Dzikevics M. “Solar energy accumulation with packed bed phase change materials” is a thesis in engineering science for a doctoral degree in engineering science (Dr. sc. ing.). Riga: Riga Technical University, 2019. Doctoral thesis has been prepared in Riga Technical University, Faculty of Power and Electrical Engineering, Institute of Energy Systems and Environment in the time period from 2014 till 2019. The hypothesis is that the use of stratified PCM in solar thermal storage will increase annual solar fraction and increase solar collector efficiency compared to traditional sensible thermal energy storage. The aim of the doctoral thesis is to evaluate the application of phase change materials (PCM) in thermal storage to increase solar fraction and solar collector efficiency. To achieve the aim of the thesis, the following objectives were set: 1. To carry out research analysis on solar thermal collector systems and phase change materials to design experimental set-up to be built within the laboratory and lead the installation of the system; 2. To carry out research analysis and experiments to obtain data for mathematical model validation; 3. To develop a short term simulation model for a thermal energy storage tank and validate it with obtained experimental data; 4. To develop a long term simulation model of solar domestic heating system and evaluate PCM use in a domestic solar thermal system with thermal storage. Introduction part contains novelty, relevance, tasks and aim of the thesis, formulation of the problem, applied research methods and practical value. First chapter summarizes the findings of the literature review about thermal storage and the use of phase change materials in solar heating systems. Main parameters describing sensible and latent heat energy storage systems are given. Second chapter describes the experimental system – facility, main object – storage tank, control and choosing of PCM and analysing PCM’s parameters. Third chapter contains energy storage experiments – methodology, heat loss experiments and cooling experiments. Experiments are needed to obtain data for mathematical model validation. Fourth chapter describes the principles of the mathematical model of the thermal energy storage tank. Fifth chapter describes the validation process of the short term model, which uses data from experiments. Sixth chapter describes the long term mathematical model and its analysis. The chapter contains sensitivity and optimization analysis descriptions. The main results of the research within the thesis are presented in this chapter.