NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

High Temperature Phase 

Change Materials for Thermal 

Energy Storage Applications

Preprint

Judith Gomez, Greg C. Glatzmaier, 

Anne Starace, Craig Turchi, and Jesus Ortega

To be presented at SolarPACES 2011

Granada, Spain

September 20-23, 2011

Conference Paper

NREL/CP-5500-52390

August 2011
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1 

HIGH TEMPERATURE PHASE CHANGE MATERIALS

FOR THERMAL ENERGY STORAGE APPLICATIONS

Greg C. Glatzmaier1, Judith Gomez2, Jesus Ortega3, Anne Starace4, Craig Turchi5 

1

Ph.D. Chemical Engineering, Senior Engineer, National Renewable Energy Laboratory.

2

Ph.D. Metallurgy and Materials Engineering, Postdoctoral Researcher, National Renewable Energy Laboratory, 

1617 Cole Blvd. MS1626, Golden, Colorado 8040, USA, Ph: +1.303.275.4290 E-Mail: judith.gomez@gmail.com

3

Science Undergraduate Laboratory Intern (Mechanical Engineering), National Renewable Energy Laboratory.

4

Ph.D. Chemistry, Postdoctoral Researcher, National Renewable Energy Laboratory.

5

Ph.D. Chemical Engineering, Senior Engineer, National Renewable Energy Laboratory.

Abstract

To store thermal energy, sensible and latent heat storage materials are widely used. Latent heat thermal 

energy storage (TES) systems using phase change materials (PCM) are useful because of their ability to 

charge and discharge a large amount of heat from a small mass at constant temperature during a phase 

transformation. Because high-melting-point PCMs have large energy densities, their use can reduce energy 

storage equipment and containment costs by decreasing the size of the storage unit. Using cascaded PCMs, 

with equally spaced melting points and with high thermal properties, the TES is significantly enhanced. 

However, currently there is not enough information on the thermal properties of molten salt systems at high 

temperatures.

Molten salt PCM candidates for cascaded PCMs were evaluated for the temperatures near 320°C, 350°C, and 

380°C. These temperatures were selected to fill the 300°C to 400°C operating range typical for parabolic 

trough systems, that is, as one might employ in three-PCM cascaded thermal storage. Such systems require a 

series of PCMs with melting points spanning the range of TES. The molten salt systems considered in this 

study were KNO3-KCl-KBr, NaCl-KCl-LiCl and MgCl2-KCl-NaCl. Because the majority of the salts studied 

were hygroscopic, a handling and mixing protocol under controlled atmosphere was employed. The mixing 

procedure was carefully controlled because the sample weight required for the thermal property tests is on 

the order of a few milligrams, so a perfectly homogenized sample is essential for accurate results. Heat 

capacity, latent heat, transformation temperatures, thermal stability, and viscosity were measured. Before 

performing these evaluations, chemical stability (corrosion) of the container materials with the molten salts 

was evaluated in a controlled atmosphere furnace under nitrogen gas. The materials tested were 316 stainless 

steel (SS316), high purity aluminum (Al1100), aluminum-manganese alloys (Al3003) and aluminum oxide 

(Al2O3). Based on the results, the best candidate for temperatures near 320°C was the molten salt KNO3-

4.5wt%KCl. For the 350°C and 380°C temperatures, the evaluated molten salts are not good candidates 

because of the corrosiveness and the high vapor pressure of the chlorides.

Keywords: phase change material (PCM), molten salts, melting point, latent heat, heat capacity, viscosity

1. Introduction

It is clearly understood that lower overall costs are a key factor to make renewable energy technologies 

competitive with traditional energy sources. Energy storage technology is one tool that can increase the value 

and reduce the cost of all renewable energy supplies.

Concentrating solar power (CSP) technologies have the ability to dispatch electrical output to match peak 

demand periods by employing thermal energy storage (TES). This attribute increases the value of the 

renewable energy and minimizes grid integration concerns. In addition, TES can reduce the levelized cost of 

energy (LCOE) if the levelized cost of the TES system is low compared to the additional revenue provided 

by greater utilization of the power block. In order to achieve these lower costs, energy storage technologies 

require efficient materials with high energy density.
2 

More than 400 megawatts (MW) of CSP capacity are currently in place in the southwestern United States [1]

and greater capacity is in place in Spain. There are plants under construction that would add more than 650 

MW of capacity in Arizona and California. One of these plants with a capacity of 280 MW is based on the 

parabolic trough technology and thermal storage using molten salts―similar in design, but larger than the 

Andasol plants in Spain [2, 3]. 

