Quaternary nitrate-based molten salts improve heat transfer performance in concentrated solar power (CSP) systems through several key mechanisms:
- Enhanced Thermal Stability: These quaternary nitrate mixtures maintain high thermal stability up to temperatures around 500–590 °C, enabling them to operate efficiently under the high-temperature conditions typical of CSP systems without significant degradation. This stability ensures reliable and sustained heat transfer over many thermal cycles.
- Lower Melting Points: By incorporating components like calcium nitrate and lithium nitrate into a base mixture of sodium nitrate and potassium nitrate, quaternary nitrate salts achieve lower melting points compared to traditional binary solar salts. This lowers the risk of solidification during operation and extends the usable temperature range for heat transfer fluids.
- Improved Heat Capacity and Energy Storage: Quaternary nitrate molten salts exhibit increased heat capacity, allowing them to store greater amounts of heat energy per unit mass. Experimental results show a high temperature difference during cooling tests, indicating superior heat retention and slower heat loss, which is advantageous for both heat transfer and thermal energy storage in CSP systems.
- Favorable Viscosity at Operating Temperatures: Although the viscosity of quaternary salts can be higher than that of binary salts at lower temperatures, it remains manageable and does not significantly hinder fluid flow. This balance supports efficient heat transfer fluid circulation within CSP heat exchanger systems.
- Promising Heat Transfer Fluid Candidate: Due to their combination of thermal stability, heat capacity, and manageable viscosity, these quaternary nitrate-based molten salts are considered promising candidates for CSP heat transfer fluids, improving heat recovery and energy storage performance in solar thermal power plants.
In summary, quaternary nitrate molten salts enhance CSP system heat transfer primarily by offering stable operation at high temperatures, improved heat capacity for energy storage, reduced melting points to avoid freezing, and suitable fluid properties for efficient circulation, making them superior to older binary nitrate salt mixtures used in thermal solar applications.
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