General · EdgepediaOctober 22, 20268 min read

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 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.¹^•²

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 analysis³
Operating temperatures	Hot salt tank about 720 °C, cold tank 500 °C, both above the blend's roughly 400 °C freeze temperature⁴
Efficiency gain	sCO2 Brayton cycle at 720 °C predicts net-cycle efficiency ≥50%, versus about 35% for 565 °C nitrate Rankine systems¹
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/year²
Salt cost	Below 0.35 USD/kg²
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-th¹^•²
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) pathway⁵^•⁶

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.¹ Above about 550 °C, affordable stainless steels show a rapid decline in allowable stress, which constrains any attempt to run nitrate plants hotter.¹

In May 2018 the US Department of Energy started the Generation 3 Concentrating Solar Power Systems (Gen3 CSP) program, in which molten chloride salts 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.⁵ 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.⁶ The program's cost goal was electricity at a levelized cost of energy below $60/MWh.⁷

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, Crescent Dunes, Noor III, and DEWA 700.⁷

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.³ Purified salt produced in later program work melted at about 400 °C, and tank design assumes a freeze temperature near 400 °C.⁸^•⁴ Sources disagree on the upper stability limit: one journal paper reports thermal stability above 800 °C for the target blend,⁴ while a 2024 DLR abstract cites thermal stability above 1000 °C.² 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.⁸ 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.⁹

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.⁵ 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.⁵ 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.¹⁰

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.⁸ A purification protocol using anhydrous carnallite consistently produced salt with ≤0.1 wt.% MgOHCl.⁸ 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.²

Monitoring and control methods are part of the design toolkit: FTIR/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.⁵^•² Handling matters as much as chemistry: improper handling and preparation of the salt mixture significantly alters its thermal properties because of moisture absorption.³

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.⁴ 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.⁶

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.⁴ 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.¹

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, to evaluate MgCl2/KCl/NaCl mixtures for optimal melting point, high-temperature stability, energy storage, and impurity removal.¹¹

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.⁶ 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.²

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 with predicted net-cycle efficiencies of 50 percent or more, against about 35 percent for 565 °C nitrate Rankine systems.¹ The salt itself is cheap, below 0.35 USD/kg.²

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 2.5.¹ A later DLR-led design using affordable Fe-based alloys (at least 50 wt.% Fe) targets a CAPEX of 27 USD/kWh-th.² 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.⁷ The particle pathway was selected for continued funding, while the liquid pathway received separate continuation funding.⁶

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,² 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,⁸ and moisture pickup during handling is itself a documented risk to salt properties.³ Vapor pressure was identified in Gen3 program reviews as a major source of risk that had not been recognized before, with limited information available.⁸ Storage-system economics remain unresolved: the $60/kWh-th system estimate and the 27 USD/kWh-th target are not directly comparable.¹^•² Programmatically, DOE's 2021 down-selection moved primary funding to the particle pathway, leaving the chloride liquid pathway on a smaller $2 million continuation.⁶

References

Initially written Oct 22, 2026