Thermo-economic analysis of high-temperature sensible thermal storage with different ternary eutectic alkali and alkaline earth metal chlorides

Solar Energy 176 (2018) 350–357

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Thermo-economic analysis of high-temperature sensible thermal storage with different ternary eutectic alkali and alkaline earth metal chlorides

T

Gowtham Mohana, Mahesh Venkataramana, Judith Gomez-Vidalb, Joe Coventrya,

⁎

a b

Research School of Engineering, Australian National University, Canberra, ACT 2601, Australia National Renewable Energy Laboratory (NREL), Golden, CO 80401, USA

ARTICLE INFO

ABSTRACT

Keywords: High-temperature Molten salt Sensible heat storage Melting point Heat capacity Mass loss

Molten salt mixtures with alkali and alkaline earth metal chlorides were developed for high-temperature sensible thermal energy storage in support of concurrent efforts to develop high-temperature advanced power cycles for concentrating solar power applications. Four ternary chloride mixtures with different cation combinations (Na, K, Li, Mg) were designed using the FactSage® software, and for three of these, the eutectic point was experimentally validated by differential scanning calorimetry. Specific heat capacity measurements were conducted following the ASTM E1269 standard, and were measured between 1.18 J/g/K and 1.31 J/g/K. The mass loss of the molten chloride salts was studied under three different gas blankets of nitrogen, argon and air by thermogravimetric analysis. All the selected salt mixtures were stable up to 700 °C, although weight loss due to vaporisation becomes significant around this temperature due to the high vapour pressure of the chloride salt mixtures. However, it is expected that operation at a temperature up to around 750 °C will be feasible in a closed system with an inert environment. Additionally, removal of chemically-bonded water and salt purification may need to be considered for extending the operating temperature. In terms of economic performance, although the inclusion of LiCl in the ternary eutectic mixtures is advantageous for reducing melting point and increasing specific heat capacity, at current costs, these benefits are unlikely to be justified unless LiCl cost reduces by a factor of three. The NaCl-KCl-MgCl2 mixture has the lowest cost per unit energy stored, at 4.5 USD/kWh.

1. Introduction

cycles like supercritical steam Rankine, supercritical CO2 (s-CO2) Brayton, and air Brayton cycles. For example, the recent U.S. Gen3 CSP Roadmap (Mehos et al., 2017) is structured around the development of a s-CO2 Brayton power cycle operating between temperature limits 500–700 °C. Another current R&D program based around the use of the s-CO2 Brayton cycle is the Australian Solar Thermal Research Initiative (ASTRI) (ASTRI, 2017). A technoeconomic analysis, done within ASTRI, comparing turbine inlet temperatures of 560 °C, 610 °C, 700 °C and 1000 °C, found highest potential for meeting cost targets based on the 610 °C case (Meybodi et al., 2016). The selection of storage media and heat transfer fluid (HTF) plays an important role in developing high-efficiency CSP plants, as they support and integrate both the receivers and power block segments of the plant. Current generation molten salts (solar salt and HITEC) decompose around 600 °C (Olivares, 2012), making them inadequate for many advanced high-temperature power cycles. Several studies were conducted utilising solid sensible heat storage materials for high-temperature application, but this approach to TES is at a relatively early stage in terms of technology maturity and market acceptability

Solar energy is increasingly considered as one of the most favourable alternative sources of energy to conventional fossil fuels. Despite its advantages, the intermittent availability of solar energy results in a gap between supply and demand at certain times of the day unless energy storage is introduced. A distinct feature of concentrating solar power (CSP) plants is that thermal energy storage (TES) can be readily integrated between the solar receiver and the power cycle, and therefore CSP plants can operate in the absence of solar radiation, and support energy demand during these periods. Development of energy efficient and cost-effective thermal storage systems and storage media is vital for the CSP industry (Casati et al., 2015; Niyas et al., 2014). Many existing CSP plants incorporate a molten salt sensible heat storage system, using either solar salt (60 wt% NaNO3 – 40 wt% KNO3) or HITEC® salt (53 wt% KNO3 – 40 wt% NaNO2 – 7 wt% NaNO3) (Gil et al., 2010; Kuravi et al., 2013; Ushak et al., 2015; Wang et al., 2012b). There is significant research effort at present to improve the efficiency of CSP plants by increasing temperature, and making use of advanced power

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Corresponding author. E-mail address: [email protected] (J. Coventry).

https://doi.org/10.1016/j.solener.2018.10.008 Received 15 June 2018; Received in revised form 28 September 2018; Accepted 3 October 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.

