## 1 Introduction  
The contemporary societies have enhanced energy needs, leading to an increasingly intensive research for the development of energy storage technologies. Global energy consumption, along with CO2 and greenhouse gasses emissions, is accelerating at a very fast pace due to global population growth, rapid global economic growth, and the ever-increasing human dependence on energy-consuming appliances. This rapid increase in global energy consumption has particular environmental implications that pose serious challenges to human health and the environment. Another consequence of the increment of global energy consumption is the reduction in the availability of traditional energy resources, such as oil, coal, and natural gas. This outcome has created a growing need for the development of new systems for the conversion and storage of clean and sustainable energy [1, 2]. The building sector has a very large impact on the overall global energy consumption, and, furthermore, to the environment, by high emissions of greenhouse gasses, CO2, and the acidification of the oceans. The increased energy consumption does not occur only during the production of structural materials and the building construction, but mostly for the operation of the premises [3]. It has been proven that buildings' operation energy consumption for providing indoor comfort accounts for 30% of total energy consumption [4]. Thermal energy storage (TES) is a promising and sustainable method for decreasing the energy consumptions in the building sector. Systems of TES using phase change materials (PCMs) find numerous applications for providing and maintaining a comfortable environment of the building envelope, without consumption of electrical energy or fuel [5]. Phase change materials are substances that are able to absorb and store large amounts of thermal energy. The mechanism of PCMs for energy storage relies on the increased energy need of some materials to undergo phase transition. They are able to absorb sensible heat as their temperature rise, and, at the phase change temperature, absorb a large amount of heat, which is called latent heat of fusion, in order to change phase. The energy stays stored in the PCM until the temperature decreases and the material undergoes phase transition again, which also signifies the energy release [1]. The materials used as PCMs can be classified based on the type of phase change to solid-liquid, liquid-gas, and solid-solid compounds. The latent heat in solid-solid PCMs, such as polyurethanes, cross-linked polyethylene, and other polymers, is relatively low compared to the latent heat of those that are in the solid-liquid form [6]. As an outcome, the PCMs used for building applications are mostly solid-liquid type. These can be organic, such as paraffin, fatty acids, etc., inorganic, such as hydrate salts and metals, or their eutectic mixtures. Organic PCMs are the best candidates for multiple building applications, though their low thermal conductivity requires the fabrication of composite materials with enhanced thermal conductivity. The most investigated approach is the encapsulation of PCMs with thermal conductive materials. The encapsulation method presents many advantages, such as the leakage of liquid phase management and the protection of the PCM from corrosion. Another approach is the formation of form-stable composite PCMs, which consist of the PCM dispersed inside a cross-linked polymer matrix, a porous mineral material or expanded graphite or perlite [3,5]. There has been a lot of research for the utilization of these materials in active and passive building applications for the reduction of electrical energy and fuel consumption. Phase change materials are used in active applications for heating, ventilation, and air conditioning systems, domestic hot water applications, suspended ceilings, external solar facades, etc. [4].