## 1 Introduction  
The development and popularization of renewable energy source applications, the reduction in energy consumption and the optimization of energy management in order to limit environmental problems (air pollution, global warming, acid rain, etc.) and the impacts of the global energy crisis have now become a real focus of scientific and engineering research. In addition, due to the impressive increase in demands in the building and industrial sectors worldwide, there is a growing need for appropriate methods to improve the thermal performance of heating and cooling systems. The market is looking for new materials, systems and technologies that could be used to reduce the dependence of buildings on fossil fuels, to use renewable energy sources, to contribute to the greener and more efficient use of energy and to improve indoor thermal comfort in a more sustainable and cost-effective way. Apart from heating, one of the biggest energy needs in buildings is the preparation of domestic hot water. In this area, it is very important to use PCMs (phase change materials) for heat storage in order to achieve the necessary savings and a balance between peak demand and energy production. This is confirmed by the amount of research and testing of these materials and their practical applications. Here, the current practice is facing several challenges, especially regarding the economic feasibility of implementing PCMs and the ways to use them [1; 2]. Thermal energy storage (TES) and phase change materials (PCM) are efficient technologies that can store high thermal energy densities in a limited temperature range. An extensive method to reduce energy consumption is the use of phase change materials known as \"PCMs\". PCMs have attracted much attention and have become a topic of major interest in recent decades. Latent heat accumulation is a highly promising method because it can provide higher energy storage densities than thermal energy storage systems in the form of sensible heat, and has the ability to store heat at a constant (or approximately constant) temperature corresponding to the phase transition temperature of the PCM. PCMs used in TES must meet two main requirements, namely high latent heat values and high thermal conductivity, and at the same time, they must have a melting/solidification point within the practical range of application [3; 4; 5]. Phase change materials can be used as preferred materials for energy storage and have the advantage of being able to maintain a high energy density under limited temperature conditions. The issue is that PCMs have relatively low thermal conductivity, which can limit the charge and discharge rates of a thermal energy storage device. Existing research focuses on potential solutions to improve the physicochemical properties of accumulation materials and their compatibility with each other in multi-stage phase accumulation. Existing research is also focused on monitoring the properties of binary compounds. The results are aimed at improving energy efficiency by increasing the ratio of renewable sources, through energy management and by reducing energy consumption for heating and cooling. This is caused by the significant increase in energy demand worldwide due to the rapid growth of the world's population and economy, which, in turn, leads to severe environmental impacts [4; 5]. The most suitable PCMs for research and application (solid-liquid state and vice versa) are paraffins, which have many advantages over other materials. Their primary characteristics are a wide range of temperature ranges, relatively high latent heat, a noncorrosive nature, durability and a long cyclic lifetime. PCM-TES systems for heat storage are receiving a lot of attention, and various possible PCM-based technological approaches in heating systems are being investigated. Emphasis is placed on the concept of PCM systems for space heating and the use of PCMs in passive and active systems.