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(Baumann and Zunft, 2015; Fernandez et al., 2010; Khare et al., 2013; Navarro et al., 2012; Py et al., 2011), whereas the two-tank molten salt TES configuration (using nitrate salts) is already widely accepted. For high temperature sensible thermal energy storage (HTSTES), there is the potential to build upon existing experience with system design and operation of the two-tank configuration, while introducing new molten salt media compatible with the advanced power cycles under development. For the US Gen3 Roadmap example mentioned above, the hot salt would need to be at least at 720 °C to allow a power cycle HTF inlet temperature of 700 °C. For the case where the molten salt is heated directly in a solar receiver, hot-spots reaching 750 °C, or even 800 °C, are possible and therefore salts must be stable to at least those temperatures to avoid vapour formation in the receiver. Mixtures of carbonates, chlorides, fluorides and hydroxides could potentially be used for HTSTES. The majority of testing of these materials for TES has focussed on latent heat storage applications (Gomez, 2011; Kenisarin, 2010; Liu et al., 2015). HTSTES media should not only have high specific heat capacity and excellent thermal stability, but must also be economically viable. For this reason, chloride salt mixtures are promising candidates, with their combination of good thermo-physical properties (Gomez, 2011; Kenisarin, 2010) and relatively low cost. Although, corrosion is beyond the scope of the present work, the authors acknowledge the high corrosion rates of metallic materials in chloride-based molten salts. The corrosion of multi-component alloys in chlorides has been studied extensively and several corrosion mitigation strategies have been proposed, such as, use of alumina-forming Ni-based alloys (Gomez-Vidal et al., 2017), alloying with refractory elements (W/Co) (Oryshich and Kostyrko, 1985), coatings (Porcayo-Calderón et al., 2014; PorcayoCalderon et al., 2012), and cathodic protection (Garcia-Diaz et al., 2016; Mehrabadi et al., 2017). It is anticipated that corrosion rates of < 100 μm/ yr are achievable by using a combination of these approaches. Mixtures of alkali and alkaline earth metal chlorides were studied previously as coolant materials for nuclear applications as part of the next generation nuclear plant (NGNP) project (Williams, 2006). Initial evaluation of thermal properties based on this work indicates their possible suitability for HTSTES. More recently, two ternary eutectic chlorides of NaCl-MgCl2-CaCl2 (NaMgCa–Cl) (Wei et al., 2015) and NaCl-KCl-ZnCl2 (NaKZn–Cl) (Li et al., 2016) were developed and tested for HTSTES application. The NaMgCa–Cl mixture has a higher melting temperature (424 °C) than the target in this work (< 400 °C), but has low material cost, whereas the NaKZn–Cl mixture has a very low melting temperature (204 °C), but it is relatively more expensive. In previous work by the present authors, thermo-physical property characterisation of the NaCl–KCl–MgCl2 ternary and its constituent binaries was conducted (Mohan et al., 2018). The ternary mixture displayed better thermo-physical properties compared to the binaries. Addition of Li salt (LiNO3) with Solar Salt and HITEC salt has been shown to have a positive effect on both melting point and heat capacity (Fernández et al., 2014; Wang et al., 2012a,b). Similar improvements were reported for carbonates, and both the heat capacity and melting temperature of a binary carbonate (Na2CO3–K2CO3) were improved by adding LiF (Wang et al., 2015) and Li2CO3 (Chen et al., 2014; Janz et al., 1963). This motivated the present study to investigate further ternary chloride mixtures, this time with the addition of LiCl, which is added because of its expected impact on reduction of melting point (discussed further below in Section 3.1) and enhancement of specific heat capacity due to the addition of a Li salt. The four ternary eutectic salt mixtures that can be formed from the base alkali and alkaline earth metal chlorides, LiCl, NaCl, KCl and MgCl2, are tested and compared. These four salt mixtures—NaCl-KCl-MgCl2 (NaKMg–Cl), LiCl-KCl-MgCl2 (LiKMg–Cl), LiCl-KCl-NaCl (LiKNa–Cl) and LiCl-NaCl-MgCl2 (LiNaMg–Cl)—are formulated based on the eutectic proportions predicted using FactSage®, or in one case, as prescribed in literature (as discussed in below in Section 3.1). Of these four salt mixtures, three (NaKMg–Cl, LiKMg–Cl, LiKNa–Cl) are confirmed to be at the eutectic composition and are selected for further comparison of thermal properties including

