Development and characterization of a quaternary nitrate based molten salt heat transfer fluid for concentrated solar power plant

Abstract

Over the past few years, there has been growing interest in using inorganic quaternary nitrate-based molten salt mixtures as a highly effective heat transfer fluid (HTF) for concentrated power plants, primarily because they can achieve low melting temperatures. However, the high viscosity of these salt mixtures is still a significant challenge that hinders their widespread adoption. The high viscosity leads to high pumping power requirements, which increases operational costs, and reduces the efficiency of the Rankine cycle. To address this challenge, this study developed and characterized a novel quaternary molten salt, focusing on the effect of LiNO₃ additions on the salt's viscosity, thermal conductivity, melting point temperature, heat capacity, and thermal stability. The quaternary mixture comprised KNO₃, LiNO₃, Ca(NO₃)₂, and NaNO₂, with varying percentages of each salt. The study utilized various standard techniques to examine the characteristics of the developed mixture. Results showed that increasing LiNO₃ content led to a decrease in melting temperature, higher heat capacity, improved thermal stability, conductivity, and reduced viscosity at solidification temperature. The lowest endothermic peak for the new mixture emerged at 73.5 °C, which is significantly lower than that of commercial Hitec and Hitec XL, indicating better potential for use as a heat transfer fluid for concentrated solar thermal power plant applications. Furthermore, the thermal stability results showed high stability up to 590 °C for all the samples examined. Overall, the new quaternary molten salt shows promise as a potential replacement for current organic synthetic oil, offering a more efficient solution.

1. Introduction

Alternative energy sources have gained increasing attention in recent years due to rising global energy demand and concerns about climate change. Concentrated solar thermal power plants are recognized as part of the future technology for harvesting the energy of the sun to generate electricity. To address the need for peak and off-peak energy supply, these systems are typically retrofitted with thermal energy storage (TES) technology. Most commercial concentrated solar power plants make use of organic synthetic oil as working fluid. However, the limitations of these oils, including their high cost and low thermal stability, have prompted researchers to explore alternative heat transfer fluids [[1], [2], [3], [4], [5], [6], [7]].

Inorganic molten salts are one promising alternative due to their low cost, high thermal stability, and ability to store large amounts of heat. For example, the solar salt consisting of a binary combination of 40%wt KNO₄ and 60%wtNaNO₃ has been developed for usage in most commercial plant [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]]. Other commercially available inorganic salts, such as Hitec and HitecXL, have melting point temperatures of 142 °C and 133 °C respectively [[21], [22], [23], [24], [25]].

However, one of the key issues restricting the use of inorganic salt mixtures as opposed to organic synthetic oils is the fact that they have a high melting point, and is usually above 100 °C. This can lead to various operational issues such as the tendency to solidify along the piping system. The high viscous nature of these inorganic salt combinations also requires increased pumping power to sustain plant operation, leading to poor plant efficiency.

To address these challenges, various researchers have suggested unique inorganic molten salt mixtures with improved thermophysical characteristics [[26], [27], [28], [29], [30]]. For example, researchers have synthesized ternary and quaternary nitrate molten salt mixtures with low melting points, good thermal stability, and high heat capacity by adding percentages of Ca(NO₃)₂ and LiNO₃ in varied proportions [[31], [32], [33]]. However, reducing the melting point of these mixtures can also increase their viscosity, which is a serious drawback for their use as working fluid.

A study by Py et al. (2015) investigated the performance of a binary nitrate salt mixture (60 wt% NaNO₃ and 40 wt% KNO₃) and found that it has good thermal stability and heat capacity, but its high melting point limits its application in concentrated solar thermal power plants [34]. Another study by Bai et al. (2017) investigated the use of ternary nitrate salt mixtures (KNO₃, NaNO₃, and Ca(NO₃)₂) and found that adding Ca(NO₃)₂ can significantly reduce the melting point while maintaining good thermal stability and heat capacity [35].

