Molten-Salt Thermal Storage

Two tanks of liquid salt — one cool at 290 °C, one blazing at 565 °C — hold enough stored sunshine to power a city through the night without burning a gram of fuel.

Stores thermal energy collected by a concentrated solar power (CSP) plant as sensible heat in a large inventory of molten nitrate salt. The system uses two tanks: a 'cold' tank holding salt at about 290 °C and a 'hot' tank at about 565 °C. During the day, heliostats or parabolic troughs heat salt and pump it into the hot tank. When electricity is needed — in the afternoon peak, evening, or even cloudy periods — the hot salt is drawn through a heat exchanger to generate superheated steam, which drives a conventional Rankine cycle turbine. The cooled salt returns to the cold tank for reheating. The salt itself is never consumed; the same inventory circulates for the life of the plant.

The storage medium is 'solar salt' — a binary mixture of 60 wt% sodium nitrate (NaNO₃) and 40 wt% potassium nitrate (KNO₃). This eutectic mixture melts at about 220 °C and remains liquid and pumpable up to approximately 600 °C, giving a working temperature window of about 300 °C between the cold and hot tanks. The large temperature differential is the key: the energy stored is proportional to the salt mass × specific heat (~1,500 J/kg·K) × temperature difference. Nitrate salt is chosen over water, oil, or other candidates because it is chemically stable at these temperatures, has extremely low vapour pressure (<0.01 Pa), is inexpensive (~$0.40–0.90/kg), and is non-flammable — unlike the mineral oils used in earlier CSP plants. The tanks are standard atmospheric-pressure cylindrical steel vessels, externally insulated, with careful corrosion-resistant stainless steel in hot-salt service. Freeze protection heaters prevent the salt solidifying at <220 °C during cold nights or outages.

In plain terms

Imagine filling a giant thermos with superhot liquid salt during the day when the sun is shining, then pouring that heat out at night to boil water, make steam, and spin a turbine that makes electricity. The salt is so good at holding heat that it loses only about 1 degree per day — your kitchen thermos loses more warmth keeping your soup hot for lunch.

Also called: two-tank molten salt, nitrate salt TES, CSP thermal storage, solar salt storage.

Why it matters

Converts an intermittent solar resource into dispatchable power: a plant with a 10-hour hot tank can supply firm electricity from late morning through midnight.
The Crescent Dunes plant in Nevada (110 MW, 1,100 MWh storage) demonstrated that large-scale round-the-clock solar power requires no fossil backup, only thermal mass.
Storage capital cost for the thermal medium is ~$30–40/kWh-thermal, far below the cost of equivalent electrical battery storage, making long-duration storage economically viable.
Molten salts lose only about 1 °C per day from a well-insulated tank — the stored heat is theoretically recoverable after weeks, though economics favour daily cycling.
The same salt inventory recycles daily for 30+ years with minimal degradation; the technology has no electrochemical wear-out mechanism.

Where it lives

Solar Two, Barstow, California (10 MWe, 3-hour storage, operated 1996–1999 — first grid-connected molten-salt power tower, 60% NaNO₃/40% KNO₃, 290 °C cold / 565 °C hot)
Crescent Dunes, Tonopah, Nevada (110 MW, 10-hour / 1,100 MWh storage, began commercial operation 2015, SolarReserve — first full-scale tower CSP with advanced salt storage)
Gemasolar, Fuentes de Andalucía, Spain (20 MWe, 15-hour storage — first plant to achieve 24-hour solar-only operation)
Andasol 1–3, Granada, Spain (three 50 MWe parabolic trough plants with 7.5-hour two-tank indirect salt storage each)

