## 1 Introduction
Current research in the field of heat storage is focused on increasing the thermal conductivity of PCMs in a variety of ways, including the effects of PCM geometrical parameters, PCM heat exchanger configurations and the addition of nanoadditives such as carbon, graphene, graphite, metals, nanotubes, metal foams, etc., but at the same time, problems such as the sedimentation of additives also arise from this. The main three factors that influence the selection of PCMs for different applications are the melting temperature, the latent heat of fusion and the thermophysical properties of the PCM, the primary thermal conductivity [21; 22; 23]. Studies on this subject also focus on increasing the heat transfer rate in PCMs. Currently, commercially available heat storage systems are based on sensible heat, while PCM-based heat storage systems are mostly in the development or experimental commercial use phase. As this literature review shows, most works deal with paraffin-based PCMs, which indicates the importance of this substance and its techno-economic potential for further improvement and application [24; 25]. The heat transfer from the surface of a PCM to the inner layers of the PCM is dependent on its physical properties. According to [26; 27], the melting process can be described by four phases: (1) PCM in solid state, (2) PCM in solid state--phase change, (3) PCM in semi-solid state--phase change, (4) PCM in liquid state--liquid. With respect to the \"massiveness\" of the PCM, but especially the very small values of \(\lambda\) in the solid state--valid for phases (1) to (3)--the internal heat exchange is of crucial importance. Therefore, it is necessary to consider not only the conditions on the surface between the PCM and the tank in which the PCM is placed, but also the internal heat exchange process itself. Several authors, mentioned in [28], have used various methods leading to an improvement in the internal heat exchange, especially in the semi-solid state, where a liquid phase has already been formed between the container in which the PCM is stored and the solid phase of the PCM, and this separates the core of the PCM (in the solid phase from the exchange surface). From their results, it can be concluded that there is a decrease in the rate of phase change, and this is due to the low value of \(\lambda\). As the research demonstrates, it is essential to ensure an increase in the \(\lambda\) value of a PCM that is still in the solid phase. Research on the influence of the size, mass and arrangement of PCMs in thermal energy storage, as well as ways to improve the thermal conductivity of PCMs, is mostly based on mathematical, analytical and numerical models [28], which are not supported by experimental studies. Therefore, an experimental model was developed to test PCMs, in which various thermophysical parameters of such materials, as well as methods to improve the methods of thermal energy storage and transfer, were investigated. Based on the growing demand for sustainable energy solutions and the limitations of current energy storage methods, this study focuses on improving the thermal conductivity of phase change materials (PCMs). Although PCMs offer significant advantages for thermal energy storage due to their high latent heat capacity, their low thermal conductivity hinders their practical application. Existing research has explored various methods to improve the performance of PCMs, including the incorporation of nanoadditives, microencapsulation and geometric modifications (e.g., fins) [21; 22; 23]. However, a critical gap in the research remains regarding the achievement of significant improvements in cure and fusion rates without compromising other important PCM properties and without implementing complex and expensive modifications. This research aims to address this gap by exploring the implementation of readily available, uncomplicated metal structures to significantly improve the thermal conductivity and heat transfer rate of PCMs.