To further reduce the melting point of inorganic salt mixtures, several studies have investigated the use of quaternary nitrate salt mixtures. Liu et al. (2020) synthesized a quaternary nitrate salt mixture with a low melting point (144.4 °C) by adding LiNO₃ to a ternary nitrate salt mixture (KNO₃, NaNO₃, and Ca(NO₃)₂) [36]. The addition of LiNO₃ increased the heat capacity and thermal conductivity while reducing the viscosity of the salt mixture [37]. investigated the effect of adding different percentages of LiNO₃ to a quaternary nitrate salt mixture (KNO₃, NaNO₃, Mg(NO₃)_2, Ca(NO₃)₂) and found that the salt mixture with 10 wt% LiNO₃ has the lowest melting point and viscosity while maintaining good thermal stability and heat capacity. However, most of these studies have focused on analyzing the thermophysical properties of inorganic salt mixtures at temperatures above 200 °C, which is not ideal for improving pumping efficiency and achieving superior performance compared to synthetic oil. Thus, further research is needed to investigate the behavior of inorganic salt mixtures at lower temperatures to improve their pumping efficiency and overall performance in concentrated solar thermal power plant.

In this paper, we present a novel quaternary nitrate salt mixture that includes lithium nitrate as an additive to investigate the effect on melting temperature, viscosity, thermal conductivity, heat capacity and thermal stability for improved performance in concentrated solar power applications. The composition range includes: 19–30 wt% NaNO₂, 18-25 wt% KNO₃, 5-35 wt% LiNO₃, 28-40 wt%Ca(NO₃) (NO₃)₂. We believe that this research will provide valuable insights into the development of more efficient and cost-effective HTF. The novelty of this research lies in the attempt to reduce the viscosity of the inorganic salt mixture and analyze its viscosity at temperatures below 200 °C, which is crucial for improving pumping efficiency and achieving superior performance compared to synthetic oil.

2. Experimental details

The salts utilized in this investigation (with purity >99%) were provided by Alpha-Aesar Limited. The quaternary combinations were mixed in a dry environment with ultrapure argon (Ar-99.99%) using various compositions, including 19-30 wt% NaNO₂, 18-25 wt% KNO₃, 5-35 wt% LiNO₃, 28-40 wt%Ca(NO₃) (NO₃)₂. This was done to develop a salt combination that has a low melting point, is stable at high temperatures, and has a low viscosity. The salt mixture was then heated in a box furnace for 3 h and exposed to a temperature of 200 °C to eliminate all traces of moisture. The thermal condition was elevated to 400 °C for a duration of 2 h for complete integration of all the components. The blended sample was then allowed to cool naturally to room temperature before it was used for the study. Table 1 presents the quaternary salt compositions' mass ratios.

Table 1.

Quaternary mixture.

	Ca(NO3)2	LiNO3	KNO3	NaNO2
S1	40 wt%	5 wt%	25 wt%	30 wt%
S2	38 wt%	10 wt%	24 wt%	28 wt%
S3	36 wt%	15 wt%	23 wt%	26 wt%
S4	34 wt%	20 wt%	21 wt%	25 wt%
S5	32 wt%	25 wt%	20 wt%	23 wt%
S6	30 wt%	30 wt%	19 wt%	21 wt%
S7	28 wt%	35 wt%	18 wt%	19 wt%

Eq (1) was used to determine the standard uncertainty of the experimental data [33,38].

$$U=k\sqrt{\sum _{i=1}^N{\left(\frac{\partial f}{\partial x_i}\right)}^2{\left(u_{x_i}\right)}^2}+2\sum _{1\le i<j}^N\frac{\partial f}{\partial x_i}\frac{\partial f}{\partial x_{\mathrm{j}}}\epsilon iju{x}_{i}u{x}_j$$ (1)

where u_x.i & u_x.j stand for the conventional components uncertainty brought on by, x.i and x.j respectively, while U represents extended uncertainty. The value of the variables assessed in this study is denoted by the letters f. Ɛ_ij stands for the correlation coefficient of x.i and x.j, and k stands for the confidence factor with a value of 2.

