Optically functional bio-based phase change material nanocapsules for highly efficient conversion of sunlight to heat and thermal storage

Abstract

Conversion of sunlight to heat and the subsequent thermal storage by nanoencapsulated bio-based phase change material slurries (NBPCMSs) in a low temperature solar system is investigated. The influences of capsule size, shell material, tilt angle, solar heat flux, PCM mass concentration, nanoparticle and its concentration are explored. The results reveal that the useful heat gain capacity of nano-enhanced coconut oil/Ag, coconut oil/Au, coconut oil/Al, and coconut oil/Cu based slurries is respectively 3.02, 3.12, 2.7, and 3.14 times better than that of pure water, due to an enhanced interaction of light with the functional bio-based PCM nanocapsules. Consequently, the thermal energy storage is reported to be 8.85, 9.29, 7.41, and 9.19 times higher. The increment in mass concentration of PCM from 5 to 20 % and addition of blended nanoparticles further augment the solar thermal energy storage capacity. Specifically, the storage capacity of coconut oil/Au based slurry is improved by up to 74.4 % when the 20 % coconout oil is used as a core material. The energy storage improvements of Cu and Ag based slurries enhance by 4.04 and 4.87 %, respectively, with the insertion of Au nanoparticles at a volume fraction of 25 ppm. Augmenting the core/shell confinement size, on the other hand, diminishes the surface area to volume ratio, allowing agglomeration of the structures inside the slurry. The performance of solar energy storage decreases as the inclination angle of the storage cavity increases from 0° to 60°, reducing the buoyancy force and particles’ collision. Further, since Al particles have low optical characteristics and thermal conductivity, the thermal performance of coconut oil/Al nanoencapsulated slurry are at the lowest level. Finally, experiment is conducted to validate the specific heat capacity model prediction under various wind speeds, from 1 to 4 m/s, and solar illuminations, from 400 to 1000 W/m2.