This paper investigates the photothermal conversion performance of a novel heat transfer fluid containing nano-encapsulated phase change material (PCM) with metallic shell materials in a solar thermal energy storage system. The study examines the effects of shell thickness, core size, shell material type, PCM mass, and shell volume concentrations on the thermal performance of the heat storage medium. The results show that the heat transfer rates of water-based Ag, Au, Cu, and Al nanofluids are 6.89, 5.86, 7.05, and 6.99 W, respectively, while slurries formed by adding paraffin@Ag, Au, Cu, and Al nano capsules to pure water enhance heat transfer by 6.18%, 13.38%, 10.8%, and 11.33%, respectively. The metallic nanoparticle-based shell materials further enhance the temperature and energy storage gains by improving the solar radiation capture capability of the heat storage medium. Specifically, the storage capacity of paraffin@Cu slurry increases by up to 290% depending on the mass concentration of PCM. The study also explores the impact of shell thickness and core size on the system performance, finding that decreasing the shell thickness from 8 to 2 nm enhances the slurry's storage ability by 7%, while increasing the core size reduces the surface area-to-volume ratio, leading to a 5% decrease in thermal energy storage for paraffin@Cu slurry. Additionally, the volume concentration of Al particles surprisingly reduces thermal energy storage by 5%. The paper validates the specific heat capacity model of paraffin-based solid PCM under various wind speeds and solar radiation conditions, demonstrating its effectiveness as an alternative to the enthalpy method.This paper investigates the photothermal conversion performance of a novel heat transfer fluid containing nano-encapsulated phase change material (PCM) with metallic shell materials in a solar thermal energy storage system. The study examines the effects of shell thickness, core size, shell material type, PCM mass, and shell volume concentrations on the thermal performance of the heat storage medium. The results show that the heat transfer rates of water-based Ag, Au, Cu, and Al nanofluids are 6.89, 5.86, 7.05, and 6.99 W, respectively, while slurries formed by adding paraffin@Ag, Au, Cu, and Al nano capsules to pure water enhance heat transfer by 6.18%, 13.38%, 10.8%, and 11.33%, respectively. The metallic nanoparticle-based shell materials further enhance the temperature and energy storage gains by improving the solar radiation capture capability of the heat storage medium. Specifically, the storage capacity of paraffin@Cu slurry increases by up to 290% depending on the mass concentration of PCM. The study also explores the impact of shell thickness and core size on the system performance, finding that decreasing the shell thickness from 8 to 2 nm enhances the slurry's storage ability by 7%, while increasing the core size reduces the surface area-to-volume ratio, leading to a 5% decrease in thermal energy storage for paraffin@Cu slurry. Additionally, the volume concentration of Al particles surprisingly reduces thermal energy storage by 5%. The paper validates the specific heat capacity model of paraffin-based solid PCM under various wind speeds and solar radiation conditions, demonstrating its effectiveness as an alternative to the enthalpy method.