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Expanded graphite (EG) is often used to encase phase change materials (PCMs) and improve their thermal performance. However, EG's effectiveness when used with disodium hydrogen phosphate dodecahydrate (DHPD) as a hydrated salt is diminished, due to its hydrophobic property. This study proposes modifying the hydrophobic surface of EG with highly hydrophilic nanoparticles, such as SiO 2 and TiO 2 , to address this shortcoming and boost its hydrophilicity. Herein, the nanoparticles were synthesized by the sol-gel technique and mixed with EG to construct a modified EG (MEG). The microstructure, morphology, and adsorption ability of MEG were analyzed. Additionally, the thermal properties, as well as the thermal stability of form-stable composite PCMs, were investigated. The results revealed that by varying the mass ratio of SiO 2 @TiO 2 in MEG, the supercooling degree of the form-stable composite PCMs decreased by 77.36%–78.49%. With 9% SiO 2 @TiO 2 in MEG, the water contact angle of MEG was optimal, and the form-stable composite PCMs prepared (DSMEG3) exhibited a phase change temperature and latent heat of 32.88 °C and 142.13 J/g, respectively. This experimental latent heat was slightly lower than the calculated latent heat by 0.12 J/g. Meanwhile, the supercooling degree decreased to 3.01 °C, and the thermal conductivity increased to 1.23 W/(m ∙ K), which was 1.73 times than that of DHPD. Moreover, DSMEG3 also showed good shape stability, preventing leakage. Therefore, the form-stable composite PCMs, as prepared, are a great potential candidate for the energy storage applications. [ABSTRACT FROM AUTHOR]

For the first time, a complete set of the volume averaged governing equations based on a three-energy equation model has been proposed to attack the transient thermal response of a bi-disperse packed bed thermocline storage system, in which its void is filled by smaller particles (i.e. solid filler). Effective permeability, stagnant thermal conductivity and effective thermal conductivities for three individual phases, namely, the fluid phase, large particle phase and small particle phase of the bi-disperse packed bed storage system, have been evaluated taking full account of tortuosity and appealing to a volume averaging theory. Subsequently, the three individual energy equations in the three-energy model were coupled and analytically solved to reveal a transient behavior of the temperature fields of the individual phases. The effective discharging efficiency of the bi-disperse packing is found to decrease substantially as compared with that of the mono-disperse packing under equal pumping power. Though one can save a substantial initial cost by filling the void by solid fillers, one must pay a penalty of a higher cost in running the bi-disperse packed bed thermocline storage system. The analysis presented in this study serves to estimate both initial and running costs of a bi-disperse packed bed thermocline storage system. [ABSTRACT FROM AUTHOR]

Expanded graphite (EG) is often used to encase phase change materials (PCMs) and improve their thermal performance. However, EG's effectiveness when used with disodium hydrogen phosphate dodecahydrate (DHPD) as a hydrated salt is diminished, due to its hydrophobic property. This study proposes modifying the hydrophobic surface of EG with highly hydrophilic nanoparticles, such as SiO 2 and TiO 2 , to address this shortcoming and boost its hydrophilicity. Herein, the nanoparticles were synthesized by the sol-gel technique and mixed with EG to construct a modified EG (MEG). The microstructure, morphology, and adsorption ability of MEG were analyzed. Additionally, the thermal properties, as well as the thermal stability of form-stable composite PCMs, were investigated. The results revealed that by varying the mass ratio of SiO 2 @TiO 2 in MEG, the supercooling degree of the form-stable composite PCMs decreased by 77.36%–78.49%. With 9% SiO 2 @TiO 2 in MEG, the water contact angle of MEG was optimal, and the form-stable composite PCMs prepared (DSMEG3) exhibited a phase change temperature and latent heat of 32.88 °C and 142.13 J/g, respectively. This experimental latent heat was slightly lower than the calculated latent heat by 0.12 J/g. Meanwhile, the supercooling degree decreased to 3.01 °C, and the thermal conductivity increased to 1.23 W/(m ∙ K), which was 1.73 times than that of DHPD. Moreover, DSMEG3 also showed good shape stability, preventing leakage. Therefore, the form-stable composite PCMs, as prepared, are a great potential candidate for the energy storage applications. [ABSTRACT FROM AUTHOR]

For the first time, a complete set of the volume averaged governing equations based on a three-energy equation model has been proposed to attack the transient thermal response of a bi-disperse packed bed thermocline storage system, in which its void is filled by smaller particles (i.e. solid filler). Effective permeability, stagnant thermal conductivity and effective thermal conductivities for three individual phases, namely, the fluid phase, large particle phase and small particle phase of the bi-disperse packed bed storage system, have been evaluated taking full account of tortuosity and appealing to a volume averaging theory. Subsequently, the three individual energy equations in the three-energy model were coupled and analytically solved to reveal a transient behavior of the temperature fields of the individual phases. The effective discharging efficiency of the bi-disperse packing is found to decrease substantially as compared with that of the mono-disperse packing under equal pumping power. Though one can save a substantial initial cost by filling the void by solid fillers, one must pay a penalty of a higher cost in running the bi-disperse packed bed thermocline storage system. The analysis presented in this study serves to estimate both initial and running costs of a bi-disperse packed bed thermocline storage system. [ABSTRACT FROM AUTHOR]