melting point, specific heat capacity and thermal stability. The inclusion of LiCl is shown to benefit melting point and heat capacity, but as it is an expensive material, a cost study is included to allow techno-economic comparison of the three salt mixtures. 2. Experimental methods 2.1. Materials The FactSage software was used to estimate the eutectic composition of the four ternary salts. FactSage is a thermochemical software package that can predict the chemical properties of different materials and mixtures from a vast group of thermodynamic databases. It allows generation of liquidus projections for salt mixtures, and calculates the eutectic point (Bale et al., 2016, 2009). The individual salt samples—NaCl (> 99% purity), KCl (> 99% purity), LiCl (> 99% purity) and MgCl2 (> 98% purity)—were purchased from Alfa Aesar and Sigma-Aldrich. All salts were opened and stored in a glove box under a nitrogen atmosphere. The individual salts were weighed according to the desired composition, combined, and then hand-ground with a mortar and pestle continuously for 30 min until the mixture became fine and visibly well mixed. Both the weighing and mixing processes were conducted in a glove box to avoid moisture absorption. The chloride salts are hygroscopic and need to be handled in a moisture free environment. In particular, magnesium chloride (MgCl2) is extremely hygroscopic so it must be handled with utmost care, as the presence of moisture can lead to several hydrolysis and partial hydrolysis reactions. The final products of the hydrolysis reaction at high temperatures are MgO and HCl (Fife et al., 1986; Friedrich and Mordike, 2006; Shoval et al., 1986; Trahan et al., 2012). Removal of oxide impurities is a time intensive process, thus having small quantities of MgO in the mixture is inevitable (Smith et al., 2000; Trahan et al., 2012). A wide range of reactions are possible during a thermal treatment of hydrated MgCl2 (Shoval et al., 1986): (1) Dehydration (T < 200 °C) MgCl2·xH2O (s) ⇌ MgCl2·(x − y)H2O (s) + yH2O (g)

(1)

MgCl2·zH2O (s) ⇌ MgCl2 (s) + zH2O (g)

(2)

(2) Thermal hydrolysis (200 °C < T < 400 °C) MgCl2·xH2O (s) ⇌ Mg(OH)·Cl (s) + HCl (g)

(3)

(3) Dehydroxylation (T > 400 °C) Mg(OH)·Cl (s) ⇌ MgO (s) + HCl (g)

(4)

2.2. Experimental procedure The melting points and heat capacities of the eutectic salt mixtures were evaluated using a differential scanning calorimeter (Netzsch Pegasus DSC 404), which works on the heat-flux principle. Graphite crucibles (85 μl), with lids perforated by a small hole, were used in the DSC to achieve rapid heat transfer into the salt mixtures due to their high thermal conductivity. The melting point analyses were conducted with a silicon carbide (SiC) furnace at a heating/cooling rate of 10 °C/ min with 50 ml/min of nitrogen gas purge. The temperature and sensitivity calibrations for graphite crucibles were conducted with five different metal samples in the test temperature range (indium, tin, zinc, bismuth, and aluminium). For heat capacity measurements, a platinum (Pt) furnace was utilised. The heat capacity experiments were conducted with a heating rate of 20 °C/min with the same operating conditions. The heat capacities of the samples were determined using the ASTM E1269 standard with a sapphire disk as the reference. The Proteus software from Netzsch was used to analyse and evaluate the heat capacity from the DSC data. 351

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The mass loss of all salt mixtures was tested from room temperature to 1000 °C at a heating rate of 10 °C/min in atmospheres of nitrogen, argon and air. About 5–10 mg of each salt sample was loaded into the alumina crucible (85 μl) with a perforated lid, and weight change measured in a simultaneous thermal analysis device (Q600 SDT TGA/ DSC, from TA Instruments). Experiments were conducted with constant purge of gases at 50 ml/min. A high-temperature isothermal stability test was conducted in a closed system to isolate any decomposition effects from vaporisation. About 5 g of each salt mixture was loaded into a 50 ml stainless-steel tube in a glove box to avoid any moisture interaction, and sealed with Swagelok caps. The tubes were weighed, then heated to 800 °C and maintained for 100 h, then cut open to release any gaseous decomposition products and re-weighed.