The value of TES is in enabling CSP technologies to play an increasingly significant role in providing 

sustainable power generation [1]. CSP plants with TES can generate electricity when sunlight is not 

available, for example, during momentary cloud transients, which otherwise disrupt electricity generation 

and cause widely varying power output, and during evening hours when electricity is highly valued. TES 

also allows for more efficient use of the turbine and other power-block components. These features provide 

an economic incentive for the addition of TES. Without TES, CSP is an intermittent power resource that 

depends on sunlight availability.

Sensible and latent heat storage materials are widely used to store thermal energy. While sensible storage 

systems are simpler, latent heat TES systems using phase change materials (PCM) are useful because of their 

greater energy density. PCM technology relies on the energy absorption/liberation of the latent heat during a 

physical transformation. Unlike vapor-liquid transformations, solid-liquid transformations produce large 

enthalpy changes without substantial density changes. Because of this behavior, salts and metallic alloys are 

good candidates for PCMs. Ideally, these materials should have a specific melting point and high heat of 

fusion, and offer favorable characteristics such as high working temperatures (more than 500°C), low vapor 

pressure, good thermal and physical properties, low corrosivity and toxicity, and, of course, low cost.

Because high-melting-point PCMs have large energy densities, their use can reduce energy storage 

equipment and containment costs by decreasing the size of the storage unit. The optimum input and output 

temperature of the energy storage equipment is determined by the melting point of the PCM, while the heat 

capacity of the TES system is determined by the PCM latent and sensible heats.

The knowledge of the thermal properties of a particular PCM is important for its selection as a thermal 

storage material. To decrease the LCOE, CSP technologies rely on different simulations to increase the 

thermal energy storage capability. Using cascaded PCMs, with equally spaced melting points and with high 

thermal properties, the TES is significantly enhanced [7]. However, currently there is not enough information 

on the thermal properties of molten salt systems at high temperatures.

2. Goals and Objectives

The goal of this work is to create a PCM database for TES applications, in particular, to enable solar 

parabolic trough plants to operate with greater efficiency and lower electricity costs.

The focus of the research was the characterization of the thermal and chemical properties of candidate 

materials for latent heat TES. Candidate PCMs were identified in a temperature range from 300°C to 500°C, 

with initial emphasis given to materials with melting points near 320°C, 350°C, and 380°C. These 

temperatures were selected to allow for a three-temperature, cascaded PCM storage system for current 

parabolic trough power plants. The physical properties most relevant for PCM use were reviewed from the 

candidate selection list. Specific objectives for this work were: 1) preparation of homogeneous molten salt 

PCM formulations minimizing water absorption and contamination; 2) chemical stability evaluations of the 

PCMs with some container materials, like stainless steel 316 (SS316), aluminum alloys (Al1100 and 

Al3003), and aluminum oxide (Al2O3); and, 3) thermal properties characterization of the PCMs, including

identifying phase change transformation temperatures with their associated latent heats, solid and liquid heat 

capacities, and viscosities.
3 

3. Background

Accurate knowledge of the heat of fusion, melting point, thermal conductivity, heat capacity of the two 

phases, and corrosivity of the PCM are essential for the design of TES systems employing phase change. The

phase transformation should occur at only one invariant temperature to maximize the efficiency of the PCM 

when absorbing and storing the heat. For parabolic trough applications, the temperature range of interest for 

the PCMs is from 300°C to 500°C. In 2007, Michels and Pitz-Paal proposed a cascaded PCM system using a 

series of five PCMs spanning the operating range from approximately 300°C to 400°C. Such a design was 

estimated to have advantages over the two-tank sensible TES system, but was complex. Reducing the 

number of temperature steps could reduce system complexity. This work seeks to identify candidate 

materials with melting point temperatures near 320°C, 350°C, and 380°C [7]. 

Thermal properties of salt blends are highly sensitive to composition and large discrepancies have been 

found amongst the literature regarding the melting points and thermal properties associated to a particular 

chemical composition for molten salt systems. The identification of an accurate melting point of a PCM 

formulation as well as its thermal properties is key for advancing TES technologies. The candidate PCM 

identification was obtained using phase diagrams in which the eutectic reactions occur in the temperature 

range evaluated. A eutectic reaction occurs at an invariant and constant temperature in which a liquid phase 

transforms into different solid phases during cooling. The eutectic temperature represents the minimum 

temperature at which the liquid phase is stable in that particular system. The eutectic composition melts at 

only one constant temperature, which is a valuable feature for a cascaded PCM.

