siegel et al.[13] was one of the first authors reporting this material, highlighting its suitable characteristics improving the operational temperature range, although the main problem associated with lithium is its price. Indeed, lithium nitrate is more expensive than potassium, calcium and sodium nitrate, thus, cost is one of the main reasons why lithium based salts have not been used in any commercial application, as well as the even more costly AgNO3 and CsNO3 nitrates [14]. Cordaro et al. [14] developed a diagram of experimental ternary phases for the LiNO3NaNO3-KNO3 components and the first study documenting the eutectic point of this mixture dates back to 1964 [15], in which it was determined that 17.3wt% NaNO3 + 59.4wt% KNO3 + 23.3wt% LiNO3 composition had the lowest melting point. The exact eutectic point has not yet been determined due to the measurement being strongly influenced by the quality of the salt used and the types of impurities present. Wang et al. [1617] established the eutectic composition in 25.9wt% LiNO3 + 20.6wt% NaNO3 + 54.1wt% KNO3, obtaining that thermal stability depends only on the LiNO3, since this component becomes unstable and thermally decomposes forming Li2O, and Li2O2, and that the other two components remain unchanged. Olivares et al. [18] studied the thermal behavior of the mixture composed by 30wt% LiNO3 + 18wt% NaNO3 + 52wt% KNO3, using the DSC/TGA, determining that the melting points obtained using different cover atmospheres of argon, nitrogen and oxygen were, 121 °C, 122 °C and 120 °C, respectively, so this parameter were not influenced by the cover atmosphere used. Mantha et al. [19] performed a simulation model based on thermodynamic principles to predict the eutectic temperature and composition of 25.92 wt.% LiNO3-20.01 wt.%NaNO354.07 wt.% KNO3 ternary salt system, this calculation was validated using DSC. Bradshaw and Meeker [20] evaluated four ternary Li/Na/KNO3 mixtures with 12%, 20%, 27% and 30% LiNO3 as shown in table 1, where they determined their respective melting points and the maximum decomposition temperature. Table 1: Melting point and thermal stability temperature of ternary mixtures [20].
3
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% LiNO3
% NaNO3
% KNO3
Melting point (°C)
12 20 27 30
18 28 33 18
79 51 40 52
200 150 160 120
Temperature stability (upper limit) °C 550 550 550 550
Another important thermophysical property investigated by Yuan Jin, Bradshaw and Coscia [21,23] is the viscosity of nitrate molten salt mixtures used in the CSP industry. This parameter allows to evaluate the thermal power and performance of thermal solar plants, since it directly influences heat transfer affecting buoyancy of molten salt mixtures. Viscosity is the property that offers resistance to the flow, which directly affects the design of the pumps and the piping of the system. As it was mentioned before, another important parameter to be fixed in these molten salts containing LiNO3 is the thermal stability. Mohammad et al. [7] determined the melting point for the mixture of 29.63wt% LiNO3 + 13.23wt%NaNO3 + 57.14 KNO3 experimentally and through simulations using FactSage software, where the values of 122.8°C and 120.84°C respectively were obtained. This author also studied the thermal decomposition in different inert atmospheres obtaining the results shown in table 2. Table 2: Thermal decomposition of ternary 29.63wt % LiNO3 + 13.23wt % NaNO3 + 57.14wt % KNO3 at different scanning rate and cover gas [7].
