Calcium nitrate in molten salt: heat transfer and thermal storage for concentrated solar power
Calcium nitrate is used as a minority component of molten nitrate salt mixtures for concentrated solar power (CSP) heat transfer and thermal storage. Mixed with sodium and potassium nitrate it forms a ternary salt that melts at about 133 °C, and adding lithium nitrate gives quaternary fluids with liquidus below 100 °C and stability above 500 °C.[1][2] The trade-off is documented: calcium nitrate is the lowest-cost nitrate constituent and lowers the melting point, but it raises viscosity, promotes glassy melts near freezing and is the least favoured constituent for thermal stability.[1] It is a research and specialty use, not a commodity one: today’s commercial towers mostly use binary sodium–potassium “solar salt”.[5][6]
Key facts at a glance
| Item | Value / answer |
|---|---|
| Role | Melting-point depressant and low-cost constituent in multi-component nitrate salts[1] |
| Ternary Ca(NO3)2–NaNO3–KNO3 melting point | About 133 °C[1] |
| Li–Na–K nitrate eutectic (for comparison) | Melts at 120 °C[1] |
| Sandia quaternary fluid composition (wt%) | NaNO3 9–18, KNO3 40–52, LiNO3 13–21, Ca(NO3)2 20–27[2][3] |
| Quaternary liquidus / stability limit | Liquidus below 100 °C; thermal stability above 500 °C[2] |
| Quaternary viscosity | 5–6 cP at 300 °C; 2–3 cP at 400 °C (near the liquidus about 200 cP)[1] |
| Known drawbacks of Ca(NO3)2 in the melt | Higher viscosity, glassy melts near freezing, least favourable effect on thermal stability[1][9][11] |
| Commercial reference salt | Sodium–potassium nitrate “solar salt” in direct two-tank power-tower storage[5][6] |
| Typical storage duration in recent tower plants | About 10 hours (Crescent Dunes, Nevada)[7] |
| Feedstock note | Ca(NO3)2·4H2O must be heated slowly so its water of hydration evolves before the melt is used[1][18][21] |
Why molten salt, and why nitrates
Concentrated solar power plants focus sunlight to heat a fluid, then store that heat so electricity can be generated after sunset. Molten nitrate salts are the proven storage medium: in a direct two-tank molten-salt power tower the same salt collects heat in the receiver and is stored in a hot tank, then passed through a steam generator.[5][6][17] For background on thermal energy storage and tower designs see [19][20]. Recent towers such as Crescent Dunes in Nevada were built with about 10 hours of storage.[7] Nitrate salts are attractive because they are liquid at moderate temperatures, have very low vapour pressure, are non-flammable and are far cheaper than synthetic heat-transfer oil.[1][2]
Where calcium nitrate fits
The standard binary salt of sodium and potassium nitrate has a relatively high freezing point, which means a plant must spend energy and care to keep every pipe above it. Researchers therefore look for ternary and quaternary blends with lower melting points. Calcium nitrate is one of the cheapest additives that lowers the melting point: a Ca(NO3)2–NaNO3–KNO3 mixture melts at about 133 °C,[1] and optimisation studies of the ternary system map its phase diagram and thermophysical properties.[9][10][11] A Sandia-developed quaternary fluid combining sodium, potassium, lithium and calcium nitrates reaches a liquidus below 100 °C and stays stable above 500 °C.[2][3] A European patent also covers calcium potassium nitrate salt for heat transfer fluids.[12]
Comparison of nitrate salt systems
| System | Melting / liquidus | Notes | Source |
|---|---|---|---|
| Solar salt (NaNO3–KNO3) | Higher than the calcium and lithium blends | Commercial standard in direct two-tank towers | [1][5][6] |
| Li–Na–K nitrate eutectic | 120 °C | Low viscosity, but lithium nitrate is the costliest constituent | [1] |
| Ca–Na–K nitrate (ternary) | About 133 °C | Cheap; viscous, tends to supercool and form glass | [1][11] |
| Na–K–Li–Ca nitrate (quaternary) | Below 100 °C | Stable above 500 °C; 5–6 cP at 300 °C | [1][2] |
Cost: why calcium nitrate is attractive
In the Sandia study, the expected cost ranking of the constituents by cation was calcium < sodium < potassium < lithium. A study quoted there estimated binary solar salt at about 0.50 USD/kg and a calcium-containing ternary at about 0.90 USD/kg, versus roughly 3–4 USD/kg for organic heat-transfer oils.[1] These are 2008-era estimates; use current quotes for any real project.
