Renewable energy resources are gaining worldwide attention, but they suffer from unpredictable and intermittent nature. The development of thermal energy storage solutions is crucial to the flexibility of the system running by balancing the energy supply and demand. Among thermal storage technologies, phase change material (PCM) shows promising potential given its high energy density due to the phase change process. Numerous studies have indicated the importance of increasing PCM thermal conductivity and developed strategies to enhance storage performance. However, the benefits of PCM cannot be guaranteed all the time when integrated into the storage unit. This PhD thesis develops standard methodologies to analyse and compare three types of PCM storage units. To evaluate the energy storage performance, the effective energy storage ratio Est is introduced, which is based on the concept of cut-off temperature to determine the useful energy. We firstly perform the optimisation study on the most common PCM storage unit, shell-and-tube, by maximising Est. A standard methodology has been proposed to identify optimal PCM volume ratios under various geometric and operational conditions, and material properties. The methodology serves as the benchmark for our ensuing investigation of the packed bed storage unit. The concept of “thresholds of bed and PCM-capsule diameter ratio”, found to be quantitatively correlated to the flow conditions, is proposed to demonstrate the performance advantage of packed bed PCM units. By decreasing PCM size to nanometers, we demonstrate a novel stratified phase change emulsion tank for cold storage, which improves the effective energy by 60% compared with water tanks. The thesis performs a comprehensive analysis of latent heat storage units and provides guidelines for the industrial design of PCM storage units.