Sweep
Scanning rate
Gas Type
(0C/min)
Argon
10
Air Oxygen
TG rapid weight loss
TG onset
temperature
temperature (0C)
working range (0C)
545
517
422
5
533
485
410
10
600
490
477
beginning temperature (0C)
4
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In addition, in this research, it is proposed the use of LiNO3 with different purity in order to check the influence of the commercial grade of LiNO3 to be used. This purity content could be the key for a less expensive Chilean LiNO3 for future CSP applications. It is important to highlight that one of the mixtures tested was obtained directly from brines extracted in the Salar de Atacama, in the north of Chile. The mixture selected for the thermal study was 30wt%LiNO3 + 57wt%KNO3 + 13wt%NaNO3, since this formulation presents the most interesting physicochemical properties, according the literature review. 1.2 Influence of impurities: It is known that impurities tend to drive corrosion but in this case our study is only focusing on the thermal properties usually used for economical and predictive studies. The impurity content of the salt analyzed is shown in table 3. Depending on the procedure to obtain large amounts of these salts and their impurity content, the final cost for the CSP industry also varies as shown in table 3. It is important to point out that synthetic salts are used only for laboratory scale tests since they are not economically interesting for commercial applications due to their extremely high cost. Table 3: Chemical composition of ternary 30wt%LiNO3 + 13wt%NaNO3 + 17wt% KNO3 and cost estimation Component
Origin
Purity (%)
Mois ture (%)
Cl(%)
Perclo rate (%)
Mg (%)
Na (%)
Sulphate (%)
Carbonate (%)
USD/Ton
NaNO3
Natural deposit
99.5
0.01
0.04
0.01
0.02 5
0.05 4
0.242
0.028
600
KNO3
Natural deposit
99.28
0.26
0.05 6
0.01
0.02 5
0.05 4
0.237
0.041
800
LiNO3 Todini Company
Synthetic (Large amounts available s)
99
0.8
-
-
-
0.2
0.05
-
15000
LiNO3 98%
Synthetic
98
0.4
0.00 2
-
0.01
0.2
0.03
-
N.A.
Cost
5
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LiNO3 99,995%
Synthetic
99.995
0.01
0.00 1
-
0.00 5
0.00 5
0.01
-
N.A.
LiNO3 Atacama Desert
Natural deposit
99.7
10.4
0.00 2
-
-
-
0.024
0.264
N.A.
Stern [24] and Fernandez et al. [25] reported the conventional method to produce LiNO3, combining lithium carbonate or lithium hydroxide with nitric acid (HNO3). The reactions for this process are the following: Li2CO3 + 2HNO3 ↔ 2LiNO3 + H2O + CO2
(1)
LiOH + HNO3 ↔ LiNO3 + H2O
(2)
Using these reactions, the production of LiNO3 at industrial scale could be possible in the Atacama Desert deposits and it could be the key for a successful development of the next generation of CSP plants. 2.
Experimental procedure: All ternary mixtures were prepared with LiNO3 of different purity. Commercial
(98%) and extra-pure LiNO3 (99.995 %) compounds were purchased from Merck KGaA, Todini Company (99%), Atacama Desert (99.7%) and the NaNO3 (99.5%), KNO3 (99.5%) was provided by SQM. In order to evaluate the transformation of Li2CO3 from Atacama to LiNO3, an X- ray microdifractometer Bruker™ D8 Discover® was used at University of Evora (Portugal). To determine their heat capacity using DSC Q-20 equipment, the mixture was dehydrated in a pyrex crucible for 2 h and heat up to 300°C in an air atmosphere furnace. After that, the mixture was cooled down slowly inside the oven. The crucibles that were used in the DSC analysis are made of aluminium and were hermetically sealed. Tests were conducted in an inert atmosphere of nitrogen (50 ml/min) and at a heating ramp of 10°C /min, previously calibrated with indium (DSC analysis) and sapphire for Modulated DSC study. This measurement was obtained using the ASTM standard E1269 [26], obtaining the Cp value from the average of 5 analyses in the same sample. The typical sample size involved in the 6
