The objective of this work is to establish methods for storage of thermal energy at high temperatures applicable in concentrating solar power plants (CSPs). Temperatures up to 600oC are considered using encapsulated phase change materials (EPCMs), where heat transfer fluid (HTF) from the solar collector would pass through the storage system embedded with EPCM capsules.
NaNO3 and a eutectic compound of NaCl–MgCl2 are selected as phase change materials (PCM), which are tested in a special calorimeter at temperatures below and above their melting points to determine their sensible and latent heat storage capabilities as well as stability and compatibility with the encapsulation material over multiple cycles of melting and solidification.
Considering the rate of melting and solidification inside the EPCM, optimum diameter of cylindrical capsules are found to be between 50 to 100 mm. Capsules with 76 mm diameter and 260 mm length, containing phase change material (PCM) are fabricated and installed in a pilot-scale thermal energy storage (TES) system for performance tests, where compressed air is used as heat transfer fluid. Tests with ten rows of EPCM capsules successfully demonstrate the ability to transfer thermal energy to and from the transport fluid, achieving energy storage and retrieval in multiple charging and discharging cycles.
A mathematical model has been developed for the test section with EPCM capsules and its predictions are found to agree with experimental measurements within 7% discrepancy in stored energy. The dynamic performance of charging and discharging rates are also well predicted by the model, giving confidence for engineering design capabilities in future applications using EPCMs for thermal energy storage.
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