Key numbers

Hot tank temperature: 565 °C (Confirmed across Solar Two (DOE), Crescent Dunes (SolarPACES), and ORNL/davidpublisher CSP design studies)
Cold tank temperature: 285–290 °C (Above freezing point (220 °C) for safe pumpability; Solar Two and Crescent Dunes)
Salt composition: 60% NaNO₃ / 40% KNO₃ (Binary eutectic 'solar salt'; melt point 220 °C; Solar Two and Crescent Dunes)
Storage duration (commercial): 6–15 hours (Crescent Dunes: 10 h / 1,100 MWh; Gemasolar: 15 h; range confirmed by SolarPACES and Stanford CSP study)
Thermal storage cost: ~$30–40/kWh-th (Two-tank molten salt; Herrmann et al., Energy (2004); representative of parabolic trough plants)
Tank heat loss: ~1 °C/day (Well-insulated hot tank; measured at Solar Two; confirmed by SolarPACES)
Salt specific heat: ~1,500 J/kg·K (NaNO₃/KNO₃ mixture; Herrmann et al. (2004))

Surprising facts

The Crescent Dunes hot salt tank holds 32,000 tonnes of molten salt — about the weight of 300 blue whales — all kept liquid at 565 °C inside a single insulated steel vessel sitting in the Nevada desert.
Solar Two's hot storage tank was filled with salt in mid-1996 and then monitored for cooling: after one full month with no solar input the temperature had dropped only 75 °C — a rate so slow that the stored heat could theoretically be kept available for months.
The entire salt inventory at Solar Two was 1,600 tonnes; melting it from solid to liquid for the first time took two months of continuous heating. Once molten, the same salt cycled through 900 m³ tanks every operational day for three years without any chemistry change.
A molten-salt CSP plant with a solar multiple of ~2.7 can achieve an annual capacity factor of roughly 65% — matching the capacity factors of coal and combined-cycle gas plants — using only sunlight as the fuel.

Open questions

Crescent Dunes tank floor failure under thermal cycling: The hot tank at Crescent Dunes developed a leak in October 2016 caused by unexpected compressive stress in the tank floor plate. The engineering team had modelled thermal expansion of the floor correctly, but the interaction between floor plate, insulating concrete foundation, and the cyclic heating from ambient to 565 °C every day produced crack propagation that no previous smaller-scale plant had exhibited. It is a documented reminder that first-of-kind scale-up can surface failure modes invisible at smaller demonstrations — and that 'commercially proven at 10 MW' does not automatically mean 'no surprises at 110 MW'.

A short history

The use of nitrate salt as a combined heat-transfer and storage medium for solar power towers was developed through a US DOE research programme in the 1980s. A proof-of-concept 'Molten Salt Electric Experiment' at Sandia National Laboratories validated the receiver and storage components at kilowatt scale. Solar Two — a 10 MWe demonstration plant east of Barstow, California, jointly funded by DOE and eleven industry/utility partners — operated from June 1996 to April 1999 as the first complete grid-connected molten-salt power tower. It proved the two-tank system and held salt at 565 °C in commercial-scale atmospheric tanks with measured heat loss within 10% of design. The next major milestone was Gemasolar in Spain (2011), which first demonstrated round-the-clock solar-only operation. Crescent Dunes in Nevada (110 MW, first light 2015) became the largest tower CSP plant with integrated storage; it suffered a tank leak in 2016 but returned to operation in 2017, providing ten hours of dispatchable solar electricity nightly under contract with NV Energy. As of 2025, the technology is commercially mature for CSP power towers; ongoing research targets higher-temperature chloride salt systems (up to 700 °C) to improve power-cycle efficiency and reduce storage costs.

Sources

US DOE/EERE — Solar Two molten-salt power tower description, Dec 1997 (https://www1.eere.energy.gov/ba/pba/pdfs/solar_tower.pdf)
SolarPACES — How CSP thermal energy storage works (Kraemer, 2018) (https://www.solarpaces.org/how-csp-thermal-energy-storage-works/)
Herrmann et al., Energy (2004) — Two-tank molten salt storage for parabolic trough plants (peer-reviewed cost and performance analysis) (https://www.sciencedirect.com/science/article/abs/pii/S0360544203001932)
Sandia National Laboratories — Solar Two test and evaluation program summary (2000) (https://www.sandia.gov/research/publications/details/summary-of-the-solar-two-test-and-evaluation-program-2000-02-08/)