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experiments was 10 mg. The standard uncertainty for heat Capacity (J / (g °C)) were 4*10-2 (J / (g °C)) for LiNO3 98 %, 2*10-2 (J / (g °C)) for LiNO3 99.995 %, 2*10-2 (J / (g °C)) for LiNO3 99 %, and 2*10-2 (J / (g °C)) for LiNO3 99.7 Atacama Desert. The viscosities of molten salt mixtures were measured using a Brookfield DV-III viscometer (Brookfield Engineering, Middleboro, MA), taking measurements at 150, 175, 200 and 300°C. The molten salt mixtures were contained in a stainless steel crucible that was heated in a furnace maintained at constant temperature by a Brookfield Thermosel controller up to 300ºC. All measurements were performed at a single rotational speed, 149 rpm, because molten nitrates are Newtonian fluids. RHEOCALC 32 was the software that allows to record the values of viscosity, shear stress, speed gradient, temperature and torque percentage for each RPM. Thermogravimetric analysis (TGA) was performed using a Mettler Toledo TGA-DSC 1 LF/894 STARe in the temperature range 25°C to 600 °C at a heating rate of 10 K/min. A mass of approximately 10 mg to 18 mg was also sealed in a 40-ml aluminium pan. For the heat capacity and thermal stability tests, a calibration process of the equipment is previously required using certified reference materials as the In (Temperature 156.59°C ± 0.01) obtaining an error between 0.01 and 0.006 % respectively. 3. Results and discussion: 3.1
Synthesis of lithium nitrate
Producing LiNO3 directly from the lithium-rich brine solutions of the Atacama Desert could mean a considerable reduction in production costs. However, thermodynamically (HSC chemistry table 4) it is not possible to obtain this compound directly using the LiCl from salt reservoirs and NaNO3. The conventional process for producing LiNO3 could be through lithium carbonate or lithium hydroxide with nitric acid, according to reaction 1 and 2. However, this process requires high purity processed raw materials. Table 4 shows results of Gibbs free energy for the production of LiNO3 from LiCl (a) and NaNO3, it is observed that for all the temperature ranges evaluated ∆G > 0, which confirms the fact that this process can not happen spontaneously.
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Table 4: Evaluation of LiNO3 formation with LiCl + NaNO3 with HSC Chemistry software T
NaNO3 + LiCl(a)=LiNO3 +NaCl ∆S ∆G
∆H C
kcal
cal/K
kcal
0.000 100.000 200.000 300.000 400.000 500.000 600.000 700.000 800.000 900.000 1000.000
4.077 3.171 2.194 6.711 2.450 1.600 0.538 -0.754 -2.289 2.659 0.480
1.987 -0.852 -3.157 5.399 -1.872 -3.046 -4.335 -5.733 -7.233 -2.551 -4.331
3.535 3.489 3.687 3.616 3.709 3.954 4.322 4.825 5.472 5.652 5.995
K
Log(K)
1E-003 9E-003 2E-002 4E-002 6E-002 8E-002 8E-002 8E-002 8E-002 9E-002 9E-002
-2.828 -2.044 -1.703 -1.379 -1.204 -1.118 -1.082 -1.084 -1.115 -1.053 -1.029
In the present research, the lithium nitrate codified as Atacama Desert, has been obtained following reaction 1 using material from Chilean salt reservoirs.. XRD analyzes for sample 1 and 2 were performed to confirm obtaining LiNO3 from Li2CO3 produced in the Atacama Desert., reaching a higher ratio LiNO3/Li2CO3 (Table 5). Table 5: Results of tests to obtain LiNO3 from the laboratory. Atacama Desert code
Weight
Volume
Weight
Ratio
Li2CO3 [g]
HNO3 [cc]
LiNO3 [g]
(LiNO3/Li2CO3)
01
73.89
139
110
1.5
02
73.89
139
131.41
1.8
, X-ray diffraction analyzes have been performed (Figure 1 and 2). Table 6 summarizes the compounds found by XRD of the samples synthesized.
