# Molten chloride salts for concentrated solar power

Molten chloride salts for concentrated solar power are a ternary mixture of magnesium, potassium, and sodium chloride (MgCl2/KCl/NaCl) proposed as the heat-transfer and thermal-storage fluid for next-generation ([Generation](https://www.edgepedia.org/generation) 3) CSP plants operating near 720 °C, well above the 565 °C limit of the nitrate salts used in today's commercial plants. The blend is inexpensive (below 0.35 USD/kg) and thermally stable at high temperature, which allows pairing with a supercritical carbon dioxide (sCO2) Brayton power cycle with predicted net-cycle efficiencies of 50 percent or more, against about 35 percent for the 565 °C nitrate-salt Rankine systems in service.<sup>[1](https://www.osti.gov/pages/servlets/purl/1779804)</sup><sup> • </sup><sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup>

| Key fact | Detail |
|---|---|
| Composition and melt point | Eutectic at 44.7 MgCl2 – 25.8 KCl – 29.4 NaCl (mol.%), lowest melting/solidification temperature 385 °C ± 1 °C by differential scanning calorimetry and thermogravimetric analysis<sup>[3](https://pubs.aip.org/aip/acp/article/2126/1/080006/701454/Molten-chloride-technology-pathway-to-meet-the-U-S)</sup> |
| Operating temperatures | Hot salt tank about 720 °C, cold tank 500 °C, both above the blend's roughly 400 °C freeze temperature<sup>[4](https://doi.org/10.1016/j.solmat.2021.111048)</sup> |
| Efficiency gain | sCO2 Brayton cycle at 720 °C predicts net-cycle efficiency ≥50%, versus about 35% for 565 °C nitrate Rankine systems<sup>[1](https://www.osti.gov/pages/servlets/purl/1779804)</sup> |
| Corrosion control | With Mg purification, SS310 and Alloy 800H corroded below 15 µm/year at 700 °C in 2000-hour tests, against a DOE target below 30 µm/year<sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup> |
| Salt cost | Below 0.35 USD/kg<sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup> |
| Storage cost gap | One four-tank chloride TES design estimated at $60/kWh-th, with raw liner materials over $7/kWh-th against a $3/kWh-th target; a later Fe-alloy-based design targets 27 USD/kWh-th<sup>[1](https://www.osti.gov/pages/servlets/purl/1779804)</sup><sup> • </sup><sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup> |
| Program status | US DOE Gen3 program launched May 2018; in March 2021 DOE down-selected the particle pathway but gave NREL $2 million to continue the liquid (molten salt) pathway<sup>[5](https://www.sciencedirect.com/science/article/abs/pii/S0927024821000234)</sup><sup> • </sup><sup>[6](https://www.nrel.gov/news/program/2022/next-gen-concentrating-solar-power-research-heats-up-at-nrel.html)</sup> |

## Background: from nitrate salts to Generation 3 CSP

The commercial state of the art (Gen2 CSP) uses molten nitrate salt as the heat-transfer and storage fluid, with a maximum operating temperature of approximately 565 °C and net thermal-to-electric conversion efficiency limited to about 35 percent, in part because of the Rankine steam systems attached to it.<sup>[1](https://www.osti.gov/pages/servlets/purl/1779804)</sup> Above about 550 °C, affordable stainless steels show a rapid decline in allowable stress, which constrains any attempt to run nitrate plants hotter.<sup>[1](https://www.osti.gov/pages/servlets/purl/1779804)</sup>

In May 2018 the US Department of Energy started the Generation 3 Concentrating Solar Power Systems (Gen3 CSP) program, in which [molten chloride salts](https://www.edgepedia.org/molten-chloride-salt) have been extensively studied as one of the most promising thermal energy storage (TES) and heat-transfer fluid materials; research institutions in Australia, China, and Europe are also investigating them.<sup>[5](https://www.sciencedirect.com/science/article/abs/pii/S0927024821000234)</sup> DOE established three technology pathways: a liquid pathway led by NREL using molten salt, a particle pathway led by Sandia, and a gas pathway led by Brayton Energy.<sup>[6](https://www.nrel.gov/news/program/2022/next-gen-concentrating-solar-power-research-heats-up-at-nrel.html)</sup> The program's cost goal was electricity at a levelized cost of energy below $60/MWh.<sup>[7](https://www.osti.gov/biblio/1807668)</sup>