2.1. Equipment

Multifunctional Thermo Analysis tool (STA-1500) was utilized to study the heat capacity, stability, as well as melting point of the quaternary nitrate salt samples. A temperature ranges from 50.0 °C to 1300.0 °C was applied to calibrate the apparatus using Zn and Al reference material. Also, alterations in heat flow and weight reduction were identified using the instrument. Specimens were positioned in experimental dish and heated to a temperature between twenty-eight degrees and two hundred degrees in an Ar atmosphere. The data were obtained at a thermal ramp of 10.0 °C/min and at a rate of 51.0 mL/min. Experimental data was measured twice to minimize inaccuracies. Errors in heat and temperature measurement, especially those associated to weight of the sample, quantification accuracy, and ambient temperature, have a considerable effect on the margin of error associated with the STA's measurements. Deviation of 1.0 °C was observed in the temperature readings. The capacity for heat absorption in an Ar atmosphere with a 10 °C/min heating rate was determined using the heat capacity ratio approach. This analysis follows the measurement technique used in earlier investigations [8,33]. Prior to the experiment, a baseline was established by heating a graphite crucible to 500.0 °C. This was filled with the regular sapphire material and heated. This supplied a reference for comparison. Subsequent measurements were done on each sample combination under the same conditions. The primary sources of uncertainty are the consistency of the test, the heat capacity of the base material, the sample mass and heating ramp. The heat capacity variability in the study is 0.090 J/g^oC. The samples were heated in a melting dish at 10.0 °C/min for thermal stability test, and were retained in a molten form for 48 h to initiate loss of weight.

Viscosity of the quaternary mixtures were determined at 90.0 °C–180.0 °C using a Malvern Instruments Kinexus rheometer with a coaxial cylinder setup. A shear rate varied between 1 and 400 1/s was applied to 40 g of heated material over a 15-min ramp period. This range of shear rates was created so that molten salt may flow to low viscosities during pumping operations. Viscosity measurements obtained by the device indicated a ±10% uncertainty. Archimedes approach, was used to determine the density of the quaternary mixture. Laser flash experiments were done to determine thermal diffusivity (α). Each mixture combinations were placed in a crucible that was heated in increments of 10.0 °C from 200.0 °C to 500.0 °C.

The test specimens were then transported to the LFA apparatus after being degassed 3 times in a vacuum furnace. Equation (2) was used to obtain the thermal conductivity (Λ) using the specific heat and density measurements. Error associated with the thermal conductivity was obtained as ±0.022 W/mK

$$\mathrm{\Lambda }={\mathrm{C}}_{\mathrm{p}.}\mathrm{\alpha }·\mathrm{\rho }$$ (2)

3. Results and discussion

3.1. Melting temperature

Fig. 1 depicts the characteristics of nitrate salt mixtures in a molten state for S1–S7. Table 2 summarizes the values of the endset temperature. In order to prevent variations during the liquefaction process resulting from the moisture attracting behavior of LiNO₃ salt, several heating and cooling runs were performed. The rate of heating affects the onset of melting and the size and enthalpy of the peaks, therefore, the endpoint temperature of the main endothermic peak on the differential scanning calorimetry curve can be considered as the melting temperature of the sample. The melting temperature of the quaternary mixture with increasing LiNO₃ concentration for samples S1–S7 was observed to be in order of decrease. According to Refs. [32,33,38] and comparing with the commercial molten salt mixtures such as the Hitec and Hitec XL which has a melting point temperature of 142 °C and 127 °C respectively, it is observed that melting point of the novel quaternary mixtures for S1–S7 is significantly lower. The lowest endothermic peak for S7 mixture emerged at 73.5 °C, with an endset temperature of 78.67 °C. The lowest endothermic peak for S6 emerged at 75.2 °C with an endset temperature of 79.34 °C. Similarly, the lowest endothermic peak and endset temperatures for S1, S2, S3, S4 and S5 were obtained as 100.1 °C and 123.4 °C, 89.3 °C and 102 °C, 86.5 °C and 89.8 °C, 77.7 °C and 83.4 °C 76.4 °C and 82.6 °C. The plots depicting the thermal behavior of the quaternary mixture revealed further mild endothermic changes occurring subsequent to the melting temperature. In accordance with [[39], [40], [41]], elements of systems with peritectic behavior do not meet thermodynamic conditions; yet, the combination of salts has the potential to liquify within the range of temperatures. Furthermore, the results suggest that incorporating LiNO₃ can result in a reduction of the melting temperature and not affect the thermal characteristics. For practical applications in concentrated solar thermal power plant, a safety margin of 30.1 °C over the measured quantity is recommended and maintained when examining the lowest operational temperatures of the liquified salts. To avoid pipe blockage or catastrophic failure, thermal power plants must sustain temperatures higher than the quaternary eutectic points necessary to ensure stability. A low melting point can increase the rate of heat transfer while minimizing the charge/discharge length of the cycle, enhancing the effectiveness of concentrated solar power as a whole. According to Refs. [8,42] melting points below 100 °C necessitate less energy input for plant operation. A melting temperature exceeding 100 °C is needed to avoid crystallization of the molten salt mixture, thus a sufficient amount of input energy is necessary.