likelihood of erroneous readings due to volatilisation, as has been reported previously for specific heat capacity measurements of similar chloride salts (Wei et al., 2015). This can occur because the system is under constant gas flow, which removes the vapour formed above the molten salt due its high vapour pressure at elevated temperatures. The heat flow measurements were recorded as a function of temperature, in three continuous steps with the temperature scanning step conducted between two isothermal steps. The isothermal steps are included to avoid any heat fluctuations in the DSC signal during the temperature scan recording. The method of specific heat capacity measurement by DSC was validated by comparing published experimental data for the specific heat capacity of a single component salt (NaCl) (Chase, 1998) with the measured data. Agreement within 2% was achieved throughout this temperature range. Specific heat capacity was measured over 10 continuous cycles, and averaged to give the values shown in Fig. 4. Values for average specific heat capacity shown in Table 2 are first averaged across all 10 cycles, and then averaged across the full temperature range of measurement. The error value in Table 2 is the standard deviation of the mean heat capacity values across the temperature range. A series of two tail t-tests were conducted between the three possible pairings of the three salt samples. The results proved that the heat capacity values were statistically different, with probability of success in the t-tests always greater than 99% (P = 0.01). LiKNa–Cl has the highest heat capacity of the three mixtures. This can be attributed to the presence of a large proportion of LiCl in the mixture, as this salt has the highest heat capacity among the metal chlorides considered in this study. For convenience of use, empirical relationships are derived for specific heat capacity for the three chloride salt mixtures measured. Polynomial regression is used to fit curves to the cp data. The curves for LiKNa–Cl are split into two regions to improve the fit. Agreement is within an R2 value of 0.95, and therefore within the nominated temperature ranges, specific heat capacity can be taken as follows:

3. Results and discussions 3.1. Determination of the eutectic point The phase diagrams predicted using FactSage® for all four ternary mixtures (NaKMg–Cl, LiKNa–Cl, LiKMg–Cl, and LiNaMg–Cl) are shown in Fig. 1(a–d), with the predicted eutectic composition and melting point given in Table 1. Formation of a eutectic for the NaKMg–Cl mixture based on this FactSage prediction was demonstrated by the authors in previous work, with the melting point within 4 °C of the predicted value (Mohan et al., 2018). For the LiKNa–Cl mixture, FactSage is inaccurate in its liquidus projections, as discussed by Raade et al. (Raade et al., 2012). This is clear by comparing the FactSage derived diagram in Fig. 1(b) (with melting point 317 °C) with the experimentally-derived phase diagram (Levin et al., 1964; Williams, 2006) in Fig. 2 (with melting point 346 °C). The latter experimental case was used for determining the eutectic proportions of the LiKNa–Cl mixture in the present work. Among the mixtures considered, mixtures with LiCl form a deep eutectic point or melting point due its high polarisation (which leads to covalent bonding characteristic according to Fajan’s rule). Adding LiCl with other ionic bonded salts (NaCl, KCl, MgCl2) leads to highly mismatched molecular size, shape and bonding of different species, which in turn allow formation of a deep eutectic (Li et al., 2016, 2017). The ternary salt mixtures were prepared, and melting points determined by DSC, as described above in Section 2. Four continuous heating and cooling cycles were performed. Fusion between the salts in the mixture was achieved during the first heating cycle. A further three continuous heating and cooling cycles were performed to demonstrate reproducibility. Fig. 3 shows the DSC heating and cooling curves for the ternary chloride mixtures, with the data taken from the second cycle for each composition. The eutectic temperature was taken as the onset melting point in the heating phase of the cycle. At the eutectic point, the solid salt mixture melts congruently, whereas for off-eutectic compositions, melting of the mixture begins at the eutectic temperature and finishes at the liquidus temperature. The eutectic and liquidus points in are taken as the average of all data, and the maximum variation of the liquidus across all cycles is indicated, showing a high level of consistency was achieved. For the NaKMg–Cl, LiKMg–Cl and LiKNa–Cl salt mixtures, one endothermic peak in the DSC curve indicates proper eutectic formation. The melting temperatures closely matched the expected eutectic temperatures (within 5 °C), as predicted by FactSage for NaKMg–Cl and LiKMg–Cl, and according to the literature for LiKNa–Cl. For the LiNaMg–Cl mixture, multiple endothermic DSC peaks were observed, indicating offeutectic composition. The final melting point is over 400 °C, making this mixture less suited for the HTSTES application. It was therefore excluded from specific heat capacity and thermal stability testing.