The Gen3 liquid design pairs a high-efficiency liquid-sodium receiver operating at about 740 °C with a two-tank molten-chloride storage system; the chloride salt is dispatched to a supercritical CO2 power cycle. The design builds on operating nitrate-salt plants such as [Gemasolar](https://www.edgepedia.org/gemasolar), Crescent Dunes, Noor III, and DEWA 700.<sup>[7](https://www.osti.gov/biblio/1807668)</sup>

## Thermophysical properties

The lowest melting and solidification temperature measured for the MgCl2–KCl–NaCl system is 385 °C ± 1 °C, at the eutectic composition 44.7 MgCl2 – 25.8 KCl – 29.4 NaCl (mol.%), determined by differential scanning calorimetry and thermogravimetric analysis.<sup>[3](https://pubs.aip.org/aip/acp/article/2126/1/080006/701454/Molten-chloride-technology-pathway-to-meet-the-U-S)</sup> Purified salt produced in later program work melted at about 400 °C, and tank design assumes a freeze temperature near 400 °C.<sup>[8](https://www.energy.gov/sites/default/files/2021-08/Gen3CSPSummit_Zhao_33870.pdf)</sup><sup> • </sup><sup>[4](https://doi.org/10.1016/j.solmat.2021.111048)</sup> Sources disagree on the upper stability limit: one journal paper reports thermal stability above 800 °C for the target blend,<sup>[4](https://doi.org/10.1016/j.solmat.2021.111048)</sup> while a 2024 DLR abstract cites thermal stability above 1000 °C.<sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup> Either figure leaves large headroom above the 720 °C operating point.

Within the Gen3 operating range of 500–720 °C, the properties needed for plant design, including density, heat capacity, viscosity, vapor pressure, thermal diffusivity, and thermal conductivity, have been measured.<sup>[8](https://www.energy.gov/sites/default/files/2021-08/Gen3CSPSummit_Zhao_33870.pdf)</sup> For related binary chloride salts, experimental maximum operating temperatures deviated from calculated values by about 6.02 percent, a measure of how far prediction and measurement still diverge.<sup>[9](https://www.mdpi.com/1996-1073/15/7/2516)</sup>

## Corrosion and salt purification

**Impurities drive the corrosion problem.** MgCl2 is strongly hygroscopic and hydrates after short contact with ambient air; on heating or melting the hydrated compounds hydrolyze to form corrosive HCl and hydroxide species such as MgOHCl.<sup>[5](https://www.sciencedirect.com/science/article/abs/pii/S0927024821000234)</sup> Corrosion of commercial Fe–Cr–Ni structural alloys in molten chlorides is driven mainly by these hydroxyl-based impurities (for example MgOH+Cl−) and by oxygen gas dissolved in the melt.<sup>[5](https://www.sciencedirect.com/science/article/abs/pii/S0927024821000234)</sup> Reviews conclude that impurities in chlorides affect metal corrosion more strongly than in other molten-salt systems, so purification effectively reduces corrosion; adding excess Mg to KCl–MgCl2 alleviates selective dissolution of alloying elements.<sup>[10](https://scijournals.onlinelibrary.wiley.com/doi/full/10.1002/ese3.2029)</sup>

**Purification works, quantitatively.** With dehydrated carnallite, corrosion was reduced from more than 1000 µm/year to under 100 µm/year by adding elemental Mg, and corrosiveness was found to correlate directly with MgOHCl content measured by analytical titration.<sup>[8](https://www.energy.gov/sites/default/files/2021-08/Gen3CSPSummit_Zhao_33870.pdf)</sup> A purification protocol using anhydrous carnallite consistently produced salt with ≤0.1 wt.% MgOHCl.<sup>[8](https://www.energy.gov/sites/default/files/2021-08/Gen3CSPSummit_Zhao_33870.pdf)</sup> At DLR, with Mg purification, corrosion rates of SS310 and Alloy 800H in molten MgCl2-KCl-NaCl at 700 °C were held below 15 µm/year in 2000-hour static laboratory immersion tests, against the DOE target of below 30 µm/year for structural alloys above 700 °C.<sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup>

Monitoring and control methods are part of the design toolkit: FTIR/[Raman spectroscopy](https://www.edgepedia.org/raman-spectroscopy), cyclic voltammetry, and potentiodynamic polarization are used to track MgOHCl concentration and salt corrosivity, and DLR's test loop integrates electrochemical impurity monitoring with in-situ purification by electrolysis or Mg addition.<sup>[5](https://www.sciencedirect.com/science/article/abs/pii/S0927024821000234)</sup><sup> • </sup><sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup> Handling matters as much as chemistry: improper handling and preparation of the salt mixture significantly alters its thermal properties because of moisture absorption.<sup>[3](https://pubs.aip.org/aip/acp/article/2126/1/080006/701454/Molten-chloride-technology-pathway-to-meet-the-U-S)</sup>

