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Molten Salt Thermal Storage for CSP Simulator
Size the Solar Salt thermal energy storage subsystem at the heart of a Concentrated Solar Power (CSP) plant. Vary the plant rating, storage hours and hot / cold tank temperatures and watch the required thermal input, storage capacity, salt mass, tank diameter and storage cost update in real time.
Parameters
Plant rating P_e
MW
Net electrical output of the power block (steam turbine)
Storage hours t_s
h
Hours the storage can sustain the plant at full rating
Hot tank T_hot
°C
Upper limit for Solar Salt (≈585 degC max)
Cold tank T_cold
°C
Must stay above the salt freezing point (≈220 degC)
Capacity factor CF
Annual average duty (≈0.20 for PV, 0.40–0.60 for CSP+storage)
Power block efficiency η
Rankine cycle efficiency (≈0.40 at 565 degC)
Salt unit cost c_salt
USD/kg
Solar Salt bulk price (0.5–1.5 USD/kg in bulk)
Results
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Thermal input (MW_th)
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Storage capacity (MWh_th)
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Salt mass (ton)
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Per-tank diameter (m)
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Salt cost (M USD)
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Storage cost (USD/kWh_th)
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CSP plant schematic — solar field, two-tank storage, power block
Day: sunlight heats the receiver and cold salt (blue) becomes hot salt (orange) stored in the hot tank. Night: hot salt flows to the steam generator and returns cold to the cold tank.
Storage capacity vs storage hours
Tank volume vs plant rating
Theory & Key Formulas
$$E_{store} = m \cdot c_p \cdot \Delta T,\quad m = \frac{E_{th} \cdot 3600}{c_p \cdot (T_{hot}-T_{cold})}$$
Sensible-heat storage. m: salt mass (kg), c_p: salt specific heat (1530 J/kg/K), ΔT: hot−cold temperature difference. The salt mass needed for a target storage capacity E_th (MWh) is obtained directly.
P_th: required thermal input (MW_th), η: power-block efficiency, V_salt: salt volume (m³), ρ_salt=1850 kg/m³, H=12 m (assumed tank height).
$$\text{Cost}_{salt} = m \cdot c_{salt},\quad \text{LCOS}_{th} = \frac{\text{Cost}_{salt}}{E_{th}}$$
Total salt cost and per-kWh_th storage cost. A target under 30 USD/kWh_th keeps CSP storage competitive with batteries.
Molten Salt Thermal Storage for Concentrated Solar Power (CSP)
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Professor, I keep hearing about "CSP" in renewables news. How is it different from regular photovoltaic solar? Both use sunlight, right?
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Good question. PV (photovoltaic) converts sunlight directly into electricity with silicon cells. CSP — Concentrated Solar Power — uses mirrors to focus sunlight onto a receiver and heat a working fluid to very high temperature, typically a molten salt. That heat then drives a steam turbine. Think of a fossil-fuel boiler replaced by a tower of sunlight. Andasol in Spain, Crescent Dunes in Nevada, Noor in the UAE and the Dunhuang plant in China are the textbook examples.
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Converting light to heat to electricity feels indirect. Why bother with that extra step?
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Because heat is much easier to store than electricity. Drag the storage-hours slider from 6 to 12 and you will see the storage capacity in MWh double. That is the killer feature: CSP can sell electricity at night using sunlight collected during the day. PV stops when clouds pass or the sun sets; CSP with storage is dispatchable — you produce power when the grid actually needs it. That changes its market value completely.
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Got it! So what exactly is this "molten salt"? Not table salt, I assume?
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The industry standard is Solar Salt — 60% sodium nitrate plus 40% potassium nitrate. It melts at about 220 degC and is stable up to roughly 585 degC. Specific heat is 1530 J/(kg·K), about one third of water, but the large operating ΔT (565−290 = 275 K) gives a very dense thermal storage per cubic metre. With the default 100 MW / 6 h settings you need over 12,000 tons of it, split between a hot tank and a cold tank.
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12,000 tons!? The tank diameter shows 27 m. That's enormous — is it economically viable?