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Figure 1: XRD study in sample Atacama Desert 01 of LiNO3
Figure 2: XRD study in sample Atacama Desert 02 of LiNO3
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Table 6: Summary of XRD analysis in samples 1 and 2 Component LiNO3 LiNO3 *3H2O Total 3.2
% Sample 1 92.15 7.85 100
% Sample 2 96.41 3.59 100
Thermal storage properties measurement 3.2.1 Melting point and Heat Capacity The DSC analysis obtained for the Li based salt with a composition of 30wt% LiNO3
+ 57wt% KNO3+13wt% NaNO3 (Figure 3), using LiNO3 with 98% of purity, showed an initial signal at 85.33°C corresponding to solid-solid type transformation from the mixture, together with the signal corresponding to the fusion of the mixture at 128.27°C. The energy required to melt the mixture was 121.5 J/g. The crucibles that were used for these assays were hermetically sealed; consequently, the water vapour released was retained in the upper part of the crucibles. When the sample was cooled, a lower solidification point was observed (77.24ºC). Consequently, for all the cycles conducted in this study, the same solidification point was obtained. The reason of this important reduction in the solidification point could be explained as being a sub-cooling problem. This behaviour is an important drawback for latent heat storage using phase change materials [27-30], however it could be a key factor when sensible heat storage systems are proposed.
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Figure 3: DSC test of ternary 30wt%LiNO3 (98%) + 13wt%NaNO3 + 57wt%KNO3
Heat capacity measurement for the ternary mixture with LiNO3 (98%) were conducted through modulated differential scanning calorimetry, MDSC study showed that heat capacity of the salt at 390°C was 1.718 J/g °C (Figure 4).
Figure 4: MDSC test of ternary 30wt%LiNO3 (98%) + 13wt%NaNO3 + 57wt%KNO3 at 390ºC
As it was mentioned before, LiNO3 codified as Atacama, was obtained through reaction 1, using Salar de Atacama brines and a similar thermal study was developed in this 11
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case. The results of the thermal analysis of the ternary lithium mixture with a composition of 30wt% LiNO3 + 57wt% KNO3+13wt% NaNO3 (Figure 5) showed a melting point of 124.14°C and the energy required to melt the mixture was 118.5 J/g. The heat capacity (Figure 6) measured was 1.740 J/g °C at 390ºC as shown in Table 6. A similar behaviour was found regarding the solidification point obtained: 76.48ºC.
Figure 5: DSC test of ternary 30wt%LiNO3 (Atacama Desert) + 13wt%NaNO3 + 57wt%KNO3
Figure 6: MDSC test of ternary 30wt%LiNO3 (Atacama Desert) + 13wt%NaNO3 + 57wt%KNO3
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It is important to highlight the reduction in the solidification points obtained, since this will be the reference temperature to take into account in the solar thermal industry in order to avoid salt freezing in the system. This fact means a significant reduction of the lower working temperature, with the consequent reduction of the costs associated to maintain the salt liquid (heat tracing costs). Current technology using “Solar Salt” presents melting and solidification points around 223ºC [31] with an operation security temperature above 290ºC.
3.2.2 Thermal stability mixture with different LiNO3 purity grades In the thermal stability analysis of the ternary Li/K/Na nitrate mixture for different qualities of LiNO3, the following results can be observed: from Figures 8 to 11 four zones were defined where zone 1 and 2 correspond to the loss of supramolecular and intramolecular water mass respectively, zone 3 is a stable zone where there is no mass loss and zone 4 that corresponds to the loss of mass by decomposition of nitrate salts. Results are shown in Table 7, which is in accordance with results reported by other authors [7, 32]. Table 7: Thermal Stability Zones for the ternary mixture. Zone 1 (Supramolecular water losses)
Zone 2 (intramolecular water losses)
Zone 3 (Stability)
Zone 4 (decomposition)
Total Weight loss (wt%)
Ternary Mixture
T °C
Weight loss (wt%)
T °C
Weight loss (wt%)
T °C
∆T
T °C
Weight loss (1- 3 wt%)
Li 98% (Merck)
29 -123
1.69%
123-373
1.624%
373-424
51
424-596
1.75%
5.1
Li 99 % (Todini)
40 - 100
1.39%
100-264
0.068%
264-368
104
368-596
2.84%
4.3
Li 99,7 % (Atacama Desert)
31-135
1.19%
135-326
0.35%
326-475
149
475-594
2.03%
3.6
Li 99,995% (Merck)
35 -132
1.86%
132-343
0.60%
343-446
103
446 -596
1.19%
3.6
The highest weight loss (wt%) of all ternary mixtures is 5.1% and it corresponds to the ternary mixture composed by LiNO3 98%, with the highest amount of impurities according to table 3. In turn, this mixture appears with the highest weight loss in zone 2 with 1.62%, more than all the other LiNO3 mixtures (Figure 7). The second mixture with the 13
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highest loss is Li 99 % (Todini) with 4.3% followed by Li Atacama Desert and the Lithium with a purity of 99.995%.