## Storage-tank and plant engineering

The hot salt tank is expected to hold salt at about 720 °C and the cold tank at 500 °C. The salt in both tanks sits above the blend's roughly 400 °C freeze temperature, and no freeze plane develops at the hot face of the hot tank.<sup>[4](https://doi.org/10.1016/j.solmat.2021.111048)</sup> The high freeze point still shapes the plant: NREL describes the salts as freezing at "a not-so-chilly 400 °C", requiring large, expensive storage tanks and freeze protection in pipes.<sup>[6](https://www.nrel.gov/news/program/2022/next-gen-concentrating-solar-power-research-heats-up-at-nrel.html)</sup>

At 720 °C the strength of stainless steel is compromised and the alloy is highly susceptible to chloride corrosion; nickel superalloys would withstand the conditions but are prohibitively expensive at scale. Internal insulation with refractory ceramic liners allows carbon steel tank shells to be used instead, a significant cost saving, drawing on liner experience from the magnesium electrolysis industry.<sup>[4](https://doi.org/10.1016/j.solmat.2021.111048)</sup> A two-tank system may hold enough hot salt to supply the power block for up to 16 hours of generation during off-sun periods.<sup>[1](https://www.osti.gov/pages/servlets/purl/1779804)</sup>

## US DOE Gen3 funding and demonstrations

Under the Gen3 CSP Lab Call, DOE's Solar Energy Technologies Office awarded NREL $1,000,000 for the project "Molten Chloride Thermophysical Properties, Chemical Optimization, and Purification" in [Golden, Colorado](https://www.edgepedia.org/golden-colorado), to evaluate MgCl2/KCl/NaCl mixtures for optimal melting point, high-temperature stability, energy storage, and impurity removal.<sup>[11](https://www.energy.gov/eere/solar/project-profile-national-renewable-energy-laboratory-2-gen3-csp-lab-call)</sup>

In March 2021 DOE down-selected the particle pathway for continued funding but created an opportunity for NREL to develop the liquid pathway over the following two years, awarding $2 million to build a prototype molten-salt tank on the mesa above NREL's Golden campus to evaluate its integrity when filled with molten salt.<sup>[6](https://www.nrel.gov/news/program/2022/next-gen-concentrating-solar-power-research-heats-up-at-nrel.html)</sup> Internationally, DLR designed the MOCTEF test facility, with a molten-salt pump and two tanks holding up to 50 kg of salt, a designed maximum test temperature of 750 °C, and integrated corrosion monitoring and in-situ purification; it was under construction as of the 2024 abstract.<sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup>

## Chloride salts by the numbers

The efficiency case is the core argument for the chloride pathway: a ternary chloride salt stable well above 720 °C enables an sCO2 closed-loop [Brayton cycle](https://www.edgepedia.org/brayton-cycle) with predicted net-cycle efficiencies of 50 percent or more, against about 35 percent for 565 °C nitrate Rankine systems.<sup>[1](https://www.osti.gov/pages/servlets/purl/1779804)</sup> The salt itself is cheap, below 0.35 USD/kg.<sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup>

The storage-system economics are less settled. An early four-tank chloride TES design was estimated at $60/kWh-th overall, with raw liner material costs over $7/kWh-th, exceeding the $3/kWh-th liner-material target by a factor of about 2.5.<sup>[1](https://www.osti.gov/pages/servlets/purl/1779804)</sup> A later DLR-led design using affordable Fe-based alloys (at least 50 wt.% Fe) targets a CAPEX of 27 USD/kWh-th.<sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup> These figures are not directly comparable, since one is a full-system estimate from a specific design study and the other a design target, but together they bracket the cost question the pathway must close. Within the Gen3 liquid pathway, the sodium receiver at about 740 °C and the overall LCOE goal below $60/MWh define the system the chloride store must serve.<sup>[7](https://www.osti.gov/biblio/1807668)</sup> The particle pathway was selected for continued funding, while the liquid pathway received separate continuation funding.<sup>[6](https://www.nrel.gov/news/program/2022/next-gen-concentrating-solar-power-research-heats-up-at-nrel.html)</sup>

