Thermal energy encounters a huge demand in the world, part of which can be met by renewable energy sources, such as solar energy, and storage of thermal energy surplus from industrial processes. For this purpose, thermal energy storage (TES) units, in which heat is stored, are developed. The energy is accumulated by phase change materials (PCM) characterized by high phase transition enthalpy. PCMs have poor thermal conductivity; therefore, to take full advantage of their capabilities and to accelerate the charging and discharging cycle, metallic structures are used. These structures are manufactured using investment casting technology. Creating models with additive methods, such as 3D printing, allows obtaining complex shapes with high accuracy, such as thin-walled castings. At a large scale, the method may not be cost-effective. In this paper, the heat exchanger models were made from PLA and the castings - from AC44200 aluminum alloy. Investment casting requires the proper selection of parameters, such as the right material for the model, the selection of the firing temperature, the adjustment of the temperature of the molten metal, the temperature of the mold, and the pressure in it. Misaligning any of the parameters can lead to imperfections on the finished casting. Based on the model roughness study, it was found that minor roughness and higher accuracy are presented by the lower parts of the casting, while weaker performance is observed for the upper parts. Metal castings in a salt PCM environment may be subjected to corrosion. Therefore, the authors proposed to produce protective coatings on aluminum castings by the PEO method - plasma electrolytic oxidation. Porous ceramic thin films consisting mainly of alumina were obtained. The next tests will be aimed to confirm whether this layer will not negatively influence the thermal conductivity of the thermal energy storage.