Fig. 1.

Differential scanning calorimetry characteristics of nitrate salt mixtures in a molten state for S1–S7.

Table 2.

Average result for melting point temperature.

Molten salt mixture	Melting temperature (0C)	Endset temperature (0C)
S1	100.1	123.4
S2	89.3	102.3
S3	86.5	89.8
S4	77.7	83.4
S5	76.4	82.6
S6	75.2	79.34
S7	73.5	78.67

3.2. Heat capacity

The heat capacity of molten salt is an essential metric that should be optimized in order to lower the amount of salt to be used for thermal storage [43]. A modulated differential scanning calorimetry test was performed between temperatures of 50 °C to 280 °C to measure the heat capacity. As shown in Fig. 2, the heat capacity of samples S1, S2, S3, S4, S5, S6 and S7 was obtained in the range of 1.674 J/g^oC, 1.893 J/g^oC, 1.962 J/g^oC, 1.975 J/g^oC, 1.898 J/g^oC, 2.00 J/g^oC, 2.010 J/g^oC respectively. The heat capacity is enhanced by the addition of lithium nitrate as observed in the result of the samples S1 to S7. The average heat capacity of the novel salt mixtures achieved higher value, which is greater than the heat capacity of the Hitec commercial salt with a value of approximately 1.55 J/g^oC.

Fig. 2.

Heat capacity for various samples.

Wang [44] developed a NaNO₃–NaNO₂–KNO₃–LiNO₃ quaternary salts for concentrated solar power applications having a capacity 2.86 J/g degree Celsius at 300.0 °C [45]. published a NaNO₃–NaNO₂– KNO₃–LiNO₃- quaternary salt having 2.56 J/g °C heat capacity and direct rise in temp., while [46] found 1.0911 J/g degree Celsius at 380 °C for the NaNO₃–KNO₃–LiNO₃. In this study, higher heat capacity of 2.01 J/g°C at 280 °C, for S7 was obtained, when compared to the other nitrate and nitrite salts quaternary mixture in literature. This suggest that molten salt mixtures have a higher potential for advancing concentrated solar thermal power plant technology.

3.3. Viscosity

The viscosity of the heat transfer fluid has a direct influence on pumping power and operational costs of a solar thermal plant. The higher the viscosity, the higher the amount of pumping force or amount of energy needed to maintain the operational temperature of the plant. The viscosity of each sample is shown in Fig. 3. The mixtures were heated to 175 °C to guarantee that any moist absorbed during loading was removed. Each point reflects the average outcome for 1-h observation period at a step constant temperature. The viscosity of the quaternary samples S1–S7 decreases as the concentration of lithium nitrate increases. High level of calcium nitrate-containing mixtures exhibits greater viscosity as observed from each sample composition.

Fig. 3.

Viscosity of sample mixtures.

The viscosity for S1 with 5 wt% LiNO₃ was 14.75 cp at 120 °C. With the addition of 10 wt%LiNO₃ for sample S2, the viscosity was further reduced to 14.65cp at similar temperature. Reduction of the viscosity was further observed for samples S3, S4, S5, S6 and S7 with values 13.92cp, 13.1cp, 12.98cp, 11.98 and 11.56 cp respectively at 120 °C. The variation of these viscosities tends to reduce further with temperature increase. For the ternary mixture 45 wt% KNO₃, 12 wt% LiNO₃ and 43 wt% Ca (NO₃)₂, developed by Ref. [27], the viscosity at 92.4 °C was 370cp. The quaternary mixture developed by Sandia Lab had a viscosity of 450cp at 98 °C. By comparing the viscosity obtained in this study with literature findings, clearly, there is tremendous decrease in the viscosity with the addition of more lithium nitrate. This mixture can be better suited as heat transfer fluid with reduced pumping power and improved overall efficiency of the Rankine cycle in solar thermal power applications. The cost implication is discussed in the proceeding section. Notably, the molten salt tends to turn crystalline at temperatures just above each melting point of the various samples. Hence, the mixture must be maintained above 170 °C for power plant applications.