## Outlook and challenges

Several hurdles separate laboratory results from a bankable plant. Corrosion data below the DOE target exist from 2000-hour static laboratory tests,<sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup> but lifetime proof at commercial scale and flow conditions remains to be demonstrated. Salt chemistry control must be maintained at commercial purity and volume: the purification protocol achieving ≤0.1 wt.% MgOHCl was demonstrated at laboratory scale,<sup>[8](https://www.energy.gov/sites/default/files/2021-08/Gen3CSPSummit_Zhao_33870.pdf)</sup> and moisture pickup during handling is itself a documented risk to salt properties.<sup>[3](https://pubs.aip.org/aip/acp/article/2126/1/080006/701454/Molten-chloride-technology-pathway-to-meet-the-U-S)</sup> [Vapor pressure](https://www.edgepedia.org/vapor-pressure) was identified in Gen3 program reviews as a major source of risk that had not been recognized before, with limited information available.<sup>[8](https://www.energy.gov/sites/default/files/2021-08/Gen3CSPSummit_Zhao_33870.pdf)</sup> Storage-system economics remain unresolved: the $60/kWh-th system estimate and the 27 USD/kWh-th target are not directly comparable.<sup>[1](https://www.osti.gov/pages/servlets/purl/1779804)</sup><sup> • </sup><sup>[2](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)</sup> Programmatically, DOE's 2021 down-selection moved primary funding to the particle pathway, leaving the chloride liquid pathway on a smaller $2 million continuation.<sup>[6](https://www.nrel.gov/news/program/2022/next-gen-concentrating-solar-power-research-heats-up-at-nrel.html)</sup>

## References

1. [Technical and Economic Feasibility of Molten Chloride Salt Thermal Energy Storage Systems (DOE national laboratory report, OSTI)](https://www.osti.gov/pages/servlets/purl/1779804)
2. [Molten chloride salt TES for next-generation CSP plants: R&D progress in corrosion control and process upscaling (DLR, SolarPACES 2024)](https://elib.dlr.de/210312/3/SolarPACES_2024_Abstract_Ding_DLR.pdf)
3. [Molten chloride technology pathway to meet the U.S. DOE SunShot initiative with Gen3 CSP (AIP Conference Proceedings)](https://pubs.aip.org/aip/acp/article/2126/1/080006/701454/Molten-chloride-technology-pathway-to-meet-the-U-S)
4. [Internal insulation and corrosion control of molten chloride thermal energy storage tanks (Solar Energy Materials and Solar Cells)](https://doi.org/10.1016/j.solmat.2021.111048)
5. [Molten chloride salts for high-temperature thermal energy storage: Continuous electrolytic salt purification with two Mg-electrodes and alternating voltage for corrosion control (Solar Energy Materials and Solar Cells)](https://www.sciencedirect.com/science/article/abs/pii/S0927024821000234)
6. [Next-Gen Concentrating Solar Power Research Heats Up at NREL (NREL, 2022)](https://www.nrel.gov/news/program/2022/next-gen-concentrating-solar-power-research-heats-up-at-nrel.html)
7. [CSP Gen3: Liquid-Phase Pathway to SunShot (DOE/NREL technical report, OSTI)](https://www.osti.gov/biblio/1807668)
8. [A KCl-MgCl2-NaCl ternary molten chloride salt could be purified and optimized to suit Gen3 CSP applications (DOE Gen3 CSP Summit presentation)](https://www.energy.gov/sites/default/files/2021-08/Gen3CSPSummit_Zhao_33870.pdf)
9. [Thermal Stability Calculation and Experimental Investigation of Common Binary Chloride Molten Salts Applied in Concentrating Solar Power Plants (Energies)](https://www.mdpi.com/1996-1073/15/7/2516)
10. [A Review of High-Temperature Molten Salt for Third-Generation Concentrating Solar Power (Energy Science & Engineering)](https://scijournals.onlinelibrary.wiley.com/doi/full/10.1002/ese3.2029)
11. [PROJECT PROFILE: National Renewable Energy Laboratory 2 (Gen3 CSP Lab Call), US Department of Energy](https://www.energy.gov/eere/solar/project-profile-national-renewable-energy-laboratory-2-gen3-csp-lab-call)

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*Topic: Encyclopedia › Technology and the built world › Energy technology › Solar power › Concentrating solar power technologies*

*Initially written Oct 22, 2026*

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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license