This thesis investigates the integration of phase change materials (PCMs) for thermal energy storage in solar collector systems. Addressing solar energy intermittency, the research explores innovative PCM applications across different solar collector configurations. Three experimental case studies are analyzed, including conventional evacuated tube heat-pipe systems and direct-absorption collectors featuring shape-stabilized PCMs (SSPCMs). The experimental investigations evaluate the system's charging and discharging processes under varied solar irradiance conditions and the implementation of heat transfer enhancement techniques, such as porous media and finned configurations. Results from Case Study I show that integrating PCM with a thermal battery and porous media results in a system efficiency of 50% ± 9.3%, with the system heating over 120 L of water per day to 38 °C under sunny conditions. In Case Study II, the use of SSPCM in a direct-absorption collector improved thermal efficiency from 66 to 82%, achieving an optimal balance of charging and discharging performance at moderate water flow rates. Case Study III, comparing finned and un-finned configurations, revealed that the un-finned system achieved higher daily efficiency, while the finned configuration provided more uniform heat distribution and prolonged heat release. These findings suggest that optimal PCM integration, coupled with appropriate system modifications, offers a cost-effective, sustainable solution for domestic and industrial heating applications. The work contributes to understanding latent heat energy storage mechanisms and highlights the potential for future PCM-based solar thermal systems.