Why molten salt is a popular thermal storage medium
Molten salt is widely used in CSP plants as both a heat transfer fluid and a thermal energy storage medium. It typically consists of a mixture of sodium nitrate and potassium nitrate, which remains liquid over a useful temperature range and stores heat with good efficiency.
Key benefits of molten salt:
- High operating temperature: Molten salts can be heated to 500–600°C or more, enabling efficient power cycles and compact storage.
- Thermal storage capability: Using two tanks—hot and cold—molten salt stores heat from the receiver and releases it later to generate steam, allowing dispatchable electricity after sunset.
- Cost-effectiveness: Compared to batteries for bulk, multi-hour storage, molten salt offers competitive costs per stored megawatt-hour for large systems.
Typical system components and flow:
- Receiver heating: Molten salt circulates through the receiver, where concentrated sunlight raises its temperature.
- Hot tank storage: Heated salt is stored in an insulated hot tank until needed for power generation.
- Heat exchange: When electricity is required, hot salt flows through a heat exchanger to generate steam for a turbine, then returns to the cold tank.
Operational and material considerations:
- Freeze point: Molten salts freeze at relatively high temperatures (~120–220°C depending on composition), so freeze protection and heating are required during outages and cold conditions.
- Corrosion and materials: Salts are corrosive at high temperatures, so compatible materials and coatings are essential for pipes, tanks, and receivers.
- Thermal losses: Insulation and careful design limit heat loss from storage tanks, improving round-trip efficiency.
Because of its thermal and cost advantages at scale, molten salt is a cornerstone of many modern CSP plants that require multi-hour storage and reliable, dispatchable renewable power.