4. Approach

The identification of candidate PCMs was performed by a thermochemical evaluation using different 

sources: Phase Diagrams for Ceramists [4], the NIST Phase Diagrams Database [5], FactSage 

thermochemistry software [6], and the open literature [7-11]. Discrepancies were found not only with the 

chemical compositions but also with the eutectic and other invariant reaction temperatures.

The lack of information regarding the molten salts at high temperatures, as well as the discrepancies in the 

invariant temperatures and compositions, has made the characterization of candidate molten salts of interest.

The characterization was performed following strict protocols for handing unstable samples to obtain 

perfectly mixed and dried samples. Molten salt candidates for cascaded PCMs were evaluated for 

temperatures between 308°C and 380°C. The molten salt systems considered in this study are shown in Table 

1.

Because the majority of the salts studied were hygroscopic, a handling protocol under controlled atmosphere 

was employed. The individual salts, acquired from Alfa-Aesar and Sigma Aldrich with purity levels greater 

than 99%, were placed in a muffle furnace at 120°C for at least 24 hours. Once dried, they were placed inside 

a dry box under nitrogen gas to be weighted in a balance with a resolution of 0.0001g, and combined at a 

specific ratio. To mix them, they were placed in a ceramic container. This jar was sealed inside the dry box to 

avoid exposure to air during the 30-minute mixing process that was performed in a powder mixer. The 

mixing procedure was carefully controlled because the sample weight required for the thermal property tests 

is on the order of a few milligrams, and a perfectly homogenized sample is essential for accurate results.

Heat capacities, latent heats, transformation temperatures and thermal stability of the PCMs were evaluated 

using a Mettler-Toledo Differential Scanning Calorimeter (DSC) and Thermogravimetric Analyzer with DSC 

(TGA/DSC). Viscosity measurements were obtained using a TA Instruments Rheometer AR2000ex. Before 

performing these evaluations, chemical stability (corrosion) of the DSC and TGA/DSC container materials 

with the molten salts was evaluated in a controlled atmosphere furnace under nitrogen gas at 450°C over two 

hours. The materials tested were 316 stainless steel (SS316), high purity aluminum (Al1100), aluminummanganese alloys (Al3003) and aluminum oxide (Al2O3). The selected time of two hours corresponds with 

the longest exposure time of the crucibles with the molten salts during the DSC and TGA/DSC tests. 
4 

Sample Molten Salt System, wt.% Reported values Source

Tm,°C ∆Hm, J/g

6A1 92.81KNO3-7.18KCl 308 - [4] 

6A2 97KNO3-3KCl ~315 - [4]

6A3 95.5KNO3-4.5KCl 320 74 [7]

6B 90.72KNO3-9.28KBr 328 - [4]

6C1 89.98KNO3-2.95KCl-7.06KBr 320 - [4]

6C2 80.69KNO3-7.44KCl-11.87KBr 342 140 [8]

1A 7.87NaCl-50.88KCl-41.25LiCl 344 - [4]

1B 34.81NaCl-32.29KCl-32.90LiCl 346 281 [8]

1C 9.491NaCl-48.43KCl-42.08LiCl 350 - [4]

3B 60.00MgCl2-20.4KCl-19.60NaCl 380 400 [7]

Table 1: Melting points (Tm) and heats of fusion (∆Hm) of the molten salt PCMs systems. 

Values are those reported by the open literature.

Once the chemical stability of the container materials with the molten salts was established, the DSC and 

TGA evaluations were performed. To do this evaluation, the samples were placed and sealed in the DSC 

crucibles inside the dry box. The sealed crucibles were then tested in the DSC instrument. Some formulations 

were stable only in alumina, which cannot be sealed airtight. For such formulations, the TGA was initially 

performed to evaluate mass losses during heating caused by the vapor pressure of the molten salt 

components. After the thermal stability evaluation in the TGA, DSC was performed using the alumina 

crucibles. Because the rheometer is made of stainless steel, only the compositions that were found to be 

compatible with stainless steel could be run on the rheometer.