c p = aT 2 + bT + c

(5)

where temperature T is the salt temperature in °C and the coefficients are given in Table 3. The R2 values from the regression analysis are also given. 3.3. Thermal stability and mass loss Both thermal stability and mass loss due to vaporisation of the molten salt are important considerations for the solar thermal application. Here, we distinguish between the chemical decomposition of the salt (thermal stability) and mass loss due to vaporisation caused by high vapour pressure (Williams, 2006). In the case of a nitrate molten salt mixture, the vapour pressure of the salt mixture at an elevated temperature (575 °C; close to its decomposition point) is as low as 14 Pa (Glazov et al., 2002). The mass loss of the three mixtures was studied by measuring weight loss when heating samples from 20 °C and 1000 °C in the DSC-TGA, under blankets of nitrogen, argon and air, at a flow rate of 50 ml/min. An upper limit working temperature is one of the most important parameters in evaluation of the suitability of salt mixtures for HTSTES. In general, chlorides possess high vapour pressure leading to the evaporation of salt mixtures, instead of decomposition. In this study, the feasibility of using chloride mixtures with an open (or nonpressurised) system design similar to the nitrate salts is considered. Fig. 5(a–c) shows the stability limit for all three ternary mixtures in all three gas environments. The three salts show a weight loss step below 300 °C, due to dehydration and hydrolyses. This is particularly pronounced for the MgCl2 containing mixtures, which reduce weight by 3.1–5.3% at around 230 °C. As explained earlier, MgCl2 is extremely hygroscopic and is likely to have absorbed some moisture during the sample preparation. Due to this absorbed moisture, the chloride salt mixtures containing MgCl2 undergo several hydrolysis reactions. Fig. 6, from (Mohan et al., 2018), shows the mass loss curve for pure MgCl2 in

3.2. Specific heat capacity determination The specific heat capacity of each eutectic salt mixture was measured using DSC between 455 °C and 600 °C in a constant nitrogen gas flow of 50 ml/min. The measurement range was capped at 600 °C to reduce the 352

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Fig. 1. Phase diagram modelled with FactSage, with the eutectic proportion predicted for (a) NaKMg–Cl (24.5–20.5–55 wt%), (b) LiKNa–Cl (39.2–46.23–14.5 wt%), (c) LiKMg–Cl (32.6–55.1–12.3 wt%), and (d) LiNaMg–Cl (26.3–32.6–41.1 wt%).

Ar-atmosphere. A 3% mass loss is observed at ∼230 °C, which explains the higher initial mass loss for LiNaMg–Cl and NaKMg–Cl salt mixtures compared to LiKNa–Cl. Care was taken to minimise water absorption during sample preparation and storage, by ensuring an inert and moisture-free environment in a glove box. However, the samples are briefly exposed to air during transfer to the DSC equipment, and thus some moisture will be absorbed making at least partial hydrolysis unavoidable. At a temperature of 700 °C, LiKNa–Cl was the most stable mixture with around 5% weight loss. There was a gradual weight loss in

all the three mixtures in the liquid state, due to the high vapour pressure of the chloride salts. To validate this claim, a mass spectrometer was coupled with the DSC-TGA to analyse the output gas flow from 350 °C (to avoid water vapour from initial hydrolysis) up to 1000 °C. The output gas signal had no trace of chlorine, which confirms that there was no decomposition of chlorides in this temperature range. The mass loss of the salt mixtures at higher temperatures is mostly influenced by the vapour pressure of the chloride salt mixtures. The vapour pressure of the ternary metal chlorides mixtures and individual

Table 1 Melting point comparison. Experimental values are for the onset of melting during the second heating cycle. Mixture

Proportion (wt%)

