Analysis of HTFs, PCMs and fins effects on the thermal performance of shell–tube thermal energy storage units

• The working processes of PCM and HTF in TES unit were modeled simultaneously.
• The conjugated thermal behavior between PCMs and HTFs were quantified.
• Molten salts are more efficient than synthetic oil in thermal transport.
• The thermal resistance of TES units is dominated by PCM side.
• The composite PCM is better than extended fins in improving TES performance.

In solar thermal power plant, the thermal energy storage (TES) system is a key component which includes thermal energy storage unit, thermal energy storage material and heat transfer fluid (HTF). We numerically simulate the dynamical thermal performances of shell–tube TES units with diverse geometric configurations, phase change materials (PCMs) including pure NaNO3 and a composite of NaNO3/expanded graphite (EG), and HTFs such as synthetic oil and molten salts. In the charging/discharging process, the conjugated heat transfer behavior between the thermal storage unit and thermal carriage channel is specially concerned. Effects of the extended fin, PCMs and HTFs on improving heat performance are examined and quantified with total heat flux and effective Nusselt numbers. Together with the presented diagrams of PCM phase evolution, the roles of heat conduction and natural convection in influencing the thermal behavior of the TES units are clearly revealed. The nonuniform phase distribution of PCM due to natural convection results in a considerable time spent in the end of the charging/discharging processes. The results show that both the full melting time and solidification time can be shortened at least 14%, benefited from using the extended fins and the composite. This study is expected to provide an apprehensive interpretation of thermal transport mechanism and rational advices in optimizing the TES system operation.