3.4. Thermal stability

The stability of low-melting salt mixtures at high temperatures determines the consistency of operation and safety of solar thermal energy storage system. Dispersion occurs in bits and is influenced by the O₂ level of the quaternary salt's environment [8,12]. Fig. 4a–g depicts the results of the thermal stability tests conducted on the samples S1–S7. Table 3 shows the values of the thermal decomposition for the various samples. The quaternary nitrate salt sample S1 showed high stability below 590 °C and gradually decompose at 600 °C for all sample mixtures. Weight loss accelerates dramatically at an average value over 620 °C. S2, S3, S4, S5, S6 and S7 showed high stability below 590 °C, 602 °C, 610 °C, 613 °C, 618 °C and 620 °C respectively. The weight loss of each sample accelerated at 620 °C, 621 °C, 623 °C, 625 °C, 630 °C and 632 °C respectively. This shows that, the quaternary mixture has a strong propensity to increase the performance of the organic Rankine cycle. In literature, the commercial Hitec salt achieved a stability temperature between 535.0 °C −538.0 °C [8,27,33,38], while the salt mixture in this study achieved stability between 590 °C - 600 °C. Hence this shows great improvement when compared to the novel mixtures developed in this study.

Fig. 4.

Thermal decomposition plot.

Table 3.

Thermal decomposition values.

Sample	Thermal decomposition °C
S1	590
S2	592
S3	602
S4	610
S5	613
S6	618
S7	620

Several studies have reported on the long-term stability of molten salt mixtures for thermal energy storage applications. For example [44], investigated the thermal stability and cyclic performance of a quaternary nitrate salt mixture (KNO₃, Ca(NO₃)_2, NaNO₃, , and Mg(NO₃)₂) with 10 wt% LiNO₃. The salt mixture was subjected to 1000 thermal cycles between 200 °C and 550 °C, and the results showed that the salt mixture maintained good thermal stability and cyclic performance with no significant degradation over time. Li et al. (2019) also investigated the long-term stability of a quaternary nitrate salt mixture (NaNO₃, Ca(NO₃)_2, KNO₃ and Mg(NO₃)₂) for solar thermal power generation.

The salt mixture was stored in stainless steel containers at 300 °C for 2000 h, and the results showed that the salt mixture maintained its thermal stability and heat capacity over time with no significant degradation [43]. While this study provides short-term stability data that shows excellent performance, the long-term stability of molten salt mixtures depends on various factors. Further studies are needed to investigate the long-term stability of various salt mixtures under variable conditions to assess their overall performance throughout the operational life of a concentrated solar thermal power plant.

3.5. Thermal conductivity

From Table 4, the quaternary salt samples is shown to have higher conductivity with an average value of 0.372 W/m.K, then by Hitec XL with 0.316 W/mK, and organic oil with 0.091 W/mK. Although there were little variations in thermal conductivity between 80^oC-280 °C for sample S1–S7 as seen in Fig. 5, they did exhibit a significant improvement over commercial organic oil. Higher thermal conductivity heat transfer fluid will be a superior solution for solar power plant applications since it will give a higher heat transfer rate inside the system. Also, the connection between temperature and thermal conductivity is seen to be directly proportional. Thermal conductivity is shown to increase with increasing temperature. The high thermal conductivity of samples S1–S7 suggests that the salt has strong heat transfer properties compared to HitecXL salt.

Table 4.

Thermal conductivity values.

