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Solidification of eicosane with and without nanoadditives is experimentally investigated in spherical enclosures subject to convective cooling in water and air. The effects of additive volume fraction and external convective cooling conditions (i.e., the heat transfer medium, subcooling, and flow velocity) on the solidification process are examined. The results are compared with a conduction-controlled thermal network model accounting for the enclosure and PCM resistances, as well as the convective subcooling. The experimentally determined solidification time is found to be consistently lower than the model prediction, likely due to asymmetric and dendritic solidification, as well as natural convection inside the enclosure and possible thermocouple position errors. A simple correlation is proposed to predict the solidification time of a phase change material (PCM) in a spherical enclosure subject to convective cooling based on the same enclosure subject to a constant temperature boundary.Results show that the solidification time decreases with the volume fraction of nanoadditives due to the improved PCM conductivity. In addition, the nanoadditives are found to be more effective for solidification in water than in air, due to the large air-side convective resistance that does not benefit from improving PCM conductivity.

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A two-dimensional numerical model is developed to simulate the transient response of a heat pipe-assisted latent heat thermal energy storage (LHTES) unit integrated with dish-Stirling solar power generation systems. The unit consists of a container which houses a phase change material (PCM) and two sets of interlaced input and output heat pipes (HPs) embedded in the PCM. The LHTES unit is exposed to time-varying concentrated solar irradiance. A three-stage operating scenario is investigated that includes: (i) charging only, (ii) simultaneous charging and discharging, and (iii) discharging only. In general, it was found that the PCM damps the temporal variations of the input solar irradiance, and provides relatively smooth thermal power to the engine over a time period that can extend to after-sunset hours. Heat pipe spacing was identified as a key parameter to control the dynamic response of the unit. The system with the greatest (smallest) heat pipe spacing was found to have the greatest (smallest) temperature drops across the LHTES, as well as the maximum (minimum) amount of PCM melting and solidification. Exergy analyses were also performed, and it was found that the exergy efficiencies of all the systems considered were greater than 97%, with the maximum exergy efficiency associated with the system having the minimum heat pipe spacing.

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