TEutectic FactSage (°C)

TEutectic Experimental (°C)

Variation between prediction and experiments ΔT (°C)

NaCl–KCl–MgCl2 (Mohan et al., 2018) LiCl–KCl–MgCl2 LiCl–NaCl–MgCl2

24.5–20.5–55.0 32.6–55.1–12.3 26.3–32.6–41.1

383 335 384

387.0 ± 0.3 334.9 ± 0.3 492.2 ± 0.3a

4 0.1 108.4

LiCl–KCl–NaCl – FactSage – From ref. (Levin et al., 1964)

26.3–32.6–41.1 40.5–50.6–8.9

317 346

– 349.4 ± 0.4

– 3.4

a

The FactSage predicted composition for LiNaMg–Cl was found to be off-eutectic, hence the liquidus temperature is reported here. 353

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Fig. 4. Specific heat capacity vs. temperature of the ternary mixtures, with vertical bars indicating standard deviation of the measurements across the temperature (averaged over 10 values).

salt mixtures was assessed using FactSage, as shown in Fig. 7. The vapour pressure predictions were validated with known experimental values for single chloride salts (NaCl, KCl (Fiock and Rodebush, 1926)) with good agreement. Vapour pressure increases sharply over 800 °C for all three salt mixtures, particularly for LiKNa–Cl and LiKMg–Cl. The higher vapour pressure of LiCl containing salts is expected, based on the vapour pressure of the individual salts shown in Fig. 7. As mentioned in Section

Fig. 2. Phase diagram (in molar percentages) of LiKNa–Cl as determined experimentally in (Levin et al., 1964), with the eutectic proportion measured as 53.5–38–8.5 mol%, or 40.52–50.61–8.87 wt%.

Fig. 3. Experimental melting point of the four ternary mixtures (a) NaKMg–Cl (Mohan et al., 2018) (b) LiKNa–Cl (c) LiKMg–Cl (d) LiNaMg–Cl 354

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Table 2 Average heat capacity of the ternary mixtures, cp = aT2 + bT + c. Mixture

Temperature range, °C

Average specific heat capacity, cp J/g/K

Standard deviation (from 10 cycles) J/g/K

NaKMg–Cl (Mohan et al., 2018) LiKMg–Cl LiKNa–Cl

450–600

1.180

± 0.013

450–600 450–600

1.220 1.310

± 0.009 ± 0.017

Table 3 Polynomial coefficients of the heat capacity functions. Mixture

Validity range, °C

a

b

c

R2

NaKMg–Cl (Mohan et al., 2018) LiKMg–Cl LiKNa–Cl LiKNa–Cl

450–600

5.0326E−06

−5.5206E−03

2.6819

0.983

450–600 450–510 510–600

4.9094E−06 4.8718E−06 −7.3861E−07

−5.1820E−03 −4.6924E−02 2.3126E−04

2.5782 12.626 1.4072

0.954 0.961 0.988

1, molten salts need to stable up to 750 °C, hence a closed system at a pressure a little above atmospheric may be required to achieve dynamic vapour-liquid equilibrium and avoid weight loss. To isolate vaporisation and decomposition, a simple experiment was carried out where each salt mixture was sealed in a stainless steel tube, weighed, then held at 800 °C for 100 h. The tubes were then cut open and re-weighed. No weight change was measured, indicating that no gaseous decomposition products were formed and released. A sample from each tube was tested in the TGA-DSC to check the melting temperature for each salt, and there was no change, further evidence of compositional stability. 3.4. Storage media cost analysis The economic viability of the thermal storage media is one of the most important selection criteria for HTSTES. Some salt mixtures might have high heat capacity, which is an advantage in terms of energy density, but the cost of the salt mixtures in USD/kWh is the most decisive factor in the final assessment of HTSTES media selection. A comparison of the three ternary chloride mixtures is shown in Fig. 8, showing both USD/tonne and USD/kWh, and assuming an operating temperature range between 520 °C and 720 °C. The costs of the individual salts considered in the study are shown in Table 4. All the values shown in the table are adjusted to April 2018 currency using the PPI (producer price index) commodity data for industrial chemicals provided by the U.S. Bureau of Labour Statistics (BLS) (BLS, 2018)). LiCl is the most expensive salt considered in the study, at almost 15 times more expensive than the other salts considered. The influence of the cost of Li is clear in Fig. 8, and the improved specific heat capacity does not compensate for the additional material costs. Industry and government are presently taking steps to reduce the extraction costs of lithium salt, driven by the expansion in use of lithium ion batteries, so the cost of LiCl is expected to reduce in coming years. Fig. 9 shows how the cost of LiCl impacts the cost of the overall ternary mixtures. LiCl would need to be less than 2000 USD/tonne for LiKNa–Cl to reach costs below 15 USD/kWh for the storage media alone, noting that the cost target for the full TES system (i.e. storage media plus tanks) under the U.S. Sunshot program is 15 USD/kWh.