The engineering catch: viscosity, stability and corrosion
- Viscosity. Calcium nitrate additions significantly increase viscosity, and calcium-containing melts tend to form glassy, viscous liquids near the freezing temperature, with about 200 cP near the liquidus in the quaternary tested.[1][11]
- Thermal stability. Sandia concluded that calcium nitrate is the least favoured constituent for thermal stability, while lithium nitrate has only a modest effect on maximum usable temperature.[1] Stability of nitrate/nitrite salts also depends strongly on cover-gas atmosphere and pre-heating history, and thermodynamic property data for nitrate melts is published openly.[13][14][15][16]
- Corrosion. Nitrate melts decompose to nitrite and oxygen, and the resulting oxide ions are far more corrosive than nitrate itself; corrosion resistance of Fe–Cr–Ni alloys deteriorates sharply at higher temperatures.[4] Corrosion-resistant coatings are an active research area.[8]
What this means for a buyer of calcium nitrate
- Thermal-storage salts are blended from high-purity, low-water feedstock; the lab work cited here started from reagent-grade Ca(NO3)2 tetrahydrate that was dehydrated by slow heating.[1]
- Chloride, sulphate, magnesium and insolubles matter because they change corrosion behaviour and melt chemistry — set limits in writing before quoting.[4]
- Plan on pilot-scale melting and thermal-cycling tests; the literature itself says further development is needed to tune proportions.[1][10]
- Volumes are project-based and lumpy; this is a specialty, not a spot-market, use.[5]
When calcium nitrate is NOT the right choice
- Commercial CSP tank fills today. Operating molten-salt towers use binary sodium–potassium solar salt, not calcium-containing blends.[5][6]
- High-temperature (>550 °C) service — nitrate decomposition and corrosion accelerate and calcium nitrate is the least stable constituent.[1][4]
- Anyone expecting a ready-to-melt grade from a fertilizer product. Fertilizer-grade calcium nitrate carries water of hydration and has not been qualified for thermal-storage use.[1]
What HRSU supplies — and what it does not
HRSU Indore supplies fertilizer-grade calcium nitrate powder (18.5–19% calcium, 15.5% minimum nitrogen, 100% water-soluble, 0% insoluble residue). HRSU does not claim this product is a certified thermal-storage or heat-transfer-fluid grade, and the molten-salt literature above uses dehydrated, high-purity material. If you are developing a nitrate blend, send HRSU your moisture, chloride and trace-metal requirements and request a sample and COA to test against.
Developing a nitrate salt blend? Request a sample
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Buy online WhatsApp usFrequently asked questions
Is calcium nitrate used in molten salt for solar power?
What is the melting point of a calcium nitrate molten salt mixture?
Why add calcium nitrate to molten salt?
It lowers the melting point and is the lowest-cost nitrate constituent, which reduces the energy needed to keep the salt molten and the cost of the fluid inventory.[1]
What are the downsides of calcium nitrate in molten salt?
How much heat storage do molten-salt towers provide?
Recent molten-salt power towers such as Crescent Dunes were designed for about 10 hours of storage.[7]
Can I use fertilizer-grade calcium nitrate for molten salt?
Not without qualification. The research uses dehydrated, high-purity feedstock, and you would need to set and verify moisture, chloride and trace-metal limits first.[1]
Related guides
- Uses of calcium nitrate: the complete guide
- How calcium nitrate is made
- Calcium nitrate safety, handling & storage
- Buying bulk calcium nitrate from India
References
- Molten nitrate salt development for thermal energy storage in parabolic trough solar power systems (Bradshaw et al., ASME Energy Sustainability 2008) — Sandia National Laboratories via U.S. DOE OSTI. Government lab paper
- Low-melting point inorganic nitrate salt heat transfer fluid — Sandia National Laboratories, Intellectual Property. Government lab
- US 7,828,990 — Low-melting point heat transfer fluid — U.S. Patent and Trademark Office. Patent
- Corrosion of alloys and metals by molten nitrates (Bradshaw & Goods, SAND2000-8727) — Sandia National Laboratories via U.S. DOE OSTI. Government lab report
- Power tower system: concentrating solar-thermal power basics — U.S. Department of Energy. Government
- Thermal storage system: concentrating solar-thermal power basics — U.S. Department of Energy. Government
- EERE success story: Energy Department investment brings Crescent Dunes to life — U.S. Department of Energy. Government
- Influence of corrosion-resistant coatings on the post-corrosion behaviour of alloys in molten salt — U.S. DOE OSTI. Government lab paper
- Digital phase diagram and thermophysical properties of KNO3-NaNO3-Ca(NO3)2 ternary system for solar energy storage — University of Arizona. University research
- Calcium based ternary nitrate salts for concentrating solar power applications — AusIMM, 12th International Conference on Molten Slags, Fluxes and Salts (MOLTEN 2024). Conference paper
- Ca(NO3)2-NaNO3-KNO3 molten salt mixtures for direct thermal energy storage systems in parabolic trough plants — Khalifa University (Khazna repository). University research
- EP 2898039 A1 — Use of a calcium potassium nitrate salt for the manufacture of a heat transfer fluid — European Patent Office, via Google Patents. Patent
- Effect of pre-heating on thermal stability of molten nitrate salt — Journal of Energy Storage, Elsevier. Peer-reviewed
- The thermal stability of molten nitrite/nitrates salt for solar thermal energy storage in different atmospheres — Solar Energy, Elsevier. Peer-reviewed
- Probing thermal decomposition mechanism of molten nitrite/nitrates salt by time of flight mass spectrometry — Solar Energy, Elsevier. Peer-reviewed
- Thermodynamic properties of molten nitrate salts — U.S. DOE OSTI. Government lab paper
- Power tower (concentrating solar-thermal power) — U.S. DOE Office of Energy Efficiency & Renewable Energy. Government
- Calcium nitrate (PubChem compound summary) — U.S. National Library of Medicine / NCBI. Government database
- Thermal energy storage — Wikipedia. Encyclopedia
- Solar power tower — Wikipedia. Encyclopedia
- Calcium nitrate — Wikipedia. Encyclopedia