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That is exactly where the designer's skill counts. Bulk salt is cheap (around 0.5–1.5 USD/kg) but 12,000 tons is still 12.8 million USD, or about 8.5 USD per kWh_th of stored energy. Compare that with lithium-ion batteries at 150–300 USD/kWh — CSP is dramatically cheaper per stored kilowatt-hour, which is why it wins in regions with valuable night-time power. This tool flags the design as a warning above 30 USD/kWh_th and a no-go above 50. Narrowing ΔT explodes the salt mass, so keeping ΔT as wide as possible is the golden rule.
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One last question — I read that CSP is "losing" to PV+battery. Does it still have a future?
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Honestly, PV+battery LCOE has crashed and now leads in many sunny regions. But CSP wins on three fronts: very low-cost storage at the gigawatt-hour scale, true 24-hour operation, and hybridisation with gas or other fuels. China, Morocco and the UAE are still building large plants. The next generation pursues supercritical CO₂ Brayton cycles and 700 degC chloride salts to push power-block efficiency past 50%. If storage cost can be cut to under 20 USD/kWh_th, the gap with PV+battery will close again.
Frequently asked questions
PV converts sunlight directly into electricity, so it stops producing when the sun goes down. CSP first heats a working fluid (molten salt) and stores the heat in insulated tanks. With 6 to 15 hours of storage the plant can keep generating long after sunset, making CSP a dispatchable renewable energy source. Per stored kilowatt-hour, molten salt storage is far cheaper than lithium batteries and helps grid stability. The trade-off is that PV+battery LCOE has dropped sharply, so CSP now wins mostly in regions with strong night-time electricity demand such as the UAE, Morocco and inland China.
Solar Salt has a freezing point near 220 degC, is stable up to about 585 degC and has a high heat capacity (c_p approx 1530 J/kg/K). It is only mildly corrosive, so standard stainless steel pipes and tanks work. Sodium and potassium nitrates are mass-produced commodities priced at 0.5 to 2 USD/kg. The main drawback is its high freezing point: a cold pipe or a cold-start can solidify the salt and ruin the plant, so trace heating is mandatory everywhere. Next-generation research targets 700 degC chloride salts and liquid metals.
Storage hours are defined as the time the plant can run at full rating using only stored heat, typically 6 to 15 h. Too few (e.g. 3 h) cannot cover the evening peak; too many (e.g. 20 h) scale up salt cost linearly without a useful revenue boost. Parabolic trough plants in Spain and the US use 6 to 7.5 h. UAE Noor 700 MW and Morocco's Noor III use 7 to 8 h. Chinese tower plants (Crescent Dunes, Cerro Dominador, Dunhuang) push toward 10 to 15 h. Drag the slider in this tool and you will see total salt cost rise nearly linearly with storage hours.
A two-tank system keeps hot salt (565 degC) and cold salt (290 degC) physically separated, so there is no thermocline to manage and operation is simple: during discharge salt flows from the hot tank through the steam generator and returns cold; during charge it flows in reverse through the receiver. Single-tank thermocline storage saves about 30% of the salt inventory and cost, but mixing degrades efficiency, so commercial plants almost universally use two tanks. The two-tank approach was proven at Solar Two (1996) and adopted by Andasol, Gemasolar and the Noor complex.
Real-world applications
Commercial CSP plant sizing: Andasol-1 (Spain, 2008, 50 MW trough with 7.5 h storage), Crescent Dunes (USA, 2015, 110 MW tower with 10 h), Noor Energy 1 (UAE, 2020, hybrid 700 MW including 15 h CSP storage), Dunhuang 100 MW (China, 2018, 11 h tower) and Noor III (Morocco, 2018, 150 MW with 7 h) all start their thermal storage design from the same formulas built into this tool. Capacity sizing begins from plant rating and storage hours, then iterates on salt mass, tank diameter and storage cost.
Decarbonising industrial process heat: Mid- to high-temperature process heat (200–600 degC) for cement, chemicals, steel and food drying accounts for roughly 40% of industrial CO₂ emissions worldwide. Companies such as Heliogen (US) and Synhelion (Switzerland) are commercialising small-scale concentrated solar storage modules that use Solar Salt or similar fluids. This tool is directly applicable to sizing small units in the 1–10 MW_th range.
