Molten Salt for thermal energy storage: 5 Advantages and Disadvantages

Introduction : Molten salt is a high temperature liquid substance formed by heating solid salts. It is typically a mixture of sodium nitrate (NaNO3) and potassium nitrate (KNO3) until they melt. These salts can remain in a liquid state at temperatures typically between 260 degreeC to 565 degreeC. In solar energy systems, particularly Concentrated Solar Power (CSP) plants, molten salt is used as a thermal energy storage medium.

When solar energy is available, it heats the molten salt through a heat exchanger. This hot molten salt is then stored in insulated tanks. When electricity is needed (e.g., during the night or cloudy periods), the heat from the stored molten salt is used to produce steam, which drives a turbine to generate electricity. The figure-1 depicts solar thermal power plant made by China using molten salt.

Advantages of Molten Salt as thermal energy storage

Following are some of the benefits of Molten Salt in thermal energy storage.

1. Molten salts have a high specific heat, allowing them to store large amounts of thermal energy per unit mass, improving overall system efficiency.
2. Energy stored in molten salt can be used to generate electricity even after sunset, enabling 24/7 power generation in solar thermal plants.
3. Compared to other thermal storage media, molten salts are relatively low cost and abundant, particularly nitrate salts.
4. Molten salts remain thermally stable at high temperatures, enabling efficient heat transfer and storage without decomposing quickly.
5. With molten salt storage, solar thermal power plants can reduce dependence on fossil fuel based backup systems.

Disadvantages of Molten Salt

Following are some of the drawbacks or limitations of Molten Salt in solar energy storage.

1. Molten salt solidifies at temperatures around 220 degreeC to 250 degreeC, requiring constant heating or insulation to prevent freezing, which adds complexity and operational cost.
2. Molten salts can be corrosive to pipes, tanks, and heat exchangers, demanding the use of expensive, corrosion resistant materials.
3. Despite insulation, some thermal energy is lost over time during storage, especially during long-term holding.
4. CSP plants with molten salt storage require significant upfront investment in infrastructure and materials.
5. Maintaining the right operating temperature and managing the flow of molten salt requires sophisticated control systems and maintenance.

Conclusion: Molten salt is a reliable and efficient thermal energy storage material used in solar thermal power plants to extend power generation beyond daylight hours. While it offers significant advantages in terms of energy efficiency, cost and dispatchability, it also presents challenges related to freezing risk, material corrosion, and operational complexity. Nevertheless, molten salt technology remains a key enabler for making solar power more consistent and dependable.