5. Results and Analysis

The corrosion test made it clear that all the crucible materials were stable in the nitrate formulations because 

they did not show any chemical attack from the KNO3-KBr-KCl molten salt. Prior work with pure nitrates 

had established their compatibility with the same container materials. The chlorides severely dissolved 

aluminum metal (Al1100), and attacked the metal alloys to some degree but did not react with Al2O3. The 

NaCl-KCl-LiCl system, samples 1A and 1B, attacked stainless steel (SS316) and incurred mass losses of 

0.54% and 1.38%, respectively. The system did not attack Al2O3 and the Al-Mn alloy (Al3003) during the 

testing period. MgCl2-NaCl-KCl reacted in a small degree with Al3003 (mass loss of 0.02%) but severely 

attacked SS316 (mass losses of 1.90%).

DSC tests for the nitrates were performed using the aluminum (Al1100) crucibles to maximize the heat 

conduction between the sample and the DSC sensor. This way, the measured latent heat was more accurate. 

Three heating and cooling cycles were performed per sample. At least three samples of each nitrate’s 

formulation were tested. The averages of the results are shown in Table 2. 

As predicted by the FactSage software and the Phase Diagrams database, samples 6C1 and 6C2, from the 

ternary mixture KNO3-KCl-KBr, are not eutectics. This system does not have a ternary eutectic. In fact, the 

composition 6C2 relies on a stability region for two phases under equilibrium: liquid and solid. The melting 

process for these compositions occurs inside a range of temperatures instead of at a fixed temperature, which 

is the case for eutectic points. This temperature range occurs between the solidus boundary (Tmi: beginning 

of melting during heating, or Tsf: end of solidification during cooling) and the liquidus boundary (Tmf: end of 

melting during heating, or Tsi: begining of solidification during cooling). These samples showed a melting 

temperature span of about 36°C and 77°C, respectively.
5 

Sample Heating Cooling SC, 

Tmi, °C Tmf, °C ∆Tm, °C ∆Hm, J/g Tsi, °C Tsf, °C ∆Ts, °C ∆Hs, J/g °C

6A1 321.18 333.15 11.97 70.75 327.43 321.78 5.65 70.48 5.72

6A2 320.67 - - 87.62 324.40 321.92 2.48 88.04 3.73 

6A3 319.68 - - 82.05 320.20 - - 83.66 0 

6B 335.08 372.05 36.98 81.87 367.13 336.54 30.59 82.87 4.92

6C1 327.73 363.45 35.72 75.29 359.35 329.34 30.01 76.93 4.10

6C2 326.58 403.74 77.16 75.89 404.37 324.41 79.96 77.30 0 

Table 2: Melting and solidification thermal properties of KNO3-KCl-KBr system. (Tmi, Tmf: initial and 

final melting temperatures; Tsi, Tsf: initial and final solidification temperatures; ∆Tm, ∆Ts: melting and 

solidification temperature range; ∆Hm, ∆Hs: heats of melting and solidification; SC: supercooling) 

The predicted eutectic points for composition 6A1, KNO3-KCl at 308°C and composition 6B, KNO3-KBr at

328°C were not obtained. These formulations melted in a temperaure range of about 12°C and 37°C, 

respectively. 

For all the KNO3-KCl samples, the beginning of melting ocurred on average at 320°C. The approach to find 

the eutectic composition was to change the chemical composition by additing small amounts of KCl and then 

evaluating the melting temperature range. Those samples in which only one temperature is resolved in DSC 

are the ones closest to the eutectic composition. The DSC peak for these formulations should be sharp with 

no bumps and no changes in the slopes, which represent changes in the phase transformation.

Formulations 6A2 and 6A3 were prepared and characterized using 95.5wt%KNO3-4.5wt%KCl (6A3) as the 

posible eutectic composition with 320°C as the invariant eutectic point, with no supercooling behavior. This 

sample produced the sharpest melting and solidification peaks with an average heat of fusion/solidification of 

about 82.86 J/g. This value is higher than the reported heat of fusion of 74 J/g [7]. This discrepancy could be 

related to the purity level of the single components and the NREL’s strict protocols followed for mixing and 

handling of the molten salts that produced homogenous, contamination-free, and moisture-free materials. 

Because of the presence of KCl in this formulation it is recommended that a long-term corrosion test with 

container materials be performed to corroboratethe use of KCl as one of the PCM candidates for the cascaded 

thermal storage system.

