This paper investigates the photothermal conversion performance of a novel heat transfer fluid containing nano-encapsulated phase change materials (PCMs) 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. Results show that 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 improve the temperature and energy storage gains by enhancing the solar radiation capture capability of the heat storage medium. The storage capacity of paraffin@Cu slurry is increased by up to 290% depending on the PCM mass concentration. However, increasing the core size from 10 to 40 nm reduces the slurry's thermal energy storage by 5%, and increasing the volume concentration of Al particles in the shell reduces the thermal energy storage by 5%. The study also validates the specific heat capacity model of paraffin-based solid PCM under various wind speeds and solar radiation conditions. The paper also presents a mathematical model for analyzing the photothermal conversion behavior of a nano-encapsulated PCM slurry-based directly heated solar system. The model considers the effects of radiation, convection, and the optical and thermophysical properties of the PCM and shell materials. The study concludes that nano-encapsulated PCM slurries offer improved thermal performance compared to pure water, with the best performance achieved by paraffin@Cu slurry. The results demonstrate that the thermal performance of the system is significantly influenced by the shell material type, core size, and PCM mass concentration. The study also highlights the importance of optimizing the shell thickness and core size to enhance the thermal energy storage capacity of the system. The findings suggest that nano-encapsulated PCM slurries can be used as an effective energy storage medium in solar thermal systems, with the potential to improve the efficiency and reliability of solar energy storage.This paper investigates the photothermal conversion performance of a novel heat transfer fluid containing nano-encapsulated phase change materials (PCMs) 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. Results show that 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 improve the temperature and energy storage gains by enhancing the solar radiation capture capability of the heat storage medium. The storage capacity of paraffin@Cu slurry is increased by up to 290% depending on the PCM mass concentration. However, increasing the core size from 10 to 40 nm reduces the slurry's thermal energy storage by 5%, and increasing the volume concentration of Al particles in the shell reduces the thermal energy storage by 5%. The study also validates the specific heat capacity model of paraffin-based solid PCM under various wind speeds and solar radiation conditions. The paper also presents a mathematical model for analyzing the photothermal conversion behavior of a nano-encapsulated PCM slurry-based directly heated solar system. The model considers the effects of radiation, convection, and the optical and thermophysical properties of the PCM and shell materials. The study concludes that nano-encapsulated PCM slurries offer improved thermal performance compared to pure water, with the best performance achieved by paraffin@Cu slurry. The results demonstrate that the thermal performance of the system is significantly influenced by the shell material type, core size, and PCM mass concentration. The study also highlights the importance of optimizing the shell thickness and core size to enhance the thermal energy storage capacity of the system. The findings suggest that nano-encapsulated PCM slurries can be used as an effective energy storage medium in solar thermal systems, with the potential to improve the efficiency and reliability of solar energy storage.