	Thermal conductivity	Temperature range
Organic-oil [47,48]:	0.080–0.100 (W/mK)	80–280 °C
Hitec-XL [[47], [48], [49]]	0.311–0.313 (W/mK)	80–280 °C
S1	0.325–0.419 (W/mK)	80–280 °C
S2	0.323–0.419 (W/mK)	80–280 °C
S3	0.324–0.408 (W/mK)	80–280 °C
S4	0.321–0.407 (W/mK)	80–280 °C
S5	0.315–0.406 (W/mK)	80–280 °C
S6	0.315–0.406 (W/mK)	80–280 °C
S7	0.315–0.400 (W/mK)	80–280 °C

Fig. 5.

Thermal conductivity of various sample mixtures.

This study suggests that the influence of LiNO₃ on the properties of the molten salt mixture could be due to its ability to modify the microstructure of the salt mixture. The addition of LiNO₃ disrupted the crystal structure of the other nitrate salts, resulting in a more homogeneous and amorphous microstructure. This, in turn, reduced the viscosity of the salt mixture, making it easier to pump and requiring less energy for circulation. Furthermore, the addition of LiNO₃ facilitated the movement of ions in the mixture, improving heat transfer and leading to higher thermal conductivity. Another possible reason is that LiNO₃ increased the heat capacity of the salt mixture. LiNO₃ has a high heat capacity and can absorb more heat than other nitrate salts, possibly increasing the overall heat capacity of the salt mixture and making it more efficient in storing and releasing thermal energy. The addition of LiNO₃ also improved the thermal stability of the salt mixture by acting as a stabilizer and preventing the decomposition of other nitrate salts at high temperatures. The exact mechanism by which LiNO₃ influences the properties of the molten salt mixture is complex and may involve several factors, including the crystal structure, ionic interactions, and the thermodynamic properties of the individual salts. Therefore, further theoretical and experimental studies are needed to fully understand the influence of LiNO₃ on the properties of molten salt mixtures.

3.6. Cost assessment

The cost of the salt mixture is crucial in analyzing the commercial usefulness, and market competitiveness. The price influences the possible application of a recommended material for thermal energy storage to some extent [50]. The cost value reported in Table 5 for each salt was gathered from the Alpha-Aesar Company's website [51]. As observed, the Lithium nitrate salt is the most expensive salt, costing much more than Ca(NO₃)₂, NaNO₂ and KNO₃. The cost of the quaternary samples was computed according to its unique ratio using Equation (3). Table 6 compares the cost parameters of each quaternary salt developed in this work with comparable data accessible in literature. The salt mixtures developed in the present study may not at the initial instant be of substantial economic advantage compared to the other cost of the existing salt mixtures, notably with S7 mixture. However, the low melting point and low viscosity will serve as a key benefit to improve the performance of concentrated solar thermal power plants. In this experiment, the S7 quaternary mixture in particular has greatly boosted the thermal stability and decreased the viscosity compared to other mixtures in Table 6, which may be beneficial in the long term.

$$\mathrm{C}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{t}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{a}\mathrm{l}\mathrm{t}\mathrm{m}\mathrm{i}\mathrm{x}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}=\sum \%\mathrm{o}\mathrm{f}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{d}\mathrm{u}\mathrm{a}\mathrm{l}\mathrm{s}\mathrm{a}\mathrm{l}\mathrm{t}\mathrm{x}\mathrm{C}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{p}\mathrm{e}\mathrm{r}/\mathrm{k}\mathrm{g}$$ (3)

Table 5.

Cost per salt.

Salt	Ca(NO3)2	NaNO2	KNO3	LiNO3
Cost ($/kg)	0.20	0.30	0.40	0.80

Table 6.

Price of molten salt.

