Molten Salt and Thermal Oil Comparison
Molten salt and thermal (hot) oil are both used as heat transfer and thermal storage media in systems like compressed air energy storage (CAES), but they differ significantly in operating temperature range, stability, safety, and system design considerations.
Temperature Range and Stability
- Molten Salt operates stably at higher temperatures, typically above 400°C, and can reach up to about 550°C or more. This high-temperature stability makes it suitable for applications requiring high-temperature thermal storage and heat transfer without decomposition.
- Thermal Oil generally has a maximum stable operating temperature around 400-430°C. Beyond this range, thermal oils tend to decompose chemically, limiting their use in high-temperature thermal storage systems.
Thermal Storage and Heat Transfer Use
- In solar thermal plants and similarly in CAES systems, molten salt is commonly used as the thermal energy storage medium due to its ability to store heat at higher temperatures and retain that heat efficiently over time with insulated tanks. Molten salt storage often involves a two-tank system (hot and cold tanks) to enable continuous heat supply even when the heat source is intermittent.
- Thermal oil is often used as the heat transfer fluid in the loop delivering heat to steam generators because it remains liquid at lower temperatures and is less prone to solidifying. However, because of its lower temperature limit, the maximum system temperature and thus thermal efficiency is limited.
Operational and Safety Considerations
- Molten salt presents some operational challenges, such as being solid at room temperature, which requires careful heating during startup and shutdown to avoid solidification and potential pipe blockages. However, it is safer in case of leaks because it is non-flammable.
- Thermal oil systems typically offer easier startup and shutdown compared to molten salt, but thermal oil is flammable and poses higher safety risks in the event of leakage or fire.
Summary Table
| Feature | Molten Salt | Thermal Oil |
|---|---|---|
| Maximum Operating Temperature | 400-550°C+ | ~410-430°C |
| Thermal Stability | High at high temperatures | Decomposes above ~430°C |
| Phase at Room Temperature | Solid (needs heating to melt) | Liquid |
| Safety | Non-flammable, safer in leaks | Flammable, higher risk in leaks |
| Startup and Shutdown | Longer, careful heating required | Faster |
| Use in CAES & Thermal Storage | Preferred for high-temp storage | Used as heat transfer fluid |
In conclusion, molten salt is preferred over hot oil for thermal storage in CAES systems requiring high-temperature operation and longer heat retention due to its superior thermal stability and safety. Thermal oil, while easier to handle and less prone to solidification, is limited by lower maximum temperatures and higher fire risk.
Original article by NenPower, If reposted, please credit the source: https://nenpower.com/blog/how-does-molten-salt-compare-to-hot-oil-for-thermal-storage-in-caes-systems/
