Molten Salt Thermal Storage Discharge Calculator
Molten Salt Energy Storage Basics
Concentrated solar power plants frequently employ molten salt tanks to dispatch energy after sunset. A blend of sodium and potassium nitrates remains liquid between roughly 290 °C and 565 °C, storing tremendous sensible heat that can later drive steam turbines. Determining how long a tank can supply power hinges on its volume, the temperature differential between hot and cold tanks, the heat capacity and density of the salt, and the plant’s power draw.
This calculator estimates stored thermal energy using the sensible heat equation:
The result in kilojoules is converted to kilowatt-hours by dividing by 3600. Multiplying by the optional efficiency field accounts for thermal and conversion losses in heat exchangers, piping, and the steam cycle. Dividing the deliverable energy by plant power (converted to kilowatts) yields a discharge duration. You can also enter a desired duration to see the probability of falling short, using a logistic risk curve centered on that target.
Design Considerations
| Parameter | Influence |
|---|---|
| Volume | Directly proportional to stored energy |
| Temperature Difference | Higher ΔT increases capacity but raises material stress |
| Specific Heat | Varies with salt composition; higher values store more energy |
| Density | Affects mass and thus total heat |
| Efficiency | Accounts for losses in piping, heat exchangers, and turbine conversion |
Engineers must also consider heat losses through insulation, stratification within the tank, pump efficiency, and parasitic loads like freeze protection heaters. Although not explicitly modeled here, these factors reduce usable energy over time. By adjusting the inputs, you can explore how system design impacts discharge duration and risk of shortfall. For example, lowering efficiency to 80 % shows how poorly insulated piping or fouled heat exchangers erode output.
Practical Example
Imagine a plant with a 1,000 m³ tank, hot and cold temperatures of 565 °C and 290 °C, and a 100 MW turbine. With 90 % efficiency, the deliverable energy is about 446,400 kWh, providing roughly 4.5 hours of full-power output. If a grid contract requires six hours, the risk metric climbs above 70 %, signaling that either more volume or a lower power draw is necessary. Increasing volume to 1,400 m³ cuts the risk to around 20 % and extends discharge to 6.3 hours.
Operational Insights
Molten salt systems demand diligent maintenance. Salts solidify below about 220 °C, so piping and tanks need trace heating even when idle. Corrosion and nitrate decomposition can contaminate the salt over time, reducing effective heat capacity. Operators periodically sample and filter the salt to maintain performance. The efficiency field lets you approximate these real-world degradations by reducing the usable energy fraction.
Dispatch strategy also matters. Some plants opt to discharge at less than full power to stretch output through the evening peak. You can experiment by lowering the power draw input; halving it doubles discharge time and dramatically improves the risk metric. Conversely, running at maximum capacity for short bursts may satisfy ancillary services but requires a large, expensive tank.
Planning for the Future
Molten salt storage offers a pathway to dispatchable renewable energy. Understanding discharge characteristics aids in sizing tanks, planning maintenance, and evaluating economic feasibility. Use the copy button to save results for project reports or sensitivity analyses. As research explores alternative salts and higher operating temperatures, this calculator can evolve to model advanced chemistries, phase-change materials, or hybrid thermal-electric systems.
Keeping a Record
Project developers often compare multiple design scenarios. Copying and pasting the calculated duration and risk into spreadsheets preserves a trail of assumptions for future review. Documenting these figures helps stakeholders revisit why particular tank volumes or efficiencies were chosen during early planning.
Worked Comparison of Salt Choices
Different salts yield different capacities. Consider replacing the traditional sodium–potassium nitrate mixture with a chloride-based salt that has a specific heat of 1.1 kJ/kg·K and density of 2000 kg/m³. Holding the same volume and temperature range, the deliverable energy becomes or roughly 168,000 kWh before efficiency losses. With an 85 % efficient system, only 142,800 kWh would reach the grid, supplying a 100 MW turbine for about 1.4 hours. The example illustrates how material choices dramatically affect storage requirements and why emerging high-temperature salts are attracting research funding.
Storage Technology Comparison
| Technology | Typical Efficiency | Discharge Duration | Cost per kWh (approx.) |
|---|---|---|---|
| Molten Salt | 70–90% | 4–12 h | $50–$100 |
| Pumped Hydro | 70–85% | 8–24 h | $100–$200 |
| Li-ion Batteries | 85–95% | 1–4 h | $150–$400 |
Molten salt occupies a sweet spot between the long duration of pumped hydro and the high round-trip efficiency of batteries. The table helps planners decide whether the additional complexity of a thermal system is justified compared with electrochemical or gravitational alternatives.
Limitations and Assumptions
The calculator assumes uniform temperature distribution, constant specific heat, and negligible thermal losses during discharge. In practice, stratification can reduce effective , and specific heat varies with temperature. The logistic risk curve is a simplified representation of reliability; real-world dispatch depends on grid demand, equipment availability, and weather forecasts. The tool also ignores capital costs, permitting requirements, and maintenance downtime. Use the results as preliminary estimates and consult detailed engineering models for final design decisions.
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