Total weigh loss (wt%)
5.0% 4.0% 3.0% 2.0% 1.0% 0.0% Li 98% (Merck) Li 99 % (Todini)
Li 99,7 Li 99,995% (Atacama (Merck) Desert) Ternary mixtures with different purity of LiNO3
Figure 7: Total weight loss of the eutectic mixture of 30 % LiNO3–13 % NaNO3–57 % KNO3 ternary system.
In the case of the LiNO3 99.995%, presents the largest water loss temperature, from an initial temperature of 35 °C until 132 °C in zone 1 (Figure 11), showing supramolecular water losses in this zone. Zone 4 is focusing on the maximum decomposition temperature that the proposed ternary mixtures can reach. The maximum temperatures obtained are around 594 and 596°C. In this zone the decomposition could be given by the formation of lithium oxides [16] in zone 4, due to the instability of LiNO3, and the evolution at high temperature of such gases N2, NO, O2, N2O, and NO2 [7,32].
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Figure 8: Thermal ternary salt decomposition curve of 30% LiNO3 + 13% NaNO3 + 57% KNO3 mixture with 98 % LiNO3 purity
Figure 9: Thermal ternary salt decomposition curve of 30% LiNO3 + 13% NaNO3 + 57% KNO3 mixture with 99 % LiNO3 purity
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Figure 10: Thermal ternary salt decomposition curve of 30% LiNO3 + 13% NaNO3 + 57% KNO3 mixture with 99, 7 % LiNO3 purity
Figure 11: Thermal ternary salt decomposition curve of 30% LiNO3 + 13% NaNO3 + 57% KNO3 mixture with 99,995 % LiNO3 purity
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The use of the ternary molten salts incorporating lithium nitrate proposed in this research, would reduce the lowest temperature point by almost 100ºC when comparing with Solar Salt. Table 8 shows a summary of the main thermal properties obtained for LiNO3 ternary mixture, using different LiNO3 purity grades compared with solar salt [33]. Table 8: Thermal results obtained in ternary 30wt% LiNO3 + 13wt% NaNO3 + 57wt% KNO3 using different LiNO3 purity grades Mixture
Melting point (°C)
Descomposition temperature (°C)
Solidification point (°C)
Heat Capacity (J / (g °C))
(% wt ) Heat Capacity/ Heat Capacity solar salt
Li 98
128
-
92
596
77.24
1.718
+14.5
Li 99
125
-
95
596
76.05
1.824
+21.6
124
-
96
76.48
1.740
+16.0
Li 99,995
128
-
92
596
75.13
1.828
´+21.8
Solar Salt
223
565
-
1.50
0
Li 99,7 Atacama Desert
Melting point (°C)- Melting point solar Salt (°C)
0
596
It is observed that the heat capacity of the different ternary mixtures at different qualities of lithium nitrate has an increase between 14 to 21% compared to the conventional binary solar salt, which increases the storage capacity of the lithium mixture for a similar volume using solar salt.