Fig. 5. Mass loss of ternary chloride mixtures in atmospheres of (a) nitrogen, (b) argon and (c) air.

cycles for CSP plants, due to a suitable combination of thermal properties and the potential for low cost. In this paper, four ternary chloride salt mixtures were considered, formulated from a selection of four alkali and alkaline earth metal chlorides (LiCl, NaCl, KCl and MgCl2). Eutectic

3.5. Conclusions Chloride salts have been identified as promising high temperature energy storage materials to support the development of next-generation power 355

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Table 4 Cost of selected chloride salts, indexed to April 2018 currency using the producer price index (PPI) for industrial chemicals provided by the U.S. Bureau of Labour Statistics, (BLS, 2018). Salt

Cost (USD/ tonne)

Reference

NaCl

60

KCl

485

LiCl

8600

MgCl2

382

52 USD/tonne in Mar 2016 (Minerals, 2016) PPI = 7.6% 450 USD/tonne in Oct 2015 (SQM, 2015) PPI = 3.2% 7900 USD/tonne in Oct 2015 (SQM, 2015) PPI = 3.2% 330 USD/tonne in Dec 2015 (USGS, 2016) PPI = 6.3%

Fig. 6. Mass loss of MgCl2 in argon (Mohan et al., 2018).

Fig. 9. Cost of ternary mixtures (USD/kWh) vs. cost of LiCl.

dependent on the vapour pressure, with significant weight loss due to vaporisation at the upper end of the expected working temperature range (around 700–750 °C). However, it is predicted that in a closed system, at a pressure a little above atmosphere, these ternary salt mixtures could operate in a stable manner. Compared to the ternary salt that excludes LiCl (NaKMg–Cl), the presence of LiCl in the eutectic ternary mixtures is shown to reduce melting temperature by as much as 50 °C (for LiKMg–Cl), which can be advantageous for practical operation of CSP plants. LiCl is also shown to improve the specific heat capacity in the range 450–600 °C by up to 11% (for LiKMg–Cl). However, at current prices the cost of including LiCl in the ternary mixtures appears to outweigh the benefits, with cost reductions by a factor of three required before ternary salts based on LiCl reach an acceptable cost range. Nonetheless, lithium costs are expected to significantly reduce, driven by the battery market, so lower melting point ternaries based on inclusion of lithium chloride may be viable in the future.

Fig. 7. Vapour pressure of the three ternary chlorides and individual chlorides predicted using FactSage. LiKMg–Cl and LiKNa–Cl data sets are identical and overlapping.

Acknowledgement This project is funded by Australian Solar Thermal Research Initiative (ASTRI), a project supported by the Australian Renewable Energy Agency (ARENA). We also thank Mr. Mark Mehos for facilitating the experimental facilities in the Thermal Storage Materials Lab at the National Renewable Energy Laboratory (NREL). The work at NREL was financially supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308.

Fig. 8. Cost analysis of the ternary mixtures.

References compositions for three of these ternary mixtures (NaKMg–Cl, LiKMg–Cl, LiKNa–Cl) were accurately determined, using a combination of predictions from the software package FactSage, and past experimental results. The mass loss of all three ternary eutectics examined was shown to be highly

ASTRI, 2017. Australian Solar Thermal Research Initiative (ASTRI). < www.astri.org. au > (cited on 07/06/2017). Bale, C.W., Bélisle, E., Chartrand, P., Decterov, S.A., Eriksson, G., Gheribi, A.E., Hack, K., Jung, I.H., Kang, Y.B., Melançon, J., Pelton, A.D., Petersen, S., Robelin, C., Sangster,

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