Solar fuels and hydrogen production: Next-generation CSP that reaches 1000+ degC enables thermochemical water splitting (sulfur–iodine, CeO₂ redox) and high-temperature electrolysis for green hydrogen. DLR's HydroSol and Sandia's CR5 reactor are well-known examples. Molten salt storage is the natural buffer for these processes, and the sizing logic in this tool transfers directly.
Feasibility-study quick estimates: Full bankable CSP feasibility uses integrated tools such as NREL's System Advisor Model (SAM) with hour-by-hour irradiance data. But for early-stage scoping — answering "Does this plant make any economic sense at all?" — the salt mass / tank diameter / storage USD per kWh given by this simulator is enough to make a go/no-go call before committing to detailed modelling.
Common pitfalls and design notes
The first pitfall is letting the temperature difference ΔT shrink too far. Salt mass scales as m ∝ 1/ΔT, so dropping ΔT from 275 K (565→290 degC) to 150 K roughly doubles the salt inventory and its cost. The usual trap is engineers raising T_cold to "stay safely away from freezing", for example to 320 degC, which silently doubles the bill. The correct response is to commit to robust trace heating from day one and keep T_cold close to the Solar Salt floor (250–290 degC). Drag T_cold in this simulator and watch the salt mass and storage cost change dramatically — that sensitivity is real.
The second pitfall is treating the power-block efficiency η as a free parameter. Rankine-cycle efficiency depends strongly on T_hot: about 0.40 at 565 degC, 0.36 at 500 degC, 0.42–0.44 above 600 degC. Tomorrow's supercritical CO₂ Brayton cycles aim for 0.50+ at 700 degC, but every commercial CSP plant today uses superheated steam Rankine. In this tool η is a separate slider for clarity, but in reality T_hot and η move together. If you drop T_hot you also drop η, which raises P_e/η, which expands every downstream quantity: storage capacity, salt mass, tanks, cost. Always check the consistency of T_hot and η in real design work.
The third pitfall is looking only at salt cost and forgetting the rest of the plant. This tool sizes only the storage subsystem. In a real project, heliostats / mirrors account for 30–40% of CAPEX, the tower and receiver 15–20%, the storage system (salt + tanks + trace heat) 10–15%, the power block (turbine + condenser) 15–20%, and balance-of-plant the remainder. Salt itself is a slice. A genuine LCOE figure requires annual energy yield (MW × CF × 8760 h), full CAPEX and OPEX, and an NPV calculation. Use this simulator as a fast first-cut for the storage subsystem, then move on to NREL SAM, IEA SolarPACES data and the IRENA cost database for project-grade numbers.
How to Use
Enter power block capacity (50–500 MW_e) and desired storage duration (4–15 hours) to set baseline thermal requirements.
Specify hot tank temperature (565–600°C typical for Solar Salt) and cold tank temperature (290–350°C) to define the usable enthalpy swing.
Click Calculate to generate salt mass, tank diameters, storage capacity in MWh_th, and levelized storage cost in USD/kWh_th.
Worked Example
For a 100 MW_e power block with 10 hours storage, hot tank at 580°C, cold tank at 290°C: system requires 1,100 ton of Solar Salt (60% NaNO₃/40% KNO₃), stores 1,050 MWh_th, needs two tanks each 12.5 m diameter, with total salt cost approximately 33 M USD and levelized storage cost of 31 USD/kWh_th including tank construction, insulation, and piping.
Practical Notes
Longer storage duration (12+ hours) enables CSP plants to supply power during evening peak demand; verify grid interconnection agreements for dispatch flexibility.
Hot tank temperature above 585°C increases corrosion rates and material costs; use Inconel 625 or stainless steel 316 for containment.
Cold tank stratification reduces effective capacity; implement internal diffusers or thermocline baffles to maintain 50°C minimum separation zone.
Salt freezing risk below 220°C requires trace heating and insulation (0.5 m mineral wool); verify winter ambient conditions at site.