3.2.3 Viscosity study Viscosity evaluation of different lithium ternary mixtures with a composition of 30 wt.% LiNO3 + 57 wt. % KNO3 + 13 wt. % NaNO3, has been carried out, using the different LiNO3 grades previously reported. Samples were measured from 150°C to 300 °C and tests were performed with controlled shear rates from 2.94 rpm to 191 rpm and the reported viscosities were obtained at a shear rate of 149 rpm (Table 9).
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Table 9: Viscosity results of ternary molten nitrate salt mixtures composed by 30% LiNO3 57% KNO3, and 13% NaNO3. Viscosity (cP) T ( °C)
Viscosity (cP) LiNO3
Viscosity (cP) LiNO3
Viscosity (cP)
LiNO3 99.7
99.995 %
99 %
LiNO3 98 %
Atacama Desert
Viscosity (cP) Solar salt
150
20.4
20.4
20.8
19.3
-
170
14.51
13.81
12.62
13.5
-
200
10.01
9.78
9.75
8.6
6.7
230
9.76
8.74
9.61
7.2
5.51
260
9.84
8.55
8.59
5.7
4.83
300
8.71
7.13
6.75
5.4
4.68
The viscosity measurements are shown in Figure 12. Viscosity results for different lithium nitrate grades drops from 21 cP, at 150 °C until 10 cP at 200°C. Above 200°C the viscosity values showed a slight reduction until 300ºC.
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Figure 112: Viscosity study for ternary molten nitrate salt mixtures composed by 30% LiNO3 +57% KNO3 + 13%NaNO3.
Binary solar salt mixture shows lower viscosities than the ternary mixtures, however, the lithium nitrate salts presents reasonable viscosity values from 150°C upwards, with a maximum value about 25 cP, which are in agreement with publications by other authors [22, 34]. The ternary mixtures obtained at the Atacama Desert shows a similar viscosity behavior compared with the rest of synthetic lithium nitrates and compared with the binary solar salt. For all mixtures with different qualities of LiNO3, it is observed that the tendency to decrease the viscosity is maintained as the temperature increases. A summary of the viscosity values reported in the literature for lithium ternary salts, is provided in Table 10, including the values obtained in this work [21, 14, 22, 23, 35].
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Table 10: Viscosity of ternary molten nitrate salt mixtures composed of lithium, sodium and potassium nitrate Viscosity Melting
Wt. %
Wt. %
Wt. %
Point °C
LiNO3
NaNO3
KNO3
from 133°C to 150°C
Viscosity
Viscosity
from 190°C
from 300°C
to 200°C
to 450°C
(cP)
(cP)
(cP)
Reference
128
30 (99,995)
13
57
20.4 (150°C)
10.01 (200°C)
8.71 (300°C)
125
30 (99,9)
13
57
20.4 (150°C)
9.78 (200°C)
7.13 (300°C)
128
30 (98)
13
57
20.8 (150°C)
9.75 (200°C)
6.75 (300°C)
80
29.1
22.6
48,3
25(133°C)
10(197°C)
6 (450°C)
[14]
95
20
40
40
25(133°C)
10.2(197°C)
5.8 (450°C)
[14]
105
15
42
42.5
24.5(133°C)
8.5(197°C)
4.7 (450°C)
[14]
120
37
18
45
19(150°C)
7.5(200°C)
-
[22]
170
33
33
34
19(150°C)
7.5(200°C)
-
[22]
140
30
18
52
19(150°C)
8(200°C)
-
[22]
140
23.4
17.3
59.3
13.3(133°C)
6.4(197°C)
2.8 (300°C)
[21]
127
30
21
49
20 (150°C)
10 (200°C)
4 (300°C)
[8]
Viscosity values obtained in this research are in the same range of the values reported by other authors for synthetic commercial salts. Finally, an estimation of the cost breakdown of the TES sub-system has been performed for the proposed Li ternary mixture compare with solar salt.
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4.