Previous evaluations in the DSC using KNO3 reported a melting point of 335.32°C with a heat of fusion of 

97.25 J/g. Adding small amounts of KCl (3wt.% and 4.5wt%) to KNO3 decreased the heat of fusion of pure 

KNO3 to 87.62 J/g and 82.05 J/g, respectively. This decrease of 10% to 16% is substantial for such small 

quantities of chloride additions. KNO3 is a viable candidate for PCM but has a higher melting point.

The formulations with only chloride anions were thermally and chemically unstable. Mass losses occurred 

during the tests caused by the vaporization of the salts. These samples were extremelly difficult to handle and 

characterize. One of the big inconveniences was the presence of adsorbed water that can react with the salt, 

generating hydrochloric gas during water removal at high temperatures. TGA/DSC and DSC results for these 

samples for three heating and cooling cycles are shown in Table 3. There were mass losses while performing

the heating cycles for each sample. The associated heat of fusion obtained from the TGA/DSC analysis was 

normalized to the real mass value obtained from the test. Even after adjusting the latent heats there was a 

decrease of these values among cycles, which meant that the samples are thermally unstable.The NaCl-KClLiCl system was also tested in the DSC because it was less aggressive and more stable than MgCl2-KClNaCl. 

The melting point of the sample 3B (60.00wt%MgCl2-20.4 wt%KCl-19.60wt%NaCl) was an average

387.60°C, which is around 8 degrees higher than the value reported by the literature (380°C). The 
6 

solidification occurred with 10.90°C of supercooling to 376.60°C. The sample’s heat of fusion was roughly 

199 J/g, which is only 50% of the reported value (400 J/g) and 68% of the calculated value from FactSage 

(290 J/g). The measured values using the TGA were not as accurate as the values obtained in the DSC. The 

latter has 120 thermocouples to control and measure the temperatures that are related to the heat flow 

measurement. This feature makes this instrument more acccurate than the TGA, which has only one 

thermocouple.

DSC results from the NaCl-KCl-LiCl system (Table 3) showed the decreasing behaviour of the latent heat, 

which is related to the mass losses amongst heating cycles. The melting point of the sample 1B (34.81 

wt%NaCl-32.29 wt%KCl-32.90wt%LiCl) was on average 352.89°C, while the value reported in literature is 

346°C. Solidification occurred with 2.18°C of supercooling to 350.70°C. The heat of fusion of this sample

was around 138 J/g which is only 49% of the reported value (281 J/g). This measured value is closer to the 

one predicted by FactSage for this composition, which was 146 J/g. This formulation is good for a 350°C

melting point PCM, but considerations must be made for its vaporization and thermal instability. Al3003 

seems to be resistent to this chloride system during the testing period (two hours), but long-term corrosion 

tests must be performed with this alloy to evaluate its real chemical stability in the presence of this PCM.

The discrepancies in the latent heats are most likely related to the fact that these chlorides are thermally and 

chemically unstable and are hygroscopic materials. The MgCl2-KCl-NaCl system, with a melting point of 

387.60°C, is probably too high for use in current parabolic trough plants operating with biphenyl/diphenyl 

oxide synthetic-oil heat transfer fluids. Additionally, this chloride system is extremelly unstable and 

chemically aggressive.

Sample-cycle Heating Cooling SC, °C Analyzer Tm, °C ∆Hm, J/g Ts, °C ∆Hs, J/g

1B-1 359.63 144.93 350.88 135.22 - DSC

1B-2 353.40 142.99 350.35 119.92 3.05 DSC

1B-3 352.37 127.33 350.88 114.25 1.49 DSC

1B-Average 352.89 - 350.70 - 2.18 DSC

1B-1 360.42 164.18 347.33 124.05 13.09 TGA/DSC

1B-2 356.73 138.55 347.39 124.35 9.34 TGA/DSC

1B-3 356.26 125.37 347.33 122.73 8.93 TGA/DSC

1B-Average 356.50 - 347.35 - 9.14 TGA/DSC

3B-1 393.90 197.92 376.30 198.30 17.60 TGA/DSC

3B-2 387.80 199.50 376.27 195.91 11.53 TGA/DSC

3B-3 387.39 197.59 377.13 183.74 10.26 TGA/DSC

3B-Average 387.60 - 376.60 - 10.90 TGA/DSC

Table 3: Melting and solidification of 34.81wt.%NaCl-32.28wt.%KCl-32.91wt.%LiCl (1B) and 

60.00wt%MgCl2-20.4 wt%KCl-19.60wt%NaCl (3B) using DSC and TGA/DSC. Due to mass losses 

during cycles, the latent heats of melting and solidification (∆Hm, ∆Hs) were adjusted. (Tm, Ts: melting 

and solidification temperatures; SC: supercooling). 