Sample	Composition	Tm (oC)	Td (oC)	Cost ($/kg)	Source
Hitec	KNO3–NaNO2–NaNO3 (53-7-40 wt%)	142.0	535.0 566	0.29	[49]
Solar Salt	KNO3–NaNO3 (40-60 wt%)	240.0	566.0	0.30	[50]
CaNaKNO3	Ca(NO3)2 NaNO3–KNO3 (36-48-16 wt%)	108.0	500	0.29	[49]
KNO3LiNa	KNO3LiNO3–NaNO3 (52-18-30 wt%)	123.0	545	0.46	[49]
Sample Btt	LiNO3–KNO3–Ca(NO3)2 (7.8-57.4-34.8 wt%)	68.81	469	0.37	[8]
Hitec XL	Ca(NO3)2 –NaNO2–KNO3 (48-7-45 wt%)	128	516	0.29	[49]
Sample Dtt	LiNO3–KNO3–Ca(NO3)2 (13.4-69.2-17.4 wt%)	78.9	472	0.43	[8]
Sample Ett	LiNO3=KNO3–Ca(NO3)2 (11.4-65.4-23.2 wt%)	79.9	469	0.40	[8]
Sample Aqq	NaNO2–LiNO3–Ca(NO3)2–KNO3 (20.6-19.6-40.4-19.4 wt%)	83.3	529	0.38	[8]
Sample Bqq	NaNO2–LiNO3 Ca(NO3)2 KNO3 (28.7-23.9-37.2-10.3 wt%)	82.2	531	0.39	[8]
S1	NaNO2–LiNO3–Ca(NO3)2 KNO3 (30-5-40-25 wt%)	100.1	580	0.34	This study
S2	LiNO3 Ca(NO3)2 KNO3 NaNO2 (10-38-24-28 wt%)	89.3	590	0.336	This study
S3	LiNO3 Ca(NO3)2 KNO3 NaNO2 (15-36-23-26 wt%)	86.5	602	0.362	This study
S4	NaNO2LiNO3 Ca(NO3)2 KNO3 (25-34-21-21 wt%)	77.7	610	0.387	This study
S5	LiNO3 Ca(NO3)2 KNO3NaNO2 (25-33-20-23 wt%)	76.4	613	0.413	This study
S6	NaNO2LiNO3 Ca(NO3)2 KNO3 (21-30-19-30 wt%)	75.2	618	0.439	This study
S7	NaNO2LiNO3 Ca(NO3)2 KNO3 (19-28-18-30 wt%)	73.5	620	0.465	This study

3.7. Hygroscopic effect and corrosion

The high working temperature of concentrated solar technology generates severe corrosion of the materials utilized. The molten salt cations and anions are associated with corrosion processes. Numerous experiments have revealed that some alloys with large metal concentrations may be damaged by molten salts at extreme temperatures, most typically leading to alloy disintegration [[52], [53], [54]]. Investigating the corrosivity of molten salt and the effects on various metals at high temperatures is often one of the key research focuses for solar thermal applications. It is therefore necessary to carry out further investigations to examine the characteristics or performance of the novel quaternary salt formulated through this investigation to completely asses the practical suitability in renewable thermal power generation.

4. Conclusion

In this study, a novel quaternary nitrate salt mixture was developed and tested for its thermophysical characteristics as a potential heat transfer fluid in concentrated solar thermal power plants. The addition of lithium nitrate was found to significantly reduce the viscosity of the mixture, with a maximum reduction observed in S7 at 11.56 cp. Moreover, the heat capacity of the samples was found to increase with an increasing concentration of lithium nitrate. The quaternary mixture exhibited higher thermal stability than Hitec and Hitec XL commercial salt, and its lowest endothermic peak was observed at 73.5 °C, which is significantly lower than existing commercial salts. The range of thermal conductivity for the quaternary salt samples was also found to be higher than that of Hitec XL and organic oil. However, the economic viability of the new quaternary salt mixture may be a challenge initially, particularly for S7. Nevertheless, the low melting point and viscosity of the mixture make it advantageous for use in concentrated solar thermal power plants. The quaternary mixture with S7, in particular, exhibited superior thermal stability compared to other available mixtures. Further studies are needed to evaluate the corrosion and hygroscopic behavior of the mixture, as well as its cycle performance over time, which are crucial for future research. Overall, this study provides important insights into the potential use of the developed mixture for Rankine cycle.

Author contribution statement

Collins Chike Kwasi-Effah: Conceived and designed the experiments; performed the experiments; analyzed and interpreted the data; wrote the paper.

Henry O. Egware; Osarobo I. Ighodaro: Performed the experiments; analyzed and interpreted the data.

Albert I. Obanor: Performed the experiments; analyzed and interpreted the data; contributed reagents, materials, analysis tools and data.

Data availability statement

Data will be made available on request.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.