Estimated cost analysis for TES The estimation of the cost analysis for the Li/K/Na nitrate mixture has been based in
a previous report from NREL based on solar salt (60%NaNO3 + 40% KNO3) [4]. This study is used as a reference to extrapolate costs of the TES sub-systems. In [4], NREL carried out a cost analysis for a conventional two tank system considering a capacity of 2703 MWht based on a previous study carried out by Abengoa Solar [36] that considered three pairs of tanks with a total capacity of 8110 MWht. For the calculation of the cost of the Li/K/Na nitrate mixture, the same thermal storage capacity (2703 MWht) was used (Table 11). The operating hot salt tank temperature in Li ternary mixture was studied at 500, 550 and 565 ° C and the cold salt tank considers an operating temperature of 190 ° C, considering that the melting point temperature to avoid is 124 °C. Considering the thermal properties of the solar salt and ones obtained for the lithium mixture experimentally, a reduction in the volume of about 35% would be necessary to reach a similar TES capacity compared to solar salt. Table 11: Thermal properties of the solar salt and the ternary mixture composed by 30% LiNO3 + 57% KNO3 + 13% NaNO3
TES CAPACITY ( MWh-t) density(Kg/m3) Cp (KJ/Kg °C) T1 (°C) T2 (°C) Volume m3 % Volume respect Solar Salt
Solar Salt 2703 1808.5 1.50 288 565 12947 -
Mixture with Li/Na/K (500 °C) 2703 1784.0 1.74 190.0 500 10112 -22
Mixture with Li/Na/K (550°C) 2703 1784.0 1.74 190 550 8708 -33
Mixture with Li/Na/K (565 °C) 2703 1784.0 1.74 190.0 565.0 8359 -35
For the reference case, Abengoa reported a solar salt cost of 1100 US$/Ton along with a cost of US $ 19.6 / Kwh-t for the TES that does not consider the indirect or contingency costs. This data is provided in Table 12. 21
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Table 12: TES cost for Two tank solar salt from Abengoa [4] TES Cost for Two - Tank solar salt at 565°C from Abengoa Scaling exp. TES CAPACITY Salt Cost Cold Tank Hot Tank Salt Inventory Instrumentation Structural steel Tank insulation Electrical Concrete Foamglass Refractory Sitework Painting
1 0 0.8 0.8 1 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8
2703 MWh-t 1100 U$S/ton $4361 $10016 $30122 $212 $666 $3724 $481 $1560 $959 $531 $339 $8 $52979 $19.6
8% 19% 57% 0% 1% 7% 1% 3% 2% 1% 1% 0% MUS$ US$/Kwh-t
In the case of the TES Direct Cost of the Lithium mixture (Table 12), the scaling exponent was set by NREL in 0.8 considering a decrease in the cost of total system by economy of scale. Since the volume required for the Li-based salt is lower than the reference solar salt, a correction is made to the tank cost factor associated with the volumes of the mixtures, which is given by the following volume scale ratio C1 = (V/VSS) 0.8, which results is 0.705 and where V refers to the volume of the ternary lithium salt and VSS refers to the volume of the solar salt. The alloy and tensile requirements remains constant for hot and cold tanks, since the Alloy Multiple is set in 1. To determine the market price that lithium nitrate should have, it was considered a value of US $ 19.6.Kwh-t. which corresponds to the equivalence between the cost of the TES for two tanks with solar salt of Table 12 and the cost of TES for two tanks with nitrate mixture of Li/K/Na (Tmax 565°C) of Table 13. On the other hand, considering that the ternary mixture
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is composed by 30% LiNO3 + 57% KNO3 + 13% NaNO3, the estimated price of KNO3 and NaNO3 was determined, based on the price of solar salt of 1100 US $ / ton. Finally, an estimation of the TES Direct cost in Li/K/Na v/s LiNO3 Prices mixture was calculated (Figure 13). Matching the cost of TES for both mixtures to US $ 19.6/ Kwht, the price of lithium nitrate contained in the ternary mixture should have a maximum price of US$ 7520/Ton to reach the storage objectives. Above this price it would fit into a noncompetitive zone, thus the proposal to use the ternary mixture would not be competitive against the current mixture of solar salt. Table 13: TES cost for two tank Li/Na/K mixture from Atacama TES Cost for Two - Tank Li/K/Na nitrates Mixture 0,705 TES CAPACITY Salt Cost Mixture (Li/K/Na) Scaling exp Rel. Size Cold Tank 0.8 0.705 Hot Tank 0.8 0.705 Salt Inventory 1 Instrumentation 0.8 0.705 Structural steel 0.8 0.705 Tank insulation 0.8 0.705 Electrical 0.8 0.705 Concrete 0.8 0.705 Foamglass 0.8 0.705 Refractory 0.8 0.705 Sitework 0.8 0.705 Painting 0.8 0.705