Heat capacity (Cp) was measured only for the KNO3-KCl-KBr system because these PCMs are thermally 

and chemically stable under the test conditions. The Cp was measured following the standard ASTM E1269-

5. Three heating and cooling cycles were performed per sample. The average of the solid and liquid heat 

capacities per each PCM are shown in Figure 1. For all the samples, the solid heat capacity increased

proportionally with temperature, and the liquid heat capacity had a constant behavior up to 500°C. The 

candidate PCM KNO3-4.5wt%KCl has high solid and liquid heat capacities compared with the other PCMs 

evaluated in the KNO3-KCl-KBr system. The heat capacities of pure KNO3 were 1.42 J/g.K at 350°C for the 
7 

solid and 1.47 J/g.K for the liquid. Adding 4.5wt%KCl to KNO3 decreased the liquid heat capacity to about 

1.31 J/g.K. This decrease of 11% substantially affects the energy density of this candidate PCM when 

compared with pure KNO3. 

The viscosity of the samples KNO3-3wt%KCl and KNO3-4.5wt%KCl was evaluated at 335°C. The average 

values were 3.19 cP and 3.23 cP, respectively. These values are 18% to 20% higher than the viscosity of 

KNO3 which is about 2.7 cP at 350°C. The viscosities of the evaluated PCMs are approximately threefold 

higher than the viscosity of the synthetic oil (Therminol VP-1®) used for parabolic trough heat transfer fluid. 

However, these candidate PCMs have an acceptable viscosity because they are not required to flow in the 

storage system.

Figure 1: Averaged heat capacity of solid and liquid phases for KNO3-KCl-KBr system. The solid heat 

capacities are the lines with a positive slope while the liquid heat capacities are the constant values.

Heat capacity spikes to infinity during phase change around 320°C.

6. Conclusions and Recommendations

Based on the results, the best molten salt candidate for a 320°C PCM was the KNO3-4.5wt%KCl

formulation. This salt had a defined eutectic behavior with no supercooling. The melting/solidification 

process occurred at ~320°C with a latent heat of ±82.86 J/g. The heat capacities were relatively high: solid 

heat capacity was 1.40 J/g.K at 270°C and liquid heat capacity was 1.31 J/g.K at greater than 380°C. The 

viscosity was 3.23 cP which is relatively good for a PCM candidate. However, comparing this formulation 

with the sensible heat solar salts currently employed in indirect two-tank TES systems for the parabolic 

trough, the thermal properties are relatively poor. The values should be greater than 100 J/g for latent heat 

and greater than 1.50 J/g.K for heat capacity. When comparing the thermal properties of KNO3-4.5wt%KCl

with those of pure KNO3 (melting point of 335°C), the reduction in the heat of fusion and the liquid heat 

capacity decreases the energy density of the KNO3 when adding small amounts of KCl.

The molten salts evaluated for the target melting point temperatures of 350°C and 380°C are not good 

candidates because of the corrosiveness and the high vapor pressure of the chlorides. For these temperatures 

other candidates must be considered. NREL is currently evaluating non chloride salt blends with melting 

0.90

1.00

1.10

1.20

1.30

1.40

150 200 250 300 350 400 450 500

Heat Capacity of Solid and Liquid, J/g.K

Temperature, C

KNO3-7.18wt.%KCl

KNO3-3wt.%KCl 

KNO3-4.5wt.%KCl

KNO3-9.28wt.%KBr

KNO3-2.95wt.%KCl-7.06wt.%KBr
8 

points between 380°C and 390°C. Finding a salt with a melting point of the intermediate temperature of 

350°C has been extremely difficult. All the possible candidates are mixtures of chlorides and fluorides, 

which are corrosive.

Future evaluations should consider long-term chemical stability (corrosion) of the construction materials in 

the molten salts as wells as thermal conductivity and density measurements. In this way, the effectiveness of 

the thermal storage could be evaluated based on the energy density of the candidate PCM.

Acknowledgments

This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308 

with the National Renewable Energy Laboratory.

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