This dissertation dissects the phase change material based thermal energy storage unit by applying an analytical model, numerical modelling, and experimental research. The thesis addresses several key objectives related to the effect of the thermophysical properties of phase change materials, the geometry of the heat exchanger, and the parameters of the heat transfer fluid on the operational characteristics of the phase change material (PCM) based thermal energy storage unit. The dissertation consists of an introduction, three chapters, general conclusions, lists of used literature and the author’s publications on the subject of the dissertation, and two appendices. The introduction covers the investigated problem and the relevance of the thesis, describes the object of the research, formulates the aim and tasks of the thesis, describes the research methodology, the scientific novelty of the thesis, the practical significance of its results and defended statements. The introduction closes with the overview of the author’s publications and conference reports on the subject of the dissertation and the outline of the dissertation structure. Chapter 1 deals with the literature analysis and closes with a formulation of conclusions and specification of the objectives of the dissertation. Chapter 2 covers the research methodology for the PCM thermal energy storage unit providing the foundation for the integrated study that includes numerical modelling, application of analytical expressions, and experimentation. The research results for the PCM thermal energy storage unit are presented in Chapter 3. It analyses the outcomes of numerical modelling whilst studying the distribution of temperature and the duration of phase change at the selected points and plane of the PCM thermal energy storage unit. A determination of the heat transfer coefficient is conducted by applying analytical expressions, and the results obtained are compared to the results of the experiments. The further description and analysis delve into the results of experimental research covering the effects of the change in the PCM density, the geometry of the heat exchanger, and the mass flow rate and temperature of the heat transfer fluid on the operational characteristics of the unit.