2703 U$S /Ton 2689 U$S/ton 40107 $2459 5% $5647 11% $40107 76% $120 0% $375 1% $2099 4% $271 1% $879 2% $541 1% $299 1% $191 0% $5 0% $52993 MUS$ $19.6 US$/Kwh-t
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Figure 12: TES cost ternary mixture Li/K/Na v/s LiNO3 price estimation
According to the point of view of cost analysis, the results obtained in Table 4 should be evaluated at a more exhaustive level. It was obtained that for a mass unit of Li2CO3 it is possible to obtain almost twice as much LiNO3, which could mean a reduction in costs of the process to obtain lithium nitrate from lithium carbonate.
5.
Conclusions:
Lithium nitrate could be one of the most promising additives in TES materials that are under study for their application on CSP plants. The present paper confirms the excellent thermal properties evaluated for a novel mixture containing 30wt% of LiNO3 + 57 wt% KNO3 + 13 wt% NaNO3 with different purity grades for LiNO3. From the work done it is possible to obtain the following conclusions: The specific heat and melting point was detected using the DSC Q-20 for all ternary mixtures with different purity grades of LiNO3. The heat capacity is between 14% to 21% higher than 24
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the solar salt currently used. Melting point was also reduced between 92 and 96°C lower compared to solar salt. Regarding the thermal stability, it was found that for all the ternary mixtures with the different purity grades of LiNO3 evaluated, the highest weight loss (wt%) of all ternary mixtures is 5.1% corresponding to the ternary mixture composed by LiNO3 98%, with higher amount of impurities. The maximum temperatures obtained in the zone 4 (decomposition) are around 594 and 596 ° C and the decomposition could be given by the evolution at high temperature of volatile N2, NO, O2, N2O, and NO2 species. According the viscosity values, it was found that for all the ternary mixtures with the different purity grades of LiNO3 evaluated, there is not a clear relation between viscosity values and the different purity grades used. Moreover, it is observed that the tendency to decrease the viscosity is maintained as the temperature increases, behavior similar to the solar salt. Finally, regarding the cost analysis for TES, a reduction in the volume (about 35%) could be achieved to reach a similar TES capacity compared to solar salt. Matching the cost of TES for both mixtures to US $ 19.6/ Kwh-t, the price of lithium nitrate contained in the ternary mixture should have a maximum price of US$ 7520/Ton.
Acknowledgement The authors would like to acknowledge the financial support provided by CONICYT / FONDAP 15110019 "Solar Energy Research Center" SERC-Chile, FIC-R 30413089 funded by Antofagasta Government and Minera Escondida Fundation.
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Declaration of interest statement: We declare to be aware of the interest statement of this document and not conflicto of interest has been reported
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Highlights:
Lithium based salts present a higher work temperature range compared to solar salt Higher purities show a decrease in viscosity with the temperature 35% of reduction could be achieved using LiNO3, to reach the current TES capacity Melting point shows a reduction close to 90°C with respect to solar salt The heat capacity increases between 14 to 21% respect solar salt